We demonstrate the capability to experimentally measure fluctuations of the convective heat transfer coefficient at the wall in a turbulent boundary layer. To achieve this, we measure two-dimensional fields of wall-temperature fluctuations beneath a zero-pressuregradient turbulent boundary layer at two moderate friction Reynolds numbers (Reτ ≈ 990 and Reτ ≈ 1800). Spatiotemporal data of wall temperature are acquired by means of a heated-thin-foil sensor as sensing hardware and an infrared camera as a temperature detector. At the lower Reτ condition, the fields of Nusselt number fluctuations (Nú) exhibit elongated features comprising streamwise and spanwise length scales comparable to those of near-wall streaks. At higher Reτ, the effective width and length of the streaks of Nu fluctuations increase. These findings are based on two-point correlations as well as streamwise-spanwise energy spectra of Nu fluctuations at the wall. The convective velocities of the Nu fluctuations are also computed using the available temporal resolution. This allows for resolving the multiscale nature of convective footprints of wall-bounded turbulence: Our experimental data reflect that larger streaks in the footprint convect at velocities in the order of the free-stream velocity, whereas more energetic smaller-scale features move at velocities in the order of 10uτ. Measurements of the kind presented here offer a promising method for wall-based sensing of turbulence and thus for usage as input to flow control systems.

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.