Resolving convective velocities of turbulent boundary layer-induced convective heat transfer fluctuations at the wall

Abstract

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.