Large Eddy Simulation of Transitional Separated-Reattached Flow over Geometries Characterized by Different Aspect Ratios and with Different Intensities of Free Stream Turbulence

Abstract

In the current study, changes in the physics of transitional separated-reattached flow due to changes of a geometry nature and an increase of intensity of free stream turbulence have been investigated numerically using the large eddy simulation approach. Numerical simulations have been carried out using the Open FOAM tool box. Six case studies are selected and divided into two groups of the flows: a low level of intensity of free stream turbulence (< 0.2%) and a high level of intensity of free stream turbulence (3.7%). Each group involves three geometrical shapes: a two-dimensional flat plate, a three-dimensional geometry with an aspect ratio value of 1 and a three-dimensional geometry with an aspect ratio value of 2. To the best of the author’s knowledge, the current study is the first work to explore transitional separated-reattached flow over three-dimensional geometries. In a comparison among the case studies, the separation bubble that formed on the flat plate is longer than that on other geometries, leading to longer temporal and spatial evolution of the transition. In addition, maximum values of the Reynolds stresses in the flat plate are larger than that in other geometries. Furthermore, all case studies show that the transition in the free shear layer is driven by the Kelvin-Helmholtz instability mechanism. Spectral analysis is carried out to cover all the computational domains employing both Fourier transform and wavelet power transform methods. In the current geometries for both incoming flows (with high and low levels of intensity of free stream turbulence), the regular shedding frequencies are in a good agreement with that reported in the literature. In addition, these frequencies are compatible with the Kelvin-Helmholtz instability conditions. Moreover, the spectral analysis indicates that the low frequency of the free shear layer flapping is absent. The evolution of coherent structures is identified by performing flow visualisation techniques. Different evolution processes of transformation of large-scale structures from Kelvin-Helmholtz rolls to hairpin structures are observed depending on the geometry shapes and on the level of intensity of free stream turbulence. The development of the turbulent boundary layer after the reattachment is also examined. For all case studies used here, a dominant observation is that there is no apparent effect of the geometry nature on the delay in the recovery of the reattached turbulent boundary layer.Iraq Governmen