In my experiment set up, I build a 2 DoF vibration system using springs and low friction slides. I've attached the force sensor on the vibration system and then link it with the cylinder, then conducted the test in a 20cmx20cm water channel, the Reynolds number is around 3000. At low reduced velocity near the initial branch, I can find that the phase lag between the vibration and the force is around 0. But, once the reduced velocity is larger and reaches the lower branch, the phase lag is very chaos and I cannot find it to be around 180 degree. Also I tried to correct the force data by subtracting the inertia force of vibrating cylinder, but I still cannot find the 180 degree phase lag. Could someone give me some tips on my set up and data pre-processing?