Selection criteria of optimal conditions for supersonic tests in a blowdown wind tunnel

Abstract

A natural desire of a wind tunnel test engineer is to plan and perform a test that will resemble the actual flight conditions as closely as possible. Geometric similarity and identical flight and wind tunnel Mach numbers are the primary requirements and they can be achieved without much problems, but the reduced size of the model and the limits of the operating envelope of the wind tunnel prevent the achievement of desired similarity with respect to the Reynolds number, which is generally much lower in a wind tunnel than in flight. Pressurized blowdown wind tunnels, like the T-38 in the Military Technical Institute (Vojnotehnički institut), Belgrade, were designed to reduce this discrepancy and achieve high Reynolds numbers by raising the stagnation pressure of the test-section flow. It is shown, however, that not the Reynolds number but instead the constraints related to model size, load range of available instrumentation, available run time, high aerodynamic loads, etc., are often decisive in the selection of the conditions for a high-speed wind tunnel test.

Keywords

• wind tunnel
• aerodynamic testing
• supersonic flow
• test plan