Charles Mockett

Two Non-zonal Approaches to Accelerate RANS to LES Transition of Free Shear Layers in DES

We present two novel approaches to improve the behaviour of DES in the region where an attached boundary layer (handled with RANS) flows into a separated shear layer (to be resolved using LES). The approaches aim to be generally-applicable and retain the non-zonal nature of DES. Furthermore, the formulations are local and can be readily implemented in general-purpose solvers. One approach introduces an adaptive grid scale definition, sensitised to the local vorticity orientation. The second approach, which can be combined with the first, involves the incorporation of alternative SGS model formulations that discern between quasi 2D and developed 3D flow states. Both modifications lead to a strong reduction of eddy viscosity in the early shear layer. Consequently, a significant acceleration of RANS to LES transition is demonstrated for a plane shear layer, a backwards-facing step and a round jet, with results from two different flow solvers shown. The greatest improvement is seen when the approaches are applied in combination.

Detached-Eddy Simulation of Landing-Gear Noise

In the present paper, compressible IDDES at Mach numbers of 0.115 and 0.23, coupled with FWH integration, were carried out for a rudimentary landing gear configuration. The obtained Mach number scaling of around M confirms the expectation that the contribution of dipoles is dominant. Comparison with experimental far-field sound measurements has also been made. Both the permeable-surface and the solid-surface compressible results agree well with the experimental data to within 2 dB. Incompressible calculations are often applied to complex industrial configurations in the low-Mach regime. To assess the validity of this practice, incompressible calculations were also conducted. Provided that wave reflections were taken into account in the far-field integration, the incompressible results agreed well with the compressible case, being at most 2 dB quieter. Modest savings in computational resources were achieved, which could have been higher with an adjusted grid. However, the applicability of such incompressible calculations is expected to be strongly case-dependent.

Demonstration of Improved DES Methods for Generic and Industrial Applications

This paper presents an overview of the DES methods implemented at the Institute of Fluid Mechanics and Engineering Acoustics (ISTA) at the TU-Berlin during the course of the European DESider project. As well as the validation of these methods on the basis of simplified, academic flow cases presented in the first part, their suitability and necessity for complex industrial applications is demonstrated using results from other research projects. The methods prove robust and reliable for a wide range of applications, ranging from external to internal flows, from bluff bodies with massive separation to the partial resolution of attached boundary layers.

Go4Hybrid: A European Initiative for Improved Hybrid RANS-LES Modelling

An overview is given of a new EU-funded project targeting the mitigation of a key remaining issue with hybrid RANS-LES methods. The grey area problem arises due to inconsistencies in the transition between RANS and LES paradigms and has a detrimental impact on the early separated shear layer, particularly for e.g. shallow separating/reattaching flows. Progress on this front is seen as central to improving industrial confidence in CFD for situations typical of the limits of machine performance.

Noise Prediction of a Rudimentary Landing Gear Using Detached-Eddy Simulation

Detached-Eddy Simulation (DES) is a promising method for efficient simulation of broadband noise at minimal computational cost. Here, results from a study of broadband noise simulation using state-of-the-art DES methods are presented for a rudimentary landing gear configuration. The DDES and IDDES variants are compared with experiments in incompressible simulations. IDDES shows mild improvement in agreement and some increase in the resolution of high frequencies. An attempt is made to independently verify published results for far-field sound prediction, using a compressible simulation coupled with Ffowcs-Williams/Hawkings (FWH) integration. In contrast to the published results, our results do not provide evidence of unexpectedly strong roles played by the ceiling or by quadrupoles. Our results furthermore predict much lower far-field noise levels than the published results. Good agreement between solid and permeable FWH surfaces is found as long as the permeable surfaces are open downstream.

Evaluation of Time Sample and Span Size Effects in DES of Nominally 2D Airfoils beyond Stall

A detailed investigation of detached-eddy simulation (DES) applied to the flow around nominally 2D airfoils in deep stall has been conducted. The sensitivity of the flow to a wide range of computational parameters has been assessed. The principal parameters of influence were found to be the length of the time sample computed for Reynolds averaging and the size of the periodic computational domain in the spanwise direction. In contrast, wind tunnel wall effects, the choice of underlying RANS model and the boundary layer transition treatment counted among the parameters for which a low to negligible level of sensitivity was observed. The magnitude of the error introduced by short time samples has been estimated using a statistical analysis technique. This combined with very long time samples (in excess of 2000 non-dimensional units) allows a reliable description of the span size effect independent of the time sample error. The study significantly enhances the body of knowledge concerning this standard validation flow for hybrid RANS-LES methods. The original DES study of Shur et al. (1999), which was conducted for such a flow, is reassessed in light of the new information and its principal conclusions are upheld. The strong level of scatter observed between different experimental sources represents the chief remaining uncertainty.

Turbulence Modelling Effects on Tandem Cylinder Interaction Flow and Analysis of Installation Effects on Broadband Noise Using Chimera Technique

The NASA tandem cylinder benchmark case (L/D = 3.7) is studied numerically using novel variants of DES the Delayed Detached Eddy Simulation (DDES) and the Improved Delayed Deteched Eddy Simulation (IDDES). The ow Mach number is 0.1285 and the Reynolds number is set to 166.000 to match th corresponding experiments at the NASA Langley Research Center (Jenkins et al., Lockard et al.). Incompressible simulations are carried out on a mandatory grid from the EC ATAAC project with approx. 9.5 million cells with a spanwise extent of 3.0D and ow normal domain extent of approx. 17.8D with symmetry boundary condition to mimic the closed experimental test section. In a second step a compressible IDDES simulation is done to evaluate the broadband noise of the con guration. Measurements are available from the QFF open jet facility. To capture the installation e ects a combined grid of above mentioned cylinder core grid and a single stream jet grid is used. The communication between the grids is done by an overset chimera technique. The noise in the far eld is calculated by a standard FW-H method. The IDDES approach is designed to extend the LES region of the original DES approach (hybrid RANS/LES) from Spalart et. al. (1997) to the turbulent boundary layer, as proposed rst by Travin et. al. in 2006. The non-zonal blending occures therefore inside the boundary layer the RANS model acts as a wall model for the LES. The comparison of the simulations show the improvement of the results by using a IDDES.

Advanced DES Methods and Their Application to Aeroacoustics

Detached eddy simulation (DES) is shown to be a suitable method for the simulation of the sound generation of turbulent flows, because it provides access to the resolved turbulent scales at minimal computational cost. The near-wall region is solved efficiently by RANS while LES is applied to all regions containing scales important for the noise generation. In addition to the usual LES resolution requirements, for acoustics the smallest of these scales are defined by the highest frequencies of interest. The sound radiation is generally computed by solving an integral over a data surface surrounding the source region outside the turbulent flow. The grid must be fine enough to resolve the sources and the propagation between the sources and the data surface. Examples for the simulation of noise emission problems are presented for the influence of nozzle serrations on jet mixing noise and three airframe noise sources, which are rod-airfoil interaction noise, the noise of high-lift devices, and airfoil self noise.

Turbulence Modelling in Application to The Vortex Shedding of Stalled Airfoils

ABSTRACT A thorough investigation is conducted into some open questions remaining in the Detached–Eddy Simulation (DES) technique. Principle among these are the questions of grid fineness, and the extent of dependency on the background turbulence model inherent in DES. The investigation takes the case of the NACA0012 airfoil beyond stall, for which a large number of grid fineness and turbulence model combinations are computed. In order to investigate the influence of high resolution in the LES zone, a new highly–refined grid is constructed. The results of the investigation demonstrate a strong deterioration of the results due to the uneven grid fineness distribution, and the reasons for this in relation to a general grid sensitivity present in DES are discussed. However, the physical character of the DES calculations remains encouraging, and motivated by this, an analysis of the phenomenon of stochastic weak vortex shedding cycles is undertaken.

Adaptive Wall Function for the Prediction of Turbulent Flows

Conventionally, two types of wall boundary condition are available for the solution of turbulence transport equations in CFD. These exhibit very different requirements on the wall normal distance of the first grid point and any violation of these requirements leads to a drastic degeneration in the solution quality. This places a very high level of importance on the design of the numerical grid, and contributes to the excessive human resources typically spent on this task. Furthermore, these criteria depend strongly on the local flow field quantities, which means that prior knowledge of the solution is required for correct grid design. In practice this often means that an iterative grid design process is required, causing further grid generation expenditure.