Vorticity Production Research Articles

Prediction of sound absorption characteristics of orifice plates with mean flow using the lattice Boltzmann method.

The purpose of this study was to evaluate the sound absorption characteristics of orifice plates in turbulent mean flows using numerical simulations. The production of vorticity at the orifice edge governs the sound absorption phenomena. The LBM-LES methodology was used to perform detailed numerical simulations of the unsteady turbulent field. The lattice Boltzmann method has several advantages over continuum based CFD methods for situations where the configurations of the duct, the orifice, and the flow structures through or in contact with porous/perforated media are geometrically complex. The absorption coefficient and the impedance of the perforated plate were obtained in a virtual three dimensional (3-D) impedance tube apparatus. Both the mean flow velocity profiles and the acoustic characteristics of a simple circular orifice were found to be in good agreement with available experimental data. The model was then exercised to investigate the acoustic properties of orifices with complex shapes over a range of mean flow velocities.

Vorticity production through rotation, shear, and baroclinicity

In the absence of rotation and shear, and under the assumption of constant temperature or specific entropy, purely potential forcing by localized expansion waves is known to produce irrotational flows that have no vorticity. Here we study the production of vorticity under idealized conditions when there is rotation, shear, or baroclinicity, to address the problem of vorticity generation in the interstellar medium in a systematic fashion. We use three-dimensional periodic box numerical simulations to investigate the various effects in isolation. We find that for slow rotation, vorticity production in an isothermal gas is small in the sense that the ratio of the root-mean-square values of vorticity and velocity is small compared with the wavenumber of the energy-carrying motions. For Coriolis numbers above a certain level, vorticity production saturates at a value where the aforementioned ratio becomes comparable with the wavenumber of the energy-carrying motions. Shear also raises the vorticity production, but no saturation is found. When the assumption of isothermality is dropped, there is significant vorticity production by the baroclinic term once the turbulence becomes supersonic. In galaxies, shear and rotation are estimated to be insufficient to produce significant amounts of vorticity, leaving therefore only the baroclinic term as the most favorable candidate. We also demonstrate vorticity production visually as a result of colliding shock fronts.

Large eddy simulation of gas-particle turbulent channel flow with momentum exchange between the phases

This paper presents results of a large eddy simulation (LES) combined with Lagrangian particle tracking and a point-force approximation for the feedback effect of particles on the downward turbulent gaseous flow in a vertical channel. The LES predictions are compared with the results obtained by direct numerical simulation (DNS) of a finer computational mesh. A parametric study is conducted for particles with two response times in simulations with and without streamwise gravitational settling and elastic, binary interparticle collisions. It is shown that the classical and the dynamic Smagorinsky turbulence models adequately predict the particle-induced changes in the mean streamwise velocity and the Reynolds stresses of the carrier phase for the range of parameters studied. However, the largest discrepancies between the LES and DNS results are found in the cases of particle-laden flows. Conditional sampling of the instantaneous resolved flow fields indicates that the mechanisms by which particles directly oppose the production of momentum and vorticity of the organized fluid motions are also observed in the LES results. However, the geometric features of the near-wall quasistreamwise vortices are overestimated by the use of both turbulence models compared to the DNS predictions.

Hydrodynamic Effects of Compliance Mismatch in Stented Arteries

Cardiovascular diseases are the number one cause of death in the world, making the understanding of hemodynamics and development of treatment options imperative. The most common modality for treatment of occlusive coronary artery diseases is the use of stents. Stent design profoundly influences the postprocedural hemodynamic and solid mechanical environment of the stented artery. However, despite their wide acceptance, the incidence of stent late restenosis is still high (Zwart et al., 2010, "Coronary Stent Thrombosis in the Current Era: Challenges and Opportunities for Treatment," Current Treatment Options in Cardiovascular Medicine, 12(1), pp. 46-57), and it is most prevailing at the proximal and distal ends of the stent. In this work, we focus our investigation on the localized hemodynamic effects of compliance mismatch due to the presence of a stent in an artery. The compliance mismatch in a stented artery is maximized at the proximal and distal ends of the stent. Hence, it is our objective to understand and reveal the mechanism by which changes in compliance contribute to the generation of nonphysiological wall shear stress (WSS). Such adverse hemodynamic conditions could have an effect on the onset of restenosis. Three-dimensional, spatiotemporally resolved computational fluid dynamics simulations of pulsatile flow with fluid-structure interaction were carried out for a simplified coronary artery with physiologically relevant flow parameters. A model with uniform elastic modulus is used as the baseline control case. In order to study the effect of compliance variation on local hemodynamics, this baseline model is compared with models where the elastic modulus was increased by two-, five-, and tenfold in the middle of the vessel. The simulations provided detailed information regarding the recirculation zone dynamics formed during flow reversals. The results suggest that discontinuities in compliance cause critical changes in local hemodynamics, namely, altering the local pressure and velocity gradients. The change in pressure gradient at the discontinuity was as high as 90%. The corresponding changes in WSS and oscillatory shear index calculated were 9% and 15%, respectively. We demonstrate that these changes are attributed to the physical mechanism associating the pressure gradient discontinuities to the production of vorticity (vorticity flux) due to the presence of the stent. The pressure gradient discontinuities and augmented vorticity flux are affecting the wall shear stresses. As a result, this work reveals how compliance variations act to modify the near wall hemodynamics of stented arteries.

Numerical simulation for turbulence-driven secondary motion over a streamwise external corner

Numerical simulation, based on a Reynolds Stress Model (RSM), of turbulent flow over a streamwise external corner is presented. This work is an extension of an earlier experimental study (Moinuddin et al., 2004), which identified a pair of counter-rotating vortices placed around the corner of a 6 m long model. Experimental data measured at an early station is used as the inlet condition for the numerical simulation. Mean flow and turbulence statistics from numerical simulation are compared with the experimental data at a downstream station and they are found to be in excellent qualitative agreement. For the scaled mean flow data, quantitative agreement is also very good. Investigation reveals that vorticity production by secondary shear stress is dominant in generating secondary flow over an external corner, which is opposite for the case of an internal corner as found by Xu and Pollard (2001).

Large Eddy Simulation of Turbulent-Cavitation Interactions in a Venturi Nozzle

Large eddy simulation of turbulent cavitating flow in a venturi nozzle is conducted. The fully compressible Favre-filtered Navier–Stokes equations are coupled with a homogeneous equilibrium cavitation model. The dynamic Smagorinsky subgrid-scale turbulence model is employed to close the filtered nonlinear convection terms. The equations are numerically integrated in the context of a generalized curvilinear coordinate system to facilitate geometric complexities. A sixth-order compact finite difference scheme is employed for the Navier–Stokes equations with the AUSM+-up scheme to handle convective terms in the presence of large density gradients. The stiffness of the system due to the incompressibility of the liquid phase is addressed through an artificial increase in the Mach number. The simulation predicts the formation of a vapor cavity at the venturi throat with an irregular shedding of the small scale vapor structures near the turbulent cavity closure region. The vapor formation at the throat is observed to suppress the velocity fluctuations due to turbulence. The collapse of the vapor structures in the downstream region is a major source of vorticity production, resulting into formation of hair-pin vortices. A detailed analysis of the vorticity transport equation shows a decrease in the vortex-stretching term due to cavitation. A substantial increase in the baroclinic torque is observed in the regions where the vapor structures collapse. A spectra of the pressure fluctuations in the far-field downstream region show an increase in the acoustic noise at high frequencies due to cavitation.

Numerical study of the turbulent mixing of a convergent shock tube

In this paper, we are concerned with physical aspects of three-dimensional Richtmyer–Meshkov instability (RMI) and resulting turbulent mixing phenomena in a convergent geometry. A system of two fluids, a heavy gas (sulfur hexafluoride, SF6) enclosed by a light gas (helium, He), is initially separated by two randomly perturbed interfaces. An artificial feature is imposed at the outer surface of these interfaces to enunciate the presence of a macroscopic perturbation.Numerical results obtained from TURMOIL3D simulations and physical insights into the RMI phenomena in such a configuration are presented and discussed. The density, mass fraction, vorticity, baroclinic vorticity production and simulated density Schlieren fields are provided to qualitatively describe the turbulent mixing process. It is concluded that the discrete feature, in conjunction with random perturbations, significantly influences the RMI and turbulent mixing characteristics.

On interaction of shock wave with elliptic gas cylinder

AbstractThe interaction of a planar shock with one elliptic heavy-gas (SF 6 ) cylinder surrounded by air is investigated experimentally. By changing the aspect ratio of the elliptic cylinder, the influence of the initial shape on the evolution of the interface is visualized by a series of dynamic photos utilized by a high-speed camera. It is found that the longer the axis perpendicular to the shock front, the faster and the severer the deformation of the gas cylinder. This can be explained mainly by the different amount of vorticity produced by the misalignment between the density gradient and the pressure gradient. When the vertical axis is much longer than the horizontal axis, the vorticity production is mainly concentrated at the upper and lower corners, which rolls up in time, and results in a structure of big vortex-pair. When the horizontal axis is much longer than the vertical axis, the baroclinic vorticity production distributes at almost every position along the interface, which leads to a faster rolling up of vortices, and even second vortex may develop at later times.Graphical Abstract[IMAGE]

On the Reynolds number scaling of vorticity production at no-slip walls during vortex-wall collisions

Recently, numerical studies revealed two different scaling regimes of the peak enstrophy Z and palinstrophy P during the collision of a dipole with a no-slip wall [Clercx and van Heijst, Phys. Rev. E 65, 066305, 2002]: Z ∝ Re0.8 and P ∝ Re2.25 for 5 × 102 ≤ Re ≤ 2 × 104 and Z ∝ Re0.5 and P ∝ Re1.5 for Re ≥ 2 × 104 (with Re based on the velocity and size of the dipole). A critical Reynolds number Rec(here, Rec ≈ 2 × 104) is identified below which the interaction time of the dipole with the boundary layer depends on the kinematic viscosity ν. The oscillating plate as a boundary-layer problem can then be used to mimick the vortex-wall interaction and the following scaling relations are obtained: Z ∝ Re^3/4, P ∝ Re^9/4, and dP/dt ∝ Re11/4 in agreement with the numerically obtained scaling laws. For Re ≥ Rec the interaction time of the dipole with the boundary layer becomes independent of the kinematic viscosity and, applying flat-plate boundary-layer theory, this yields: Z ∝ Re1/2 and P ∝ Re3/2.

Viscous tilting and production of vorticity in homogeneous turbulence

Viscous depletion of vorticity is an essential and well known property of turbulent flows, balancing, in the mean, the net vorticity production associated with the vortex stretching mechanism. In this letter, we, however, demonstrate that viscous effects are not restricted to a mere destruction process, but play a more complex role in vorticity dynamics that is as important as vortex stretching. Based on the results from three dimensional particle tracking velocimetry experiments and direct numerical simulation of homogeneous and quasi-isotropic turbulence, we show that the viscous term in the vorticity equation can also locally induce production of vorticity and changes of the orientation of the vorticity vector (viscous tilting).

DNS of the thermal effects of laser energy deposition in isotropic turbulence

The interaction of a laser-induced plasma with isotropic turbulence is studied using numerical simulations. The simulations use air as the working fluid and assume local thermodynamic equilibrium. The numerical method is fully spectral and uses a shock-capturing scheme in a corrector step. A model problem involving the effect of energy deposition on an isolated vortex is studied as a first step towards plasma/turbulence interaction. Turbulent Reynolds number Reλ = 30 and fluctuation Mach numbers Mt = 0.001 and 0.3 are considered. A tear-drop-shaped shock wave is observed to propagate into the background, and progressively become spherical in time. The turbulence experiences strong compression due to the shock wave and strong expansion in the core. This behaviour is spatially inhomogeneous and non-stationary in time. Statistics are computed as functions of radial distance from the plasma axis and angular distance across the surface of the shock wave. For Mt = 0.001, the shock wave propagates on a much faster time scale compared to the turbulence evolution. At Mt of 0.3, the time scale of the shock wave is comparable to that of the background. For both cases the mean flow is classified into shock formation, shock propagation and subsequent collapse of the plasma core, and the effect of turbulence on each of these phases is studied in detail. The effect of mean vorticity production on the turbulent vorticity field is also discussed. Turbulent kinetic energy budgets are presented to explain the mechanism underlying the transfer of energy between the mean flow and background turbulence.

Comparison between Two Case Studies of Developing and Nondeveloping African Easterly Waves during NAMMA and AMMA/SOP-3: Absolute Vertical Vorticity Budget

Abstract Two West African disturbances observed in August and September 2006 during the National Aeronautics and Space Administration African Monsoon Multidisciplinary Analysis (NAMMA) and the Special Observing Period 3 (AMMA/SOP-3) have been simulated using the Méso-NH numerical model with explicit convection. The first disturbance spawned Hurricane Helene (2006) off the West African coast, and the second one, referred to as perturbation D, though relatively intense, failed to develop. Over the continent, each case was associated with a well-defined African easterly wave (AEW) trough with embedded growing and decaying convective activity of various size, duration, and intensity. The aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the AEW trough, with respect to the synoptic-scale processes. The absolute vorticity budgets are analyzed during the “continental” and “oceanic transition” stages of these AEW troughs in order to highlight the similarities and differences between the developing pre-Helene disturbance and the nondeveloping perturbation D. For the developing case, low- to midlevel cyclonic vorticity was produced by convective processes through tilting and stretching. Cyclonic vorticity was then transported upward through vertical advection associated with convection and outward through horizontal advection mostly induced by the large-scale midlevel diverging circulation related to the downstream AEW ridge. For the nondeveloping case, low- to midlevel cyclonic vorticity production through stretching and tilting, and its vertical transport were relatively similar over the continent but smaller over the oceanic transition because of weaker convective activity. The outward transport through horizontal advection was also weaker as there was little midlevel divergence induced by the downstream AEW ridge in this case.

Evolution and decay of a rotating flow over random topography

The evolution and decay of a homogeneous flow over random topography in a rotating system is studied by means of numerical simulations and theoretical considerations. The analysis is based on a quasi-two-dimensional shallow-water approximation, in which the horizontal divergence is explicitly different from zero, and topographic variations are not restricted to be much smaller than the mean depth, as in quasi-geostrophic dynamics. The results are examined by comparing the evolution of a turbulent flow over different random bottom topographies characterized by a specific horizontal scale, or equivalently, a given mean slope. As in two-dimensional turbulence, the energy of the flow is transferred towards larger scales of motion; after some rotation periods, however, the process is halted as the flow pattern becomes aligned along the topographic contours with shallow water to the right. The quasi-steady state reached by the flow is characterized by a nearly linear relationship between potential vorticity and transport function in most parts of the domain, which is justified in terms of minimum-enstrophy arguments. It is found that global energy decays faster for topographies with shorter horizontal length scales due to more effective viscous dissipation. In addition, some comparisons between simulations based on the shallow-water and quasi-geostrophic formulations are carried out. The role of solid boundaries is also examined: it is shown that vorticity production at no-slip walls contributes for a slight disorganization of the flow.

Eastern Mediterranean Transient and reversal of the Ionian Sea circulation

Maps of the remotely sensed Absolute Dynamic Topography (ADT) and Sea Surface Temperature (SST) over the Eastern Mediterranean in the period June 1993 to June 2001 show a dramatic and sudden change in the sea surface structure. This change, which is presumably associated with the relaxation of the Eastern Mediterranean Transient (EMT), had taken place by the middle of 1997 and consisted of a complete reversal of the Ionian upper‐layer circulation from anticyclonic to cyclonic. In the EMT phase, SST and ADT maps suggest enhanced communication between the Ionian and the Aegean basins as well as Aegean waters spreading northward along the eastern Ionian flank. Negative wind vorticity in the Ionian suggests that the switch of the upper‐layer circulation from anticyclonic to cyclonic cannot be explained in terms of the wind forcing. Therefore, we suggest the predominance of the baroclinic vorticity production term in determining the upper‐layer circulation inversion.

Low‐frequency characteristics of wave group–forced vortices

The dynamics of vorticity motions forced by wave groups incident on an alongshore‐uniform barred beach are analyzed. For both normally and obliquely incident wave groups, the potential vorticity and enstrophy equations reveal that the temporal variability of wave group–forced vortices is directly linked to the variability in the incoming wave groups rather than bottom friction, as previously hypothesized. Analysis of the lifespan of individual vortices further shows that the wave group forcing is responsible for not only the temporal variations of the vortices but also their eventual demise. Vortices in the simulations persist for 5 to 45 min, which is consistent with recent field observations. For oblique wave groups, the resulting vortices are advected by the mean current, yielding a signature in the frequency–wave number spectrum that is similar to that usually attributed to shear instabilities of the alongshore current. These results may explain previous observations of alongshore‐propagating vorticity motions in the presence of a stable alongshore current. For simulations involving an unstable alongshore current, we find that the inclusion of wave group forcing results in velocity spectra that are much broader compared to the simulations that neglect wave grouping, which could explain discrepancies between previously observed and modeled spectral widths of propagating vorticity motions. Finally, the potential enstrophy balance shows that vorticity production due to wave groups may be as important as that due to the instability process and that not all low‐frequency vortical motions observed during oblique wave incidence should be attributed to shear instabilities of the alongshore current.

A high energy density shock driven Kelvin–Helmholtz shear layer experiment

Radiographic data from a novel and highly successful high energy density Kelvin–Helmholtz (KH) instability experiment is presented along with synapses of the theory and simulation behind the target design. Data on instability growth are compared to predictions from simulation and theory. The key role played by baroclinic vorticity production in the functioning of the target and the key design parameters are also discussed. The data show the complete evolution of large distinct KH eddies, from formation to turbulent break-up. Unexpectedly, low density bubbles comparable to the vortex size are observed forming in the free-stream region above each vortex at late time. These bubbles have the appearance of localized shocks, possibly supporting a theoretical fluid dynamics conjecture about the existence of supersonic bubbles over the vortical structure [transonic convective Mach numbers, D. Papamoschou and A. Roshko, J. Fluid Mech. 197, 453 (1988)] that support localized shocks (shocklets) not extending into the free stream (P. E. Dimotakis, Proceedings of the 22nd Fluid Dynamics, Plasma Dynamics and Lasers Conference, 1991, Paper No. AIAA 91-1724). However, it is also possible that these low density bubbles are the result of a cavitationlike effect. Hypothesis that may explain the appearance of low density bubbles will be discussed.

Effect of density step on stirring properties of a strain flow

The influence of steep density gradient on stirring properties of a strain flow is addressed by considering the problem in which an interface separating two regions with different constant densities is stabilized within a stagnation-point flow. The existence of an analytic solution for the two-dimensional incompressible flow field allows the exact derivation of the velocity gradient tensor and of parameters describing the local flow topology. Stirring properties are affected not only by vorticity production and jump of strain intensity at the interface, but also by rotation of strain principal axes resulting from anisotropy of pressure Hessian. The strain persistence parameter, which measures the respective effects of strain and effective rotation (vorticity plus rotation rate of strain basis), reveals a complex structure. In particular, for large values of the density ratio, it indicates dominating effective rotation in a restricted area past the interface. Information on flow structure derived from the Okubo–Weiss parameter, by contrast, is less detailed. The influence of the density step on stirring properties is assessed by the Lagrangian evolution of the gradient of a passive scalar. Even for a moderate density ratio, alignment of the scalar gradient and growth rate of its norm are deeply altered. Past the interface effective rotation indeed drives the scalar gradient to align with a direction determined by the local strain persistence parameter, away from the compressional strain direction. The jump of strain intensity at the interface, however, opposes the lessening effect of the latter mechanism on the growth rate of the scalar gradient norm and promotes the rise of the gradient.

The hydrodynamics of ship-like sections in heave, sway, and roll motions predicted using an unsteady Reynolds-averaged Navier—Stokes method

The application of strip theory for predicting ship seakeeping response in waves relies on sectional added mass and viscous damping. A Reynolds averaged Navier—Stokes (RANS) approach is adopted to obtain two-dimensional hydrodynamic coefficients that include viscous and rotational flow effects as well as free surface wave generation. Calculations are made for circular and rectangular sections of cylinders swaying, heaving, and rolling in the presence of a free surface, for a range of frequencies. The predicted hydrodynamic coefficients are compared with available experimental and numerical results. The method is successfully validated for sway of a submerged circular cylinder. Good agreement is obtained for sway and heave of a rectangular cylinder at a free surface. For roll, the results are highly sensitive to the mesh applied due to the vortex shedding that occurs at the sharp corners, even at relatively low roll amplitudes. The appropriate density of mesh influences the convection of the vortex, the production of vorticity, and the position of the vortex core. These all have a large impact on fluid damping. A hybrid-based zonal mesh generation approach is proposed to maintain a compromise between necessary mesh refinement and the overall number of cells. Using this technique and a small fixed time step leads to improved predictions of the roll hydrodynamic coefficients, particularly for fluid damping.

PIV investigation of flow over a transverse square rib in pressure gradients

This paper reports on experiments conducted to study the characteristics of separated and reattached turbulent flows experiencing adverse and favorable pressure gradients. The adverse and favorable pressure gradients were produced, respectively, in asymmetric diverging and converging channels, and the flow separation was caused by transverse square rib attached to the lower wall of the channels. A high-resolution particle image velocimetry was used to conduct detailed velocity measurements in the separated and redevelopment regions. Reference measurements were also conducted in a parallel-walled channel for which the pressure gradient is small compared to those produced in the diverging and converging for comparison. From these measurements, iso-contour plots of the mean velocity, spanwise vorticity and turbulence production in the vicinity of the rib were reported. The profiles of the mean velocities and vorticity, Reynolds stresses, triple correlations, production terms in the transport equations for the turbulent kinetic energy and Reynolds shear stress, mixing length and eddy viscosity were obtained in the separated and redevelopment regions as far downstream as 120 rib heights.

가스발생기 비-반응 유동의 난류 혼합에 대한 LES 해석

LES analysis was conducted with in-house CFD code to investigate the turbulence evolution and interaction due to turbulence ring and splash plate in the gas generator. Though chemical reaction was not accounted for, the results can be useful in determining the turbulence characteristics generated by ring and plate. The calculation results show that the installation of turbulence ring can introduce additional turbulences and improve turbulent mixing in the downstream flow. However, the addition of splash plate in the downstream of TR brings totally different shape of perturbation energy and enstrophy distribution for turbulent mixing. This enhancement can be done by the formation of the intensively strong vorticity production and mixing behind the plate. Pressure drop was found to be a reasonable level of about 1% or less of initial pressure in all calculation cases. Also, calculation results revealed that the variation of TR shape and intrusion length did not change the characteristics of turbulent mixing in the chamber. Even though the effect of installation location of splash plate on the turbulent mixing is not investigated yet, calculation results conclude the addition of splash plate leads to the increase in turbulent mixing with an acceptable pressure drop.