Interaction of a rectangular jet with a slotted plate in presence of a control mechanism: Experimental study of the aeroacoustic field

Abstract

Abstract. The acoustic comfort inside residential buildings is of high interest. HVAC systems employ different shapes of diffusers to ensure air mixing. The interaction between the airflows and the blades of these terminals may result in intense noise radiation. In this work, an experimental study was carried out to investigate the aero-acoustic production of a rectangular jet impinging on a rectangular plate with a slot. For certain flow regimes, such configuration results in whistles with high acoustic levels, called "self-sustaining tones". These tones result from the interaction between the Aerodynamic modes of the jet and acoustic modes. The impact of the vortical structures on the rectangular plate results in pressure waves that re-excite the jet near its exit. This feedback mechanism and the aero-acoustic coupling are responsible for the high-energy tones and can lead to structural fatigue through vibrations. A control mechanism consisting of a thin rod was introduced between the jet nozzle and the impinging wall to disturb the vortex dynamics responsible for the loop of the self-sustaining tones. A total of 1085 positions of the rod were tested between the nozzle and the impinged plate to identify positions of optimal noise reduction. Simultaneous Stereoscopic Particle Image Velocimetry (SPIV) and unsteady pressure measurements were conducted to characterize both the kinematic and the acoustic fields. Two zones were distinguished in terms of control efficacy. In the first one, the sound pressure level dropped by 19 dB, while in the second zone, the sound pressure level increased by 14 dB. The velocity fields show that the presence of the rod divides the main jet into two lateral jets from both sides of the axis of the convergent. The presence of the cylinder creates an artificial expansion of the jet and divides it into two shear flows or jet-like flows. The outer part of these flows expands radially with less interaction with the plate as compared to the case without control. This behavior affects the deformation of vortices against the slot and results in a disappearance of the loop of self-sustaining tones. The main novelty of this work relates to the implementation and analysis of a control mechanism using 2D3C (SPIV) velocity measurements simultaneously with the acoustic radiation produced by the interaction of this flow with a slotted impinging wall.