Martin Skote - Profile on Academia.edu (original) (raw)

Papers by Martin Skote

The computational fluid dynamic model of a live-sized dragonfly (Sympetrum flaveolum) hindwing is... more The computational fluid dynamic model of a live-sized dragonfly (Sympetrum flaveolum) hindwing is simulated according to the in-flight flapping motions measured in kinematic experiments. The flapping motion of the simulated wing is accomplished by dynamically re-gridding the wing-fluid mesh according to the established kinematic model for each flapping pattern. Comparisons between two distinct flapping patterns (double figure-eight and simple figure-eight) are studied via analysis of the aerodynamic forces and flow field structures. The result shows that additional lift is generated during supination and upstroke for the double figure-eight pattern, while maximum thrust is generated during pronation for the simple figure-eight pattern. In addition, through our comparisons of the different kinematics, we are able to reveal the mechanism behind the leading edge vortex stabilization prior to supination and the kinematic movement responsible for additional lift generation during supination. By increasing the translational deceleration during stroke-end rotations in the double figure-eight flapping pattern, a trailing edge vortex is formed which is stronger as compared to the single figure-eight flapping pattern, thus enhancing the lift. C 2015 AIP Publishing LLC. [http://dx.033604-2 Y. H. Chen and M. Skote Phys. Fluids 27, 033604 (2015)

A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifuga... more A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifugal effects, produced by rotation of the sample disc, in order to increase internal circulatory rates, and with them temperature homogenization and mixing speeds. A proof of concept has been presented by testing a rapid 45-cycle PCR DNA amplification protocol. During the repeated heating and cooling that constitutes a PCR process, the 100 lL samples were rotated at a speed equivalent to an effective acceleration of gravity of 7,000 g. A cycle time of 20.5 s gave a total process time of 15 min to complete the 45 cycles. A theoretical and numerical analysis of the resulting flow, which describes the increased mixing and temperature homogenization, is presented. The device gives excellent reaction speed efficiency, which is beneficial for rapid PCR.

13 A model of two-phase non-Newtonian horizontal annular flows, which predicts film 14 thickness ... more 13 A model of two-phase non-Newtonian horizontal annular flows, which predicts film 14 thickness and pressure gradient from flowrates only, is presented. In the model, the gas and 15 non-Newtonian liquid flows are calculated separately based on the independent governing 16 equations. The shear stress balance at the gas-liquid interface is calculated in order to link 17 two phases together. The non-Newtonian fluid is assumed as a power-law shear-thinning 18 liquid. The logarithmic velocity distribution is chosen to calculate the turbulent velocity 19 profile in the gas core. The influences of entrainment and aeration are included in the model. 20 The pressure drop, film thickness, void fraction, the frictional multiplier, and 21 Lockhart-Martinelli parameter are predicted. The analytical model is compared with the 22 published experimental investigations, and the results show that the model can predict the 23 film thickness and pressure gradient simultaneously based on the flowrates of liquid and gas. 24 The frictional multiplier and Lockhart-Martinelli parameter are calculated at the same time, 25 and the predicted values are comparable with the experimental data. The difference between 26 the analytical model and the experiments is lower than 10%. 27 28

Direct numerical simulations have been performed to study the drag reduction resulting from spati... more Direct numerical simulations have been performed to study the drag reduction resulting from spatial oscillations of a segment of the wall under a turbulent boundary layer. The oscillating motion is imposed by utilizing a streamwise modulated spanwise wall forcing. The results are compared with earlier simulations using temporal oscillations with an identical segment and forcing amplitudes, and with a frequency related to the wavelength through a convective velocity. Two different oscillation amplitudes with equal oscillation wavelength have been used, which allows for a direct comparison between a relatively weak and strong forcing of the flow. The weaker forcing results in 25 % drag reduction while the stronger forcing, with twice the amplitude, yields 41 % drag reduction. Comparison with the temporal cases reveals drastically improved energy savings for the spatial oscillation technique, in accordance with earlier channel flow investigations. The streamwise variation of spanwise shear is shown to follow the analytical solution to the laminar Navier-Stokes equations derived under the assumption of constant friction velocity. Furthermore, the spanwise velocity profiles at various phases are compared with the analytical solution, and show very good agreement. The downstream development of the spatial Stokes layer thickness is theoretically estimated to be ∼x 1/15 , in general agreement with the simulation data. The spatial variation of the spanwise Reynolds stress is investigated and compared with the variation in time for the temporal wall forcing cases. The controversy regarding a zero or non-zero production of spanwise Reynolds stress in the temporal case is elucidated. In addition, comparison with the spatial case reveals that a second production term originating from the downstream variation of the spanwise wall velocity has a negative contribution to the production, and hence relates to the larger drag reduction in the case of spatial forcing.

An investigation of a model of turbulence generation in the wall region of a turbulent boundary l... more An investigation of a model of turbulence generation in the wall region of a turbulent boundary layer is made through direct numerical simulations. The model is based on the varicose instability of a streak. First, a laminar boundary layer disturbed by a continuous blowing through a slot is simulated in order to reproduce and further investigate the results reported from the experiments of Acarlar and Smith ͓J. Fluid Mech. 175, 43 ͑1987͔͒. An isolated streak with an inflectional profile is generated that becomes unstable, resulting in a train of horseshoe vortices. The frequency of the vortex generation is equal to the experimental results. Comparison of the instability characteristics to those predicted through an Orr-Sommerfeld analysis are in good agreement. Second, a direct numerical simulation of a turbulent boundary layer is performed to point out the similarities between the horseshoe vortices in a turbulent and a laminar boundary layer. The characteristics of streaks and the vortical structures surrounding them in a turbulent boundary layer compare well with the model streak. The results of the present study show that one mechanism for the generation of horseshoe vortices in turbulent boundary layers is related to a normal inflectional instability of the streaks.

Direct numerical simulation of two turbulent boundary layer flows has been performed. The boundar... more Direct numerical simulation of two turbulent boundary layer flows has been performed. The boundary layers are both subject to a strong adverse pressure gradient. In one case a separation bubble is created while in the other the boundary layer is everywhere attached. The data from the simulations are used to investigate scaling laws near the wall, a crucial concept in turbulence models. Theoretical work concerning the inner region in a boundary layer under an adverse pressure gradient is reviewed and extended to the case of separation. Excellent agreement between theory and data from the direct numerical simulation is found in the viscous sub-layer, while a qualitative agreement is obtained for the overlap region.

Direct Numerical Simulations have been performed to study the effect of an oscillating segment of... more Direct Numerical Simulations have been performed to study the effect of an oscillating segment of the wall on a turbulent boundary layer flow. Two different oscillation amplitudes with equal oscillation period have been used, which allows a direct comparison between a relatively weak and strong forcing of the flow. The weaker forcing results in 18% drag reduction while the stronger forcing, with twice the amplitude, yields 29% drag reduction. The downstream development of the drag reduction is compared with earlier simulations and experiments. In addition, a simulation with identical oscillation parameters as in previous numerical and experimental investigations allows for an estimation of the effect of the Reynolds number on the drag reduction.

Spatiotemporal quantification of the blood flow in the human vasculature has been greatly aided w... more Spatiotemporal quantification of the blood flow in the human vasculature has been greatly aided with the non-invasive proposition offered by Doppler ultrasound. However, relatively large deviation of the blood flow measurement from the actual value is expected, owing to a number of contributing factors. The research work expounded here attempts to ameliorate the accuracy of the blood flow output quantification by combining the Doppler measurement with the computational fluid dynamics modeling based on Navier-Stokes equation. In vitro assessment of the integrated approach was carried out with custom made phantom and probe positioning Mechatronic system. Analysis of the experimental results showed that, compared to the stand alone Doppler ultrasound measurement, the integrated model gives better accuracy in quantifying the volume flow rate.

Direct numerical simulations have been performed to study the effect of a stationary distribution... more Direct numerical simulations have been performed to study the effect of a stationary distribution of spanwise wall-velocity that oscillates in the streamwise direction on a turbulent boundary layer. For the first time, a spatially developing flow with this type of forcing is studied. The part of the boundary layer which flows over the alternating wall-velocity section is greatly affected with a drag reduction close to 50% which exhibits an oscillatory distribution with a wavenumber which is twice that of the imposed wall-velocity. The maximum in drag reduction occurs where the wall velocity is at its maximum (or minimum) and the minimum occurs where the wall velocity is zero. Comparisons of the mean spanwise velocity profiles with the analytical solution to the laminar Navier-Stokes equations show very good agreement. The streamwise velocity profile indicates a thickening of the viscous sub-layer when scaled with the local friction velocity and an upward shifting of the logarithmic region when scaled with the reference (unmanipulated) friction velocity. An estimation of the idealized power consumption shows that-with the present wall forcing magnitude-more energy is required for the spatial oscillation than what is saved by drag reduction.

Direct numerical simulation of a turbulent boundary layer under a strong adverse pressure gradient

In recent years direct numerical simulation (DNS) of turbulent boundary layers has become an impo... more In recent years direct numerical simulation (DNS) of turbulent boundary layers has become an important complement to experiments for obtaining turbulence data. The rst DNS of turbulent boundary layers in an adverse pressure gradient (APG) were the temporal simulations performed by Spalart & Leonard 13] where they used a similarity coordinate system. Spalart & Watmu 14] compared experiments and DNS of an APG turbulent boundary layer in a varying pressure gradient and they found good agreement. Skote et al.

Large Scale Parallel Direct Numerical Simulation of a Separating Turbulent Boundary Layer Flow over a Flat Plate Using NAL Numerical Wind Tunnel

Direct numerical simulation(DNS) of fundamental fluid flow simulation using 3-dimensional Navier-... more Direct numerical simulation(DNS) of fundamental fluid flow simulation using 3-dimensional Navier-Stokes equations is a typical large scale computing which requires high performance computer with vector and parallel processing. In the present paper a turbulent boundary layer flow simulation with strong adverse pressure gradient on a flat plate was made on NAL Numerical Wind Tunnel. Boundary layer subjected to a strong adverse pressure gradient creates a separation bubble followed by a region with small, but positive, skin friction. This flow case contains features that has proven to be difficult to predict with existing turbulence models. The data from present simulation are used for investigation of the scalings near the wall, a crucial concept with respect to turbulence models. The present analysis uses spectral methods and the parallelization was done using MPI(Message-Passing Interface). A good efficiency was obtained in NWT. To compare with other machine performances, previous computations on T3E and SP2 are also shown.

European Biophysics Journal With Biophysics Letters, Jan 1, 2006

A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifuga... more A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifugal effects, produced by rotation of the sample disc, in order to increase internal circulatory rates, and with them temperature homogenization and mixing speeds. A proof of concept has been presented by testing a rapid 45-cycle PCR DNA amplification protocol. During the repeated heating and cooling that constitutes a PCR process, the 100 μL samples were rotated at a speed equivalent to an effective acceleration of gravity of 7,000 g. A cycle time of 20.5 s gave a total process time of 15 min to complete the 45 cycles. A theoretical and numerical analysis of the resulting flow, which describes the increased mixing and temperature homogenization, is presented. The device gives excellent reaction speed efficiency, which is beneficial for rapid PCR.

Reynolds Stress Budgets in Couette and Boundary Layer Flows

Reynolds stress budgets for both Couette and boundary layer flows are evaluated and presented. Da... more Reynolds stress budgets for both Couette and boundary layer flows are evaluated and presented. Data are taken from direct numerical simulations of rotating and non-rotating plane turbulent Couette flow and turbulent boundary layer with and without adverse pressure gradient. Comparison of the total shear stress for the two types of flows suggests that the Couette case may be regarded as the high Reynolds number limit for the boundary layer flow close to the wall. The limit values of turbulence statistics close to the wall for the boundary layer for increasing Reynolds number approach the corresponding Couette flow values. The direction of rotation is chosen so that it has a stabilizing effect, whereas the adverse pressure gradient is destabilizing. The pressure-strain rate tensor in the Couette flow case is presented for a split into slow, rapid and Stokes terms. Most of the influence from rotation is located to the region close to the wall, and both the slow and rapid parts are affected. The anisotropy for the boundary layer decreases for higher Reynolds number, reflecting the larger separation of scales, and becomes close to that for Couette flow. The adverse pressure gradient has a strong weakening effect on the anisotropy. All of the data presented here are available on the web [36].

The performance of a pseudo spectral turbulence simulation code on various supercomputers, with e... more The performance of a pseudo spectral turbulence simulation code on various supercomputers, with either shared memory or distributed memory, is presented. The communication with the memory is intense, and careful consideration of the two memory configurations is needed to obtain high performance. The investigations of the performance show that the scaling with the number of processors is excellent for both memory systems. Also, vector processors are compared with super scalar processors, and the performance is generally higher for the vector processor since the code vectorizes well. However, the computers with the scalar processors, and distributed memory, have a much larger number of processors which gives an overall better performance for the total machine. The numerical code, with e.g. 24 · 10 6 degrees of freedom, was run at 3.5 Gflop/s on 64 processors on an IBM SP2 machine.

Eleventh Symposium on Turbulent Shear Flows : Grenoble, France, September 8-10, 1997, 33:13 - 33:18

Four classes of turbulence models (algebraic, k − , k − ω and a differential Reynolds-stress mode... more Four classes of turbulence models (algebraic, k − , k − ω and a differential Reynolds-stress model) are applied to boundary layers under adverse pressure gradient with a constant equilibrium parameter β = δ * τ w dp dx . Numerical solutions up to Re θ = 10 8 give the classical scalings in the inner and outer layer for all models. Comparison is made with experiments of Clauser at β ≈ 2 and 8 and with recent experiments by Skåre and Krogstad at β = 20. We have also performed new direct numerical simulations at β ≈ 0.25 and 0.65 up to Re θ = 700. The differential Reynolds-stress model shows the best agreement with the experiments and the DNS.

Atmospheric Environment, Jan 1, 2005

The purpose of the paper is to examine the relation between urban morphology (three-dimensional s... more The purpose of the paper is to examine the relation between urban morphology (three-dimensional structure) and windiness. We regard a city as a porous obstacle, which is open at the top. We consider the interaction between the atmospheric boundary layer and a city to be both a function of the overall shape (silhouette or skyline) and the internal resistance to the flow caused by the friction when the wind flows over the urban surfaces. We regard the street pattern as an interconnected flow network with the crossings as nodes. Flow along the streets is generated by pressure differences.We here use a highly idealized city model consisting of a circular block divided into two or four equally large sectors. Two types of cases are studied, the first with only one street through the city model with different angles between the street and the oncoming wind. The second case also contains a perpendicularly crossing street (through the center). Both wind tunnel experiments and numerical flow computations (computational fluid dynamics, CFD) are used and compared. The general agreement between the two is good and the CFD method offers new possibilities for quantifying the urban wind environment.

The objective has been to study turbulent boundary layers under adverse pressure gradients (APG) ... more The objective has been to study turbulent boundary layers under adverse pressure gradients (APG) through direct numerical simulation (DNS). The numerical code is based on a pseudo-spectral technique which is suitable for the simple geometry (flat plate) considered here. A large effort has been put into the optimization of the numerical code on various super computers. Five large simulations have been performed, ranging from a zero pressure gradient boundary layer to a separating flow. The simulations have revealed many features of APG turbulent boundary layers which are difficult to capture in experiments. Especially the near-wall behavior has been investigated thoroughly, both through the statistical and instantaneous flow.

Journal of Fluid Mechanics, Jan 1, 2002

Direct numerical simulation of a turbulent boundary layer under a strong adverse pressure gradient

Large Scale Parallel Direct Numerical Simulation of a Separating Turbulent Boundary Layer Flow over a Flat Plate Using NAL Numerical Wind Tunnel

European Biophysics Journal With Biophysics Letters, Jan 1, 2006

A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifuga... more A novel polymerase chain reaction (PCR) method is presented that utilizes Coriolis and centrifugal effects, produced by rotation of the sample disc, in order to increase internal circulatory rates, and with them temperature homogenization and mixing speeds. A proof of concept has been presented by testing a rapid 45-cycle PCR DNA amplification protocol. During the repeated heating and cooling that constitutes a PCR process, the 100 μL samples were rotated at a speed equivalent to an effective acceleration of gravity of 7,000 g. A cycle time of 20.5 s gave a total process time of 15 min to complete the 45 cycles. A theoretical and numerical analysis of the resulting flow, which describes the increased mixing and temperature homogenization, is presented. The device gives excellent reaction speed efficiency, which is beneficial for rapid PCR.

Reynolds Stress Budgets in Couette and Boundary Layer Flows

Eleventh Symposium on Turbulent Shear Flows : Grenoble, France, September 8-10, 1997, 33:13 - 33:18

Atmospheric Environment, Jan 1, 2005

Journal of Fluid Mechanics, Jan 1, 2002