DRAG & ITS EFFECTS

Thinking of fluids moving in different layers of different velocities, shear stress acts between adjacent layers, just like how friction acts between 2 surfaces in contact at different velocities.

The walls exert such large shear stress onto the liquid which causes liquid at the edges to have 0 velocity, a no-slip condition. Since shear stress is transmitted through the different layers of liquid, the presence of the wall causes a U-shape velocity profile across the cross-section.

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Drag depends on the density of the air, the square of the velocity, the air's viscosity and compressibility, the size and shape of the body, and the body's inclination to the flow. In general, the dependence on body shape, inclination, air viscosity, and compressibility is very complex.