[Paper Review] Closed-loop control of forced turbulent jets

Igor A. Maia, Peter Jordan+3 more|arXiv (Cornell University)|Sep 19, 2020

Aerodynamics and Acoustics in Jet Flows16 references4 citations

TL;DR

This study demonstrates real-time, closed-loop wave-cancellation control in a forced turbulent jet using system-identified transfer functions to suppress axisymmetric wavepackets. The method achieves order-of-magnitude reductions in velocity fluctuations across an extended streamwise region, validating wave cancellation as a viable strategy for jet noise reduction in high-Reynolds-number flows.

ABSTRACT

Closed-loop control of turbulent flows is a challenging problem with important practical and fundamental implications. We perform closed-loop control of forced, turbulent jets based on a wave-cancellation strategy. The study is motivated by the success of recent studies in applying wave cancellation to control instability waves in transitional boundary layers and free-shear flows. Using a control law obtained through a system-identification technique, we successfully implement wave-cancellation-based, closed-loop control, achieving order-of-magnitude attenuations of velocity fluctuations. Control is shown to reduce fluctuation levels over an extensive streamwise range.

Motivation & Objective

• To develop and validate a closed-loop control strategy for suppressing coherent disturbances in turbulent jets.
• To apply wave-cancellation control based on system-identified transfer functions to a forced turbulent jet.
• To assess the performance and spatial extent of attenuation in velocity fluctuations downstream of the control objective.
• To evaluate the feasibility of extending this approach to unforced, naturally occurring turbulent jets.

Proposed method

• A forced turbulent jet with Re = 5×10⁴ and Ma = 0.05 was used, with axisymmetric disturbances introduced via synthetic jets at the nozzle lip.
• Sensors consisting of six microphones in the near-field measured axisymmetric pressure fluctuations to provide real-time feedback.
• A control law was derived using system identification to model the transfer function between actuation and sensor response.
• Actuators—six synthetic jet speakers at 1.5D downstream—generated counteracting disturbances based on the identified model.
• The control objective was defined at a streamwise position of 1.5D, with velocity fluctuations measured via a hot-wire anemometer on the jet centerline.
• The wave-cancellation strategy used real-time feedback to generate destructive interference against targeted axisymmetric wavepackets.

Experimental results

Research questions

• RQ1 Can wave-cancellation control be effectively implemented in a closed-loop configuration for a turbulent jet?
• RQ2 To what extent does the control reduce velocity fluctuations across the streamwise direction?
• RQ3 How does the performance depend on coherence between sensors, actuators, and the control objective?
• RQ4 Can system-identified transfer functions enable effective real-time control in a high-Reynolds-number turbulent flow?
• RQ5 What are the spatial limitations and unintended effects of localized wave cancellation in a turbulent jet?

Key findings

• The control achieved order-of-magnitude reductions in velocity fluctuation levels at the objective position (1.5D downstream).
• Significant attenuation of wavepacket amplitudes was observed over an extended streamwise region, not limited to the control location.
• Local amplification of fluctuations occurred at 0.25 ≤ r/D ≤ 0.5 due to high-frequency content, but this effect diminished downstream as energy convected away.
• The control performance was strongly correlated with coherence between sensors and the objective, and between actuators and the objective.
• The results confirm that wave-cancellation control based on system-identified models is effective for suppressing coherent structures in turbulent jets.
• The study provides a foundation for future closed-loop control of unforced turbulent jets, with coherence metrics guiding optimal sensor and actuator placement.