How to prevent backflow when airfoil starts heaving
 

Hi,

I am trying to simulate an airfoil doing pure heaving, up and down.

My udf is:

/* Compiled udf for modelling plunging airfoil in Fluent*/
#include "udf.h"

/*
The equation of the plunging motion y(t)=amp*sin(2*pi*f*t)

amp = plunge amplitude
f = plunge frequency
*/

DEFINE_CG_MOTION(plunge,dt,cg_vel,cg_omega,time,dt ime)
{
Thread *t;
face_t f;
real k = 0.58;
real amp = 0.175;
real Uinf = 1.;

real omega;
real vel;
real freq;

freq = 0.58;
omega = 2.0*M_PI*freq;

/*reset velocities*/
NV_S(cg_vel, =,0.0);
NV_S(cg_omega, =,0.0);

/*get the thread pointer for which the motion is defined*/
t=DT_THREAD(dt);
vel = omega*amp*cos(omega*time);
cg_vel[1]=vel;
Message("vel %f time %f k %f h %f\n",cg_vel[1],time,k,amp);
}

The BC at outlet is simply outflow or pressure outlet.

My initial simulation was at Re=10000. All went fine.

However, when I tried to simulate at Re=100, backflow happens, after about 200 timesteps. Tried to reduce the time interval but it still happens.

What can I do to prevent this from happening?

Thanks!