Results are presented of high-fidelity scale-resolving simulations of an asymmetric curved turbulent wake subjected to adverse pressure gradient based on the zonal RANS-IDDES approach to turbulence representation. The work is performed within the framework of a joint German-Russian project "Complex Wake Flows" and presents a continuation of the compu-tational/experimental studies of the straight symmetric wakes carried out by the same team during 2017-2019. The addition of asymmetry and longitudinal curvature to the flow model made it more representative in terms of reproducing the real wakes behind the three-element high-lift wing configurations used during takeoff and landing of an aircraft. The reliability of the obtained results (both the mean flow parameters and the turbulent statistics) is supported by the grid-refinement study, whereas their comparison with the similar results of the RANS computations reveals a considerable discrepancy. This suggests the necessity of the improve-ment of the RANS models, which is planned for future work.

The paper presents validation results of a hybrid simulation method for aeroacoustics in turbulent flows at low Mach numbers. The hybrid method implemented in the Ansys Fluent® CFD package applies a scale-resolving turbulence model to compute the noise sources in an incompressible flow, while the noise propagation is modeled by a wave equation formulated for the acoustic potential. The selected test case deals with a flow and a sound field around a generic side view mirror of a car. The SBES model by Menter, which belongs to the class of the RANS-LES models, is used for the flow simulation. It switches to the Large Eddy Simulation (LES) mode in separated mixing layers and recirculation zone behind the mirror as well as in the following wake, where flow develops intensive turbulence and dominating noise sources. The acoustics wave equation is formulated in the model form of Kaltenbacher et al. and is applied in the time domain. The overall calculation is performed as a transient co-simulation on the same mesh using the finite-volume discretization method for both the flow and the acoustics parts. The wave equation is advanced in time using the HHT-α method. Obtained distribution of the mean wall pressure over the mirror surface closely matches the experimental one. Rich content of the resolved turbulent vortices in the separation zone and good agreement of the calculated and measured wall pressure spectra at sensor locations downstream the mirror evidence a proper LES resolution quality of noise sources. Comparison of the computed noise spectra at the remote microphones with the experimental data demonstrates the sound propagation accuracy and validates the overall aeroacoustics simulation method.

Results are presented of a series of RANS computations aimed at creating a new experimental flow model of a curved turbulent wake evolving under adverse pressure gradient. In the course of the computations, key geometric parameters of the model (the angle of attack of a flat plate generating the wake and the shape and the angles of attack of liner foils creating the pressure gradient) were varied in a wide range. The purpose was to find the parameters ensuring desirable features of the flow, namely, a considerable wake curvature and its strong deceleration leading to formation of a large stagnation or even a reversal flow region, on the one hand, and no flow separation either from the flat plate or from the surfaces of the liner foils, on the other hand. As a result, the design satisfying all these demands has been found. This design will be implemented and studied in the framework of recently launched joint German-Russian project “Complex Wake Flows” which presents a continuation of an earlier similar project devoted to symmetric wakes.

We present an analysis of RANS turbulence models for a new turbulent wake flow case in adverse pressure gradient. The reference data are obtained from an LES. RANS simulations using the Spalart-Allmaras model, the SST model and the SSG/LRR-\(\omega \) model show a tendency to underpredict the onset of flow reversal in the wake. The comparison with the LES data reveals that the SSG/LRR-\(\omega \) model underpredicts the turbulent transport of the Reynolds stresses and overpredicts the ratio of production to dissipation. Therefore two modifications of the SSG/LRR-\(\omega \) model are studied, i.e., the modification of the coefficient of the gradient diffusion hypothesis for the turbulent transport of the Reynolds stresses and the sensitization of the dissipation rate to irrotational strains. Both modifications improve the agreement with the reference data.

The paper presents a bilateral German-Russian project launched in 2017 and aimed at investigation of turbulent wakes in the presence of Adverse Pressure Gradient (APG). Such wakes are a common feature of high-lift wing flows near the maximum lift conditions (take-off and landing), when the wake of the main wing is subjected to APG created by flaps. This type of flow is known to be poorly predicted by available RANS models. Hence, an ultimate goal of the project is their improvement based on a detailed experimental dataset and on results of high-fidelity turbulence resolving simulations providing relevant second moment closure terms not accessible by measurements. After a brief overview of the experimental and numerical parts of the project, the paper focuses on the first zonal RANS-IDDES computations of a wake of the flat plate in APG created by a plane diffuser. These computations performed in the initial stage of the project (before obtaining experimental data) are aimed at evaluating the capability of this approach to ensure the required accuracy with reasonable computational resources. Results of the simulations conducted on 3 grids (18, 30, and 50 million cells) support the credibility of the approach and suggest that it ensures not only virtually grid-independent prediction of the mean flow characteristics of the wake but also the dissipation-rate which is a key quantity in the context of improvement of the Reynolds Stress Transport RANS models. This is achieved, despite a relatively large grid step in the wake region (about 75 Kolmogorov length scales), thanks to computing this quantity based on the balance of the separate terms of the Reynolds stress transport equations.

Results of simulations of the noise generated by a turbulent flow around a simplified rain gutter of a car windshield are presented. The hybrid approach of ANSYS FLUENT used the scale-resolving SBES model for calculating a turbulent flow under an incompressible flow assumption, and the wave equation model for the propagation of acoustic perturbations. The mean static pressure and the sound intensity obtained in the simulations were compared with experimental data and were found to be well predicted.

Results are presented of high-fidelity scale-resolving simulations of the wake flow exposed to adverse pressure gradient (APG). Specifically, zonal RANS-IDDES computations are performed of the flow model designed and manufactured at the Technische Universität Braunschweig in the framework of a joint German-Russian project “Wake in Adverse Pressure gradient”. The model includes a flat plate as the wake generator and two pairs of thin liner foils creating APG. Results of the computations of mean flow characteristics agree with currently available experimental data and differ from those of the RANS predictions. This suggests the necessity of RANS models improvement which is planned to be done with the use of the combined experimental/numerical database on the mean flow and turbulence statistics of the wake accumulated in the course of the project.

Present work is dedicated to development of the software for interactive visualization of results of simulation of gas dynamics problems on meshes of extra large sizes. Kitware ParaView visualization tool, which is popular among engineers and scientists is used as a frontend. The coupling of client and server instances of ParaView is used in the project. The crucial feature of the work is an application of Apache Hadoop and Apache Spark for distributed retrieving of simulation data from files on hard disk. The data is stored on the cluster in Hadoop Distributed File System (HDFS) managed by Apache Hadoop and is provided to ParaView server by Apache Spark data processing tool.

Experimental set-up for study of 2D wall-mounted hump flow

Results are presented of scale-resolving Zonal RANS-IDDES and pure RANS computations of a turbulent wake exposed to adverse pressure gradient. The RANS-IDDES is performed with the use of the NTS code of SPBPU, and the RANS computations are carried with the use of the TAU code of DLR. It is shown that none of the considered RANS models (two linear eddy viscosity models and a full differential Reynolds stress transport model) is capable of reproducing the mean wake characteristics and turbulent statistics predicted by the RANS-IDDES. Accumulated RANS-IDDES database may be used for improvements of RANS models.

An assessment of two approaches aimed at acceleration of the RANS-LES transition in separated flows has been performed in the framework of Delayed Detached Eddy Simulation (DDES). The former approach is based on the use of the shear-layer adapted subgrid scale (DDES.SLA) and the latter involves an alternative subgrid model (s-DDES). Simulations of two separated flows, namely the backward-facing step flow and flow over a wall-mounted hump have been performed with the use of ANSYS-FLUENT. The results were compared with those obtained with the use of the in-house NTS code and with experimental data. It is shown that both RANS-LES acceleration techniques implemented in ANSYS-FLUENT allow more accurate and less code-sensitive results to be obtained in comparison to the original DDES formulation.

Two models, an algebraic WMLES and IDDES, were tested on their ability to predict wall-bounded flows with pressure gradient. A developed flow between two moving flat plates under an adjusted pressure gradient (Couette-Poiseulle flow) was used for testing. Within two considered test cases three types of boundary layers were examined: boundary layer with adverse pressure gradient (APG), boundary layer with favorable pressure gradient (FPG) and a wall-bounded flow with negligible skin-friction. It was shown that both models are capable of accurate prediction of the flow structure and mean parameters of boundary layers with APG and FPG and of frictionless wall-bounded flow. The difference between simulation results and DNS data is less than 7%.

A comparative study is conducted between the original versions of Delayed Detached-Eddy Simulation (DDES) and Improved DDES (IDDES) and these approaches combined with a new (shear layer adapted) definition of the subgrid length-scale recently proposed in Shur et al. (Flow Turbul. Combust. 95(4), 709–737, 2015). This definition is aimed at accelerating the transition to resolved turbulence in separated shear-layers, which significant delay is typical of the non-zonal hybrid RANS-LES models, in general, and DES-like approaches, in particular. An objective of the study is widening the validation database of the new solutions-dependent definition of the length-scale compared to that employed in the original work of Shur et al. In order to reach this, three different complex separated flows with well-understood flow physics were considered, which all are widely used for the validation of different CFD approaches. These flows are: a flow with non-fixed pressure-induced separation and reattachment (wall-mounted hump), a massively separated flow (NACA 0021 airfoil beyond stall), and a supersonic separated flow (wake behind a cylindrical forebody). The results of simulations suggest that the DDES and IDDES models combined with the shear-layer adapted subgrid length-scale perform according to their design (no unforeseen interactions of the shear-layer adapted length-scale with the empirical functions involved in the DDES and IDDES formulations are observed) and considerably mitigate the delay of transition from fully modeled to partially resolved turbulence in the separated shear layers compared to the standard DES definition of the length-scale (maximum local grid-spacing).

A new automatic high-order hybrid central-difference/upwind scheme is developed for the finite-volume approximation of the inviscid fluxes within global hybrid RANS-LES approaches. Performance of the scheme is illustrated by examples of computations of three types of flow: a flow with massive separation with the use of Delayed Detached-Eddy Simulation (DDES), a flow with separation and reattachment with the use of DDES with shear-layer adapted subgrid length scale, and a fully attached flow with the use of Improved DDES (IDDES).

Hybrid RANS-LES computations of the separated flow over a wall-mounted hump are presented, which employ different grey-area mitigation techniques in the framework of a structured and an unstructured flow solver. Two zonal approaches using different synthetic-turbulence generators at the RANS-LES interface, as well as a non-zonal approach based on a shear-layer-adapted subgrid scale are compared in detail with validation data from a wind-tunnel experiment. Irrespective of the applied flow solver, the different methods are shown to be similarly effective in reducing the grey area compared to the basic hybrid RANS-LES model, and thus provide satisfying mean-flow predictions of the pressure-induced separation. (C) 2017 Elsevier Inc. All rights reserved.

A comparative study is conducted of the original versions of Delayed Detached- Eddy Simulation (DDES) and Improved DDES (IDDES) and these approaches combined with "shear-layer-adapted" (SLA) subgrid length-scale proposed recently for resolving the issue of delayed RANS-to-LES transition in separated shear layers in global hybrid RANS-LES approaches. Computations were carried out of two separated flows: a transonic flow past M 219 cavity and a subsonic flow over NASA wall mounted hump. Results of the computations suggest that the use of the SLA subgrid length-scale considerably accelerates transition to resolved three-dimensional turbulence in the separated shear layers and substantially improves agreement with the experimental data.