Steven Frankel | Technion Israel Institute of Technology (original) (raw)

Papers by Steven Frankel

Research paper thumbnail of Accuracy Assessment of RANS Predictions of Active Flow Control for Hydrofoil Cavitation

Processes

In this work, we numerically investigate the cavitating flow on the scaled-down 2D model of guide... more In this work, we numerically investigate the cavitating flow on the scaled-down 2D model of guided vanes. Furthermore, the effects of wall injection on both the cavitation and on the hydrodynamic performance of the guided vane are studied. The numerical simulations are performed using OpenFOAM v1906. We used a 2D k- ω SST model for modeling the turbulence in the present set of simulations. We studied the flow for two angles of attack, viz. 3 ∘ and 9 ∘ . For the 3 ∘ angle of attack, the present numerical work is in good agreement with the previous experimental work, but for the larger angle of attack, because of flow separation, the present simulations do not capture the flow correctly.

Research paper thumbnail of High-Order Implicit-Explicit Multi-block Time-stepping Method for Hyperbolic PDEs

52nd Aerospace Sciences Meeting, 2014

This work seeks to explore and improve the current time-stepping schemes used in computational fl... more This work seeks to explore and improve the current time-stepping schemes used in computational fluid dynamics (CFD) in order to reduce overall computational time. A high-order scheme has been developed using a combination of implicit and explicit (IMEX) time-stepping Runge-Kutta (RK) schemes which increases numerical stability with respect to the time step size, resulting in decreased computational time. The IMEX scheme alone does not yield the desired increase in numerical stability, but when used in conjunction with an overlapping partitioned (multi-block) domain significant increase in stability is observed. To show this, the Overlapping-Partition IMEX (OP IMEX) scheme is applied to both one-dimensional (1D) and two-dimensional (2D) problems, the nonlinear viscous Burger's equation and 2D advection equation, respectively. The method uses two different summation by parts (SBP) derivative approximations, second-order and fourth-order accurate. The Dirichlet boundary conditions are imposed using the Simultaneous Approximation Term (SAT) penalty method. The 6-stage additive Runge-Kutta IMEX time integration schemes are fourth-order accurate in time. An increase in numerical stability 65 times greater than the fully explicit scheme is demonstrated to be achievable with the OP IMEX method applied to 1D Burger's equation. Results from the 2D, purely convective, advection equation show stability increases on the order of 10 times the explicit scheme using the OP IMEX method. Also, the domain partitioning method in this work shows potential for breaking the computational domain into manageable sizes such that implicit solutions for full three-dimensional CFD simulations can be computed using direct solving methods rather than the standard iterative methods currently used.

Research paper thumbnail of A Study of Coupled Turbulent Mixing, Soot Chemistry, and Radiation Effects Using the Linear Eddy Model

Combustion and Flame, 1998

Transient simulations of strongly radiating, acetylene-air, nonpremixed flames in stationary, hom... more Transient simulations of strongly radiating, acetylene-air, nonpremixed flames in stationary, homogeneous turbulence are conducted in order to study coupled turbulence, soot chemistry, and radiation interactions. The linear eddy model is used to simulate turbulent advection. A laminar flamelet state relationship combustion model is employed along with two different soot models. The first soot model involves an extension of the laminar flamelet concept to soot using a soot volume fraction state relationship. The second soot model involves transport equations for soot mass fraction and soot number density, which include finite rate source terms to account for soot nucleation, surface growth, agglomeration, and oxidation. Radiation effects are accounted for by including the appropriate source/sink terms in the conservation of energy equation. The effects of a presumed surrounding large scale field which radiates with the spectral properties of soot at an assumed effective temperature are also included. Simulations are conducted for two values of the surrounding temperature and the model large eddy turnover time. The results capture several unique aspects of strongly radiating turbulent flames. In particular, an inflection point in the temperature versus mixture fraction profile is observed near the soot region which highlights the effects of radiative cooling. The large difference between radiation source terms calculated using mean properties and those calculated using instantaneous properties highlights the important interactions between turbulence and radiation.

Research paper thumbnail of Study of Spectral Noise Emissions from Standard Turbulent Nonpremixed Flames

Aiaa Journal, 2004

The occurrence of oscillating combustion and combustion instability has led to resurgence of inte... more The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in causes, mechanisms, suppression, and control of flame noise. Nonpremixed flame noise is low frequency and difficult to control using conventional acoustic liner and so ...

Research paper thumbnail of Numerical simulation of turbulence transition and sound radiation for flow through a rigid glottal model

Journal of The Acoustical Society of America, 2007

Large eddy simulation (LES)-based computational aeroacoustics techniques were applied to a static... more Large eddy simulation (LES)-based computational aeroacoustics techniques were applied to a static model of the human glottis, idealized here as a planar channel with an orifice, to study flow-acoustic interactions related to speech. Rigid models of both converging and diverging glottal passages, each featuring a 20 deg included angle and a minimal glottal diameter of 0.04 cm, with an imposed transglottal pressure of 15 cm H2O, were studied. The Favre-filtered compressible Navier-Stokes equations were integrated for this low-Mach-number flow using an additive semi-implicit Runge-Kutta method and a high-order compact finite-difference scheme with characteristic-based nonreflecting boundary conditions and a multiblock approach. Flow asymmetries related to the Coanda effect and transition to turbulence, as well as the far-field sound, were captured. Acoustic-analogy-based far-field sound predictions were compared with direct simulations and showed that dipole sources, arising from unsteady flow forces exerted on the glottal walls, are primarily responsible for the tonal sound observed in the divergent glottis case.

Research paper thumbnail of Stochastic Inlet Conditions for Large-Eddy Simulation of a Fully Turbulent Jet

Aiaa Journal, 2003

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Research paper thumbnail of Effects of Spatial Filtering on Sound Radiation from a Subsonic Axisymmetric Jet

Aiaa Journal, 2000

ABSTRACT

Research paper thumbnail of Experimental verification of the quasi-steady approximation for aerodynamic sound generation by pulsating jets in tubes

Journal of The Acoustical Society of America, 2002

Voice production involves sound generation by a confined jet flow through an orifice (the glottis... more Voice production involves sound generation by a confined jet flow through an orifice (the glottis) with a time-varying area. Predictive models of speech production are usually based on the so-called quasi-steady approximation. The flow rate through the time-varying orifice is assumed to be the same as a sequence of steady flows through stationary orifices for wall geometries and flow boundary conditions that instantaneously match those of the dynamic, nonstationary problem. Either the flow rate or the pressure drop can then be used to calculate the radiated sound using conventional acoustic radiation models. The quasi-steady approximation allows complex unsteady flows to be modeled as steady flows, which is more cost effective. It has been verified for pulsating open jet flows. The quasi-steady approximation, however, has not yet been rigorously validated for the full range of flows encountered in voice production. To further investigate the range of validity of the quasi-steady approximation for voice production applications, a dynamic mechanical model of the larynx was designed and built. The model dimensions approximated those of human vocal folds. Airflow was supplied by a pressurized, quiet air storage facility and modulated by a driven rubber orifice. The acoustic pressure of waves radiated upstream and downstream of the orifice was measured, along with the orifice area and other time-averaged flow variables. Calculated and measured radiated acoustic pressures were compared. A good agreement was obtained over a range of operating frequencies, flow rates, and orifice shapes, confirming the validity of the quasi-steady approximation for a class of relevant pulsating jet flows.

Research paper thumbnail of Computational aeroacoustics of phonation, Part I: Computational methods and sound generation mechanisms

Journal of The Acoustical Society of America, 2002

The aerodynamic generation of sound during phonation was studied using direct numerical simulatio... more The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered ͑125 Hz͒. The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

Research paper thumbnail of Broadband sound generation by confined turbulent jets

Journal of The Acoustical Society of America, 2002

Sound generation by confined stationary jets is of interest to the study of voice and speech prod... more Sound generation by confined stationary jets is of interest to the study of voice and speech production, among other applications. The generation of sound by low Mach number, confined, stationary circular jets was investigated. Experiments were performed using a quiet flow supply, muffler-terminated rigid uniform tubes, and acrylic orifice plates. A spectral decomposition method based on a linear source-filter model was used to decompose radiated nondimensional sound pressure spectra measured for various gas mixtures and mean flow velocities into the product of ͑1͒ a source spectral distribution function; ͑2͒ a function accounting for near field effects and radiation efficiency; and ͑3͒ an acoustic frequency response function. The acoustic frequency response function agreed, as expected, with the transfer function between the radiated acoustic pressure at one fixed location and the strength of an equivalent velocity source located at the orifice. The radiation efficiency function indicated a radiation efficiency of the order (kD) 2 over the planar wave frequency range and (kD) 4 at higher frequencies, where k is the wavenumber and D is the tube cross sectional dimension. This is consistent with theoretical predictions for the planar wave radiation efficiency of quadrupole sources in uniform rigid anechoic tubes. The effects of the Reynolds number, Re, on the source spectral distribution function were found to be insignificant over the range 2000ϽReϽ20 000. The source spectral distribution function approximately obeyed a St Ϫ3 power law for Strouhal number values StϽ0.9, and a St Ϫ5 power law for StϾ2.5. The influence of a reflective open tube termination on the source function spectral distribution was found to be insignificant, confirming the absence of a feedback mechanism.

Research paper thumbnail of Aerodynamic transfer of energy to the vocal folds

Journal of The Acoustical Society of America, 2005

The aerodynamic transfer of energy from glottal airflow to vocal fold tissue during phonation was... more The aerodynamic transfer of energy from glottal airflow to vocal fold tissue during phonation was explored using complementary synthetic and numerical vocal fold models. The synthetic model was fabricated using a flexible polyurethane rubber compound. The model size, shape, and material properties were generally similar to corresponding human vocal fold characteristics. Regular, self-sustained oscillations were achieved at a frequency of approximately 120 Hz. The onset pressure was approximately 1.2 kPa. A corresponding two-dimensional finite element model was developed using geometry definitions and material properties based on the synthetic model. The finite element model upstream and downstream pressure boundary conditions were based on experimental values acquired using the synthetic model. An analysis of the fully coupled fluid and solid numerical domains included flow separation and unsteady effects. The numerical results provided detailed flow data that was used to investigate aerodynamic energy transfer mechanisms. The results support the hypothesis that a cyclic variation of the orifice profile from a convergent to a divergent shape leads to a temporal asymmetry in the average wall pressure, which is the key factor for the achievement of self-sustained vocal fold oscillations. me rica.

Research paper thumbnail of Sound generation by steady flow through glottis-shaped orifices

Journal of The Acoustical Society of America, 2004

Although the signature of human voice is mostly tonal, it also includes a significant broadband c... more Although the signature of human voice is mostly tonal, it also includes a significant broadband component. Quadrupolelike sources due to turbulence in the region downstream of the glottis, and dipolelike sources due to the force applied by the vocal folds onto the surrounding fluid are the two primary broadband sound generating mechanisms. In this study, experiments were conducted to characterize the broadband sound emissions of confined stationary jets through rubber orifices formed to imitate the approximate shape of the human glottis at different stages during one cycle of vocal fold vibrations. The radiated sound pressure spectra downstream of the orifices were measured for varying flow rates, orifice shapes, and gas mixtures. The nondimensional sound pressure spectra were decomposed into the product of three functions: a source function F, a radiation efficiency function M , and an acoustic response function G. The results show that, as for circular jets, the quadrupole source contributions dominated for straight and convergent orifices. For divergent jets, whistling tonal sounds were emitted at low flow rates. At high flow rates for the same geometry, dipole contributions dominated the sound radiated by free jets. However, possible source-load acoustic feedback may have hampered accurate source identification in confined flows.

Research paper thumbnail of Computational aeroacoustics of phonation, Part II: Effects of flow parameters and ventricular folds

Journal of The Acoustical Society of America, 2002

The aerodynamic generation of sound during phonation was studied using direct numerical simulatio... more The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered ͑125 Hz͒. The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

Research paper thumbnail of Modeling transition to turbulence in eccentric stenotic flows

Journal of Biomechanical Engineering-transactions of The Asme, 2008

Mean flow predictions obtained from a host of turbulence models were found to be in poor agreemen... more Mean flow predictions obtained from a host of turbulence models were found to be in poor agreement with recent direct numerical simulation results for turbulent flow distal to an idealized eccentric stenosis. Many of the widely used turbulence models, including a large eddy simulation model, were unable to accurately capture the post-stenotic transition to turbulence. The results suggest that efforts towards developing more accurate turbulence models for low Reynolds number, separated transitional flows are necessary before such models can be used confidently under hemodynamic conditions where turbulence may develop.

Research paper thumbnail of Numerical Modeling of Pulsatile Turbulent Flow in Stenotic Vessels

Journal of Biomechanical Engineering-transactions of The Asme, 2003

Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-aver... more Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number kturbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k -⑀ turbulence model and the standard k -⑀ model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred. Fig. 7 Streamlines for the smooth stenosis from the low Reynolds number k -model at different phases in the flow cycle. The streamfunction increment is 0.0018 s À1 .

Research paper thumbnail of Direct numerical simulation of stenotic flows, Part 1: Steady flow

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) s... more Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predicted a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking the symmetry of the post-stenotic flowfield by causing the jet to deflect towards the side of the eccentricity and at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned into turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a −5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent puff observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of thirty but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to manifest itself in large temporal and spatial gradients of WSS, with significant axial and circumferential variations in the turbulent section.

Research paper thumbnail of Direct numerical simulation of stenotic flows. Part 1. Steady flow

Journal of Fluid Mechanics, 2007

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) s... more Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predicted a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking the symmetry of the post-stenotic flowfield by causing the jet to deflect towards the side of the eccentricity and at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned into turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a −5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent puff observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of thirty but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to manifest itself in large temporal and spatial gradients of WSS, with significant axial and circumferential variations in the turbulent section.

Research paper thumbnail of Direct numerical simulation of stenotic flows. Part 2. Pulsatile flow

Journal of Fluid Mechanics, 2007

Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were d... more Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were discussed in Part 1 of this study. DNS of pulsatile flow through the 75% stenosed tube (by area) employed for the computations in Part 1 is examined here. Analogous to the steady flow results, DNS predicts a laminar post-stenotic flowfield in the case of pulsatile flow through the axisymmetric stenosis model, in contrast to previous experiments, in which intermittent disturbed flow regions and turbulent breakdown were observed in the downstream region. The introduction of a stenosis eccentricity, that was 5% of the main vessel diameter at the throat, resulted in periodic, localized transition to turbulence. Analysis in this study indicated that the early and mid-acceleration phases of the time period cycle were relatively stable, with no turbulent activity in the poststenotic region. However, towards the end of acceleration, the starting vortex, formed earlier as the fluid accelerated through the stenosis at the beginning of acceleration, started to break up into elongated streamwise structures. These streamwise vortices broke down at peak flow, forming a turbulent spot in the post-stenotic region. The early part of deceleration witnessed intense turbulent activity within this spot. Past the middeceleration phase, through to minimum flow, the inlet flow lost its momentum and the flowfield began to relaminarize. The start of acceleration in the following cycle saw a recurrence of the entire process of a starting structure undergoing turbulent breakdown and subsequent relaminarization of the post-stenotic flowfield. Peak wall shear stress (WSS) magnitudes occurred at the stenosis throat and close to the reattachment location during most of the acceleration phase, with the rest of the vessel experiencing much lower levels of WSS. Turbulent breakdown at peak flow resulted in a sharp amplification of WSS levels across the region corresponding to the turbulent spot, accompanied by large axial and circumferential fluctuations. Magnitudes dropped rapidly after the mid-deceleration phase, when the relaminarization process took over.

Research paper thumbnail of Performance of a 3Bladed Propeller Pump to Provide Cavopulmonary Assist in the Failing Fontan Circulation

Annals of Thoracic Surgery, 2008

Purpose. We hypothesized that a propeller pump design would function optimally to provide cavopul... more Purpose. We hypothesized that a propeller pump design would function optimally to provide cavopulmonary assist in a univentricular Fontan circulation.

Research paper thumbnail of Mechanical Cavopulmonary Assist for the Univentricular Fontan Circulation Using a Novel Folding Propeller Blood Pump

Asaio Journal, 2007

A blood pump specifically designed to operate in the unique anatomic and physiologic conditions o... more A blood pump specifically designed to operate in the unique anatomic and physiologic conditions of a cavopulmonary connection has never been developed. Mechanical augmentation of cavopulmonary blood flow in a univentricular circulation would reduce systemic venous pressure, increase preload to the single ventricle, and temporarily reproduce a scenario analogous to the normal two-ventricle circulation. We hypothesize that a folding propeller blood pump would function optimally in this cavopulmonary circulation. The hydraulic performance of a two-bladed propeller prototype was characterized in an experimental flow loop using a blood analog fluid for 0.5-3.5 lpm at rotational speeds of 3,600-4,000 rpm. We also created five distinctive blood pump designs and evaluated their hydraulic performance using computational fluid dynamics (CFD). The two-bladed prototype performed well over the design range of 0.5-3.5 lpm, producing physiologic pressure rises of 5-18 mm Hg. Building upon this proof-of-concept testing, the CFD analysis of the five numerical models predicted a physiologic pressure range of 5-40 mm Hg over 0.5-4 lpm for rotational speeds of 3,000-7,000 rpm. These preliminary propeller designs and the two-bladed prototype achieved the expected hydraulic performance. Optimization of these configurations will reduce fluid stress levels, remove regions of recirculation, and improve the hydraulic performance of the folding propeller. This propeller design produces the physiologic pressures and flows that are in the ideal range to mechanically support the cavopulmonary circulation and represents an exciting new therapeutic option for the support of a univentricular Fontan circulation.

Research paper thumbnail of Accuracy Assessment of RANS Predictions of Active Flow Control for Hydrofoil Cavitation

Processes

In this work, we numerically investigate the cavitating flow on the scaled-down 2D model of guide... more In this work, we numerically investigate the cavitating flow on the scaled-down 2D model of guided vanes. Furthermore, the effects of wall injection on both the cavitation and on the hydrodynamic performance of the guided vane are studied. The numerical simulations are performed using OpenFOAM v1906. We used a 2D k- ω SST model for modeling the turbulence in the present set of simulations. We studied the flow for two angles of attack, viz. 3 ∘ and 9 ∘ . For the 3 ∘ angle of attack, the present numerical work is in good agreement with the previous experimental work, but for the larger angle of attack, because of flow separation, the present simulations do not capture the flow correctly.

Research paper thumbnail of High-Order Implicit-Explicit Multi-block Time-stepping Method for Hyperbolic PDEs

52nd Aerospace Sciences Meeting, 2014

This work seeks to explore and improve the current time-stepping schemes used in computational fl... more This work seeks to explore and improve the current time-stepping schemes used in computational fluid dynamics (CFD) in order to reduce overall computational time. A high-order scheme has been developed using a combination of implicit and explicit (IMEX) time-stepping Runge-Kutta (RK) schemes which increases numerical stability with respect to the time step size, resulting in decreased computational time. The IMEX scheme alone does not yield the desired increase in numerical stability, but when used in conjunction with an overlapping partitioned (multi-block) domain significant increase in stability is observed. To show this, the Overlapping-Partition IMEX (OP IMEX) scheme is applied to both one-dimensional (1D) and two-dimensional (2D) problems, the nonlinear viscous Burger's equation and 2D advection equation, respectively. The method uses two different summation by parts (SBP) derivative approximations, second-order and fourth-order accurate. The Dirichlet boundary conditions are imposed using the Simultaneous Approximation Term (SAT) penalty method. The 6-stage additive Runge-Kutta IMEX time integration schemes are fourth-order accurate in time. An increase in numerical stability 65 times greater than the fully explicit scheme is demonstrated to be achievable with the OP IMEX method applied to 1D Burger's equation. Results from the 2D, purely convective, advection equation show stability increases on the order of 10 times the explicit scheme using the OP IMEX method. Also, the domain partitioning method in this work shows potential for breaking the computational domain into manageable sizes such that implicit solutions for full three-dimensional CFD simulations can be computed using direct solving methods rather than the standard iterative methods currently used.

Research paper thumbnail of A Study of Coupled Turbulent Mixing, Soot Chemistry, and Radiation Effects Using the Linear Eddy Model

Combustion and Flame, 1998

Transient simulations of strongly radiating, acetylene-air, nonpremixed flames in stationary, hom... more Transient simulations of strongly radiating, acetylene-air, nonpremixed flames in stationary, homogeneous turbulence are conducted in order to study coupled turbulence, soot chemistry, and radiation interactions. The linear eddy model is used to simulate turbulent advection. A laminar flamelet state relationship combustion model is employed along with two different soot models. The first soot model involves an extension of the laminar flamelet concept to soot using a soot volume fraction state relationship. The second soot model involves transport equations for soot mass fraction and soot number density, which include finite rate source terms to account for soot nucleation, surface growth, agglomeration, and oxidation. Radiation effects are accounted for by including the appropriate source/sink terms in the conservation of energy equation. The effects of a presumed surrounding large scale field which radiates with the spectral properties of soot at an assumed effective temperature are also included. Simulations are conducted for two values of the surrounding temperature and the model large eddy turnover time. The results capture several unique aspects of strongly radiating turbulent flames. In particular, an inflection point in the temperature versus mixture fraction profile is observed near the soot region which highlights the effects of radiative cooling. The large difference between radiation source terms calculated using mean properties and those calculated using instantaneous properties highlights the important interactions between turbulence and radiation.

Research paper thumbnail of Study of Spectral Noise Emissions from Standard Turbulent Nonpremixed Flames

Aiaa Journal, 2004

The occurrence of oscillating combustion and combustion instability has led to resurgence of inte... more The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in causes, mechanisms, suppression, and control of flame noise. Nonpremixed flame noise is low frequency and difficult to control using conventional acoustic liner and so ...

Research paper thumbnail of Numerical simulation of turbulence transition and sound radiation for flow through a rigid glottal model

Journal of The Acoustical Society of America, 2007

Large eddy simulation (LES)-based computational aeroacoustics techniques were applied to a static... more Large eddy simulation (LES)-based computational aeroacoustics techniques were applied to a static model of the human glottis, idealized here as a planar channel with an orifice, to study flow-acoustic interactions related to speech. Rigid models of both converging and diverging glottal passages, each featuring a 20 deg included angle and a minimal glottal diameter of 0.04 cm, with an imposed transglottal pressure of 15 cm H2O, were studied. The Favre-filtered compressible Navier-Stokes equations were integrated for this low-Mach-number flow using an additive semi-implicit Runge-Kutta method and a high-order compact finite-difference scheme with characteristic-based nonreflecting boundary conditions and a multiblock approach. Flow asymmetries related to the Coanda effect and transition to turbulence, as well as the far-field sound, were captured. Acoustic-analogy-based far-field sound predictions were compared with direct simulations and showed that dipole sources, arising from unsteady flow forces exerted on the glottal walls, are primarily responsible for the tonal sound observed in the divergent glottis case.

Research paper thumbnail of Stochastic Inlet Conditions for Large-Eddy Simulation of a Fully Turbulent Jet

Aiaa Journal, 2003

RefDoc Bienvenue - Welcome. Refdoc est un service / is powered by. ...

Research paper thumbnail of Effects of Spatial Filtering on Sound Radiation from a Subsonic Axisymmetric Jet

Aiaa Journal, 2000

ABSTRACT

Research paper thumbnail of Experimental verification of the quasi-steady approximation for aerodynamic sound generation by pulsating jets in tubes

Journal of The Acoustical Society of America, 2002

Voice production involves sound generation by a confined jet flow through an orifice (the glottis... more Voice production involves sound generation by a confined jet flow through an orifice (the glottis) with a time-varying area. Predictive models of speech production are usually based on the so-called quasi-steady approximation. The flow rate through the time-varying orifice is assumed to be the same as a sequence of steady flows through stationary orifices for wall geometries and flow boundary conditions that instantaneously match those of the dynamic, nonstationary problem. Either the flow rate or the pressure drop can then be used to calculate the radiated sound using conventional acoustic radiation models. The quasi-steady approximation allows complex unsteady flows to be modeled as steady flows, which is more cost effective. It has been verified for pulsating open jet flows. The quasi-steady approximation, however, has not yet been rigorously validated for the full range of flows encountered in voice production. To further investigate the range of validity of the quasi-steady approximation for voice production applications, a dynamic mechanical model of the larynx was designed and built. The model dimensions approximated those of human vocal folds. Airflow was supplied by a pressurized, quiet air storage facility and modulated by a driven rubber orifice. The acoustic pressure of waves radiated upstream and downstream of the orifice was measured, along with the orifice area and other time-averaged flow variables. Calculated and measured radiated acoustic pressures were compared. A good agreement was obtained over a range of operating frequencies, flow rates, and orifice shapes, confirming the validity of the quasi-steady approximation for a class of relevant pulsating jet flows.

Research paper thumbnail of Computational aeroacoustics of phonation, Part I: Computational methods and sound generation mechanisms

Journal of The Acoustical Society of America, 2002

The aerodynamic generation of sound during phonation was studied using direct numerical simulatio... more The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered ͑125 Hz͒. The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

Research paper thumbnail of Broadband sound generation by confined turbulent jets

Journal of The Acoustical Society of America, 2002

Sound generation by confined stationary jets is of interest to the study of voice and speech prod... more Sound generation by confined stationary jets is of interest to the study of voice and speech production, among other applications. The generation of sound by low Mach number, confined, stationary circular jets was investigated. Experiments were performed using a quiet flow supply, muffler-terminated rigid uniform tubes, and acrylic orifice plates. A spectral decomposition method based on a linear source-filter model was used to decompose radiated nondimensional sound pressure spectra measured for various gas mixtures and mean flow velocities into the product of ͑1͒ a source spectral distribution function; ͑2͒ a function accounting for near field effects and radiation efficiency; and ͑3͒ an acoustic frequency response function. The acoustic frequency response function agreed, as expected, with the transfer function between the radiated acoustic pressure at one fixed location and the strength of an equivalent velocity source located at the orifice. The radiation efficiency function indicated a radiation efficiency of the order (kD) 2 over the planar wave frequency range and (kD) 4 at higher frequencies, where k is the wavenumber and D is the tube cross sectional dimension. This is consistent with theoretical predictions for the planar wave radiation efficiency of quadrupole sources in uniform rigid anechoic tubes. The effects of the Reynolds number, Re, on the source spectral distribution function were found to be insignificant over the range 2000ϽReϽ20 000. The source spectral distribution function approximately obeyed a St Ϫ3 power law for Strouhal number values StϽ0.9, and a St Ϫ5 power law for StϾ2.5. The influence of a reflective open tube termination on the source function spectral distribution was found to be insignificant, confirming the absence of a feedback mechanism.

Research paper thumbnail of Aerodynamic transfer of energy to the vocal folds

Journal of The Acoustical Society of America, 2005

The aerodynamic transfer of energy from glottal airflow to vocal fold tissue during phonation was... more The aerodynamic transfer of energy from glottal airflow to vocal fold tissue during phonation was explored using complementary synthetic and numerical vocal fold models. The synthetic model was fabricated using a flexible polyurethane rubber compound. The model size, shape, and material properties were generally similar to corresponding human vocal fold characteristics. Regular, self-sustained oscillations were achieved at a frequency of approximately 120 Hz. The onset pressure was approximately 1.2 kPa. A corresponding two-dimensional finite element model was developed using geometry definitions and material properties based on the synthetic model. The finite element model upstream and downstream pressure boundary conditions were based on experimental values acquired using the synthetic model. An analysis of the fully coupled fluid and solid numerical domains included flow separation and unsteady effects. The numerical results provided detailed flow data that was used to investigate aerodynamic energy transfer mechanisms. The results support the hypothesis that a cyclic variation of the orifice profile from a convergent to a divergent shape leads to a temporal asymmetry in the average wall pressure, which is the key factor for the achievement of self-sustained vocal fold oscillations. me rica.

Research paper thumbnail of Sound generation by steady flow through glottis-shaped orifices

Journal of The Acoustical Society of America, 2004

Although the signature of human voice is mostly tonal, it also includes a significant broadband c... more Although the signature of human voice is mostly tonal, it also includes a significant broadband component. Quadrupolelike sources due to turbulence in the region downstream of the glottis, and dipolelike sources due to the force applied by the vocal folds onto the surrounding fluid are the two primary broadband sound generating mechanisms. In this study, experiments were conducted to characterize the broadband sound emissions of confined stationary jets through rubber orifices formed to imitate the approximate shape of the human glottis at different stages during one cycle of vocal fold vibrations. The radiated sound pressure spectra downstream of the orifices were measured for varying flow rates, orifice shapes, and gas mixtures. The nondimensional sound pressure spectra were decomposed into the product of three functions: a source function F, a radiation efficiency function M , and an acoustic response function G. The results show that, as for circular jets, the quadrupole source contributions dominated for straight and convergent orifices. For divergent jets, whistling tonal sounds were emitted at low flow rates. At high flow rates for the same geometry, dipole contributions dominated the sound radiated by free jets. However, possible source-load acoustic feedback may have hampered accurate source identification in confined flows.

Research paper thumbnail of Computational aeroacoustics of phonation, Part II: Effects of flow parameters and ventricular folds

Journal of The Acoustical Society of America, 2002

The aerodynamic generation of sound during phonation was studied using direct numerical simulatio... more The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered ͑125 Hz͒. The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

Research paper thumbnail of Modeling transition to turbulence in eccentric stenotic flows

Journal of Biomechanical Engineering-transactions of The Asme, 2008

Mean flow predictions obtained from a host of turbulence models were found to be in poor agreemen... more Mean flow predictions obtained from a host of turbulence models were found to be in poor agreement with recent direct numerical simulation results for turbulent flow distal to an idealized eccentric stenosis. Many of the widely used turbulence models, including a large eddy simulation model, were unable to accurately capture the post-stenotic transition to turbulence. The results suggest that efforts towards developing more accurate turbulence models for low Reynolds number, separated transitional flows are necessary before such models can be used confidently under hemodynamic conditions where turbulence may develop.

Research paper thumbnail of Numerical Modeling of Pulsatile Turbulent Flow in Stenotic Vessels

Journal of Biomechanical Engineering-transactions of The Asme, 2003

Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-aver... more Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number kturbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k -⑀ turbulence model and the standard k -⑀ model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred. Fig. 7 Streamlines for the smooth stenosis from the low Reynolds number k -model at different phases in the flow cycle. The streamfunction increment is 0.0018 s À1 .

Research paper thumbnail of Direct numerical simulation of stenotic flows, Part 1: Steady flow

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) s... more Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predicted a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking the symmetry of the post-stenotic flowfield by causing the jet to deflect towards the side of the eccentricity and at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned into turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a −5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent puff observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of thirty but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to manifest itself in large temporal and spatial gradients of WSS, with significant axial and circumferential variations in the turbulent section.

Research paper thumbnail of Direct numerical simulation of stenotic flows. Part 1. Steady flow

Journal of Fluid Mechanics, 2007

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) s... more Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predicted a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking the symmetry of the post-stenotic flowfield by causing the jet to deflect towards the side of the eccentricity and at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned into turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a −5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent puff observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of thirty but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to manifest itself in large temporal and spatial gradients of WSS, with significant axial and circumferential variations in the turbulent section.

Research paper thumbnail of Direct numerical simulation of stenotic flows. Part 2. Pulsatile flow

Journal of Fluid Mechanics, 2007

Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were d... more Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were discussed in Part 1 of this study. DNS of pulsatile flow through the 75% stenosed tube (by area) employed for the computations in Part 1 is examined here. Analogous to the steady flow results, DNS predicts a laminar post-stenotic flowfield in the case of pulsatile flow through the axisymmetric stenosis model, in contrast to previous experiments, in which intermittent disturbed flow regions and turbulent breakdown were observed in the downstream region. The introduction of a stenosis eccentricity, that was 5% of the main vessel diameter at the throat, resulted in periodic, localized transition to turbulence. Analysis in this study indicated that the early and mid-acceleration phases of the time period cycle were relatively stable, with no turbulent activity in the poststenotic region. However, towards the end of acceleration, the starting vortex, formed earlier as the fluid accelerated through the stenosis at the beginning of acceleration, started to break up into elongated streamwise structures. These streamwise vortices broke down at peak flow, forming a turbulent spot in the post-stenotic region. The early part of deceleration witnessed intense turbulent activity within this spot. Past the middeceleration phase, through to minimum flow, the inlet flow lost its momentum and the flowfield began to relaminarize. The start of acceleration in the following cycle saw a recurrence of the entire process of a starting structure undergoing turbulent breakdown and subsequent relaminarization of the post-stenotic flowfield. Peak wall shear stress (WSS) magnitudes occurred at the stenosis throat and close to the reattachment location during most of the acceleration phase, with the rest of the vessel experiencing much lower levels of WSS. Turbulent breakdown at peak flow resulted in a sharp amplification of WSS levels across the region corresponding to the turbulent spot, accompanied by large axial and circumferential fluctuations. Magnitudes dropped rapidly after the mid-deceleration phase, when the relaminarization process took over.

Research paper thumbnail of Performance of a 3Bladed Propeller Pump to Provide Cavopulmonary Assist in the Failing Fontan Circulation

Annals of Thoracic Surgery, 2008

Purpose. We hypothesized that a propeller pump design would function optimally to provide cavopul... more Purpose. We hypothesized that a propeller pump design would function optimally to provide cavopulmonary assist in a univentricular Fontan circulation.

Research paper thumbnail of Mechanical Cavopulmonary Assist for the Univentricular Fontan Circulation Using a Novel Folding Propeller Blood Pump

Asaio Journal, 2007

A blood pump specifically designed to operate in the unique anatomic and physiologic conditions o... more A blood pump specifically designed to operate in the unique anatomic and physiologic conditions of a cavopulmonary connection has never been developed. Mechanical augmentation of cavopulmonary blood flow in a univentricular circulation would reduce systemic venous pressure, increase preload to the single ventricle, and temporarily reproduce a scenario analogous to the normal two-ventricle circulation. We hypothesize that a folding propeller blood pump would function optimally in this cavopulmonary circulation. The hydraulic performance of a two-bladed propeller prototype was characterized in an experimental flow loop using a blood analog fluid for 0.5-3.5 lpm at rotational speeds of 3,600-4,000 rpm. We also created five distinctive blood pump designs and evaluated their hydraulic performance using computational fluid dynamics (CFD). The two-bladed prototype performed well over the design range of 0.5-3.5 lpm, producing physiologic pressure rises of 5-18 mm Hg. Building upon this proof-of-concept testing, the CFD analysis of the five numerical models predicted a physiologic pressure range of 5-40 mm Hg over 0.5-4 lpm for rotational speeds of 3,000-7,000 rpm. These preliminary propeller designs and the two-bladed prototype achieved the expected hydraulic performance. Optimization of these configurations will reduce fluid stress levels, remove regions of recirculation, and improve the hydraulic performance of the folding propeller. This propeller design produces the physiologic pressures and flows that are in the ideal range to mechanically support the cavopulmonary circulation and represents an exciting new therapeutic option for the support of a univentricular Fontan circulation.