Wall-resolved Large Eddy Simulation Research Articles

Numerical investigation of flows around an axisymmetric body of revolution by using Reynolds-stress model based hybrid Reynolds-averaged Navier–Stokes/large eddy simulation

Flows around an axisymmetric body of revolution at a zero yaw angle were studied using a hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) approach, which employed a full Reynolds stress model (RSM) in the RANS branch with the aim of accounting for the Reynolds stress anisotropy, streamline curvature, and flow separations in the boundary layer. The SUBOFF model without appendages was applied to conduct the simulations, and the Reynolds number based on the free-stream velocity and the length of the body is ReL=1.2×106. The results, including time-averaged Cp, Cf, and velocity statistics, were compared with the experimental data and wall-resolved LES results available in the literature, and the overall agreement of the comparisons was satisfactory. To assess the performance of the RSM-based hybrid RANS/LES approach, we carried out shear-stress transport-based hybrid RANS/LES approach simulations under identical free-stream conditions for comparison. The sensitivity of the hybrid RANS/LES approach to the RANS models was observed for separated flow with surface curvature and adverse pressure gradient-induced separation. The RSM-based hybrid RANS/LES approach was found to provide a better prediction for the unsteady flows near the stern. That is because the effects of the streamline curvature and the strong interactions between individual stresses can be captured by the exact production terms in the RSM-based hybrid RANS/LES approach. These effects are important for predicting the development of turbulent boundary layers along the stern.

Broadband noise prediction for aerofoils with a serrated trailing edge based on Amiet’s theory

Trailing-edge serrations are used as means to reduce aerofoil broadband self-noise in various industrial applications. However, a reliable prediction of the reduced noise level has not been established to date. In this paper, the authors suggest that Amiet’s theory can be used to accurately predict the sound radiated from serrated trailing edges. A plausible explanation as to how the classical theory, originally derived from non-serrated (straight) trailing edges, also works for the serrated trailing edges is provided. Besides, the authors propose a directivity correction to the Amiet’s original model to account for realistic aerofoil geometry. The directivity correction provides a significantly improved prediction of the noise level in the upstream and downstream directions for both serrated and baseline trailing edge cases. This study is based on wall-resolved large-eddy simulations of Joukowski aerofoils with multiple serration geometries and in different flow conditions.

Flow features and thermal stress evaluation in turbulent mixing flows

The present paper depicts a numerical study of flow and thermal features in T-junctions with circular cross-section. The numerical simulation is based on the experimental validated wall-resolved Large-Eddy Simulation method. The Reynolds number in the warm main pipe flow covers a range of Rem=27000 to Rem=134000 and the temperature difference of the mixing fluids is up to ΔT=249 K. The location of maximum fluctuations does not change due to a nearly identical flow pattern type in all simulations. This condition allows the investigation of the flow evolution, mixing process and heat transfer for the same thermal flow pattern type. The numerical simulations are coupled with the heat conduction in the structure due to the thermal fluid-structure-interaction. Further, this study offers a deeper insight into the hidden physics of turbulent mixing flows and the transport as well as damping of thermal fluctuations in the near-wall fluid/solid region. Our study considers also thermal stress analysis by Finite-Element Method by utilizing the wall temperature distribution of previously performed numerical simulations. Therefore, fatigue damage is evaluated based on the stress received from the Finite-Element analysis. The results show that the highest Reynolds number case corresponds to the highest flux intensity and turbulence. This affects the flow field and the thermal features in the core flow and the transport of fluctuations close to the wall. The locations with peak fluctuation intensity in the fluid generates also high fluctuating temperature areas in the structure. The finite element analysis demonstrates that the highest temperature difference case leads to the highest alternating stress intensity which has potential for thermal fatigue. The generated maximal alternating stress is coupled with the flow conditions of the mixing fluids. With an increasing Reynolds number of the main and branch pipe flow as well as temperature difference raises also the resulting maximum alternating stress intensity.

Effect of the liquid–gas interface curvature for a superhydrophobic surface with longitudinal grooves in turbulent flows

A superhydrophobic (SH) surface has shown great potential in reducing flow resistance and saving energy in hydrodynamic applications. In this paper, we have tried to investigate the effects of liquid–gas interface curvature of a SH wall in turbulent flows with the wall-resolved large eddy simulation (LES). The LES is first validated against direct numerical simulation results before the curvature shape is parameterized and examined at various Reynolds numbers (Reτ = 180, 395, and 590). The parametric study shows that a positive curvature angle leads to a higher flow rate, while the effect of a negative curvature angle on the flow resistance is minimal. In addition, the effect of the interface curvature on the flow rate is weakly dependent on the Reynolds number. Analysis shows that larger flow rate can be obtained by reducing the spanwise momentum exchange. A positively curved interface bows into the liquid and shifts the transverse flow circulation (in the cross-sectional plane) away from the solid wall, which helps to reduce spanwise momentum exchange and thus the flow resistance significantly. In contrast, a negatively curved interface does not change the location of the transverse circulation but deforms its shape, which hardly affects the spanwise momentum exchange or the flow rate. The near-wall streak patterns above the SH wall distribute with roughly the same spacing of the surface texture. In addition, the absolute distance plays a more important role than the viscous distance in the variation of the streaks with the distance from the SH wall.

Influence of free-stream turbulence on the aerodynamic performance of a three-dimensional airfoil

Influence of free-stream turbulence on flows past a static three-dimensional SD7003 airfoil is numerically investigated via wall-resolved Large-Eddy Simulation (wall-resolved LES). The Reynolds number and Mach number are 60 000 and 0.2, respectively, with the angle of attack (AoA) varying from 4° to 14°. The purpose of this investigation is to clarify the effects of intensities and integral length scales of turbulence in the approaching flow on the aerodynamic performance of the airfoil. To this end, a synthetic digital filtering technique and a specific source term concept are employed to generate turbulent fluctuations within the incoming flow. It turns out that turbulence with higher intensity in the approaching flow may cause the variations in effective AoAs, which not only postpone flow development but also promote the diversity of flow classifications from an instantaneous point of view. Furthermore, it is shown that the integral length scale has significant and systematic impacts on turbulent fluctuations in the boundary layer. Larger integral length scales tend to result in higher levels of turbulence intensities, which thus induce larger separations at all AoAs.

Application of an Immersed Boundary Method on an Instrumented Turbine Blade With Large Eddy Simulation

Abstract Leading edge instrumentation used in compressor and turbine blades for jet-engine test rigs can cause significant obstruction and lead to a marked increase in downstream pressure loss. Typical instrumentation used in such a scenario could be a Kiel-shrouded probe with either a thermocouple or pitot-static tube for temperature/pressure measurement. High fidelity analysis of a coupled blade and probe requires the generation of a high-quality mesh which can take a significant amount of an engineers time. The application of an immersed boundary method (IBM) and large eddy simulation (LES) is shown in this paper to enable the use of an extremely simple mesh to observe the primary flow features generated due to the blade and probe interaction effects, as well as quantify downstream pressure loss to within a high level of accuracy. IBM is utilized to approximately model the probe, while fully resolving the blade itself through a series of LES simulations. This method has shown to be able to capture downstream loss profiles as well as integral quantities compared to both experiment and fully wall-resolved LES without the need to spend a significant amount of time generating the ideal mesh. Additionally, it is also able to capture the turbulence anisotropy surrounding the probe and blade regions.

Predictions of Conjugate Heat Transfer in Turbulent Channel Flow Using Advanced Wall-Modeled Large Eddy Simulation Techniques.

In this paper, advanced wall-modeled large eddy simulation (LES) techniques are used to predict conjugate heat transfer processes in turbulent channel flow. Thereby, the thermal energy transfer process involves an interaction of conduction within a solid body and convection from the solid surface by fluid motion. The approaches comprise a two-layer RANS–LES approach (zonal LES), a hybrid RANS–LES representative, the so-called improved delayed detached eddy simulation method (IDDES) and a non-equilibrium wall function model (WFLES), respectively. The results obtained are evaluated in comparison with direct numerical simulation (DNS) data and wall-resolved LES including thermal cases of large Reynolds numbers where DNS data are not available in the literature. It turns out that zonal LES, IDDES and WFLES are able to predict heat and fluid flow statistics along with wall shear stresses and Nusselt numbers accurately and that are physically consistent. Furthermore, it is found that IDDES, WFLES and zonal LES exhibit significantly lower computational costs than wall-resolved LES. Since IDDES and especially zonal LES require considerable extra work to generate numerical grids, this study indicates in particular that WFLES offers a promising near-wall modeling strategy for LES of conjugated heat transfer problems. Finally, an entropy generation analysis using the various models showed that the viscous entropy production is zero inside the solid region, peaks at the solid–fluid interface and decreases rapidly with increasing wall distance within the fluid region. Except inside the solid region, where steep temperature gradients lead to high (thermal) entropy generation rates, a similar behavior is monitored for the entropy generation by heat transfer process.

Evaluation and application of advanced CFD models for rotating disc flows

This paper addresses limitations of widely used Reynolds-averaged turbulence models (RANS) for prediction of gas turbine internal air systems. Results from direct numerical simulation (DNS), wall-resolved large-eddy simulation (LES), wall-modelled large-eddy simulation (WMLES), and RANS for benchmark test cases are compared. For rotor-stator disc cavity flows results for mean velocities, velocity fluctuations, rotor torque and laminar-turbulent transition are considered and compared with published data. For cavities between co-rotating discs attention is focused on buoyancy-driven convection in the centrifugal force field. It is concluded that WMLES is suitable for application in engine conditions, offering better accuracy than RANS in some critical applications. This confirms recently published results for turbine rim sealing and is further illustrated by application to convection in a sealed cavity at higher Rayleigh number than is practical with DNS or wall-resolved LES. The results show that the approximate near-wall treatment gives reasonable results for complex flows and extend previous studies to higher speed rig conditions where Eckert number effects become significant.

Wall model based on neural networks for LES of turbulent flows over periodic hills

A data-driven wall model for turbulent flows over periodic hills is developed using a feedforward neural network and wall-resolved large-eddy simulation data. The developed wall model employs wall-normal distance, near-wall velocities, and pressure gradients as input features, and the wall shear stresses as output labels, respectively. In the $a$ $p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i$ test, the accuracy of the trained wall model is examined using periodic hill cases at different Reynolds numbers and with different hill geometries. In the $a$ $p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i$ test, the trained wall model is applied to the flow over periodic hills and turbulent channel flows.

Study on the role of soot and heat fluxes in upward flame spread using a wall-resolved large eddy simulation approach

The present study aims to obtain further understandings of vertical flame spreading phenomena by analysing the influences of soot and individual heat flux components on PMMA walls using large eddy simulation. Total heat flux consists of convective and radiative components, but it is not clear which one has a significant role in fire spread. The computational code used is an in-house version of FireFOAM 2.2.x, which has recently undergone specific development and validation for flame spread studies by the authors. The present study has conducted numerical simulations for flame spread and full wall fire configurations. By scale-up of the PMMA size from 0.4 to 1.0 m, the convective heat flux decreased by 41.4% at the location of the pyrolysis front, radiative heat flux increased by 86.9%, and radiative heat flux due to soot grew by 215.2%. As the pyrolysis height increases from 0.3 to 1.0 m, the convective heat flux decreased by 26.8% at the location of the pyrolysis front. The radiative heat flux increased by 96.8%, and its components of combustion of the gaseous fuel and soot grew by 55.9% and 233.3%, respectively. Moreover, the ratio of radiative heat flux to total heat flux increased by 66.5%, and that of soot to radiative heat flux grew by 73.9%. The contribution of soot to radiative heat flux almost linearly increased against the pyrolysis height and that was higher at a higher pyrolysis height.

Wall-Resolved Large Eddy Simulations of the Transient Turbulent Fluid Mixing in a Closed System Replicating a Pressurized Thermal Shock

Wall-Resolved Large Eddy Simulations of the Transient Turbulent Fluid Mixing in a Closed System Replicating a Pressurized Thermal Shock

On the use of a two-layer model for large-eddy simulations of supersonic boundary layers with separation

The two-layer modeling approach has become one of the most promising and successful methodology for simulating turbulent boundary layers in the past ten years. In the present study, a mixed wall model for large-eddy simulations (LES) of high-speed flows is proposed which combine two approaches; the thin-Boundary Layer Equations (TBLE) model of Kawai and Larsson (1994) and the analytical wall-layer model of Duprat et al. (2011) for streamwise pressure gradients. The new hybrid model has been efficiently implemented into a three-dimensional compressible LES solver and validated against DNS of a spatially-evolving supersonic boundary layer (BL) under moderate and strong pressure gradients, before being employed for the prediction of nozzle flow separations at different flow conditions, ranging from weakly to highly over-expanded regimes. A good agreement is obtained in terms of mean and fluctuating quantities compared to the DNS results. Particularly, the current wall-modeled LES results are found to perfectly match the DNS data of supersonic BL with/out pressure gradient. It is also shown that the model can account for the effect of the large-scale turbulent motions of the outer layer, indicating a good interaction between the inner and the outer part of the wall layer. In terms of simulations costs and improvements of computing power, the obtained results highlight the capability of the current wall-modeling LES strategy in saving a considerable amount of computational time compared to the wall-resolved LES counterpart, allowing to push further the simulations limits. Furthermore, the application of these computationally low-costly LES simulations to nozzle flow separation allows to clearly identify the origin of the shock unsteadiness, and the existence of broadband and energetically-significant low-frequency oscillations (LFO) in the vicinity of the separation region.

Reynolds stress tensor measurements using magnetic resonance velocimetry: expansion of the dynamic measurement range and analysis of systematic measurement errors

This study presents magnetic resonance velocimetry (MRV) Reynolds Stress measurements in a periodic hill channel with a hill Reynolds number of Re = 29,500. The velocity encoding scheme is based on the ICOSA6 method with six icosahedral encoding directions and multiple encoding values are measured to increase the dynamic range. The full Reynolds stress tensor is obtained from a voxel-wise three-dimensional Gaussian fit using the magnitude data of all acquisitions. The MRV results are compared to a wall-resolved large eddy simulation and laser Doppler velocimetry measurements conducted in the same channel. It is shown that the MRV Reynolds stress data have excellent precision and agree qualitatively with the reference data. However, there are apparent systematic deviations. One of the most prominent error contributions is the signal attenuation caused by higher orders of motion, which leads to an overestimation of the turbulence level. Another fundamental error is identified in the assumption that the turbulence is Gaussian distributed. With the presented reconstruction technique, the MRV data are fitted to a statistical model, and depending on the examined flow setup, the Gaussian model can lead to considerable errors. Possible ways of how to reduce all identified errors are presented. In summary, this technique enables Reynolds stress tensor measurements in complex internal flows with high dynamic range and excellent precision. However, several issues need to be resolved to make the turbulence quantification more accurate.Graphic abstract

Investigation of turbulent flow in square duct with heated foil thermometry

Fully developed turbulent flow of water in a square duct has been studied with thermography of the 25 μm heated steel foil that covers part of the duct wall. Thermal fluctuations observed in the specific case fall in-between the ideal isothermal boundary condition, which is achieved with a thick heater cooled with water, and ideal isoflux boundary condition, which could be theoretically achieved with an infinitely thin heated foil. Results in the present paper are obtained at fixed Reynolds number 10000 and heat flux at the thin foil around 6 kW/m2, which was considered to be the upper limit, where the water can be treated as a passive scalar at Prandtl number 5.4 with temperature increments around 5 °C above the inlet value. Main experimental results are 2D temperature fields on the outer side of the foil, which were averaged over time to predict the mean foil temperature, temperature fluctuations and the corresponding spectra. These parameters provide accurate description of the turbulence near the heated foil. Simple geometry of the experiment and fully developed velocity field are convenient for accurate numerical simulation, which was performed with wall-resolved Large Eddy Simulation approach. Average magnitude of the measured and computed temperature fluctuations was roughly 0.5 °C and 0.6 °C, respectively, which corresponds to about 10% of the temperature increment. Measured thermal streak spacing was around 60 wall units, and was together with the power spectra of the temperature fluctuations in close agreement with the LES numerical predictions that included the foil with conjugate heat transfer. Despite the square cross-section of the duct, which contains around dozen thermal streaks on each side of the duct, the spectra and the streak spacing remain similar as in the channel flow between the parallel planes.

On the Flow and Passive Noise Control of an Open Cavity at Re = 5000

The interaction of turbulent flows with the external structure of ground vehicles generates uncomfortable noise and a lot of attention is devoted to find new mechanisms for its suppression. The present work is concerned with open cavity flows, very often found in the automotive industry. A three-dimensional rectangular very wide open cavity with aspect ratio \(L/D=4\) at Reynolds number \(Re_D=5000\) and Mach number \(M=0.1\) is considered. The passive control technique is based on eight different geometrical modifications: the length of the cavity, the radius of the trailing, leading and bottom edges and the difference in heights between the left and right wall of the cavity. Wall-resolved Large Eddy Simulations (LES) are used to obtain the flow fields and a post-process based on Curle’s analogy is applied to evaluate the acoustic radiation and the effectiveness of the control mechanisms. The results show that the modifications on the trailing edge are the most effective to control the flow. They allow to reduce the pressure fluctuations produced by the recirculation confined inside the cavity and the abrupt ejection of the flow at the trailing edge. As a consequence, the overall sound pressure level can be decreased up to 9 dB.

A wall-resolved large-eddy simulation of deep cavity flow in acoustic resonance

Abstract

Wall-Modeled Large Eddy Simulations of Axial Turbine Rim Sealing

Abstract This paper presents wall-modeled large-eddy simulations (WMLES) of a chute-type turbine rim seal. Configurations with an axisymmetric annulus flow and with nozzle guide vanes fitted (but without rotor blades) are considered. The passive scalar concentration solution and WMLES are validated against available data in the literature for uniform convection and a rotor–stator cavity flow. The WMLES approach is shown to be effective, giving significant improvements over an eddy viscosity turbulence model, in prediction of rim seal effectiveness compared to research rig measurements. WMLES requires considerably less computational time than wall-resolved LES, and has the potential for extension to engine conditions. All WMLES solutions show rotating inertial waves in the chute seal. Good agreement between WMLES and measurements for sealing effectiveness in the configuration without vanes is found. For cases with vanes fitted, the WMLES simulation shows less ingestion than the measurements, and possible reasons are discussed.

Unified wall-resolved and wall-modeled method for large-eddy simulations of compressible wall-bounded flows

Wall-resolved large eddy simulations and wall-modeled large eddy simulations are still separate strategies in the field of wall-turbulence applications, and no significant attempts have been documented to blend them in order to exploit the full potential of the two methods. This work enables a smooth transition between these two techniques, designing a robust algorithm that dynamically adapts the wall treatment. In particular, we propose a unified method that employs augmented turbulent viscosity and diffusivity at the wall location and allows for preservation of both the no-slip and isothermal/adiabatic conditions.

Effect of the Reynolds and Richardson numbers on thermal mixing characteristics

This paper presents a detailed numerical study regarding the influence of Reynolds and Richardson numbers in T-junctions with circular cross-section. The numerical investigation is based on the wall-resolved Large-Eddy Simulation (LES), which has been experimentally validated. The Reynolds number in the main pipe covers a range from Re=78400 to Re=235200, which is high enough to approach the engineering applications. This also belongs to the primary contribution of the current study. Furthermore, the temperature difference between the main pipe and branch pipe (Tm−Tb) is 180 K and the mass flow rate ratio (m˙m/m˙b) is 3. The primary focus is on thermal mixing characteristics in the downstream as well as on the evolution of the flow by consideration of gravity and buoyancy effects. Additionally, our study considers the thermal field and its corresponding fluctuations in the near-wall region. In addition, this paper also includes simulations with different T-junction configurations therefore we can examine the influence of the entrance orientation of the heavier cold branch pipe fluid on the mixing process and heat transfer. The analysis of our results indicates that gravity effects can be neglected in cases with increased Reynolds number and simultaneously decreased Richardson number. The area with high fluctuation intensities becomes larger by the higher Reynolds number and lower Richardson number of the mixing flow therefore they shows a higher potential for High Cycle Thermal Fatigue (HCTF). Furthermore, the highest near-wall thermal fluctuations are 40% of the temperature difference between the mixing flows.

Grid-point and time-step requirements for direct numerical simulation and large-eddy simulation

We revisit the grid-point requirement estimates in Choi and Moin [“Grid-point requirements for large eddy simulation: Chapman’s estimates revisited,” Phys. Fluids 24, 011702 (2012)] and establish more general grid-point requirements for direct numerical simulations (DNS) and large-eddy simulations (LES) of a spatially developing turbulent boundary layer. We show that by allowing the local grid spacing to scale with the local Kolmogorov length scale, the grid-point requirement for DNS of a spatially developing turbulent boundary layer is N∼ReLx2.05 rather than N∼ReLx2.64, as suggested by Choi and Moin, where N is the number of grid points and Lx is the length of the plate. In addition to the grid-point requirement, we estimate the time-step requirement for DNS and LES. We show that for a code that treats the convective term explicitly, the time steps required to get converged statistics are Nt∼ReLx/Rex06/7 for wall-modeled LES and Nt∼ReLx/Rex01/7 for wall-resolved LES and DNS (with different prefactors), where Rex0 is the inlet Reynolds number. The grid-point and time-step requirement estimates allow us to estimate the overall cost of DNS and LES. According to the present estimates, the costs of DNS, wall-resolved LES, and wall-modeled LES scale as ReLx2.91, ReLx2.72, and ReLx1.14, respectively.