[Darius]

Originally Posted by ghorrocks

He he he. I cannot count the number of times I have been told that on the forum. Trust me, your mesh is the cause of the error.

Simply adding more mesh will not help. Have a look on the web at airfoil meshes (google it) and you will see some very nicely crafted meshes.

When I check the quality the leading edge is the main issue of not getting better mesh.... I wanted to build the exact model as in the wind tunnel otherwise if I make an edge on the trailing edge then I could make the mesh finer... I know it's not about having too many elements but I still think the results are way too off from W.T test that could be fixed by just a bit finer mesh

[flotus1]

Originally Posted by Darius

This's the whole CFX expression for NACA 23012 AIRFOIL on zero angle of attack model to get CL, CD and CM:

LIBRARY:
CEL:
&replace EXPRESSIONS:
AOA = 20[deg]
Denom = 0.5*massFlowAve(Density)@Inlet*Uinf^2*0.129295 [m^2]
Drag = cos(AOA)*Fx+sin(AOA)*Fy
Fx = force_x()@Airfoil
Fy = force_y()@Airfoil
Lift = cos(AOA)*Fy-sin(AOA)*Fx
Mz = torque_z()@Airfoil
Moment = Mx+My+Mz
Mx = torque_x()@Airfoil
My = torque_y()@Airfoil
Uinf = 35[m s^-1]
Ux = Uinf*cos(AOA)
Uy = Uinf*sin(AOA)
cD = Drag/Denom
cL = Lift/Denom
cM = Moment/Denom*0.18[m]
END
END

Maybe it's good for students to use it

That does not really answer my question.

Originally Posted by flotus1

And how exactly does that alter the geometry or the mesh of an airfoil with initially zero angle of attack?

Show a screenshot of the whole computational domain you used in the second simulation where the AoA was miraculously changed through a CFX expression.
We need a screenshot, another mobile phone photo wont help.

[Darius]

Originally Posted by flotus1

That does not really answer my question.


Show a screenshot of the whole computational domain you used in the second simulation where the AoA was miraculously changed through a CFX expression.

Sorry I don't get your question... I've got two but the same airfoils, one has been rotated 20 degrees in solidworks and the other one was used the expression to apply the AOA.... I've uploaded the domain picture and the both domains are just the same so I don't get what you mean by how the expression alters the geometry

[flotus1]

Let me recapitulate what I understood: You have two DIFFERENT geometries.

In geometry 1, the airfoil is rotated by 20 degrees around the z-axis.
The flow direction at the inlet is in x-direction so the angle of attack of the flow is actually 20 degrees.
You evaluate lift and drag forces in Y- and X-direction respectively. I agree with this setup.

In geometry 2, the airfoil in not rotated.
If the flow enters in X-direction, there is actually 0 angle of attack.
Even if you account for an angle of attack by changing the inflow direction, the flow will be straightened by the side walls and the airfoil will not "see" the AoA you entered at the inlet.
It seems to me that all you did was changing the vectors of force evaluation.

So from what I understood so far, your two computational setups do not represent the same physical setup.

[Darius]

Originally Posted by flotus1

Let me recapitulate what I understood: You have two DIFFERENT geometries.

In geometry 1, the airfoil is rotated by 20 degrees around the z-axis.
The flow direction at the inlet is in x-direction so the angle of attack of the flow is actually 20 degrees.
You evaluate lift and drag forces in Y- and X-direction respectively. I agree with this setup.

In geometry 2, the airfoil in not rotated.
If the flow enters in X-direction, there is actually 0 angle of attack.
Even if you account for an angle of attack by changing the inflow direction, the flow will be straightened by the side walls and the airfoil will not "see" the AoA you entered at the inlet.
It seems to me that all you did was changing the vectors of force evaluation.

So from what I understood so far, your two computational setups do not represent the same physical setup.

Thanks for the reply.
You got it right for the first part. I didn’t claim that the AOA for the non-rotated model has not been accounted or applied in the simulation. Its results are different to the zero AOA but very different to the first one.

The AOA has been defined by the expression, and everything else such as the mesh, domain, inlet and outlet pressare and etc.. are just the same so I expect to get the same results.
The only difference is that one AOA was made in solidworks and the other one was accounted by expression the rest are all the same... that's all.

I don't know what you mean by "the flow will be straightened by the side walls"

[flotus1]

Leaving aside expressions for force evaluation for a moment:
Are the two setups identical in terms of geometry and boundary conditions?

[Darius]

Originally Posted by flotus1

Leaving aside expressions for force evaluation for am moment:
Are the two setups identical in terms of geometry and boundary conditions?

Yes... all the geometry, sizings domains, mesh, mesh quality, boundary layer, boundary conditions, flow set-up are all the same.

I accidentally discovered that these two models are giving different results cuz I couldn't get the similar results as the wind tunnel test I made earlier in the university's lab. So I wanted to try different way and I got surprised that I get different values. The lift on the rotated model simulation gave 52N while the non-rotated model gave 21N and the wind tunnel gave 98N. That's why I got so confused.

I tried to get the result by much simpler softwares like Designfoil and javafoil and their results were very closer to the wind tunnel test data than the CFX... I also tried to simulate it with fluent but I still got the similar results as the CFX.

[ghorrocks]

I have deleted a number of posts from this thread as the discussion went off-topic.

Please ensure that all future posts on this thread remain on topic.

[singer1812]

You guys are wasting time trying to come up with a solution on why the solutions dont match each other, and why they dont match wind tunnel.

Glenn answered your question in detail, your mesh is crap. Fix that and you will get better results. If you are just calling this into Ansys mesh in WB, setting a few sizes and hit the mesh button, you are going to get a terrible mesh, like what you have shown.

You need to control the inflation rate, the expansion rate and the size on the surface.