Story by Susan Maas
Mrugank Bhatt is immersed in bubbles. Specifically, the Ph.D. student in the University of Minnesota’s Department of Aerospace Engineering and Mechanics (AEM) studies cavitation—the formation of vapor bubbles in liquid. Cavitation typically happens when a liquid is exposed to a rapid drop in pressure.
“If you have a container of water and you heat it up to 100 degrees Celsius, the water becomes vapor and you have bubbles. The same can happen if you drop the pressure significantly without changing the temperature,” Bhatt explained. “The latter is why you see bubbles forming on rapidly rotating marine propulsors.”
His research aims to yield a better understanding of when this process starts and how it develops. Cavitation is significant in several contexts. In marine applications, it’s typically problematic. It’s a cause of noise, vibrations, and material damage.
Finding a way to predict precisely when and how cavitation will occur—thereby helping design efficient marine propulsion systems—is the purpose of this project Bhatt’s been working on for the past year, funded by the Office of Naval Research (Dr. Ki-Han Kim program manager) under the U.S. Department of Defense’s Multidisciplinary University Research Initiative (MURI).
“What we learn here can be applied in many other fields,” Bhatt explained.
“For instance, in biomedical contexts, we can use ultrasound to form and direct the collapse of these cavitation bubbles to break up kidney stones,” he said. “And sonoporation—using bubbles to increase the permeability of cell membranes—can be used for directed drug delivery to a particular organ.”
AEM Professor Krishnan Mahesh, a recipient of the college’s George W. Taylor Award for Distinguished Research and Guillermo E. Borja Award, is leading the MURI project. The University of Minnesota team is collaborating on it with researchers from Caltech, UC Santa Barbara, University of Iowa, University of Michigan, MIT, Johns Hopkins, and the Australian Maritime College.
In Minnesota, Bhatt said, their focus is on computational modeling—conducting simulations on supercomputers. Their numerical work fuels and complements the experiments performed by team members at Michigan, Johns Hopkins, and the Australian Maritime College, who are working on the experimental side, Bhatt said.
AEM graduate students Aditya Madabhushi and Filipe Brandão are also part of the study. Madabhushi is exploring cavitation inception in vortex interaction, while Brandão is working on how non-condensable gas can influence bubble formation.
To learn more about the project titled ‘MURI: Predicting Turbulent Multi-Phase Flows with High Fidelity – A Physics-Based Approach,’ see the AEM department website.
Making calculations happen faster
Most of what’s known about cavitation, Bhatt said, has only been learned in the past century, starting with the work of British physicist Lord Rayleigh.
“The majority of the computational work has been done in the past two or three decades,” he explained, with the advent of supercomputing. “It really speeds up the calculations—you get results much more quickly. Part of my work is taking the current base code and enabling it to do a higher time step. I’ve worked on something called implicit time marching, which can allow faster calculation of cavitation equations.”
“It’s much easier when you have a single phase in the flow— either just water or just vapor,” Bhatt said. “When you have a mixture of both, interestingly the sound speed drops significantly, which makes the problem more challenging.”
Although the MURI project is just over a year old, Bhatt’s been studying cavitation with Professor Mahesh for nearly four years. He’s enjoying the chance to collaborate with researchers from different universities and different disciplines.
“We have people from chemistry working on this, looking at the molecular properties that then can be used for computations at macroscopic level…people from computer science using the latest machine-learning algorithms, people from mathematics, and so on,” he said.
They “meet” biweekly via WebEx, with faculty and students from various universities taking turns presenting, and the whole team gathers in person once a year, Bhatt said.
“I like that people with expertise in different areas are coming together and sharing their knowledge.”