Axially varying sand grain roughness in turbulent Taylor-Couette flow

We investigate the effect of axially alternating rough and smooth surfaces in a Taylor-Couette apparatus with Reynolds numbers up to 2×10⁶. We employ realistic roughness by applying industrial, waterproof sandpaper (grit P36 with a measured standard deviation of the roughness height k_rms = 170 μm) on the inner cylinder of the Twente Turbulent Taylor-Couette facility. We vary s/d from 0.42 to 5.05 where d is the gap width of the Taylor-Couette and s the axial length scale of the rough and smooth patches. By varying s/d we can tune the size of the Taylor rolls and thereby the overall drag (C_f or, equivalently, Nu_ω) the inner cylinder experiences. We will discuss how the drag as a function of the driving speed depends on s/d and we will connect these results to our local PIV measurements.