Synthetic Jet And Its Application In Jet Vectoring

The ability to control a flow field to improve efficiency or performance is of the great technological importance. Recently synthetic jet and its application in active flow control are one of the leading areas of many scientists and engineers in aerodynamics. Experimental measurements and numerical simulations are performed in this paper to investigate the flow characteristics of synthetic jet and its application in jet vectoring.With Hot Wire Anemometry (HWA) and Particle Image Velocimetry (PIV), the flow field characteristics of piezoelectric actuator are studied. Experiments show that for the piezoelectric actuator, two resonant frequencies exist, on which the velocity and momentum flux of synthetic jet are larger. Comparisons of different input signals reveal that rectangular wave maximizes the velocity of synthetic jet due to the energy input in half oscillation cycle is the biggest. Detailed studies also show that in the time-averaged flow field of piezoelectric synthetic jet, the half width and overspreading angle of synthetic jet are larger than the 2D conventional jet, while the decreasing ratio of streamwise centerline velocity in the developed section is faster than the 2D conventional jet. Also the profile of distributions of cross stream velocity in the time-averaged flow field shows the similarity like the 2D conventional jet. In order to investigate the effects of different orifice aspect ratios on the characteristics of synthetic jet, one circular and five rectangular orifices with the same output areas are studied in detail. The measurements show that with the rising aspect ratio of rectangular orifices, the soaked area increases, thus the wetted perimeter extends, which enhances the surface friction in the orifice channel. Consequently the stroke length L0 and time-averaged orifice velocity U0 decrease. The instantaneous vorticity fields indicate that with the aspect ratio rising up the local corner stress increases, which motivates the rectangular jet itself to more stable axis-symmetric structure. Hence the vortex pairs rapidly break up and the existing time for coherence becomes shorter.Due to the distinct variation of cylinder volume in one oscillation cycle, the traditional unsteady velocity inlet boundary condition disagrees with the actual flow situation. Dynamic mesh refreshing method of FLUENT commercial software is introduced, and the moving function of mesh is identical to the piston movement. Compared with the PIV measurements, the instantaneous flow structure, vortex trajectories and distributions of time-averaged streamwise centerline velocity are consistent with the experiments, which confirm the numerical simulations.To achieve efficient jet vectoring using synthetic jet indicates the potential application in aero dynamical engineering. In the thesis two spatial layouts are investigated in detail by both experiments and numerical simulations. For the piezoelectric actuator parallel to main flow, the absorption of low pressure region near the orifice and the interactions between the vortex and main flow are the primary reasons that cause the deflection of the main flow. In the current experiment the maximum 45 degree vectoring turning 5.5m/s main flow is achieved by the specially designed piezoelectric actuator. Experiments manifest that optimum space between the synthetic jet and main flow exists. Numerical simulations of flow field structures reveals that circulation area develops when the space between the actuator and main flow increases, which directly baffles the entrainment of the actuator, correspondingly the effect of jet vectoring reduces. For the piezoelectric actuator vertical to the main flow, both experiments and numerical simulation illustrate the main flow deflects to the actuator due to the absorption near the orifice and the Coanda effect near the surface of the actuator, even the synthetic jet directly impinges the main flow. Obviously to realize the main jet deflecting to the downstream of the actuator, larger momentum of synthetic jet is essential; however the momentum of piezoelectric synthetic jet is not enough to manipulate the main flow. As a result a piston-driven actuator is fabricated, which provides significantly greater mass and momentum flux of synthetic jet compared with the piezoelectric actuator. In the experiment the rectangular synthetic jet impacts the main flow, and with phase-locked technology the PIV captures the sequences of the interactions between the main flow and synthetic jet, during which a source point appears and it divides the flow field into two branches: the continuous one and the undisturbed one. The contour of time-averaged velocity flow field indicates that due to the periodical impingement of the synthetic jet in the half cycles, a low velocity triangle zone appears, however with the increasing velocity of main flow the triangle zone fades out. The analysis of PIV images demonstrates that the vectoring angle increases with the ratio of momentum flux of synthetic jet to momentum flux of main flow, thus to achieve jet vectoring successfully in vertical spatial layout, the momentum of main flow and synthetic must be in the same order of magnitudes.