# Piston motion in CFX

Member Posts: 1
edited January 15 in Fluids

Hi,

I'm trying to simulate piston (linear & reciprocating) motion in CFX.

Can I carry it out without FSI?

I want to use only CFX, not Mechanical.

Thanks!

• Posts: 40Ansys Employee

Yes, this transient simulation can be done in CFX using just the Moving/Deforming Mesh model. Here are the steps to set this up, assuming that you're modeling a piston in a cylinder and using a cylindrical fluid domain:

-Generate an appropriate mesh. Using either hex or prismatic (i.e. extruded triangles) mesh aligned with the mesh motion direction will work. When these elements are compressed/stretched, the mesh skewness isn't affected - only the aspect ratio. Using tetrahedral mesh is not recommended as this type of element can only be stretched/compressed a small amount before the mesh skewness becomes an issue.

-In CFX-Pre, under the Basic Settings tab of the domain, change the Mesh Deformation option from None to Regions of Motion Specified (Periodic Regions of Motion is also an available option that is applicable in this case if the motion is periodic). Leave The "Displacement Rel. To" at "Previous Mesh".

-For the wall boundary representing the top of the piston, this will be a moving wall. Under Boundary Details, select "Specified Displacement". Assuming motion is in the z-direction, set the x and y displacement components to 0. For the z component, use an expression like zz=Vel*t, where Vel=Lmax/tmax, where Lmax is the mx length for the piston stroke and tmax is the length of the transient simulation.

-For the wall boundaries representing the cylinder walls, the mesh motion can be set as "Unspecified". This means that the elements on that boundary will deform to accommodate the motion of the piston wall boundary.

-For the wall boundary opposite to the piston wall, set this mesh motion option to "Stationary". This boundary will not move.

During the simulation the volume mesh will deform in order to accommodate the motion of the piston wall.

-