Separated Flow Regimes Research Articles

The Turbulence Characteristics of Two-Dimensional Wall-Jet and Wall-Wake Flows

Measurements of mean and fluctuating hydrodynamic properties of two-dimensional, constant property, wall-jet and wall-wake flows are reported. The mean properties include static pressure, velocity and wall shear stress; the fluctuating properties include the three fluctuating components of velocity, the local turbulent shear stress, uv, and u-spectra. The measurements were carried out for four values of the velocity ratio, UC/UG, viz. 0.75, 0.91, 1.33, and 2.3; for values of the ratio of lip thickness to slot height, t/yC, of 0.126 and 1.14; and for values of nondimensional distance, x/yC, between zero and 150. The results demonstrate the difficulties involved in deriving a satisfactory prediction procedure for flows of this type. In the upstream region, normal pressure gradients and separated flow regimes imply the need for elliptic equations. Also in the upstream region, the u-spectra measurements reveal preferred frequencies although the energy contained at the preferred frequency is never more than 25 percent of the total energy. Prandtl mixing length and the Prandtl-Kolmogorov turbulence length scale are deduced from measurements and shown to be very nonsimilar for all values of x/yC. The measurements are reported in sufficient detail to permit the detailed testing of prediction procedures.

The Part Played by Wind Tunnels in Modern Aeronautics

Summary:The wind tunnel has been the main instrument for experimental research in aeronautics since the turn of the century and its history is as brief as powered flight itself. It has always played a leading rdle in aeronautics right from the time of the Wright Brothers to the present day.The early form of wind tunnels as developed at the National Physical Laboratory and other research establishments have been replaced over the years by larger and more elaborate facilities, each having a particular speed range and specific role in aeronautical research and development. Modern wind tunnels having continuous operation of the type installed at the Royal Aircraft Establishment involve powers of up to 100 000 h.p. and cost in the neighbourhood of £10 000 000. In spite of this cost it is a sobering thought that, if anything, the overall cost of each data point is less today than it was in the early days of aeronautics. Such tunnels are highly complex instruments for research and development and although they play a rôle similar in many respects to that of a computer, they have the advantage of always dealing with a real fluid. Indeed no known computer could cope with the range of problems that can be solved in a wind tunnel, ranging from such complex problems as the structure of turbulent flow to the pressures in separated flow regimes, to quote just two examples. This fact must always be kept in perspective when arguments are presented for more and more work on digital computers to solve basic fluid motion problems. Of course we cannot overlook the problems of interpretation of wind tunnel data and the difficulties encountered in extrapolating these data to flight conditions.The development of theories for such problems together with answers is of ever increasing importance, especially as the problems in aeronautics are becoming more and more complex. These solutions have the important function not only of high-lighting certain aspects of the flow, but of pin-pointing the important parameters in a problem together with providing better than order of magnitude results. However, theories must be based on physical facts and these can only be determined in the final analysis from experiments.The wind tunnel has three highly significant roles in modern aeronautics:(i) Experimental research as a forerunner to future theoretical research.(ii) Experimental research as a confirmation and extension of theory.(iii) Current research and development work on a given aircraft.The current trend in bringing strong theoretical and experimental groups together, plus the employment of rapid measurement techniques in wind tunnels, promises the greatest possible productive utilisation of these highly important and expensive facilities. In most cases today they are playing a strong, if not the leading, role in aeronautical development.