A nearly isotropic turbulence generated by a pair of vibrating grids

A pair of vertically oscillating grids were applied to experimentally produce a region of nearly isotropic stationary turbulence in homogeneous fluids contained in a water tank. With the appropriate choice of grid mesh size (M), oscillating frequency (f) and stroke (S), and the distance between the grids (H), two distinct flow regions are generated: a highly turbulent region near each grid in the form of turbulent wakes and an approximately isotropic stationary turbulence located in the core region between the grids, as verified by extensive laser-Doppler velocimetry. The former is similar to that generated by a single vibrating grid, which was commonly used in mixed layer experiments. The latter flow region has essentially zero mean velocities, nearly equal magnitude of root-mean-square turbulent intensities in all three directions, and nearly −5/3 energy decay slopes, indicating that the turbulence has some properties of isotropic turbulence. This region of interest is chosen to be a domain where the variations among values of horizontal and vertical root-mean-square velocities are less than 15% and its height is found to be linearly proportional to H. In this region, the effective (overall) turbulent intensity may be represented by an empirical relation of the form q = CfS1.5M0.5H−n, where the experimental constant C ≈ 0.89 and the power constant n = 1.5 at least for 4 ≤ H/M ≤ 6. The present results confirm the expectation of Villermaux et al. [13] that the energy released in the system per unit time by each grid is additive. Other parameters of interest, such as the autocorrelation, the flow integral length scale, and the experimental uncertainties also are reported. Finally, the current flow apparatus can be conveniently adopted for many experimental studies such as particle or scalar dispersion in isotropic turbulence and simulation of premixed turbulent combustion [12].