arXiv subjects

Jibu Tom Jose

Publications and source records attributed to Jibu Tom Jose.

5 recordsLinked to original sources

arXiv2026

High-Density Monocular 3D Particle Image Velocimetry by Wavefront Shaping and Deep Learning

Three-dimensional (3D) Particle Image Velocimetry (PIV) measures flow velocities by imaging laser-illuminated tracer particles seeded in a fluid, and is widely used in both academia and industry. Many applications require a compact setup for optical accessibility, ideally with a single camera, while also demanding high seeding densities for accurate velocimetry. These requirements, however, are typically incompatible: monocular methods break down at high densities; high-density measurements instead generally rely on multi-camera tomographic systems. Here, we introduce Point-spread-function-Engineering Training-based PIV (PET-PIV), a compact monocular 3D velocimetry approach that resolves this long-standing compactness-density trade-off through a minimal optical modification and deep-learning-based algorithms. PET-PIV requires only the insertion of a single thin phase mask to an otherwise conventional PIV setup, beneath the objective in microscopy or at the lens iris in macro-scale imaging, and calibrates the resulting imaging system in situ, making the approach straightforward to implement and readily scalable. Computationally, PET-PIV operates in two complementary regimes: a tracking/Lagrangian mode that localizes and links individual particles, and, more importantly, a field mode for ultra-high densities which directly reconstructs 3D velocity fields from 2D image sequences. In realistic experimental validations using a tomographic PIV system, PET-PIV demonstrates strong agreement with ground-truth references, with correlation coefficients (CC) exceeding 0.97, alongside an order-of-magnitude improvement in computational speed. Notably, PET-PIV enables monocular 3D macro-scale PIV at densities previously achievable only with multi-camera setups, dramatically expanding the applicability of 3D velocimetry in space-constrained and optically restricted environments.

physics.optics↗

arXiv2026

Dynamic masking for boundary-aware velocity reconstruction in volumetric particle tracking with moving solids

Volumetric particle tracking velocimetry (PTV) produces scattered Lagrangian tracks that must be reconstructed on an Eulerian grid before velocity gradients, pressure, or hydrodynamic loads can be evaluated. This step is usually performed on a domain treated as entirely fluid. When a solid body lies within the measurement volume, its surface kinematics are not imposed and the reconstruction is weakest in the steep-gradient region next to the body. We introduce LE-DM (Lagrangian-to-Eulerian reconstruction with Dynamic Masking), a constrained reconstruction framework for moving solid boundaries. A time-dependent signed-distance function classifies grid nodes as open fluid, boundary shell, or solid interior. The particle data, incompressibility constraint, prescribed surface velocity, and regularization terms are then assembled on the masked domain within a single solve. The method requires only a signed-distance field and a surface velocity, allowing stationary walls, translating, rotating, multiple, and deforming bodies to be represented in the same formulation. LE-DM is assessed using an analytical oscillating sphere, synthetic tracks from a CFD rising-sphere simulation, and a refractive-index-matched tomographic-PTV experiment on a freely rising sphere. The surface kinematics are enforced to solver tolerance, while the bulk reconstruction remains unchanged where no body is present. In the analytical case, the first-cell error is reduced from 14\% to 3\% of the body speed. In the experiment, LE-DM recovers the independently measured surface velocity, whereas an all-fluid reconstruction does not. The result is a divergence-free, boundary-consistent velocity field for pressure and force estimation.

physics.flu-dyn↗

arXiv2026

Coherent Structure Transport in Turbulent Axisymmetric Pipe Expansions

Turbulent separated flows in axisymmetric expansions can sustain fundamentally different transport organization despite nearly identical mean topology. Using stereo-PIV and time-resolved planar PIV, we compare abrupt $90^\circ$ (step) and gradual $45^\circ$ (wedge) axisymmetric expansions at step height Reynolds numbers of 25000 and 35000. Despite similar reattachment lengths, near-separation turbulence differs, with the wedge exhibiting higher turbulent kinetic energy over a broader shear layer, while the step confines production to a thinner region near the corner, where a secondary vortex weakens momentum and fluctuations. The spatial spectra reveal a pronounced spectral hump in the out-of-plane velocity fluctuations near separation. This feature is consistently observed across all cases and reflects the expansion effects on the redistribution of fluctuation energy associated with the interaction between the separating shear layer and the recirculating flow. Temporal spectra show no geometry-specific dominant frequencies, and space-time correlations indicate similar normalized convection velocities across all cases. The primary effect of geometry, therefore, does not lie in the characteristic scales or transport speeds, but in the spatial organization and persistence of coherence. The step cases exhibit stronger spectral concentration, longer local integral time scales, and a broader distribution of space-time correlations in convection velocities associated with momentum-depleted return flow. Finite-time Lyapunov exponent (FTLE) fields confirm that these differences extend to material transport, as the wedge produces larger and less fragmented deformation regions, while the step yields a more segmented pattern that persists downstream of reattachment.

physics.flu-dyn↗

arXiv2025

Effect of Expansion Geometry on Turbulence in Axisymmetric Pipe Flows

We investigate the influence of expansion geometry on the flow field and turbulence structure in axisymmetric pipe flows through comparative analysis of abrupt ($90^\circ$) and gradual ($45^\circ$) area expansions with an area ratio of 2.56 at step-height Reynolds numbers of 25000 and 35000. Utilizing refractive index-matched stereo Particle Image Velocimetry, we resolve the three-component velocity fields and extract turbulence statistics with high spatial fidelity. Both configurations exhibit full flow separation, recirculation, and shear layer development; however, the gradual expansion consistently yields elevated turbulence levels, broader shear layers, enhanced Reynolds stress anisotropy, and stronger out-of-plane fluctuations. In contrast, the abrupt expansion generates a secondary vortex that disrupts the return flow, reducing shear layer interaction and turbulent kinetic energy (TKE) production. The governing mechanism is attributed to the geometry-induced modulation of the return flow. In the gradual case, the return flow remains attached to the sloped surface and impinges obliquely on the free-stream, generating a distributed region of high shear and sustained turbulence production leading to intensified TKE and anisotropy in the near-expansion region. The abrupt case confines this interaction, limiting turbulence generation spatially and structurally. These findings reconcile prior observations of increased pressure loss in sloped expansions and reveal the fundamental role of expansion slope in controlling turbulence generation and energy redistribution in separated flows. The observed trends suggest a generalizable mechanism relevant to a broader range of expansion angles and flow conditions.

physics.flu-dyn↗

arXiv2025

On the application of refractive index matching to study the buoyancy-driven motion of spheres

Refractive index matching (RIM) is a powerful tool for multiphase flow studies as it eliminates optical distortions and enables high-fidelity tomographic measurements near solid-fluid interfaces of freely moving solids in the flow. However, by improving the RIM and optical quality, the solids become effectively invisible, preventing direct identification of their location. To address this limitation, we develop a physics-informed detection framework that locates transparent spheres within time-resolved tomographic Particle Tracking Velocimetry by combining tracer voids, vertical velocity signatures, and vortex structures into a unified optimization problem. Integrated with volumetric reconstructions, the method provides simultaneous analysis of velocity, pressure, and force on the sphere. Applied to an example case of an 11.11 mm acrylic sphere rising in a RIM sodium iodide solution, the technique reveals a clear phase-locked relation between double-thread wake structures, surface-pressure distributions, and unsteady hydrodynamic forces over half a cycle of the sphere motion in the 4R vortex shedding regime. For the first time, this enables direct calculation of drag and lift histories on a freely moving sphere. The framework can be extended to dynamic masking for improved tomographic reconstruction and pressure-field calculations, to non-spherical bodies with more complex motions, and to multi-body interactions, advancing RIM from a flow-only diagnostic to a tool for fully coupled body-wake measurements.

physics.flu-dyn↗