Abstract
The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.
Highlights
The capability of CFD solvers in predicting real flows encountered in engineering applications strongly depends on the assumptions made for the closure of the Reynolds shear stress, requested into RANS based schemes
Higher values of the vorticity highlight the presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks. The breakdown of such structures leads to the transition of the boundary layer, and the knowledge of their characteristic dimension and how they change as a function of Free Stream Turbulence Intensity (FSTI) properties may be exploited to apply flow control strategy
Large Eddy Simulation (LES) data of the boundary layer developing over a flat plate have been discussed in the paper
Summary
The capability of CFD solvers in predicting real flows encountered in engineering applications strongly depends on the assumptions made for the closure of the Reynolds shear stress, requested into RANS based schemes. The reference condition has been set to match the ERCOFTAC data reported in Roach and Brierley [15], providing validation of the accuracy of the present work, as provided by Voke and Yang in several works [16,17,18] in order to validate their LES results Both the turbulence intensity magnitude and the length scale have been varied independently in order to clearly highlight the effects of these parameters on the statistical properties of the transition process, and on the coherent structures involved in the process itself. The auto-correlation of instantaneous snapshots has been computed to identify the effects of the parameter variation on the streak spacing, following the work of Matsubara and Alfredsson [20]
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