Fede Hernández-Sánchez

Researcher in Experimental Fluid Dynamics & Acoustics,

Instituto de Ciencias Aplicadas y Tecnología (ICAT),

Universidad Nacional Autónoma de México (UNAM).

Understanding through experimental evidence

About me

My interest in experimental fluid mechanics began during my undergraduate studies in Mechanical Engineering at UNAM. At the end of the Continuum Mechanics course, I encountered the Navier-Stokes equations and was fascinated to discover that they were nothing more than Newton's second law expressed in such a general way that each of its terms contained information about different physical phenomena: temporal and spatial acceleration, energy dissipation, gravitational potential, among others. All concepts, including the origin of chaos in the advective term and the nature of continuous change associated with philosophical dialectics, were present.

For months I searched for someone who could teach me how to solve the Navier-Stokes equations, a challenge so profound that even mathematics seemed to lack the tools to answer whether a general solution existed. Then, on the list of courses, Roberto Zenit's name appeared. He had a laboratory equipped with experimental techniques such as high-speed video, particle imaging velocimetry (PIV), and hot-wire anemometry.

He was the one who taught me that nature itself embodies the behaviors we try to express in equations; that we must ask nature how it solves problems, always in harmony with its own rules. I understood that experimentation is the means to establish a dialogue with nature, and that learning to interpret experimental results is an essential part of learning its language.

That language—made of observation, measurement, and reflection—is what I continue to explore as a researcher and what I now wish to pass on to the next generation of researchers.

On my research

My research explores how to use the tools of fluid mechanics and acoustics to understand and improve energy utilization in different systems, as well as propulsion at various scales. Using advanced experimental techniques, such as high-speed video recording, planar laser-induced fluorescence, the Schlieren method, shadow photography, and particle-based velocimetry (PIV/PTV) techniques, I study flows, sounds, and their interaction in confined geometries.

A central contribution of my work at UNAM is the discovery and characterization of a low-frequency resonant mode in tiny enclosures, such as cavities. This phenomenon has potential applications ranging from aerospace propulsion and thermal management in turbomachinery to compressing the dynamics of musical instrument soundboxes. One of my current projects seeks to unravel the physics of propeller propulsion in intermediate Reynolds number regimes, laying the groundwork for next-generation energy and flow systems from an experimental perspective. I am also interested in resonant modes of musical instrument sound boxes, simulating the dynamics of stochastic fragment ejection, as occurs in volcanoes, visualizing the dynamics inside Rijke tubes, as well as developing low-cost experimental techniques accessible to schools and universities with less access to resources.

Academic Trajectory

Bachelor's degree in Mechanical Engineering from the Faculty of Engineering at UNAM (2001-2008).
Master's degree in Materials Science and Engineering from UNAM (2009-2011).
PhD in the Physics of Fluids group at the University of Twente, Netherlands (2011-2015).
Postdoctoral fellow at King Abdullah University of Science & Technology (KAUST), Saudi Arabia (2015-2018).
Researcher at ICAT-UNAM (since 2018).

Experimental techniques implemented at ICAT-UNAM

High-speed imaging video recording

High-speed video recording is a technique used in experimental fluid mechanics to capture rapid flow phenomena occurring on millisecond or even microsecond timescales. By recording thousands of frames per second (fps), this method allows us to visualize and quantify transient events such as jet formation, droplet ejection, vortex shedding, and collision interactions. The resulting image sequences can be processed to extract velocity fields, oscillation frequencies, and flow structures, often in combination with other diagnostic tools such as laser illumination or particle-based velocimetry. This technique provides quantitative information about flow behavior, enabling detailed analysis of complex fluid-structure and fluid-acoustic interactions. We can record high-speed video at up to 24,000 fps. Below are some examples recorded in the laboratory.

Lighter

Spark and flame expelled from a lighter recorded at 1000 fps and played back at 30 fps. The video is slowed down more than 30 times. Spark ignition is a fundamental problem studied in combustion engines.

Lip trilling

Lip trilling recorded at 2142 fps and played back at 30 fps. The video is slowed down more than 70 times. This oscillation is the basis of the double-reed mechanism, necessary to produce sound with musical instruments such as the trumpet or the French horn.

Pinching a water balloon

A water-filled balloon punctured with a pin on the left side. Video recorded at 7135 fps and played back at 10 fps. The phenomenon was slowed down more than 700 times. The intention was to observe the fracture dynamics of a viscoelastic material and also to have some fun.

Planar laser-induced fluorescence

Planar laser-induced fluorescence (PLIF) is a visualization technique used in fluid mechanics to observe the internal behavior of complex flows. In this method, a thin sheet of laser light containing a small amount of fluorescent dye is passed through the fluid. When the dye molecules absorb the laser light, they emit light of a different color, which can be recorded by a camera. This allows visualization of a two-dimensional cross-section of the flow, revealing how the fluids mix, disperse, or move in real time. By analyzing the brightness and distribution of the fluorescence, quantitative information about properties such as concentration or temperature can even be obtained.

Vortex rings

A vortex train ejected from a Synthetic Jet Actuator (SJA) visualized through a high-speed camera and a flat laser designed and manufactured at ICAT.

Stochastic drops ejection

Here, drops are periodically and simultaneously ejected from the edges of a resonating metal container.

Edge vortex

Edge vortex forming above cargo trucks. The experiment was conducted using a model and water vapor droplets.

Shadow photography

Shadow photography is a simple yet effective technique used in various fields, including fluid mechanics, to visualize objects. In this method, a bright light source passes through the flow and projects its shadow onto a screen or camera sensor. While it may seem like a basic visualization, shadow photography can provide valuable qualitative and quantitative information about flow characteristics that would otherwise be invisible to the naked eye.

Pendulum tracker

This video was recorded with an Arduino camera (open design) that records at up to 210 fps. The light source was designed at ICAT to prevent flickering.

Schlieren imaging

Schlieren imaging is a technique that allows visualization of density variations in fluid media. These variations are usually invisible to the naked eye. When light passes through a fluid with varying density—such as air heated by a flame or a gas moving at high speed—it is slightly deflected, a phenomenon known as refraction. The Schlieren system uses lenses, mirrors, and a sharp-edged filter to detect these minute changes in the direction of light, transforming them into bright and dark patterns that reveal the structure of shock waves, thermal plumes, and acoustic fields. This method allows researchers to observe how fluids move and interact without the need to introduce probes that could disturb the flow. In our laboratory, Schlieren imaging is combined with high-speed cameras to capture rapid and transient phenomena in fluid mechanics and acoustics with exceptional clarity.

We have 15, 20, and 30 cm parabolic mirrors to implement this technique.

Slow-motion lighter

In this example of high-speed Schlieren photography, the density changes are observed due to the presence of butane gas and later due to the flame.

Particle imaging velocimetry (PIV)

Particle imaging velocimetry (PIV) is an experimental technique that allows researchers to measure entire velocity fields in a fluid without inserting flow-disrupting probes. In this technique, tiny particles are introduced into the flow, closely following the fluid's motion. A laser sheet illuminates these particles in a specific plane, and two images are captured in rapid succession using a high-speed or synchronized camera. By comparing the particle motion between the two images, we can calculate local velocity vectors across the entire field of view. The result is a detailed map of the flow structure—showing vortices, jets, and other dynamic features—that helps us understand how momentum and energy are transported in fluids. PIV is one of the key quantitative tools in our laboratory for studying unsteady and resonant flows.

Velocity field

Velocity field of an estimated vorticity ring using PIV in the cross section of a flow produced by a head bottle being squeezed inside a water tank.

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Particle tracking velocimetry (PTV)

Particle tracking velocimetry (PTV) is a complementary technique to particle induction velocity (PIV) that focuses on tracking individual particles within a fluid. Instead of measuring average displacements over small regions, PTV tracks the precise trajectories of numerous particles over time. This provides highly detailed information about the paths, velocities, and accelerations of fluid elements, revealing the fine structure of complex or turbulent flows. Because it captures the behavior of individual particles, PTV is especially useful for studying three-dimensional motion, flow instabilities, or cases with large velocity gradients. In our laboratory, PTV is used in conjunction with high-speed imaging to relate visual flow patterns to the precise quantitative motion of the fluid.

Stochastic drop tracking

Tracking of drops ejected from the edges of a resonating metal container, recorded at over 2,000 frames per second.