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 Re_Dh = 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 (t_r) and blockage ratios (σ) via piezoelectric thin-film flapping velocimetry (PTFV). Thin-film’s tip-deflection (δ_rms) and voltage response (V_rms) analysis along the grids’ centreline reveals increasing flow fluctuation strength with increasing σ, t_r 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 t_r 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 (V_rms, δ_rms) and millinewton turbulence forcing F_rms 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.