Design of Low Reynolds Number Airfoils with Trips

Abstract

A design philosophy for low Reynolds number airfoils that judiciously combines the tailoring of the airfoil pressure distribution using a transition ramp with the use of boundary-layer trips is presented. Three airfoils with systematic changes to the shape of the transition ramp have been designed to study the effect of trips on the airfoil performance. The airfoils were wind-tunnel tested with various trip locations and at Reynolds numbers of 100,000 and 300,000 to assess the effectiveness of the design philosophy. The results show that the design philosophy was successfully used in integrating a boundary-layertrip from the outset in the airfoil design process. For the Reynolds numbers and the range of airfoil shapes considered, however, airfoils designed with trips do not hold any clear advantage over airfoils designed for good performance in the clean condition.

Feather roughness reduces flow separation during low Reynolds number glides of swifts

“…The roughness elements, created by a combination of feather rachides and the overlapping feather vanes, effectively result in corrugated surface analogs to the earlier mentioned strip turbulators (Lentink and de Kat, 2014). A remarkable difference is, however, that theoretical estimates suggest that the roughness height of swift hand wings is similar to the boundary layer thickness (Lentink and de Kat, 2014), whereas in model wings it is a small fraction (Braslow and Knox, 1958;Gibbings, 1959;Kraemer, 1961;Lissaman, 1983;Simons, 1994;Lyon et al, 1997;Giguere and Selig, 1999;Gopalarathnam et al, 2003). Subsequent model wing studies with corrugated versus smooth model swift wings suggest that feather-like roughness reduces drag at Re=15,000, but not at higher Re (Lentink and de Kat, 2014).…”

“…However, neither a control experiment with a smooth model hummingbird wing nor experiments at intermediate α were performed in this study. Such experiments would be insightful, because similar roughness created by strip turbulators is known to trigger transition at the surface and reduce flow separation of model airplanes and drones operating at higher Re up to 100,000 and beyond (Braslow and Knox, 1958;Gibbings, 1959;Lissaman, 1983;Simons, 1994;Giguere and Selig, 1999;Gopalarathnam et al, 2003). Model wing studies have, therefore, not fully resolved the influence of feather roughness on boundary layer flow separation, transition and reattachment in bird flight.…”

“…Laminar flow-based drag-reduction strategies are therefore within reach for animals swimming and flying at Reynolds numbers below a million. Whereas a low Re is a requisite for low drag based on laminar flow, it is not sufficient, because laminar boundary layers are more sensitive to boundary flow separation than turbulent ones (Lissaman, 1983;Simons, 1994;Lyon et al, 1997;Giguere and Selig, 1999;Gopalarathnam et al, 2003;Spedding et al, 2008). Flow separation mostly occurs on the upper side of a wing at a positive angle of attack, due to the strong adverse pressure gradient in the boundary layer (Schlichting, 1979;White, 1991;Mueller, 2002;Shyy, 2013;Kundu and Cohen, 2007).…”

Design and Experimental Validation of Swirl-Recovery Vanes for Propeller Propulsion Systems

“…1, after having determined the optimal circulation distribution from the lifting-line method, the airfoil sections need to be designed to achieve minimum drag on the vanes. In the wind-tunnel test, the typical Reynolds number on the vane sections was on the order of 10 5 , at which serious deterioration of the lift and drag may occur due to the laminar separation bubbles [31,32]. On the other hand, the inflow of the SRVs is highly turbulent because of the presence of the propeller upstream of it.…”

2D Simulation Studies on Use of Boundary Layer Trip for Airfoil E216

“…LSB forms a recirculation region of low pressure air between separation and reattachment locations [3]. The separated flow may not reattach to the surface if the Reynolds number is very low, and hence LSB will not be formed [4]. The pressure measurements on an airfoil surface can be effectively used to find the locations of separation, transition and reattachment points of a LSB [5].…”