On the time-average, the direction of the mixing layer midline remains essentially the same, with rich conclusions under the Cartesian coordinate system (CCS). However, the static pressure mismatch state of the jet leads to a strong coupling effect of expansion and compression in the mixing process. This causes the midline of the mixing layer to become irregularly curved in shape. It is not convenient to describe with fixed CCS anymore. Therefore, a post-processing method of the flow field is proposed based on streamline tracing using the Lagrangian viewpoint to establish a coordinate system for mixing layer evaluation based on the streamline as the coordinate axis, which is called the Followed Body Coordinate System (FBCS) in this study. The following conclusions are obtained for the velocity thickness, momentum thickness, and Pitot pressure thickness treated by FBCS, and they are in good agreement with each other. In addition, the correction of Pitot pressure thickness after two methods of processing is less than 0.08, and the scalar thickness is insensitive to the choice of coordinate system. The consistency of the corrected velocity thickness and momentum thickness is significantly enhanced. This is due to the fact that the processing in FBCS significantly reduces the effects of airflow velocity and density variations caused by flow expansion and compression. The problem of inaccurate vector thickness evaluation in this state is improved.
The flow structure of the multi-strut ejector based on jet segmentation has an obvious relationship with the transverse characteristic scale of the jet. Therefore, this paper studies the relationship between jet transverse characteristic size and mixing layer. Through the simulation of multi-strut ejectors with different number of struts, the development of mixing layer in the mixing process of jets with different transverse characteristic sizes is compared under the same boundary conditions. It is concluded that when there is no obvious coupling effect between the mixing layer and the shock & expansion wave, there is a linear relationship between the transverse characteristic scale of the jet and the mixing length of the flow direction, and the jet mixing with different transverse characteristic scales is consistent. When the shockwaves and expansion waves are strongly coupled with the mixing layer, the linear relationship between the jet transverse characteristic scale and the mixing layer length is not strong.
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