Relating Skin-Friction Drag Reduction to Riblet Tip Radius

Water channel experiments of a prototype riblet film are performed to measure the effect of riblet tip radius on skin-friction drag reduction (DR). Experiments show that the change in drag reduction due to the tested finite riblet tip radius R = R_1 > 0 is DR_R=0 − DR_R=R_1 ≈ 3.5 ± 1.0% at a near-optimal riblet size. These experimental results are also compared to a riblet performance model. For the same tested finite tip radius, the model predicts DR_R=0 − DR_R=R_1 ≈ 2.6%, which falls within the ± 1.0% uncertainty of the experimental results at this riblet size. The performance model is further applied to predict the performance losses due to a finite tip radius for flight conditions corresponding to a single-aisle passenger aircraft in cruise. These performance losses are up to 22% for optimally sized riblets with tip radii up to 4 microns.