Velocity measurements of streamwise roll cells in rotating plane Couette flow

For the first time, quantitative velocity measurements in rotating plane Couette flow are demonstrated. Particle image velocimetry is used at a low Reynolds number with anti-cyclonic, i.e. destabilising rotation, where the instability is expected to give rise to steady streamwise-oriented roll cells. The streamwise and spanwise velocities of the roll cells were measured on the centreplane of the flow and at two planes on either side. The streamwise velocity is spanwise periodic with an amplitude variation approximately ±42 % of half the velocity difference between the moving walls. The wall-normal velocity was estimated by assuming steady, laminar and streamwise-independent flow. Despite the large amplitude of the disturbance, both the spanwise wave length and amplitude ratio between the streamwise and wall-normal components were close to what is obtained from linear theory. A splitting event of a roll cell was also captured by the velocity measurements and its development followed in time.