According to the call topic, the goal of the project was to optimise the design of a subsonic jet pump, to reduce the noise level as much as possible. The optimisation of the subsonic jet pump was done from the aerodynamic point of view and two type of analyses were used, a RANS analysis as a “fast-screening” procedure to be able to identify using such techniques a trend in the flow behaviour when performing a parameter study and a time dependent analysis with the more time-consuming but more accurate and more reliable, Large Eddy Simulation, for only the most promising or thought-provoking configurations.
The acoustic treatment for further reducing the noise levels of the jet pump, in any configuration, is the application of noise treatment in the mixing part, and/or in the diffuser. From the acoustic treatment standpoint, there are two ways of attaching the noise reduction problem. First, the passive acoustic treatment, and second, which was considered in the following, composed of resonant - absorbent structures. Taking into consideration the aerodynamic optimisation and the acoustic treatment a demonstrator with different configurations were tested in the anechoic chamber where the velocity field and acoustics were measured and compared with the numerical results.
The goal of the project was to optimise the design of a subsonic jet pump, to reduce as much as possible the noise level. The optimisation of the subsonic jet pump was done from the aerodynamic point of view and two type of numerical analyses were be used: a Reynolds Averaged Navier - Stokes (RAND) analysis as a “fast-screening” procedure to be able to identify trends in the flow behaviour when performing a parameter study, and a time dependent analysis using the more time-consuming but more accurate and more reliable Large Eddy Simulation (LES), for only the most promising or thought-provoking configurations.
For the RANS analysis, a set of candidate geometries was defined, based on a baseline geometry provided by the SGO member. Each configuration was derived from the baseline geometry by modifying relevant geometrical parameters for optimisation purposes. Next, the fast and robust RANS technique was used to select a couple of these configurations. The RANS numerical simulations were carried out using the SST k-ω turbulence model and the ANSYS FLUENT commercial software. The numerical results of the simulations were analysed, particularly from the standpoint of to the production of turbulence kinetic energy, the wall pressure distribution, the uniformity index, and the vorticity components, in order to obtain information on the location and the strength of the acoustic sources as well as on the mixing performance. Based on this analysis, three jet pump configurations showing the greatest potential for noise reduction and increasing mixing performance were selected for further experimental and numerical testing.
Next, the baseline and the selected optimised jet pump demonstrators were designed based on the previously defined geometry and manufacture. A modular design of the demonstrators was be carried out, such that to maximise the number of common components. Also, test rig adaptation parts required to carry out experimental measurements were designed and manufactured. An experimental program aimed for acoustic and aerodynamic measurements was defined. Two types of experiments were planned and carried out: acoustic measurements on the three selected configurations and on the baseline configuration, to quantify and assess the impact of modifications, and aerodynamic tests on the three selected configurations and on the baseline configuration for the determination of the instantaneous flow velocity field using Particle Image Velocimetry (PIV). The acoustic and the aerodynamic data were analysed.
The directivity tests revealed that the best results were obtained for Configuration 4, which yielded a noise reduction, determined using the global sound level calculated from the levels directivity microphones of 2.5dB(A). The global acoustic intensity shows that for Configurations 2 and 3 the area with high noise level shifted towards the ejector exit in comparison with the baseline (Configuration 1). For Configuration 4, the high acoustic intensity area is completely isolated at the injection area. During the intercorrelation measurements, a low vibration level at the exit of the primary flow was obtained for all configurations, at a distance of 4D the pump natural frequencies work for Configurations 1 – 3, while for Configuration 4 a new frequency is excited, because of the injector geometry. The correlation function revealed a substantial contribution of the high frequency's vibration in the sound signal.
The highest axial velocity component value on the centreline is measured for Configuration 1, while the lowest is measured for Configurations 2 and 3. The fastest momentum mixing between the primary stream and the entrained air is registered for Configuration 4 which is an advantage from the noise production standpoint. In the shear layer, large fluctuations of the mean axial velocity are observed, indicating a high level of turbulence in the region, with a maximum value noted for Configuration 3. The tapering of the central air stream is found minimal for Configuration 4. Vortical structures are observed in the mean flow in the transversal and spanwise profiles, due to the detachment of the boundary layer on the mixing segment duct. The vortex is the strongest of Configuration 4. In the transversal direction the central, high velocity region of the flow expands towards downstream, due to the spreading of the primary jet. The spreading rate is slightly larger for Configurations 2 and 3, with only marginal differences between them, and the smallest for Configuration 4. The momentum mixing rate is roughly similar, with a slightly larger value registering for Configuration 4, where the transversal and spanwise velocity profiles indicate a ring shaped jet that progresses radially towards the centreline as it moves downstream.
Finally, unsteady, compressible LES numerical simulations of the confined jet flows characterising the baseline and the three optimised solutions were carried out, reproducing the experimental tests conditions and geometries. The Dynamic Mode Decomposition (DMD) technique was used for post-processing the LES data, to quantify the developed instabilities in the duct, to characterise the energetic content of the dominant modes, and to identify the flow structures at dominant frequencies correlating the flow and the acoustics. Acoustic propagation at the diffuser outlet, up to 20 exit diameters downstream was also calculated based on the LES data.