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.
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