# Pressure distribution on a pier by CFX

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February 22, 2016, 05:50
Pressure distribution on a pier by CFX
#1
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muhamed
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Hello friends..
I did a simulation to get the pressure distribution on a bridge pier, which is resulted by the water drag force. I drew the pressure contours, as shown in the first picture, but the pressure I got is the water static pressure, gama* h. How can I get the pressure contours of the water force using CFX?
Attached Images
 Rectangular pier pressure.jpg (68.5 KB, 34 views) Rectangular pier.jpg (53.1 KB, 32 views)

 February 24, 2016, 07:05 #2 New Member   Join Date: Aug 2014 Posts: 12 Rep Power: 5 Hello, in your plot there is both, pressure from the flow and the hydrostatic pressure form the depth. the pump in the middle of the pier comes form the stagnation point. if you want the force on the pier, use areInt(Pressure)@ . One time on the front and once on the back and subtract them. that should give you the force. if you need a plot of the pressure du too flow, you can create an new Expression where you subtract the hydrostatic pressure: "Pressure - Density * 9.81 [m/s^2] * H", H is your depth form the surface. i hope, this helps you.

February 25, 2016, 05:35
#3
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muhamed
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Quote:
 Originally Posted by Cloudseeker Hello, in your plot there is both, pressure from the flow and the hydrostatic pressure form the depth. the pump in the middle of the pier comes form the stagnation point. if you want the force on the pier, use areInt(Pressure)@ . One time on the front and once on the back and subtract them. that should give you the force. if you need a plot of the pressure du too flow, you can create an new Expression where you subtract the hydrostatic pressure: "Pressure - Density * 9.81 [m/s^2] * H", H is your depth form the surface. i hope, this helps you.
Thank you for your reply. Frankly, I don't know how to add this expression and how to get the plot. Do yo have a tutorial showing how to do that? Thank you again for your help. Regards.

 February 25, 2016, 06:53 #4 New Member   Join Date: Aug 2014 Posts: 12 Rep Power: 5 this can help you a bit: http://www.padtinc.com/blog/the-focu...essions-part-1 after making the expression you need to make a variable to use it in plots: in cfd-post left of the expression tab there is a variable tab --> right click: new... ---> Method: Expression, Expression: your new one After that you can use this variable like any other in your plots

February 25, 2016, 07:25
#5
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muhamed
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Quote:
 Originally Posted by Cloudseeker this can help you a bit: http://www.padtinc.com/blog/the-focu...essions-part-1 after making the expression you need to make a variable to use it in plots: in cfd-post left of the expression tab there is a variable tab --> right click: new... ---> Method: Expression, Expression: your new one After that you can use this variable like any other in your plots
Thank you very much for your help. I will do that and see how it goes. Best regards.

 March 10, 2016, 04:41 #6 Member   muhamed Join Date: Jun 2013 Posts: 67 Rep Power: 6 Please, How it is possible to get the drag coefficient on the bridge pier using ANSYS CFX? I know that it is possible to get the drag force, which is force-x, from the CFD-post and In FLUENT it is possible to get that coefficient but how can I get or calculate the drag coefficient CFX? Regards.

 March 10, 2016, 05:25 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,721 Rep Power: 106 Define a CEL expression which evaluates the drag coefficient. CD is defined differently for lots of different applications so there is no universal definition - so you need to define it yourself for your case.

March 11, 2016, 03:34
#8
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muhamed
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Quote:
 Originally Posted by ghorrocks Define a CEL expression which evaluates the drag coefficient. CD is defined differently for lots of different applications so there is no universal definition - so you need to define it yourself for your case.
Thank you for your reply. Apart from the CEL, after getting the drag force (F-x) from CFD- post by using CFX, is it possible to calculate the drag coefficient using the following formula?

Cd = (Fx) / (0.5 * ρ * v^2 * A)

where cd is the drag coefficient, Fx is the water drag force on the pier in the flow direction, ρ is the water mass density, v is the flow velocity and A is the projected frontal area of the pier.

 March 11, 2016, 06:07 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,721 Rep Power: 106 That's why CFX has CEL - to implement equations like this with a simple method. If you don't understand CEL then do some of the CFX tutorials and see how they do it. It is quite straightforward. Mohammad80 likes this.

March 15, 2016, 05:17
#10
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muhamed
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I used the CFX program and I have a problem with the resulted drag force. the flow velocity is very low (0.0125 m/sec) while the resulted drag force on the pier is very high (1095.98 N) as shown in the attached picture. I checked all the units and there is no problem. the pier diameter (the pier width) is (0.4 m) and the water depth at the inlet and at the outlet is (0.8 m). The channel sides are supposed to be "symmetry" and the bottom was assumed to be a wall and the top side was "opening". I'm confused and don't know where is the problem...
Attached Images
 Problem with a pier.jpg (79.4 KB, 13 views)

 March 15, 2016, 08:19 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,721 Rep Power: 106

March 25, 2016, 04:16
#12
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muhamed
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In the attached picture I did the same simulation of the pier into a channel using FLUENT but the drag force I got is negative. I used a pressure inlet and pressure outlet with giving the water and bottom levels by ticking on the open channel choice. Any tips please?
Attached Images
 problem1.jpg (105.5 KB, 7 views) problem11.jpg (102.9 KB, 6 views) problem111.JPG (120.4 KB, 7 views)

October 5, 2016, 04:30
#13
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muhamed
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Hi,
It's a long time since the last comment of you....
I have a problem with the wind pressure distribution on a building. As you can see from the attached picture (Picture 1), which I downloaded it from the internet, the pressure at the lower part of the building is much less than that at the upper part and this is expected to be happened because the wind velocity near the ground is much less than that at the top.
The problem I faced is that the wind pressure be the same along the height of the building (Picture 2). Could you please help me?

Do I have to use a UDF for a parabolic inlet flow velocity? if yes, Please, do you have a UDF for a 3-D open channel problem? Regards
Attached Images
 Picture 1.jpg (50.6 KB, 6 views) Picture 2.jpg (57.0 KB, 6 views)

 October 5, 2016, 05:50 #14 New Member   Join Date: Aug 2014 Posts: 12 Rep Power: 5 Hello, in your last post you mentioned using Fluent. So if you still use it, you better ask in the Fluent section. if you use CFX: You need to create a profile boundary condition file. look here: http://www.cfd-online.com/Forums/cfx...ile-inlet.html

October 5, 2016, 06:13
#15
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muhamed
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Quote:
 Originally Posted by Cloudseeker Hello, in your last post you mentioned using Fluent. So if you still use it, you better ask in the Fluent section. if you use CFX: You need to create a profile boundary condition file. look here: http://www.cfd-online.com/Forums/cfx...ile-inlet.html
Thank you for your reply. Frankly, I tried the both programs. Do you think that I need a UDF file to get the same result as in the first picture (Picture 1)? or just I have to play with some variables inside the program to get it?

 October 5, 2016, 06:19 #16 New Member   Join Date: Aug 2014 Posts: 12 Rep Power: 5 I only tried fluent for an short example. But I think you need an UDF, an input file, or something like that.

October 5, 2016, 06:26
#17
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muhamed
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Quote:
 Originally Posted by Cloudseeker I only tried fluent for an short example. But I think you need an UDF, an input file, or something like that.
Thank you again dear Cloudseeker. Just one more question, is it possible to divide the inlet face into small divisions and give each one a velocity value in order to get a parabolic profile of the inlet flow velocity instead of writing a UDF ? if yes, this will be more easier for me than writing a UDF.

 October 5, 2016, 06:31 #18 New Member   Join Date: Aug 2014 Posts: 12 Rep Power: 5 I would go with the UDF, since you are likely to need them in the future for more complicated problems. https://www.youtube.com/watch?v=LQ2y0sIXgio

October 5, 2016, 06:57
#19
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muhamed
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Quote:
 Originally Posted by Cloudseeker I would go with the UDF, since you are likely to need them in the future for more complicated problems. https://www.youtube.com/watch?v=LQ2y0sIXgio
I have seen that tutorial before, it talks about a 2-D case while mine is a 3-D case. Anyway, thank you very much for your help. Best regards.

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