Order Out of Chaos: Slowly Reversing Mean Flows Emerge from Turbulently Generated Internal Waves

We demonstrate via direct numerical simulations that a periodic, oscillating mean flow spontaneously develops from turbulently generated internal waves. We consider a minimal physical model where the fluid self-organizes in a convective layer adjacent to a stably stratified one. Internal waves are excited by turbulent convective motions, then nonlinearly interact to produce a mean flow reversing on timescales much longer than the waves' period. Our results demonstrate for the first time that the three-scale dynamics due to convection, waves, and mean flow is generic and hence can occur in many astrophysical and geophysical fluids. We discuss efforts to reproduce the mean flow in reduced models, where the turbulence is bypassed. We demonstrate that wave intermittency, resulting from the chaotic nature of convection, plays a key role in the mean-flow dynamics, which thus cannot be captured using only second-order statistics of the turbulent motions

Human pains and animal pains

Nociception and pain, its most elaborate cognitive expression, are an alarm system designed to protect the body: they elicit reactions that aim to decrease their initial cause and therefore limit its consequences. A nociceptive stimulus activates a series of sensory organs called “nociceptors”, which are free endings of non-myelinated fibres spread throughout the tissues. These nociceptors are polymodal in the sense that they are activated by stimuli of various physical natures (thermal, mechanical, chemical). Nociceptive signals travel to the dorsal horn of the spinal cord before being distributed to the ventral and lateral horns (for somatic and vegetative reflexes) and to numerous brain structures, including the reticular formation, the thalamus and several cortical areas. In man, this latter category includes the primary and secondary somaesthetic cortices, and especially the cingular and insular cortices belonging to the limbic system, which plays an essential role in the genesis of emotions. Nociceptive systems are very old phylogenetically. Only in species that acquired an emotional brain (limbic system) can pain be defined as a sensory and emotional experience. As the evaluation of pain in animals will always be hindered by the absence of verbal communication, the anthropomorphic approach is our only option, but our understanding may be improved by the additional use of neuropsychological data.La nociception et son expression la plus élaborée sur le plan cognitif, la douleur, constituent un signal d’alarme qui protège l’organisme: elles déclenchent des réactions dont la finalité est de diminuer la cause qui les a provoquées et d’en limiter ainsi les conséquences. Un stimulus nociceptif active un ensemble discret d’organes sensoriels, les nocicepteurs, terminaisons libres de fibres amyéliniques qui tapissent l’ensemble des tissus. Ces nocicepteurs sont polymodaux, c’est-à-dire qu’ils répondent à des stimulus de différentes natures physiques (thermique, mécanique, chimique). Le message nociceptif est relayé dans la corne dorsale de la moelle épinière pour être distribué vers les cornes ventrale et latérale (réflexes somatiques et végétatifs) et dans de nombreuses structures cérébrales, notamment la formation réticulée, le thalamus et plusieurs aires corticales. Ces dernières incluent chez l’homme les cortex somesthésiques primaire et secondaire et surtout les cortex cingulaire et insulaire qui appartiennent au système limbique, essentiel dans la genèse des émotions. Les systèmes nociceptifs sont très anciens dans la phylogenèse. Ce n’est qu’avec l’apparition du cerveau émotionnel (limbique), que l’on peut parler de douleur au sens d’une expérience sensorielle et émotionnelle. Comme l’absence de communication verbale sera toujours un obstacle pour évaluer la douleur de l’animal, il est suggéré que, pour mieux la comprendre, l’incontournable anthropomorphisme se nourrisse des données neuropsychologiques

Experimental study of internal wave generation by convection in water

We experimentally investigate the dynamics of water cooled from below at 0^oC and heated from above. Taking advantage of the unusual property that water's density maximum is at about 4^oC, this set-up allows us to simulate in the laboratory a turbulent convective layer adjacent to a stably stratified layer, which is representative of atmospheric and stellar conditions. High precision temperature and velocity measurements are described, with a special focus on the convectively excited internal waves propagating in the stratified zone. Most of the convective energy is at low frequency, and corresponding waves are localized to the vicinity of the interface. However, we show that some energy radiates far from the interface, carried by shorter horizontal wavelength, higher frequency waves. Our data suggest that the internal wave field is passively excited by the convective fluctuations, and the wave propagation is correctly described by the dissipative linear wave theory

Shape and size of large-scale vortices: A generic fluid pattern in geophysical fluid dynamics

Planetary rotation organizes fluid motions into coherent, long-lived swirls, known as large-scale vortices (LSVs), which play an important role in the dynamics and long-term evolution of geophysical and astrophysical fluids. Here, using direct numerical simulations, we show that LSVs in rapidly rotating mixed convective and stably stratified fluids, which approximates the two-layer, turbulent-stratified dynamics of many geophysical and astrophysical fluids, have a generic shape and that their size can be predicted. We show that LSVs emerge in the convection zone from upscale energy transfers and can penetrate into the stratified layer. At the convective-stratified interface, the LSV cores have a positive buoyancy anomaly. Due to the thermal wind constraint, this buoyancy anomaly leads to winds in the stratified layer that decay over a characteristic vertical length scale. Thus LSVs take the shape of a depth-invariant cylinder with a finite-size radius in the turbulent layer and of a penetrating half dome in the stratified layer. Importantly, we demonstrate that when LSVs penetrate all the way through the stratified layer and reach a boundary that is no-slip, they saturate by boundary friction. We provide a prediction for the penetration depth and maximum radius of LSVs as a function of the LSV vorticity, the stratified layer depth, and the stratification. Our results, which apply for cyclonic LSVs, suggest that LSVs in slowly rotating stars and Earth's liquid core are confined to the convective layer, while in Earth's atmosphere and oceans they can penetrate far into the stratified layer

The energy flux spectrum of internal waves generated by turbulent convection

We present three-dimensional direct numerical simulations of internal waves excited by turbulent convection in a self-consistent, Boussinesq and Cartesian model of convective--stably-stratified fluids. We demonstrate that in the limit of large Rayleigh number ($Ra\in [4\times 10^7,10^9]$) and large stratification (Brunt-V\"{a}is\"{a}l\"{a} frequencies $f_N \gg f_c$, where $f_c$ is the convective frequency), simulations are in good agreement with a theory that assumes waves are generated by Reynolds stresses due to eddies in the turbulent region (Lecoanet \& Quataert 2013 MNRAS 430 (3) 2363-2376). Specifically, we demonstrate that the wave energy flux spectrum scales like $k_{\perp}^4f^{-13/2}$ for weakly-damped waves (with $k_{\perp}$ and $f$ the waves' horizontal wavenumbers and frequencies), and that the total wave energy flux decays with $z$, the distance from the convective region, like $z^{-13/8}$.Comment: 13 pages, 6 figure

The Fine Tuning of Pain Thresholds: A Sophisticated Double Alarm System

Two distinctive features characterize the way in which sensations including pain, are evoked by heat: (1) a thermal stimulus is always progressive; (2) a painful stimulus activates two different types of nociceptors, connected to peripheral afferent fibers with medium and slow conduction velocities, namely Aδ- and C-fibers. In the light of a recent study in the rat, our objective was to develop an experimental paradigm in humans, based on the joint analysis of the stimulus and the response of the subject, to measure the thermal thresholds and latencies of pain elicited by Aδ- and C-fibers. For comparison, the same approach was applied to the sensation of warmth elicited by thermoreceptors. A CO2 laser beam raised the temperature of the skin filmed by an infrared camera. The subject stopped the beam when he/she perceived pain. The thermal images were analyzed to provide four variables: true thresholds and latencies of pain triggered by heat via Aδ- and C-fibers. The psychophysical threshold of pain triggered by Aδ-fibers was always higher (2.5–3°C) than that triggered by C-fibers. The initial skin temperature did not influence these thresholds. The mean conduction velocities of the corresponding fibers were 13 and 0.8 m/s, respectively. The triggering of pain either by C- or by Aδ-fibers was piloted by several factors including the low/high rate of stimulation, the low/high base temperature of the skin, the short/long peripheral nerve path and some pharmacological manipulations (e.g. Capsaicin). Warming a large skin area increased the pain thresholds. Considering the warmth detection gave a different picture: the threshold was strongly influenced by the initial skin temperature and the subjects detected an average variation of 2.7°C, whatever the initial temperature. This is the first time that thresholds and latencies for pain elicited by both Aδ- and C-fibers from a given body region have been measured in the same experimental run. Such an approach illustrates the role of nociception as a “double level” and “double release” alarm system based on level detectors. By contrast, warmth detection was found to be based on difference detectors. It is hypothesized that pain results from a CNS build-up process resulting from population coding and strongly influenced by the background temperatures surrounding at large the stimulation site. We propose an alternative solution to the conventional methods that only measure a single “threshold of pain”, without knowing which of the two systems is involved

Peripheral and Central Determinants of a Nociceptive Reaction: An Approach to Psychophysics in the Rat

BACKGROUND: The quantitative end-point for many behavioral tests of nociception is the reaction time, i.e. the time lapse between the beginning of the application of a stimulus, e.g. heat, and the evoked response. Since it is technically impossible to heat the skin instantaneously by conventional means, the question of the significance of the reaction time to radiant heat remains open. We developed a theoretical framework, a related experimental paradigm and a model to analyze in psychophysical terms the "tail-flick" responses of rats to random variations of noxious radiant heat. METHODOLOGY/PRINCIPAL FINDINGS: A CO(2) laser was used to avoid the drawbacks associated with standard methods of thermal stimulation. Heating of the skin was recorded with an infrared camera and was stopped by the reaction of the animal. For the first time, we define and determine two key descriptors of the behavioral response, namely the behavioral threshold (Tbeta) and the behavioral latency (Lbeta). By employing more than one site of stimulation, the paradigm allows determination of the conduction velocity of the peripheral fibers that trigger the response (V) and an estimation of the latency (Ld) of the central decision-making process. Ld (approximately 130 ms) is unaffected by ambient or skin temperature changes that affect the behavioral threshold (approximately 42.2-44.9 degrees C in the 20-30 degrees C range), behavioral latency (<500 ms), and the conduction velocity of the peripheral fibers that trigger the response (approximately 0.35-0.76 m/s in the 20-30 degrees C range). We propose a simple model that is verified experimentally and that computes the variations in the so-called "tail-flick latency" (TFL) caused by changes in either the power of the radiant heat source, the initial temperature of the skin, or the site of stimulation along the tail. CONCLUSIONS/SIGNIFICANCE: This approach enables the behavioral determinations of latent psychophysical (Tbeta, Lbeta, Ld) and neurophysiological (V) variables that have been previously inaccessible with conventional methods. Such an approach satisfies the repeated requests for improving nociceptive tests and offers a potentially heuristic progress for studying nociceptive behavior on more firm physiological and psychophysical grounds. The validity of using a reaction time of a behavioral response to an increasing heat stimulus as a "pain index" is challenged. This is illustrated by the predicted temperature-dependent variations of the behavioral TFL elicited by spontaneous variations of the temperature of the tail for thermoregulation

Is it relevant to use the english word "sentience" in french?

Depuis Aristote les animaux sont considérés comme des êtres sensibles, ce qui a été reconnu sur le plan juridique dans le code rural en 1976 puis le code civil en 2017. Par ailleurs, les philosophes du XVIIIe siècle ont introduit le concept de « sentience » - la capacité de ressentir - pour l’opposer à la raison - la capacité de penser -. Cette notion s’est précisée ultérieurement entre les mains des philosophes de l’esprit anglo-saxons pour exprimer la capacité d’éprouver des expériences subjectives (« qualia », « conscience phénoménale »), un concept clair et relativement « simple » mais que seule l’introspection permet d’approcher. En dépit de cette difficulté ontologique, certains penseurs en éthique animale souhaitent introduire ce concept dans la langue française, sans s’interroger sur sa polysémie qui, déjà, brouille sa signification dans la littérature anglophone. En effet, la « sentience » englobe selon les auteurs les sensations, les perceptions, les émotions, les affects, les sentiments, la cognition, la conscience, etc. Chacun de ces termes étant polysémique et appartenant à la fois aux champs philosophiques et scientifiques, le terme générique apparait comme un fourre-tout sémantique obérant toute approche raisonnée. Pour s’en convaincre, il suffit d’envisager la confusion qu’entraînerait le remplacement de l’affirmation « l’animal est un être sensible » par « l’animal est un être sentient ». Sur le plan législatif, il serait source de regrettables malentendus. Soulignons en outre le piège sémantique que dissimule l’utilisation de l’article défini et du singulier, négligeant ainsi l’extraordinaire diversité du monde animal qui s’est épanouie dans l’espace - notre planète - et le temps - 600 millions d’années -.Since Aristotle animals are considered as sensitive beings, what was recognized at the legislative level in the French rural and civil codes in 1976 and 2017, respectively. The XVIIIth century philosophers introduced the concept of «sentience» - the ability to feel - to differ from the reason - the capacity to think -. This notion was then clarified by the Anglo-Saxon philosophers of mind to express the capacity to feel subjective experiences («qualia», «phenomenal consciousness»), a clear and relatively «simple» concept but only approachable by introspection. In spite of this ontological difficulty, some animal ethics thinkers wish to introduce this concept into the French language, without wondering about its polysemy which, already, blurs its meaning in the English-speaking literature. According to the authors, «sentience» includes sensations, perception, emotions, feelings, affects, feeling, cognition, consciousness, etc. Each of these terms being polysemic and belonging to both the philosophic and scientific fields, the generic name appears as a semantic oldall compromising any reasoned approach. To convince oneself, it is enough to envisage the confusion driven by the replacement of the asser- tion «the animal is a sensitive being» by «the animal is a sentient being». At the legislative level, it would be the source of regrettable misunderstandings. Let us also underline the semantic trap that hides the grammatical use of the defined article and the singular, so neglecting the extraordinary diversity of the animal kingdom that spread out over space -our planet- and time -600 millions years-

Numerical Simulations of Internal Wave Generation by Convection in Water

Water's density maximum at 4C makes it well suited to study internal gravity wave excitation by convection: an increasing temperature profile is unstable to convection below 4C, but stably stratified above 4C. We present numerical simulations of a water-like fluid near its density maximum in a two dimensional domain. We successfully model the damping of waves in the simulations using linear theory, provided we do not take the weak damping limit typically used in the literature. In order to isolate the physical mechanism exciting internal waves, we use the novel spectral code Dedalus to run several simplified model simulations of our more detailed simulation. We use data from the full simulation as source terms in two simplified models of internal wave excitation by convection: bulk excitation by convective Reynolds stresses, and interface forcing via the mechanical oscillator effect. We find excellent agreement between the waves generated in the full simulation and the simplified simulation implementing the bulk excitation mechanism. The interface forcing simulations over excite high frequency waves because they assume the excitation is by the "impulsive" penetration of plumes, which spreads energy to high frequencies. However, we find the real excitation is instead by the "sweeping" motion of plumes parallel to the interface. Our results imply that the bulk excitation mechanism is a very accurate heuristic for internal wave generation by convection.Comment: Accepted to PR

Freezing of Enkephalinergic Functions by Multiple Noxious Foci: A Source of Pain Sensitization?

BACKGROUND:The functional significance of proenkephalin systems in processing pain remains an open question and indeed is puzzling. For example, a noxious mechanical stimulus does not alter the release of Met-enkephalin-like material (MELM) from segments of the spinal cord related to the stimulated area of the body, but does increase its release from other segments. METHODOLOGY/PRINCIPAL FINDINGS:Here we show that, in the rat, a noxious mechanical stimulus applied to either the right or the left hind paw elicits a marked increase of MELM release during perifusion of either the whole spinal cord or the cervico-trigeminal area. However, these stimulatory effects were not additive and indeed, disappeared completely when the right and left paws were stimulated simultaneously. CONCLUSION/SIGNIFICANCE:We have concluded that in addition to the concept of a diffuse control of the transmission of nociceptive signals through the dorsal horn, there is a diffuse control of the modulation of this transmission. The "freezing" of Met-enkephalinergic functions represents a potential source of central sensitization in the spinal cord, notably in clinical situations involving multiple painful foci, e.g. cancer with metastases, poly-traumatism or rheumatoid arthritis