Strange values in drag coefficient of a fuselage
 

Hi everyone,

This is my first thread but I have been reading you for long time. Now I have a problem with one simulation and I am writing to you in the hope that you can help me.

First let me explain my case.

I have to compute the drag and lift coefficients of a fuselage of a helicopter for some values of angle of atack. Next picture is the shape of the fuselage made by CATIA:

http://s10.postimg.org/ldix6bpux/geometry_catia.jpg

I have created a symmetric boundary volume surronding the fuselage like this:

http://s28.postimg.org/hgi5iarbx/mesh.jpg

blue face: velocity inlet (50 m/s)
red face: pressure outlet
hollow with the shape of the fuselage (white mesh): wall

After set the turbulunce model and the boundary conditions I have changed de reference values.

http://s27.postimg.org/4uag665zn/reference_values.jpg


The projected area of half fuselage in Z-axis is 0.9 m2 and the length of the fuselage 3 m

Then, I create the monitors of drag and lift coefficient and I run the case. After reach a "good" convergence this is the value of my coefficients:

http://s12.postimg.org/3qhz0rz8t/con...efficients.jpg


As you can see the value of drag is extremely small (0.05). The normal drag coefficient for a fuselage of an helicopter like this must be between 0.5 and 0.1. Is almost 10 times smaller.
I am a absolute begginer in fluent, therefore I think that I am doing a big mistake in some step: maybe I have use a wrong reference values or I have used a different scale or dimensions. I don't know.
Does anyone know what I am doing wrong? Does anyone have any ideas to improve my results?

All help or idea is welcome! Thank you!

hi
You are saying that drag coefficient value should be 0.1.Your value is coming 0.04995 i.e 0.05 X 2=0.1 As you have used symmetry zone,you have to multiply the value by 2.

Rest you can improve your result by refining the mesh.

If I carry out the same simulations using the whole fuselage (non symmetric case) I get the same results: 0.05.
I think that the coeffcients values are independent of that.

I have the same problem with parabolic trough
 

I have the same problem with parabolic trough. Please any explanation for that !

Hi,
Have u scaled ur model after reading mesh in fluent? i.e you have to tell Fluent that ur model is modeled in 'mm' scale or 'meter' scale.
you do one thing
read mesh file> scale > select spallart allmaras turbulent model > use ideal gas condition >give symmetry wall as symmetry boundary condition ..rest fluid domain to pressure far field. initialize the flow and solve it....then check the value.

For getting drag force it is needed to be multiplied by 2. But in drag coefficient its not needed because increase of the frontal area compensate this.
As you have set the reference values correctly, just check your scale as gunjan said to be realistic.