On May 20, the recovery operations of the last two divers (Giorgia Sommacal and Muriel Oddenino) who died after being trapped in the Thinwana Kandu cave in the Maldives were successfully completed. In the meantime, a hypothesis is making its way in these hours to explain the dynamics of the tragedy that we at Geopop had already presented in recent days: that of the Venturi effect, a phenomenon known in fluid dynamics which may have forcefully dragged the five Italian divers who died during the dive on May 14th into the cave. The hypothesis is proposed by Alfonso Bolognini, president of the Italian society of underwater and hyperbaric medicine (SIMSI). This phenomenon would have been activated by underwater currents causing a “suction” that would have forcefully pushed the divers into the cave. In reality, from a physical point of view it is not a suction: there would not have been a force originating from inside the cave that attracted the divers; rather, it would have been the sea outside the cave that pushed the divers into the underground environments, where they would have finished the breathing mixture in the cylinders.
What is the Venturi effect and how it works from a physical point of view
You may have noticed that the wind, when channeled into a narrow road, suddenly becomes faster. Or you may have noticed that by partially blocking the hole in the garden watering hose, water begins to spray out of the hose much faster. These are examples of the Venturi effect in action.
The phenomenon was described at the end of the eighteenth century by the Italian physicist Giovanni Battista Venturi and essentially states two things: the first is that a fluid in a current that decreases its cross-section increases in speed, as happens to the wind in a narrow alley or to water that comes out of a partially blocked hole; the second – fundamental to understand what could have happened in the Maldives – is that the pressure of the fluid in a current decreases the higher its speed.
A narrowing current becomes faster because moving fluids, like water in an underwater current, must follow a very precise law: as much water must enter a certain section in a certain time as must exit. In other words, the incoming flow rate must be equal to the outgoing flow rate. Water, like liquids in general, is incompressible, so when it encounters a constriction or constriction it must necessarily become faster to ensure that the same amount of water exits the constriction section as enters in the same period of time.
It remains to be understood why a faster flow has a lower pressure. Here we have to bring into play the energy of this fluid current. Let’s put ourselves in the case of a horizontal flow, so we can ignore the effects of depth. The current therefore has two types of energy: one associated with its speed (kinetic energy) and one associated with its pressure (pressure energy). We associate energy with pressure because pressure makes the fluid capable of doing work. For example, let’s imagine we have a syringe without a needle, filled with water. If we plug its hole with a finger and push the piston, we increase the pressure on the water; the moment we remove our finger from the hole, the water splashes out because the pressure energy we have provided has the opportunity to convert into kinetic energy, giving speed to the water.
The important fact here is that the total energy of a fluid stream must be conserved. This means that the sum of its kinetic energy and its pressure energy must remain constant over time (we are always ignoring the energy linked to depth, i.e. the potential energy). Translated: if we increase the speed, the pressure must reduce. A small increase in speed can cause a large decrease in pressure.
This, incidentally, is the mechanism that makes airplanes fly: the airfoil is made in such a way as to produce a faster flow of air above the wing, and therefore at a lower pressure. Because the pressure under the wing is higher, the wing feels a net upward force that offsets the weight of the plane and keeps it in the air.
How the fluid dynamic phenomenon could have been lethal for the 5 Italian divers
The hypothesis that is circulating at the moment is that something similar happened to the divers who died in the Maldives, but with a net downward force. The mouth of the submerged cave of Thinwana Kandu may have offered the conditions to generate a powerful flow of water via the Venturi effect which would have pushed the 5 Italian divers into the underground system, with lethal consequences. Let’s see how.
The current passes through the mouth of the cave where it enters by channeling itself into a relatively narrow passage. Due to the Venturi effect, the speed of the flow entering the cave is higher than the speed of the current outside the cave. As a result, its pressure is significantly lower than that outside the cave.
The divers near the cave entrance find themselves at this point in a strong pressure gradient: above them, outside the cave, the water has a higher pressure; below them, inside the cave, the pressure is lower. It is the pressure difference between above and below that drags them down, because it generates a net force that pushes towards the low pressure area, i.e. into the cave. In fact, the fluids tend to move in this direction to reset the gradient and restore the pressure balance. We are talking about “suction” by the cave, but it is a misnomer: technically, from a physical point of view it was not the water in the cave that attracted the bodies of the divers, but the water outside the cave that pushed them inside.
The same massive flow of water may have raised the sediments inside the cave, bringing visibility to practically zero and making it even more difficult for divers to ascend.
According to Bolognini, the current could have dragged all five divers into the cave or even just one: at that point, the others would have entered to save the first and all would have died from asphyxiation and cardiac arrest trapped in the underground environment.
Investigations and evidence still to be analysed
Confirmation of this hypothesis could arrive shortly: the Finnish divers involved in the body recovery operations have handed over GoPro cameras and wrist computers recovered inside the cave system to the police. The material will be examined by Maldivian investigators and the Rome prosecutor’s office, who are reconstructing the dynamics of the incident.
For investigators, the devices could provide key elements to clarify the last moments: the path followed by the divers, the depth reached, the visibility conditions and any technical difficulties encountered inside the caves.
