High Fidelity Compressible and Incompressible Flow Simulations of a Turbo-machinery Geometrical Configuration

Authors

João Isler  - Department of Aeronautics, Imperial College London, UK
Guglielmo Vivarelli - Department of Aeronautics, Imperial College London, UK
Francesco Montomoli - Department of Aeronautics, Imperial College London, UK
Spencer Sherwin - Department of Aeronautics, Imperial College London, UK
Paolo Adami - Rolls Royce Deutschland, Dahlewitz, DE
Raul Vazquez - Rolls Royce plc., Derby, UK

Abstract

High fidelity numerical simulations at high Reylnolds number (106), although extremely expensive, are nowadays possible due to the efficiency of new algorithms on highly parallel systems. Among the computational fluid dynamics methodologies employed, high-order methods are becoming increasingly popular since the industrial sector strives to increase fidelity and accuracy for design purposes while retaining computational efficiency. To this end, the well-known highly scalable spectral/hp element framework Nektar++ is used. This approach therefore brings the possibility to have access to a wide range of spatial and temporal scales in turbulent flows and also study instability mechanisms of shear flows with a lower computational cost. In this work, we investigated the impact of compressibility in an engine intake for a range of high Reynolds numbers in absence of transonic effects and under strong adverse pressure gradients by means of computational simulations. The case consists of a flow over a flat plate which is subjected to a strong adverse pressure gradient imposed by employing an inviscid liner set as an upper boundary. Additional computational resource saving was, in this study, achieved by reducing the computational domain, so that, although only a small part of the domain was computed, it was possible to reproduce the physical behaviour expected in the whole computational domain. This methodology combined with the high-order methods demonstrated to be highly efficient and drastically reduced the computational time to perform the three-dimensional numerical simulations. In order to assess the compressibility effects in the vicinity and over the flat plate surface, compressible and incompressible high-order Implicit Large Eddy Simulations (iLES) of the Navier-Stokes (NS) equations were performed. With this methodology, we are able to assert what are the physical mechanisms behind the turbulent transition processes which the flows go through. In addition, and most important, what is the role that compressibility is playing in this flow configuration, in order to promote the design of improved geometries using high-order methods. The energetic flow conditions due to the high Reynolds numbers combined with the strong adverse pressure gradient imposed by the liner make the flow field separate. The separated flow gives rise to a separation bubble, which leads to a turbulent flow after going through a transition region. Furthermore, the compressible and incompressible instabilities were triggered by the same physical mechanisms. The Tollmien-Schlichting (T-S) instabilities trigger the Kelvin-Helmholtz (K-H) behaviour, which in turn causes the turbulent transition. Therefore, a consistent investigation of the compressibility effects and boundary layer behaviours of the compressible and incompressible solutions provided the necessary understanding of the viscous driven separation that governs the flow on an engine intake at moderate fan speed.