M. C. So - Academia.edu (original) (raw)

Papers by M. C. So

Vox Sanguinis, Jul 10, 2023

Fluid Mechanics and its Applications, 2004

Page 191. THREE DIMENSIONAL MODELING OF FLOW INDUCED VIBRATION FOR AN ELASTIC CYLINDER IN A CROSS... more Page 191. THREE DIMENSIONAL MODELING OF FLOW INDUCED VIBRATION FOR AN ELASTIC CYLINDER IN A CROSS FLOW Y. LIU, RMC SO AND CH ZHANG Abstract: A fully three dimensional flow induced vibration ...

Journal of Fluids Engineering, 1977

Journal of Fluids and Structures, 2003

This note reports an experimental study of the fluid damping of a long slender cylinder, fixed at... more This note reports an experimental study of the fluid damping of a long slender cylinder, fixed at both ends (no rotation and displacement), in a cross air-flow. The structural dynamic strain was measured in the lift direction over a range of reduced velocity U r using a fibre-optic Bragg grating sensor. An auto-regressive moving average technique was used to deduce the effective damping ratios (including structural and fluid damping) from the strain data. The modal damping ratios corresponding to the first-, second-and third-mode vibrations over a range of U r from 3 to 45 have been characterized and discussed. The results show that the value of fluid damping varied significantly at resonance when the vortex-shedding frequency coincides with one of the natural frequencies of the combined fluid-structure system.

Proceeding of Proceedings of Symposium on Energy Engineering in the 21st Century (SEE2000) Volume I-IV

Journal of Applied Mathematics and Physics, 2022

The nature of turbulent swirling and rotating flow in a straight pipe is investigated using a fam... more The nature of turbulent swirling and rotating flow in a straight pipe is investigated using a family of near-wall two-equation models. Specifically, the viability of three different near-wall two-equation models is assessed. These models are asymptotically consistent near the wall. The first two models, one with isotropic and another with anisotropic eddy viscosity invoked, solved a dissipation rate equation with no explicit correction made to account for swirl and flow rotation. The third model assumes an isotropic eddy viscosity but solves an improved dissipation rate equation that has explicit corrections made to account for swirl and flow rotation. Calculations of turbulent flows in the swirl number range 0.25-1.3 with and without a central recirculation region reveal that, with the exception of the third model, neither one of the other two models can replicate the mean field at the swirl numbers tested. Furthermore, taking stress anisotropy into account also fails to model swirl effect correctly. Significant improvements can be realized from the third model, which is based on an improved dissipation rate equation and the assumption of isotropic eddy viscosity. The predicted mean flow and turbulence statistics correlate well with measurements at low swirl. At high swirl, the two-equation model with an improved dissipation rate equation can still be used to model swirling and rotating pipe flows with a central recirculation region. However, its simulation of flows without a central recirculation region is not as satisfactory.

AIAA Journal, 2007

A lattice Boltzmann method that can recover the first coefficient of viscosity and the specific h... more A lattice Boltzmann method that can recover the first coefficient of viscosity and the specific heat ratio correctly has been adopted for one-step aeroacoustic simulations because it can recover the speed of sound correctly. Instead of solving the Navier-Stokes equations as in the case of direct numerical simulation, the lattice Boltzmann method only needs to solve one transport equation for the collision function. Other flow properties are obtained by integrating this collision function over the particle velocity space. The lattice Boltzmann method is effective only if appropriate nonreflecting boundary conditions for open computational boundaries are available, just like the direct numerical simulation. Four different nonreflecting boundary conditions are commonly used with direct numerical simulation for one-step aeroacoustic simulations. Among these are the characteristics-based method, the perfectly matched layer method, the C 1 continuous method, and the absorbing layer method. Not all nonreflecting boundary conditions are applicable when used with the lattice Boltzmann method; some might not be appropriate, whereas others could be rather effective. This paper examines some existing nonreflecting boundary conditions plus other new proposals, their appropriateness, and their suitability for the lattice Boltzmann method. The assessment is made against two classic aeroacoustic problems: propagation of a plane pressure pulse and propagation of acoustic, entropy, and vortex pulses in a uniform stream. A reference solution is obtained using direct numerical simulation assuming a relatively large computational domain without any specified nonreflecting boundary conditions. The results, obtained with a computational domain half the size of that used for the direct numerical simulation calculations, show that the absorbing layer method and the extrapolation method with assumed filter perform the best.

International Journal of Heat and Mass Transfer, 1996

An explicit algebraic heat-flux (EAHF) model is derived by invoking the assumption of equilibrium... more An explicit algebraic heat-flux (EAHF) model is derived by invoking the assumption of equilibrium turbulence for both the velocity and the thermal fields. Further modifications are achieved by applying an approximation technique to render the implicit heat flux vector relation explicit. Thus derived, the heat flux vector yields two terms. The first term is identical to that given by a simple thermal eddy diffusivity rrtodel, while the second term provides a correction to the streamwise heat flux. This second term is non-zero even when the streamwise mean temperature gradient is zero. It allows the modeling of the generation of a streamwise heat flux due to the interaction of the turbulent eddies with the mean temperature gradient normal to the streamwise direction. Previously derived near-wall corrections to the equations of the temperature variance and its dissipation rate are found to be equally valid for the EAHF model. The near-wall EAHF model is validated against direct numerical simulation data and experimental measurements. In addition, the calculations are also compared with those obtained from a second-order model and the simple thermal eddy diffusivity model. Two different near-wall Reynolds-stress models are used to calculate the velocity field and they are found to have little effect on the thermal field predicted by the EAHF model. In general, the results for the temperature field are in good agreement with data and are essentially unaffected by the second term in the EAHF model. On the other hand, the prediction of the streamwise heat flux is in good agreement with that given by a second-order model which correlates well with data.

1st National Fluid Dynamics Conference, 1988

Journal of Fluids and Structures, 1999

The present investigation examines a simple fluid–structure interaction problem, which is represe... more The present investigation examines a simple fluid–structure interaction problem, which is represented by the unsteady response of an airfoil/blade to a Karman vortex street in an inviscid uniform flow. Two different cases were examined; one with a rigid airfoil/blade, where the structural stiffness is infinite, another with an elastic blade. In both cases, the flow remains attached to the airfoil/blade surface. A time-marching technique solving the Euler equations and a two-degree-of-freedom structural dynamic model is used to examine the interactions between the fluid and the structure. The interactions between the convected vortices and the structure modify the shed wake whose energy, in turn, feeds into the forces and moments acting on the structure. For a rigid airfoil/blade, it is found that the amplitude of the aerodynamic response is not proportional to the density of the oncoming vortex street, but depends on c/d , the ratio of the chord length (c) to the axial spacing (d) of the convected vortices. When the number of vortices per unit length is increased, the amplitudes of the aerodynamic response increase and then decrease even though the density of the vorticity keeps increasing and so is the energy of the excitation wake. Maxima are observed at c/d=0·5, 1·5 and 2·5. This behaviour is analogous to the structural resonance phenomenon and is labeled “aerodynamic resonance”. The existence of such an “aerodynamic resonance” is important to turbomachinery applications where the blade is elastic, the flow is unsteady and the shed vortices from the previous row are convected downstream by the mean flow. Thus, “aerodynamic resonance” alone or in conjunction with structural resonance could impact negatively on the fatigue life of turbine blades and their combined effects should be accounted for in blade design. A preliminary attempt to assess this impact has been carried out. It is found that the relative fatigue life of a blade could be reduced by four orders of magnitude as a result.

Proceeding of First Symposium on Turbulence and Shear Flow Phenomena

Proceeding of International Heat Transfer Conference 7

Journal of Applied Mathematics and Physics

An explicit algebraic stress model (EASM) has been formulated for two-dimensional turbulent buoya... more An explicit algebraic stress model (EASM) has been formulated for two-dimensional turbulent buoyant flows using a five-term tensor representation in a prior study. The derivation was based on partitioning the buoyant flux tensor into a two-dimensional and a three-dimensional component. The five-term basis was formed with the two-dimensional component of the buoyant flux tensor. As such, the derived EASM is limited to two-dimensional flows only. In this paper, a more general approach using a seven-term representation without partitioning the buoyant flux tensor is used to derive an EASM valid for two-and three-dimensional turbulent buoyant flows. Consequently, the basis tensors are formed with the fully three-dimensional buoyant flux tensor. The derived EASM has the two-dimensional flow as a special case. The matrices and the representation coefficients are further simplified using a four-term representation. When this four-term representation model is applied to calculate two-dimensional homogeneous buoyant flows, the results are essentially identical with those obtained previously using the two-dimensional component of the buoyant flux tensor. Therefore, the present approach leads to a more general EASM formulation that is equally valid for two-and three-dimensional turbulent buoyant flows.

International Journal of Modern Physics C, 2007

This paper reports a study of the ability of an improved LBM in replicating acoustic interaction.... more This paper reports a study of the ability of an improved LBM in replicating acoustic interaction. With a BGK model with two relaxation times approximating the collison term, the improved LBM is shown not only able to recover the equation of state, but also replicates the specific heat ratio, the fluid viscosity and thermal conductivity correctly. With these improvements, the recovery of full set of unsteady compressible Navier-Stokes equations is possible. Two complex aeroacoustic interaction problems, namely the interaction of three fundamental aeroacoustic pulses and scattering of short wave by a zero circulation vortex, are calculated. The LBM solutions are compared with DNS results. In the first case it has been shown that the improved LBM is as effective as the DNS in simulating aeroacoustic interaction of three pulses. Both methods obtain essentially same results using same truncated domains. In the scattering problem, LBM is able to replicate the directivity of scattered acou...

Applied Mechanics and Biomedical Technology, 2003

Two numerical methods for the determination of the pth moment Lyapunov exponents of a two-dimensi... more Two numerical methods for the determination of the pth moment Lyapunov exponents of a two-dimensional system under bounded noise or real noise parametric excitation are presented. The first method is an analytical-numerical approach, in which the partial differential eigenvalue problems governing the moment Lyapunov exponents are established using the theory of stochastic dynamical systems. The eigenfunctions are expanded in double series to transform the partial differential eigenvalue problems to linear algebraic eigenvalue problems, which are then solved numerically. The second method is a Monte Carlo simulation approach. The numerical values obtained are compared with approximate analytical results with weak noise amplitudes.

ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2002

The numerical method developed previously to analyze two-dimensional vortex-blade interaction pro... more The numerical method developed previously to analyze two-dimensional vortex-blade interaction problems is validated using recently measured vortex-induced blade vibration data. It assumed a vortex lattice method to calculate the flow field assuming a distribution of sources and discrete vortices on the blade surfaces and a free wake model for the wake flow. A discrete vortex tracking technique in Lagrangian frame is used to track the path of the vortices. The blade is modeled as elastic structures with two-degree-of-freedom in plunging and pitching direction. The fully coupled fluid-structure interaction problem is resolved by means of a time-marching technique. The flow-field is assumed to be inviscid, incompressible and two-dimensional, with no flow separation occurring on the surfaces of the blade. Two cases were examined and they included a blade-vortex interaction and a blade vortex street interaction problem. In the blade-vortex interaction case, the blade is modeled as rigid; therefore, the response of the structure is purely aerodynamics. The calculated variation of the lift coefficient of the blade with the horizontal missed distance of the convected vortex compares well with known experimental results. In the blade vortex street interaction case, the blade is modeled as elastic and is under the unsteady excitation from a Karman vortex street. The calculated blade responses due to vortex-induced vibration are compared with some recently measured vibration characteristics of a flat plate placed behind a cylinder at different separation distance. Good agreement between calculations and measured vibration amplitudes of the plate at its mid-span is obtained, thus indicating that the numerical method gives a viable model for the analysis of the aerodynamics and structural response in vortex/blade interaction problems.

Journal of Sound and Vibration, 2007

A numerical investigation of sound absorption by an induct orifice with and without flow was carr... more A numerical investigation of sound absorption by an induct orifice with and without flow was carried out using a sixthorder finite difference direct numerical simulation (DNS) scheme with explicit fourth-order time marching to solve the governing Navier-Stokes equation. The DNS scheme has previously been validated against benchmark aeroacoustic problems and good agreement was obtained. Thus, it was applied to simulate the acoustic impedance of a circular orifice with different openings and a laminar flow through the same orifice. Both discrete frequency and broadband excitations were studied. When the induct orifice is exposed to discrete frequency sound wave in the absence of flow, alternate vortex shedding on both sides of the orifice is observed. The strength of shed vortices is stronger at low frequencies and thus the reduction of sound energy is higher. These vortices dissipate while moving away from the orifice. Therefore, the process provides a mechanism for adsorption of incident sound. The numerical results of broadband excitation indicate that small orifice opening is a more efficient sound absorber whereas a large opening is more or less transparent to the incident wave. The absorption, reflection and transmission coefficients of the induct orifice are calculated by a transfer function method. It is found that the sound coefficients are strongly dependent on the orifice opening size and frequency. In the presence of a flow, only alternate vortex shedding on one side of the orifice is observed. In spite of this, the results show that sound absorption behavior is very similar to the no flow case, i.e., sound absorption is more effective with small orifice.

Journal of Fluids Engineering, 2009

Conventional lattice Boltzmann method (LBM) is hyperbolic and can be solved locally, explicitly, ... more Conventional lattice Boltzmann method (LBM) is hyperbolic and can be solved locally, explicitly, and efficiently on parallel computers. The LBM has been applied to different types of complex flows with varying degrees of success, and with increased attention focusing on microscale flows now. Due to its small scale, microchannel flows exhibit many interesting phenomena that are not observed in their macroscale counterpart. It is known that the Navier–Stokes equations can still be used to treat microchannel flows if a slip-wall boundary condition is assumed. The setting of boundary conditions in the conventional LBM has been a difficult task, and reliable boundary setting methods are limited. This paper reports on the development of a finite difference LBM (FDLBM) based numerical scheme suitable for microchannel flows to solve the modeled Boltzmann equation using a splitting technique that allows convenient application of a slip-wall boundary condition. Moreover, the fluid viscosity i...