Smooth wall separation and reattachment at high Reynolds numbers

Semi-confined flows

= Underlying Flow Regime 3-34

Abstract

Separation of a turbulent boundary layer from a smooth wall caused by Adverse Pressure Gradient (APG) and subsequent reattachment of the separated shear layer and relaxation of the reattached boundary layer are extremely complex phenomena encountered in numerous flows of high interest for the aerospace industry, civil engineering, ground transportation etc. Its accurate prediction presents a serious challenge for the RANS modelling as well as for the scale-resolving approaches, such as LES and hybrid RANS-LES methods. For the latter, a major difficulty is associated with so called “grey area issue”, which shows up as a strong delay of transition from RANS to LES in the separated shear layer resulting in a significant deviation of CFD predictions from experimental data. The flow past the 2D NASA Wall-Mounted Hump (2DWMH) studied in the experiments of Greenblatt et al. [‌1] and by Naughton et al. [‌2] possesses all the above mentioned features, thus presenting, in terms of geometry, a relatively simple but nonetheless quite representative Test Case (TC) for the considered UFR. The experiments had been designed for CFD validation, and their results were used for this purpose in numerous computational studies including those carried out in the framework of specially organized international workshops http://cfdval2004.larc.nasa.gov/case3expdata.html [‌3] and in the course of two EU projects, ATAAC http://cfd.mace.manchester.ac.uk/twiki/bin/view/ATAAC/WebHome [‌4] and Go4Hybrid http://go4hybrid.mace.manchester.ac.uk/go4hybrid/bin/view/Main/WebHome [‌5]. The purpose of this document is to summarise results of the systematic numerical studies of the 2DWMH flow carried out within the Go4Hybrid project with the use of enhanced hybrid methods in a comparison with similar results obtained by Uzun and Malik [‌6] who employed Wall-Resolved LES of the flow on extremely large grids. Hence these results may serve as an additional dataset for validation of less expensive approaches. This analysis combined with a brief overview of RANS models performance for the 2DWMH test case presented at the NASA Turbulence Modeling Resource Portal https://turbmodels.larc.nasa.gov/nasahump_val.html [‌7] gives a clear idea on capabilities and restrictions of different turbulence modelling/simulation strategies with regard to the considered UFR, in general, and the 2DWMH test case in particular