urgent problem : fluid height

ounifiras · December 9, 2013, 10:16

Dear experts :

I am trying to simulate a flow through a labyrinth weir. I put a condition to limit the left limit Xmin specific pressur (fluid height = 5.34 m) for X max It was puted as an overflow, Ymin Ymax & & Zmin Were puted as a Wall, Zmax WAS puted as symmetry , the initial condition for the fluid height was 5.34m. after completing the simulation OF RESULTS are shown in figures, the problem is that the height of the fluid does not remain constant, but for the example Weir height the fluid remains constant. i want to know how to have an initial height of fluid constant for all time

JBurnham · December 9, 2013, 10:47

It sounds like you set it up right, just like the Example simulation (Flow Over a Weir). Did you make an initial fluid region for fluid at t = 0? You should do that as well to speed up the time to steady state. After you do that, check your boundary conditions again and make sure they're what you described, because they sound correct. Good luck.

ounifiras · December 9, 2013, 10:55

Dear Jeff Burnham

I did not understand the initial fluid for fluid region at year t = 0. I have a condition that limits Xmin I put as specific pressur (Fluid height = 5.34m) and I created a fluid Region with ZLow = 0 and Zhight = 5.34m. i did not Understand the initial fluid for fluid Region at year t = 0.

Thanks greatly

JBurnham · December 9, 2013, 11:00

An initial fluid region is the same. It is fluid location at t = 0. Check that you have Gravity physics active, and that gz is negative (pointing downward), e.g. gz = -9.81 (if you're using SI units).

ounifiras · December 9, 2013, 11:32

yes i use the SI and gz = -9.81 . for the Xmax wich is puted as Outflow, can it cause a problem? In the exemple (Weir) I found this remark in the '' &bcdata'' : pbctyp=1.0, remark='specified boundary p=0.0 is stagnation pressure' AND fbct(1,5)=0., remark='no fluid below bottom boundary', . do you think that they have any influence in this problem

thank you greatly .

JBurnham · December 9, 2013, 12:26

Outflow boundaries are only correct when Froude number Fr > 1 (supercritical flow) at the boundary. If flow is subcritical (Fr <= 1), then Outflow will drain out the flow, so use a pressure-type boundary instead, with stagnation option checked and specified fluid elevation.

ounifiras · December 9, 2013, 17:17

thank you greatly

Mr Burnham do you think that the lenght of the chanel influence in the initial condition because this problem appear just with the channel wich has 100m as a lenght ?

December 9, 2013, 10:16	urgent problem : fluid height	#1
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	Dear experts : I am trying to simulate a flow through a labyrinth weir. I put a condition to limit the left limit Xmin specific pressur (fluid height = 5.34 m) for X max It was puted as an overflow, Ymin Ymax & & Zmin Were puted as a Wall, Zmax WAS puted as symmetry , the initial condition for the fluid height was 5.34m. after completing the simulation OF RESULTS are shown in figures, the problem is that the height of the fluid does not remain constant, but for the example Weir height the fluid remains constant. i want to know how to have an initial height of fluid constant for all time

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December 9, 2013, 10:47		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	It sounds like you set it up right, just like the Example simulation (Flow Over a Weir). Did you make an initial fluid region for fluid at t = 0? You should do that as well to speed up the time to steady state. After you do that, check your boundary conditions again and make sure they're what you described, because they sound correct. Good luck.

December 9, 2013, 10:55		#3
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	Dear Jeff Burnham I did not understand the initial fluid for fluid region at year t = 0. I have a condition that limits Xmin I put as specific pressur (Fluid height = 5.34m) and I created a fluid Region with ZLow = 0 and Zhight = 5.34m. i did not Understand the initial fluid for fluid Region at year t = 0. Thanks greatly

December 9, 2013, 11:00		#4
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	An initial fluid region is the same. It is fluid location at t = 0. Check that you have Gravity physics active, and that gz is negative (pointing downward), e.g. gz = -9.81 (if you're using SI units).

December 9, 2013, 11:32		#5
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	yes i use the SI and gz = -9.81 . for the Xmax wich is puted as Outflow, can it cause a problem? In the exemple (Weir) I found this remark in the '' &bcdata'' : pbctyp=1.0, remark='specified boundary p=0.0 is stagnation pressure' AND fbct(1,5)=0., remark='no fluid below bottom boundary', . do you think that they have any influence in this problem thank you greatly .

December 9, 2013, 12:26		#6
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	Outflow boundaries are only correct when Froude number Fr > 1 (supercritical flow) at the boundary. If flow is subcritical (Fr <= 1), then Outflow will drain out the flow, so use a pressure-type boundary instead, with stagnation option checked and specified fluid elevation.

December 9, 2013, 17:17		#7
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	thank you greatly Mr Burnham do you think that the lenght of the chanel influence in the initial condition because this problem appear just with the channel wich has 100m as a lenght ?