Engineering

Q&A: Do momentum and heat always go with the flow?

Tamy Guimarães stands in front of the low-speed wind tunnel that will be used to investigate the ways in which momentum and heat are transported through air flows. Credit: Caleb Craig/Penn State . All Rights Reserved.

UNIVERSITY PARK, Pa. — Turbulent flows of air and water play a vital role transporting momentum, heat and substances such as moisture, pollutants and nutrients in everything from the environment and energy systems to medical technologies and aerospace engineering. However, the ways in which momentum and heat are transported within these flows appear to be different — contrary to what was previously believed, particularly when the flow is over a rough surface, such as a forest canopy or a textured material.  

A team of researchers at Penn State led by Tamy Guimarães, assistant professor of mechanical engineering, recently received a three-year, $710,741 U.S. National Science Foundation grant to investigate why and how this discrepancy occurs, reshaping our understanding of how momentum and heat move through the world. 

In the Q&A below, Guimarães discussed the goals of the project and the potential impact of the knowledge the team hopes to gain from this work.  

Q: What are your goals for this research project? 

Guimarães: We want to understand how momentum and scalar transport happen over rough surfaces. In simple terms, scalar quantities only have magnitude — think concentration of particles or how hot it is. Momentum transport, however, also has a direction, which can strongly modify flows by producing vortices and turbulence, for example. For smooth surfaces at certain flow conditions, the mechanisms responsible for transferring momentum and heat are analogous, known as the Reynolds analogy. Recent research, however, indicates that when the surface is rough, the Reynolds analogy no longer holds true.  

Q: Why does it matter that momentum and heat transfer differently through and over rough surfaces? 

Guimarães: Heat, or rather temperature, can be considered a scalar quantity. The equations that describe momentum transport have a pressure forcing term, while nothing analogous exists in the heat transfer equations. By assuming a similar behavior for both quantities, we are missing key information when describing phenomena that have both momentum and scalar transfer present. This can have a huge impact on applications ranging from intravenous drug delivery to machine design and weather forecast.  

Q: What are some examples of real-life applications of this knowledge?  

Guimarães: This knowledge can be applied to a wide range of flows. In blood flows, for example, we are interested in how oxygen and other nutrients, which are scalars, are transported through our body. In weather research, as another example, we want to know how temperature and pollutant particles are transported. Intuitively, we can imagine that particles move differently over a football field versus over a forest canopy. And, given the correct scale, the football field would be a smooth surface, while the forest canopy acts as a rough surface. 

Q: How will you investigate the differences in these types of flows in different circumstances?  

Guimarães: We will combine computational simulations and experiments in two different facilities at Penn State: the glycerin tunnel at the Applied Research Lab, and the water tunnel in my laboratory. Penn State has world-class facilities and instruments to measure flows in the detail needed to capture the differences in the transport phenomena that we are looking for. This, combined with the expertise on our team, makes Penn State uniquely positioned to conduct this research. The data collected in these experiments will be used to validate numerical simulations and help formulate appropriate models to describe the behavior of these phenomena. 

Other researchers on this project are Robert Kunz, professor of mechanical engineering; Xiang Yang, Kenneth K. and Olivia J. Kuo Early Career Professor of Mechanical Engineering; Jeff Harris, associate research professor at the Penn State Applied Research Lab and in the engineering science and mechanics department; and Matthew Bross, associate research professor at the Penn State Applied Research Lab. 

Last Updated October 15, 2025

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