arXiv subjects

E. Herbert

Publications and source records attributed to E. Herbert.

5 recordsLinked to original sources

arXiv2020

Thermodynamics of animal locomotion

Muscles are biological actuators extensively studied in the frame of Hill's classic empirical model as isolated biomechanical entities, which hardly applies to a living organism subjected to physiological and environmental constraints. Here we elucidate the overarching principle of a \emph{living} muscle action for locomotion, considering it from the thermodynamic viewpoint as an assembly of actuators (muscle units) connected in parallel, operating via chemical-to-mechanical energy conversion under mixed (potential and flux) boundary conditions. Introducing the energy cost of effort, $COE_-$, as the generalization of the well-known oxygen cost of transport, $COT$, in the frame of our compact locally linear non-equilibrium thermodynamics model, we analyze oxygen consumption measurement data from a documented experiment on energy cost management and optimization by horses moving at three different gaits. Horses adapt to a particular gait by mobilizing a nearly constant number of muscle units minimizing waste production per unit distance covered; this number significantly changes during transition between gaits. The mechanical function of the animal is therefore determined both by its own thermodynamic characteristics and by the metabolic operating point of the locomotor system.

physics.bio-ph↗

arXiv2019

Thermodynamics of metabolic energy conversion under muscle load

The metabolic processes complexity is at the heart of energy conversion in living organisms and forms a huge obstacle to develop tractable thermodynamic metabolism models. By raising our analysis to a higher level of abstraction, we develop a compact -- i.e. relying on a reduced set of parameters -- thermodynamic model of metabolism, in order to analyze the chemical-to-mechanical energy conversion under muscle load, and give a thermodynamic ground to Hill's seminal muscular operational response model. Living organisms are viewed as dynamical systems experiencing a feedback loop in the sense that they can be considered as thermodynamic systems subjected to mixed boundary conditions, coupling both potentials and fluxes. Starting from a rigorous derivation of generalized thermoelastic and transport coefficients, leading to the definition of a metabolic figure of merit, we establish the expression of the chemical-mechanical coupling, and specify the nature of the dissipative mechanism and the so called figure of merit. The particular nature of the boundary conditions of such a system reveals the presence of a feedback resistance, representing an active parameter, which is crucial for the proper interpretation of the muscle response under effort in the framework of Hill's model. We also develop an exergy analysis of the so-called maximum power principle, here understood as a particular configuration of an out-of-equilibrium system, with no supplemental extremal principle involved.

cond-mat.stat-mech↗

arXiv2016

Closed-loop approach to thermodynamics

We present the closed loop approach to linear nonequilibrium thermodynamics considering a generic heat engine dissipatively connected to two temperature baths. The system is usually quite generally characterized by two parameters: the output power $P$ and the conversion efficiency $η$, to which we add a third one, the working frequency $ω$. We establish that a detailed understanding of the effects of the dissipative coupling on the energy conversion process, necessitates the knowledge of only two quantities: the system's feedback factor $β$ and its open-loop gain $A_{0}$, the product of which, $A_{0}β$, characterizes the interplay between the efficiency, the output power and the operating rate of the system. By placing thermodynamics analysis on a higher level of abstraction, the feedback loop approach provides a versatile and economical, hence a very efficient, tool for the study of \emph{any} conversion engine operation for which a feedback factor may be defined.

cond-mat.stat-mech↗

arXiv2012

Dual local and non-local cascades in 3D turbulent Beltrami flows

We discuss the possibility of dual local and non-local cascades in a 3D turbulent Beltrami flow, with inverse energy cascade and direct helicity cascade, by analogy with 2D turbulence. We discuss the corresponding energy spectrum in both local and non-local case. Comparison with a high Reynolds number turbulent von Karman flow is provided and discussed.

physics.flu-dyn↗

arXiv2011

Susceptibility divergence, phase transition and multistability of a highly turbulent closed flow

Using time-series of stereoscopic particle image velocimetry data, we study the response of a turbulent von Kármán swirling flow to a continuous breaking of its forcing symmetry. Experiments are carried over a wide Reynolds number range, from laminar regime at $Re = 10^{2}$ to highly turbulent regime near $Re = 10^{6}$. We show that the flow symmetry can be quantitatively characterized by two scalars, the global angular momentum $I$ and the mixing layer altitude $z_s$, which are shown to be statistically equivalent. Furthermore, we report that the flow response to small forcing dissymetry is linear, with a slope depending on the Reynolds number: this response coefficient increases non monotonically from small to large Reynolds number and presents a divergence at a critical Reynolds number $Re_c = 40\,000 \pm 5\,000$. This divergence coincides with a change in the statistical properties of the instantaneous flow symmetry $I(t)$: its pdf changes from Gaussian to non-Gaussian with multiple maxima, revealing metastable non-symmetrical states. For symmetric forcing, a peak of fluctuations of $I(t)$ is also observed at $Re_c$: these fluctuations correspond to time-intermittencies between metastable states of the flow which, contrary to the very-long-time-averaged mean flow, spontaneously and dynamically break the system symmetry. We show that these observations can be interpreted in terms of divergence of the susceptibility to symmetry breaking, revealing the existence of a phase transition. An analogy with the ferromagnetic-paramagnetic transition in solid-state physics is presented and discussed.

cond-mat.stat-mech↗