When a jet impinge on ground or flat plate, a parallel flow to the ground is produced. This flow is called wall jet flow. When two or more jets impinge on ground, collision of the wall-jet flows takes place in the domain between the jets. Such collision produces a upward fan-like flow between the jets. This flow is called upwash flow. As the upwash flow proceeds with flow line from the jets intersect forming a jet-like flow, what is called fountain flow appears. The direction and strength of this fountain flow is strongly dependent on the parent jets strength and orientation assuming a uniform ground surface. The fountain flow is known to involve complex flow structures.

This fountain flow is of remarkable significance concerning the overall lift power provided by the jets. When the jets impinge on the ground, they entrain ambient medium (air for V/Stol airctraft) with them.  When this happens below the plate surface (V/STOL aircraft fuselage), a suck down phenomenon is said to takes place. This suck down of ambient air around the jets results in great lift loss. The fountain flow helps in reducing this lift loss.