Developement of a wall shear stress measurement methodology for automotive purposes

Wall shear stress measurements have always been challenging to be operated in experimental aerodynamics field. Non-intrusive and high-accuracy probes are surely necessary in order to compute such quantity in a region where flow field is most of times not foregone to be predicted. Primary aim of my master’s degree thesis project was to develop a specific measurement technology which was able to acquire pressure information from near wall flow field to compute local wall shear stress values. Designated probe and its relative measuring chain were tested on a simple spherical body, of which aerodynamics behaviour has been widely studied in the past through numerical and experimental tests. Lots of literature references are indeed available about such topic. The whole project has been carried out in collaboration with Dallara Automobili, with the purpose of designing and validating a technique which the company might eventually make use of for its future works. In a first instance a brief outline of existing pneumatic techniques was carried out. A particular focus was done on a triangular-shaped block probe, which provided for computing information concerning wall shear stress 3D vector direction in addition to its absolute magnitude. This peculiar feature was the discriminant parameter which led to designate such probe among others. Wind tunnel tests were carried out by placing 5 wall shear stress probes and additional 4 static pressure probes on spherical body’s surface. These latter were located in order to map pressure field and compare it with numerous literature references existing on this topic. Furthermore, thanks to pressure distributions, an interesting even though approximative evaluation of drag coefficient was possible to be computed. Three different tests’ Reynolds numbers have been picked, respectively belonging to a sub-critical, critical and post-critical regime for a spherical geometry case. One of the main aims was in fact to experiment various separation conditions, making sure to obtain reliable results both in laminar and turbulent flow regimes, characterised by remarkable differences in terms of wall shear stress and static pressure values. First results obtained appeared quite reliable since first comparisons with past literature works; however, a further confirm came in a second time thanks to a validation carried out through Dallara’s CFD models. A great results overlap with CFD models was indeed achieved, not only sustaining experimental setup reliability, but providing Dallara Automobili for a solid and robust numerical methodology to eventually accelerate results computation in similar future projects. In conclusion, wall shear stress measurement technique developed resulted in an undoubtable success for multiple reasons. Its main positive aspects surely reside in its affordability and measurements repeatability. Furthermore, a quite high accuracy was noticeable even for the most chaotic and unpredictable flow zones, as for example separated ones. This hence allows Dallara to eventually make use of such technology even for more complex geometries without incurring in gross errors.