Experimental study of boundary layer in compressible flow using hot film sensors through statistical and qualitative methods

The boundary layer state, laminar, transitional or turbulent, greatly affects an aircraft’s aerodynamic performance, specifically the skin friction force. In the present research, the characteristics of the boundary layer and the transition region have been studied experimentally through the statistical methods (RMS and Skewness), and also qualitative and frequency analysis using hot film sensors output. The tests have been performed at Mach numbers from 0.25 to 0.85 with the maximum Reynolds number of 11.6 million per meter, and the maximum angle of attack of 9° on a supercritical airfoil. The validation of the methods mentioned, which are usually used for incompressible flow, has been investigated for compressible flow with shock waves with detailed assessment. Results have shown that in the shock- free case, the boundary layer regions are identifiable by statistical analysis such as RMS and Skewness, as in incompressible flow. However, in the presence of shock waves, because of sudden, local and instantaneous changes in the output voltage level of hot films, determination of the dominant pattern in the boundary layer becomes impossible using the RMS of the total data. Therefore, first, the signal must be divided into different regions and then, with identifying the cause of voltage fluctuations in each region and its effect on the behavior of the steady state boundary layer, appropriate intervals for RMS calculation can be determined. The frequency analysis is valid and can be used in all conditions tested. In these tests the trip strip performance in forming forced transition was also evaluated (through using hot films and pressure coefficient distribution) and the characteristics of the boundary layer were studied. Results have illustrated that the trip strip, forced transition, compared with the free transition, causes sudden boundary layer turbulence, limitation of the transitional region, a change in the shock structure and pressure distribution.