Large Eddy Simulation Solution Research Articles

Jet Installation Noise Modelling for Round and Chevron Jets

AbstractWall-Modelled Large Eddy Simulations (LES) are conducted using a high-resolution CABARET method, accelerated on Graphics Processing Units (GPUs), for a canonical configuration that includes a flat plate within the linear hydrodynamic region of a single-stream jet. This configuration was previously investigated through experiments at the University of Bristol. The simulations investigate jets at acoustic Mach numbers of 0.5 and 0.9, focusing on two types of nozzle geometries: round and chevron nozzles. These nozzles are scaled-down versions (3:1 scale) of NASA’s SMC000 and SMC006 nozzles. The parameters from the LES, including flow and noise solutions, are validated by comparison with experimental data. Notably, the mean flow velocity and turbulence distribution are compared with NASA’s PIV measurements. Additionally, the near-field and far-field pressure spectra are evaluated in comparison with data from the Bristol experiments. For far-field noise predictions, a range of techniques are employed, ranging from the Ffowcs Williams–Hawkings (FW–H) method in both permeable and impermeable control surface formulations, to the trailing edge scattering model by Lyu and Dowling, which is based on the Amiet trailing edge noise theory. The permeable control surface FW–H solution, incorporating all jet mixing and installation noise sources, is within 2 dB of the experimental data across most frequencies and observer angles for all considered jet cases. Moreover, the impermeable control surface FW–H solution, accounting for some quadrupole noise contributions, proves adequate for accurate noise spectra predictions across all frequencies at larger observer angles. The implemented edge-scattering model successfully captures the mechanism of low-frequency sound amplification, dominant at low frequencies and high observer angles. Furthermore, this mechanism is shown to be effectively consistent for both $$M=0.5$$ M = 0.5 and $$M=0.9$$ M = 0.9 , and for jets from both round and chevron nozzles.

Large Eddy Simulation of Separated Flows on Unconventionally Coarse Grids

Abstract We examine and benchmark the emerging idea of applying the large eddy simulation (LES) formalism to unconventionally coarse grids where Reynolds-averaged Navier–Stokes (RANS) would be considered more appropriate at first glance. We distinguish this idea from very large eddy simulation and detached eddy simulation, which require switching between RANS and LES formalism. LES on RANS grid is appealing because first, it requires minimal changes to a production code; second, it is more cost-effective than LES; third, it converges to LES; and most importantly, it accurately predicts flows with separation. This work quantifies the benefit of LES on RANS-like grids as compared to RANS on the same grids. Three canonical cases are considered: periodic hill, backward-facing step, and jet in cross flow. We conduct direct numerical simulation (DNS), proper LES on LES grids, LES on RANS-quality grids, and RANS. We show that while the LES solutions on the RANS-quality grids are not grid converged, they are twice as accurate as the RANS on the same grids.

On the propeller wake evolution using large eddy simulations and physics-informed space-time decomposition

A novel modal analysis methodology, denoted as the physics informed sparsity-promoting dynamic mode decomposition (pi-SPDMD) model, was introduced for the reduction and reconstruction analysis of intricate propeller wake flows, aiming to provide insight into the inherent flow structures spanning diverse temporal and spatial scales. Large-Eddy Simulation (LES) was employed to numerically model the wake dynamics of a four-bladed propeller, providing a comprehensive resolution from the proximate to the distant wake regions. The findings indicate that the pi-SPDMD model enhances the efficiency of the sparse-promoting algorithm, producing modes that gravitate towards stability, and the resulting decomposition maintains commendable physical fidelity. Integrating the results from the LES solution and the modal decomposition of pi-SPDMD, the tip vortex exhibits a uniform topological configuration with notable coherence in the proximate domain. In this region, the large-scale vortex is the dominant feature of the propeller wake, and there is a marked intermittency in the turbulence. In the mid-field, the tip vortex system transitions into fine-scale vortices, rapidly diminishing in coherence due to the onset of elliptic instability and subsequent secondary vortex generation. As the tip vortex structures related to physical quantities become fully discretized, the small-scale turbulent patterns quickly intermingle, leading to a more homogeneous distribution in the distant wake.

Wall-modeled large-eddy simulation of turbulent non-Newtonian power-law fluid flows

For high-fidelity predictions of turbulent flows in complex practical engineering problems, the Wall-Modeled (WM) Large-Eddy Simulation (LES) has aroused great interest. In the present study, we prove that the conventional Wall-Stress Models (WSMs) developed for WMLES of Newtonian fluids fail to predict the shear-thinning-induced drag reduction in power-law fluids. Therefore, we propose novel algebraic, integrated, and Ordinary-Differential-Equation (ODE) WSMs, for the first time, for WMLES of power-law Non-Newtonian (NN) fluids and assess their performance against reference Wall-Resolved (WR) LES solutions. In addition, the effects of the key model parameters, including the WSM type, sampling height, sampling cell, and axial grid resolution are explored, and it is revealed that turbulent NN flow predictions have a much higher sensitivity to the choice of WSM, compared to their Newtonian counterparts. It is manifested that, in contrast to WRLES, accurate modeling of the mean apparent and subgrid-scale NN viscosities in the Non-Newtonian ODE (NNODE) model can improve the predictions considerably. Therefore, closures with lower uncertainties on coarse WMLES grids are sought for these terms. Finally, the best performance for the present test cases is obtained via the integrated NN WSM, sampling at the lower edge of the log layer within the third off-the-wall grid cell. Nevertheless, the new NNODE WSM can have an advantage in the presence of non-equilibrium effects in more complex problems.

Wind Tunnel and Grid Resolution Effects in Large-Eddy Simulations of the High-Lift Common Research Model

This paper describes the application of a compressible large-eddy simulation (LES) solver to the flow over the High-Lift Common Research Model (CRM-HL) in landing configuration, a complex external aerodynamic flow configuration with deployed slats, flaps, a flow-through nacelle, and the associated brackets/fairings on the high-lift devices. The bulk Mach number is (0.2) and the mean-aerodynamic-chord-based Reynolds number is typical of a wind tunnel experiment (5.49E6). A key development in these simulations relative to previous work is that this work serves to establish robustness of LES methods to Reynolds number and aircraft configuration. Previous studies focused on the Japan Aerospace Exploration Agency Standard Model, which featured a nearly 3× lower Reynolds number than the present CRM-HL investigation and less aggressive wing/slat leading-edge curvature than the CRM-HL geometry, which led to lower leading-edge pressure suction peak magnitudes, both of which led to less stringent grid resolution requirements. In these LES simulations, an algebraic equilibrium wall modeling approach is employed along with a dynamic implementation of the Smagorinsky subgrid-scale model. The calculations are carried out in both a free air setting and one that includes the wind tunnel facility at seven angles of attack at five grid resolution levels, ranging from ≈10 to 1500 million control volumes. In free air, the solutions show decreasing sensitivity to the grid with each successive refinement level and systematically approach the experimental lift coefficient data as the grid is refined, with the 1.5 billion control volume case showing excellent agreement with the corrected experimental data. The simulations in both free air and in the wind tunnel predict a stall mechanism featuring a large inboard juncture stall and a nose-down break in the pitching moment curve, both of which agree with the experimental observations. The accuracy of the simulations is assessed via comparisons of integrated forces/moments, surface pressures, and surface skin friction visualizations. Graphics processing unit (GPU) acceleration of the charLES solver results in tractable turnaround times that make LES a useful tool in the aerospace industry design cycle. Recent GPU acceleration of the flow solver has made LES solutions that are highly accurate in lift/drag/moment for relevant high-lift aircraft flows achievable within about 5 h of wall time on 600 GPU cores.

Stable Boundary Layers and Subfilter-Scale Motions

Recent high-resolution large-eddy simulations (LES) of a stable atmospheric boundary layer (SBL) with mesh sizes N=(5123,10243,20483) or mesh spacings ▵=(0.78,0.39,0.2) m are analyzed. The LES solutions are judged to be converged based on the good collapse of vertical profiles of mean winds, temperature, and low-order turbulence moments, i.e., fluxes and variances, with increasing N. The largest discrepancy is in the stably stratified region above the low-level jet. Subfilter-scale (SFS) motions are extracted from the LES with N=20483 and are compared to sonic anemometer fields from the horizontal array turbulence study (HATS) and its sequel over the ocean (OHATS). The results from the simulation and observations are compared using the dimensionless resolution ratio Λw/▵f where ▵f is the filter width and Λw is a characteristic scale of the energy-containing eddies in vertical velocity. The SFS motions from the observations and LES span the ranges 0.1<Λw/▵f<20 and are in good agreement. The small, medium, and large range of Λw/▵f correspond to Reynolds-averaged Navier–Stokes (RANS), the gray zone (a.k.a. “Terra Incognita”), and fine-resolution LES. The gray zone cuts across the peak in the energy spectrum and then flux parameterizations need to be adaptive and account for partially resolved flux but also “stochastic” flux fluctuations that represent the turbulent correlation between the fluctuating rate of strain and SFS flux tensors. LES data with mesh 20483 will be made available to the research community through the web and tools provided by the Johns Hopkins University Turbulence Database.

Multifidelity Approach to Sensitivity Estimation in Large-Eddy Simulation

A novel approach to compute affordable approximate sensitivities in a large-eddy simulation (LES) is proposed and assessed. The approach is based on solving a Reynolds-averaged Navier–Stokes (RANS) problem that has been linearized around the mean LES solution, with closure modeling required for the linearized changes in turbulent Reynolds stresses. In the present study, the closure modeling is based on a linearized algebraic turbulence model. The method is assessed for the flow over a NACA0012 airfoil at a fixed angle of attack, with the Reynolds number as the variable parameter. The results show that, provided an accurate linearized closure model, the method predicts the correct sensitivity of the skin friction coefficient and the mean velocity field at the cost of a linearized RANS, which provides an important proof-of-concept for this approach. The linearized algebraic turbulence model with standard model coefficients produces a reasonably accurate sensitivity, but the results also suggest that accuracy could be gained from recalibrating the model coefficients for this new use of the model.

Impact of Wall Temperature on Aerothermal Characteristics of an Array of Surface Microstructures

Abstract The aero-thermal behavior of surface microstructures is of wide relevance, especially given the development of additive manufacturing (AM). Of particular interest is the interaction between fluid flow and heat transfer. In this work, two contrasting configurations, a flat plate boundary layer and an array of hemispheric microstructures are examined at three wall-inflow temperature ratios (TR): cooled (TR = 0.5), adiabatic (TR = 1) and heated wall (TR = 1.5). Due to compensation between fluid viscosity and velocity gradient in the boundary layer, the heat transfer effects may appear deceptively small if judged using the common aerothermal parameters (Cf, Nu). The authors find instead the local Reynolds number to be more usefully indicative of such aerothermal interaction. The scale-resolving large eddy simulations (LES) simulations at a range of Reynolds numbers show that the cooled wall case is characterized by a markedly earlier transition which takes place at a much lower (by 50%) bulk flow Reynolds number compared to a near-adiabatic case. Furthermore, it is shown that the incompressible flow LES solutions fail to capture the early transition under the same cooling condition. Finally, a regrouping of the nondimensional parameters (CD, Nu) with TR is proposed leading to a more unified characterization for easier scaling of wall heat transfer effects in practical applications.

A grid-adaptive simulation model for turbulent flow predictions

Hybrid Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) methods, abbreviated as hybrid RANS-LES, have been rapidly developed and increasingly used for predicting complex turbulent flows. In this study, a new high-fidelity hybrid RANS-LES strategy that modifies the turbulent viscosity equation using the ratio of grid length scale to turbulent integral length scale based on the Kolmogorov energy spectrum, termed the grid-adaptive simulation (GAS) model, is proposed to achieve high accuracy for turbulent flows using different grid resolutions. Using the shear-stress transport (SST) k–ω model as the baseline turbulence model, the GAS-SST model is validated by predicting three typical turbulent flows with coarse and fine meshes, including periodic hill flow, circular cylinder flow, and simplified tip leakage flow. As a reference, the scale-adaptive simulation (SAS) and delayed detached-eddy simulation (DDES) models are also employed to predict the above three turbulent flows. Solutions of GAS-SST, SAS-SST, and DDES-SST are compared against the high-fidelity data from the experiments or LES solutions. Detailed comparisons show that the GAS-SST model could achieve high accuracy with different grid resolutions for all three validation cases, which means that the GAS model has strong grid-adaptive ability. The results predicted by the GAS-SST model using coarse meshes are usually much more in agreement with the high-fidelity data than those predicted by SAS-SST and DDES-SST models. The GAS model demonstrates the potential to address the accuracy and computational efficiency requirements for predicting turbulent flows.

Hybrid large‐eddy simulation with adaptive downstream anisotropic eddy viscosity model

AbstractTurbulence pervades most flows of engineering interest, and its prediction remains a challenge on both accuracy and cost. One promising predictive approach that reduces cost combines large‐eddy simulation (LES) with simulation based on Reynolds averaged Navier–Stokes equations (RANS). This study presents a method to overcome stability and accuracy issues associated with these hybrid LES‐RANS methods. The method developed involves extracting the Reynolds stresses from the upstream LES solution and then using that information to convert the downstream RANS model from a scalar eddy‐viscosity model to an anisotropic nonlinear eddy‐viscosity model. The method developed differs from the downstream tensorial eddy‐viscosity model by being independent of the coordinate system. The method developed was evaluated by computing film cooling of a flat plate with the coolant injected through one row of circular holes. Results obtained show instabilities at the LES‐to‐RANS interface to be eliminated. Also, the method developed yielded solutions that compare reasonably well with those from LES, even though a significant portion of the flow is computed by the adapted anisotropic RANS model instead of LES, which significantly reduced the number of grid points and computational time needed. Since modification of the downstream RANS model is based on information extracted from the upstream LES solution, the method developed is adaptive to the problem being studied.

Linear and nonlinear modelling of far-field propagation of broadband shock-associated noise

A triple-scale computational model is implemented to simulate noise generated by a supersonic under-expanded screeching jet corresponding to the LTRAC (Laboratory for Turbulence Research in Aerospace and Combustion) experiment at different distances from the source. The investigation is focused on the broadband-associated noise, which is a prominent feature of the acoustic field of the LTRAC jet. In the jet near-field, the compressible Navier–Stokes equations are solved using the high-resolution CABARET Large Eddy Simulation (LES) method accelerated on Graphics Processing Units. The LES solution is substituted in the Ffowcs Williams–Hawkings (FW–H) model to obtain the noise solution in the acoustic mid field at 20 initial jet diameters from the jet nozzle exit. The mid-field acoustic solution is used as the input for the spherical generalised Burgers’ equation. The general form of Burgers’ equation is solved numerically in the frequency domain for a wide range of observer distances up to 18 million initial jet diameters, where viscous dissipation fully dominates for most frequencies. To answer the question if the nonlinear acoustic wave propagation effects for the LTRAC jet are important, the nonlinear and linear solutions of Burgers’ equation are compared.

Wall-model integrated computational framework for large-eddy simulations of wall-bounded flows

The wall-modeled large-eddy simulation (WMLES) computational framework generally includes a wall-model solver outside the large-eddy simulation (LES) infrastructure, with the two solvers communicating only at the matching location and the wall. Having a wall-model solver outside the LES jeopardizes the performance of WMLES: first, the wall-model solver adds significant computational overhead; second, the LES solution in the wall-adjacent cell is ambiguous; and third, it is very difficult to utilize the emerging high-order numerical schemes. This paper addresses the above issues by abandoning wall-model solvers altogether and integrating wall models into LES solvers. We will employ a set of physics-inspired bases for LES solution reconstruction in the wall-adjacent cell. The methodology gives rise to a computational framework that effortlessly accounts for non-equilibrium effects in a high-order code without a stand-alone wall-model solver. We consider channel flow for a proof of concept and periodic hill for validation.

Broadband shock-associated noise modelling for high-area-ratio under-expanded jets.

Broadband shock-associated noise (BBSAN) is an important component of supersonic jet noise for jets at off-design conditions when the pressure at the nozzle exit is different from the ambient. Two high-area-ratio under-expanded supersonic jets at nozzle pressure ratios (NPRs) 3.4 and 4.2 are considered. The jets correspond to conditions of the experiment in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) in the Supersonic Jet Facility of Monash University. Flow solutions are obtained by the large eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) methods. The solutions are validated against the particle image velocimetry (PIV) data. For noise spectra predictions, the LES solution is combined with the time-domain Ffowcs Williams-Hawkings method. To probe the accuracy of the reduced-order method based on acoustic analogy, the RANS solutions are substituted in the Morris and Miller BBSAN method, where different options for modelling of the acoustic correlation scales are investigated. The noise spectra predictions are compared with the experimental data from the non-anechoic LTRAC facility and the NASA empirical sJet model. Apart from the low frequencies influenced by the jet mixing noise, the RANS-based acoustic predictions align with those from LES for most frequencies in the range of Strouhal numbers (St) 0.4 < St < 2 within 1-2 dB.

Analysis and Interpretation of Data-Driven Closure Models for Large Eddy Simulation of Internal Combustion Engine

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;We present an automatic data-driven machine learning (ML) approach for the development, evaluation and interpretation of deep neural networks (DNNs) for turbulence closures and demonstrate their usage in the context of cold-flow large-eddy simulation (LES) of the four-stroke Darmstadt engine using an open-source compressible multi-dimensional CFD solver OFICE, in a hybrid PDE-ML framework. Rather than explicitly using canonical formulations of closure terms, these DNNs robustly discover the functional relationships between the large-scale features of the resolved flow (cell Re, strain and rotation rate tensors etc.) obtained by solving the Navier Stokes to the small-scale unresolved terms. Experimentally validated high-fidelity LES solutions of the engine at different crank angles are utilized as the ground truth to train the DNN based closure models. Since optimizing these DNNs can be a laborious process for scientific datasets, and often require specialized expertise, we propose a Bayesian optimization framework that automatically determines the best set of network parameters, including the architecture and training hyperparameters - batch size, regularization etc. for optimum performance. We compare and contrast various networks for their effectiveness in an &lt;i&gt;a-priori&lt;/i&gt; testing setting. Finally, the best ‘learnt’ network is integrated with the open-source CFD solver (OFICE), and solutions are obtained over several injection cycles. These experiments reveal that the DNN models temporally track resolved scalar variance with a good accuracy. Additionally, we interpret the artificial neural networks with sensitivity analysis to determine the relevant large-scale features for the learning process.&lt;/div&gt;&lt;/div&gt;

Assessing the Applicability of the Structure-Based Turbulence Resolution Approach to Nuclear Safety-Related Issues

The accuracy of computational fluid dynamics (CFD) predictions plays a fundamental role in supporting the operation of the current nuclear reactor fleet, and even more importantly the licensing of advanced high-efficiency reactor concepts, where local temperature oscillations driven by thermal striping, cycling and stratification can limit the structural performance of vessels and components. The complexity of the geometrical configurations, coupled to the long operational transients, inhibits the adoption of large eddy simulation (LES) methods, mandating the acceptance of the more efficient Reynolds-averaged Navier-Stokes (RANS)-based models, even though they are unable to provide a complete physical description of the flow in regions dominated by complex unsteady coherent structures. A new strategy has been proposed and demonstrated at Massachusetts Institute of Technology (MIT) toward the enhancement of unsteady Reynolds-averaged Navier-Stokes (URANS) predictions, using local resolution of coherent turbulence, to provide higher fidelity modeling in support of safety-related issues. In this paper, a comprehensive assessment of the recently proposed Structure-based (STRUCT-ε) turbulence model is presented, starting from fundamental validation of the model capabilities and later focusing on a representative safety-relevant application, i.e., thermal mixing in a T-junction. Solutions of STRUCT-ε, the widely used Realizable k−ε model (RKE) and Large Eddy Simulation with Wall-Adapting Local Eddy-viscosity subgrid scale closure (LES-WALE) are compared against the experimental data. Both the velocity and temperature fields predicted by the STRUCT-ε model are in close agreement with the high-fidelity data from the experiments and reference LES solutions, across all validation cases. The approach demonstrates the potential to address the accuracy requirements for application to nuclear safety-related issues, by resolving the turbulent flow structures, while the computational efficiency provides the ability to perform consistent uncertainty quantification.

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.

Large eddy simulations of wind loads on an external floating-roof tank

Wind load of an external floating-roof tank is analyzed using large eddy simulation (LES). First, the governing equations and a LES solution procedure employing FLUENT software are presented. Subsequently, using two benchmark tank models, the accuracy of LES is examined and the advantages of LES over wind Reynolds-averaged Navier-Stokes (RANS) method in computing the unsteady wind loads and vortex structures around a tank are validated. Finally, using LES, the turbulent flows and wind loads around an external floating-roof tank at varying Reynolds numbers and liquid levels are analyzed. The DSM SGS model was found to have better accuracy when LES is applied to the tank flow. For an external floating-roof tank, the Reynolds number has little influence on the wind load at Re > 106, while the liquid level affects significantly both the wind loads on the tank and the vortex structures in the flow, and the wind pressure on the internal wall distributes more evenly than that on the external wall, due to the recirculation of the flow in the cavity above the roof. In all cases, the pressure on the external floating-roof tank oscillates apparently with time, which indicates the necessity of applying LES to the wind load analysis of tanks.

Error scaling of large-eddy simulation in the outer region of wall-bounded turbulence

We study the error scaling properties of large-eddy simulation (LES) in the outer region of wall-bounded turbulence at moderately high Reynolds numbers. In order to avoid the additional complexity of wall-modeling, we perform LES of turbulent channel flows in which the no-slip condition at the wall is replaced by a Neumann condition supplying the exact mean wall-stress. The statistics investigated are the mean velocity profile, turbulence intensities, and kinetic energy spectra. The errors follow (Δ/L)αReτγ, where Δ is the characteristic grid resolution, Reτ is the friction Reynolds number, and L is the meaningful length-scale to normalize Δ in order to collapse the errors across the wall-normal distance. We show that Δ can be expressed as the L2-norm of the grid vector and that L is well represented by the ratio of the friction velocity and mean shear. The exponent α is estimated from theoretical arguments for each statistical quantity of interest and shown to roughly match the values computed by numerical simulations. For the mean profile and kinetic energy spectra, α≈1, whereas the turbulence intensities converge at a slower rate α<1. The exponent γ is approximately 0, i.e. the LES solution is independent of the Reynolds number. The expected behavior of the turbulence intensities at high Reynolds numbers is also derived and shown to agree with the classic log-layer profiles for grid resolutions lying within the inertial range. Further examination of the LES turbulence intensities and spectra reveals that both quantities resemble their filtered counterparts from direct numerical simulation (DNS) data, but that the mechanism responsible for this similarity is related to the balance between the input power and dissipation rather than to filtering.

Identifying Weaknesses in Eddy-Viscosity Models for Predicting Film Cooling via Large-Eddy Simulations

Large-eddy simulations (LES) and Reynolds-averaged Navier–Stokes (RANS) simulations were performed to investigate modeling weaknesses in two popular RANS models, realizable and shear-stress transport (SST), in predicting film cooling of a flat plate. The film-jets issue from a plenum through one row of circular holes of diameter and length that are inclined at 35° relative to the plate. The LES and RANS solutions generated were validated by comparing with data from PIV and thermal measurements. The LES results agree well, whereas RANS results show large errors. Results obtained show the turbulent and thermal structure of the jets predicted by the two RANS models to differ considerably. However, both models are consistent in underpredicting the spread of the film-cooling jet. The counter-rotating vortex pair dominates the interaction of the jet and crossflow in the near-wall region, and neither RANS model could predict the strength and structure of that interaction. The gradient-diffusion and Boussinesq hypotheses were evaluated by using the LES data. Comparing LES and RANS results shows that tends to overpredict eddy viscosity, whereas SST tends to underpredict the eddy viscosity. Additionally, both models predict very low values of eddy viscosity near the wall, which leads to incorrect Reynolds stresses. Although regions of counter-gradient diffusion and stress-strain misalignment were found in the near-wall region, further above the wall, the jet behaved according to the hypotheses.

Effects of Density and Blowing Ratios on the Turbulent Structure and Effectiveness of Film Cooling

Large eddy simulations (LES) were performed to investigate film cooling of a flat plate, where the cooling jets issued from a plenum through one row of circular holes of diameter D and length 4.7D that are inclined at 35 deg relative to the plate. The focus is on understanding the turbulent structure of the film-cooling jet and the film-cooling effectiveness. Parameters studied include blowing ratio (BR = 0.5 and 1.0) and density ratio (DR = 1.1 and 1.6). Also, two different boundary layers (BL) upstream of the film-cooling hole were investigated—one in which a laminar BL was tripped to become turbulent from near the leading edge of the flat plate, and another in which a mean turbulent BL is prescribed directly. The wall-resolved LES solutions generated were validated by comparing its time-averaged values with data from PIV and thermal measurements. Results obtained show that having an upstream BL that does not have turbulent fluctuations enhances the cooling effectiveness significantly at low velocity ratios (VR) when compared to an upstream BL that resolved the turbulent fluctuations. However, these differences diminish at higher VRs. Instantaneous flow reveals a bifurcation in the jet vorticity as it exits the hole at low VRs, one branch forming the shear-layer vortex, while the other forms the counter-rotating vortex pair (CRVP). At higher VRs, the shear layer vorticity is found to reverse direction, changing the nature of the turbulence and the heat transfer. Results obtained also show the strength and structure of the turbulence in the film-cooling jet to be strongly correlated to VR.