We are pleased to announce that our article “The water entry of slender axisymmetric bodies” was just published in Physics of Fluids!!! Congratulations to Kyle, Stephen and Tadd. This work was really fun and was the first time we utilized an Inertial Measurement Unit synchronized with our imaging data. The IMU was developed by Stephen and processed by Kyle. Kyle was able to show that the IMU was more accurate in determining forces, accelerations, velocity and position than the images. He also showed that the IMU was able to determine the center of pressure for the rigid objects! This was an incredible break through for our group and we thank the reviewers for encouraging us to try this. Link.
We present a study of the forces, velocities, and trajectories of slender (length/diameter = 10) axisymmetric projectiles using an embedded inertial measurement unit (IMU). Three nose shapes (cone, ogive, and flat) were used. Projectiles were tested at vertical and oblique impact angles with different surface treatments. The trajectory of a half-hydrophobic and half-hydrophilc case impacting vertically was compared to the trajectory of symmetrically coated projectiles impacting the free surface at oblique angles. The oblique impact cases showed significantly more final lateral displacement than the half-and-half case over the same depth. The amount of lateral displacement was also affected by the nose shape, with the cone nose shape achieving the largest lateral displacement for the oblique entry case. Instantaneous lift and drag coefficients were calculated using data from the IMU for the vertical, half-and-half, and oblique entry cases. Impact forces were calculated for each nose shape and the flat nose shape experienced the largest impulsive forces up to 37 N when impacting vertically. The impact force of the flat nose decreased for the oblique entry case. The location of the center of pressure was determined at discrete time steps using a theoretical torque model and values from the IMU. Acoustic spectrograms showed that the sound produced during the water entry event predominately arises from the pinch-off for the cone and ogive nose shapes, with additional sound production from impact for the flat nose shape. Each test run was imaged using two Photron SA3 cameras.