Rob Watson

Modifying an unstructured Roe solver for large eddy simulation

© 2015 American Institute of Aeronautics and Astronautics Inc. All rights reserved. With the ever increasing availability of computing power, attention is being turned to large eddy simulation as a design tool at the component level. To maintain integration with pre-existing design processes based on solving the Reynolds Averaged Navier-Stokes equations, legacy RANS codes are being pressed into service with minimal adaptation to perform large eddy simulations. Often, these legacy RANS solvers are based around the approximate Riemann solver of Roe. It is shown here that the Roe scheme is not ideally suited to large eddy simulations, and may require excessive user care to extract worthwhile information — a process not always possible within design timeframes. A different discretisation scheme, which is shown to conserve kinetic energy, but which also shares some of the favourable structural behaviour of Roe’s is implemented, and is then tested on a range of geometries, both canonical and industrial. One of the advantages of this approach is its ability to eliminate the smoothing terms which have a strong deleterious affect on the LES performance of Roe. This allows explicit subgrid models to be sensibly applied. The new scheme, with explicit turbulence modelling, is found to outperform the Roe scheme for LES in all of the test cases.

Temporal Stabilisation of Flux Reconstruction on Linear Problems

Filtering is often used in Large Eddy Simulation with a global filter width, instead here a filter width in the reference domain of high order Flux Reconstruction is considered. It is shown via Von Neumann analysis how filtering effects the dispersion and dissipation of the scheme when spatially and temporally discretised. With it being shown that filtering stabilises the scheme temporally by upto

Optimal multi-block mesh generation for CFD

25%

Improved Hierarchical Modelling for Aerodynamically Coupled Systems

for forth order FR. The impact of filtering on error production is calculated, highlighting the reduction in convective velocity caused and showing numerically the impact on order of accuracy. Finally, the turbulent Taylor-Green case is used to understand the effect of reference domain filtering on the transition to turbulence, and a filter Reynolds number is defined that is shown to be useful in understanding the effect of filtering on simulations.

Perfectly Parallel Optimization for Cutback Trailing Edges

ABSTRACT An assessment of various automatic block topology generation techniques for creating structured meshes has been performed in the first part of the paper. The objective is to find out optimal blocking methods for generating meshes suitable for flow simulations. The comparison has been carried out using an adjoint-based error analysis of the meshes generated by these block topologies. Different objective functions and numerical schemes have been used for this assessment. It is found that, in general, the medial axis-based approaches provide optimal blocking and yields better accuracy in computing the functional of interest. This is because the medial axis-based methods produce meshes which have better flow alignment specially in case of internal flows. In the second part of the paper, the adjoint-based error indicator has been used to adapt the block topology in the regions of large error.

Towards robust unstructured turbomachinery large eddy simulation

© 2017 ASME. Strong aerodynamic coupling can make the high fidelity simulation of a number of critical aero-engine components prohibitively expensive - particularly within the timeframes of industrial design cycles. This paper develops a body force based hierarchy of approaches to modelling the effects of blade rows. These are envisaged as allowing the computationally expensive parts of coupled systems to be resolved much more cheaply, rendering the cost of the overall simulation as more manageable. Simulation of the coupling that exists between the flow around an aero-engine intake and its fan is particularly emphasised, as this is becoming stronger and more performance critical with the modern trends towards the reduction of the relative diffuser length. The use of the viscous smeared geometry level of fidelity is initially shown to be an effective model over a number of cases a simple compressor blade row, a modern high bypass fan, and the Darmstadt rotor. After this, it is shown working as part of a coupled system in an intake experiencing crossflow. Higher fidelity geometry representations are then considered, which mimic the effect of struts. Finally, a mix of various fidelity geometry representations and turbulence modelling approaches is shown to bring otherwise hugely expensive calculations within the realm of practical computation, in the form of a full fan-to-flap calculation.

High Order Flux Reconstruction on Stretched and Warped Meshes

Previous attempts have been made to optimize the performance of film-cooling slots for cutback trailing edges, but these involved the use of steady calculation methods, which have been shown to be inappropriate for accurately capturing the behavior of this class of flows. Here, an unsteady method (large-eddy simulation on a coarse grid, or very large-eddy simulation) is used to compute the flow. To take advantage of the enormous parallel capacity of modern supercomputers and distributed computing nets, as well as the relatively low cost of very large-eddy simulation, while at the same time mitigating its lower scope for significant parallelization, a perfectly parallel evolutionary optimization process was undertaken. A relatively crude optimization target of maximizing the adiabatic wall film-cooling effectiveness averaged over the entire exposed cutback surface was used as a proof of concept. The optimizing heuristic then used an evolutionary approach to design a turbulator planform, subject to some imp...