An Experimental Study Of Surface Pressure Fluctuations In A Separating Turbulent Boundary Layer

Experimental surface pressure fluctuations measurements over a wide range of Reynolds number (1400

An experimental investigation is described which shows relationships between the noise spectra of a layer of air bubbles in a turbulent flow and a maximum stable bubble size which can exist in the same flow. An air bubble layer with individual bubble sizes greater than a maximum stable size was introduced into the boundary layer of water flowing along a smooth flat plate. It was found that the maximum stable bubble size was related to the turbulent kinetic energy dissipation as had been previously shown by other investigators for a variety of turbulent flows. A unique 'corner frequency' of the noise spectra which resulted from the bubble splitting down to a maximum stable size could be also related to the turbulent kinetic energy dissipation of the boundary layer flow. (Author).

An experimental study of techniques for modifying the surface shear stress, both mean and fluctuating, in turbulent boundary layers is reported. The surface shear along the test section of a 45.7cm square wind tunnel, with a downstream expansion section, was determined by means of surface-hot-wire-heat-transfer-gauges. Turbulent boundary layers in both zero and adverse pressure gradients were evaluated. The mean and fluctuating surface shear in a zero pressure gradient can be effectively reduced by employing a set of closely spaced fins. The fins protruded from the surface up through the sublayer. The fins produced an effective momentum defect for the boundary layer near the surface, which reduced the surface shear fluctuations by as much as 75 percent, two boundary layer thicknesses downstream, and 52 percent 15 boundary layer thicknesses downstream (the limit of the present experiment). The mean surface shear was also reduced by 35 to 25 percent for the same 2 to 15 boundary thicknesses downstream. The fins do not appear to produce a large increase in the thickness of the layer. No small size device, which could be mounted in the sublayer, was found that could develope a persistent increase in either the mean or fluctuating surface shear. Small scale vortex generators had no measureable effect on the flow for downstream distances greater than approximately 4 to 5 boundary layer thicknesses. Only the large scale vortex generator, normally use to delay boundary layer separation, produced an increase in surface shear at large distances downstream. (Author).

This report documents an experimental study of detaching and reattaching boundary layers and boundary layers downstream of a reattachment. Three low Mach number, fully turbulent boundary layers flowing on a flat surface have been.

This volume contains 37 contributions in which the research work is summarized which has been carried out between 1984 and 1990 in the Priority Research Program "Physik abgeloster Stromungen" of the Deutsche Forschungsgemeinschaft (DFG, German Research Society). The aim of the Priority Research Program was the inten sive research of the whole range of phenomena associated with separated flows. Physi cal models as well as prediction methods had to be developed based on detailed experi mental investigations. It was in accordance with the main concept of the research program that scientists working on problems of separated flows in different technical areas of application participated in this program. The following fields have been represented in the program: aerodynamics of wings and bodies, aerodynamics of auto mobiles, turbomachinery, ship hydrodynamics, hydraulics, internal flows, heat exchan gers, bio-fluid-dynamics, aerodynamics of buildings and structures. In order to concentrate on problems common in all those areas the emphasis of the program was on basic research dealing with generic geometric configurations showing the fundamental physical phenomena of separated flows. The engagement and enthusiasm of all participating scientists are highly appreciated. The program was organized such that all researchers met once a year to report on the progress of their work. Special thanks ought to go to Prof. E. A. Muller (Gottingen), Prof. H. Oertel jun. (Braunschweig), Dr. W. Schmidt (Dornier), Dr. H. -W. Stock (Dornier) and Dr. B. Wagner (Dornier), who had the functions of referees on those annual meetings.