Abstract:
The validity and accuracy of method's of determining corrections to the measured velocity in a wind tunnel to compensate for the constraining effect of the wails are reviewed following recent experimental evidence from the R.A.E. 10 x 7 ft subsonic wind tunnel. It is concluded that such corrections, commonly known as 'blockage' corrections, can be successfully applied at Mach numbers up to 0.96 but some modifications are necessary to the formulae at present in use. The more important of these are outlined below. (1) The compressibility factor should be based on the corrected Mach number of the stream. (2) The ratio of 'solid' blockage (i.e., the blockage due to model excluding wake) to the peak wall velocity increment is not constant but depends on the length of the model and the Mach number of the stream. (3) The calculated solid blockage of a wing must be increased to allow for the presence of local supersonic flow. For wings of usual plan form, this may be done by an empirical factor which is a function of the rise in drag coefficient. (4) Addition of corner fillets to the tunnel gives rise to a larger percentage increase of the solid blockage than of the wall velocity increments. Formulae for the calculation of the longitudinal distribution of blockage increment due to any model, necessary to check the validity of the method in particular cases, are presented in a form which, it is hoped, will facilitate their use in any 10 x 7 wind tunnel. Formulae for the corresponding wall velocity increments, used to check the accuracy of the method by comparison with measured wail pressures, are also given.
