| The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. The acknowledged MIRA vehicle model is characterized by similar real vehicle geometry, thus especially suitable for this purpose.An experimental investigation has been conducted to measure the near wake flow structure of the MIRA 1/8 scale mode using Particle Image Velocimetry(PIV). The experiments were conducted at a Reynolds number was 9.18×105 based on the model length and incident flow velocity. The flow is highly three-dimensional. In order to capture accurately the flow structure, PIV measurements were performed in three orthogonal planes, including the x-z plane at two spanwise locations, the x-y plane at four transverse locations, and the y-z plane at five longitudinal locations, where x, y, and z are the coordinates along longitudinal, transverse and spanwise direction, respectively.The PIV data are processed to obtain not only the instantaneous flow field but also the mean flow by averaging 1200 instantaneous images. It’s worth noting that the instantaneous flow is quite different to the time-average flow. It shows many particular interest and complexity vortical structures which can be seen in the time-average flow.Firstly, in the near wake of the notchback, the A-pillar vortices extend in size and toward the deck lid center, then become two small size streamwise vortices downstream and merge with the bubble E vortices. The tail of the model has a large and small two vortices, namely E and F. The shape of the E vortex is more like "L" type rather than the horseshoe. And the F vortex is a row of alternating small vortex structure, which is characterized by the transverse vortex with rib structures, and like a wave shape.Secondly, in the near wake of the fastback, there are three typical longitudinal vortices: the A-pillar produced from the front A column of the fastback along the middle of the window back gradually extend backward; the C-pillar is a typical trailing vortices; the D vortex forms from the diffuser angle at the bottom of the car. The airflow over the top of the car is completely attached to the window back, and it produces just like the wave of vortex structure, and separates many sub vortex.Thirdly, in the near wake of the squareback, the biggest feature is to produce a "n" type recirculation vortex and its characteristics is that in the vertical direction the vortex ratates from the lateral inward, in the longitudinal direction the vortex from the inside outward and the recirculation vortex occurred mainly in the window back, also has a vacuum region between the window back and the vortex.Fourthly, in the near wake of the pickup, the wake vortex distribution is very similar with the notchback, such as the A-pillar, the C-pillar, the D vortex and E vortex and so on. It dues to the airflow through the top of the pickup as the notchback, occurs completely separation on the window back, and not attaches to the window back.Fifthly, from the instantaneous flow we can see the C-pillar and the vortex “D” are counter-rotating, at the junction of the two vortices occurs strong interlacing, leading to produce many respective sub-vortices. Moreover, due to the size and vorticity of the C-pillar are much larger than the vortex “D”, and plays a dominant role relative to the vortex “D”, so that most of the sub-vortices rounded by the C-pillar core.This study not only analyzed the vortex shedding characteristics of turbulent near wake, but more importantly provided insight into the complex characteristic three-dimensional features of the flow structure in the wakes of MIRA model. |
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