Learning Forum

[CAP_NJ]

Dear all,

I have simulated a ducted fan config (see figure) with CFX AND Fluent with the same boundary conditions - pt Inlet and p Outlet and the same RPM. I have used the default turbulence settings etc. in both cases, hence they differ slighlty to each other. However, when I am calculating the overall force (thrust) in z direction I get significantly different results for both solvers. I am aware of the importance in setting the reference pressure in Fluent correctly, since it is sensitive to the results (see second equation). I dont get the advantage of relating the pressure to a reference pressure (why is it reducing round off errors). As far as I know, CFX is estimating the force by the accumulation of pressure and viscous force on each surface but I coulnt find any mathematical definition in user guides. In Fluent I get -212N and in CFX -68N which means the force in cfx is only about a third as high as the one calculated in fluent. Even when I include the reference pressure in CFX as done in fluent, the result is completely different (changing from -68N to +266.25N). I dont think the difference can be explained by the differences in turbulence settings (while both are using SST) etc. Do you know where the difference comes from??? I am really grateful for every help you can provide!!!

 

[CFD_Friend]

Hi CAP_NJ,

How did you calculate the axial forces in CFX? You could do an area integral of the pressure multiplied by the vector normal component of surface in the axial direction, that if thrust will be acting in the z direction we can use the Normal Z as the vector.

[CAP_NJ]

Hi CFD_Friend,

thanks a lot for your answer!!! I tried a lot a of ways to calculate the force (even the way you mentioned). In the end the problem was, that I have had set the option "include reference pressure" to "t" (true?) in CFX. In my understandng I would expect the force would be defined as shown on the figure above, just like in fluent, but it was different. I still had to define the relative pressure (pt_inlet-p_ref) at the inlet and outlet (p_outlet-p_ref). It does not make sense to me, but it works like that.

Thanks again and best regards!