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Do fluids always flow smoothly?
No, fluids move in either a laminar (smooth) flow or a turbulant flow. In laminar flow the motion of all the fluid particles is very orderly, with all particles moving in straight lines parallel to the flow. At low velocities the fluid tends to flow without mixing. In turbulent flow the fluid moves in an unpredictable manner controlled by pressure and temperature differences. Turbulent flow occurs at higher velocities where fluid particles form leading or lateral mixing. Laminar flow reduces skin friction, but tends to separate from the wing easily, which causes pressure drag. Turbulent flow increases skin friction, but tends to move faster and remain close to the surface of the wing, which reduces pressure drag.