Abstract:
The present research programme of the National Physical Laboratory is concerned with the influence exerted on model forces and surface pressures by the thick, laminar boundary layers characteristic of rarefied, near-continuum gas flows. Two earlier N.P.L. reports discussed aspects of this viscous interaction problem, for circular cones at zero incidence m and for circular cylinders placed normal to the oncoming stream (2). In both cases the authors were concerned with predicting the measured pressure distribution from a knowledge of the inviscid (i.e. high Reynolds number) pressure distribution and the boundary-layer growth in the low-density flow. For attached flows away from the stagnation region reasonably good agreement could be obtained between simple viscous-interaction theories and experiment. These investigations were of a rather preliminary nature and their scope was partly limited by some of the experimental difficulties inherent in low-density aerodynamics. A natural development of this work would be to look at the structure of a simple laminar boundary layer and its influence on the surface pressure distribution, and then to investigate the flow changes which occur when separation is provoked. This is the aim of the experiment described in the present report. A flat plate at zero incidence was used as the basic surface; separation was induced by forward-facing steps of different heights placed normal to the stream.
