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Abstract
Experimental studies were completed to determine the effect of freestream turbulence on stagnation region heat transfer and to investigate the final stages of boundary layer transition to turbulence. The objectives of the first investigation were to determine the separate effects of turbulence intensity, integral length scale and Reynolds number on the heat transfer in the stagnation region of a circular cylinder. Five different turbulence-generating grids were used to obtain a wide range of freestream turbulence characteristics. The ratios of integral length scale to cylinder diameter ranged from 0.05 to 0.5 and turbulence intensities were in the range 1 to 15 percent. Mean heat transfer measurements were made using a heated cylindrical model and tests were performed at five cylinder Reynolds numbers ranging from 40000 to 175000. Heat transfer augmentation was found to be highly dependent on the integral length scale of turbulence, with smaller scales resulting in higher heat transfer. In addition, a new time-resolved heat transfer measurement technique was developed to investigate the relation between unsteady fluid motions in the vicinity of the stagnation point and corresponding surface heat flux fluctuations. Simultaneous velocity and heat flux measurements made at the stagnation point revealed a clear crosscorrelation between the two signals.
The objective of the second series of experiments was to obtain quantitative estimates of the growth and convection rates of naturally-occurring turbulent spots in a transitional boundary layer. These were estimated by use of wide-bandwidth, high spatial resolution heat transfer measurements on an instrumented flat plate in a compression-heated transient aerodynamic facility. Thirty-two thin-film heat transfer gauges were located along the model surface and high-speed multi-channel data sampling was used to track the coherent transitional activity along the plate. Experiments were performed in a zero pressure gradient at Mach numbers of 0.05 and 0.1. Freestream turbulence intensity was on the order of 0.5 percent and gas-to-wall temperature ratios of 1.1 and 1.3 were used. Special data reduction techniques were used to calculate the convection velocities of the leading and trailing edges of the turbulent spots and these results compared well with those for artificially-generated spots. The intermittency distribution in the transition zone showed good agreement with the "universal" distribution of Narasimha.
