[Liam]

Hello all, I have been teaching myself to use ANSYS over the past 3 months for use in my thesis and have found some of the posts on this forum to be very helpful. I am a first time poster and would really appreciate help from anyone who's had sucess with momentum sources in the past.

I have read a lot of posts on the subject of representing axial fans with momentum source subdomains, with the most helpful answers as always coming from Ghorrocks. I am having a few problems with my model and was hoping someone could help out as the documentation is fairly scant on the subject of momentum sources.

I am modelling a large axial fan whose operating point is 650m^3/s providing a static pressure rise of 135 Pa. I have access to the rest of its performance curve and have entered this as a user function named 'Fan Function' whose input is a volumetric flow rate and output is a static pressure rise. I have made a model which consists of a small cylinder representing the fan sub domain which is surrounded by a large 'enclosure' air domain.

I have then written a user expression:
"(Fan Function((massFlow()@F19.21)/(Rho))) / (0.1[m])"
This expression evaluates the user function by inputting the measured mass flow rate at face 'F19.21' which is the inlet circle face of the fan subdomain. This mass flowrate divided by 'Rho' (which is defined in another expression, Rho=1.185) gives a volumetric flowrate which is input into the 'fan function' user function to give pressure rise (based on the fan performance curve). This pressure rise is then divided by 0.1m which is the 'height' of the fan subdomain giving what I understand to be the momentum source term in the vertical (or 'y') direction.

I have set this user expression (which outputs the momentum source) as the vertical momentum source in the subdomain window (with transverse components being 0). My understanding is that this will create a model of the fan which 'reacts' to resistance downstream to settle on an operating point that the real life fan would settle on. I face the following problems.

- When the solution starts, the mass flowrate through the fan inlet face 'F19.21' is zero. This means that the expression attempts to read pressure rise at zero mass flow rate from my user function which is below the region of interpolation. I have tried to specify intitial vertical velocity components in the air domain 'initialization' tab but no value has worked. Is there a better way to setup either my user expression, user function or initial conditions so that I do not recieve this error? Bearing in mind I have no data for the fan below 300m^3/s flowrate

-In an attempt to see how the fan model would work if the user expression was working I took out the user expression and input a constant as the vertical momentum source. I found It would work for a very low value (~100) but if I change the momentum source to the expected value at operating point (~1300) I cannot get the model to solve without hitting an 'overflow' error. I have tried different 'large negative' values for the momentum source and cannot reach convergence. I have tried a lot of different mesh approaches. What exactly do the 'v' and 'vspec' refer to in the documentation when it talks about the source term being = -C(v-vspec)? How would I factor my user expression into this equation? Are there any general rules of thumb on 'stable' or 'realistic' momentum sources?


Again, any help would be greatly appreciated.