# Research Spotlight:

Professor Tamer Zaki

## Meet Professor TAMER ZAKI

Tamer Zaki is a professor of Mechanical Engineering who focuses on transitional and turbulent flows. He received his Ph.D. (2005) in Flow Physics and Computational Engineering from Stanford University.

His research group, the Johns Hopkins’ Flow Science and Engineering (FSE), capture detailed flow instabilities and turbulence through high fidelity simulations.

## Q: What is the focus of your research?

TZ: “My research is focused on turbulence and how it comes about and evolves. The implications of turbulence are endless: our atmosphere, oceans, and our own bodies involve fluid motion and, in more cases than not, turbulence. Our group specializes in two classes of problems related to studying turbulence: forward and inverse. In forward problems, we try to predict how the flow evolves dynamically over time. In inverse problems, rather than predicting the evolution, we try to predict the events that led to certain observations.”

## Q: Your research is known to specialize in simulations to capture detailed flow instabilities and turbulence. What is the process of creating these simulations?

TZ: “We start from the equations that govern the dynamics of these flows. To solve these equations, we develop state of the art algorithms. We then deploy these algorithms in the ARCH facility. If you have state of the art algorithms and hardware, you can solve these equations for increasingly complex problems with higher levels of fidelity. We can then accurately simulate the phenomena of interest that occur in real life. For example, think of hypersonic flight. Physical experiments are notoriously difficult to perform, so instead we use computer simulations. We look at the region around the flight vehicle, put a grid around it, divide it into smaller regions, discretize the equation and solve for how the flow behaves in different flight conditions.”

## Q: What is the importance of these simulations in your research?

TZ: “Simulations are essential. Everybody is familiar with the idea of scientists being in a physical lab doing physical experiments. In contrast, our group has computational laboratories like ARCH, and we do computational experiments. These experiments give us data that is otherwise very difficult or even impossible to measure in a physical experiment. We can access data without any intrusive measurement techniques, we don’t have to physically probe but instead ask the computer to provide us with the data. We then use these facilities to test our theories using computational experiments.

One unique feature about this computational laboratory is that you can dream up a computational experiment that is impossible to do in a physical lab, and you are able to execute and learn from it. You’ve heard the expression “thought experiment”, which is an experiment that you don’t have to physically realize in a lab. A computational experiment is the same way: we can take an equation that we know governs a physical phenomenon and we can artificially change one of the variables. You get to learn about how the flow or turbulence behaves when you modify the behavior of an equation. These numerical experiments are powerful tools in the study of the dynamics of turbulence.”

## Q: How has computing changed the way in which we study transitional and turbulent flows?

TZ: “Being able to perform these experiments with computing has allowed us to explore transitional flows and to discover the different ways they become turbulent. Transition to turbulence is a chaotic process and turbulence itself is chaotic: changes are dramatic even due to small variation in the system. Having access to supercomputers allows you to study these problems with machine precision, a very high precision that is difficult to achieve in a physical lab. Computing gives you the capacity to study these phenomena with very high accuracy which is needed when studying chaotic systems.”

## Q: One of the topics that your research group focuses on is about Hypersonic Flight. Could you give a brief overview of the research you do to understand Hypersonic Flight and how simulations impact that area of your research?

TZ: “When you think of hypersonic flight, these are fluids that are moving at multiples of the speed of sound. Therefore, making physical measurements is difficult since changes happen at very short time scales. We instead do simulations to explore this regime. What is unique about our research is that we also solve the inverse problem: we’ve taken the measurements and we were able to predict the multitudes of possibilities that could have generated them. In a recent paper, we demonstrated how we used the ARCH facilities to discover the flow that led to measurements from an experiment at one of the Department of Defense research labs. It’s very exciting because our work has changed the conversation about how to interpret these measurements. There’s a long-standing view of how transition to turbulence takes place in these experiments, and we demonstrated using our simulations that this established mechanism is not viable, and additionally discovered the most likely cause of the measurements. We started from few measurement locations and gave the community the entire three-dimensional and time-dependent flow field that generated these isolated measurements.

To me these activities are very exciting because they are a marriage of two pillars of scientific discovery: the experiments, which are very difficult, and the simulations that interpret the measurements. Our group fuses the experiment and simulations together, which is unique and only possible because there are many brilliant experimentalists that we collaborate with, and we have these algorithms that we deploy in the ARCH facility that can infuse the measurements in our calculations.”

## Q: What inspired you to pursue a research career in transitional and turbulent flows?

TZ: “It is easy to be fascinated by how a flow that is moving in a very smooth or laminar way can suddenly become turbulent. It’s as chaos is lurking the background waiting to pounce or manifest as turbulence. It’s intriguing how there’s so many ways a laminar flow can become turbulent: when you finally understand one of the mechanism and successfully control it, another one emerges. Turbulence itself seems very chaotic and disorganized, but it in fact has some order within it, with certain statistical quantities that are reproducible.

Perhaps the most fascinating aspect is how the multitude of processes that take the flow laminar to turbulence, and turbulence itself, are all governed by the same set of equations. These equations have encapsulated in them the dynamics of all laminar, transitional, and turbulent flows. In a nutshell, you can say the problem of turbulence is mathematically interesting, computationally challenging, and physically fascinating.“

## Q: What is the impact or value of Rockfish on your research?

TZ: “It’s fair to say that it would have been impossible to achieve the advancements that we contributed to the field at Hopkins today if we didn’t have this facility. Our simulation algorithms on Rockfish are our windows into the dynamics of these flows and allow us to understand, predict, and control them.”

## Q: What other resources would you like ARCH to provide?

TZ”: “High-performance computing at leading institution is part of the infrastructure. Expanding ARCH would empower our team to tackle more computationally challenging problems.”