Research at the Experimental Hydrodynamics Laboratory

Free Surface Hydrodynamics

Water Entry by Spheres      Breaking Waves     Naval Ballistics

Water Entry of Naval Ballistics T. T. Truscott & A. H. Techet  Above is a video excerpt from the Discovery Channel “Time Warp” episode titled “Breakdancing” which features research from the MIT EHL group! (First air date November 19, 2008) Summary The water-entry of ballistic projectiles is investigated using high-speed digital imaging to capture the subsurface cavity dynamics. Specially designed, .22 caliber projectiles were fired into the water at shallow angles to the free surface (5o to 15o) at Mach numbers between 0.3 and 1.0. Projectile tip geometry can be varied to allow projectiles to successfully enter the water and travel large distances underwater, due to the subsurface air-cavity that forms after impact, dramatically decreasing drag on the projectile. Projectile dynamics, critical entry angle, and cavity formation are discussed, and results show that successful water-entry is a factor of tip shape and length-to-diameter ratio. The cavity formed upon water-entry is compared to a modified theoretical model based on the classic Logvinovich model for supercavitating projectiles. The modified, theoretical cavity model compares well with the experimental data.  Experimental Setup at the MIT Rifle Range  Modified 0.22 Caliber bullets (example). APS Poster: Impact of Ballistics at Shallow Angles 2007 (pdf)  Bullet Entering the water at a large-scale testing at Aberdeen test facility  Video of modified 22 caliber slug water entry  Images of the cavities formed by a standard .22 bullet water entry (a) and a modified .22 (aluminum) with a length to tip diameter ratio of l/dtip = 7.2 (b). The bullets impacted the free surface at roughly the same speed of 250 m/s and are captured at the same time after impact. The standard 0.22 bullet tumbles and behaves erratically, whereas the slug-type bullet flies on a straighter path. http://web.mit.edu/ahtechet/www/posters/Truscott_Techet_Beal_APS2007b.pdfshapeimage_2_link_0