Session: 10.4.3 - Vortex Dynamics III

Paper Number: 158567

158567 - Impact of Vortex Generators on Shear Layer and Separation Bubble Dynamics Under Unsteady Inflow 

Abstract: 

Vortex generators are widely used as passive flow control devices to alleviate boundary layer separation and stall. These devices generate streamwise vortices that re-energize the near-wall flow by bringing high-speed flow away from the surface. Most studies of a vortex generator are carried out under a steady framework of either laminar inflow or focus on the time-averaged statistics of turbulent flow (Lin, Progress in Aerospace Sciences 38 (4-5), 389-420, 2002, Panaras & Lu, Progress in Aerospace Sciences, 74, 16-47, 2015). In many real-world applications, however, there is a strong unsteady, often periodic component associated with the flow approaching the vortex generator, which has the potential to generate an unsteady component to the streamwise vorticity. Examples include (1) cases in which the inflow boundary layer is transitional or turbulent (Chan & Chin, Journal of Fluid Mechanics 932: A29., 2022), (2) vortex generators used in turbomachinery applications (Li & Ji, Journal of Turbomachinery 141 (8), 081005, 2019), and (3) vortex generators used in conjunction with other active or passive flow control technologies such as microcavity actuators (e.g. Visbal & Garmann, Theoretical and Computational Fluid Dynamics 37 (3), 289-303, 2023). To better utilize vortex generators under such circumstances, a deeper understanding of the interaction of the unsteady streamwise vortices with the separated flow is required. The present work aims to provide relevant insights by considering the prototypical configuration of a backward-facing step. ANSYS Fluent, a commercial finite volume solver, is used to carry out transient simulations. The underlying natural frequencies of the system consisting of unsteady inflow, vortex generator, and backward-facing step are studied. Our preliminary simulations (Paudyal et al., 76th Annual Meeting of the Division of Fluid Dynamics, 2023) of a backward-facing step without vortex generators and a periodically perturbed inflow of different frequencies, carried out at a Reynolds number of 800 and expansion ratio of two showed interesting unsteady dynamics. The addition of a triangular vortex generator upstream of the backward-facing step reveals it generates streamwise vortices of fluctuating strength on interaction with the unsteady inflow. These unsteady streamwise vortices interfere with the shear layer and the separation bubble dynamics downstream. The full paper will carry out a detailed analysis of this interplay between unsteady streamwise vortices and the different instability modes of the separated flow and will also consider cases with different Reynolds numbers and stronger inflow perturbations that is likely to exhibit strong nonlinear behavior. Instantaneous, time-averaged and RMS statistics of velocity and vorticity fields will be examined for the different cases with the objective of acquiring a mechanistic understanding. 

Presenting Author: Santosh Paudyal The University of Akron

Presenting Author Biography: Santosh Paudyal is a PhD student at the University of Akron working with Dr. Saikishan Suryanarayanan in the Department of Mechanical Engineering. Mr. Paudyal has a Bachelor of Technology in Mechanical Engineering from Uttarakhand Technical University, India and a Master of Science in Mechanical Engineering from University of North Dakota, USA. He is interested in Fluid Mechanics, Structural dynamics, Fluid structure interactions and Computational Methods.