Visualisation of turbulent wedges under the influence of favourable pressure gradients using both shear-sensitive and temperature sensitive liquid crystals

Emmons discovered in 1951 that boundary layer transition occurred when some arrow shaped disturbances, or turbulent spots, formed randomly and grew as they propagated downstream until they merged and covered the entire flow field. Using Emmons’ spot hypothesis, and with a knowledge of the spot’s propagation rate and spreading angle, one could model the boundary layer intermittency distribution more accurately, hence the transition process

On the spreading of turbulent spots in nonisothermal laminar boundary layers under the influence of favourable pressure gradients

A comprehensive experimental investigation of the aerodynamic and thermal spreading of turbulent spots in favourable pressure gradients. Measurements were taken with shear stress and temperature sensitive paint, surface hot films, and hot wires on a flat plate in a low speed test facility. The turbulent spots were triggered with a frequency of 120 Hz at a location with a momentum thickness Reynolds-number of 400. The thermal spread was measured using a heated plate. The spreading angle of turbulence was found to depend on the acceleration parameter K = (n/U*U).dU/dx AC --- 0 0.25E-6 1.00E-6 Aerodynamic deg 6.9 5.9 3.3 Thermal deg 6.8 4.8 2.5 Spreading is slower in accelerating flow and the momentum boundary layer establishes the turbulent flow state faster than the thermal boundary layer. The measured spreading angles in zero pressure gradient are lower than the 10 deg identified in other experiments. A comparison between surface hot film and hot wire measurements away from the wall indicate that this is due to a lateral "overhang" of the turbulent regime which spreads slower close to the wall than some distance form it. <br/