An experimental investigation of the coherent turbulent structures in a controlled and uncontrolled turbulent three-dimensional wall jet
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Date
2023-10
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University of New Brunswick
Abstract
The turbulent three-dimensional wall jet exhibits significant disparity in growth, with lateral growth rate five to eight times larger than vertical growth rate. The mechanisms that cause this strong asymmetric growth are not fully understood but are known to be linked to the passage of coherent structures. The purpose of the present investigation was twofold: to further the understanding of coherent structure development in the wall jet and to apply targeted active flow control to alter these structures and the flow field.
The development of coherent structures was explored experimentally through synchronized simultaneous measurements of fluctuating wall pressure and flow velocity at a Reynolds number of ReD ≈ 134, 000. Fluctuating wall pressure was measured using a two-dimensional lateral-streamwise array of 89 microphones, positioned symmetrically about the centreline from x/D = 5 to 15 between z/D = ±4. Particle Image Velocimetry was used to measure the lateral and streamwise flow velocities along a two-dimensional plane at y/D = 0.5. This plane has never been measured in the wall jet and was chosen to investigate continuous streamwise structural evolution.
The measured velocity contours showed the large lateral growth. Low-order reconstructions of the instantaneous unsteady pressure field showed significant lateral asymmetry and the growth of large angled chevron structures. The estimated instantaneous velocity fields showed that these structures were intermittent, changed speed, slowed down, surged forward, and convected laterally, while growing and strengthening with downstream development. It was determined that these structures caused flapping in the jet and the large lateral growth.
Targeted active control was performed using eight synthetic jets actuated around the nozzle. Experimental measurements showed that the control had a significant impact on jet development, causing an increase in lateral half-width of 80%. This level of control has not been achieved in this field to date. Compared to the uncontrolled jet, the controlled jets exhibited stronger fluctuations in the near-field, larger lateral growth at comparable streamwise positions, stronger instantaneous lateral velocities, shorter potential cores, and larger angled structures.