Hybrid RANS-LES Models Research Articles

Capturing the drag crisis in the flow around a smooth cylinder using a hybrid RANS-LES model on coarse meshes

A hybrid strategy combining Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) methods is nowadays seen as an efficient way to simulate turbulent flows of practical relevance. In this work, the scale-resolving hybrid (SRH) model proposed by Manceau (2018) is compared with a conventional unsteady RANS model, and LES and experimental data from the literature for the flow around a smooth cylinder in the flow regime around the drag crisis. Based on a temporal filtering formalism, this approach has seen limited testing for turbulent separated flows. The drag crisis phenomenon is dominated by complex near-wall physics and is challenging to simulate. The predictive accuracy and the robustness to mesh coarsening for the SRH model are assessed for this test case, with the aim to demonstrate that this hybrid approach can be a credible cost-saving alternative to LES for separated turbulent flows. The meshes considered in this numerical study are far coarser than the ones used in the LES reference data, yet, the results for the time mean drag are found in good agreement with the reference data. Other features of the flow, such as the presence and sizes of the laminar separation bubbles and the consequent magnitude of the time mean lift are not as well captured. In general, the qualitative behaviour of the SRH model is good when considered in the context of questions previously raised in the literature about hybrid models. The mesh savings are achieved by coarsening the spatial resolution in the wake, whereas the resolution required in the near-wall area and shear layers remains high, though much reduced compared to the reference LES data. The key point taken from this study is that the SRH model is an attractive option to produce higher-fidelity data on coarse meshes.

On hybrid RANS-LES of transition in a separated boundary layer

The numerical simulation of laminar separation followed by transition and re-attachment of a turbulent boundary layer has been investigated with particular focus on the prediction of the transition process. The turbulence has been modeled using hybrid RANS-LES models, namely the variants DDES and IDDES. For reference, a computation with nearly DNS-like resolution is featured. Using differently fine meshes, the prediction of the shear layer vortex breakup process into turbulence has been observed in terms of turbulence intensities and turbulence length scales. It is shown how these properties develop during the transition process going along with flow re-attachment. Slight differences arise between DDES and IDDES, the latter one operating in a mode of wall-modeled LES. To describe the impact on the aerodynamic properties, the probability of flow separation along the transition process is presented and the correlation between length scales, turbulence intensity and flow separation is discussed. Conclusions are drawn on the applicability and qualification of the turbulence models as well as on requirements concerning mesh resolution and discretization schemes.

A comprehensive CFD study of the spray combustion, soot formation and emissions of ternary mixtures of diesel, biodiesel and gasoline under compression ignition engine-relevant conditions

In this research, the spray combustion, soot formation and exhaust emissions of diesel, biodiesel, gasoline fuels and their mixtures are analysed in a constant volume chamber. A multicomponent kinetic mechanism (CDBG) suitable for diesel-biodiesel-gasoline mixtures developed by our research group is utilised, and the associated physicochemical properties are thoroughly calculated. Adaptive mesh refinement scheme with appropriate mesh independency analysis are applied. Liquid penetration length, lift-off length, ignition delay and soot formation have been benchmarked against experimental data in the literature. A hybrid RANS-LES model, known as DES model, is used to simulate the turbulent condition. The effects of different ambient temperature/oxygen levels on the flame structure, soot formation and emissions of different ternary mixtures of D75|BD20|G5, D70|BD20|G10 and D65|BD20|G15 were analysed. D65|BD20|G15 resulted in a lower soot mass yield than that of BD100 (pure biodiesel) and D100 (pure diesel) for about 35% and 27%, respectively, at T = 900 K | O2 = 15%. Greater soot mass reductions for the tested fuels were captured by the decrease in ambient temperature from 900 K to 800 K by a factor of ∼1/3 (same ambient O2 concentration). Lower nitrogen oxides (NOx) emissions were obtained for D100 by factors of ∼1/2 at T = 900 K | O2 = 15% compared to BD100. Gasoline-added mixtures revealed lower NOx compared to BD100 (∼20%) yet still higher than D100. Lower carbon dioxide (CO2) and carbon monoxide (CO) emissions were captured for D65|BD20|G15 compared to BD100 and D100.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine

Today, hydropower plants received increased attention as they have the ability to stabilize the electrical grid. However, this is accompanied by more start-stop sequences as well as operation in off-design conditions. One big task is to develop improved runner designs that can resist the increased pressure fluctuations. In the scope of this study, a Francis turbine is simulated for a deep part load operating point with the use of the SAS model and the more recently developed SBES model with ANSYS CFX. Both turbulence models are hybrid RANS-LES models. The simulation results are compared against model test data from a measurement at high cavitation number. For the evaluation of the pressure fluctuations comparison is furthermore made against experimental data at low cavitation number. All in all, SAS and SBES model predict similar flow characteristics. Both, the velocity profiles in the PIV measurement plane and the predicted pressure fluctuations, show only minor differences. The comparison against experimental data shows that the amplitude of the broadband frequency spectrum, which results from the occurrence of vortices, is overestimated at the investigated locations on the runner blades.

Near Stall Unsteady Flow Responses to Morphing Flap Deflections

The unsteady flow characteristics and responses of an NACA 0012 airfoil fitted with a bio-inspired morphing trailing edge flap (TEF) at near-stall angles of attack (AoA) undergoing downward deflections are investigated at a Reynolds number of 0.62 × 106 near stall. An unsteady geometric parametrization and a dynamic meshing scheme are used to drive the morphing motion. The objective is to determine the susceptibility of near-stall flow to a morphing actuation and the viability of rapid downward flap deflection as a control mechanism, including its effect on transient forces and flow field unsteadiness. The dynamic flow responses to downward deflections are studied for a range of morphing frequencies (at a fixed large amplitude), using a high-fidelity, hybrid RANS-LES model. The time histories of the lift and drag coefficient responses exhibit a proportional relationship between the morphing frequency and the slope of response at which these quantities evolve. Interestingly, an overshoot in the drag coefficient is captured, even in quasi-static conditions, however this is not seen in the lift coefficient. Qualitative analysis confirms that an airfoil in near stall conditions is receptive to morphing TEF deflections, and that some similarities triggering the stall exist between downward morphing TEFs and rapid ramp-up type pitching motions.

Comparison study of URANS and hybrid RANS-LES models on predicting vertical axis wind turbine performance at low, medium and high tip speed ratio ranges

Comparison study of unsteady Reynolds-averaged Navier-Stokes (URANS) and hybrid RANS-LES models is carried out for predicting the performance of three-straight-bladed vertical axis wind turbine operating within tip speed ratios (TSRs) ranges of (1.44–3.3). The evaluation is focused on power coefficient, moment coefficient and vortex structure generation, growth and transportation predictions. It was found that URANS turbulence modelling is sufficient for averaged power coefficient prediction and specific range of TSR evaluation, and it also benefits from short simulation run time. To further evaluate flow field details and to understand the underlying flow physics such as dynamic stall behaviour, hybrid RANS-LES turbulence modelling is necessary. Comparing between hybrid models adopted, stress-blended eddy simulation based on transition shear-stress transport (TSST) model has achieved an overall better performance, such as a reduction of simulation discrepancy by 50% in low TSRs range compared to TSST turbulence model. In both medium and high TSRs ranges, the modelling discrepancies are less than 3% compared to TSST turbulence models at about 25% extra computational time. Furthermore, in high TSRs range, hybrid RANS-LES models are able to predict the appearance of vortex shedding at high azimuthal angles (θ ≥ 180°) for which URANS models failed to capture.

Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations

The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the automotive industry, as it increases the importance of efficient aerodynamics and demands how variations of optional extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource-consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are investigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions. • Three hybrid RANS-LES models are compared to wind tunnel measurements for two DrivAer car body configurations. • The turbulence models RANS shielding and transition to LES region properties are essential for achieving consistent accuracy. • The SBES model is the only model to capture the complex flow over the rear window of the notchback car configuration. • The DDES and IDDES models suffer from insufficient shielding of the RANS region and premature switch to WMLES, respectively.

Modification to Improved Delayed Detached-Eddy Simulation Regarding the Log-Layer Mismatch

The wall-modeled large-eddy simulation branch of the widely used improved delayed detached-eddy simulation (IDDES) method is supposed to resolve the problems of log-layer mismatch and underprediction of skin friction, as claimed by the authors of the IDDES (Shur, M. L., Spalart, P. R., Strelets, M. K., and Travin, A. K., "A Hybrid RANS-LES Model with Delayed DES and Wall-Modeled LES Capabilities," International Journal of Heat and Fluid Flow, Vol. 29, 2008, pp. 1638–1649). But, some researchers did meet these problems when using the IDDES. Aiming at finding out the reason and seeking a remedy, an analysis of the IDDES is carried out that focuses on the relation between the wall-parallel grid size and the length scale of the energetic turbulent structures at the Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) interface, and it reveals that the flaw of the IDDES arises from the fact that, in the IDDES approach, the energetic, large-scale, and stress-carrying turbulent structures in the vicinity of the RANS/LES interface are essentially impossible to well resolve by the LES mode. On the basis of the analysis, a modification to the IDDES is proposed by moving the RANS/LES interface further away from the wall so that, at the RANS/LES interface, the wall-parallel grid size is sufficient to resolve the turbulent structures. The modification is simple in form but has clear physical meaning. Through a series of sensitivity tests, it can be concluded that the modified IDDES does resolve the problems of log-layer mismatch and underprediction of skin friction, and it is applicable to low-, moderate-, and high-Reynolds-number flat plate boundary-layer flows without exception.

Two-equation hybrid RANS–LES models: a novel way to treat k and ω at inlets and at embedded interfaces

ABSTRACTA novel method for prescribing k and ω at inlets and RANS–LES interfaces in embedded LES is presented and evaluated. The method is based on the proposal by Hamba to use commutation terms at RANS–LES interfaces. Commutation terms are added to the k and ω equations in the region near the inlet (i.e. the RANS–LES interface). The proposed method can also be used when prescribing inlet values for k and ω in hybrid LES–RANS. The commutation terms are added in embedded LES at the LES side of the RANS–LES interface. The influence of the extent of the region where the commutation terms are added is investigated. It is found that it is most efficient to add commutation terms in only one cell layer adjacent to the interface; in this way, tuned constants are avoided. The commutation term in the ω equation is derived from transformation of the k and ϵ equations. When the commutation terms are used in only one cell layer, the commutation term in the k equation corresponds to a negative convection term. Hence, the commutation term can be omitted and a homogeneous Neumann inlet boundary condition can be used. The commutation term in the ω equation is retained. The novel method is evaluated for channel flow (Reτ = 8000), boundary layer flow (Reθ = 11, 000) and backward-facing step flow (ReH = 28, 000). Hybrid LES–RANS is used for the first two flows and embedded LES for the backward-facing step flow.

A computational study of the flow around the KVLCC2 model hull at straight ahead conditions and at drift

The flow around the KVLCC2 model tanker hull at 0°, 12° and 30° drift is investigated using classical Reynolds Averaged Navier–Stokes (RANS) models from two different proprietary codes, hybrid RANS-LES models from one of these proprietary codes, and Large Eddy Simulation (LES) models from a third, semi-proprietary code. Understanding the three-dimensional (3D) flow around a ship hull and in the wake of the hull on straight course or during maneuvering is important for many design and operational aspects, as well as from an environmental perspective. Tetrahedral grids of 13 million and 74 million cells respectively are used for the RANS and hybrid RANS-LES; for the LES, grids of between 120 and 200 million tetrahedral cells are used. The objectives are to generally increase knowledge of the flow past a ship hull both at straight-ahead and at static-drift conditions, and to assess the predictive capabilities of RANS, hybrid RANS-LES and LES computational methods. Using these computational results, the focus is specifically on understanding the complex vortical topology and associated surface-flow characteristics, and how these change when the drift angle increases. The RANS and time-averaged hybrid RANS-LES and LES predictions are compared against each other and against experimental data to further validate the simulation models and to provide additional insight into the flow physics.

Time-accurate Numerical Simulations of Swirling Flow with Rotor-stator Interaction

A series of numerical simulations is undertaken to study a highly swirling turbulent flow generated by rotor-stator interaction in a swirl generator. The purpose is to assess the applicability of different turbulence models in swirling flow with a high level of unsteadiness and a significant production and dissipation of turbulence in the flow away from the wall. Nine turbulence models are compared: four high-Reynolds URANS, two low-Reynolds URANS and three hybrid URANS-LES. These are the standard k−𝜖, SST k−ω, realizable k−𝜖, R N G k−𝜖, Launder-Sharma k−𝜖, Lien-Cubic k−𝜖, delayed DES Spalart-Allmaras, DDES SST k−ω and improved DDES-SA. The URANS models are capable of capturing the main unsteady feature of this flow, the so-called helical vortex rope, which is formed by the strong centrifugal force and an on-axis recirculation region. However, the size of the on-axis recirculation region is overestimated by the URANS models. Although the low-Reynolds URANS formulations resolve the boundary layers in the runner and the draft tube more accurately, they still encounter difficulties in predicting the main flow features in the adverse pressure gradient in the draft tube. It is shown that a more detailed resolution, which is provided by the hybrid URANS-LES methods, is necessary to capture the turbulence and the coherent structures. The flow contains a strong disintegration of the vortex rope which is predicted well by the hybrid RANS-LES models. The hybrid methods also capture the blade wakes better than the other models, elucidating the wake interaction with the vortex rope. The frequency of the vortex rope is predicted well and the total turbulence (resolved and modeled), suggested by DDES-SA, corresponds reasonably well to the experimental results.

The PANS [formula omitted] model in a zonal hybrid RANS–LES formulation

A new approach to use the partially averaged Navier–Stokes (PANS) model as a hybrid RANS–LES model is presented. It is evaluated in fully developed channel flow and embedded LES in a hump flow. For the channel flow, the two RANS–LES interfaces are parallel to the walls. In the URANS region, fk is set to one. In the LES region, fk is set to a constant value (the baseline value is fk=0.4) or it is computed. It is found that the new model gives good results for channel flow for a large span of Reynolds numbers (4000⩽Reτ⩽32,000). In the channel flow simulations, three different grids are used in the wall-parallel planes, 322,642 and 1282, and the model yields virtually grid-independent flow fields and turbulent viscosities. Embedded LES is used for the hump flow which is well predicted. The RANS–LES interface is normal to the flow from the inlet. RANS is used upstream of the interface. Downstream this interface, RANS is used near the wall and LES is used away from the wall.