Measurement of the viscous sublayer in near-separated flows

This paper investigates experimentally the development of the viscous sublayer in a two-dimensional incompressible turbulent wall boundary layer under severe pressure gradients. The wall was also moderately heated and the influence of heat transfer on the development of the viscous sublayer was included. A semi-empirical equation for the thickness of the viscous sublayer: $$\delta _s = 11.5\frac{\nu }{{U_\tau }}(0.61\frac{{T_\omega }}{{T_s }})^{1/2}$$ was derived, which holds everywhere except closely near the separation point of the boundary layer. The measurements were made on a flat plate in a test section 1.7 m long and 0.8 m wide. The height and shape of the top surface of the test section could be varied, and thus it was possible to imposed different pressure gradients on the flow. Specially designed fine probes facilitated the measurement of the velocity distribution very close to the wall.