A skin-friction estimation method based on time-resolved TSP data and a Taylor-hypothesis derived algorithm

There are at least two direct links between the friction acting on the surface of a (slightly warmer or colder) body under the influence of a surrounding flow and the temperature distribution on the surface of the body itself. The first is the so-called Reynolds’ analogy, which uses the energy equation to connect the thermal and momentum boundary layers by ascribing the evolution of the temperature distribution at the wall to the action of the skin-friction vector field. On theother hand, a relaxation of the “frozen turbulence” condition at the wall leads to an intriguing derivation of the Taylor Hypothesis, which supplies an operative tool for the skin friction evaluation based on the direct relationship between the celerity of propagation of thermal blobs and the friction velocity. The measurement of friction velocity and skin friction brings a meaningful description of the vorticity generation due to the interaction of the flow field with the body’s surface. Moreover, the point of view of the surface allows going deeper inside the onset of flow separation and reattachment in regions where the flow topology strongly alters the boundary layer paradigm.
The presentation reports on the theoretical connection between the celerity of passive displacement of temperature disturbances at the wall and skin friction. The seminar discusses the accuracy and critical issues of the method by illustrating some of its most recent applications. Among them will be considered new results about the case of a lifting NACA 0015 hydrofoil coated by a functional layer of Temperature Sensitive Paint and placed in the wake of a marine propeller.