What causes flow separation and how can it be prevented?

Flow separation occurs when boundary layer detaches from a surface due to adverse pressure gradient (pressure increasing in flow direction). The boundary layer loses momentum to overcome pressure rise, velocity near wall reverses, and flow separates forming recirculation zones. Effects include increased drag, pressure loss, vibration, and reduced efficiency. Prevention methods: streamlined body shapes with gradual area changes, boundary layer suction, vortex generators to energize boundary layer, slot or blown jets to add momentum, dimples (golf ball effect) to trip turbulent boundary layer earlier, and maintaining favorable pressure gradients in diffuser design (expansion angle < 7°).