An Efficient CAA Strategy for Studying Flow-Acoustics Coupling Phenomenon in Low-Speed Closed-Return Open-Jet Wind Tunnels

The low-frequency flow-acoustics coupling phenomenon pertaining to low-speed closed-return open-jet wind tunnels (OJWT) is studied using an efficient computational aero-acoustics (CAA) method. The computer model is a 2-D axisymmetric approximation of a realistic wind tunnel geometry, and it renders a linear airline ductwork that retains the longitudinal variation of the cross-sectional area. The looping of the airline flow is modeled using cyclic boundary conditions together with a fan model and a heat exchanger model to maintain energy balance for the closed system of the wind tunnel flow. This modeling strategy is aimed at achieving an efficient CAA analysis tool for the challenging aeroacoustic problem with a full-scale whole-circuit OJWT geometry. The result of the method is carefully examined to ensure the solution quality that pertains to the basic acoustic and aerodynamic aspects of the problem setting. The CAA analysis reveals the staged or conditional response behavior, which is understood to arise from a matched frequency condition between the open-jet vortex shedding motion and longitudinal eigenmode of the airline geometry. In a most severe coupling condition of very elevated pressure oscillation in the test section, a longitudinal standing wave pattern is revealed that it spans over the airline length in resemblance to the response of a frequency-matched acoustic eigenmode on the same airline geometry. In contrast, the standing wave pattern no longer sustains when the coupling intensity subsides. The time-averaged solution shows the expected trend of adverse axial pressure gradient toward the aft end of the test section. The predicted pressure gradient is not sensitive to the varying coupling intensity at different airspeeds in the simulated airline geometry, which is contrary to physical observations. Potential reasons for this discrepancy are discussed in the paper but have not yet been investigated in detail. Sufficient mitigation of flow-acoustic coupling intensity and adverse axial pressure gradient is an important design objective of an OJWT design. The demonstrated CAA strategy is foreseeably significantly more computationally efficient than a 3-D counterpart, which makes it a good complement to experimental methods in the development of airline design optimization.