Numerical investigation of diffuser sensitivity and test section flow in a supersonic free-jet wind tunnel

초록

This study presents a comprehensive numerical investigation of the influence of diffuser geometry on the nonlinear flow behavior within the test section of a supersonic free-jet wind tunnel. The research focused on flow development at the nozzle exit, pressure recovery at the diffuser outlet, and flow uniformity inside the test section. Large-scale CFD simulations were conducted in which operating conditions were kept constant whereas diffuser geometric parameters were systematically varied using design of experiments technique for an axisymmetric aerodynamic circuit. The results demonstrated that the diffuser geometry strongly governed the inherently nonlinear flow characteristics, where small geometric modifications led to disproportionate changes in pressure recovery and flow uniformity. Notably, nearly half of the simulated configurations failed to establish stable flow. The second-throat diameter and scoop angle were identified as critical parameters for initiating the flow, whereas the subsonic diffuser length was found to be the primary determinant of pressure recovery. The analysis of the test section confirmed that the jet assumes a natural conical shape, a phenomenon driven by the pressure imbalance between the jet core and the test chamber. Analytical expressions for diffuser efficiency and Mach uniformity index were introduced to quantify this behaviour. This study highlights nonlinear diffuser sensitivity as a key factor in achieving pressure recovery and flow uniformity, offering both theoretical insights and practical guidance for the design of supersonic free-jet wind tunnels.

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