David C.S. Kuhn

Numerical study of pulsatile channel flows undergoing transition triggered by a modelled stenosis

In this research, we numerically investigate the physics of pulsatile flows confined within a 3-dimensional channel with a modelled stenosis formed eccentrically on the upper wall using the method of large-eddy simulation (LES). An advanced dynamic nonlinear subgrid-scale stress model was utilized to conduct numerical simulations and its predictive performance was examined in comparison with that of the conventional dynamic model. The Womersley number tested in the simulation was fixed at 10.5 and the Reynolds numbers tested were set to 750 and 2000, which are characteristics of human blood flows in large arteries. An in-house LES code, based on curvilinear Cartesian coordinates, has been developed to conduct the unsteady numerical simulations using three different grid systems. The physical characteristics of the flow field have been studied in terms of the resolved mean velocity, turbulence kinetic energy, viscous wall shear stress, resolved and subgrid-scale turbulent shear stresses, local kinetic ener...

The Relationship Between Pulsatile Flow Impingement and Intraluminal Thrombus Deposition in Abdominal Aortic Aneurysms

Direct numerical simulations were performed on four patient-specific abdominal aortic aneurysm (AAA) geometries and the resulting pulsatile blood flow dynamics were compared to aneurysm shape and correlated with intraluminal thrombus (ILT) deposition. For three of the cases, turbulent vortex structures impinged/sheared along the anterior wall and along the posterior wall a zone of recirculating blood formed. Within the impingement region the AAA wall was devoid of ILT and remote to this region there was an accumulation of ILT. The high wall shear stress (WSS) caused by the impact of vortexes is thought to prevent the attachment of ILT. WSS from impingement is comparable to peak-systolic WSS in a normal-sized aorta and therefore may not damage the wall. Expansion occurred to a greater extent in the direction of jet impingement and the wall-normal force from the continuous impact of vortexes may contribute to expansion. It was shown that the impingement region has low oscillatory shear index (OSI) and recirculation zones can have either low or high OSI. No correlation could be identified between OSI and ILT deposition since different flow dynamics can have similar OSI values.

Surface roughness effects on the turbulence statistics in a low Reynolds number channel flow

A high-resolution particle image velocimetry was used to characterize a low Reynolds number turbulent flow in a channel. Experiments were conducted over a sand grain-coated surface of large relative roughness, and the results were compared with measurements over a smooth surface. The roughness perturbation significantly modified the outer layer. Even though the streamwise Reynolds stress shows less sensitivity in the outer layer to the boundary condition, significant enhancements were observed in the wall-normal Reynolds stress and the Reynolds shear stress. These modifications were considered as footprints of the larger-scale eddies transporting intense wall-normal motions away from the rough wall. A quadrant decomposition shows that strong and more frequent ejections are responsible for the larger values of the mean Reynolds shear stress over the rough wall. The results also indicate that spanwise vortex cores with mean vorticity of the same sign as the mean shear are the dominant smaller-scale vortical structures over the smooth and rough walls. A linear stochastic estimation-based analysis shows that the average larger-scale structure associated with these vortices is a shear layer that strongly connects the outer layer flow to the near-wall flow. A proper orthogonal decomposition of the flow suggests that the large-scale eddy is more energetic for the rough wall, and contributes more significantly to the resolved turbulent kinetic energy and the Reynolds shear stress than the smooth wall.

Free surface effects on the statistical properties of a submerged rectangular jet

The results of an experimental investigation of turbulent rectangular jets offset from a free surface are presented. Two rectangular jets of aspect ratio 2 and 4 were examined and the results were compared to a square jet at the same offset height ratio of h/De ≈ 2.7, where De is the circle-equivalent diameter of the nozzle. A particle image velocimetry (PIV) was used to measure the mean flow and turbulent characteristics at a Reynolds number of 7900 and a Froude number of 1.29. The results indicate significant enhancements in the jet spreading rate and mean streamwise velocity decay rate for the larger aspect ratio nozzles. The results reveal that the jet-free surface interaction had a greater impact on the mean surface-normal velocity than the mean streamwise velocity. The values of the turbulence intensities, Reynolds shear stress and structure parameter were found to be nearly independent of the nozzle aspect ratio. Surface mean velocity and turbulence intensities were also measured to characterize th...

Effects of sedimenting particles on the turbulence structure in a horizontal channel flow

This work presents the results of experiments conducted in a horizontal channel to characterize low Reynolds number turbulent flows in the presence of small solid particles. The particle diameter relative to the integral length scale, dp/Λx, is approximately 0.02. Particles and fluid turbulence characteristics are measured for three average solid volume fractions of approximately ϕv = 2.0 × 10−4, 4.0 × 10−4, and 8.0 × 10−4 under conditions where the particle number density is evolving due to deposition. The results indicate that the mean slip between particles and the fluid is important only close to the wall. Away from the wall, the particles and unladen fluid mean velocities are similar. Differences between particles and the unladen fluid statistics are more pronounced in the wall-normal velocity fluctuations than the streamwise velocity fluctuations and Reynolds shear stress due to the stronger effect of the gravitational force in the wall-normal direction. The fluid turbulent intensities show no depen...

Study of laminar–turbulent flow transition under pulsatile conditions in a constricted channel

In this paper, direct numerical simulation is performed to investigate a pulsatile flow in a constricted channel to gain physical insights into laminar–turbulent–laminar flow transitions. An in-house computer code is used to conduct numerical simulations based on available high-performance shared memory parallel computing facilities. The Womersley number tested is fixed to 10.5 and the Reynolds number varies from 500 to 2000. The influences of the degree of stenosis and pulsatile conditions on flow transitions and structures are investigated. In the region upstream of the stenosis, the flow pattern is primarily laminar. Immediately after the stenosis, the flow recirculates under an adverse streamwise pressure gradient, and the flow pattern transitions from laminar to turbulent. In the region far downstream of the stenosis, the flow becomes re-laminarised. The physical characteristics of the flow field have been thoroughly analysed in terms of the mean streamwise velocity, turbulence kinetic energy, viscous wall shear stresses, wall pressure and turbulence kinetic energy spectra.

Large-Eddy Simulation of Physiological Pulsatile Flow Based on a Dynamic Nonlinear Subgrid-Scale Stress Model

Pulsatile laminar-turbulent transitional flow in a three-dimensional (3D) constricted channel represents a challenging topic and has many important applications in bio-medical engineering. In this research, we numerically investigate the physics of a physiological pulsatile flow confined within a 3D channel with an idealized stenosis formed eccentrically on the top wall using the method of large-eddy simulation (LES). The advanced dynamic nonlinear subgrid-scale stress (SGS) model of Wang and Bergstrom [1] was implemented in the current LES approach to properly resolve the unrealistic SGS dissipation effects and numerical instabilities that are intrinsic to the Smagorinsky type dynamic models (DM). The Reynolds numbers tested in the simulation are 1700 and 2000 , which are characteristic of human blood flows in large arteries. An in-house 3-D LES code has been modified to conduct our unsteady numerical simulations, and the results obtained have been validated using two different grid arrangements and the experimental results of Ahmed and Giddens [2]. The numerical results have been examined in terms of the resolved mean velocity, turbulence kinetic energy, viscous wall shear stress, resolved and subgrid-scale Reynolds stresses, as well as the local kinetic energy fluxes between the filtered and subgrid scales.Copyright

Adverse and Favourable Pressure Gradient Turbulent Flows over Smooth and Rough Surfaces

An experimental investigation was undertaken to study the salient features of adverse and favourable pressure gradient turbulent flows over a smooth wall and gravel roughness in asymmetric diverging and converging channels. Reference experiments were also performed in a parallel walled channel for which the pressure gradient was nearly zero. A high resolution particle image velocimetry system was used to conduct the velocity measurements. From these measurements, both one-point and two-point statistics were extracted and used to determine the effects of combined roughness and pressure gradient on the turbulence structure. It was found that adverse pressure gradient and surface roughness increased the turbulence intensities and Reynolds shear stress over the entire boundary layer, while favourable pressure gradient increased the turbulent intensities in the wall region and decreased the turbulence level in the outer layer. The Reynolds shear stress was decreased substantially by the favourable pressure gradient resulting in a considerable decay in the levels of the stress ratios over the smooth surface and gravel roughness. The distributions of the turbulent diffusion terms show considerable transport of turbulent kinetic energy and Reynolds shear stress towards the wall in the presence of adverse pressure gradient and surface roughness, while these terms are attenuated by favourable pressure gradient. In the diverging channel, it was found that surface roughness increases the spatial extents of the two-point streamwise velocity auto-correlation contour in the inner layer and increases the extents of the wall-normal velocity correlation in the outer layer.