Unsteady Periodic Forcing of Passive Grid-Generated Turbulence

Experimental wind tunnel measurements of grid-generated turbulence were carried out to better understand the dynamic response and to assess the scaling laws for non-stationary, non-equilibrium turbulence evolving within a flow composed of large, time-periodic fluctuations of the freestream velocity. The experiments were conducted within the unsteady low-speed wind tunnel at the University of Colorado Boulder using a conventional rectangular grid with a mesh size of M = 0.10 m and a blockage ratio of 44%. Synchronized measurements were collected from a single-component hotwire anemometer and a high-speed planar particle image velocimetry system at a streamwise position of x = 2.90 m downstream from the grid (x/M = 29). The freestream velocity within the wind tunnel test section was dynamically varied in a periodic fashion using a set of louvers situated far upstream of the test-section, producing a time-varying periodic velocity with a mean of U = 13.25 m/s and an amplitude of σ_u = 2.70 m/s at a frequency of f = 1 Hz. These conditions provided a mean turbulence Reynolds number based upon the Taylor microscale of Re_λ ≈ 350. The hysteretic nature of grid-generated turbulence was analyzed and will be discussed, for example quantifying the velocity variances, turbulent kinetic energy, integral length scales, and Taylor microscale as a function of the unsteady periodic freestream velocity and the associated streamwise pressure gradient. In addition, the local equilibrium hypothesis (Gotto & Vassilicos, Fluid Dyn. Res., 2016) and unsteady dissipation scaling (Zheng et al., JFM, 2023) will also be applied and presented.