Experimental study on the trajectory characteristics of cylinders during asynchronous parallel high-speed vertical water entry

This study conducted asynchronous parallel high-speed vertical water entry experiments, acquiring trajectory for dual cylinders under varying lateral spacings and time intervals. The findings reveal that instability and tail slap phenomena are predominantly observed within the range of dimensionless time intervals less than 1 for the first cylinder. Instability manifests primarily as rotational instability. During the clockwise rotation of the first cylinder, the deflection angle exhibits an inverse relationship with the increasing time interval. When the first cylinder rotates anticlockwise, the deflection angle is proportional to the increase in the time interval. Regarding the second cylinder, the instability manifests primarily in three forms: collision instability, pressure difference instability, and puncture instability. As both lateral spacing and time interval increase, the second cylinder exhibits enhanced ballistic stability, concomitant with a diminished likelihood of tail slap occurrence. While the puncture process contributes to drag reduction for the second cylinder, it simultaneously introduces unpredictable forces acting upon the cylinder, potentially inducing instability. Furthermore, this study delineates the critical conditions of lateral spacings, and time intervals associated with the onset of puncture instability in the second cylinder. The research further revealed that the second cylinder consistently exhibits a lower drag coefficient than the first one, with a reduction of approximately 6.3%. Notably, during the puncture phenomenon of the second cylinder, the drag coefficient experiences its maximum reduction, decreasing by approximately 56.6%.