arXiv subjects

M. S. Triantafyllou

Publications and source records attributed to M. S. Triantafyllou.

3 recordsLinked to original sources

arXiv2026

Turbulence Closure in RANS and Flow Inference around a Cylinder using PINNs and Sparse Experimental Data

Traditional Reynolds-averaged Navier-Stokes (RANS) closures, based on the Boussinesq eddy viscosity hypothesis and calibrated on canonical flows, often yield inaccurate predictions of both mean flow and turbulence statistics. Here, we consider flow past a circular cylinder over a range of Reynolds numbers (3,900-100,000) and Mach numbers (0-0.3), encompassing incompressible and weakly compressible regimes, with the goal of improving predictions of mean velocity and Reynolds forces. To this end, we assemble a cross-validated dataset comprising hydrodynamic particle image velocimetry (PIV) in a towing tank, aerodynamic PIV in a wind tunnel, and high-fidelity spectral element DNS and LES. Analysis of these data reveals a universal distribution of Reynolds stresses across the parameter space, which provides the foundation for a data-driven closure. We employ physics-informed neural networks (PINNs), trained with the unclosed RANS equations, to infer the velocity field and Reynolds-stress forcing from boundary information alone. The resulting closure, embedded in a forward PINN solver and the numerical solver OpenFOAM, significantly improves RANS predictions of both mean flow and turbulence statistics relative to conventional models.

physics.flu-dyn↗

arXiv2014

Ultra-fast escape maneuver of an octopus-inspired robot

We design and test an octopus-inspired flexible hull robot that demonstrates outstanding fast-starting performance. The robot is hyper-inflated with water, and then rapidly deflates to expel the fluid so as to power the escape maneuver. Using this robot we verify for the first time in laboratory testing that rapid size-change can substantially reduce separation in bluff bodies traveling several body lengths, and recover fluid energy which can be employed to improve the propulsive performance. The robot is found to experience speeds over ten body lengths per second, exceeding that of a similarly propelled optimally streamlined rigid rocket. The peak net thrust force on the robot is more than 2.6 times that on an optimal rigid body performing the same maneuver, experimentally demonstrating large energy recovery and enabling acceleration greater than 14 body lengths per second squared. Finally, over 53% of the available energy is converted into payload kinetic energy, a performance that exceeds the estimated energy conversion efficiency of fast-starting fish. The Reynolds number based on final speed and robot length is $Re \approx 700,000$. We use the experimental data to establish a fundamental deflation scaling parameter $σ^*$ which characterizes the mechanisms of flow control via shape change. Based on this scaling parameter, we find that the fast-starting performance improves with increasing size.

physics.flu-dyn↗

arXiv2014

Efficiency of Fish Propulsion

It is shown that the system efficiency of a self-propelled flexible body is ill-defined unless one considers the concept of quasi-propulsive efficiency, defined as the ratio of the power needed to tow a body in rigid-straight condition over the power it needs for self-propulsion, both measured for the same speed. Through examples we show that the quasi-propulsive efficiency is the only rational non-dimensional metric of the propulsive fitness of fish and fish-like mechanisms. Using two-dimensional viscous simulations and the concept of quasi-propulsive efficiency, we discuss the efficiency two-dimensional undulating foils. We show that low efficiencies, due to adverse body-propulsor hydrodynamic interactions, cannot be accounted for by the increase in friction drag.

physics.flu-dyn↗