Reappearance of azimuthal waves in turbulent Taylor–Couette flow at large aspect ratio

Velocity field data were acquired for Taylor–Couette flow in the annular gap between an inner rotating cylinder and a stationary concentric outer cylinder using particle image velocimetry (PIV) in a meridional plane of the annulus. Data were acquired for several rotational Reynolds numbers with the ratio of the rotational and critical Reynolds numbers ranging from 6 to 200, corresponding to flow states ranging from laminar wavy Taylor vortex flow to turbulent Taylor vortex flow. Spatial correlations of velocity fluctuations were found to exhibit a sharp decrease as RR, the ratio of Reynolds number to the critical Reynolds number, increases from 16, indicating the disappearance of azimuthal waves and the onset of turbulence, reaching a minimum at R=18R=18. However, correlation lengths subsequently increase with increasing RR, displaying a secondary peak from 20⩽R⩽3820⩽R⩽38, suggesting the reappearance of azimuthal waves. The reemergence of azimuthal waves was confirmed through other methods including analysis of the axial velocity. At still higher Reynolds numbers, correlation lengths decay once again. The magnitude and Reynolds number associated with the secondary peak in the fluctuation velocity correlations were found to be dependent on the location of the basis point used in the calculations. Specifically, correlation lengths were longest near the outer cylinder in the inflow boundary and near the inner cylinder in the outflow boundary. This was shown to be due to the spatial dependence of Reynolds stresses in turbulent Taylor–Couette flow.