STUDIES ON THE EFFECTS OF FREE-STREA TURBULENCE ON BOUNDARY LAYER CHARACTERISTICS

Abstract

The knowledge of the turbulent boundary layer growth on e smooth flat plate particularly under low level of turbulence in the free;.Stream is available in plenty since it has been subjected time and again to quite detailed investigations in the past. However9 there are many practical fluid flow situations wherein the turbulent boundary layer exists under high free-stream turbulence. Information regarding this aspect is found insufficient mainly because of the following reason: Intensity and scale of turbulence are the two essential parameters which'have to clearly specified in order to understand the structure of turbulence. Obviously each of the above parameters will influence the boundary layer growth in its own characteristic manner. Investigations on the behaviour of the turbulent boundary layer on a flat plate under high intensities of turbulence in the free stream are adequate, however, studies quantifying the influence of the scale of turbulence are found to be almost non-existent. This has rendered the studies esta-blishing the behaviour of flat plate turbulent boundary layer under high free-stream turbulence incomplete. In the present study, efforts have been made to isolate the individual influences of the intensity and scale of turbulence on the behaviour of a flat plate turbulent boundary layer. Variations in the velbcity (v) profiles, boundary layer thicknesses and the skin-friction coefficient for various intensities and scales of Wrbule-nce in the free-stream have been investigated: On the basic. of above study, the following main conclusions have been drawn: 1. Rapid flattening of the velocity profiles is obser-ved with increase in the macro-scale of turbulence: however, an inconsistent trend is observed with progressively increasing intensities of turbulence in this regard. 2. The boundary layer thicknesses are observed to be relatively insensitive to the scale of turbulence but arc affected significantly by the intensity of turbulence. 3. The local skin-friction coefficient cf is signifi-cently affected by the macro-scale of turbulence. Progressively increasing intensities of turbulence in the free-stream corresponded with en inconsistent trends in the value of cf.