Global Entrainment Research Articles

Entrainment at multi-scales in shear-dominated and Rayleigh–Taylor turbulence

A partially turbulent flow continuously incorporates irrotational fluid into the turbulent region, a phenomenon known as entrainment. Although entrainment locally acts at viscous scales, the thin interface separating the turbulent from the irrotational region is extremely convoluted, and twisted in such a way that renders the global entrainment flux scale-independent. Despite turbulent entrainment being widely recognized as a multi-scale process, the theoretical basis for quantifying the entrainment flux at multi-scales is lacking. In this paper we derive an equation that allows us to quantify the local entrainment velocity at multi-scales. This is done by defining the local entrainment velocity as the propagation speed of an iso-surface of filtered enstrophy relative to the coarse-grained velocity field, and using the filtered enstrophy budget to split the total velocity into its individual components, i.e. viscous, inviscid, baroclinic and sub-filter. The equation is used to investigate the entrainment at multi-scales in simulated turbulent mixing layers, where turbulence is sustained by either a mean shear or an unstable buoyancy gradient (Rayleigh–Taylor turbulence).

Lattice-Boltzmann modeling of buoyancy-driven turbulent flows

The pressure-based hybrid lattice-Boltzmann method presented by Farag et al. [“A pressure-based regularized lattice-Boltzmann method for the simulation of compressible flows,” Phys. Fluids 32, 066106 (2020)] is assessed for the simulation of buoyancy driven flows. The model is first validated on Rayleigh–Benard and Rayleigh–Taylor two-dimensional cases. A large-eddy simulation of a turbulent forced plume is then carried out, and results are validated against experiments. Good overall agreement is obtained, both for mean and fluctuation quantities, as well as global entrainment. The self-similarity characteristic of the plume in the far-field is also recovered.

Optimization of periodic input waveforms for global entrainment of weakly forced limit-cycle oscillators

We propose a general method for optimizing periodic input waveforms for global entrainment of weakly forced limit-cycle oscillators based on phase reduction and nonlinear programming. We derive averaged phase dynamics from the mathematical model of a limit-cycle oscillator driven by a weak periodic input and optimize the Fourier coefficients of the input waveform to maximize prescribed objective functions. In contrast to the optimization methods that rely on the calculus of variations, the proposed method can be applied to a wider class of optimization problems including global entrainment objectives. As an illustration, we consider two optimization problems, one for achieving fast global convergence of the oscillator to the entrained state and the other for realizing prescribed global phase distributions in a population of identical uncoupled noisy oscillators. We show that the proposed method can successfully yield optimal input waveforms to realize the desired states in both cases.

Unified description of turbulent entrainment

Abstract

Adaptação acústico-prosódica entre falantes

This paper presents a global analysis of entrainment in map-task dialogues in European Portuguese, including 48 dialogues, between 24 speakers. Our main goal is to analyze the acoustic-prosodic similarities between speaker pairs, namely if there are global entrainment cues displayed in the dialogues, if entrainment is manifested in distinct sets of features shared amongst the speakers, if entrainment depends on the gender and role of the speaker (giver or follower), and if speakers tend to entrain more with specific interlocutors regardless of the role. Results show that globally speakers tend to be more similar to their partners than to their own speech in the majority of the analyzed features, a strong evidence for entrainment. Moreover, almost all the pairs of speakers display cues of global entrainment, even though in different degrees (speakers entrain but in distinct features). Additionally, the role and gender effects tend to be less striking than the specific interlocutor effect. Our results support the fact that all prosodic parameters are monitored by the speakers in our corpus, contrarily to studies for other languages, which indicate that the main cues are energy related.

Global and local aspects of entrainment in temporal plumes

To date, the understanding of the role buoyancy plays in the entrainment process in unstable configurations such as turbulent plumes remains incomplete. Towards addressing this question, we set up a flow in which a plume evolves in time instead of space. We demonstrate that the temporal problem is equivalent to a spatial plume in a strong coflow and address in detail how the temporal plume can be realized via direct numerical simulation. Using numerical data of plume simulations up to $Re_{\unicode[STIX]{x1D706}}\approx 100$, we show that the entrainment coefficient can be determined consistently using a global entrainment analysis in an integral framework as well as via a local approach. The latter is based on a study of the local propagation of the turbulent/non-turbulent interface relative to the fluid. Locally, this process is dominated by small-scale diffusion which is amplified by interface convolutions such that the total entrained flux is independent of viscosity. Further, we identify a direct buoyancy contribution to entrainment by baroclinic torque, which accounts for 8 %–12 % of the entrained flux locally, comparable to the 15 % buoyancy contribution at the integral level. It appears that the baroclinic torque is a mechanism that might explain higher values of the entrainment coefficient in spatial plumes compared with jets.

Convection at an isothermal wall in an enclosure and establishment of stratification

In this experimental–theoretical investigation, we consider a turbulent plume generated by an isothermal wall in a closed cavity and the formation of heat stratification in the interior. The buoyancy of the plume near the wall and the temperature stratification are measured across a vertical plane with the temperature laser induced fluorescence method, which is shown to be accurate and efficient (precision of $0.2\,^{\circ }$C) for experimental studies on convection. The simultaneous measurement of the velocity field with particle image velocimetry allows for the calculation of the flow characteristics such as the Richardson number and Reynolds stress. This enables us to give a refined description of the wall plume, as well as the circulation and evolution of the stratification in the interior. The wall plume is found to have an inner layer close to the heated boundary with a laminar transport of hardly mixed fluid which causes a relatively warm top layer and an outer layer with a transition from laminar to turbulent at a considerable height. The measured entrainment coefficient is found to be dramatically influenced by the increase in stratification of the ambient fluid. To model the flow, the entrainment model of Morton, Taylor & Turner (Proc. R. Soc. Lond. A, vol. 234 (1196), 1956, pp. 1–23) has first been adapted to the case of an isothermal wall. Differences due to their boundary condition of a constant buoyancy flux, modelled with salt by Cooper & Hunt (J. Fluid Mech., vol. 646, 2010, pp. 39–58), turn out to be small. Next, to include the laminar–turbulent transition of the boundary layer, a hybrid model is constructed which is based on the similarity solutions reported by Worster & Leitch (J. Fluid Mech., vol. 156, 1985, pp. 301–319) for the laminar part and the entrainment model for the turbulent part. Finally, the observed variation of the global entrainment coefficient, which is due to the increased presence of an upper stratified layer with a relatively low entrainment coefficient, is incorporated into both models. All models show reasonable agreement with experimental measurements for the volume, momentum and buoyancy fluxes as well as for the evolution of the stratification in the interior. In particular, the introduction of the variable entrainment coefficient improves all models significantly.

2A1-A07 グローバルエントレインメントを生起させるCPGと身体の連関様式の設計方策 : 四脚ロボットを用いた事例研究

2A1-A07 グローバルエントレインメントを生起させるCPGと身体の連関様式の設計方策 : 四脚ロボットを用いた事例研究

GLOBAL ENTRAINMENT EFFECT ON BIPED ROBOT LOCOMOTION ENERGY

In recent years, many researchers have studied the generation of rhythmic movement emerge as a stable limit cycle from global entrainment between biological neural networks that include central pattern generators (CPGs) and physical systems (i.e. robot arm) interacting with external environment. However, the ei¬�ect of global entrainment on biped locomotion energy has yet to be addressed. This paper delineates an approach to minimize bipedâ��s locomotion energy by means of global entrainment. The rhythmic movements for biped locomotion such as walking and running are generated by CPG using coupled nonlinear oscillators of Van Der Pol (VDP). Tuning VDP oscillator parameters to meet certain criteria (i.e. minimum energy) is a dii¬�cult task to accomplish. This is due to the nonlinearity and the coupling of the oscillators. To overcome these dii¬�culties, response surface methodology (RSM) has been proposed to characterize VDP parameters to achieve ei¬�cient energy for biped locomotion. Then, global entrainment has been implemented between i¬�ve-link biped robot and the constructed CPG. As a result, the sensory information modulates VDP oscillatorsâ�� frequencies and tuned them to the resonance frequencies of the biped link lengths. The obtained results show an evidence of the importance of global entrainment in achieving lower energy for biped locomotion while maintaining the same or even higher forward velocity.

Investigation of entrainment and thermal properties of a cryogenic dense-gas cloud using optical measurement techniques

Cryogenic dense-gas clouds have been investigated in a heavy-gas channel under controlled source and ambient conditions. Advantage is taken from new, non-intrusive optical measurement techniques (e.g. image correlation velocimetry, ICV, and background oriented Schlieren, BOS) providing detailed pictures of the temperature and velocity field in relevant regions of the cloud. The ice particles in the cloud, formed by nucleation, represent a natural seeding to be used as tracers, which have the advantage of behaving passively. Two layers can be identified in a cryogenic gas cloud: a lower cold layer, which is visible due to the presence of ice particles, and an invisible upper layer, where the ice particles have melted, mostly due to heat addition by air entrainment into the upper layer.A two-layer model has been applied to a generic element of the cloud, where detailed experimental data regarding velocity and temperature are available. Thermal- and dilution behaviour can be interpreted by means of the model which is presented in detail. A global entrainment parameter is deduced allowing a simple comparison with existing experimental information obtained by other traditional experimental techniques. The numerical values of the present entrainment parameter agree well with the correlations proposed by other authors. Thermal effects, such as heat transfer from the ground, appear to be very important. In addition, the visible height of the cloud can be predicted in relative good agreement with the experimental observations, by means of a thermal balance including the phase transition of the ice particles.

Optical density and velocity measurements in cryogenic gas flows

This paper presents the application of optical measurement techniques in dense-gas flows in a heavy-gas channel to determine planar two-component (2C) velocity profiles and two-dimensional (2D) temperature profiles. The experimental approach is rather new in this area, and represents progress compared with the traditional techniques based on thermocouple measurements. The dense-gas flows are generated by the evaporation of liquid nitrogen. The optical measurement of both the velocity and density profiles is accomplished by the implementation of particle image velocimetry (PIV) and background-oriented schlieren (BOS) systems. Supplemental thermocouple measurements are used as independent calibrations to derive temperatures from the density data measured with the BOS system. The results obtained with both systems are used to quantify the dilution behavior of the propagating cloud through a global entrainment parameter β. Its value agrees well with the results obtained by earlier studies.

STOCHASTIC RESONANCE IN ALPHA OSCILLATORS IN THE HUMAN BRAIN

We experimentally study stochastic resonance (SR) in the human brain through a noise effect for entrainment dynamics of the alpha (α) wave. The measurement has been carried out under the following conditions in order to obtain clear evidence of the SR phenomenon in the central nervous system. The periodic and noisy stimuli are respectively applied to the right and the left eyes of the subject independently. When only periodic and constant (but weak enough) stimulus is applied to the right eye, it does not induce any global entrainment of α-oscillators to the stimulus frequency. In this situation increasing the amplitude of the noise stimulus (frequency bandwidth from 15 to 60 Hz), harmonic entrainment occurs for a certain range of the noise amplitude and shows a sharp peak in the power spectrum of brain waves. Plotting the amplitude of the peak against the noise amplitude, a bell-type shape is seen, i.e. the maximum signal amplitude at an optimum noise. It clearly indicates the SR phenomenon. Taking account of our experimental conditions and set-up, the present SR phenomenon occurs not on the retina but on the visual processing area in the central nervous systems beyond the optic chiasma.

1P1-E11 Global Entrainment に基づいた 2 足ロボットの歩行運動

1P1-E11 Global Entrainment に基づいた 2 足ロボットの歩行運動

Effects of heat release and buoyancy on flow structure and entrainment in turbulent nonpremixed flames

Particle image velocimetry was used to examine the velocity fields in the near- and far-field of the axisymmetric jet in co-flow under nonreacting and reacting conditions. The jets studied include nonreacting jets at Re o = 10,000 and 2,000 and flames at Re o = 10,000 and 37,500. Qualitatively, heat release was seen to impact the instantaneous flow structure over the first third of the flame length by restricting the large-scale movement of the jet. The instantaneous flow structure over the remainder of the flame length was less affected by heat release, a possible consequence of the lower density gradients. Heat release was seen to impact the mean characteristics of the flow by reducing the local Reynolds number by a factor of 10 over the length of the flame and reducing the global entrainment rate by ∼50%. In addition, heat release acted to narrow the jet width up to 20%, reduce the turbulence intensities up to 40%, and increase the centerline-velocities by factors of 2 to 3. The effects of buoyancy resulting from heat release were also examined and tended to counteract the effects of heat release by promoting entrainment and increasing turbulence intensity.

A model of the neuro-musculo-skeletal system for human locomotion

Adaptive gaits of humans were produced as a result of emergent properties of a model based on the neurophysiology of the central pattern generator and the biomechanics of the human musculoskeletal system. We previously proposed a neuromusculoskeletal model for human locomotion, in which movements emerged as a stable limit cycle that was generated through the global entrainment among the neural system, composed of neural oscillators, the musculoskeletal system, and the environment. In the present study, we investigated the adaptability of this model under various types of environmental and task constraints. Using a computer simulation, it was found that walking movements were robust against mechanical perturbations, loads with a mass, and uneven terrain. Moreover, the speed of walking could be controlled by a single parameter which tonically drove the neural oscillators, and the step cycle could be entrained by a rhythmic input to the neural oscillators.

Emergence of bipedal locomotion through entrainment among the neuro-musculo-skeletal system and the environment

A principle of locomotor control in an unpredictably changing environment is presented on the basis of neurophysiology and biomechanics from the perspective of nonlinear dynamics theory. Locomotor movements emerge as a limit cycle generated through global entrainment among the neuro-musculo-skeletal system and the environment. A computer simulation of a specific model of bipedal locomotion shows its ability to adapt to a changing environment in real-time. The stability of the limit cycle is maintained despite the presence of time delays in transporting and processing information between the neural rhythm generator and the musculo-skeletal system. With considerable time delays, however, the locomotor pattern becomes chaotic, which is compared with a gait of patients with neural deficits. A general framework for motor control is discussed toward the control of movements in an unpredictable environment.

The role of streamwise vorticity in the near-field entrainment of round jets

The role of streamwise vortex structures in the near-field (x/d < 10) evolution of a round jet is examined. In free shear layers the streamwise vorticity develops into Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the streamwise vorticity in entraining fluid increases relative to that of the azimuthal vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by streamwise vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.

Self-organized control of bipedal locomotion by neural oscillators in unpredictable environment

A new principle of sensorimotor control of legged locomotion in an unpredictable environment is proposed on the basis of neurophysiological knowledge and a theory of nonlinear dynamics. Stable and flexible locomotion is realized as a global limit cycle generated by a global entrainment between the rhythmic activities of a nervous system composed of coupled neural oscillators and the rhythmic movements of a musculo-skeletal system including interaction with its environment. Coordinated movements are generated not by slaving to an explicit representation of the precise trajectories of the movement of each part but by dynamic interactions among the nervous system, the musculo-skeletal system and the environment. The performance of a bipedal model based on the above principle was investigated by computer simulation. Walking movements stable to mechanical perturbations and to environmental changes were obtained. Moreover, the model generated not only the walking movement but also the running movement by changing a single parameter nonspecific to the movement. The transitions between the gait patterns occurred with hysteresis.

Functional role of the epicardium in postinfarction ventricular tachycardia: Observations derived from global epicardial activation mapping, entrainment, and epicardial laser photoablation

Functional role of the epicardium in postinfarction ventricular tachycardia: Observations derived from global epicardial activation mapping, entrainment, and epicardial laser photoablation

Dispersion of initially compact dense gas clouds

Many field and laboratory experiments on the dispersion of isothermal dense gas clouds use initial cloud shapes having nearly equal vertical and horizontal dimensions, termed initially compact clouds. The earliest stage of mixing, dominated by the initial spreading motion of the cloud, is modeled in this paper as one having a constant global entrainment rate, the empirical value of which is obtained from comparisons with dense gas cloud releases in still air. By adding to this initial entrainment that due to wind turbulence (as modified by cloud stratification), a good correlation is found for dispersion in wind tunnel tests of initially compact dense gas clouds. This initial high rate of entrainment was found to be missing in wind tunnel experiments using non-compact clouds. Although no quantitative criterion is available to distinguish among these initial shapes, their effects are eventually erased by the wind driven mixing.