Reactive experimental control of turbulent jets

TL;DR

This paper demonstrates a novel linear control approach in the frequency domain to suppress coherent structures in turbulent jets, significantly reducing velocity fluctuation power spectra in both forced and natural jets.

Contribution

It introduces an optimal causal control law based on transfer functions and Wiener-Hopf formalism for turbulent jet suppression, improving performance over previous methods.

Findings

Achieved order-of-magnitude reduction in velocity fluctuation spectra in forced jets.

Attained about 60% attenuation in natural turbulent jets at key frequencies.

Validated the effectiveness of frequency-domain linear control in turbulent flow management.

Abstract

We present an experimental study of reactive control of turbulent jets. We target axisymmetric disturbances associated with coherent structures, which are known to underpin the peak sound radiation of turbulent jets. We first consider a forced jet flow case, such that the coherent structures can be amplified above background levels, which makes it easier to detect them by the sensors. We then consider the more challenging case of a natural jet, i.e., without artificial forcing. The control strategy explores linear convective mechanisms in the initial jet region, which justifies application of linear control theory. The control law is constructed in the frequency domain, based on empirically determined transfer functions. The Wiener-Hopf formalism is used to enforce causality, providing an optimal causal solution, thus, preventing the drop in performance that may be observed in flow control applications that use simpler wave-cancellation methods. With this approach, we could improve the control performance of forced turbulent jets compared to results obtained in previous studies, attaining order-of-magnitude attenuation of power spectra of velocity fluctuations. Furthermore, we could obtain substantial levels of attenuation of natural turbulent jets, of about 60% in power spectra for the most amplified frequencies. These results open new directions for the control of turbulent flows.