Dynamic Hybrid RANS-LES Research Articles

Scale-Resolving Simulations of a Statistically Targeted Forcing Method for Synthetic Turbulence Generation in a Freestream Turbulence

Abstract Computational fluid dynamics results for synthetic turbulence generation of freestream turbulence by a proposed statistically targeted forcing (STF) method are presented. The STF method has been previously documented for homogeneous isotropic and anisotropic turbulence and formulated to introduce a fluctuating velocity field with a distribution of first and second moments that match a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. This study extends its applicability to generation of freestream turbulence in scale-resolving simulations in which flow is spatially developing. The method provides flexibility in regions where synthetic turbulence needs to be generated or damped, for use in engineering level scale-resolving simulations such as large eddy simulation (LES), and hybrid Reynolds-averaged Navier–Stokes (RANS)-LES. The objective of this study is to evaluate the performance of the proposed STF method in simulations that incorporate monotonically integrated LES (MILES), Smagorinsky (SMAG) LES subgrid stress model, and dynamic hybrid RANS-LES (DHRL) model in reproducing inflow freestream turbulent flow, and the capability of each model to reproduce proper energy decay characteristics downstream of forcing. Results are inter-rogated and compared to target statistical velocity and turbulent stress distributions for inflow turbulence and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the STF method can successfully reproduce desired statistical distributions in a nearly isotropic freestream turbulent flow.

Application of scale-resolving simulation to a hydraulic coupling, a hydraulic retarder, and a hydraulic torque converter

The paper describes the qualification and validation of large eddy simulation (LES) and hybrid Reynolds-averaged Navier–Stokes (RANS)/LES, the so-called scale-resolving simulation (SRS) approaches, which are currently employed in transient simulations of internal flow for fluid machineries. Firstly, the application of various turbulence models in ANSYS FLUENT is briefly introduced to acquire the external performance of three hydrokinetic devices and to compare it with experimental data. It was found that a remarkable improvement in external performance was achieved. The best results could be as low as 4% for the absolute error in hydraulic coupling, 2%–5% for the error for the hydraulic retarder, and 2%–4% for the hydraulic torque converter. Basically, all models had better error levels than that of around 10%–15% obtained by RANS. Then four typical SRS simulations were applied to conduct numerical simulations of the internal flow fields for hydraulic coupling, the hydraulic retarder, and the hydraulic torque converter. The results provided two indisputable facts, firstly, that SRS models are more accurate in certain flow situations than RANS models and, secondly, that SRS models can give additional information compared with RANS simulations. Finally, the BSL SBES DSL model, a dynamic hybrid RANS/LES (DHRL) turbulence model, was applied to simulate and analyze the flow mechanism of the hydraulic coupling to deepen our understanding of it. The detailed flow structure in hydraulic coupling was determined and was used to understand the flow mechanism.

Application of Hybrid RANS/LES turbulence models in rotor-stator fluid machinery: a comparative study

Purpose This paper aims to improve performance prediction and to acquire more detailed flow structures so as to analyze the turbulence in complex rotor-stator flow. Design/methodology/approach Hydraulic retarder as typical fluid machinery was numerically investigated by using hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) models CIDDES Algebraic Wall-Modeled Large Eddy Simulation (LES) (WMLES) S-Ω and dynamic hybrid RANS/LES (DHRL). The prediction results were compared and analyzed with a RANS model shear stress transport (SST) k-omega which was a recommended choice in engineering. Findings The numerical results were verified by experiment and indicated that the predicted values for three hybrid turbulence models were more accurate. Then, the transient flow field was further analyzed visually in terms of turbulence statistics, Reynolds number, pressure-streamline, vortex structure and eddy viscosity ratio. The results indicated that HRL approaches could capture unsteady flow phenomena. Practical implications This study achieves both in performance prediction improvement and better flow mechanism understanding. The computational fluid dynamics (CFD) could be used instead of flow visualization to a certain extent. The improved CFD method, the fine computational grid and the reasonable simulation settings jointly enhance the application of CFD in the rotor-stator flow. Originality/value The improvement was quite encouraging compared with the reported literatures, contributing to the CFD playing a more important role in the flow machinery. DHRL provided the detailed explanation of flow transport between rotor and stator, which was not reported before. Through it, the flow mechanism can be better understood.

Evaluation of hybrid RANS/LES models for prediction of flow around surface combatant and Suboff geometries

Predictive capabilities of hybrid RANS/LES models are compared for single-phase flow over a surface combatant at Re=5.3×106 and an appended DARPA Suboff model at Re=1.2×107. The turbulence models used in the study are: k–ω shear stress transport (SST)-URANS; Spalart Allmaras based detached eddy simulation (SA-DDES); k–ω based improved delayed detached eddy simulation (KW-IDDES); and a dynamic hybrid RANS/LES (DHRL) model coupling SST and implicit LES. For the surface combatant case, both SA-DDES and KW-IDDES predicted <1% resolved turbulence, whereas DHRL predicted up to 70% resolved turbulence. The SST-URANS, SA-DDES, KW-IDDES and DHRL predictions compared within 4.23%, 4.14%, 5.88% and 3.88% of the experimental data, respectively. For the Suboff case, both the SA-DDES and KW-IDDES predicted only limited resolved turbulence in the sail wake and downstream of the fins, whereas DHRL predicted resolved turbulence levels comparable to LES. The averaged error for the mean velocity profile at propeller plane for SST-URANS, SA-DDES, KW-IDDES and DHRL predictions was 6.4%, 8.9%, 13.1% and 3.0%, respectively. The corresponding errors for the turbulence variables were 44%, 70%, >100% and 28%, respectively. Overall, the DHRL model performed best among the turbulence models tested, and KW-IDDES performed worst. The study indicates that the DHRL approach has the potential to provide accurate mean flow predictions while resolving small-scale turbulent structures. Results also highlight the importance of the wall function formulation for accurately resolving mean skin friction coefficient, especially over smooth regions of the hull.