Synthetic jets generated by four typical and two novel exciting signals are simulated in the present study. The vortex structures and velocity characteristics are analyzed in detail to evaluate the performance of the exciting signal. The synthetic jets excited with the four typical signals, i.e., the triangle signal, sinusoidal signal, trapezoid signal and square signal, are simulated in the first place. It is found that stronger synthetic jets and higher entrainment can be realized by signals with higher peak velocity. Among the typical signals, the most satisfactory performance is observed in the case with the triangle signal. Two novel signals, i.e., the bi-frequency signal and signal with varying duty cycle are subsequently simulated. The numerical result shows that, even with the same peak velocity, the two novel signals have better performance than the triangle signal. The optimal result is achieved in the case with the varying signal. The signal momentum is investigated to fundamentally explain the mechanism behind the different performance of the synthetic jets generated with different signals with the same characteristic velocity. A new parameter, i.e., characteristic momentum is subsequently proposed. The synthetic jets generated with the signals of higher characteristic momentum are found to manifest better performance under the condition of the same frequency and characteristic velocity.
The flow around a square cylinder with a synthetic jet positioned at the rear surface is numerically investigated with the unsteady Reynolds-averaged Navier-Stokes (URANS) method. Instead of the typical sinusoidal wave, a bi-frequency signal is adopted to generate the synthetic jet. The bi-frequency signal consists of a basic sinusoidal wave and a high-frequency wave. Cases with various amplitudes of the high-frequency component are simulated. It is found that synthetic jets actuated by bi-frequency signals can realize better drag reduction with lower energy consumption when appropriate parameter sets are applied. A new quantity, i.e., the actuation efficiency Ae, is used to evaluate the controlling efficiency. The actuation efficiency Ae reaches its maximum of 0.266 8 when the amplitude of the superposed high-frequency signal is 7.5% of the basic signal. The vortex structures and frequency characteristics are subsequently analyzed to investigate the mechanism of the optimization of the bi-frequency signal. When the synthetic jet is actuated by a single-frequency signal with a characteristic velocity of 0.112 m/s, the wake is asymmetrical. The alternative deflection of vortex pairs and the peak at half of the excitation frequency in the power spectral density (PSD) function are detected. In the bi-frequency cases with the same characteristic velocity, the wake gradually turns to be symmetrical with the increase in the amplitude of the high-frequency component. Meanwhile, the deflection of the vortex pairs and the peak at half of the excitation frequency gradually disappear as well.
The unsteady characteristics and mechanisms of the front stagnation point oscillation for the flow around a square cylinder are investigated with the two-dimensional time-resolved particle image velocimetry technique in the range of 1420 {less than or equal to} Re {less than or equal to} 8600. The front stagnation point oscillates with the frequency equal to the wake vortex-shedding frequency. The Reynolds number has no effect on the amplitude and the dimensionless frequency of the front stagnation point. It is revealed that the front stagnation point moves from the centerline to one side when the wake vortex from the opposite side is forming and moving towards the centerline. This growing wake vortex stays attached to the rear surface of square cylinder. After the wake vortex is detached from the rear surface, the front stagnation moves from the maximum displacement back towards the centerline. Furthermore, the quantitative correspondence between the location of front stagnation point and the wake pattern are revealed via time-lag correlation. The movement of the front stagnation point and the variation of typical proper orthogonal decomposition coefficient of the wake are nearly opposite in phase.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.