Research | Xinyu Si

My research explores how fluid motion, external forcing, and active swimmers shape transport and energy transfer. I combine laboratory experiments with advanced flow diagnostics to study these interactions.

Rotating convection

Rotation and heating together drive flows inside planets, oceans, and stars. In Earth’s core, the boundary with the mantle is likely uneven — so how does the shape of a boundary change the way a rotating fluid carries heat?

To study this, our team in the Kelley lab built ToRoCo (Topography in Rotating Convection), a laboratory facility in which a rotating, heated water layer can be bounded by interchangeable textured plates. ToRoCo measures the overall heat transport while tracking tracer particles in three dimensions, so changes in heat transfer can be tied directly to the flow structures behind them.

Design model of the ToRoCo rotating-convection apparatus, with numbered component callouts. — The ToRoCo rotating-convection apparatus: design model (a) and laboratory setup (b).

Laboratory apparatus of the ToRoCo rotating-convection apparatus, with numbered component callouts. — The ToRoCo rotating-convection apparatus: design model (a) and laboratory setup (b).

Tracer particles followed in 3D inside ToRoCo, tracing warm fluid rising and cool fluid sinking. Color shows vertical speed (mm/s); played at 5× speed.

ToRoCo running in the lab, real speed. Press play to watch.

Tensor geometry and energy transfer

Turbulence passes energy between eddies of different sizes. In three dimensions it usually cascades from large eddies to small ones; in two-dimensional flows it climbs from small to large — a direction long thought to be set by the flow itself.

We found that the direction depends on geometry: whether small-scale stirring works with or against the stretching of the larger flow. Swimming brine shrimp revealed this first, sending energy up or down depending on how they faced the flow. We then turned it into a control knob, flipping the direction of energy transfer by orienting the stirring, and found it works even in slow, viscous flows (featured in Quanta Magazine).

Papers: PNAS Nexus (2024) · Science Advances (2025) · Journal of Fluid Mechanics (2026)

Compact five-panel comparison: energy-cascade schematic at top left with E(k) and k axes, energy-injection labels, and direct and inverse energy flux; strain and stress eigenvector alignment at top right; velocity, strain, and stress fields across the bottom — How flow geometry sets the direction of energy transfer. Top left: energy can move toward smaller eddies (red) or larger eddies (blue). Bottom left: the measured flow, shaded by the local direction of energy transfer in the same colors. Bottom middle and right: the stretching directions of the larger flow (purple) and the stirring directions of small-scale motion (green). Top right: an enlarged view of the circled region, where the two line up and energy moves toward larger eddies (blue).

Two-way coupling between active matter and flow

Plankton, swimming microbes, and swarms of small animals all move through flowing water — and they push back on it. My work follows both directions of this coupling.

Flows influence the transport of active-matter. Hidden pathways and barriers in a flow, known as Lagrangian coherent structures, shape where swimmers travel and where they gather. Swimmers also reshape the evolution of flow. Even a sparse swarm can shift and twist these structures, with consequences for mixing.

Papers: Physics of Fluids (2021) · Physical Review Fluids (2022) · Physical Review Fluids (2024)

Compact two-by-two comparison: background-flow FTLE field at top left, deformed FTLE field with swarming A. salina at packing fraction 0.074 at top right, and cropped flow velocity arrows and swimmer images below — Brine shrimp reshape the flow's hidden structure. Top: the flow's transport barriers without (left) and with (right) a swarm. Bottom: the background flow (left) and the swimming shrimp (right).

Velocity vectors showing the jet and surrounding flow generated by a swimming brine shrimp, A. salina, with a one-centimeter scale bar — A single brine shrimp (*Artemia salina*) pushes a jet of water behind it; arrows show the surrounding flow.

A brine shrimp swimming through a sheared flow. Press play to watch.

Crowd dynamics

How does a crowd move when everyone tries to keep their distance? Treating pedestrians like particles in a gas, we built a social distance model in which each person heads for a goal while keeping a preferred spacing from others. Tested against experiments, it captures how distanced crowds actually walk.

The model reveals a surprise at doorways: once people’s preferred spacing exceeds the door width, the exit jams. A small angled sidewall beside the door breaks the crowd’s symmetry, and a 30° wall more than triples the outflow without pushing people closer together. As in my work on active matter, simple geometry reshapes collective motion.

Read the paper in Scientific Reports (2021).

Animated simulation of pedestrians passing a bottleneck beside a 20-degree angled sidewall — With a 20° sidewall, the crowd drains through the door much faster.

Animated simulation of pedestrians passing a bottleneck beside a straight sidewall — With a straight wall, neighbours compete for the door and the exit jams.