#Boundary layer airfoil skin#And this is what causes skin friction drag. The boundary layer is essentially our concept of how far this effect is significant above the surface. The layer of fluid closest to the surface sticks to the surface and an average momentum exchange with molecules above it leads to deceleration of flow and consequently the growth of the boundary layer. The one parallel to the free stream is thrust or pressure drag (depending on direction). The force normal to free stream direction is lift (or could be negative lift depending on direction). The pressure difference creates a net force in different directions. If it were not for viscosity, the fluid wouldn't want to follow a curvature, leading to acceleration and decelerate and consequently decrease or increase the pressure. I think the better way of putting it would "viscosity" is essential for lift rather than boundary layer is essential. With higher viscosity the particles won't be able to follow such high velocity gradients and the distance between the no-slip surface to the freestream flow around the airfoil will be bigger: the boundary layer will be thicker.No viscosity at all means no energy loss and therefore no drag.The velocity gradient can be infinite and the layer thickness is zero.Its thickness depends mainly on the viscosity the fluid around the airfoil: The no-slip condition on the airfoil surface creates a layer with a high velocity gradient (from zero on the surface to $V_\infty$- the freestream velocity). However, you do need a boundary layer to explain drag. Lift is created due to a pressure difference between the upper and lower side of an airfoil and you don't need any boundary layer to explain this pressure difference. Why? Take a look at airfoils in potential flow: they have lift but neither drag nor a boundary layer. I wouldn't say that the boundary layer is responsible for creating lift, even if the real world friction enables the creation of circulation and therefore lift.
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