Mechanisms and Identification of Unsteady Separation Development and Remediation

• Main Author: Melius, Matthew Scott
• Format: Others
• Published: PDXScholar 2018
• Subjects: Unsteady flow (Aerodynamics); Fluid dynamics; Stalling (Aerodynamics); Aerodynamics and Fluid Mechanics; Fluid Dynamics
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/4064; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=5074&context=open_access_etds

Unsteady flow separation represents a highly complex and important area of study within fluid mechanics. The extent of separation and specific time scales over which it occurs are not fully understood and has significant consequences in numerous industrial applications such as helicopters, jet engines, hydroelectric turbines and wind turbines. A direct consequence of unsteady separation is the erratic movement of the separation point which causes highly dynamic and unpredictable loads on an airfoil. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation especially in response to a sudden change in the effective angle of attack. To capture the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. The flow fields are examined through the application of Eulerian techniques such as proper orthogonal decomposition and empirical mode decomposition to capture unsteady separation characteristics within the flow field. Then, for a higher order description, coherent structures such as vortices and material lines are resolved to fully characterize the flow during a full pitching cycle in a Lagrangian framework. The Eulerian and Lagrangian methods described in the present analysis is extended to investigate the spanwise characteristics within the root section of a three dimensional airfoil. Furthermore, statistical information of the separation point is pursued along four spanwise positions during two cases of unsteady separation. The results of the study describe a critical role of surface vorticity accumulation in unsteady separation and reattachment. Evaluation of the unsteady characteristics of the shear layer reveal evidence that the build-up and shedding of surface vorticity directly influence the dynamic changes in separation point. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic design, but also has broader implications on the larger discipline of unsteady fluid dynamics.