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August 26, 2008, 05:12 
drag coefficient of cylinder

#1 
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hello friends
i want to get the drag coefficient of cylinder2D. my boundary conditions are velocity_inlet, pressure_outlet, sidewall and wallcylinder. i use laminar model because Re Number is 500. i checked my REFERENCE VALUES yet. but my calculation is wrong compare with experimets data. please teach me what should i check the anything else. thank you for your help. 

August 27, 2008, 09:18 
Re: drag coefficient of cylinder

#2 
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i am not sure if you can use pressure outlet with velocity inlt. try changing it to out flow.


August 27, 2008, 12:25 
Re: drag coefficient of cylinder

#3 
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Hi joh, thanks for your advise. but i have already tried it.so have i made a mistake fundamentally?
as for Cd graph, it form fluctuation, isn't it? thanks a lot 

August 28, 2008, 12:12 
Re: drag coefficient of cylinder

#4 
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what do ya mean by fluctuation. if you a taking results at given conditions, all you should get is a number.
what is the dimension of the cylinder and what is the distance of your first cell from cylinder. 

August 28, 2008, 12:19 
Re: drag coefficient of cylinder

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The flow around a cylinder at Re=500 is unsteady and three dimensional. I am not sure if a 2D steady simulation can accurately calculate the CD.


August 28, 2008, 12:21 
Re: drag coefficient of cylinder

#6 
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but consider a cylinder between two walls in ideal fluid (inviscid). still 3D flow? i dont know


August 29, 2008, 03:30 
Re: drag coefficient of cylinder

#7 
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im doing a 2D simulation and set a cylinder by making circular hole in a rectangle. so i get the Cd with solve>monitors>force>drag.
i show my setup details. i used laminar model cylinders diameter is 0.1m. a rectangle area 1*2 m^2 velocity_inlet v=0.0734m/s. material is air(dencity is constant). time step size is 0.0005 i set Unsteady Formulation>second order Implicit Gradient option >GreenGauss Node Based Residual monitors>Absolute Criteria is 1e^6 Residual monitors>Relative Criteria is 0.001 i did not use adapt and surface tool. i set the Boundary layer which first row from cylinder is 0.001 in GAMBIT. Is this correct? thanks for your help 

August 29, 2008, 05:32 
Re: drag coefficient of cylinder

#8 
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i had a problem lately with my 2d results. and it turned out that the distance of my first cell was too little. increase the distance of the first cell to 0.2 mm and see if it works. i think it should.


August 29, 2008, 10:43 
Re: drag coefficient of cylinder

#9 
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Did you check your y+ values and compare those to the expected values for the turbulence model you selected?
Thanks, Jason 

August 29, 2008, 13:37 
Re: drag coefficient of cylinder

#10 
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Are you sure your reference values are set properly?


August 29, 2008, 21:56 
Re: drag coefficient of cylinder

#11 
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thanks your response i used cylinders diameter 0.1(m). A rectangle area 1*2(m^2). velocity_inlet v=0.0734(m/s).
so i show my REFERENCE VALUES. area is 0.1 m^2. depth is 1m. Length is 0.1m. velocity is 0.0734m/s. Pressure is 0 Pa. Density and Viscocity are default of air. Is this correct? and the distance of the first cell from cylinder is 0.1mm in my simulation. as for y+, i used laminer model. is it possible to check the y+ in laminer model simulation? my calculation is wrong. please help me 

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