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Direct numerical simulation of backward-facing step flow at $Re_{\unicode[STIX]{x1D70F}}=395$ and expansion ratio 2

Published online by Cambridge University Press:  24 January 2019

A. Pont-Vílchez*
Affiliation:
Heat and Mass Transfer Technological Centre (CTTC), Universitat Politècnica de Catalunya BARCELONA Tech (UPC) C/ Colom, 11 08222 Terrassa (Barcelona), Spain
F. X. Trias
Affiliation:
Heat and Mass Transfer Technological Centre (CTTC), Universitat Politècnica de Catalunya BARCELONA Tech (UPC) C/ Colom, 11 08222 Terrassa (Barcelona), Spain
A. Gorobets
Affiliation:
Keldysh Institute of Applied Mathematics, 4A, Miusskaya Sq., Moscow 125047, Russia
A. Oliva
Affiliation:
Heat and Mass Transfer Technological Centre (CTTC), Universitat Politècnica de Catalunya BARCELONA Tech (UPC) C/ Colom, 11 08222 Terrassa (Barcelona), Spain
*
Email address for correspondence: arnau@cttc.upc.edu

Abstract

Backward-facing step (BFS) constitutes a canonical configuration to study wall-bounded flows subject to massive expansions produced by abrupt changes in geometry. Recirculation flow regions are common in this type of flow, driving the separated flow to its downstream reattachment. Consequently, strong adverse pressure gradients arise through this process, feeding flow instabilities. Therefore, both phenomena are strongly correlated as the recirculation bubble shape defines how the flow is expanded, and how the pressure rises. In an incompressible flow, this shape depends on the Reynolds value and the expansion ratio. The influence of these two variables on the bubble length is widely studied, presenting an asymptotic behaviour when both parameters are beyond a certain threshold. This is the usual operating point of many practical applications, such as in aeronautical and environmental engineering. Several numerical and experimental studies have been carried out regarding this topic. The existing simulations considering cases beyond the above-mentioned threshold have only been achieved through turbulence modelling, whereas direct numerical simulations (DNS) have been performed only at low Reynolds numbers. Hence, despite the great importance of achieving this threshold, there is a lack of reliable numerical data to assess the accuracy of turbulence models. In this context, a DNS of an incompressible flow over a BFS is presented in this paper, considering a friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}$) of 395 at the inflow and an expansion ratio 2. Finally, the elongation of the Kelvin–Helmholtz instabilities along the shear layer is also studied.

Type
JFM Papers
Copyright
© 2019 Cambridge University Press 

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