Learning Forum

[Ray5049]

Hello All,

I am running a simulation of a NACA0012-Airfoil in a rectangular domain with polyhedral mesh from ANSYS Fluent. For this simulation I need to compare the lift and drag coefficient of the airfoil with the experimental result from XFoil Data (http://airfoiltools.com/polar/details?polar=xf-n0012-il-1000000). For example, by 4degree the lift coefficient is 0.06932798 and the drag coefficient is 0.012277595. The percentage error for these values are more than 60% from the experimental result (cl=0.4276 and cd=0.00728). I would assume the mesh is fine enough as the minimum orthogonal quality reaches 0.30014. The Reynolds number that i used to compare in this simulation is 1.000.000 and the velocity of the flow is 146.07m/s. The boundary conditions and solution methods that I used are as shown from the screenshots. Is there a possible way that i could make my solution closer with the experimental results at this point? Thank you for your help.

 

With Regards,

Aimran 

 

 

[Rob]

I can't tell if you've used inflation: that will have a huge effect on lift & drag values. Also note that you're looking at coefficients: read the definitions for these in Fluent (in the documentation) and the paper.  

There are several tutorials/YouTube videos on NACA airfoils so see how they've done it. 

[Ray5049]

Hi rwoolhou,

Thank you for the reply. I used prisms layer at the wing of airfoil with the following settings:

 

I followed the steps as from the YouTube videos on NACA airfoils https://www.youtube.com/watch?v=WCpUsh6otmc&list=PLEBoXla1uL0FH78AvyXfHirtnNuD-uJ1U&index=11&t=0s but still the result does not satisfy the experimental results.

 

With regards,

Aimran

[Rob]

What y+ value are you seeing? How do the definitions of coefficient compare?

[Ray5049]

Hi rwoolhou,

the wall y+ value of the airfoil is between 10 and 130. Are the coeffcients from ANSYS fluent compared with the defined reference values? I have changed the area according to area of the airfoil (which is calculated through the surface integrals from results) and length with the chord length of the airfoil. The coefficents are defined from force reports as shown.

 

With regards,

Aimran

[Rob]

What does the manual/theory say about y+ values? How are the coefficients defined in the link?  

As an aside: staff are not permitted to open/download files, so I've not read any of it. 

[Ray5049]

Hi rwoolhou,

the y+ value shown i supposed should be resolve using the strategy: resolving the viscous sublayer. Therefore, the SST k-w model is used for this simulation. The drag coefficent are defined as x=cos(degree) and y=sin(degree). As for the lift coefficient, x=-sin(degree) and y=cos(degree).

With regards,

Aimran

[Rob]

Ah, but check the exact definition in Fluent, and in the paper. Just because both are lift/drag coefficients doesn't mean they're calculated in the same way.

k-w is looking for y+ of around 1: this will also improve lift/drag force calculations. Does the experimental work have any force data?

[Ray5049]

Where can i check the exact definition of these coefficients?

For the y+ value, should I remesh the entire geometry? The experimental work does not have any force data.

[Rob]

In Fluent, look in the User's Guide, in the experiment I have no idea. 

Read the section on adaption. You'll need to change the setting using the TUI for poly elements 

/mesh/adapt/set method    (I think).