# flow problem upon a step

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 July 17, 2013, 08:11 flow problem upon a step #1 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 4 Hello. I am modeling a channel that has a step for entering air into the flow. After simulation, the water is fulling the middle of step and the jet doesn't form. I don't know why after simulation the flow doesn't cross properly upon the step? In the stream line it looks like the flow condition is good. What is difference between volume fraction and streamline or particle? I'll be grateful if you can help Thanks Last edited by flow_CH; July 19, 2013 at 03:45.

 July 17, 2013, 16:59 #2 Member   Stephan Langenberg Join Date: Sep 2011 Location: Germany Posts: 73 Rep Power: 5 Your aim is that the air forms a jet (a stramline)strough the water? I'm not that fit in the difference between multi-phase modells, but you should proove if the VOF modell is more usefull than Eulerian... I've made good results for phase separation with the explict geo reconstruct solver of VOF models. Volume Fraction is a weightage for every cell. If mp-1 is 0,5 you have 50% of this phase in your controll-volume. If Ansys did it as the common fluid dynamic definitions a streamline is a geometrical visualisation of the flow (common fluid dynamic literature may help).

 July 18, 2013, 00:05 #3 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 4 This is VOF multiphase model. My question is why jump is not form upon the step? The physics of flow before the step is good but after the step flow going to middle of channel and not going straight to downstream.

July 18, 2013, 02:14
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shubham
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Streamlines
Definition: Streamlines are the Geometrical representation of the of the flow velocity.
Description: In the Eulerian method, the velocity vector is defined as a function of time and space coordinates.
If for a fixed instant of time, a space curve is drawn so that it is tangent everywhere to the velocity vector, then this curve is called a Streamline.
Therefore, the Eulerian method gives a series of instantaneous streamlines of the state of motion
Alternative Definition:A streamline at any instant can be defined as an imaginary curve or line in the flow field so that the tangent to the curve at any point represents the direction of the instantaneous velocity at that point.
In an unsteady flow where the velocity vector changes with time, the pattern of streamlines also changes from instant to instant.
In a steady flow, the orientation or the pattern of streamlines will be fixed.

Pathlines
Definition: A path line is the trajectory of a fluid particle of fixed identity. Pathlines are the trajectories that individual fluid particles follow. These can be thought of as "recording" the path of a fluid element in the flow over a certain period. The direction the path takes will be determined by the streamlines of the fluid at each moment in time.

Volume Fraction
Definition: In simple terms it is defined as the fraction on volume occupied by a particular thing.
Suppose we have a glass half filled with water than we can say that 50% of total volume of glass or 0.5 fraction of total volume of glass is filled with water and rest 50 % is air.
Important to note that sum of all volume fraction values is always 1 or 100%

Similar concept is for CFD. for example, imagine two phase flow (Ph1 and Ph2) than the fraction of cell volume occupied by phase Ph1 is "alpha1" than fraction of cell volume occupied by phase Ph2 is "1 -alpha1"

For better understanding you can go through these links
http://www.princeton.edu/~asmits/Bic...treamline.html
http://web.mit.edu/fluids-modules/ww...rial-spsl.html
http://www.nptel.iitm.ac.in/courses/...streamline.htm
Attached Images
 Pathlines in two phase flow.jpg (56.6 KB, 4 views) Pathlines.jpg (19.8 KB, 3 views) Streamline.jpg (13.3 KB, 3 views)

July 18, 2013, 04:48
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Saeed
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Quote:
 Originally Posted by shubham208011 Streamlines Definition: Streamlines are the Geometrical representation of the of the flow velocity. Description: In the Eulerian method, the velocity vector is defined as a function of time and space coordinates. If for a fixed instant of time, a space curve is drawn so that it is tangent everywhere to the velocity vector, then this curve is called a Streamline. Therefore, the Eulerian method gives a series of instantaneous streamlines of the state of motion Alternative Definition:A streamline at any instant can be defined as an imaginary curve or line in the flow field so that the tangent to the curve at any point represents the direction of the instantaneous velocity at that point. Comments: In an unsteady flow where the velocity vector changes with time, the pattern of streamlines also changes from instant to instant. In a steady flow, the orientation or the pattern of streamlines will be fixed. Pathlines Definition: A path line is the trajectory of a fluid particle of fixed identity. Pathlines are the trajectories that individual fluid particles follow. These can be thought of as "recording" the path of a fluid element in the flow over a certain period. The direction the path takes will be determined by the streamlines of the fluid at each moment in time. Volume Fraction Definition: In simple terms it is defined as the fraction on volume occupied by a particular thing. Suppose we have a glass half filled with water than we can say that 50% of total volume of glass or 0.5 fraction of total volume of glass is filled with water and rest 50 % is air. Important to note that sum of all volume fraction values is always 1 or 100% Similar concept is for CFD. for example, imagine two phase flow (Ph1 and Ph2) than the fraction of cell volume occupied by phase Ph1 is "alpha1" than fraction of cell volume occupied by phase Ph2 is "1 -alpha1" For better understanding you can go through these links http://www.princeton.edu/~asmits/Bicycle_web/streamline.html http://web.mit.edu/fluids-modules/ww...rial-spsl.html http://www.nptel.iitm.ac.in/courses/...streamline.htm
Then can i say: According to my image, water is collected into the step (stair) with approximately volume fraction=0.85 ?
If yes then can you tell me why the flow can not jump over the step?

 July 18, 2013, 05:31 #6 Member   shubham Join Date: Mar 2009 Posts: 48 Rep Power: 8 In my opinion its not like what u r saying. I don't know the exact geometry and boundary conditions of ur problem but from seeing the pic you attached things i can predict for flow time t =1.77 are: 1.) in upper portion VoF of air is maximum (red color) means in that particular region amount of water is minimum (or nil as vof for air is 1) 2.) in the region shown by bright blue color; vof of air is negligible means in these regions of amount of water is much much higher than air (mostly water only) 3.) in the region shown by light blue, green and yellow colors water and air co-exist To draw a solid conclusion from vof of air or water, i recommend you to view contours from different view angles so that you can visualize the whole volume and use clip to range option to see the contours according to your requirement.

July 18, 2013, 05:46
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Saeed
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Quote:
 Originally Posted by shubham208011 In my opinion its not like what u r saying. I don't know the exact geometry and boundary conditions of ur problem but from seeing the pic you attached things i can predict for flow time t =1.77 are: 1.) in upper portion VoF of air is maximum (red color) means in that particular region amount of water is minimum (or nil as vof for air is 1) 2.) in the region shown by bright blue color; vof of air is negligible means in these regions of amount of water is much much higher than air (mostly water only) 3.) in the region shown by light blue, green and yellow colors water and air co-exist To draw a solid conclusion from vof of air or water, i recommend you to view contours from different view angles so that you can visualize the whole volume and use clip to range option to see the contours according to your requirement.
I know all things that you said. I tested the clip for solid conclusion too but the water is collected into the step. I am very confused

 July 18, 2013, 05:51 #8 Member   shubham Join Date: Mar 2009 Posts: 48 Rep Power: 8 can you send me the geometry file and exact boundary conditions u used at different inlet and outlet??

 July 18, 2013, 08:47 #9 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 4 The size of my geometry is high that can i send. My slope of channel is about 31 degree and velocity of water is 4 m/s. But i don't know why the flow can not cross over the step. It seems the flow of air in the aerator is pulling the inner flow to middle of channel but this is not possible because the volume of water into the channel is high.

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