  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.
