not another motorbike question | nutilda - nut fields
 

Hi guys



I have a question about the motorbike tutorial, in particular I need some clarifications about the effects of the eddy viscosity fields nut / nutilda on the flow velocity U. So far so good I understand that nutilda is a spalartallmares factor that is calculated (https://www.cfd-online.com/Wiki/Spalart-Allmaras_model). I have the understanding that nut is a measure for the turbulent viscosity which one can derive by e.g, with k and epsilon using the turbulence properties calculator and the eddy viscosity ratio (https://www.cfd-online.com/Tools/turbulence.php). In the motorbike case nutilda is initialized, which effects nut and probably the flow field U.



So here comes my problem, since I intended to modify the case a bit, I did not initialize nutilda at the inlet. Basically, I set the values to zero. If I do this I also get a nice turbulences in the flow field behind the motorbike, however the nut / nutilda files do not show any effect. Sine there are no turbulent parameters (and k is not calculated) in the case what is the measure for turbulence if nut / nutilda don’t show anything, which factor can I use to derive the turbulent intensity from ?
What is the effect of nut / nutilda on the flow velocity U? I had the idea that nut is affected by the motorbike and hence would cause in the flow field to be come turbulent, but this is seems not the case since the nut field is shows nothing and the U field has nice turbulences behind the motorbike…


Apologies if my question appears somehow stupid to you but I just don’t get and it is late …



Thank you so much I appreciate any help.


Best H

no ideas ??? why I dont get values in the nut (set to calculated) and the nutilda field using a SpalartAlmares| DES | DDES or IDDES? Even if I calculated a turblent intensity of 21% ??? Best H

Hi all,
unfortunately I did not get any response... However, for those who might end up with a similar issue like me in future... here is what I did to get a nice nut / nuTilda field. The main thing is that one cannot simply set the nuTilda field to zero, it always requires a value, in order to get field data. So one has to initialize nuTilda by a value. I tried several cases zero doesn’t work and the field does not show any values but e.g. 1e-20 works. In addition, such a small number is not realistic … but one can estimate nut and by the turbulence properties calculator and then initialize nutilda by the factor fv in the equation or multiply nut by 0.5 which if read in the forum somewhere....
I hope that helps anybody Best H