Isotropic scaling and interface refinements in the design of low-speed wind tunnel bellmouths and contractions

The aerodynamic performance of low-speed wind tunnels depends critically on bellmouth and contraction geometry. Although systematic design methodologies have previously been established, the influence of three-dimensional isotropic geometric scaling on their aerodynamic performance has not been investigated. The present study systematically examines the effects of isotropic scaling and targeted geometric refinements in the form of converging wall tapers using steady-state Reynolds-averaged Navier–Stokes (RANS) simulations coupled with the shear-stress-transport (SST) k-ω turbulence model. Near-optimal bellmouth geometries were identified through systematic parametric investigations, while a previously established near-optimal contraction geometry with a contraction ratio (CR) of 9 was adopted. Scaling effects were evaluated by isotropically reducing the bellmouth and contraction geometries by factors of 25 and 12.5, respectively, and comparing the aerodynamic performance of the scaled and unscaled configurations over the corresponding Reynolds-number ranges. The results demonstrate that the previously established near-optimal non-dimensional geometric parameters remain applicable under isotropic scaling, with the scaled and unscaled configurations exhibiting consistent aerodynamic performance over the investigated Reynolds-number ranges. The recommended contraction produced a highly uniform exit flow, with velocity non-uniformity below 0.4% at all three investigated velocities (10, 25, and 50 m/s), while replacing the square cross-section of the near-optimal bellmouth intake duct with a regular octagonal cross-section reduced the velocity non-uniformity at the bellmouth exit by approximately 33%. Furthermore, introducing a 1° inward taper downstream of the octagonal bellmouth exit reduced streamwise velocity non-uniformity and boundary-layer thickness, with the final proposed bellmouth design achieving an overall reduction in exit-flow non-uniformity of approximately 50% compared with the original near-optimal bellmouth configuration with a square cross-section. For the scaled and unscaled configurations of the near-optimal contraction, a 0.5° inward taper upstream of the contraction reduced the magnitude of the adverse pressure gradient near the contraction entrance, while a 0.25° inward taper downstream of the contraction further improved downstream flow quality, reducing streamwise velocity non-uniformity by approximately 30–40% and shortening the downstream distance required to achieve a highly uniform flow before the test section. These findings provide generalized non-dimensional design guidelines for geometrically similar laboratory-, industrial-, and research-scale low-speed wind tunnels.