Suppression of Turbulence by Self-Generated and Imposed Mean Flows

The first direct experimental evidence of the suppression of quasi-two-dimensional turbulence by mean flows is presented. The flow either is induced externally or appears in the process of spectral condensation due to an inverse cascade in bounded turbulence. The observed suppression of large scales is consistent with an expected reduction in the correlation time of turbulent eddies due to shearing. At high flow velocities, sweeping of the forcing-scale vortices reduces the energy input, leading to a reduction in the turbulence level.

Figure 1

(a) Time evolution of the total kinetic energy and instantaneous velocity fields measured at (b) $t=13\mathrm{s}$ and (c) $t=71\mathrm{s}$. (d) Spectrum of the total spectral energy of the flow at $t=(9–17)\mathrm{s}$. (e) Spectra of the turbulent velocity fluctuations before, $t=(9–17)\mathrm{s}$ (open squares, $N=200$), and after the formation of a single large vortex, $t=(61–79)\mathrm{s}$ (solid circles, $N=400$).

Figure 2

Instantaneous velocity fields of unbounded turbulence (a) and in the presence of an externally generated large-scale azimuthal flow. The size of the box corresponds to the analyzed fraction of the fluid cell $0.18\times 0.18{\mathrm{m}}^2$. Spectra of turbulence are computed with mean flow subtracted: before the large flow is imposed (open squares, $N=400$) and in the presence of the mean flow (solid circles, $N=200$). Third-order structure functions without (open squares) and with (solid circles) externally imposed mean flow (d).

Figure 3

Mean azimuthal velocity of the flow after the large-scale vortex generation (a),(c) and the derivative of its angular velocity (b),(d) during spectral condensation of the bounded turbulence (a),(b) and in the case of externally forced unbounded flow (c),(d).