麻豆传媒

News — While Olympic divers strive for minimal splash as they enter the water, a popular diving style of the M膩ori people in New Zealand, known as Manu jumping, looks to do just the opposite! Instead of receiving points for a clean entry into the water, divers, colloquially, “popping the Manu” are judged based on the size and height of the splash they create. Drs. Pankaj Rohilla and Daehyun Choi, both Postdoctoral Fellows in the Bhamla Lab at Georgia Tech, spearheaded the investigation into the fluid dynamics that create the largest splash.

To answer this question, Pankaj and Daehyun first analyzed publicly available YouTube videos with an ideal side view of humans executing the Manu. “We estimated the kinematics assuming the average height of a person is about 1.71 m in New Zealand, so using that data, we estimated the splash height,” Rohilla explained.

This video analysis revealed four distinct stages of a Manu jump: (1) the body enters the water in a V-formation, (2) the body opens underwater to enlarge the air cavity, (3) the cavity closes and collapses, and (4) the air cavity displaces the water forcing it upward, known as a Worthington jet, to produce the resulting splash.

Based on these videos, Rohilla explains, “we realized that the median V-angle is nearly 46 degrees amongst all 50 videos, so that was a key finding.” This observation prompted the next portion of their study to find the optimal angle of entry using solid wedges of varying V-angles. By dropping these wedge projectiles from varying heights, they determined that a 45-degree V-angle creates the deepest air cavity and the tallest Worthington splash. This angle is notably similar to the average angle of humans from the videos!

Because the solid wedges were passive, researchers wanted to mimic the active underwater movements of the Manu divers. Olympic divers create an L-shape with their body under the water to break the air cavity apart and, therefore, minimize the splash. Manu divers, on the other hand, roll back and kick underwater to open their bodies, creating a larger, deeper air cavity and larger splash. To determine the optimal entry angle and timing of this body opening, Choi created a Manubot – a free-falling robot designed with a hinge mechanism to actively open, mimicking the underwater opening of humans. By dropping the Manubot at varying angles and opening times, researchers determined the optimal combination. The Manubot trials revealed that the optimal entry angle to create the largest splash was 30 degrees and optimal opening time occurred at a depth of about half the arm length of the robot.

Using videos of humans performing Manu jumping, passive solid projectile trials, and a dynamic maneuvering robot, Rohilla, Choi, and colleagues discovered what it takes to make a big splash. Future investigations will include greater details in the complexity of human anatomy and potentially high-speed imaging of trained divers to more accurately capture the splash dynamics of “popping the Manu.”