• Energy Research

Abstract Ventilation strategies play an important role of enhancing energy conservation, Indoor Air Quality (IAQ) and acoustic comfort in any conditioned space. Impinging jets are used in such application and should be understood to be controlled. This paper focuses on the flow physics and the aero-acoustic coupling of an impinging jet ventilation system. Experimental data coupling are obtained thanks to simultaneous measurements of the velocity and the acoustic fields using respectively time-resolved particle image velocimetry (PIV) and a microphone. Spatio-temporal cross-correlations between the transverse velocity and the acoustic signals give us a better understanding of the self-sustained tones generated in impinging jets. A pre-whitening technique is used to investigate the coupling between the acoustic and the velocity signals. This method is useful for analysing small random signals superimposed on a high amplitude pure tone.

Abstract Impinging jets are widely used in ventilation systems in order to improve indoor air quality and energy consumption. Thus the flow dynamics involved in these configurations is very important and should be understood and controlled. In the present study, a plane jet issuing from a rectangular nozzle and impinging on a slotted plate was investigated experimentally by means of 3D-stereoscopic PIV. The velocity field was experimented for thirty parallel planes. The whole field was obtained by an interpolation between the reconstructed planes. The 3D reconstruction of the flow was obtained by using the phase averaging technique applied to each of the instantaneous fields. Self-sustained tones generated in this configuration were measured simultaneously with the PIV fields by the use of a microphone. The obtained acoustical signal was used as reference in the phase averaging technique.

Impinging jets on plates with specific geometries, found in habitable enclosures, require a fundamental knowledge of the flow topology in order to reduce any acoustic or aeraulic discomfort. However, the flow in this type of jets is three-dimensional, time-dependent and turbulent. Thus, a high-speed volumetric velocity measurement technique is required to investigate the complete topology of the three-dimensional unsteady coherent flow structures. The research in the present paper has as aim the analysis of the flow from a volumetric point of view by using Time Resolved-Tomographic Particle Image Velocimetry (TR-TPIV) measurements. For this reason, an impinging jet on a slotted plate experimental set up has been developed and implemented. The kinematic fields of the studied volume are measured by TPIV acquisitions. Afterwards, the flow’s most energetic features were understood by the use of the snapshot Proper Orthogonal Decomposition (POD) technique which was applied on the kinematic fields. The results have shown that the first two modes were the most energetic and reflected the convective motion through the volume.

In order to know the origin of the noise generation when an impinging jet hit a specific geometry, an experimental setup was used allowing the generation of the flow and the adjustments of its parameters (such as Reynolds number, confinement, alignment, etc…). The vortex dynamics in case of a high acoustic level for two different Reynolds numbers Re = 5684 and Re = 6214 are considered here. Indeed, many configurations allow self-sustaining sound loop to take place in confined spaces between the nozzle and the impinged surface. This feedback loop optimizes the energy transfer between the aerodynamic field and the acoustic field and creates a source of noise that can become very noisy. Thus, to control these phenomena, it is necessary to understand the aero-acoustic coupling in such configurations. As a result, we measured the 2C kinematic instantaneous fields (vx, vy) of the flow by the technique of Particle Image Velocimetry (PIV) with a sampling rate of 1 KHz and the acoustic field is obtained using a B&K Microphone. For the two considered Reynold numbers, we can distinguish two patterns for the vortices travelling from the nozzle toward the plate of impact.

Abstract Ventilation systems are of vital importance for buildings, not only to provide acceptable thermal conditions and air quality for occupants, but also with regards to energy usage. Impinging jets can be encountered in many ventilation strategies which have major impacts on the acoustic environment and energy performance. The self-sustaining tones can be generated in such applications where a feedback loop is installed in the system. This phenomenon is explained by the corollary of Howe who shows that the origin of noise in such configurations can be attributed to fluid rotations. Howe highlights the role of phase conditions between the vorticity, the velocity of the flow and the acoustic velocity for the optimization of energy transfers between the turbulent kinetic energy and the sound field. In this work, we use 2D-PIV technique and a microphone respectively to measure the kinematic fields simultaneously with the acoustic generation for a rectangular jet impinging on a slotted plate. This study aims to investigate the transfers between the turbulent kinetic energy and the sound field for two Reynolds numbers presenting a high and a low noise levels. It is shown that phase conditions are necessary for the optimization of energy transfer which allows the installation of the self-sustained loop in the flow. It was found also that the change of the aerodynamic mode which is directly related to the self-sustained frequency amplifies the sound intensity and promotes the transfer of energy to the acoustic field.

Impinging jets are encountered in ventilation systems in order to ensure air mixing and thermal comfort such as in residential buildings. These jets represent, under certain configuration, a source of noise which degrades the acoustic comfort. This source can be reinforced by the presence of self-sustaining tones which occurs when the impinged wall has a slot or other abrupt geometry. In order to reduce the acoustic level, many techniques of passive or active control could be adapted. In this work a cylinder is introduced between the exit nozzle and the impinged wall. This aims to disturb the vortex dynamics of the flow and to break the self-sustaining tones loop what will drastically decrease the acoustic generation as it is shown in this study. Thus, in this paper we present the influence of the introduced cylinder on the acoustic field generated by a flow impinging on a slotted plate for a Reynolds number Re=4600. An acoustic cartographic is illustrated to present of the noise level with respect to the cylinder position between the nozzle and the impinged wall. A total number of 1085 spatial positions for the cylinder with a step of 1 mm were experimentally investigated to find out the optimal position to adapt for the best noise attenuation.