Fine flow structure and mixing characteristic in supersonic flow induced by a lobed mixer

Abstract

In a supersonic suction type of mixing layer wind tunnel, by employing nanoparticle-based planar laser scattering (NPLS) method, contrast experiments are carried out with the emphasis on the fine flow structures of planar mixing layer and the mixing layer induced by triangular lobed mixer. The normal-shock equation, isentropic equation and sound speed relationship are utilized to calculate the flow parameters. The calculated Mach numbers are 1.98 and 2.84 for upper and lower airstreams respectively with a convective Mach number of 0.2. The NPLS images clearly shows the Kelvin-Helmholtz vortices, streamwise vortices, shock waves and the pairing processes of large-scale vortex structures. The unsteady properties of development and evolution for large-scale vortices are obtained by contrasting the NPLS images at different times. Also, it has been demonstrated by the present experimental investigation that in supersonic mixing layer with low convective Mach number, the small shock waves are still existing. These small shock waves that occur have negative effects on the mixing process. It is because the convection flow process of upper and lower airstreams is non-isentropic, causing the total pressure to lose. Based on the NPLS results, flow structures and mixing characteristics are analyzed quantitatively by using fractal and intermittency theory. The results show that the mixing efficiency increases obviously with the introducing of large-scale streamwise vortices. The nibbling of vortex clusters induced by large-scale streamwise vortices obviously increases the interface area of mixing. Meanwhile, compared with planar mixing layer, larger spanwise structures roll up in triangular lobed mixing layer, leading to more entrainment of upper and lower airstreams. In the present investigation of supersonic planar mixing layer, the value of fractal dimension of fully turbulent region is stable at 1.55-1.6. Whereas the value of fractal dimension for triangular lobed mixing layer reaches 1.88 at the flow field far away downstream, which breaks through the value of fully developed turbulence for planar mixing layer. Besides, in triangular lobed mixing layer, the shear action between streamwise vortices and spanwise structures plays a leading role in promoting mixing. The mixing flow shows the property of apparent crushability and three-dimensional behavior, which plays a positive role in promoting mixing at a scalar level. The analysis of intermittency indicates that the interaction between streamwise and spanwise vortices dominates the mixing characteristics, and due to the entrainment of streamwise vortices, the mixing region induced by triangular lobed mixer becomes larger, and more fluids are engulfed into the mixing region to complete the mixing process.