Abstract
The effect of streamwise plasma vortex generators on the convective heat transfer of a turbulent boundary layer is experimentally investigated. A Dielectric Barrier Discharge (DBD) plasma-actuator array is employed to promote pairs of counter-rotating, streamwise-aligned vortices embedded in a well-behaved turbulent boundary layer over a flat plate. The study aims at elucidating the mechanism of interaction between the plasma-induced vortical structures and the convective heat transfer process downstream of them. The full three-dimensional mean flow field is measured with planar and stereoscopic PIV. The convective heat transfer is assessed with infrared thermography over a heat-flux sensor located downstream of the actuators. The combination of the flow field and heat transfer measurements provides a complete picture of the fluid-dynamic interaction of plasma-induced flow with local turbulent transport effects. The results show that the streamwise vortices are stationary and confined across the spanwise direction due to the action of the plasma discharge. Flow-field measurements show that the opposing plasma discharge causes a mass- and momentum-flux deficit within the boundary layer, leading to a low-velocity region that grows in the streamwise direction and which is characterised by an increase in displacement and momentum thicknesses. This low-velocity ribbon travels downstream, promoting streak-alike patterns of reduction in the convective heat transfer distribution. Near the wall, the plasma-induced jets divert the main flow. This phenomenon is a consequence of the DBD-actuator momentum injection and, thus, the suction caused on the surrounding fluid by the emerging jets. The stationarity of the plasma-induced vortices makes them persistent far downstream, reducing the convective heat transfer.
Highlights
A Dielectric Barrier Discharge (DBD) plasma-actuator array is employed to promote pairs of counter-rotating, streamwise-aligned vortices embedded in a well-behaved turbulent boundary layer over a flat plate
A common solution for convective heat transfer enhancement in turbulent flows consists of producing embedded streamwise vortices in the boundary layer [1] by the use of passive elements such as vortex generators [2] or wall-mounted obstacles [3,4]
Limited contributions can be found in the literature regarding convective heat transfer reduction in turbulent flows, in spite of the fact that it is of paramount importance in several engineering applications, requiring the use of technologies such as film-cooling in turbomachinery
Summary
A common solution for convective heat transfer enhancement in turbulent flows consists of producing embedded streamwise vortices in the boundary layer [1] by the use of passive elements such as vortex generators [2] or wall-mounted obstacles [3,4]. According to the numerical investigation by Zhang and Collins [33], embedded vortices split into two main categories: those deeply embedded in the boundary layer and those covering up to the outer region of the TBL The latter is the most common in flow-control techniques, generally induced by physical vortex generators, promoting an effective heat transfer enhancement (and an increase of skin friction). The experimental study of Whalley and Choi [54] shows the possibility of inducing co- and counter-rotating streamwise vortices that interact to generate a spanwise travelling wave and the formation of wide ribbons of low-speed streamwise velocity within the viscous sublayer In their experimental work, Jukes et al [55] achieve up to 45% skin-friction drag reduction downstream of the actuator, by introducing a spanwise oscillation in the near-wall region of a turbulent boundary layer. A parametric study is carried out, evaluating these effects for several streamwise vortex intensities, as a function of the actuation momentum coefficient
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