Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields ...Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields around the cylinder were measured by Particle Image Velocimetry (PIV). The Keulegan-Carpenter number (KC) varied in the range from 5 to 20 and the viscous parameter beta = Re / KC was set at 500 (Re is Reynolds number). It was found that the strength and frequency of the lift force increased with KC number, the main frequency of the lift force being three times the frequency of the oscillatory flow at KC = 20. The movement and strength of the vortices around the cylinder are discussed for different KC numbers.展开更多
The wind tunnel experiments is conducted to get inspiration for understanding the mechanism of the asymmetric flow pattern and developing an innovative flow control technique for a slender body at high angle of attack...The wind tunnel experiments is conducted to get inspiration for understanding the mechanism of the asymmetric flow pattern and developing an innovative flow control technique for a slender body at high angle of attack. The bi-stable situation of the side forces is observed, which could be easily switched by a tiny disturbances either from coming flow or from artificial disturbances at nose tip (including manufacturing defect). In turbulent flows the side forces switched randomly between positive and negative. There exists a hysteresis loop of side force with the rolling angle. A rod in front of the slender body is used to change the vortex pattern, which could be kept even the rod is moved out from the stream. A miniature strake attached to the nose tip of the model can be moved to different circumferential position. When the strake is stationary, the hysteresis loop disappears and the side force does not change with the turbulent fluctuation of coming flow. The results from dynamic measurements of section side force indicates that when the strake swung at lower frequency the side force can follow the cadence of the swinging strake. With increasing frequency, the magnitude of the side force decreases. At still high frequency, the side force diminishes to zero. If the strake is swinging, while the middle position can be changed to different circumferential angle Фs on either left or right side, the side forces can be changed proportionally with the angle Фs. On the basis of the experimental results, the mechanism of the asymmetry is discussed.展开更多
Three supercell storms on 24 June 2004(0624),28 June 2003(0628),and 27 September 2002(0927) induced different damages in Shandong Province.Storm 0927 was inferior in size and intensity to storms 0628 and 0624.Th...Three supercell storms on 24 June 2004(0624),28 June 2003(0628),and 27 September 2002(0927) induced different damages in Shandong Province.Storm 0927 was inferior in size and intensity to storms 0628 and 0624.The structure and evolvement of the three storms were analyzed in detail based on the WSR98D radar data in combination with weather charts.The results show that mesoscale surface convergence triggered release of instable energy,which resulted in severe convection.During the development stage,storms 0927,0628,and 0624 displayed multi-cell propagation,single-cell evolution,and multi-cell mergence,respectively.The storm tracks were similar:they were all right-moving supercell storms,i.e.,moving at an angle of 30°-70° to the right of the mean wind and at a speed of about 45%-70% of the mean wind speed.In the mature stage,the maximum reflectivity appeared at the low level in storm 0927,mid level in storm 0628,and mid-upper level in storm 0624.These storms possessed almost all typical features of supercell storms:weak echo region(WER),bounded weak echo region(BWER),and mesocyclone.An organized mesocyclone formed at the middle height of an updraft,deepened gradually downward and upward,and became a typical mid-level mesocyclone with strong updrafts.The vertical structures of airflows in the three storms were similar,i.e.,significant convergence at low level,nearly pure rotation at mid level,and divergent rotation at upper level.However,signatures of mid-level horizontal airflows in the three storms were different:at mid level,there was a single vortex in storm 0628,but a double-vortex flow pattern was seen in storms 0927 and 0624.The horizontal structure of the double-vortex flow was hard to be blown away by the environmental airflow,and thus the storms could persist for a longer period of time than the single vortex storm.展开更多
In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) nu...In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.展开更多
基金National Science Foundation of China and British Council
文摘Force measurements of oscillatory flow acting on a single circular cylinder have been carried out. The experiments were done by oscillating a circular cylinder in still water. Instantaneous forces and velocity fields around the cylinder were measured by Particle Image Velocimetry (PIV). The Keulegan-Carpenter number (KC) varied in the range from 5 to 20 and the viscous parameter beta = Re / KC was set at 500 (Re is Reynolds number). It was found that the strength and frequency of the lift force increased with KC number, the main frequency of the lift force being three times the frequency of the oscillatory flow at KC = 20. The movement and strength of the vortices around the cylinder are discussed for different KC numbers.
文摘The wind tunnel experiments is conducted to get inspiration for understanding the mechanism of the asymmetric flow pattern and developing an innovative flow control technique for a slender body at high angle of attack. The bi-stable situation of the side forces is observed, which could be easily switched by a tiny disturbances either from coming flow or from artificial disturbances at nose tip (including manufacturing defect). In turbulent flows the side forces switched randomly between positive and negative. There exists a hysteresis loop of side force with the rolling angle. A rod in front of the slender body is used to change the vortex pattern, which could be kept even the rod is moved out from the stream. A miniature strake attached to the nose tip of the model can be moved to different circumferential position. When the strake is stationary, the hysteresis loop disappears and the side force does not change with the turbulent fluctuation of coming flow. The results from dynamic measurements of section side force indicates that when the strake swung at lower frequency the side force can follow the cadence of the swinging strake. With increasing frequency, the magnitude of the side force decreases. At still high frequency, the side force diminishes to zero. If the strake is swinging, while the middle position can be changed to different circumferential angle Фs on either left or right side, the side forces can be changed proportionally with the angle Фs. On the basis of the experimental results, the mechanism of the asymmetry is discussed.
基金Supported by the National Natural Science Foundation of China under Grant No. 40575012Department of Science and Technology of Shandong Province under Grant No. 2007GG20008001
文摘Three supercell storms on 24 June 2004(0624),28 June 2003(0628),and 27 September 2002(0927) induced different damages in Shandong Province.Storm 0927 was inferior in size and intensity to storms 0628 and 0624.The structure and evolvement of the three storms were analyzed in detail based on the WSR98D radar data in combination with weather charts.The results show that mesoscale surface convergence triggered release of instable energy,which resulted in severe convection.During the development stage,storms 0927,0628,and 0624 displayed multi-cell propagation,single-cell evolution,and multi-cell mergence,respectively.The storm tracks were similar:they were all right-moving supercell storms,i.e.,moving at an angle of 30°-70° to the right of the mean wind and at a speed of about 45%-70% of the mean wind speed.In the mature stage,the maximum reflectivity appeared at the low level in storm 0927,mid level in storm 0628,and mid-upper level in storm 0624.These storms possessed almost all typical features of supercell storms:weak echo region(WER),bounded weak echo region(BWER),and mesocyclone.An organized mesocyclone formed at the middle height of an updraft,deepened gradually downward and upward,and became a typical mid-level mesocyclone with strong updrafts.The vertical structures of airflows in the three storms were similar,i.e.,significant convergence at low level,nearly pure rotation at mid level,and divergent rotation at upper level.However,signatures of mid-level horizontal airflows in the three storms were different:at mid level,there was a single vortex in storm 0628,but a double-vortex flow pattern was seen in storms 0927 and 0624.The horizontal structure of the double-vortex flow was hard to be blown away by the environmental airflow,and thus the storms could persist for a longer period of time than the single vortex storm.
基金Sponsored by the National Natural Science Foundation of China(Grant No.50538020)
文摘In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.