Acoustic excitation as a flow control technique in a high-speed compressor cascade

A numerical investigation of the effectiveness of acoustic excitation is carried out to control flow separation in a NACA65-K48 linear compressor cascade (LCC) in the absence of cascade endwalls. The operating conditions for the LCC are Ma = 0.67 and Rec = 560 × 10^3, which are representative of an aeroengine. For the numerical investigations, Improved Delayed Detached Eddy Simulation (IDDES) is used due to its capability to capture flow separation and unsteady flow characteristics with reasonable computational requirements among the high fidelity approaches. The flow through the cascade passage does not experience any separation at the aerodynamic design point (ADP) of the cascade. Therefore, an incidence angle of i = 8° is used in this study where flow separation is observed over blade the suction surface at x/c ≈ 0.6. For acoustic excitation, both external and internal acoustic excitation techniques are investigated. In external acoustic excitation, the sound source is distributed at the inlet boundary of the computational domain whereas in internal excitation, sound waves are introduced into the computational domain from a thin slot located at the onset of flow separation on the blade suction surface. Sound waves are introduced in the flow field at the modal frequencies in the uncontrolled flow, which are Stc = 0.187, 0.409, and 0.618 in terms of Strouhal number based on the blade chord length at a constant excitation amplitude of SPL = 151dB. The results have indicated that external acoustic excitation has no major effect in controlling flow separation in the LCC. Internal acoustic excitation, on the other hand, has a strong effect in modulating the flow separation in the LCC passage such that a reduction of Δζmax ≈ 16% is observed in the total pressure loss coefficient on the measurement plane located 0.4Cax downstream of the cascade exit. Further investigations are also carried out for the effect of sound amplitude on the effectiveness of internal acoustic excitation. In overall, acoustic excitation can serve as a means of flow control in aeroengine compressor cascades.