Experimental control of installed jet noise

In the last few years, flow control has become increasingly important for the aeronautical field, since it is seen as a promising tool to design safer and more efficient aircraft. In this regard, noise emission is still a major concern in the aviation industry. Specifically, for the under-wing configuration currently adopted in civil aircraft. When the jet interacts with a nearby surface, such as the wing, hydrodynamic structures are scattered into the acoustic field, drastically increasing the emitted noise. Within this context, a feed-forward control scheme is considered for the attenuation of jet installation noise in the far field. Since non-causality is observed in several flow control problems solved in the frequency domain, we compare a wave-cancelling approach, where causality is imposed via the truncation of the control kernel, to the Wiener-Hopf approach, where the causality constraint is imposed a priori. The latter provides an optimal causal solution, and with this, prevents the drop in performance that may be observed in flow control applications that use a truncated solution. The results presented here show a significantly better performance of the Wiener-Hopf method with respect to that of a truncated Kernel, where the control was performed based on microphones measurements, which provided the axisymmetric mode in the near field of the jet as the input signal for the controller. An attenuation of up to 5dB of the broadband spectral hump related to installation effects is obtained.