Abstract
An experimental study has been conducted to investigate the effects of surface roughness on the profile loss of a flat-plate with a contoured wall. All of the measurements have been conducted for the suction side pressure gradient of a high-lift low pressure turbine airfoil at the fixed Reynolds number (Rec) and freestream turbulence intensity (Tu) of 1.2 · 105 and 3.2%, respectively, representing a cruise condition. The time-resolved streamwise and wall-normal velocity fields for three different surface roughness values of Ra/C · 105 = 0.065, 4.417 and 7.428 have been measured with a 2D hot-wire probe. For the smooth surface, a laminar separation bubble forms from about 60% of the chord; and laminar-to-turbulent transition occurs during reattachment. Since the portion of turbulent flow over the flat-plate is relatively small, the overall profile loss is mainly determined by the momentum deficit generated during transition. Increased roughness decreases the maximum height and length of the separation bubble but does not affect the separation bubble onset location. The beneficial effects of increased surface roughness on the profile loss appear in the separated shear layer and reattachment. Increased surface roughness increases turbulent mixing in the separated shear layer. Thus, the shear layer thickness and momentum deficit are reduced. In addition, increased surface roughness reduces the length scale and turbulence intensity of the shed vortices. Consequently, turbulent mixing and momentum deficit during reattachment of boundary layers are decreased, resulting in a lower profile loss.
Experimental investigation of separated shear layer from a leading edge subjected to various angles of attack with tail flap deflections
