Question

why circular cloud is craeted around a supersonic aircraft? give reason

Answer 1

It forms a cone because it depends on a shock wave, and the region enclosed by the shock wave appears conical in shape.

From the introduction to the article about vapor cones:

Atmospheric water then condenses, and thus becomes visible, as air pressure decreases suddenly across shock waves associated with supersonic flow speed.

It's also put nicely on the page for the Prantl-Glauert singularity:

The reason for the condensation cloud that is being observed is that humid air is entering a low-pressure region, which also reduces local density and temperature, sufficiently so to cause condensation. The vapour vanishes as soon as the pressure increases again to ambient levels.