Experimental evaluation of square bar and fractal grid-generated turbulent flow inside recirculating water tunnel

High resolution, two dimensional PIV measurements of grid-generated turbulence in the US Naval Academy's recirculating water tunnel (1.8m test section with 0.41m x 0.41m cross sectional area) are presented for two different grid designs. The first grid is a uniform square bar grid with mesh width, M$=$3.9cm, bar thickness t$_{\mathrm{0}}=$1cm, a streamwise thickness of 1cm and resulting solidity of 44{\%}, similar to the conventional grid used by Krogstad and Davidson (2012). The other is Mazellier {\&} Vassilicos' (2010) square fractal grid, SFG17, with fractal iteration count, N$=$4, thickness ratio t$_{\mathrm{r}}=$17 and length ratio L$_{\mathrm{r}}=$8. Grid patterns differ from the published designs by a circular hole with 4.30cm diameter in the middle that will accept, in future experiments, a shaft connected to an axisymmetric rotating wake generator with diameter, D. Grids were designed to generate turbulence of specific integral length scale of O(D) and intensity of 6{\%} at the prescribed downstream location. Mean tunnel centerline velocity is 2 m/s and measurements are made in a streamwise vertical center plane with nominal individual field of view (FOV) of 12x8 cm$^{\mathrm{2}}$. Spatial coverage in the test section is accomplished by ``tiling'' individual FOV with approximately 2cm overlap. Results will focus on characterizing resulting turbulence in the test section and discussion will include comparison between published results and the present measurements.