Rotating plane Couette flow at high rotation number

Flow structures in the rotating plane Couette flow facility at KTH (described in Tsukahara, et al. {\it J. Fluid Mech.} vol.~{\bf 648}) have been studied at high rotation numbers. The test section is 20~mm wide with a length of 1500~mm in the streamwise ($x$) and 360~mm in the spanwise ($z$) directions and can be rotated in the spanwise direction up to angular velocities of $\Omega_z \approx 0.6$~rad/s. The flow is characterised by: (1) the Reynolds number $Re$ based on the test section's half-width $(h)$ and half of the velocity difference between the moving walls, (2) the rotation number $\Omega = 2\Omega_z h^2/\nu$. For low rotation numbers the primary instability consists of streamwise-oriented roll cells, but Tsukahara, et al. showed the secondary instability in the form of wavy streamwise oriented roll-cells at $Re=100$ and $\Omega = 3 - 12$, whereas for higher $\Omega$, the flow structures again stabilize to streamwise-oriented roll cells. Here we find that at even higher $\Omega$ in the range $40 - 70$, a new type of secondary instability develops in the form of counter-rotating helical roll-cells. The structure of this instability, as well as other instabilities, are investigated by flow visualization as well as two-dimensional PIV-measurements in several $xz$-planes.