Separation is an adverse aerodynamic phenomenon resulting in loss of aerodynamic performance. Previous studies on separation have mainly dealt with a two-dimensional analysis due to the assumption that the third dimension was negligible or that it was too complicated to analyze, leading to an incomplete analysis. Literature has indicated however, that this is insufficient in understanding separation and needs to be studied as such as spanwise instabilities have been shown to play a major role in the flow field and physics. The goal of this project is to understand this phenomenon by varying parameters such as sweep, aspect ratio, Reynolds number, and angle of attack and observing how these parameters affect the flow field, particularly at low Reynolds numbers where there is a lack of experimental work in this regime.

The project is a collaborative effort between three universities. Here at CeFPaC, experimental research is being done on three-dimensional separation with cross-collaboration being done with computational fluid dynamics at UCLA under Prof. Kunihiko (Sam) Taira and Triglobal stability analysis at the University of Liverpool under Prof. Vassilis Theofilis. The idea behind the collaboration is to be able to learn from each other as well as to validate the results in understanding this phenomenon.

Two sets of experiments were conducted at CeFPaC: dye flow visualizations and Stereo-Particle Image Velocimetry (SPIV). A wide range of parameters, such as sweep, aspect ratio, Reynolds number, and angle of attack were studied in order to gain a deeper understanding on how certain features would affect separation. The dye flow, using fluorescent dye inject upstream of the models, would be used in order to obtain a qualitative understanding of the flow field whereas the SPIV was used to obtain quantitative conclusions on a certain condition. The SPIV was used in order to identify prominent vortical features, such as the tip vortex and horseshoe vortex. These features would be studied cases by case to observe how the parameters, such as aspect ratio and sweep, would affect these structures. The idea behind this research is to obtain the fundamental knowledge to understanding three-dimensional separation and be able optimally place flow control actuators on vehicles in order to mitigate the phenomenon.

### Related Publications:

- Hayostek, S., & Amitay, M. (2020). Vortical Structures on Low Aspect Ratio Finite Wings at Low Reynolds numbers. Bulletin of the American Physical Society.
- Hayostek, S., & Amitay, M. (2019, November). Effect of boundary conditions on 3-D separation over an airfoil. In APS Division of Fluid Dynamics Meeting Abstracts (pp. G10-002).
- Hayostek, S., Amitay, M., Zhang, K., Taira, K., He,W., Burtsev, A.,&Theofilis, V, "Collaborative Investigation of 3-D separation on Low Aspect Ratio Finite Span Wings." The 59th IACAS, Haifa and Tel Aviv, Israel, March 4-6, 2019
- Taira, K., Zhang, K., Amitay, M., Hayostek, S., Theofilis, V., He,W.,&Burtsev, A. Separated Flows over Finite-Aspect Ratio Wings: Computational, Experimental, and Stability Analyses. In International Symposium on Turbulence and Shear Flow Phenomena (2019).
- Hayostek, S., Amitay, M., Zhang, K., Taira, K., He, W., & Theofilis, V. (2019). Wake Dynamics of Finite Aspect Ratio Wings. Part I: An Experimental Study. In AIAA Scitech 2019 Forum (p. 1384).
- Hayostek, S., & Amitay, M. (2018). Three-dimensional separation on finite aspect ratio swept back wings. In 2018 Fluid Dynamics Conference (p. 3729).