2D PIV measurements in the near field of grid turbulence using stitched fields from multiple cameras

Abstract

We present measurements of grid turbulence using 2D particle image velocimetry taken immediately downstream from the grid at a Reynolds number of Re M = 16500 where M is the rod spacing. A long field of view of 14M 9 4M in the down-and cross-stream directions was achieved by stitching multiple cameras together. Two uniform biplanar grids were selected to have the same M and pressure drop but different rod diameter D and crosssection. A large data set (10 4 vector fields) was obtained to ensure good convergence of second-order statistics. Estimations of the dissipation rate e of turbulent kinetic energy (TKE) were found to be sensitive to the number of meansquared velocity gradient terms included and not whether the turbulence was assumed to adhere to isotropy or axisymmetry. The resolution dependency of different turbulence statistics was assessed with a procedure that does not rely on the dissipation scale g. The streamwise evolution of the TKE components and e was found to collapse across grids when the rod diameter was included in the normalisation. We argue that this should be the case between all regular grids when the other relevant dimensionless quantities are matched and the flow has become homogeneous across the stream. Two-point space correlation functions at x/M = 1 show evidence of complex wake interactions which exhibit a strong Reynolds number dependence. However, these changes in initial conditions disappear indicating rapid cross-stream homogenisation. On the other hand, isotropy was, as expected, not found to be established by x/M = 12 for any case studied.

Evolution of turbulence and in-plane vortices in the near field flow behind multi-scale planar grids

“…The experiment was conducted in the facility previously reported by Cardesa et al 4 This water channel is made of 10 mm thick perspex and has an 8 m long working section with a 0.9 × 0.5 m 2 cross section. Three mono-plane grids made of 2 mm thick stainless steel were installed at the entrance of the working section.…”

“…Other important ratio, the size of the thinnest bar to the width of the smallest hole, which we define here as ζ, were 33.45%, 16.67% and 11.43%, respectively. In order to make direct comparison with the turbulent flow behind the bi-plane grids of Cardesa et al, 4 the water channel was operated at the free stream velocity of U ∞ = 0.5 ± 0.02 ms −1 , producing a Reynolds number based on M 1 of CONV of Re M1 = U ∞ M 1 /ν = 18 750 ± 750. This gave Re λ ∼ 840 measured at x ≈ 1M 1 downstream of the bar, which was the streamwise position where the streamwise fluctuation velocity was at its maximum.…”

“…In terms of near field grid turbulence, some examples of DNS studies must be acknowledged, 8,10,22 although their Reynolds numbers are limited, typically one order of magnitude smaller than those in the experiments. This work is inspired by the near field PIV study of bi-plane grid turbulence by Cardesa et al 4 and builds on the LDA measurements of Krogstad and Davidson. 20 We carry out statistically independent two dimensional 2DPIV investigations in the near field turbulent wake generated by a conventional and two multi-scale planar grids.…”

“…Cardesa et al 4 studied the flow near bi-plane grids and Gomes-Fernandes et al 12 used monoplane multi-scale grids. In the former study, well converged velocity statistics in the near field of two types of conventional grids are reported.…”

“…There will normally be a considerable recirculation zone behind the grid bars, where strong reversed flows are found. At Reynolds numbers based on M (Re M ) of the order of 10 4 , the recirculation zone, determined by the distance from the grid to the apparent stagnation point (if any), may typically extend from x/M ≈ 1 4 to 1.75 20 depending on the grid geometry. Even though the resolution and signal-to-noise ratio of HWA is much better than most other techniques commonly adopted in turbulence measurements, its limitation of not being able to measure reversed flows makes it an unsuitable tool in regions very close to the grid.…”