Suppression of large-scale azimuthal modulations in a von Kármán flow using random forcing

Abstract

A stable flow structure in a fully turbulent von Kármán swirling flow with counter-rotating disks is examined experimentally using global characterization methods (particle image velocimetry and ultrasonic profiling) and local methods at the center (ultrasonic Lagrangian tracking). The flow exhibits an azimuthal modulation in the central transverse plane, rotating slowly in the azimuthal direction. Contrary to previous studies, the bifurcation does not emerge from jumps of the shear layer plane but is symmetric to the transverse plane. To illustrate the underlying flow topology, a low-order model consisting of three superimposed modes is presented. A pseudo-random reversal of the disk rotation suppresses this dominant flow structure, retaining only the prototypical mean inward-pumping mode. Variations of the forcing method are studied for Reynolds numbers between 25 000 and 100 000, characteristic reversal times between 13 and 3000 Lagrangian integral times, and two reversal patterns. In contrast to a regular disk reversal, the employed pseudo-random sequence does not introduce any spurious timescales. The simple, yet efficient method is shown to robustly suppress the low-frequency signature of the azimuthal modulations over all Reynolds numbers under investigation. Globally, this yields a strong improvement in axisymmetric homogeneity and local statistical stationarity at moderate timescales. Also in the center of the cell, the removal of the highly anisotropic large-scale modulations enhances the transverse isotropy and homogeneity of the fully resolved turbulent flow and otherwise leaves the small-scale turbulent features largely unaffected. A description of the flow with the low-order model consequently reduces to only a single mode.