Discussion Forum

Hi all!

I am new in COMSOL and in this moment I am trying to create flow waves in a pipe filled with a compressible fluid using the PDE Coefficients. The problem has been analliticaly solve using some approximations and boundary conditions for the Continuity Equation and Navier Stokes Equations, giving as a result that the Pressure and the axial and radial Velocity are traveling waves with a complex velocity, produccing an oscillation.

The problem consists in coupling three differential equations and I think I did that part well (I selected three physics and in each one I wrote the equation with its corresponding coefficients). Now I have doubt about the boundary conditions, in the module of Fluid Dynamics (Laminar Flow) the No Slip boundary condition (U(R)=V(R)=0, axial and radial velocity to be zero at the walls of the pipe) is already prescribed, but in the PDE Coefficients would be the equivalent for No Slip the Zero-Flux? Furthermore, to produce the oscillations I need to apply a Pressure Oscillation at the entrance of the Pipe so I decided to used Dirichlet BC at the top of my pipe and I prescribed the Pressure equation to be sin(wt), the result is not what I am expecting, For the Pressure I am getting for all the times the same Pressure I prescribed and also the results have an unreallistic order of magnitudes (10^150).

I already worked with the module of Fluid Dynamics and I tried to reproduce the same as the paper and I never obtained the oscillations, so then I decied to use the PDE coefficients to write exactly the equation that are used in the paper with the approximations considered there.

I would appriciate any idea and advice,

Victoria Jimenez.