Análise de estol dinâmico por meio de abordagens baseadas em dados e princípios físicos

Author(s)

Lucas Feitosa de Souza

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Mecânica
• Defense date: 2025-01-31

Examining board members

William Roberto Wolf; Marcello Augusto Faraco de Medeiros; Bernardo Luiz Rocha Ribeiro

• Advisor: William Roberto Wolf; Renato Fuzaro Miotto

Abstract

The main focus of the present study lies on the dynamic stall phenomenon, emphasizing the application of modal decomposition techniques for intermittent and transient features, besides flow control strategies to improve aerodynamic performance of unsteady airfoils. Wall-resolved large eddy simulations are carried out solving the compressible Navier-Stokes equations. The first case considered includes a periodic plunging SD7003 airfoil under deep stall condition, with Reynolds and Mach numbers Re = 60, 000 and M8 = 0.1, respectively, and reduced frequency ? = 0.5. The second case investigated is a NACA0018 airfoil under the characteristic motion of a vertical axis wind turbine (VAWT) operating under a Reynolds number Re? = 50, 000 based on the blade rotational velocity, and tip speed ratio ? = 3. Two Mach numbers are studied, being M8 = 0.025 and 0.1. Considering the periodic plunging airfoil, modal decomposition techniques are employed to analyze the transient flow features leading to the onset and evolution of the dynamic stall vortex (DSV). A comparison between the modes obtained with a multidimensional empirical mode decomposition (EMD) and the multi-resolution dynamic mode decomposition (mrDMD) is provided highlighting the compact spatial support representation of the modes given by the intrinsinc mode functions (IMFs) produced by EMD. The EMD provides improved modal decomposition capabilities compared to the mrDMD since the IMFs are able to represent multiple frequencies in the same mode. Moreover, they capture flow features in greater detail, being able to accurately represent the process of growth of the DSV through Kelvin-Helmholtz instabilities arising in the shear layer that forms on the leading edge of the airfoil at high effective angles of attack. This observation becomes pivotal for flow control, leveraging an analysis that combines Lagrangian coherent structures and local stability analysis. The intersection of ridges in the positive and negative Lyapunov exponent fields identifies the shear layer dynamics as analogous to a saddle point. Stability analysis at this location determines the moment when the flow becomes unstable and identifies the optimal frequency to disturb the shear layer. This targeted intervention mitigates the growth of the dynamic stall vortex, significantly reducing aerodynamic drag while minimizing the actuator’s duty cycle. For the case of VAWTs, this work verified the numerical framework used to simulate the dynamic stall setup through grid convergence studies and an assessment of the impact of computational domain size in the spanwise direction. The results showed that a spanwise domain of 40% of the chord is sufficient to capture the critical physical phenomena. A study of compressibility effects revealed the influence of this parameter on the development of two-dimensional coherent structures, along with a downstream displacement of the dynamic stall vortex formation point. These effects contribute to increased aerodynamic loading in the quasi-incompressible case (M8 = 0.025) due to an extended blade-vortex interaction. An active flow control strategy involving suction and blowing at the leading edge, within the frequency range of 8 = St = 16, was tested. This approach significantly reduced flow separation, mitigating the formation of both dynamic stall and trailing-edge vortices. The strategy enabled the extraction of useful energy from the turbine by reducing the aerodynamic drag associated with dynamic stall. Moreover, the energy required for the jet actuation was minimal compared to the energy extracted by the turbine, demonstrating the efficiency of the proposed control mechanism (AU)

• FAPESP's process: 22/08567-9 - Numerical investigation of an airfoil under light dynamic stall
• Grantee: Lucas Feitosa de Souza
• Support Opportunities: Scholarships in Brazil - Master