pressure drop in flow between two plates 2D

Hirayoki · July 11, 2018, 05:21

Hi ! i am totally beginner with fluent and i need to create very simple model, but i didnt found any tutorial which i could used for this case.

I have two plates with know dimension - hight is 4mm and lenght is 600m
i also know the velocity in the beginning of model, which is 3,6m/s and

in the first step i need to create model with this conditions and pressure 1,38bar in inlet and outlet to see decrease of velocity due the
frictional losses

i made mesh, but now i have little bit problem with boundary conditions, because when i use something like velocity inlet with that 3,6m/s and pressure-outlet or outflow, in both case i will get result with greater velocity in the outlet and in inlet, which is wrong

pls can you help me ?

LuckyTran · July 11, 2018, 13:06

It depends what equation of state you use for density whether you need to use an outflow or a pressure outlet. Pressure outlet is safer.

The boundary layer will accelerate the flow in between the two plates to make up for the slower fluid near the plates and maintain mass balance.

But how do you get a faster velocity at the inlet when you have specified the velocity? It should be 3,6 m/s uniformly on the inlet.

Hirayoki · July 12, 2018, 02:10

Quote:

Originally Posted by LuckyTran

It depends what equation of state you use for density whether you need to use an outflow or a pressure outlet. Pressure outlet is safer.

The boundary layer will accelerate the flow in between the two plates to make up for the slower fluid near the plates and maintain mass balance.

But how do you get a faster velocity at the inlet when you have specified the velocity? It should be 3,6 m/s uniformly on the inlet.

Its not faster in the inlet but in outlet, a just want to create simulation with specific velocity in the inlet of tube and see how big dissipation of velocity will be in outlet due to friction. But i think problem is somewhere in pressure, because for this case i need constant pressure in whole tube, but if you will look in picture fluent.jpg you can see that is not constant which is probably reason why is that velocity greater in the outlet

LuckyTran · July 12, 2018, 09:00

Maybe you should review your fluid mechanics. Why do you expect velocity to dissipate? That would violate conservation of mass & momentum. Consider that the tube is infinitely long, then do you expect 0 velocity the outlet? Then where does the flow go if there is 3,6 m/s entering the tube? And why do you think the pressure should be constant?

Leonardo.flores · July 13, 2018, 03:10

Just graph the X-velocity vectors. You will see how the velocity is high in the center and gets lower in the near-wall region, until 0 velocity in the wall. Check also that your wall boundaries (upper and lower plates) are with no-slip condition.

You can also draw a 600m line in the center and XY plot X-velocity vs Position. What do you get there?

July 11, 2018, 05:21	pressure drop in flow between two plates 2D	#1
Hirayoki New Member nvm Join Date: Jul 2018 Posts: 2 Rep Power: 0	Hi ! i am totally beginner with fluent and i need to create very simple model, but i didnt found any tutorial which i could used for this case. I have two plates with know dimension - hight is 4mm and lenght is 600m i also know the velocity in the beginning of model, which is 3,6m/s and in the first step i need to create model with this conditions and pressure 1,38bar in inlet and outlet to see decrease of velocity due the frictional losses i made mesh, but now i have little bit problem with boundary conditions, because when i use something like velocity inlet with that 3,6m/s and pressure-outlet or outflow, in both case i will get result with greater velocity in the outlet and in inlet, which is wrong pls can you help me ?

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July 11, 2018, 13:06		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	It depends what equation of state you use for density whether you need to use an outflow or a pressure outlet. Pressure outlet is safer. The boundary layer will accelerate the flow in between the two plates to make up for the slower fluid near the plates and maintain mass balance. But how do you get a faster velocity at the inlet when you have specified the velocity? It should be 3,6 m/s uniformly on the inlet.

July 12, 2018, 09:00		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	Maybe you should review your fluid mechanics. Why do you expect velocity to dissipate? That would violate conservation of mass & momentum. Consider that the tube is infinitely long, then do you expect 0 velocity the outlet? Then where does the flow go if there is 3,6 m/s entering the tube? And why do you think the pressure should be constant?

July 13, 2018, 03:10		#5
Leonardo.flores Member Leonardo Join Date: Nov 2017 Posts: 37 Rep Power: 8	Just graph the X-velocity vectors. You will see how the velocity is high in the center and gets lower in the near-wall region, until 0 velocity in the wall. Check also that your wall boundaries (upper and lower plates) are with no-slip condition. You can also draw a 600m line in the center and XY plot X-velocity vs Position. What do you get there?