We've all been mesmerized for years by that thing of beauty rising out of the water majestically in that James Bond movie... If you're thinking Ursula Andress or Halle Berry or Daniel Craig, think again. I'm an Engineer and to me, the submarining Lotus Espirit from The Spy who Loved Me is my equivalence of a teenager's infatuation with water entry of matinee idols. In my fantasy world, my car would run on road, swim in water, fly in air and even brew me a nice, cold beer while I watch football. The Lotus Espirit ticked off two of those boxes quite handsomely.
When I was tasked with showing off new application areas with STAR-CCM+’s Overset Mesh feature, I scoured the internet for a model of the Lotus Espirit. In the end, I had to settle for a worthy substitute, the Assault Amphibious Vehicle (AAV7A1) used by the United States Marine Corps. The engineering design challenges for an amphibious vehicle are primarily the reason they are few and far between. In the case of vehicles like the AAV7A1, requirements of dual capability of speed in water and land in extreme conditions combined with a light armor to keep the weight down result in working within a tight design space. One of the major concerns when designing such a vehicle is the crossing from land to water or vice versa. Safety of the crew and the vehicle as a whole is of paramount importance when crossing into a different environment.
Numerous parameters play a role in the vehicle’s behavior when entering water or land: vehicle speed, direction relative to water and slope of the beach being the most important. Key factors in assessing the safety of the beach ingress are the maximum pitch and roll angles as the vehicle enters water, maximum accelerations on the vehicle and flooding of engine and passenger compartment. The body shape, weight distribution and size of the vehicle are already defined based on the mission requirements to achieve proper performance for land or sea. Safety tests are usually done on a prototype in a test basin using varying angles of ground slope and speed for entry into the water.
Simulation of water entry is currently being used in analysing the vehicle body shape; correcting for proper trim, body accelerations and flooding behavior before testing a prototype. Amphibious vehicles are well known for ballooning design times and project costs stemming from extensive testing for adequate safety. Simulation can reduce both time and cost, while giving insight on vehicle behavior.