An experimental study is conducted to examine the impact of internal geometry of a fluidic oscillator on its working mechanisms, i.e., the widths of the inlet wedge W1, mixing chamber W2, and exit throat W3, normalized by the width of the inlet throat W0. Using time-resolved particle image velocimetry, the flow dynamics both inside and outside the oscillator are measured simultaneously. The phase-averaged flow fields are obtained using proper orthogonal decomposition analysis based on which the pressure fields are computed. It is found that the external jet spreading angle and the oscillation frequency are proportional to the width of the inlet wedge up to W1/W0 = 2. This is because the inlet wedge controls the feedback flow and accordingly the recirculation bubble in the mixing chamber. At a critical lower value of W1/W0 = 0.8, there is no feedback flow with a stable external sweeping jet. The mixing chamber width W2/W0 controls the size of the recirculation bubble, which has a notable proportional control on the spreading angle. With a small mixing chamber of W2/W0 = 2.9, it is also found that the strong feedback flow can still produce a stable sweeping jet motion but with a small spreading angle. The exit throat width W3/W0 has non-monotonous control on the external jet spreading angle and the oscillation frequency. It is noteworthy that the jet can still produce a stable sweeping motion even with a large value of W3/W0 = 4.2, which can significantly reduce the blocking effect of the exit.
The Reynolds number (Re) of the low-pressure turbine blades is low in the cruising state, and the laminar boundary layer of the suction surface is prone to separate from the action of the reverse pressure gradient. The separated laminar boundary layer may undergo transition and reattachment, and form laminar separation bubbles. Both situations will cause a lot of energy loss. Therefore, it is necessary to control the boundary layer of the low-pressure turbine with low Re Number. As a relatively new type of active flow control technology, the sweeping jet oscillator does not require moving parts and can generate high-frequency unsteady excitation at the outlet. With the capability of generating jets with a wide range of speeds and high frequencies and adapting to harsh environments with strong electromagnetic interference and radiation, it is an effective means of boundary layer control. From the results of experiments, we found that when Re is 10000, turbulence is 1.8% and the momentum coefficient controlled by the oscillating jet is 0.95%, the boundary layer can be reattached.
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