| As the vehicle energy issue becomes increasingly significant,people realize the necessary to improve fuel efficiency,which pushes the study of vehicle aerodynamic drag reduction.At present,the aerodynamic drag reduction methods include the passive and active drag reduction,of which the passive drag reduction has good application,but there are less research on active drag reduction.In order to obtain the efficient active drag reduction,we need firstly analyze the flow field,especially the transient flow field,which can reflect the flow field more accurately,and explore the formation mechanism of the aerodynamic drag.Based on the understanding of the flow characteristics,we can apply the flow control method specifically and thus effectively reduce aerodynamic drag,which is very important for improving the vehicle fuel efficiency.The international standard MIRA notchback model is used in this article to simulate the instantaneous flow.The instantaneous flow date can be averaged by time into the timeaveraged flow date.The macro flow condition with strong regularity can be obtained through the averaged flow,and the small size vortex structures which is completely complex and random can be observed through the transient flow.Based on the research of the real flow,the jet technology are applied to control the flow around the model using the Power FLOW software which is based on the LBM theory.This will be a guidance for the application of the jet flow control method on the vehicle.There are two parts in this article: the flow structure and jet drag reduction research.The following is the results of this article.1?The flow structures of the notchback MIRA model from the averaged date: There are complex vortex flow structures in the vehicle tail and their contribution to the vehicle aerodynamic drag is very big.Due to the influence of the C-pillar vortex,the separation vortex of the rear window presents the complex three-dimensional structure,forming a large negative pressure zone.The C-pillar vortex becomes larger in the process of backward development,and its energy goes up,which influences aerodynamic drag significantly.Thus,effective control of the separation vortex and the C-pillar vortex will promote the vehicle drag reduction.2?The flow structures of the notchback MIRA model from the transient date: The transient vorticity is more complicated and it's more close to the real flow.The large-scaled and steady vortex(such as the C-pillar vortex)can't be figured from the transient flow.The flow field around the wheels,rear window,and the vehicle rear is formed with many smallscaled vortex.From the spectrum analysis of the monitoring points,there is an obvious vibration frequency f = 23 Hz at the engine,roof,side window and body side and the vibration frequency f = 12 Hz after the rear.Through the pressure and velocity pulsation,we found there is larger vibration at the vehicle rear,the rear wheel and the bottom of the side body.3?In order to improve the wake of the MIRA model,especially to weaken the separate vortex in the rear window and C-pillar vortex,five positions are set jet in the vehicle rear.By changing the jet diameter and the jet velocity,we discuss the drag reduction effect in different jet conditions,and explore the drag reduction mechanism: At A position(rear of the roof),we get a 7% jet drag reduction.The A position jet reduces the separation of the rear window,which improves the negative pressure at the rear window and the vehicle rear.At B position(sides of the rear window),we get a 10% jet drag reduction.The B position jet reduces strength of C-pillar vortex,which is one pair of trailing vortex and it contributes much into the aerodynamic drag.At C position(lower edge of the rear window),we get a 10% jet drag reduction.The C position jet breaks the one big separation vortex at the rear window into two smaller separation vortex,the strength of which is weaker.Thus we get a lower aerodynamic drag.At D position(sides of the trunk),we get a 8.7% jet drag reduction.The D position jet blocks the flow from the side into the up of the trunk,and weakens the flow energy into the separation flow at the rear window and the C-pillar vortex,which reduces the aerodynamic drag.At E position(rear of the trunk),we get a 6.7% jet drag reduction.The E position jet corresponds to erect a baffle at the vehicle rear,which reduces the whole flow velocity.And according to Bernoulli's principle,the negative pressure in the vehicle rear is increased.So it can reduces the aerodynamic drag.4?Setting jets in all five positions tentatively,a 9.1% jet drag reduction is obtained which is to say the combination drag reduction effect is less than single jet.In the combination jet condition,the jets in ABC positions increase negative pressure after the rear window,which is beneficial to reduce the air drag.But when the air flows back to the vehicle rear,it's influenced by DE jets,decreasing the negative pressure.The DE jets suppress the effect of ABC jets,which makes the whole vehicle is unable to obtain a better drag reduce effect.Through the analysis of transient flow field,this paper makes a further understanding of the flow field characteristics of MIRA model.And good effect of aerodynamic drag reduction is achieved through jet,which is of great guiding significance for the development of automobile aerodynamic drag reduction. |
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