High-order Large Eddy Simulations Research Articles

A comparison of three error indicators for adaptive high-order large eddy simulation

We use a discontinuous high-order method to evaluate three error indicators, specifically designed for mesh adaptation in large eddy simulation (LES). The indicators are based on specific physical and numerical arguments. The first indicator is based on the unsteady residual. The second indicator is based on a local smoothness indicator. The third indicator is based on an estimate for small scale turbulent kinetic energy. We test the three error indicators at driving autonomous mesh refinement on implicit LES, evaluating the error indicators on two widely used benchmark simulations of the T106 LP turbine cascade. We compare the adaptively meshed simulations against direct numerical simulation (DNS) results to judge their effectiveness at reducing solution error in LES.

High-Order Large Eddy Simulations of a Wind Turbine in Ducted and Open-Rotor Configurations

Abstract High-order large eddy simulations are performed to study the performance and flow fields of a ducted wind turbine (DWT) operating at different tip speed ratios. To evaluate the effects of the duct, simulations with the same tip speed ratios are also performed on the corresponding open-rotor turbine. It is found that the ducted turbine consistently obtains higher power outputs than the open-rotor counterpart, and the duct itself enhances flow turbulence and blade trailing-edge vortices but weakens tip and hub vortices. Flow bifurcation is observed at the largest tip speed ratio and is identified to be caused by blade blockage effects. Comparative simulations are also performed on both turbines under different yaw angles. It is noticed that the ducted configuration is insensitive to small yaw angles and maintains higher power outputs than the open-rotor configuration at all yaw angles. Moreover, it is observed that the wakes of both configurations recover more quickly as the yaw angle increases.

Validation of a High-Order Large Eddy Simulation Solver for Acoustic Prediction of Supersonic Jet Flow

A high-order large eddy simulation (LES) code based on the flux reconstruction (FR) scheme is further developed for supersonic jet simulation. The FR scheme provides an efficient and easy-to-implement way to achieve high-order accuracy on an unstructured mesh. The order of accuracy and the shock capturing capability of the solver are validated with the isentropic Euler vortex and Sod’s shock tube problem. A heated under-expanded supersonic jet case from NASA’s Small Hot Jet Acoustic Rig (SHJAR) database is used for validation. The turbulence statistics along the nozzle centerline and lip-line are examined. We predict the acoustic radiation with the Ffowcs Williams and Hawkings method, which is integrated with our solver. The far-field acoustic predictions show reasonable agreement with the experimental measurement in the upstream and downstream directions, where the shock-associated noise and the large-scale turbulent mixing noise are dominant, respectively.

Isothermal and heated subsonic jet noise using large eddy simulations on unstructured grids

Jet noise remains the major contributor to airplane noise at take-off. In the past half-century of research, sound generation mechanisms in supersonic jets have been extensively characterized, while the sound sources in subsonic jets are still to be clearly identified. For the last two decades, Large Eddy Simulations (LES) have become a major tool for investigating jet noise sources due to their ability to capture detailed information in turbulent flows and their moderate cost that allows industrial applications. However, many challenges still arise when dealing with complex nozzle geometries and heating effects in the jet core. In this paper, subsonic jets with or without nozzle geometry at Mach number of 0.9 and moderately high Reynolds numbers ranging from 2 × 105 to 1 × 106 are computed using LES, providing a base of validation for different nozzle configurations and operating conditions. In this work, the high-order unstructured LES solver AVBP is combined with Ffowcs Williams and Hawkings’ acoustic analogy on unstructured grids. The vortex pairing phenomenon is evidenced without properly triggering the turbulence in the jet at the nozzle exit. Hence, a non-geometrical tripping, where the firsts prism layers of the mesh at the nozzle wall are removed, is proposed and shown to be a successful method for triggering proper turbulence development in shear layers for the cases with nozzle. Moreover, it is more easily implemented because it does not require any geometrical modifications and it generates more natural turbulence than previous methods, leading the path to actual industrial dual-stream configurations. Both isothermal and heated jet flow cases are performed and validated with existing experimental data in terms of aerodynamics and acoustics, which demonstrates the capacity of an unstructured LES solver to correctly simulate both cold and heated jet noise phenomena.

Multiblock High Order Large Eddy Simulation of Powered Fontan Hemodynamics: Towards Computational Surgery.

Children born with only one functional ventricle must typically undergo a series of three surgeries to obtain the so-called Fontan circulation in which the blood coming from the body passively flows from the Vena Cavae (VCs) to the Pulmonary Arteries (PAs) through the Total Cavopulmonary Connection (TCPC). The circulation is inherently inefficient due to the lack of a subpulmonary ventricle. Survivors face the risk of circulatory sequelae and eventual failure for the duration of their lives. Current efforts are focused on improving the outcomes of Fontan palliation, either passively by optimizing the TCPC, or actively by using mechanical support. We are working on a chronic implant that would be placed at the junction of the TCPC, and would provide the necessary pressure augmentation to re-establish a circulation that recapitulates a normal two-ventricle circulation. This implant is based on the Von Karman viscous pump and consists of a vaned impeller that rotates inside the TCPC. To evaluate the performance of such a device, and to study the flow features induced by the presence of the pump, Computational Fluid Dynamics (CFD) is used. CFD has become an important tool to understand hemodynamics owing to the possibility of simulating quickly a large number of designs and flow conditions without any harm for patients. The transitional and unsteady nature of the flow can make accurate simulations challenging. We developed and in-house high order Large Eddy Simulation (LES) solver coupled to a recent Immersed Boundary Method (IBM) to handle complex geometries. Multiblock capability is added to the solver to allow for efficient simulations of complex patient specific geometries. Blood simulations are performed in a complex patient specific TCPC geometry. In this study, simulations without mechanical assist are performed, as well as after virtual implantation of the temporary and chronic implants being developed. Instantaneous flow structures, hepatic factor distribution, and statistical data are presented for all three cases.

LES of longitudinal and transverse self-excited combustion instabilities in a bluff-body stabilized turbulent premixed flame

Combustion dynamics of a V-flame in an afterburner-type configuration are investigated using high-order compressible large eddy simulations (LES) and compared to experimental results. Both self-excited longitudinal (100 Hz) and transverse (1400 Hz) modes observed in the experiments are captured by LES and instability mechanisms are discussed. LES results for all modes are compared to a Helmholtz solver output, showing that the transverse mode appearing in the LES is the 1Lx-2Ty-0Tz eigenmode of the chamber, affecting the velocity field symmetrically. The 1Lx fluctuation causes a symmetric flame roll-up which increases heat release rate fluctuations, closing the feedback loop. The 2Ty component of the mode is active along the flame holder axis and causes not only transverse fluctuations but also a reorganization of the mean flame along two main zones located on both sides of the zero acoustic velocity plane, a feature that has not been reported before. Dynamic mode decomposition (DMD) is used to extract the structure of the transverse mode from LES snapshots which is found to match the Helmholtz solver prediction. This study confirms the capacity of high-order LES to capture not only low-frequency oscillations but also high-order frequency transverse modes in combustion chambers.

LES and analyses on the vortex structure behind supersonic MVG with turbulent inflow

In this paper, an implicitly implemented high order large eddy simulation by using the fifth order bandwidth-optimized WENO scheme is applied to make comprehensive studies on ramp flows with and without control at Mach 2.5 and Reθ=5760. Flow control in the form of microramp vortex generators (MVG) is applied. The mechanism of vortex ring generation behind MVG has been studied in detail and shear layer instability has been studied and found as the mechanism of K–H vortex ring generation. A series of new observations on the flow around supersonic MVG have been made including inflection points (surface in 3-D), vorticity conservation, interaction of the primary vortex and new generated K–H vortex rings, and the K–H vortex ring structure. The numerical observations have been confirmed by the experimental work.

Large eddy simulation of powered Fontan hemodynamics

Children born with univentricular heart disease typically must undergo three open heart surgeries within the first 2–3 years of life to eventually establish the Fontan circulation. In that case the single working ventricle pumps oxygenated blood to the body and blood returns to the lungs flowing passively through the Total Cavopulmonary Connection (TCPC) rather than being actively pumped by a subpulmonary ventricle. The TCPC is a direct surgical connection between the superior and inferior vena cava and the left and right pulmonary arteries. We have postulated that a mechanical pump inserted into this circulation providing a 3–5 mmHg pressure augmentation will reestablish bi-ventricular physiology serving as a bridge-to-recovery, bridge-to-transplant or destination therapy as a “biventricular Fontan” circulation. The Viscous Impeller Pump (VIP) has been proposed by our group as such an assist device. It is situated in the center of the 4-way TCPC intersection and spins pulling blood from the vena cavae and pushing it into the pulmonary arteries. We hypothesized that Large Eddy Simulation (LES) using high-order numerical methods are needed to capture unsteady powered and unpowered Fontan hemodynamics. Inclusion of a mechanical pump into the CFD further complicates matters due to the need to account for rotating machinery. In this study, we focus on predictions from an in-house high-order LES code (WenoHemoTM) for unpowered and VIP-powered idealized TCPC hemodynamics with quantitative comparisons to Stereoscopic Particle Imaging Velocimetry (SPIV) measurements. Results are presented for both instantaneous flow structures and statistical data. Simulations show good qualitative and quantitative agreement with measured data.

Studies on Unsteady Flow Characteristics in a High Pressure Turbine Cascade Based on a High-Order Large Eddy Simulation Turbulence Model

In the present paper, unsteady viscous flow analysis around turbine blade cascade using a high-order LES turbulence model is carried out to investigate the basic physical process involved in the pressure loss mechanism. This numerical analysis is assessed to the wind tunnel cascade test. Basically, all the physical phenomena occurring in nature are the effect of some cause, and the effect can somehow be measured. However, to understand the cause, detail information regarding the visualization of the phenomena, which are difficult to measure, are necessary. Therefore, in the present paper, firstly the computed results are compared with the measured data, which are the final outcome of the cause (of the phenomena under investigation), to verify whether our physics-based model could qualitatively predict the measured facts or not. It was found that the present model could well predict measured data. Therefore, the rest of the computed information, which were difficult to measure, were used to visualize the overall flow behavior for acquiring some knowledge of the physical process associated with the pressure loss mechanism. The present study led to an understanding that the interaction of the vortex generated on the suction and pressure surface of the blade and the secondary vortex generated on the end wall, downstream of the trailing edge, resulted in the formation of a large vortex structure in this region. This unsteady three-dimensional flow characteristic is expected to play an important role in the pressure loss mechanism.

Experimental and high-order LES analysis of the flow in near-wall region of a square cylinder

A coupled experimental/numerical analysis of turbulent flow past a square cylinder is performed at the ERCOFTAC Reynolds number Re = U ∞ D/ ν = 21,400, where U ∞ is the inflow velocity and D the cylinder height. Complementary Laser Doppler Velocimetry (LDV) and high-order large-eddy simulations (LES) approaches, based on a spectral vanishing technique (SVV-LES), provide a comprehensive data base including both instantaneous data and post-processed statistics. Beyond these results, an achievement of the paper is to investigate the coherent structures developing on the sides and in the wake of the cylinder with a special focus on the flow features in the near-wall region. The flow is found to separate at the leading edge of the cylinder with the occurence of three-dimensional Kelvin–Helmholtz (KH) pairings localized in the separating shear layer. The interaction between these KH vortical structures and Von Kármán vortex shedding (VK) in the near wake is discussed based on both visualisations and frequency analysis. In particular, signatures of VK and KH vortical structures are found on velocity time samples.

High-order Large Eddy Simulations of Confined Rotor-Stator Flows

In many engineering and industrial applications, the investigation of rotating turbulent flow is of great interest. In rotor-stator cavities, the centrifugal and Coriolis forces have a strong influence on the turbulence by producing a secondary flow in the meridian plane composed of two thin boundary layers along the disks separated by a non-viscous geostrophic core. Most numerical simulations have been performed using RANS and URANS modelling, and very few investigations have been performed using LES. This paper reports on quantitative comparisons of two high-order LES methods to predict a turbulent rotor-stator flow at the rotational Reynolds number Re=400000. The classical dynamic Smagorinsky model for the subgrid-scale stress (Germano et al., Phys Fluids A 3(7):1760-1765, 1991) is compared to a spectral vanishing viscosity technique (S\'everac & Serre, J Comp Phys 226(2):1234-1255, 2007). Numerical results include both instantaneous data and postprocessed statistics. The results show that both LES methods are able to accurately describe the unsteady flow structures and to satisfactorily predict mean velocities as well as Reynolds stress tensor components. A slight advantage is given to the spectral SVV approach in terms of accuracy and CPU cost. The strong improvements obtained in the present results with respect to RANS results confirm that LES is the appropriate level of modelling for flows in which fully turbulent and transition regimes are involved.

High-order LES of the flow over a simplified car model

We study the influence of the Reynolds number on the Ahmed body flow for the subcritical incidence . =25°. Large Eddy Simulations (LES) have been performed at low Reynolds number Re=8322, in agreement with the experiments of Spohn and Gillieron (2002), for the configuration used by Lienhart et al. (2002) at Re=768000. Our simulations, based on a spectral Chebyshev collocation-Galerkin Fourier method, have been carried out with a parallel multi-domain solver. The LES capability is implemented by a Spectral Vanishing Viscosity (SVV) technique. The results globally point out that the topology of the flow is essentially determined by the body geometry.

High-order LES of turbulent heat transfer in a rotor–stator cavity

The present work examines the turbulent flow in an enclosed rotor–stator system subjected to heat transfer effects. Besides their fundamental importance as three-dimensional prototype flows, such flows arise in many industrial applications but also in many geophysical and astrophysical settings. Large eddy simulations (LES) are here performed using a spectral vanishing viscosity technique. The LES results have already been favorably compared to velocity measurements in the isothermal case (Séverac, E., Poncet, S., Serre, E., Chauve, M.P., 2007. Large eddy simulation and measurements of turbulent enclosed rotor–stator flows. Phys. Fluids, 19, 085113) for a large range of Reynolds numbers 10 5 ⩽ Re = Ω b 2 / ν ⩽ 10 6 , in an annular cavity of large aspect ratio G = ( b - a ) / H = 5 and weak curvature parameter R m = ( b - a ) / ( b + a ) = 1.8 ( a , b the inner and outer radii of the rotor and H the interdisk spacing). The purpose of this paper is to extend these previous results in the non-isothermal case using the Boussinesq approximation to take into account the buoyancy effects. Thus, the effects of thermal convection have been examined for a turbulent flow Re = 10 6 of air in the same rotor–stator system for Rayleigh numbers up to Ra = 10 8 . These LES results provide accurate, instantaneous quantities which are of interest in understanding the physics of turbulent flows and heat transfers in an interdisk cavity. Even at high Rayleigh numbers, the structure of the iso-values of the instantaneous normal temperature gradient at the disk surfaces resembles the one of the iso-values of the tangential velocity with large spiral arms along the rotor and more thin structures along the stator. The averaged results show small effects of density variation on the mean and turbulent fields. The turbulent Prandtl number is a decreasing function of the distance to the wall with 1.4 close to the disks and about 0.3 in the outer layers. The local Nusselt number is found to be proportional to the local Reynolds number to the power 0.7. The evolution of the averaged Bolgiano length scale 〈 L B 〉 with the Rayleigh number indicates that temperature fluctuations may have a large influence on the dynamics only at the largest scales of the system for Ra ⩾ 10 7 , since 〈 L B 〉 remains lower than the thermal boundary layer thicknesses.

High-order LES of the flow over a simplified car model

We study the influence of the Reynolds number on the Ahmed body flow for the subcritical incidence α =25°. Large Eddy Simulations (LES) have been performed at low Reynolds number Re=8322, in agreement with the experiments of Spohn and Gillieron (2002), for the configuration used by Lienhart et al. (2002) at Re=768000. Our simulations, based on a spectral Chebyshev collocation-Galerkin Fourier method, have been carried out with a parallel multi-domain solver. The LES capability is implemented by a Spectral Vanishing Viscosity (SVV) technique. The results globally point out that the topology of the flow is essentially determined by the body geometry.