Implicit Large Eddy Simulation Method Research Articles

Numerical Simulation of Turbulent Mixing in a Shallow Lake for Periods of Under-Ice Convection

The article presents the results of application of Implicit Large Eddy Simulation method to numerical simulation of under-ice radiatively driven convection, developing in ice-covered water bodies in the moderate zone at the end of freeze-up period. Studies of the radiatively driven convection are of importance because of the role it plays in the temperature regime of lakes and the functioning of lake ecosystems at the end of freeze-up period. The simulation was carried out with the use of the finite-volume software code SINF/Flag-S, developed in SPbPU. The SIMPLEC algorithm with second-order accuracy was used for ad­vancing in time. The discretization of the convective terms was made with the use of QUICK scheme. The results of calculations were used to study variations in the temperature and pulsation velocity components with periodically varying intensity of external energy pumping during the daily cycle. The dissipation of the kinetic energy, background potential energy, and buoyancy flux were evaluated, and changes in these variables during a daily cycle of radiation impact were calculated. The efficiency mixing of water column was evaluated for the period of development of radiatively driven convection in a model domain simulating a small lake covered by ice.

An implicit large eddy simulation method based on all-speed schemes

It remains a challenge for ILES methods to provide physical consistent viscosity in real applications which comprise complex flow structures in both high and low Mach numbers regimes. For these, we analyze an Implicit Large Eddy Simulation (ILES) method based upon all-speed schemes in a framework of the finite volume method in this study. This method can accurately capture both compressible phenomena such as shock waves as well as small turbulent structures. With a detailed analysis of the numerical viscosity and effective eddy viscosities in physical and spectral spaces, the method is verified to be suitable for the simulation of compressible turbulent flows. After it, the derived ILES method is applied to the simulation of a cylinder flow and a transonic cavity flow. Both results are in good agreements with the previous LES or experimental data. These results indicate that it is promising to study the application of the all-speed scheme to the ILES of complex engineering flow problems.

Statistical characteristics of turbulent mixing in spherical and cylindrical converging Richtmyer–Meshkov instabilities

In this paper, the Richtmyer–Meshkov instabilities in spherical and cylindrical converging geometries with a Mach number of approximately 1.5 are investigated by using the high resolution implicit large eddy simulation method, and the influence of the geometric effect on the turbulent mixing is investigated. The heavy fluid is sulphur hexafluoride (SF6), and the light fluid is nitrogen (N2). The shock wave converges from the heavy fluid into the light fluid. The Atwood number is 0.678. The total structured and uniform Cartesian grid node number in the main computational domain is 20483. In addition, to avoid the influence of boundary reflection, a sufficiently long sponge layer with 50 non-uniform coarse grids is added for each non-periodic boundary. Present numerical simulations have high and nonlinear initial perturbation levels, which rapidly lead to turbulent mixing in the mixing layers. Firstly, some physical-variable mean profiles, including mass fraction, Taylor Reynolds number, turbulent kinetic energy, enstrophy and helicity, are provided. Second, the mixing characteristics in the spherical and cylindrical turbulent mixing layers are investigated, such as molecular mixing fraction, efficiency Atwood number, turbulent mass-flux velocity and density self-correlation. Then, Reynolds stress and anisotropy are also investigated. Finally, the radial velocity, velocity divergence and enstrophy in the spherical and cylindrical turbulent mixing layers are studied using the method of conditional statistical analysis. Present numerical results show that the geometric effect has a great influence on the converging Richtmyer–Meshkov instability mixing layers.

Results of numerical simulation on a thick teardrop airfoil at low Reynolds numbers

A symmetrical thick teardrop airfoil was investigated in this work. A numerical simulation was performed with a laminar flow, using the Implicit Large Eddy Simulation method. The fields of flow parameters in the vicinity of the airfoil were obtained for Reynolds numbers 3.5e+4 and 6.9 e+4.

Flow and Noise Predictions of Coaxial Jets

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computation of Coaxial Jet Noise experiment. The jet is heated and issues for a short-cowl axisymmetric nozzle with a central body at a transonic speed. The first LES method is based on the Compact Accurately Boundary-Adjusting High-Resolution Technique (CABARET) scheme, for which implementation features include asynchronous time stepping at an optimal Courant–Friedrichs–Lewy number, a wall model, and a synthetic turbulence inflow boundary condition. The CABARET LES is implemented on Graphics Processing Units. The second LES approach is based on the hybrid Reynolds-average Navier–Stokes (RANS)/implicit LES method that uses a mixture of a high-order Roe and Monotonicity-Preserving reconstruction of the 9th order (MP9 scheme) and a wall distance model of the improved delayed detached-eddy simulation type. The RANS/implicit LES method is run on a Message Passing Interface (MPI) cluster. Two grid generation approaches are considered: the unstructured grid using OpenFOAM utility snappyHexMesh and the conventional structured multiblock body-fitted curvilinear grid. The LES flow solutions are compared with the experiment and also with solutions obtained from the standard axisymmetric RANS method using the turbulence model. For noise predictions, the LES solutions are coupled with the penetrable surface formation of the Ffowcs Williams–Hawkings method. The results of noise predictions are compared with the experiment, and the effect of different LES grids and acoustic integration surfaces is discussed.

Flow separation control in a conical diffuser with a Karman-vortex generator

As a novel passive flow control device, a ring-shaped Karman-vortex generator (KVG) is employed to alleviate the severe flow separation of a conical diffuser with a total divergence angle of 36∘ at a Re=1.0×104. The baseline diffuser and 18 controlled schemes with different KVG sizes and location combinations are studied by an implicit large-eddy simulation method. The flow control mechanisms and design principles are discussed based on current simulations and relevant researches. The diameter of the KVG DK is suggested to be 3–4% of the length of the expansion section. The length to the throat and the distance from the wall are suggested to be 7.5–10.0DK and 2.0–3.0DK, respectively. For the optimal combinations, the alternately shedding Karman vortex street is seldom affected by the wall, and the unsteady wake can directly arrive downstream at the near-wall region and effectively enhance the flow mixing. The results demonstrate that a properly designed KVG can suppress massive flow separation in the expansion section and downstream region and promote pressure recovery performance.

On the stabilization of leading-edge vortices with different reduced frequencies over finite-aspect-ratio pitch-up wings

An implicit large-eddy simulation method is used to numerically simulate the flowfield around a finite-span rectangular wing in pitch motion. The effect of the reduced frequency (k) on the stability of the leading-edge vortex (LEV) is studied. The stability of LEVs is considered to include adhesion and structural stability. The wing is pitched at a position of around 1/4c (chord length), and the flow has a Reynolds number of 9053. Reduced frequencies of 0.2, 0.4, 0.8, and 1.6 are studied. For k = 0.2, the calculated results are in good agreement with experimental measurements, which demonstrates the reliability of the calculation method. The results show that the structural stability of LEVs can be significantly enhanced by increasing k. Larger values of k reduce the spatial scale of the LEVs, as well as prevent them from growing too fast and bursting. Moreover, a larger reduced frequency also delays the decrease in the LEV circulation, allowing the LEV structure to maintain stability at a larger angle of attack (α). In addition, a larger value of k also helps to enhance the adhesion stability of the LEVs. Numerical simulation results show that smaller values of k encourage the formation of secondary vortices. These accelerate the backflow between the LEVs and the wing, thus promoting the upward movement of the LEVs and reducing their adhesion stability. However, analysis of the convection terms in the vortex dynamic equations indicates that lower k is beneficial for the vorticity in the LEV to be transported outwards, but very small k will cause the reverse transport of vorticity, which is harmful to the stability of the LEV.

Aeroacoustic simulation of the high-lift airfoil using hybrid reynolds averaged Navier-Stokes/high-order implicit large eddy simulation method

A hybrid RANS/HILES method (HRILES) is developed by combining the RANS-SST model and high order implicit large eddy simulation method (HILES) and employed with the Ffowcs Williams-Hawkings (FW-H) equation to predict the slat noise of 30P30N high-lift airfoil. Comparison has been made between the HRILES method and the traditional DDES based on the full-turbulence model SST by simulating the single cylinder case with <inline-formula><tex-math id="M2">\begin{document}$Re_{{d}}=4.3\times10^4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20190777_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20190777_M2.png"/></alternatives></inline-formula>. The HRILES method is able to predict the transition phenomenon and the small-scale separation bubble in the sub-critical wake region while the DDES can't and get a better mean wall pressure distribution than DDES. The amplitude and frequency spectrum of the far-field sound pressure level are in good agreement with the experimental data. In the simulation of 30P30N high-lift airfoil, the famous IDDES model is also used for comparison, both results are compared with experimental measurements. The computational mesh is provided by Japan Aerospace Exploration Agency (JAXA) in the Workshop on Benchmark problems for Airframe Noise Computations (BANC). The HRILES method obtains quantitative agreement with experimental data in terms of mean wall pressure coefficient, frequency spectrum of pressure fluctuations on the slat surface, and the mean flow statistics in the slat cusp shear layer. The IDDES model slightly underestimate the suction effect on the upper surface of the slat, and delays the instabilities in the slat cusp shear layer. The near-field noise spectra are compared with measurements obtained in JAXA low-speed Wind Tunnel. Narrow band peaks present are well recovered by both methods, while IDDES model overestimate the broadband noise. Far-field noise directivity results of every components, filtered in the band [256Hz–10KHz], are compared with each other, and the slat cove is confirmed to dominate the sound noise levels. The slat and flap noises show a typical dipole distribution, while the main wing noise's directivity is not apparent. Computational results show that the HRILES method, as one kind of generalized Hybrid RANS/LES method, HRILES can smoothly switch between the SST model and the HILES method. HRILES has the high-resolution simulation capability of the HILES in the LES region, and can reduce the requirements of the HILES method on the near-wall grid distribution by using the SST model in the inner boundary layer. As a result, the HRILES method has advantages in simulations at high reynolds numbers and aeroacoustic problems. Further research will be carried out in the applications at higher reynolds number flows with complex geometry in the future.

Study of the Possibilities of Gasdynamic Air Flow Control in the Spatial Air Intake of a Light Supersonic Business-Class Aircraft by the RANS/ILES Method

The effect of the boundary-layer bleed system on the flow and characteristics of the spatial air intake of a supersonic business-class aircraft integrated with an airframe simulator was studied by the combined high-resolution Reynolds Averaged Navier–Stokes Implicit Large Eddy Simulation method (RANS/ILES ). Two variants of geometry with and without the boundary-layer bleed system were considered. The effect of gasdynamic flow control with the use of synthetic jets was studied for both air intake variants. The effect produced by the boundary-layer bleed system and the regime parameters of synthetic jets on the flow in the air intake and its characteristics, the level of turbulent fluctuations, and the surge boundary was studied. The level of velocity and pressure fluctuations was established to be lower at the outlet of the air intake without the boundary-layer bleed system. The application of synthetic jets made it possible to reduce the total pressure losses and level of turbulent fluctuations in the channel of the air intake and at its outlet, thus increasing the range of its stable operation.

Numerical study of an airfoil with riblets installed based on large eddy simulation

The implicit large eddy simulation method is applied in a riblet drag reduction study of a low-speed airfoil. The numerical method is tested by a channel flow with streamwise riblets installed on one side. The mean velocity profile, root mean square of velocity and Reynolds shear stress match well with reference direct numerical simulation data. Next, numerical investigation is conducted on an airfoil called Eppler E374 at free stream Mach number 0.2, angle of attack 3° and Reynolds number 2.0 × 105. The simulated flow around the airfoil without or with a numerical trip is in good agreement with the experimental data. When isosceles triangle riblets are installed on the airfoil along the streamwise direction, the lift coefficient is increased, and the friction drag is decreased. The pressure distribution on the airfoil is slightly changed, corresponding to the increased pressure drag. The Reynolds stresses are greatly reduced by the riblets at locations of strong pressure gradient. The result of the power spectrum density of pressure shows that the high-frequency fluctuations are suppressed when the riblet film is installed on the airfoil, and the vortex structures in the boundary layer are also reduced.

Spectrum study on unsteadiness of shock wave–vortex ring interaction

Shock oscillation with low-frequency unsteadiness commonly occurs in supersonic flows and is a top priority for the control of flow separation caused by shock wave and boundary layer interaction. In this paper, the interaction of the shock caused by the compression ramp and the vortex rings generated by a micro-vortex generator (MVG) in a supersonic flow at Ma = 2.5 is simulated by the implicit large eddy simulation method. The analysis of observation and the frequency of both the vortex ring motion and the shock oscillation is carried out. The results show that the shock produced by a compression ramp flow at Ma = 2.5 has a dominant non-dimensional low frequency, which is around St = 0.002, while the vortex rings behind the MVG have a dominant high frequency which is around St = 0.038. The dominant low frequency of the shock, which is harmful, can be removed or weakened through the shock-vortex ring interaction by the vortex rings which generate high frequency fluctuations. In the shock and vortex ring interaction region, a dominant high frequency St = 0.037–0.038 has been detected rather than the low frequency St = 0.002, which indicates that the vortex ring is stiff enough to break or weaken the shock. This analysis could provide an effective tool to remove or weaken the low frequency pressure fluctuation below 500 Hz, which has a negative effect on the flight vehicle structures and the environmental protection, through the high frequency vortex generation.

Study on shock wave-vortex ring interaction by the micro vortex generator controlled ramp flow with turbulent inflow

In this study, we will investigate the interaction between the vortex rings and oblique shocks generated by the micro vortex generatorʼs (MVG) controlled ramp flow at M=2.5 and Reθ=5760. An implicit large eddy simulation method is used by solving the unfiltered form of the Navier–Stokes equations with the 5th order bandwidth-optimized WENO scheme. It shows that the vortex ring structure generated by MVG is very stable, it does not break down and it keeps its original topology after penetrating the strong shock wave. However, the oblique shocks are broken when they interact with vortex rings. The bump on the 3D shock wave surface is observed and its mechanism is explained. The separation zone, which is originally generated by the shock–boundary layer interaction, is significantly reduced due to the vortex ring–shock interaction.

Dynamics of Sonic Hydrogen Jet Injection and Mixing Inside Scramjet Combustor

This paper presents the application of a Finite Volume Godunov-type implicit large eddy simulation method to study fuel injection into the combustion chamber of HyShot-II scramjet engine without chemical reaction/combustion in order to understand the fuel injection and air-fuel (hydrogen) mixing. The study is carried out in two parts; part one presents analysis of 2D HyShot-II geometry (without fuel injection) incorporating high temperature gas formulation which is validated against the NASA Thermally-Perfect-Gas code in order to obtain the combustion chamber inlet conditions. These combustor initial conditions are then utilized in part two for 3D combustion chamber simulations with hydrogen injection but cold flow where a digital filter based turbulent inflow boundary condition has been utilized. The purpose of the study is to understand the flow physics, hydrogen jet penetration and air & fuel mixing inside the HyShot-II combustor which is vital at the design stages. Various flow features are investigated such as the Mach number, velocity, pressure distributions, temperature, turbulent kinetic energy, Reynolds stresses and the effect of counter rotating vortices on mixing. The results of full geometry simulations are compared with computational results from the German Aerospace Centre, DLR, whereas due to unavailability of any data for hydrogen cold flow the validity of the results is based upon a similar validation case presented earlier (Rana et al., 2011b).

Hemodynamics of an End-to-Side Anastomotic Graft for a Pulsatile Pediatric Ventricular Assist Device

Numerical simulations are performed to investigate the flow within the end-to-side proximal anastomosis of a pulsatile pediatric ventricular assist device (PVAD) to an aorta. The anastomotic model is constructed from a patient-specific pediatric aorta. The three great vessels originating from the aortic arch--brachiocephalic (innominate), left common carotid, and left subclavian arteries--are included. An implicit large eddy simulation method based on a finite volume approach is used to study the resulting turbulent flow. A resistance boundary condition is applied at each branch outlet to study flow splitting. The PVAD anastomosis is found to alter the aortic flow dramatically. More flow is diverted into the great vessels with the PVAD support. Turbulence is found in the jet impingement area at peak systole for 100% bypass, and a maximum principal normal Reynolds stress of 7081 dyn/cm(2) is estimated based on ten flow cycles. This may be high enough to cause hemolysis and platelet activation. Regions prone to intimal hyperplasia are identified by combining the time-averaged wall shear stress and oscillatory shear index. These regions are found to vary, depending on the percentage of the flow bypass.

Numerical Study of Blood Flow at the End-to-Side Anastomosis of a Left Ventricular Assist Device for Adult Patients

We use an implicit large eddy simulation (ILES) method based on a finite volume approach to capture the turbulence in the anastomoses of a left ventricular assist device (LVAD) to the aorta. The order-of-accuracy of the numerical schemes is computed using a two-dimensional decaying Taylor-Green vortex. The ILES method is carefully validated by comparing to documented results for a fully developed turbulent channel flow at Re(tau)=395. Two different anastomotic flows (proximal and distal) are simulated for 50% and 100% LVAD supports and the results are compared with a healthy aortic flow. All the analyses are based on a planar aortic model under steady inflow conditions for simplification. Our results reveal that the outflow cannulae induce high exit jet flows in the aorta, resulting in turbulent flow. The distal configuration causes more turbulence in the aorta than the proximal configuration. The turbulence, however, may not cause any hemolysis due to low Reynolds stresses and relatively large Kolmogorov length scales compared with red blood cells. The LVAD support causes an acute increase in flow splitting in the major branch vessels for both anastomotic configurations, although its long-term effect on the flow splitting remains unknown. A large increase in wall shear stress is found near the cannulation sites during the LVAD support. This work builds a foundation for more physiologically realistic simulations under pulsatile flow conditions.

Computational and Experimental Investigation of a Nonslender Delta Wing

Computational simulations have been performed for a 50-deg-sweep delta wing with a sharp leading edge at a 15 deg angle of attack and moderate Reynolds numbers of Re = 2 x 10 5 , Re = 6.2 x 10 5 , and Re = 2 x 10 6 . A sixth-order compact-difference scheme with an eighth-order low-pass filter is used to solve the Navier-Stokes equations. Turbulence modeling has been accomplished using an implicit large eddy simulation method that exploits the high-order accuracy of the compact-difference scheme and uses the discriminating higher-order filter to regularize the solution. Computations have been performed on a baseline mesh of 11.3 x 10 6 grid points and a refined mesh of 35 x 10 6 grid points. An assessment of grid resolution showed that significantly finer-scale features of the flow could be captured on the refined mesh, providing a more accurate representation of the complex, unsteady, separated flow. Comparisons are also made with high-resolution particle image velocimetry images obtained for the two lower Reynolds numbers. The numerical results are examined to provide a description of the mean and instantaneous flow structure over the delta wing, including the separated vortical flow, vortex breakdown, surface flow features, and surface boundary-layer transition near the symmetry plane. The effect of Reynolds number on each of these features is assessed.

Contrasting a novel temporally orientated Hamilton–Jacobi‐equation‐based ILES method with other approaches for a complex geometry flow

AbstractFlow and heat transfer inside an idealized electronics system is simulated using large‐eddy simulation (LES)‐related approaches that have been validated for simpler canonical flows. These include: Yoshizawa LES, detached eddy simulation (DES), limited numerical scales (LNS) and other hybrid LES–RANS (Reynolds‐averaged Navier–Stokes) approaches including a new ILES (implicit LES)–RANS method. For the ILES, dissipation from the one legged temporal discretization is used to drain turbulence. The use of differential equations, including the Hamilton–Jacobi and Eikonal, to model turbulence distance functions is explored. The Hamilton–Jacobi is shown to be especially compatible with the zonal RANS–ILES approach and the Eikonal with DES. Performances of the LES‐related methods are compared with explicit algebraic stress unsteady RANS (URANS) results and also measurements. Considering the problem complexity, generally, for all methods, predicted mean velocities and turbulence intensities are in a reasonable agreement with measurements. Average errors are 15 and 25%, respectively. With the exception of the zonal ILES–RANS method, turbulence intensities are under‐predicted. For heat transfer, none of the models performs well giving circa 100% errors. Notably, the LNS performs poorly for both the flow field and heat transfer giving a highly complex RANS–LES interface with inappropriate upstream LES boundary conditions. DES is found impossible to converge. This is partly attributed to the irregular LES–RANS interface arising with the method. All the LES approaches significantly underpredict heat transfer and the URANS over‐predicts. Even the increased flow activity arising from use of the less dissipative ILES element does not prevent the significant heat transfer under‐prediction. Copyright © 2005 John Wiley & Sons, Ltd.