Laminar-Turbulent Transition of a Low Reynolds Number Rigid or Flexible Airfoil

Abstract

Laminar-turbulent transition can affect the aerodynamic performance of low Reynolds number flyers, such as micro air vehicles that operate at the Reynolds number of 10 4-10 5. To gain better understanding of the fluid physics and the associated aerodynamics characteristics, we coupled a Navier-Stokes solver, the e N transition model, and a Reynolds-averaged two-equation closure to study the low Reynolds number flow characterized with the laminar separation bubble and transition. A new intermittency function suitable for low Reynolds number transitional flow incurred by laminar separation is proposed and tested. With the method, we investigate the performance of a rigid airfoil and a flexible airfoil, mounted with a flexible membrane structure on the upper surface, using SD7003 as the configuration. Good agreement is obtained between the prediction and experimental measurements regarding the transition location, aerodynamic coefficients, and overall flow structures. We also examine the impact of gust on the transition process and airfoil performance. We further investigate the effect of the flexible surface on the transition. We find that the self-excited flexible surface vibration affects the separation and transition positions; however, the time-averaged lift and drag coefficients are close to those of the rigid airfoil. Nomenclature = kinematic eddy viscosity = nondimensionalized time, tc=U ! = specific dissipation rate ! g = gust frequency

Prediction of Aerodynamic Characteristics for Elliptic Airfoils in Unmanned Aerial Vehicle Applications

“…This must be due to the size of separation bubble produced in that region. As first explained by Tani (1964) and later reviewed by Shyy et al (1999) and Lian & Shyy (2007), long separation bubbles generally cover considerable portion of airfoil surface and affect inviscid pressure and velocity distributions around the airfoil, whereas, short bubbles cover small portion of surface and do not affect pressure and velocity distributions.…”

“…Figure 2 shows a LSB formed on the suction surface near the leading edge of airfoil at α = 7⁰ using the transition-sensitive k--ω (Walters and Cokljat, 2008) model. The presence of a long separation bubble and post separation behavior of the boundary layer results in increased drag and decreased lift coefficient (Lian & Shyy, 2007).…”

Numerical and Experimental Investigations of an Elasto-Flexible Membrane Wing at a Reynolds Number of 280,000

“…According to the Mayle correlation [17], the percentage of laminar flow relative to the chord length is estimated to be not negligible: it is supposed to be more than 5% of the length of the wing. It was already showed that the laminar-turbulent transition considerably affects the behavior of the elasto-flexible membrane as it directly influences the pressure distribution on the wing and, consequently, the deformation and the geometry of the wing [10,18]. Therefore, two different flow models are investigated in unsteady mode in this paper: the k-ω SST model coupled with the γ-Reθ transition model [19,20], which is used for the transition phenomenon, and the k-ω SST model without transition modeling.…”

Stochastic Bifurcation Analysis of an Elastically Mounted Flapping Airfoil

“…On the other hand, gusts having large scale eddy motion cause pitch motion. Lian and Shyy [13] observed that both the lift and drag coefficients are altered significantly under gust environment. Wind tunnel tests are also carried out to study the impact of gusty flows on MAVs [14,15].…”