periodic variation of velocity normal to flow direction using periodic BC
 

Hello again,

I'm still trying to solve a turbulent cross flow over a tube bank with heat transfer. So far I have assembled the single domains exhaust (flowing around the tubes in x-direction), tubes and thermal oil (flowing through tubes in y-direction). In order to reduce computational costs I use periodic BC for the inlet and outlet of exhaust. Being aware that the periodic BC of CFX only applies to the momentum equations, I added an source term for the energy equations at the outlet of the exhaust, which adds the amount of heat being exchanged over the tube surfaces as proposed by stumpy:

during this thread:
http://www.cfd-online.com/Forums/cfx...tml#post283314

This generally worked quite well: The solution converged. The pressure loss, the simulated heat transfer coefficients (which both are the results of most interest to me) and the absolute temperature changes of both thermal oil and exhaust are approximately as calculated by empirical correlations. BUT: Both temperatures are rising, where the exhaust should be cooled down by the calculated value...

Searching for the reason of this I found a periodically changing temperature (figure 1) and furthermore y+ (figure 2) at the outer surface of the tubes. Since the layers are of constant thickness along the tubes (fig. 3) I had a closer look at the velocity (fig. 4). The graph shows an periodically changing velocity distribution for which I can see no reasons.

Has anyone encountered a problem similar to this or any idea what can be wrong? Is this possibly related to stumpy's hint that his duct walls use a specified wall heat flux?



further BC:
exhaust - upper and lower wall (normal:y): symmetry
exhaust - side walls (normal:z): adiabatic free-slip wall
any other informations required?

Hi

http://www.cfd-online.com/Forums/cfx...onditions.html

maybe these velocity changes come from Dean-/Görtler/Floryan-eddy?(flow over/trough curved plates/channels ...)

Hello bratzinger,
thanks for the hint. I've never heard of these eddys. But why shall they occur in this direction. I mean if I understand it right the reason for theier occurance is curvature but the curvature is only in the x-y-plane (around the tubes). But if the reason for this phenomenon are eddys, their axes should be parallel to the x-y-plane where there is no curvature, or am I wrong?

Regarding your post
do you mean that you encountered the same phenomenon maybe because of a very short model in flow direction?

Hi
if i understand u correctly u got an air flow e.g. in picture 1 from the left side to the right around your pipes (-> if the fliow got enough space there could be a karman eddy street in xy-plane and maybe eddies in other planes caused be the curvature and turbulent instabilities?) and u got the oil flow from upfront to the back through your pipe with i guess transient/turbulent Re-numbers?(->periodicly occuring velocity changes near wall and therefore differences in heatflux/temperature/yplus?).

That's correct besides that the oil is flowing through the tubes from backside to the front but that should not change anything.
Yes, the oil flow should be turbulent but actually the periodic changes do not occur IN the tube (see for example y+ plotted in figure 2 in the left tube) but outside the tube (right tube in fig. 2). So as I see it, the turbulence of the oil flow should not be the reason for the periodic changes, should it?

To be honest I don't have so much experience with turbulent flows so it did not came to my mind that the eddies in x-y-plane could cause eddies in other planes.

So maybe the periodicity isn't really the problem which causes the temperature to rise. Looking at the temperature distribution (see attachment) there is a hot spot in the wake of the second tube which seems unphysical. Maybe I should extend the domain to more than two tubes... What do you think?