Hi

alpha(t) is harmonic motion and omega[2] is angular velocity.

We use angular velocity because of boundary condition.

Generally, you have 3 different motion (2D cases):

1. Pitch which is rotation about an axis.

2. Heave which is vertical translation.

3. Surge which is horizontal motion.

As I wrote before, If you have pitching motion, it rotates about an axis, in fact you need angular velocity.

Now, If you have heave motion, you have something like this:

h(t)=h0+amplitude*sin(...)

In this case you need linear velocity instead of angular velocity.

DEFINE_CG_MOTION(airfoil, dt, vel, omega, time, dtime)

In UDF, vel and omega represents linear and angular velocity respectively.

vel[0] means: linear velocity in x-direction (or surge motion)

vel[1] means: linear velocity in y-direction (or heave motion)

vel[2] means: linear velocity in z-direction

omega[2] means: angular velocity about z axis.

Base on motion, you choose which one of them must be used.

Best Regards.

Thanks so much ,Alireza .

So, we don't need to define alpha(t) & omega is fine to model pitching. also alpha0 is determined in B.C,inlet velocity. yeah?

Hi

Not exactly. Yes, you don't need to define alpha(t), also you don't need to alpha(t) as B.C. in inlet velocity.

Based on your Re, you set a velocity at inlet and use define_cg_motion (just omega) for pitching airfoil.

Best Regards

@ Alireza Beiki

Appreciate !