Separated-flow transition at low Reynolds number impacted by bi-scaled rough surface

Experiments are carried out to study the traits of laminar boundary layer separation and transition induced by the semicircular leading-edge of constant thickness airfoil model having a bi-scaled rough surface. This roughness pattern is fabricated using sandblast process, wherein the mean of centerline-averaged roughness along the leading-edge is twice that of trailing edge of the model; compares well with that observed over an operationally worn turbine blade. Surface static pressure and the velocity are measured along the suction side midspan of the model, for the Reynolds number, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>R</mml:mi> <mml:msub> <mml:mi>e</mml:mi> <mml:mi>D</mml:mi> </mml:msub> <mml:mo>≅</mml:mo> <mml:mn>0.2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>5</mml:mn> </mml:msup> </mml:mrow> </mml:math> , based on the leading-edge diameter, and freestream turbulence, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>Tu</mml:mtext> <mml:mo>∼</mml:mo> <mml:mn>0.8</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> conditions. On comparing the results of smooth ( Case - I ) and rough surface ( Case - II ), the impacts of roughness on the time-mean flow field are evident. Although it is uncommon, for the present configuration, the onset of separation and thereby, subsequent laminar portion of the separation bubble is comparable for both cases . However, the transition is accelerated due to surface roughness, and thereby, the maximum bubble height is attained at a primitive location, followed by early reattachment. Eventual outcome of this relative analysis is that the separation bubble is approximately 9.2% shorter, with enhanced resistance to bursting. Despite the topological differences, the transition in both cases occurs in the outer shear layer primarily through inviscid Kelvin-Helmholtz ( K-H ) mechanism. Notably, in Case - II , the predominant shedding frequency closely aligns with the maximum amplification frequency associated with the Tollmien-Schlichting ( T-S ) instability mechanism. It implies the prevalence of T-S waves in the transition induced by surface roughness.