This essay focuses on the phenomenon of Laminar Separation Bubble observed at Low Reynolds Numbers. The effect of this bubble on the airfoil characteristics is discussed. Airfoils used at low Reynolds numbers have to be designed taking into consideration the finite space and time occupied by the bubble. Active as well as passive mechanisms to control/delay the formation of the bubble are also mentioned.
For conventional aircraft wings, whose Reynolds number exceeds a million, the flow is typically turbulent with the boundary layer able to strengthen itself by ‘mixing’. Consequently flow doesn’t separate until high angles of attack are encountered. For lower Reynolds numbers, the flow is initially laminar and is prone to separate even under mild adverse pressure gradient. Under certain flow conditions, the separated flow reattaches and forms a Laminar Separation Bubble (Fig. 1) while transitioning from laminar to turbulent state. Laminar separation bubble could modify the effective shape of the airfoil and consequently influence the aerodynamic performance, generally in a negative manner.
The need to understand low Reynolds number (104 to 106) aerodynamics is driven by variety of applications from windmills to aircrafts. High Altitude Long Endurance (HALE) Reconnaissance aircrafts, Micro Aerial Vehicles, insect and bird’s flight fall in this Reynolds number range. Before the design or analysis of airfoils at these Reynolds Numbers can be improved, it is essential to achieve better understanding of flow physics.
A laminar separation bubble is formed when the previously attached laminar boundary layer encounters an adverse pressure gradient of sufficient magnitude to cause the flow to separate. Downstream of the point of separation, denoted by S in Fig. 1 , the flow can be roughly divided into two main regions. The first region is bounded by the mean dividing streamline ST’R and the airfoil surface. The mean dividing streamline is generally regarded as the collection of points across each velocity profile at which the integrated mass flow is zero. This first region represents the relatively slow re-circulatory flow forming the bubble.