Control of flow around a circular cylinder by synthetic jets has been experimentally investigated in a water tunnel with particle image velocimetry(PIV) technique.The synthetic jets are positioned at both the front an...Control of flow around a circular cylinder by synthetic jets has been experimentally investigated in a water tunnel with particle image velocimetry(PIV) technique.The synthetic jets are positioned at both the front and rear stagnation points.With power spectrum analysis,proper orthogonal decomposition(POD) method and other techniques for data processing,particular attention is paid to the control of vortical structures around the circular cylinder,in which the excitation frequency f e is one to three times of the natural frequency f0 and the cylinder Reynolds number and the excitation amplitude are fixed.The influenced-scope of the synthetic jet enlarges as the excitation frequency increases,and thus the synthetic jet dominates the global flow field gradually.For the natural case and the control case at fe/f0=1,the distributions of the first two POD modes and the power spectra for their POD coefficients all exhibit the characteristics of the natural shedding with antisymmetric mode.For fe/f0=2 and fe/f0=3,the variations in the third and fourth POD modes and the corresponding power spectra indicate that the wake vortex shedding mode changes and the dominant frequency becomes the excitation frequency.For fe/f0=2,the wake vortex sheds downstream with either the antisymmetric or the symmetric mode;for fe/f0=3,the synthetic jet vortex pair interacts with the near wake shear layers from both sides to induce a pair of the symmetric wake vortices,which is gradually converted into an antisymmetric mode when shedding downstream.展开更多
This study puts forward an active control method for circular cylinder flow by placing two small affiliated rotating cylinders adjacent to the main cylinder, and their effects on the drag and lift forces acting on the...This study puts forward an active control method for circular cylinder flow by placing two small affiliated rotating cylinders adjacent to the main cylinder, and their effects on the drag and lift forces acting on the main cylinder as well as the heat trans- fer effectiveness are numerically investigated. According to the diameter of the main cylinder the Reynolds number is chosen as Re=200. The well-proven finite volume method is employed for the calculation. The code is validated by comparing the present computed results of flow passing an isolated rotating cylinder with those available from the literature. To describe the present control model, two parameters are defined： the rotation direction of the two small cylinders （including co-current rota- tion and counter-current rotation） and the dimensionless rotation rate a. In the simulation, the rotation rate a varies from 0 to 2.4. The results indicate that the optimum rotation direction of the subsidiary cylinders, which is beneficial to both drag reduc- tion and beat transfer enhancement, is the co-current rotating （the upper affiliated cylinder spins clockwise and the lower affili- ated cylinder spins counter-clockwise）. We observe noticeable suppression of the vortex shedding and favorable reduction of the fluid forces acting on the main cylinder as the rotation rate increases. Besides, the pressure and viscous components of the drag force are analyzed. Energy balance between energy cost for activating the rotating cylinders and energy saving by the momentum injection is considered. In addition, the influence of the affiliated rotating cylinders on heat transfer is also investi- gated. The average Nusselt number is found to increase with the rotation rate.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 11202015 and 10832001)
文摘Control of flow around a circular cylinder by synthetic jets has been experimentally investigated in a water tunnel with particle image velocimetry(PIV) technique.The synthetic jets are positioned at both the front and rear stagnation points.With power spectrum analysis,proper orthogonal decomposition(POD) method and other techniques for data processing,particular attention is paid to the control of vortical structures around the circular cylinder,in which the excitation frequency f e is one to three times of the natural frequency f0 and the cylinder Reynolds number and the excitation amplitude are fixed.The influenced-scope of the synthetic jet enlarges as the excitation frequency increases,and thus the synthetic jet dominates the global flow field gradually.For the natural case and the control case at fe/f0=1,the distributions of the first two POD modes and the power spectra for their POD coefficients all exhibit the characteristics of the natural shedding with antisymmetric mode.For fe/f0=2 and fe/f0=3,the variations in the third and fourth POD modes and the corresponding power spectra indicate that the wake vortex shedding mode changes and the dominant frequency becomes the excitation frequency.For fe/f0=2,the wake vortex sheds downstream with either the antisymmetric or the symmetric mode;for fe/f0=3,the synthetic jet vortex pair interacts with the near wake shear layers from both sides to induce a pair of the symmetric wake vortices,which is gradually converted into an antisymmetric mode when shedding downstream.
文摘This study puts forward an active control method for circular cylinder flow by placing two small affiliated rotating cylinders adjacent to the main cylinder, and their effects on the drag and lift forces acting on the main cylinder as well as the heat trans- fer effectiveness are numerically investigated. According to the diameter of the main cylinder the Reynolds number is chosen as Re=200. The well-proven finite volume method is employed for the calculation. The code is validated by comparing the present computed results of flow passing an isolated rotating cylinder with those available from the literature. To describe the present control model, two parameters are defined： the rotation direction of the two small cylinders （including co-current rota- tion and counter-current rotation） and the dimensionless rotation rate a. In the simulation, the rotation rate a varies from 0 to 2.4. The results indicate that the optimum rotation direction of the subsidiary cylinders, which is beneficial to both drag reduc- tion and beat transfer enhancement, is the co-current rotating （the upper affiliated cylinder spins clockwise and the lower affili- ated cylinder spins counter-clockwise）. We observe noticeable suppression of the vortex shedding and favorable reduction of the fluid forces acting on the main cylinder as the rotation rate increases. Besides, the pressure and viscous components of the drag force are analyzed. Energy balance between energy cost for activating the rotating cylinders and energy saving by the momentum injection is considered. In addition, the influence of the affiliated rotating cylinders on heat transfer is also investi- gated. The average Nusselt number is found to increase with the rotation rate.
