[NateF]

Hello,

I am trying to model fluid flow through a pipe with two shutters that open and close at the end that act as a nozzle. Initially, I successfully simulated the flow and mesh movement with the shutters starting at the midpoint. I controlled the mesh movement with this equation:

ShutterDisplacement = 14[mm]*sin(2*pi*4[Hz]*t)

I change the sign for one of the shutters to get them to move in opposite directions. When I further refined the mesh, the simulation failed because the mesh folded. I then modeled the shutters to start in the closed position (4 mm gap between them), in hopes that the mesh wouldn't fold because the area in the nozzle would grow from the original position. I used this equation to control the movement:

ShutterDisplacement = 14[mm]*sin(2*pi*4[Hz]*t+pi/2)

The problem that I am running into is that the mesh jumps to the midpoint in the first time step then starts moving. For the mesh deformation option, I selected "Regions of Motion Specified" with displacement relative to "Initial Mesh." For the shutter displacement I selected "Specified Displacement." My initial conditions are all set to zero. Am I approaching this the right way? If I am, then how can I get the simulation to not make that initial jump between timesteps 0 and 1?

Attached are images of timesteps 0 and 1. The blue area is the nozzle opening, the orange areas are the shutters, and square is the pipe.

CFX_shutters_0.png
CFX_shutters_1.png

Thank you for your help,
-Nate

[ghorrocks]

Your first ShutterDisplacement equation starts at zero at time zero, but your second ShutterDisplacement equation does not start at zero displacement at time zero. That will mean the mesh will do a big jump in the first time step.

[NateF]

I added a vertical shift to my second equation and it fixed the issue. Thank you for your help.