| As considered as one of the common complex flow phenomena in wind turbine operation,Boundary layer separation of the blade will not only reduce the acquisition power of the wind rotor but also brings about influence on the working stability of the wind turbine.A certain flow control method can be used to delay or eliminate flow separation and enhance the aerodynamic performance of the blade.Zero-net-mass-flux jet(ZNMF jet)is a burgeoning active flow control technology,with unique advantages such as no need of extra air source and fast response velocity.By virtue of the significant feature that it only outputs the momentum externally and the output quality is zero,it has great potential in being applied for the flow control of the wind turbine blade.The research on ZNMF jet and its flow control is carried out by using CFD simulations and taking ZNMF jet actuator,plate,wall-mounted hump,airfoil and vertical axis wind turbine as research objects.Besides,the influence of different parameters of the actuator on flow control effect is analyzed and the reason that the ZNMF jet restrains the flow separation of the blade is also revealed,which provides a reference for the design of ZNMF jet in wind turbine blade.The simplified model of the actuator is built and two-dimensional flow field of the ZNMF jet in a stationary and cross flow are studied by taking ZNMF jet actuator as the research object.Through comparison of the calculation result from the experimental value,the accuracy in simplified model and calculation method are determined.In addition,the flow structure nearby the exit of actuator features of different excitation parameters and functional rules of ZNMF jet on boundary layer are analyzed.The simulation results show that ZNMF jet has vertex pair at orifice,which is expanded outward gradually under its own induction.With increasing of the excitation frequencies,the distance away from the vortex to the orifice and spacing between vortex cores radius and vortex pair are gradually shortened.The time-average flow field of the ZNMF jet in the cross flow is similar to that of the continuous jet.At different phases of expulsion and ingestion,ZNMF jet has different effects on the boundary layer.Meanwhile,the effect of ZNMF jet on the boundary layer is superior to that of continuous jet for the fuller velocity type of the boundary layer with the same velocity ratio.The larger the excitation velocity,the smaller the influence of cross flow on the trajectory.The larger the excitation frequency,the trajectory further approach that of the continuous jet with the same ratio.The control effect of ZNMF jet on the separation flow of a wall-mounted hump is studied by taking the wall-mounted hump as the research object.Based on comparison and confirmation with the experimental data,the influence of jet spacing on separation flow control of the wall-mounted hump is analyzed and the reason that ZNMF jet restrains the flow of the hump is also revealed.By adopting orthogonality analysis method,the influence of four control parameters on flow control of the wall-mounted hump is studied.The primary and secondary order of control parameters influencing three indicators(separation point,reattachment point and area of recirculation zone)are obtained through range difference analysis.The simulation results show that the jet produced durning excitation makes the velocity within separation zone more fullness.When blowing,the vortex behind hole entrainment the high energy fluid outside the separation zone into the separation zone,the separation area is also reduced.Durning ingesting,low velocity fluid upper boundary layers of holes is indrawn into holes,boundary layers becomes thinner,delaying the separation The incident angle of the hole has the greatest influence on the location of the separation point of the hump.Similarly,the excitation frequency also has the greatest influence on location of the reattachment point and area of recirculation zone.Two-dimensional flow field of airfoil without/with ZNMF jet under the static and dynamic stall are studied by taking the large thickness airfoils on wind turbines as the research object.Based on comparison and confirmation with the experimental data,the influence of the parameters on separation flow control of the airfoil is analyzed and the reason that ZNMF jet reduces the hysteresis effect of the aerodynamic characteristics is also revealed.The simulation results show that ZNMF jet could restrain the dynamic stall phenomenon of the airfoil.High frequent fluctuation in aerodynamic characteristic hysteresis loops of the airfoil produces with ZNMF jet control.The hysteresis effect weakens with the increasing of excitation frequency and excitation velocity.Meanwhile,the number of high frequency oscillation is increased as the excitation frequency increases.Besides,the two-dimensional unsteady flow field of H type wind turbine of two-blade without/with ZNMF jet is studied by taking vertical axis wind turbine as the research object.Based on comparison and confirmation with the experimental data,the influence of the parameters on dynamic stall of the blade is analyzed and the reason that aerodynamic load of wind turbine is increased with ZNMF control is revealed.The simulation results show that ZNMF jet can restrain the development of leading and trailing edge vortexes of the blade and substantially increases the power coefficient of the wind turbine.ZNMF jet mainly improves the work capacity of the blade of the phase angle from 90°to 180°.Fluctuation in coefficient curve of tangential force appears with ZNMF jet control.The fluctuation ranges increase with the increasing of momentum coefficient.Through comprehensive consideration into the relationship between energy consumption and output power of the blade,the momentum coefficient should not be larger than 0.015. |
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