Dynamics of Laminar Separation Bubbles on Low Reynolds Number Airfoils

Abstract

This is a detailed study on the aerodynamics of low Reynolds number airfoils. E387 airfoil that comes under the category of trailing-edge stall is used extensively in this study. The reduction in lift due to low Reynolds number effects can be modeled by the so-called viscous decambering of airfoils. In particular, we show this viscous decambering effect can be subsumed in the zero-lift angle term. It is further demonstrated that leading-edge camber angle plays an important role in explaining the stall characteristics of different airfoils. An attempt has been made to heuristically illustrate the different stall characteristics from thin-airfoil theory, and also to understand the basic design philosophy of low Reynolds number airfoils. Laminar Separation Bubble bursting is a deleterious phenomenon (in turbine blades, airfoils etc.) resulting in a loss of lift and increase of drag at relatively low Reynolds numbers. Bursting of a laminar separation bubble is characterised by a massive loss of lift in an airfoil; the surface pressure distribution departs significantly from the inviscid pressure distribution. There are some criteria in vogue in the literature to characterise onset of the bursting phenomenon. A relatively simple one-parameter criterion that was proposed in the past (Diwan, Chetan, and Ramesh 2006), has been found to be quite successful in characterising bursting and is well cited in the literature, including in unsteady flow contexts. This criterion is revisited and reassessed in the light of our present laminar separation bubble measurements over an Eppler 387 airfoil. Building on these foundations, the pursuit of a more robust bursting criterion that could also be predictive in nature, led to a new simple criterion. According to this, bursting is signalled by the ratio of the freestream velocity at reattachment to that at separation reaching a critical value of 0.86. New engineering correlations for length and height of laminar separation bubbles are also proposed. These correlations, along with ...