Speaker
Description
In aerospace transportation and propulsion systems, shock-induced flow separation leads to highly unsteady flow fields and has strong detrimental effects on the aerodynamic behavior and performance. To alleviate these effects, separation control is investigated and developed. A commonly pursued approach uses vortex generators of different design to increase the momentum transfer within the boundary layer and thus make it less prone to separation. Different designs of mechanical vortex generators, valued in aerospace engineering for their robustness and simplicity, as well as the more flexible and less drag-penalty prone air-jet vortex generators (AJVGs) have been studied. A large number of parameters influence the control effectiveness of these devices, amongst them geometrical parameters, flow parameters, and the array arrangement of multiple vortex generators. The latter aspect is particularly relevant for AJVGs, which are small enough to allow for inter-device spacings that enable interactions between the turbulent structures induced by neighboring AJVGs, and where the intensity of these interactions strongly influence their control effect. Jet / jet interactions of medium strength enhance the control effectiveness of AJVGs, whereas interactions that are too strong may even cause an increase in separation.
With this great number and variety of control parameters, it is inevitable that the vortex generators encounter off-design conditions during a flight trajectory or engine run, where the operating conditions vary and will never be as clean as in a laboratory.
An overview will be provided on the relevant control parameters and their respective influences, both for mechanical vortex generators including microramps and microvanes and for air-jet vortex generators, and we will introduce the physical mechanisms and underlying flow dynamics governing the control effect. Then, influences of off-design conditions will be discussed. For these purposes, we use results from recent experimental and numerical studies in supersonic and hypersonic turbulent boundary layers. A joint analysis of this rich experimental-numerical data set with statistical methods and modal analysis with dynamic-mode decomposition, amongst other techniques, allows an in-depth interpretation of observed flow phenomena and control mechanisms and effects. On the basis of these findings, the application potential of the investigated control devices will be assessed.
