Conditional Sampling of Dissipation in Large-Scale Rotational Regions of Grid Turbulence

Abstract

Multi-point hot-wire measurements in grid turbulence were used to conditionally average the dissipation of turbulent kinetic energy over inertial-scale features of a turbulent velocity field. Velocities were measured at 5 separate spatial points arranged in the cross-configuration shown in Figure 1. The probes were all located in a cross-stream plane. There are 4 single hot-wires at the edges, with an x-wire at the center. Six separate velocity components were thus measured simultaneously. The widest spacing between any two probes (=2L in Fig. 1) spans 0.65 of the integral length scale A. The flow features used in the conditioning described below are thereby inside the inertial range and much larger than the the worms observed at the relatively small scales (see e.g. Jimenez et al., 1993). The wind-tunnel turbulence was generated with a grid of a very large mesh-size (M =19 cm), producing turbulent Reynolds number Reλ of about 200, while retaining good transverse homogeneity of the turbulence. The dissipation was approximated with the socalled pseudo- or surrogate dissipation ε r ’ based on one or two velocity gradients, to avoid the kinematic constraints previously described by Thoroddsen (1995).