[Mohammad80]

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?

[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.

[Mohammad80]

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.

[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

[Mohammad80]

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.

[Mohammad80]

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.

[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.

[Mohammad80]

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.

[ghorrocks]

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]

Thank you for your reply.
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...

[ghorrocks]

FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

[Mohammad80]

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?

[Mohammad80]

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

[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

[Mohammad80]

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?

[Cloudseeker]

I only tried fluent for an short example. But I think you need an UDF, an input file, or something like that.

[Mohammad80]

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.

[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

[Mohammad80]

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.