Flow separation dramatically increases drag primarily because it disrupts smooth airflow, leading to a larger wake and significant pressure differences between the front and rear of an object.
Understanding Flow Separation
Flow separation occurs when the boundary layer, the thin layer of fluid directly adjacent to the surface of an object, loses momentum and detaches from the surface. This is often caused by:
- Adverse Pressure Gradient: When the pressure increases in the direction of the flow (adverse pressure gradient), the boundary layer slows down. If the pressure gradient is strong enough, the flow can stall and reverse direction near the surface, leading to separation.
- Sharp Geometries: Abrupt changes in shape, like a sharp corner on a wing, can force the flow to rapidly decelerate and separate.
- High Angle of Attack: For airfoils (like wings), a high angle of attack (the angle between the wing and the incoming airflow) can create a strong adverse pressure gradient on the upper surface, resulting in flow separation and stall.
- Surface Roughness: Rough surfaces increase friction and turbulence within the boundary layer, making it more susceptible to separation.
- Fluid Viscosity: Viscosity causes the fluid to stick to the surface, forming the boundary layer. Higher viscosity can sometimes contribute to early flow separation.
How Flow Separation Increases Drag
Flow separation increases drag through several mechanisms:
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Increased Pressure Drag: When flow separates, it creates a large, turbulent wake behind the object. The pressure within this wake is significantly lower than the pressure on the front (leading edge) of the object. This pressure difference between the front and rear creates a net force opposing the motion, which is pressure drag (also known as form drag). The larger the wake, the greater the pressure difference and, therefore, the higher the pressure drag.
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Increased Turbulence: Flow separation generates significant turbulence in the wake. This turbulent wake consumes energy from the flow, which is manifested as drag.
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Increased Skin Friction Drag: While seemingly counterintuitive, flow separation can increase skin friction drag in some circumstances. The turbulent wake region often reattaches to the object further downstream. The turbulence present within this reattached flow can enhance the rate of momentum transfer near the wall, leading to higher wall shear stress and thus, increased skin friction drag, albeit to a lesser extent than the increase in pressure drag.
Example: Airfoil Stall
A classic example is the stall of an aircraft wing (airfoil). As the angle of attack increases, the adverse pressure gradient on the upper surface becomes stronger. Eventually, the flow separates near the leading edge, creating a large turbulent wake over the wing. This significantly reduces lift and drastically increases drag, leading to a stall.
Summary
Flow separation increases drag because it disrupts the smooth flow of air, leading to a larger turbulent wake and a significant pressure difference between the front and rear of the object. This pressure difference results in increased pressure drag, which is the dominant contributor to the overall drag increase. Additionally, increased turbulence and (sometimes) skin friction drag contribute to the overall drag penalty.
