Advanced surface oil flow visualization technique applied to complex transonic flows for the verification of viscous effects in CFD

An advanced multi-color surface oil flow technique is used to visualize the wall shear stress distributions on two different wind tunnel models in transonic as well as supersonic flow conditions reaching from Mach 0.9 to Mach 2.0. The results of the visualizations are superpositioned by numerically calculated wall shear stress distributions. Not only the shear stress directions are analyzed but in addition the absolute wall shear stresses are linked to the particle deposition density on the model surfaces. The superimposed images are used to verify the accurate representation of viscous effects in complex CFD simulations concerning rotatory flows introduced by canted fins on non-spinning bodies. By adding different fluorescent color particles to the classical mixture of mineral oil and titanium dioxide, the technique was improved to facilitate the distinction of footprints of different interactional flow structures on the body surfaces in high-speed flows. A detailed analysis of the spatial patterns is performed by the use of associated numerical and experimental results to identify the dominant physical phenomena and reveal their various interactions. Due to the color coding not only the visibility of characteristics like shocks, expansion waves or flow separation lines are enhanced but also the tracing of vortical structures is possible. The technique turned out as great success concerning the high resolution of the vortical structures and shock interactions. Hence, a precise comparison of fluid mechanical phenomena between experiment and CFD simulation is possible even in the challenging field of transonic flows. Graphical Abstract