Visualization of the structure of supersonic turbulent boundary layers

Abstract

A series of flow visualizations has been performed on two flat-plate zero-pressure-gradient supersonic boundary layers. The two different boundary layers had moderate Mach numbers of z.8 and z.5 and Reo's of 8z, ooo and z5, ooo respectively. A number of new visualization techniques were applied. One was a variation of conventional schlieren employing "selective cut-off" at the knife edge plane. Motion pictures of the flow were generated with this technique. Droplet seeding was also used to mark the flow, and high speed movies were made to show structure evolution. Still pictures were also taken to show details within the large-scale motions. Finally, Rayleigh scattering was used to construct planar images of the flow. Together, these techniques provide detailed information regarding the character and kinematics of the large-scale motions appearing in boundary layers in supersonic flow. Using these data, in concert with existing hot-wire data, some suggestions are made regarding the characteristics of the "average" large-scale motion.

Three-dimensional instantaneous structure of a shock wave/turbulent boundary layer interaction

“…Based on the high-speed isosurface (0.99U ∞ ), it appears that at the boundary layer edge, the turbulent/non-turbulent interface may be characterized as a highly corrugated surface, consistent with the observations of Smith & Smits (1995), who visualized the structure of supersonic boundary layers using Schlieren and Rayleigh scattering, and characterized the outer layer as consisting of an array of regularly spaced uniform low-density bulges, separated from a uniform higher-density free stream by a sharp, instantaneously ragged interface. The average velocity inside the relatively high-vorticity regions in figure 11 is close to the free stream (0.9U ∞ ), also consistent with observations of Smith & Smits (1995), who report that structures in the outer region convect at around 0.9U ∞ . As a matter of fact, Spina et al (1991) have shown that the convection velocity of the large-scale motions is nearly constant across 80 % of the supersonic boundary layer and is equal to approximately 0.9U ∞ .…”

“…The major vorticity component is parallel with the wall and normal to the free-stream direction. Such a feature has been found by Smith & Smits (1995) in the outer layer of their supersonic boundary layer, which they refer to as a 'large-scale bulge'. These authors also discuss how such bulges may support Reynolds shear stress by acting as a pump, drawing high-momentum fluid down into the boundary layer.…”

Resolution effects in compressible, turbulent boundary layer simulations

“…In supersonic flow, the large-scale structures have been studied using hotwires [16][17][18][19], particle image velocimetry [20], wall pressure transducers [21], and flow visualization [22,23]. The primary effect of compressibility on turbulent boundary layers is the conversion of mechanical energy to heat through compression and viscous dissipation [8,9].…”

Visualization of coherent structures in a supersonic flat-plate boundary layer

“…Quasistreamwise vortices or vortex pairs were detected in the wall region of a turbulent boundary layer at Mach 4.5 by Huang et al [8], who also used a DNS database. In experimental studies, coherent structures in a supersonic turbulent boundary layer were visualized by Smiths et al [9] employing different methods. Using the recently developed nanoparticle-based planar laser scattering (NPLS) technique, the interaction between an oblique shock wave and turbu- lent boundary layer was visualized by Zhao et al [10], as well as the influence of a large vortex of the mixing layer on an oblique shock wave.…”