Numerical simulation (DNS and LES) of manipulated turbulent boundary layer flow over a surface-mounted fence

Abstract

Results from numerical simulations are presented for manipulated turbulent boundary layer flow over a surface-mounted fence, for a Reynolds number of Re h =3000 (based on fence height, h , and maximum inflow velocity, U ∞ ). First, a reference data set was provided from a Direct Numerical Simulation (DNS) using 51.6 million grid points to resolve all the relevant spatial scales of the flow. A Large-Eddy Simulation (LES), using 1.67 million grid points, was validated with this reference solution and compared with experimental data for the same Reynolds number. Then, manipulated flow cases were investigated applying time-periodic forcing through a narrow slot upstream of the flow obstacle. High-frequency forcing, with Str 1= f 1 h/ U ∞=0.60 , leads to about 10% reduction of the mean re-attachment length. A much stronger reduction of about 36% could be achieved by low-frequency forcing with Str 2= f 2 h/ U ∞=0.08 . In the latter case, large-scale coherent structures are created between the location of the disturbance and the fence, they roll over the flow obstacle (nearly unaffected) and in rolling downstream they still grow in size until they fill out the entire height of the separation zone behind the fence. In agreement with corresponding experiments of Siller and Fernholz in 1997 for a higher Reynolds number ( Re h =10500 ) the optimum forcing Strouhal number seems to be related to the low-frequency movement of the entire separation bubble and not to the instability mode of the separating shear layer.©2000 Éditions scientifiques et médicales Elsevier SAS