I am trying to solve buoyancy
 

I am trying to solve buoyancy driven flow by modifying icoFoam solver (by adding source term and solving temp with it). I am getting spurious solution near the top and bottom walls while solving buoynacy driven flow in a square cavity. Overall solution looks fine at other places.

I doubt, snGrad scheme is not taken properly? I am not sure about reason behind this spurious solution.

I require help for this problem

I'm guessing you are using zer
 

I'm guessing you are using zeroGradient pressure boundary conditions on the walls.

This is wrong for a buoyant flow. See the buoyantFoam test case for the type of boundaries required.