Surface Roughness Impact on Boundary Layer Transition and Loss Mechanisms Over a Flat-Plate Under a Low-Pressure Turbine Pressure Gradient

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 freestream turbulence intensity (Tu) of 3.2% under Reynolds numbers of Re_c = 1. 2 · 10⁵ (cruise) and Re_c = 5.2 · 10⁵ (take-off). The time-resolved streamwise and wall-normal velocity fields for three different surface roughness values of R_a/C · 10⁵ = 0.065, 4.417, and 7.428 have been measured with a 2D hot-wire probe. At the take-off condition (Re_c = 5.2 · 10⁵), attached flow transition occurs, and increased surface roughness increases the loss. For all of the surfaces, momentum deficits in the laminar to early transition region (γ ≈ 0.05) are similar. For the intermediate transition (γ ≈ 0.5), increased roughness reduces the Reynolds stress and accelerates the breakdown of large-scale turbulent spots into small-scale turbulent eddies. Therefore, turbulent energy and momentum deficit are decreased for rough surfaces. For the late transition (γ > 0.9), transitional boundary layers become similar to turbulent boundary layers, and increased surface roughness increases turbulent mixing, boundary layer thickness, and, hence, the momentum deficit. On the other hand, at the cruise condition (Re_c = 1.2 · 10⁵), separated flow transition occurs and increased surface roughness decreases loss. Since the portion of turbulent flow 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, reducing the shear layer thickness and momentum deficit. In addition, increased surface roughness reduces the length scale and turbulence intensity of the shed vortices. Consequently, turbulent mixing and momentum deficit during the reattachment of boundary layers are decreased, resulting in a lower profile loss.