# How do I get nodal force components on a surface?

## Recommended Posts

To get the nodal output on specific surfaces, indicate a non-zero value for 'Nodal time step frequency' or 'Nodal time frequency' under the SURFACE_OUTPUT command. Use 'acuTrans' as below to extract the nodal area and traction on the desired surface. The product of nodal area and nodal traction will be the nodal force components.

acuTrans -osf -osfs "Wall" -osfv node,area,traction

##### Share on other sites

Hi Yao,

I suppose that this force components are the vectorial sum of the pressure and viscous forces. Is there a way to get only the viscous forces or wall shear stresses?

##### Share on other sites

Unfortunately, no, there is really no easy way to do this in the current release.  You can use AcuFieldView to compute the wall shear based on tau_wall = mu*dv/dn.  You can also use the acuSrfShear python script that ships with the distribution to compute the wall shear.  However, there is no way to directly visualize the results of acuSrfShear and it also requires that you define your walls of interest directly with the TURBULENCE_WALL command.

##### Share on other sites

CFDGURU,

Thanks for your answer, also there is an option to get the moment of the momentum, passing "moment" to acuTrans. Is there a way to calculate this moment in around a point different of the default coordinate system?

Thanks,

##### Share on other sites

rodre28,

AcuSolve includes the moment of momentum as an integrated surface output variable.  If you have the integrated surface output frequency set to a non-zero value, you should see this quantity present in acuProbe as a time trace.  AcuSolve will always compute this quantity about the global origin, but you can easily transform it.  To do this, you'll simply need to compute the moment about the new point, then add it to the moment about the origin.  You can do this using the traction components that AcuSolve exports:

Mx'(x1,y1,z1)  =  y1*zTraction  -  z1*yTraction

My'(x1,y1,z1) =   z1*xTraction  -  x1*zTraction

Mz'(x1,y1,z1) =   x1*yTraction  -  y1*xTraction

M(x1,y1,z1)   = Mx'+Mx(0,0,0)

M(x1,y1,z1)   = My'+My(0,0,0)

M(x1,y1,z1)   = Mz'+My(0,0,0)

You'll want to check my signs on this, but hopefully the approach is clear.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.