Abstract
Three variants of schlieren techniques are employed to investigate the development of second-mode instability waves in the hypersonic boundary layer of a slender cone in a reflected shock tunnel. First, a previously proposed technique using high frame rate (i.e., at least as high as the dominant instability frequency) schlieren visualization with a continuous light source is shown to provide repeatable measurements of the instability propagation speed and frequency. A modified version of the technique is then introduced whereby a pulsed light source allows the use of a higher-resolution camera with a lower frame rate: this provides significant benefits in terms of spatial resolution and total recording time. A detailed picture of the surface-normal intensity distribution for individual wave packets is obtained, and the images provide comprehensive insight into the unsteady flow structures within the boundary layer. Finally, two-point schlieren deflectometry is implemented and shown to be capable of providing second-mode growth information in the challenging shock tunnel environment.
Highlights
1.1 Transverse wall motionsThe turbulent boundary layer can be considered as an assortment of coherent structures that are dispersed quasiperiodically in space and time (Head and Bandyopadhyay 1981; Robinson 1991; Hutchins et al 2005)
On initiation of DBD plasma, the fluid ejected laterally by plasma forcing is replenished by entrainment from above the plasma actuator
We have shown that it is possible to create spanwise travelling waves for turbulent boundary-layer control using two different types of DBD plasma actuator arrays
Summary
The turbulent boundary layer can be considered as an assortment of coherent structures that are dispersed quasiperiodically in space and time (Head and Bandyopadhyay 1981; Robinson 1991; Hutchins et al 2005). Xu and Choi (2006) performed a similar experiment in a water flume to observe up to a 30 % reduction in skin-friction drag They carried out flow visualisations in the near-wall region of the boundary layer to observe the formation of wide ribbons of low-speed fluid during spanwise travelling-wave control. A further numerical investigation was conducted by Klumpp et al (2011) In their large-eddy simulation (LES), the outof-plane motion of a spanwise surface wave was implemented in a turbulent channel flow and achieved a skinfriction drag reduction of 9 %. The standing wave translates the time-dependent forcing of spanwise-wall oscillation (1.1) into a stationary counterpart by introducing spanwise velocity that is modulated in the streamwise direction He observed a skin-friction drag reduction of 52 %, similar to the findings of Viotti et al (2009). We have investigated into the use of dielectricbarrier-discharge (DBD) plasma actuators to generate spanwise travelling waves in air
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