Changing Boundary conditions

Hello, I am working to solve a conjugate heat transfer problem. The model consider a porous pipe flow, the velocity of the fluid has a sinusoidal profile (u(t)=u0sin(2pi*t)) and i want to apply a changing boundary condition at both ends of the pipe depending on the sign of velocity;

if u(t)>0 , the inlet of the fluid is T(x=0,t)=Th while if u(t)< 0 , the inlet of the fluid is T(x=L,t)=Tc

How and where i can implement these conditions ?

Thank you in advance

Ps;I am new in COMSOL