Employing controlled vibrations to predict fluid forces on a cylinder undergoing vortex-induced vibration

In the present study, we measure the fluid forces on a vertical cylinder that is forced to vibrate transversely to a water channel flow, and compare directly to the forces encountered by freely vibrating cylinders, under conditions where we carefully match the amplitude, frequency, and Reynolds number (Re) of the two cases. A key point is that we use precisely the same cylinder and submerged flow configuration for both the free and controlled cases. Where the free vibration exhibits closely sinusoidal motion, the controlled sinusoidal motion yields forces in close agreement with the free vibration case. Although this result might be expected, previous comparisons have not been uniformly close, which highlights the importance of matching the experimental conditions precisely, and of accurately measuring the phase between the force and body motion. For a lightly damped system, which is perhaps the most significant case to analyze, one typically finds that the maximum response amplitude is quite unsteady. One might conventionally expect prediction of forces to be difficult in such cases. However, it is of practical significance that, even in this case, a quasi-steady approximation is effective. This is a significant point because it suggests that controlled vibration measurements for constant amplitude motion might remain applicable to free vibration systems undergoing even transient or intermittent motions.