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• # Doubt in the problem regarding pressure variation , contours ,vectors in mixing

Posted by on October 1, 2023 at 11:05 pm

I have some doubts which are getting me very confused :

1.We are observing the variations of parameters like temperature,velocity,pressure etc.in mixing elbow through contours and vector field on a plane(midplane) .Since plane is a 2D space , we can see the variation of these parameters which lie only on that plane(midplane) area, which is a function of location of plane inside the elbow. So like you have demonstrated in the lecture itself that we can see the variation of parameters on that plane globally ; my concern is how can it be possible to capture the variation happening inside the whole fluid domain(3D space) of a particular parameter on a plane(midplane)? Because it is not compulsory that minimum and maximum of a particular parameter( say temperature) would be happening on that particular plane(midplane), it should rather show variation happening locally i.e. variation of parameters on that particular plane , Is it ?

2. In the mixing elbow problem you considered the boundary condition of outlet pressure to be 1 bar, after simulation the contour plot is showing that pressure inside the whole fluid domain of elbow is almost same as of outlet . And near the vicnity of inlet and at the bottom of the bend it is showing negative Pressure , Is negative pressure possible in that regions ?

2 Members · 2 Replies
• 2 Replies
• ### Sandeep

October 1, 2023 at 11:14 pm

Good question!

1. Post-processing at the midplane is just one way to look at it. You can always look for volume average volume max, make several surfaces, and check the area average or area max at that surface etc. But for such problems looking at the midplane is good enough to make an engineering judgement.

2. You need to check the static pressure and the total pressure. Fluid flow is driven by pressure difference. At the inlet, you have an additional dynamic head you gave as the velocity boundary condition.

The negative pressure at the bend is a good observation. such drastic pressure drops can occur in bends and throat regions of hydraulic devices and if the pressure drops below its vapor pressure it will cavitate. In your case, you haven’t modeled cavitation that’s why it shows a negative number. So that implies you need to consider modeling the vapor formation.

• ### abhishek_k

Member
October 6, 2023 at 6:55 pm

Okk..but In 1, what if the parameter which we are observing through contours on midplane is not maximum globally; whether in that case the variation of that parameter(say temperature) will still show the global variation happening on that plane ?