Learning Forum

[Antonio Sola]

Hello! 

I'm simulating a laminar flow crossing a device in different cases (with modifications of its internal geometry). 

In case of a widening of the ducts of the internal geometry, the average velocity tends to decrease (It has sense because the diameters of the ducts are bigger). But the average pressure results show also the pressure tends to decrease and I don't know if it has sense, considering the pressure should increase with the widening of the ducts... The boundary condictions are: inlet velocity>0; inlet pressure=0; outlet pressure=0, for all cases.

My question is: what type of pressure measure Fluent? and, has it sense the pressure and the velocity decrease at the same time?

In a third case, with already bigger ducts, the effect is the same: average velocity and also pressure of the flow decrease...

Here is the system schematically:

Thank you in advaced.

[Rob]

What are you setting on the inlet in each case? 

[Antonio Sola]

I set an initial velocity, the same for all cases, and inlet and outlet pressure are zero for all cases.

[Rob]

That's fine. Why would pressure loss increase with a wider duct? Be wary of maximum values without checking where those values are. If you check the area average on the inlet how does it compare across the cases? How do the velocity & pressure contours look?

[Antonio Sola]

The average of pressure at the inlet and outlet I get are:  

[Antonio Sola]

At any point of the domain the behaior is the same.

[Rob]

So as the annular flow channel becomes larger the overall pressure loss decreases. 

[Antonio Sola]

Exactly

[Rob]

I'm lost - in the original post you thought the loss shouldn't decrease?

[Antonio Sola]

Sorry.

So as the annular flow channel becomes larger the overall pressure loss increases. That is the point.

[Rob]

But in the images the flow channel is smallest in #1 ?  Please post some Fluent results images. 

[Antonio Sola]

Beginning again... We have three cases:

 

 

Essentially are the same (same inlet and outlet channels diameters, same initial velocity at the inlet).

 

The unique difference is the diameter of the circular internal duct (circular duct diameter of case #1 is the smallest).

 

The problem I see is that increasing the diameter of the internal circular duct, the average pressure is smaller (when I think should be bigger, because the velocity should decrease and the pressure increase).

 

Here are new relations between the values gotten in the different cases:

 

 

As we can see, the average pressure decrease when circular duct diameter is bigger (it doesn't have sense for me).

 

Some images about the velocity and inlet profiles:

 

Comparison between Case #1 (left) and Case #2 (right):

 

 

 

Comparison between Case #2 (left) and Case #3 (right):

 

 

 

Thank you for your interest and best regards.

[Rob]

Inlet pressure will increase with Case 1 - it's a smaller channel so that makes sense. Domain average is a bit more complicated as it's dependent on where the pressure drop occurs. I'd also question the asymmetry in Case 1, how many cells are there in the annular gap? 

[Antonio Sola]

And what represents the pressure at the inlet? Why is bigger when the annular duct diameter is smaller?

I understand the domain average is more complecated because I don't have the same number of elements in all models and what you say, but the comparisons using the same scales show the pressure is always smaller (almost at all the points) when the annular duct increase its diameter...

In case 1 there are 13 cels in the annular gap..

Thank you!

[Rob]

What causes a pressure loss in a system? Check your notes, and possibly look up Moody Charts and discharge coefficients. 

You may want to review the mesh, it's fairly good but given the flow field you may want to look at growth rate. 

[Antonio Sola]

Ok! Thank you for your suggestions.

Could you please confirm me the pressure Fluent is measuring is the total manometric pressure, that means the sum of static pressure + dynamic pressure?

[Rob]

It's what you choose to check - "pressure" is static, but you also have other options. They're explained in the User's Guide, Chapter 42.