how to know grid independence in any problem , is it necessry to find out

it is not necessary to find out grid independence, but for supplying the time step size or the courant number, and for some turbulence parameters, knowledge of the grid dependence is necessary.

If you want to know how well you are capturing the physics - then grid independence is a definite must. (If you double the mesh count - and your answer changes by more than a few percent - then you know your original mesh was probably not causing any flow characteristics. If the answer changes by more than that - then you know the mesh itself was influencing the flow results.)