D11 Particle Image Velocimetry - PIV

Topic

The fluid surrounding an insect's wing in flight moves in a way invisible to the naked eye; particle image velocimetry—known as PIV—is the gold-standard technique for revealing the fluid's actual motion, as it measures the complete velocity field of the air around the wing during flight.

To visualize this invisible fluid motion, the fluid must first be seeded with tracer particles—tiny hollow glass microspheres or droplets of atomized olive oil—that are small and light enough to faithfully follow every movement of the fluid without altering it or lagging behind.

Once the fluid is seeded with these particles, a pulsed laser illuminates a single spatial plane at two moments in rapid succession, and a camera records the exact position of each tracer particle within that plane at both moments, yielding two nearly identical images of the same group of particles taken just instants apart.

By comparing these two images, a technique known as cross-correlation divides the illuminated plane into small, overlapping "interrogation windows" and determines, for each window, the most probable displacement of the tracer particles between the first and second moments. Dividing this displacement by the brief time interval yields a local fluid velocity vector for that sub-region of the plane.

Repeating this comparison across all interrogation windows in the illuminated plane produces a complete vector field describing the fluid velocity at every point, revealing at a glance how the air moves around the insect's wing at the captured moment.

From this complete vector field, numerical differentiation allows for the calculation of the fluid's vorticity at each point—that is, the rate and direction of the fluid's local rotation around that point. This vorticity allows for the identification of vortices forming around the insect's wing—including the leading-edge vortex, a structure of concentrated rotation that forms right above the wing's leading edge and is crucial for generating lift.

A circulation integral quantifies the intensity of each of these vortices; based on this intensity, the vortex impulse method makes it possible to estimate the total aerodynamic force generated by the insect's wing during flight, without the need to mount a mechanical balance directly onto the insect. This is particularly valuable precisely because the insect is so small: any sensor mounted directly on it would alter its natural flight, whereas a fluid seeded with particles—observed using lasers and a camera—reveals the same aerodynamic force without touching the insect at all.

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