Research On The Characteristic And Optimization Of Wall-attaching And Oscillation In External Stimulation Jet

Flow control technology is one of the research hotspots in modern fluid dynamics, and also represents the forefront of the development of fluid dynamics. Compared with the passive flow control, the active control manner has the advantages of precise action, rapid response and significant effect, etc.The research in this article is based on the study contents and patents of NSFC Projects (51276026), and is directed against the requirements and weakpoints of jet oscillation technology in static shock wave refrigeration and other fields. The systematic research has been carried out for the first time on the oscillation mechanism and process characteristics of the main jet which is stimulated to oscillate and controlled by the external micro jet. A series of CFD numerical simulation analysis and the studies of geometry optimization, stimulation parameters and experiment validation have been conducted on oscillation performance, process efficiency, oscillation jet model and other aspects. The latest research results and conclusions provide reliable basis for the research on innovation mechanism of the NSFC Project and for the declaration of independent intellectual property rights. The specific research contents and corresponding conclusions are as follows:(1) The comparative analysis and research have been completed on the stimulation mechanisms of the external stimulation and the self-stimulation by distributary of the main jet. The characteristics and differences of the oscillation model of the two manners have been explored. The numerical simulation and analysis indicate that the external stimulating jet could be accelerated by self-expansion along the direction of the main jet. The acceleration manner dissipate no kinetic energy of the main jet, so the most significant advantage of the external stimulation oscillator is the high process efficiency. The total pressure retention rate K is 15%-25% higher than self-stimulating oscillator.(2) The simulation aiming at the minimum stimulation flow rate have been finished, and the influence tendency of the feature size of the external oscillator on the oscillation performance and the minimum stimulation flow rate has been determined. Simultaneously, the optimal range of the geometric parameters has been obtained which can trigger and sustain the oscillation of the main jet stably. The optimized results suggest that the stimulation flow rate could be 10% lower than that of the main jet. The exploration and comparative analysis between the vertical or parallel stimulation and the push-and-pull stimulation explain the mechanism and characteristics of each manner. In addition, the experimental data verifies that the external stimulation oscillator is easy to oscillate and the oscillation frequency and phase can be controlled accurately in real time.(3) In order to increase the process efficiency (total pressure retention rate K), a quantity of numerical simulation and experiments have been performed. The optimization analysis concludes that the total pressure retention rate K could be higher than 85%. Furthermore, the optimized geometric dimensions and the stimulation parameters or model have been acquired which could generate a stable oscillation jet with the highest K as well as the excellent oscillation performance. It has also been found and proved that the redundant quality of the stimulation flow would increase the value of K and be favorable to oscillation. Because the energy of the stimulation flow could be merged into the main jet, and the boundary layer flow could be improved as the increase of the stimulation flow rate. Therefore, the optimal selection is the stimulation flow rate is 30%-50% higher than the minimum value, for lower energy consumption and better stimulation effect.(4) Under the condition of non-ideal stimulation mode and decreasing pressure peak value of the stimulation flow, experimental study and numerical simulation have been carried out on the influence of the required stimulation flow and the total pressure retention rate. The research indicates that the total pressure retention rate is decreased by only 3%-4% and the stimulation ability remains at a high level when the stimulation mode is changed into sine function. While decreasing the pressure peak value of the stimulation flow by 25%, the flow rate must be increased by 70% to motivate and sustain the oscillation of the main jet. In addition, the switching time ratio increases with the increasing oscillation frequency and the total pressure retention rate is reduced. Meanwhile, the reduction of the duty cycle of the stimulation function could also decrease the total pressure retention rate.(5) According to the urgent need of time-avenged jet in low duty cycle of high refrigeration efficiency, a novel approach (namely multi-stage external stimulation oscillation) that can generate multiple and time-avenged jets has been proposed. The oscillation mechanism and performance have been revealed and analyzed. It is concluded that the total pressure consumption of the second stage is 50% less than that of the first stage. The value of K is over 80% if the geometric parameters and the stimulation time are optimized, which is much higher than that of the self-stimulation manner (not more than 72%).