Paul Ronney explains fluid theory on the whiteboard. (Photo Credit: Magali Gruet/USC)
There is a book tucked under Paul Ronney’s arm. It is a hardcover, grey with gold lettering, worn at the corners in the way that only a book consulted for decades gets worn. The title: Viscous Fluid Flow, by Frank M. White. Ronney, professor and chair of the Department of Aerospace and Mechanical Engineering at USC, doesn’t explain why he brought it. Over the next hour, he’ll return to it again and again.
He leads the way into the Rapp Research Building, better known around the AME department simply as the RRB, and stops outside a set of heavy doors. “We call this,” he says, “the Fabulous Fluids Facility.” A pause. “Run, of course, by the fabulous fluids facility faculty.”
Push through the doors and the scale hits you. This is a serious research building, and most of its ground floor belongs to fluid mechanics. Three facilities, each one its own world, each one a different way of asking the same fundamental question: how does fluid move, and why?
There is a water channel, long and glassed-in, currently drained for maintenance. There is a towing tank, narrow and deep, its surface mirror-still. And further down the floor, half-hidden and enormous, is the wind tunnel.
The Water Channel: Lessons from a Shark
The Water Channel. (Photo Credit: Magali Gruet/USC)
The channel itself is a long rectangular tank, fitted with a converging section at one end and a series of fine mesh screens. When it is running, water enters from below, passes through the screens and the contraction, and emerges as a smooth, glassy flow: what fluid dynamicists call laminar, meaning it moves in orderly sheets rather than chaotic swirls. The screens do the work of calming it, the way a series of baffles in a recording studio absorb noise until the room goes quiet.
One line of research happening here involves shark skin. Not actual shark skin, but printed surfaces that replicate its microscopic geometry: tiny overlapping scales, each one angled to interact with the flow passing over them. The question the team is trying to answer is whether that texture, precisely reproduced, can reduce drag on a surface moving through water. The implications stretch well beyond marine biology. A ship’s hull. A submarine. A pipeline. Anything that moves through fluid and pays a fuel cost to do it.
PhD student Sabah is at the water channel, in her second year. Her work focuses on flow over morphable structures: how fluid motion can cause a surface to deform, and how that deformation feeds back into the flow itself. She does both modeling and experiments, then compares the two. “The interesting thing,” Ronney interjects, “is when they don’t agree.” Sabah smiles. That’s exactly when the real work begins.
She wants to end up in aviation. R&D, Boeing if she’s lucky. It is not a small ambition, and she does not say it like one.
The Stratified Towing Tank: Wakes That Last a Week
Boeing, submarines, deep ocean wakes; fluid mechanics has a way of connecting things that seem unrelated at first. The next station makes that clearer.
The towing tank is close by, and it is the strangest thing in the room. At first glance it looks like an aquarium, long and clear-sided, filled with what appears to be plain water. It is not plain water. It is a carefully prepared mixture, denser at the bottom than at the top, built up in layers using salt, water and ethanol dissolved in precise ratios. The fluid is stratified, meaning it has the same invisible internal structure as the deep ocean, where pressure, temperature, and salinity create layers that resist mixing.
Madeline, a fifth-year PhD student advised by Geoffrey Spedding Professor of Aerospace and Mechanical Engineering for AME, is at the tank, hands covered in epoxy, preparing a small oblong model to be towed through the fluid via the overhead track that runs the length of the tank.
Madeline working on a model before testing. (Photo Credit: Magali Gruet/USC)
What Madeline is studying is what happens after an object passes through stratified water. In ordinary fluid, a wake dissipates in seconds or minutes. In stratified fluid, the wake can persist for days. A week, even. The disturbance propagates outward as internal (underwater) waves, invisible from the surface but detectable by instruments, carrying a signature of the object that created them: its size, its speed, possibly even its shape. The applications are what you would expect.
“If there was an earthquake during the experiment?” someone wonders. The kind of question you only think to ask in California.”If it happened during the one-minute run, I would have to retake it,” says Madeline. “Then I would wait for all the motion to settle before I run again anyway. I’m not observing for a week afterward. We’re working at a smaller scale than the ocean. The wakes here last a couple of hours.”
She goes back to her epoxy. Ronney picks up his book and moves on.
The Dryden Wind Tunnel: A Machine With a History
The wind tunnel is at the far end of the building, and it is old. Not old in the way that equipment becomes obsolete, but old in the way that certain instruments become, with use and time, irreplaceable. The Dryden Wind Tunnel, or DWT, was built in 1918. It spent its first decades at what was then called the National Bureau of Standards in Washington, D.C., before being declared surplus in the 1970s and shipped to USC, where it has lived ever since.
What makes it valuable is not its speed (the maximum is around 35 meters per second, about 75 miles an hour, which is modest) but its quiet. The tunnel is, by any measure, one of the best low-turbulence wind tunnels in the world.
The Dryden Wind Tunnel (Photo Credit: Magali Gruet/USC)
Part of the secret is the wood. The tunnel is built almost entirely of it, not steel, and wood, unlike metal, absorbs vibration rather than transmitting it. Part of it is the screens, layers of fine mesh that smooth the incoming flow the same way the water channel does. And part of it is the shape of the turning sections, the careful geometry of the corners where the flow bends without becoming chaotic.
The Wiggles That Won a War
Ronney stops in front of a whiteboard near the tunnel’s test section, covered in equations and formulas. He opens his book before he even begins to speak.
Ronney pauses before the tunnel and offers the larger context: “Probably the most important problem in fluid mechanics is to find ways to reduce drag, because that drag increases the fuel needed to move a vehicle such as an airplane, a drone, a car, or a ship, and that drag decreases the range of the vehicle. For flow inside of a pipe, drag increases the power needed to move the fluid from one end to the other. What causes drag? Much of it is due to the turbulent flow around the vehicle or in the pipe. What causes the transition of smooth, laminar flow to turbulent flow? The first step is often wiggly but not yet turbulent flow structures called Tollmien-Schlichting waves.”
Paul Ronney demonstrate fluid theories from the Viscous Fluid Flow book. (Photo Credit: Magali Gruet/USC)
Tollmien-Schlichting waves.(Photo Credit: Magali Gruet(/USC)
“These wiggles,” he says, pointing first at the whiteboard, then at a nearly identical figure in White’s Viscous Fluid Flow, “are called Tollmien-Schlichting waves.” He says the name with the deliberateness of someone who knows they are introducing something important. The waves are the first sign that a smooth laminar flow is beginning to break down into turbulence, the very earliest tremor before the whole system becomes chaotic. For decades, theorists had predicted they should exist. No experiment had managed to confirm it, because every wind tunnel at the time was too noisy: the incoming turbulence drowned out the very signal researchers were trying to detect.
The Dryden tunnel, with its extraordinarily quiet flow, was the first one where the signal came through clean. The experiment, conducted during World War II, was the first conclusive proof that these waves existed. The same data appears in White’s textbook, which Ronney is still holding open, one thumb marking the page.
The discovery mattered beyond the laboratory. Understanding how and when laminar flow breaks down helped lead to the design of the laminar-flow airfoil used on the P-51 Mustang, the first Allied fighter with the range to escort bombers all the way from England to targets in Germany and back. The Mustang’s range changed the air war. Bombers that had been flying into Germany unescorted on the crucial last leg to the target, suffering devastating losses, suddenly had fighter cover for the entire mission. The shift in the balance of power traced back, in part, to what this tunnel revealed about air moving over a wing. Ronney mentions this the same way he mentions most things: not to brag, but because he thinks the lineage matters. A wind tunnel. A graph. A fighter aircraft. A war.
He closes the book.
The Test Section: Ari and the Singing Wing
Inside the wind tunnel’s test section, Ari is a third-year PhD student, at work. He is also advised by Spedding. He has embedded small speakers inside a wing. The speakers emit a tone at a precise frequency, and that frequency, if tuned correctly, nudges the airflow over the wing into a higher-lift state. No moving parts. No ailerons or flaps. Just a sound.
The best result so far is a doubling of the lift-to-drag ratio under certain conditions. Double the lift-to-drag ratio and you effectively double the range of an aircraft, or cut its fuel consumption in half. Ari says this calmly, in the way people in research tend to describe results that would sound extraordinary anywhere else. Ronney does not say it calmly. “That’s remarkable,” he says, and means it.
Ari and his musical wing. (Photo Credit: Magali Gruet/USC)
Then he opens White’s Viscous Fluid Flow one more time, to the section on boundary layer stability, and holds it up next to the wing sitting on the workbench; not yet in the tunnel, but headed there. The grey cover, the gold letters, the dense figures inside. The wing waiting to fly. The two things belong together in some way that doesn’t need explaining.
Ari mentions the result could apply to drones. Ronney nods. “Or birds,” he adds. A pause. “But we’d have to remake birds.” He says it lightly. It lingers anyway: a reminder that nature got here first, and these labs are still catching up.
Published on March 5th, 2026
Last updated on March 5th, 2026