Squid achieve extraordinary thrust by jetting water through a compliant funnel (the siphon) that passively reshapes jet vortex formation during each mantle contraction. The key mechanism — elastic wave propagation in the compliant nozzle wall — modulates the jet impulse beyond what a rigid nozzle can achieve. We design and characterize squid-inspired flexible nozzles for rotary underwater propulsion systems, combining PIV flow field measurements and dynamometry to quantify thrust amplification arising from this elastic wave-mediated vortex dynamics.

Deep-learning Bayesian surrogate models are developed to accelerate nozzle geometry optimization across high-dimensional parameter spaces, significantly reducing the number of expensive physical experiments required. The open-source FSI simulation framework (OpenFOAM + CalculiX + preCICE) developed in this project enables fully reproducible 3D fluid–structure interaction simulations of compliant nozzle dynamics.

Earlier foundational work at Ajou University characterized the fundamental mechanics of cephalopod-inspired soft nozzle systems — measuring thrust generation, elastic energy storage, and impulse-release dynamics in compliant structures that replicate squid funnel mechanics.

Mudskippers are among the few fish that can move across water surfaces, using a distinctive fin-assisted "galumphing" gait that exploits surface tension, inertia, and fin thrust in concert. We investigate the interfacial hydrodynamics and 3D kinematics of mudskipper water-hopping through high-speed laboratory experiments and biological fieldwork conducted in Borneo.

A custom stereo-camera 3D tracking system enables non-invasive pose estimation of freely-moving animals in natural mangrove habitats — the first such system deployed for mudskippers in the wild. Physical and robotic models are built to validate the discovered locomotion principles and bridge biological observation to engineering implementation.

Related work covers the Manu jump — how humans create large water-entry splashes through body posture control and timing — and Epineuston vortex recapture in tiny water-walking insects, where leg-generated vortices are recaptured by subsequent strokes to enhance thrust efficiency.

We developed DeepBubbleVelocimetry — the first CNN-based optical flow algorithm purpose-built for bubble velocimetry — enabling non-invasive velocity diagnostics at bubble void fractions up to 58%, far beyond the ~11% limit of conventional particle tracking velocimetry (PTV). The algorithm fine-tunes the PWC-Net optical flow network on synthetically generated bubble image datasets, achieving superior accuracy and speed across all bubble density regimes.

Liquid atomization studies of water columns broken by annular dual-nozzle gas jets revealed Rayleigh–Taylor instability as the primary atomization mechanism. Flow regime boundaries were theoretically derived using control volume analysis and validated experimentally. The resulting droplet diameter predictions match Rayleigh–Taylor cell wavelengths.

Industrial collaborations addressed spray coating film formation for marine paint applications (Daewoo Shipbuilding), and the effects of superhydrophobic surface coatings on bluff-body wakes (Samsung SAIT), where the optimal SHPo condition was found to reduce recirculation length by augmenting near-separation flow fluctuations.

A flexible nozzle modifies the vortical structure of a starting jet and amplifies thrust. By combining hydrodynamic conservation equations with linearized shell theory, we derived and validated the optimal nozzle stiffness for maximum impulse — establishing the theoretical foundation that underpins the entire squid-inspired propulsion program. This work introduced the concept of elastic wave-mediated vortex formation number control in compliant nozzles.

Multi-body wake interactions in in-line sphere arrays (Re = 1000) were systematically characterized using 2D PIV and dye visualization. Downstream spheres were found to weaken upstream wakes, reducing form drag through pressure reconstruction analysis.

Electroconvective circulating flows in asymmetric multiscale porous membranes were identified and characterized. Asymmetric coulombic force distribution among micropores drives circulating flows that enhance ion transport and lower the effective membrane resistance.

In collaboration with Yonsei Severance Hospital, we developed medical devices for urinary surgery — including a Foley catheter removal assessment apparatus and hydronephrosis-induction systems for ureteral access. These resulted in three granted patents and one PCT patent. Product demo video →