Unsteady forcing of turbulence by a randomly actuated impeller array

Abstract We investigate the unsteady forcing of turbulent flow in a well-stirred reactor using opposing arrays pitched-blade impellers which randomly and independently reverse rotation. systematically explore dependence large-scale motions homogeneity isotropy turbulence upon forcing. identify three dimensionless control parameters: source fraction (the time spent clockwise motion), period an impeller Reynolds number. find timescale motion corresponds to T , average reversal, angular speed $$\varOmega$$ Ω fraction. As jet-stirred tanks, substantially increases kinetic energy, energy dissipation, integral length scale Taylor microscale number ( $$R_\lambda$$ R λ ) improves flow, provided is chosen optimally sufficiently large $$\varOmega > 10^3$$ T > 10 3 ); has relatively small influence. The must be matched speed: decreasing below this threshold results less intense, more inhomogeneous flow. Spectra two-point velocity increments demonstrate that injection dominated by axial shear generated across becomes prominent at smaller scales. However, even $$R_\lambda \approx 354$$ ≈ 354 signature can still detected near-dissipation-range statistics. These observations provide insight into optimisation mechanism transfer when generate confined vessels. Graphical abstract