| Plasma flow control has the ability to reduce the drag and increase the lift of an aircraft,which is benefit for the energy saving and emission reduce.NS-DBD(nanosecond pulsed dielectric barrier discharge)actuator is one of the plasma flow control actuators,it has advantages of simple structure,wide frequency range,low average power consumption and wide application range,has become a hot research spot in the past decade.NS-DBD actuators have been applied in the mixing layer,bow shock,shock wave/boundary layer interaction,anti-icing and deicing control.NS-DBD involves multiple physical processes in the discharge-flow interaction,and the time and space scales are very different.The interactions of the plasma,electro-hydrodynamic,joule heating and the flow is still partly unclear.The influences of the internal and external key parameters of the actuator on the discharge characteristics are also not completely clear,which lead to the lack of effective guidelines for the flow control applications.To solving the two major problems,In this paper,a three equations drift-diffusion equation is adopted to study the key species and chemical reaction,and the effects of the internal and external parameters of the actuator on the discharge characteristics are also studied.A large eddy simulation equation is adopted to investigate the mechanism of the wake transition around a cylinder.First,the effects of the species and chemical reactions on the discharge characteristics of a one-dimensional discharge are studied.The discharge energy is mainly transferred to the gas heating and the chemical energies of the heavy species.The excitation reactions and the ions are crucial to the discharge energy.The gas heating consists of a fast and a slow processes.The fast gas heating is released in the time scale of the width of the high voltage pulse and the gas heating efficiency(η)is 41%～47%.The release time of the slow gas heating energy is larger than 7000ns,which is mainly contributed by the vibration-translational relaxation of the nitrogen molecules.After analyzing the key species and chemical reactions,a simple reaction model is proposed to study the characteristics of a two dimensional NS-DBD streamer discharge,it is found that the streamer head velocity(V)and streamer thickness(h)are around 1 mm/ns and 30～40μm,respectively.The maximum length(L)is almost the same to the length of the grounded electrode.The spatial distribution of the major gas heating power is nearly located in the stream,but the electro-hydrodynamic force is mainly distributed in the sheath layer,where the height is almost two orders of magnitudes smaller than the thickness of the streamer.In quiescent air,the gas heating induces a shock wave and vortex structures.The electro-hydrodynamic force induces a small velocity(<0.3 m/s),and the local temperature,pressure and density disturbances are also very weak,which can be neglected.Second,the effects of the discharge conditions,the dielectric layer and the voltage on the discharge characteristics are studied separately.The pressure,temperature and speed have the most obvious effects on the discharge.The pressure and temperature mainly affect the discharge characteristics by changing the gas density.The pressures p and V can be scaled as e(?),h and L can be scaled as p-0.8.The total discharge energy QD_ei and gas heating QGH be scaled as p-0.5.The speed affects the charge density and electric field by changing the movements of the positive and negative ions,and the effect is very weak in the investigated speed range(0～258 m/s)in this work.The dielectric constant(ε)and thickness(td)of the dielectric layer have the greatest effects.Either increasing ε or decreasing td increases the capacitance of the dielectric layer,V,L,QD_ei and QGH increase,but h and η decrease.Either increasing the gradient of the voltage pulse Ugrad or the width τpulse induces the increase of the voltage peak Up,the current increases,however,it is saturated after a certain value.When Ugrad keeps unchanged,V and QD_ei increase linearly with the increase of τpulse,QGH increases exponentially,but h decreases linearly.On the contrary,when τpulse keeps constant,V～(Up)-1.9,h decreases,both QD_ei and QGH can be scaled as～(Up)1.5·log(Up).The current magnitude in the negative pulse in the voltage fall period is lower than that in the positive pulse in the voltage rise phase,however,it is higher in the voltage rise pulse in the negative pulse.h is largely reduced in the negative pulse,but V and L keep almost unchanged,QD_ei and QGH are reduced.The frequency of the pulses v affects the residual charge density in the end of a pulse period.The break down voltage in the high frequency discharge is lower,and the average electric field and h are also lower,but QD_ei increases.The ions’ energy of the QGH increases,but the energy of the exothermic reactions decreases.η is almost unaffected.When v<1kHz,the residual charge density is very low,and the effects of v is weak,which can be neglected.Finally,the mechanism of NS-DBD flow control is studied.Uniform gas heating perturbation power is applied near the separation point of the boundary layer of the laminar flow around the cylinder with Re=3900,and the stream and normal Reynolds normal stresses and shear stresses production terms are induced,but the variation of the normal stresses and shear stresses production terms are very weak,resulting in the increase of the turbulent kinetic energy near the disturbance location.The transition of the laminar shear layer in the wake is promoted.The length of the wake decreases,but the average temperature,pressures and velocities of the flow field are weakly affected,and the drag coefficients and vortex shedding frequencies are also changed slightly.The present work provides theory and engineering values to the application of the NS-DBD flow control in aeronautic and aerospace. |
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