Realisation of Fractal Grid-Induced Turbulence Strength with PTFV: Effects of Grid Geometry

Abstract

The unravelling of multilength-scale insert-generated turbulence, particularly, the induced-forcing plays critical role in the fundamental comprehension of energy formation and decay as a function of grid conformation. This study experimentally investigates the flow mechanical characteristics at ReDh = 4.1 × 104 for a regular-grid (RG), single-square-grid (SSG) and six 2D planar space-filling square-fractal-grids (SFG) of different fractal iterations (N), thickness ratios (tr) and blockage ratios (σ) via piezoelectric thin-film flapping velocimetry (PTFV). Thin-film’s tip-deflection (δrms) and voltage response (Vrms) analysis along the grids’ centreline reveals increasing flow fluctuation strength with increasing σ, tr and decreasing N, owing to higher shedding intensity of lower frequency, larger scale energy-containing vortices from thicker first iteration bar. However, higher: energy dissipation rate, centreline mean velocity decrement rate and local deceleration experienced in the turbulence decay region of larger tr grid, along with additional fractal scales lead to less potent flow-structure-interplay on thin-film undulation. More importantly, SSG-generated turbulence enables the generation of average (Vrms, δrms) and millinewton turbulence forcing Frms that are respectively, 9× and 5× larger than RG of similar σ, and 2× larger than the best performing N = 3 SFG. Our findings disclose the importance of grid geometrical management for effective utilisation of turbulence-generating grids in engineering applications.