on simulating the lower portion of the natural boundary layer

Atmospheric Environment Pergamon Press 1973. Voi. 7, pp. 225-226. Printed in Great Britain.

LETTER ON

SIMULATING

TO THE EDITORS

THE LOWER PORTION OF THE NATURAL BOUNDARY LAYER

RECENTmeasurements have shown that the natural adiabatic boundary layer height is about 700 m. The maximum boundary layer height that can be generated in a wind tunnel is about 1 m. This gives a model scale of 1: 700. It shouid be noted that to generate 1 m thick boundary layer in a wind tunnel one generally needs an upstream stretch of about 30 m although it is possible to grow the boundary layer artificially in about 5 boundary layer heights, COUNIHAN (1969). For a typical high rise building of 100 m height, this gives a model height of 0.14 m which is too small to reproduce any minor details of secondary importance in the model.

X

FIG. I. Varying fraction of the boundary layer heights to be simulated. The present trend is to simulate the entire boundary layer height, namely 6, in a wind tunnel (see FIG. 1). We are not aware of any investigations as to the need of simulating the upper part of the boundary layer. For some purposes, it may be necessary only to simulate the lower portion of the boundary layer, say 3-4 times the height of the model. The veIocity profile will look like the curve 1,2, 3 or 4 (see FIG. 1) where curve 5 is the natural velocity profile. If the above conjecture is true, it should be possible to use bigger models. For example, if we need only to simulate a fraction of boundary layer, say 3 times the height of a model of a building 100 m high, the model scale can be increased up to 1: 300. allowing models of @33 m height to be used instead of 0.14 m for the same boundary fayer generating m~h~ism in the laboratory. Tests of above nature are planned in the near future in our laboratory. We intend varying the fractional boundary layer height, i.e. &‘/S up to the value of 1.0, to study flow characteristics around the model, and thereby determine the minimum fraction of the lower boundary layer needed for simulating proper flow characteristics in various applications. The above ideas have emerged out of discussions with Bengt Wiren of our laboratory, to whom appr~iation is expressed. Department of Aeronautics The Royal Institute of Technology Stockholm, Sweden

v. KR. SHARAti

REFERENCE CO-N 1. (1969) An improved method of simulating atmospheric boundary layer in a wind tunnel. Atmospheric Environment 3, 197-214. 225