I am analysing flow over wing but I found out that at low angle of attack between 0 and 10, the lift coefficient is reducing instead of it increasing with angle of attack while the drag coefficient is increasing. I will appreciate if someone can tell me what is the cause.
I am a new user of Ansys fluent and to CFD
Thanks
Because one possibility is there to give the wrong velocity component
in fluent
Firstly, check the velocity field and/or fluid pathlines maybe you will identify the mistake in the boundary conditions data. Possibly signs are changed!! check it out.
first, I think it is important to check if the boundary conditions have been prescribed accurately. then check if you are respecting the Y+ value for the specific turbulence model that you are using. Finally, make sure if the dynamic pressure in the lower side of the wing is higher than the upper side.
Thank you all for the contributions.
I have been able to set up the simulation and run the calculation. This time the lift coefficient was increasing for different angle of attack up until alpha=5 but at alpha =6 the CL becomes lower than what it is in alpha =5. Please, what could cause the drop.
Also, I calculated yplus value after several calculations(iterations) and got 764.2138. Please, is this value ok or it's too high.
I am using K-omega (2eqn) SST model.
Yplus value is too high. Please add your mesh inflation. ıf You use sst model, generally yplus must be 0-4 I think. Also, before cfd analysis. You can use xflr5 software. You can predict true values. Finally, please check your reference values.
There is a Possibility to run the same experience of changing gradually the angle of Attack using XFoil in MATLAB package. This can be more easy and useful if Ansys it's a bit difficult to set it up correctly .
The Y+ value depends on the turbulence model you are using. if you are using a turbulence model with a wall function then the Y+ value must be in a range from 30 to 100. However, if you are using a turbulence model, which doesn't need a wall function (more accurately, the turbulence model resolves the boundary layer) the Y+ value must lay in a range less than 5. Another point that must be considered here is the behavior of the fluid flow around airfoil at high angles of attack. because in this range the flow is an-isotropic and the Reynolds stress turbulence models are more favorable to be used. The usual two-equation models (such as k-e e k-w) fail to capture the fluid flow characteristics at high angles of attack, since they are based on the assumption of isotropic behavior of turbulence.
Check your reference values. Also, if you plot lift and drag values by the means of calculating sin and cos values, check that these values sign is correct.
One possibility is maybe are you applying the wrong component of wind velocity
in fluent
apply the input velocity of domain, components x direction and y- direction
v.cos theta
v.sin theta
theta is the angle of attack
v undisturbed wind speed or wind velocity
I hope this explanation will helpful for you
Lift is calculated perpendicular to freestream flow. You need to change the force vector components so that they are perpendicular to the flow.
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