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.