If you use ANSYS CFX or FLUENT, you have to select 'density ' analysis to inform the effect of the density as a function of temperature. This will get the buoyancy force effect appear through the investigation.

Good Luck

In my opinion temperature gradient makes velocity gradient. Is it true?

I also think that the occurence of the buoyancy force due to the high temperature difference around the sphere will make velocity gradient, if you also agree.

High-temperature body may result in onset of movement and convection in the surrounding fluid,

depending on the ratio of the buoyancy  force to the one produced by viscous resistance of fluid to shear.  This is similar to Stokes problem. All depends on temperature contrast, fluid density , thermal expansion coefficient and viscosity. The secondary factor is also thermal diffusivity

Thanks for your answers. 

Dear Evgene Burov suppose the fluid temperature is constant and also we can define this fluid as a source. Also the other properties are constant. Now what happen?

Dear Hasan Celik 

I agree you. The buoyancy force must be considered.

A body at higher temperature in a fluid will cause fluid movement if a gravitation field is present and if the fluid changes density with temperature. Then the gravitation field will cause a fluid movement around the body driven by buoancy forces, which are just the difference in hydrostatic pressure of low and high density fluid. The fluid movement is the reason of free convection heat transfer (instead of pure heat conduction).

If a body is immersed in a fluid and has a different temperature, this temperature difference should be taken into account. Even if the density is considered to be constant, a bouyancy term should be considered in the presence of gravity. Read about the Oberbeck-Boussinesq approximation.

The Oberbeck Boussinesq approximation includes the coefficient of thermal expansion, which is zero in a strictly constant density fluid, so no buoancy. It is an approximation for a fluid, where the density is (almost) unaffected by pressure (i.e. incompressible), but not strictly unaffected by temperature.

Thank you for the clarification. What I meant is that in the Oberbeck-Boussinesq approximation treats fluids as incompressible and considers a reference density.

I think if fluid density be constant, thermal gradient can not make velocity gradient and inverse.

exactly; but as I. Malico has correctly pointed out (I was not clear enough in my first answer) we have to distinguish between incompressible (i.e. pressure does not change density) and strictly constant density (i.e. temperature does also not change density).

In a case with zero compressibility and finite thermal expansion coefficient, there is buoancy in the presence of a gravitation field, but you can still use a constant density code which is just supplemented by an artificial buoancy force as in the Oberbeck-Boussinesq approximation.

I think we are all right and I apologize for not having been clear enough in my first answer.