GATE Mechanical Engineering (ME) 2022 Shift 1 Solved Paper

In the following two-dimensional momentum equation for natural convection over a surface immersed in a quiescent fluid at temperature T_∞ (g is the gravitational acceleration, β is the volumetric thermal expansion coefficient, ν is the kinematic viscosity, u and v are the velocities in x and y directions, respectively, and T is the temperature)
$\mathit{u}\frac{\partial \mathit{u}}{\partial \mathit{x}}+\mathit{v}\frac{\partial \mathit{u}}{\partial \mathit{y}}=\mathit{g}\mathit{\beta }\left(\mathit{T}-{\mathit{T}}_{\mathit{\infty }}\right)+\mathit{\nu }\frac{{\partial }^2\mathit{u}}{\partial {\mathit{y}}^2}$
the term gβ(T - T_∞) represent