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Internal Report

Flow control of boundary layers and wakes

Author Document Type Year Download File size
Jens Fransson Doctoral thesis 2003 Download 11.9 Mb
Id
ISSN 0348-467X
ISRN KTH/MEK/TR--03/18--SE

Abstract

Both experimental and theoretical studies have been considered on flat plate boundary layers as well as on wakes behind porous cylinders. The main thread in this work is control, which is applied passively and actively on boundary layers in order to inhibit or postpone transition to turbulence; and actively through the cylinder surface in order to effect the wake characteristics. An experimental set-up for the generation of the asymptotic suction boundary layer (ASBL) has been constructed. This study is the first, ever, that report a boundary layer flow of constant boundary layer thickness over a distance of 2 metres. Experimental measurements in the evolution region, from the Blasius boundary layer (BBL) to the ASBL, as well as in the ASBL are in excellent agreement with boundary layer analysis. The stability of the ASBL has experimentally been tested, both to Tollmien--Schlichting waves as well as to free stream turbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is found for the streamwise amplitude profiles and the phase velocity when compared with linear spatial stability theory. However, the energy decay factor predicted by theory is slightly overestimated compared to the experimental findings. The latter disturbances are known to engender streamwise elongated regions of high and low speeds of fluid, denoted streaks, in a BBL. This type of spanwise structures have been shown to appear in the ASBL as well, with the same spanwise wavelength as in the BBL, despite the fact that the boundary layer thickness is substantially reduced in the ASBL case. The spanwise wavenumber of the optimal perturbation in the ASBL has been calculated and is beta = 0.53, when normalized with the displacement thickness. The spanwise scale of the streaks decreases with increasing turbulence intensity (Tu) and approaches the scale given by optimal perturbation theory This has been shown for the BBL case as well. The initial energy growth of FST induced disturbances has experimentally been found to grow linearly as Tu^2Re_x in the BBL, the transitional Reynolds number to vary as Tu^-2, and the intermittency function to have a relatively well-defined distribution, valid for all Tu. The wake behind a porous cylinder subject to continuous suction or blowing has been studied, where amongst other things the Strouhal number (St) has been shown to increase strongly with suction, namely, up to 50% for a suction rate of 2.5% of the free stream velocity. In contrast, blowing shows a decrease of St of around 25% for a blowing rate of 5% of the free stream velocity in the considered Reynolds number range.