Non-intrusive sensing of wall-temperature fluctuations beneath a turbulent boundary layer for wall-based estimation of turbulent structures*

An experimental study was designed to capture the time-resolved wall-footprint of velocity fluctuations within a turbulent boundary layer. A central facet of this proof-of-concept study involved a non-intrusive film-based sensor at the wall, which was imaged using high-speed infrared thermography (IRT) to obtain a 2D field of the convective heat transfer fluctuations. Simultaneously, velocity fluctuations within the boundary layer were measured using planar 2D2C PIV in both wall-parallel and wall-normal-streamwise planes above the heat transfer sensor. Experiments were carried out in an open-loop wind tunnel facility at the Faculty of Aerospace Engineering of the Delft University of Technology, and comprised a flat plate of 60cm in width and 360cm in length, at a zero-pressure-gradient condition (achieved using a flexible ceiling). A turbulent boundary layer was generated with the aid of a P40 sandpaper trip and measurements were performed at 300cm downstream of the trip at two friction Reynolds numbers: 990 and 1800, at a free-stream velocity of 5m/s and 10m/s, respectively. For the heat transfer measurements, a heated-thin-foil sensor was flush-mounted within the wall. The sensor was made of a 10µm thin stainless-steel foil, and was heated by a direct current that was uniformly applied to the foil. Time-resolved heat transfer measurements yielded the instantaneous wall-temperature field and the convective heat transfer coefficient. The fluctuations of the thermal quantities at the wall depict the footprint of the wall-attached flow structures. Also, the synchronized temperature and velocity measurements allowed for a direct investigating of the correlation between the off-the-wall velocity fluctuations and the heat transfer fluctuations at the wall. The demonstration of the sensor proofs it to be particularly valuable for realizing real-time, wall-based flow control methodologies aimed at heat transfer enhancement or turbulent drag reduction.