[TwoShaft]

Hello,

i have a problem with a setup that comprises 2 domains:

Hot air flowing through an insulated pipe (Domain 1)
The pipe is cooled by natural convection (Domain 2)

Due to complex geometry around the pipe, i get non-realistic velocities (> Mach 800) in the natural convection domain after 10-15 solver iterations.

To improve convergence I would therefore like to apply the gravitational force in small steps using CEL (so i wouldn't have to do it by hand and can run the simulation overnight):

NCzapplied = -.981*(1+(1-step(Iter-aitern))+(1-step(2*Iter-aitern))+(1-step(3*Iter-aitern))+(1-step(4*Iter-aitern))+(1-step(5*Iter-aitern))+(1-step(6*Iter-aitern))+(1-step(7*Iter-aitern))+(1-step(8*Iter-aitern))+(1-step(9*Iter-aitern)))

Iter is a constant of 50, so the gravitational vector increases from -.981 m s^-2 in 9 steps to -9.81 m s^-2 over 450 solver iterations and then remains constant. CFX can plot the expressions just fine.

My problem is now, I am getting the following Expression Error and I'm not sure wheter this is intended to be unavailable for the buoyancy model (and why?) or whether I am doing something wrong:

Parameter 'Gravity Z Component' in object '/FLOW:Flow Analysis 1/DOMAIN:Air/DOMAIN MODELS/BUOYANCY MODEL' has been assigned an expression that references the following unavailable variables: aitern

Any help would be much appreciated!