The performance of almost all model aircraft is strongly influenced by laminar separation bubbles, which may occur at low Reynolds numbers. Such a separation bubble is caused by a strong adverse pressure gradient (pressure rise along the surface), which makes the laminar boundary layer to separate from the curved airfoil surface. The pressure rise is related to the velocity drop towards the trailing edge of the airfoil, which can be seen in the velocity distribution of the airfoil through Bernoulli's equation.
The boundary layer leaves the surface approximately in tangential direction, resulting in a wedge shaped separation area. The separated, but still laminar flow is highly sensitive to disturbances, which finally cause it to transition to the turbulent state. The transition region (not exactly a transition point) is located away from the airfoil at the outer boundary of the separated flow area. The thickness of the now turbulent boundary layer grows rapidly, forming a turbulent wedge, which may reach the airfoil surface again. The region where the turbulent flow touches the surface again is called reattachment point. The volume enclosed by the regions of separated laminar flow and turbulent flow is called a laminar separation bubble. Inside the bubble the flow may be circulating, the direction near the airfoil surface may even be the opposite of the direction of the outer flow. There is almost no energy exchange with the outer flow, which makes the laminar separation bubble quite stable.
Laminar flow, separation bubble and turbulent flow.
The separation bubble thickens the boundary layer and thus increases the drag of the airfoil. The drag increment can be several times the drag of the airfoil without a separation bubble. Lift and Moment are also influenced by a laminar separation bubble, which can lead to problems with stability and control of a model aircraft.
If the transition occurs quite far away from the airfoil surface, it may happen that the turbulent flow wedge cannot reach the surface again. Thus there is no reattachment and the bubble stays open. Such a flow field with a thick area of separated flow causes a high drag and usually the lift breaks down. The same happens when the angle of attack is increased beyond the maximum lift.
A very effective means to avoid the drag penalties and nonlinear behavior of lift and moment coefficients, caused by laminar separation bubbles, are turbulators.
Airfoils with reflexed mean lines (as used on flying wing models) suffer stronger from the low Reynolds number effects, because the reflex adds to the pressure gradient in their boundary layer.
Last modification of this page: 21.05.18
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