Experimental studies of turbulent spot parameters using thin-film heat-transfer gauges

An experimental investigation of the growth of turbulent spots occurring naturally in the boundary-layer transition process was conducted. Although the primary focus of the study was aimed at the gas-turbine environment (e.g., pressure gradients, Mach number, Reynolds number, and gas-to-wall temperature ratio), a wide range of freestream Mach numbers and pressure gradients were investigated which make the results of general interest. The freestream Mach numbers utilized in the study covered the incompressible and compressible subsonic ranges as well as the supersonic regime. The effects of a favorable pressure gradient upon the growth of these naturally occurring spots were also assessed. The state of the boundary layer was monitored by a large array of thin-film heat-transfer gauges on the surface of a flat plate which made it possible to track and monitor the growth of each individual spot as it convected in the streamwise direction. Turbulent spot leading-edge and trailing-edge convection velocities were calculated using a digital intermittency detector, while spot spreading angles were calculated from the rate-of-change of unsteady signal level with distance along the flat-plate model surface. The instrumentation and analysis techniques are described in detail, and some representative results are presented.