Rayleigh–Taylor mixing in an otherwise stable stratification

2011 ◽  
Vol 688 ◽  
pp. 507-527 ◽  
Author(s):  
Andrew G. W. Lawrie ◽  
Stuart B. Dalziel
Keyword(s):  
Stable State ◽  
Stable Stratification ◽  
State Density ◽  
Mixing Efficiency ◽  
Density Profiles ◽  
Physical Mechanisms ◽  
Available Energy ◽  
Ideal Mixing ◽  
Driven System ◽  
Before And After

AbstractWe seek to understand the distribution of irreversible energy conversions (mixing efficiency) between quiescent initial and final states in a miscible Rayleigh–Taylor driven system. The configuration we examine is a Rayleigh–Taylor unstable interface sitting between stably stratified layers with linear density profiles above and below. Our experiments in brine solution measure vertical profiles of density before and after the unstable interface is allowed to relax to a stable state. Our analysis suggests that less than half the initially available energy is irreversibly released as heat due to viscous dissipation, while more than half irreversibly changes the probability density function of the density field by scalar diffusion and therefore remains as potential energy, but in a less useful form. While similar distributions are observed in Rayleigh–Taylor driven mixing flows between homogeneous layers, our new configuration admits energetically consistent end-state density profiles that span all possible mixing efficiencies, ranging from all available energy being expended as dissipation, to none. We present experiments that show that the fluid relaxes to a state with a significantly lower mixing efficiency than the value for ideal mixing in this configuration, and deduce that this mixing efficiency more accurately characterizes Rayleigh–Taylor driven mixing than previous measurements. We argue that the physical mechanisms intrinsic to Rayleigh–Taylor instability are optimal conditions for mixing, and speculate that we have observed an upper bound to fluid mixing in general.

scholarly journals Efficient mixing in stratified flows: experimental study of a Rayleigh–Taylor unstable interface within an otherwise stable stratification

2014 ◽  
Vol 756 ◽  
pp. 1027-1057 ◽  
Author(s):  
Megan S. Davies Wykes ◽  
Stuart B. Dalziel
Keyword(s):  
Turbulent Mixing ◽  
Laboratory Experiments ◽  
Functional Form ◽  
Salt Water ◽  
Stable Stratification ◽  
Mixing Efficiency ◽  
Final States ◽  
Density Profiles ◽  
Rayleigh Taylor Instability ◽  
Good Agreement

AbstractBoussinesq salt-water laboratory experiments of Rayleigh–Taylor instability (RTI) can achieve mixing efficiencies greater than 0.75 when the unstable interface is confined between two stable stratifications. This is much greater than that found when RTI occurs between two homogeneous layers when the mixing efficiency has been found to approach 0.5. Here, the mixing efficiency is defined as the ratio of energy used in mixing compared with the energy available for mixing. If the initial and final states are quiescent then the mixing efficiency can be calculated from experiments by comparison of the corresponding density profiles. Varying the functional form of the confining stratifications has a strong effect on the mixing efficiency. We derive a buoyancy-diffusion model for the rate of growth of the turbulent mixing region, $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\dot{h} = 2 \sqrt{\alpha A g h}$ (where $A = A(h)$ is the Atwood number across the mixing region when it extends a height $h$, $g$ is acceleration due to gravity and $\alpha $ is a constant). This model shows good agreement with experiments when the value of the constant $\alpha $ is set to 0.07, the value found in experiments of RTI between two homogeneous layers (where the height of the turbulent mixing region increases as $h =\alpha A g t^2$, an expression which is equivalent to that derived for $\dot{h}$).

scholarly journals Physiological and Psychological Effects of Watching Videos of Different Durations Showing Urban Bamboo Forests with Varied Structures

2020 ◽  
Vol 17 (10) ◽  
pp. 3434
Author(s):  
Yuqian Wang ◽  
Mingyan Jiang ◽  
Yinshu Huang ◽  
Zhiyi Sheng ◽  
Xiao Huang ◽  
...  
Keyword(s):  
Blood Pressure ◽  
Skin Conductance ◽  
Human Body ◽  
Stable State ◽  
Mean Value ◽  
Psychological Effects ◽  
Physiological Indicators ◽  
Psychological Indicators ◽  
Before And After ◽  
Long Time

This study illustrated the physiological and psychological effects of watching videos of different durations showing bamboo forests with varied structures. Physiological indicators, including EEG (electroencephalogram), blood pressure, skin conductance, and pulse, were monitored in 180 Chinese university students (mean age: 20.72 ± 2.56 years) while they were watching bamboo videos. Before and after watching the videos, their psychological indicators, including positive and negative moods, were measured using the Profile of Mood States questionnaire. After watching the bamboo videos of different durations, all of the physiological indicators responded to the stimulation after only 1 min. The indicators showed different trends at 1, 3 and 5 min. EEG decreased and then was maintained at a stable level after 1 min, and the high β, low β, and α waves had no significant differences between 1, 3 and 5 min. Blood pressure dropped to a stable state after 3 min, and the decline was significantly different greater after 3 min than after 1 min. Skin conductance increased for 1 to 5 min, and it did not stabilize after a long time (5 min). Pulse decreased after 1 min but increased after 5 min. After watching the videos with bamboo of varying structures, the physiological and psychological indicators showed significantly different changes. Skin conductance significantly increased (mean value: 6.78%), and the amount of sweat was more effectively reduced, thereby reducing tension, when the students viewed videos of sympodial bamboo forests compared with monopodial bamboo forests. Bamboo forests with a higher canopy density (0.83–0.85) could significantly decrease α waves (mean value: 1.50 Hz), relaxing the human body. High β and low β waves showed greater decreases, with tension reduced more effectively, when bamboo forests with a low tilt ratio (< 1.5%) were viewed. Bamboo forests with neat undergrowth could have more beneficial physiological and psychological effects on the human body.

Simulation of Hydrostatic Drive System of Crawler Bulldozer

2011 ◽  
Vol 243-249 ◽  
pp. 6178-6182 ◽  
Author(s):  
Pu Xing ◽  
Shi Lin Luo ◽  
Cun Bo Liu ◽  
Cong Feng Tian
Keyword(s):  
Steady State ◽  
Stable State ◽  
Feedback Regulation ◽  
Hydraulic Motor ◽  
Operation System ◽  
Efficient Operation ◽  
Proportional Control ◽  
System Pressure ◽  
Driven System ◽  
Working Principle

According to the hydraulic driven system working principle of crawler bulldozer, the simulation model of electro-hydraulic proportional control driven system was established by using AMESim, and the simulation was carried out. The simulation result indicates that using feedback regulation in control of hydraulic motor of driven system, the system pressure stay in a relatively stable state, which is conducive to the efficient operation system. As the load increasing suddenly, the system pressure volatile during the volume of hydraulic motor changing. However, steady state can be quickly restored.

Parameter Prediction Method for Submarine Cuttings Piles in Offshore Drilling

SPE Journal ◽  
2020 ◽  
Vol 25 (03) ◽  
pp. 1307-1332
Author(s):  
Baojiang Sun ◽  
Zhi Zhang ◽  
Zhiyuan Wang ◽  
Shaowei Pan ◽  
Ze Wang ◽  
...  
Keyword(s):  
Current Velocity ◽  
Geometric Characteristic ◽  
Stable State ◽  
Prediction Method ◽  
Continuous Model ◽  
Ocean Currents ◽  
Ocean Current ◽  
Offshore Drilling ◽  
Before And After ◽  
Drilling Site

Summary The cost of offshore drilling operations can be significantly reduced by discharging drilling cuttings into the seabed. However, this leads to accumulation of cuttings piles on the seabed near the drilling site. A certain thickness of cuttings piles changes the original trend of the seabed terrain undulation condition, thus bringing potential safety hazards to the underwater installation of production manifolds. Moreover, the interaction between cuttings and ocean currents near the cuttings piles causes the geometric shape of cuttings piles to evolve over time, which makes it more difficult to accurately predict their characteristics. On the basis of the force analysis of cuttings, considering the effects of cuttings properties (cutting size, density) and ocean-current velocity on the geometric characteristic evolution of the formed cuttings piles, a continuous model for describing the evolution of the returned cuttings piles is established in this study. This model can quantitatively characterize the functional relationship between characteristics of cuttings piles and relevant parameters (current velocity, cutting size, evolution time), and predict the location and geometry characteristics of the cuttings piles evolving into a stable state in ocean currents. Comparing the measured data in laboratory experiments and at an offshore drilling field, the relative error of the model amounts to less than 10%, which demonstrates its rationality. Simulation results show that there will be significant changes in the geometry of cuttings piles before and after the evolution, in which the intensity is correlated with current velocity and cuttings size, and cuttings piles might even split into several parts under certain conditions. The simulation and analysis of the transport and deposition of cuttings returned from the wellhead on the seabed is highly significant for the guide and optimal design of underwater production manifolds.

scholarly journals A Parameterization with a Constrained Potential Energy Conversion Rate of Vertical Mixing Due to Langmuir Turbulence

2019 ◽  
Vol 49 (11) ◽  
pp. 2935-2959 ◽  
Author(s):  
Brandon G. Reichl ◽  
Qing Li
Keyword(s):  
Turbulent Mixing ◽  
Vertical Mixing ◽  
Stable Stratification ◽  
Surface Boundary ◽  
Langmuir Turbulence ◽  
Surface Boundary Layer ◽  
Eddy Simulation ◽  
Available Energy ◽  
Climate Simulations ◽  
Surface Buoyancy

AbstractIn this study we develop a new parameterization for turbulent mixing in the ocean surface boundary layer (OSBL), including the effect of Langmuir turbulence. This new parameterization builds on a recent study (Reichl and Hallberg 2018, hereafter RH18), which predicts the available energy for turbulent mixing against stable stratification driven by shear and convective turbulence. To investigate the role of Langmuir turbulence in the framework of RH18, we utilize data from a suite of previously published large-eddy simulation (LES) experiments (Li and Fox-Kemper 2017, hereafter LF17) with and without Langmuir turbulence under different idealized forcing conditions. We find that the parameterization of RH18 is able to reproduce the mixing simulated by the LES in the non-Langmuir cases, but not the Langmuir cases. We therefore investigate the enhancement of the integrated vertical buoyancy flux within the entrainment layer in the presence of Langmuir turbulence using the LES data. An additional factor is introduced in the RH18 framework to capture the enhanced mixing due to Langmuir turbulence. This additional factor depends on the surface-layer averaged Langmuir number with a reduction in the presence of destabilizing surface buoyancy fluxes. It is demonstrated that including this factor within the RH18 OSBL turbulent mixing parameterization framework captures the simulated effect of Langmuir turbulence in the LES, which can be used for simulating the effect of Langmuir turbulence in climate simulations. This new parameterization is compared to the KPP-based Langmuir entrainment parameterization introduced by LF17, and differences are explored in detail.

Thermal equilibration behind an ionizing shock

1966 ◽  
Vol 26 (3) ◽  
pp. 459-479 ◽  
Author(s):  
H. Wong ◽  
D. Bershader
Keyword(s):  
Refractive Index ◽  
Theoretical Analysis ◽  
Thermal Equilibrium ◽  
Mass Density ◽  
Relaxation Times ◽  
Ionization Mechanism ◽  
Density Profiles ◽  
Physical Mechanisms ◽  
Thermal Equilibration ◽  
Equilibrium Region

The physical mechanisms underlying the relaxation process leading to thermal equilibrium behind ionizing shock waves in argon have been studied through use of optical techniques. The non-equilibrium condition in the relaxation region was investigated experimentally by measuring the shift in the fringes due to a change in the refractive index of the medium with a Mach–Zehnder interferometer. Both electron- and mass-density profiles from the shock front to the equilibrium region were determined. The experimental work has been supplemented by a theoretical analysis of the ionization mechanism to explain the measured profiles and relaxation times.

Equilibrium-State Density Profiles of Centrifuged Cakes of Flocculated Suspensions

1992 ◽  
Vol 289 ◽  
Author(s):  
Wei-Heng Shih ◽  
Wan Y. Shih ◽  
Seong-Il Kim ◽  
Ilhan A. Aksay
Keyword(s):  
Local Density ◽  
State Density ◽  
Constant Density ◽  
Density Profiles ◽  
X Ray ◽  
Consolidation Pressure ◽  
Silica Suspension ◽  
Γ Ray ◽  
Silica Suspensions ◽  
Alumina Suspensions

AbstractMany advanced ceramics are formed by colloidal consolidation using techniques such as sedimentation, centrifugation, or pressure filtration. In all these, the minimization of density variations of various colloidally-consolidated cakes has not been examined systematically until recently. Schilling et al. used γ-ray densitometry to study the density variations in a sedimented cake and showed that the sediment of a flocculated alumina suspension exhibited significant density variations within the cake whereas the sediment of a dispersed alumina suspension showed a constant density profile. Auzerais et al. used a medical X-ray computer tomography (CT) to study the settling of dispersed and flocculated silica suspensions and obtained similar results: The sedimented cake of a dispersed silica suspension was fairly uniform whereas those of flocculated silica suspensions exhibited significant density variations. However, Shih et al. examined pressure-filtered cakes of flocculated alumina suspensions with γ-ray densitometry and showed that the density profiles of pressure-filtered cakes of flocculated alumina suspensions were uniform, in contrast to their sedimentation counterparts which showed significant density variations. These studies illustrate that how the local density varies within a consolidated cake depends not only on the suspension conditions but also on how the suspensions are consolidated, e.g., the consolidation pressure.

scholarly journals Vortex-ring-induced stratified mixing: mixing model

2017 ◽  
Vol 837 ◽  
pp. 129-146 ◽  
Author(s):  
Jason Olsthoorn ◽  
Stuart B. Dalziel
Keyword(s):  
Vortex Ring ◽  
Turbulent Mixing ◽  
Quantitative Agreement ◽  
Vortex Rings ◽  
Mixing Efficiency ◽  
Mean Flow ◽  
Density Profiles ◽  
One Dimensional ◽  
Physical Experiments ◽  
Dynamical Features

The study of vortex-ring-induced mixing has been significant for understanding stratified turbulent mixing in the absence of a mean flow. Renewed interest in this topic has prompted the development of a one-dimensional model for the evolution of a stratified system in the context of isolated mixing events. This model is compared to numerical simulations and physical experiments of vortex rings interacting with a stratification. Qualitative agreement between the evolution of the density profiles is observed, along with close quantitative agreement of the mixing efficiency. This model highlights the key dynamical features of such isolated mixing events.

scholarly journals The Influence of Feedback on Balance in Stroke Patients

2020 ◽  
Vol 1 (2) ◽  
Author(s):  
Jûratė Ramanauskaitė ◽  
Gražina Krutulytė
Keyword(s):  
Physical Therapy ◽  
Body Position ◽  
Dynamic Balance ◽  
Stable State ◽  
Cerebral Stroke ◽  
Static Balance ◽  
Stroke Patients ◽  
Eyes Closed ◽  
Physical Exercises ◽  
Before And After

Stroke has been recognized as one of the basic reasons what concerns illness, death-rate and long-term disability all over the world. Balance disorder is one of the most common disorders and one of the basic factors limiting the routine of cerebral stroke patients. Therefore the feedback is especially important to train balance in patients with the disorder to perceive their body position. The use of feedback provides patients with sensormotoric disorders ability to evaluate different physiological reactions better and to improve simetricity of their body position. There are expensive and complicated feedback-based platforms used to train balance, however, there was no evidence, whether a mirror – a simple and cheap means of feedback – equally influences the balance of stroke patients. Objectives: to assess the feedback influence on the balance of stroke patients. To evaluate the mobility of stroke patients before and after the physical therapy. To evaluate the static balance of stroke patients before and after the physical therapy To evaluate the dynamic balance of stroke patients before and after the physical therapy Methods and measures: The research was carried out from 2008–02–01 to 2009–02–02, in the 2nd Department of In-patient Rehabilitation (Centre of Physical Medicine and Rehabilitation, Vilnius University Hospital Santariškių Klinikos). There were 40 cerebral stroke patients in the survey examined, who were selected according to the following criteria: • the first cerebral stroke, • ability to understand the tasks of the survey, • ability to stand 2 minutes without assistance, • ability to stand 30 seconds without assistance ant with eyes closed, • ability to step on and step off a stair individually or with observation, • stable state of the cardiovascular system The patients were divided into two groups randomly: group 1 (n = 20) consisted of the patients whose balance was trained using platform „MTD-balance“; group 2 (n = 20) consisted of the patients whose balance was trained using a mirror. During the survey all the patients underwent a specially created program of both general and special, aimed physical exercises. The program lasted 4 weeks (5 days a week, twice a day) and consisted of basic and functional procedures of physical therapy The functional state was evaluated at the time of patients’ arrival at the department of rehabilitation and 4 weeks later. The power of muscles was evaluated according to the 5 grade system of Lovett; sensations (surface and deep), muscular tone according the spastic scale of Ashworth; general mobility (5 classes according to the scale suggested by the European Federation for Research in Rehabilitation (EFRR), legs mobility (5 classes according to the scale suggested by the EFRR), dynamic balance was evaluated using the tests of Berg and „Stand and go“, static balance (sitting and standing, with eyes closed and eyes opened) was evaluated using the „MTD-balance“ system. Conclusions: 1. During the application of the 4-week program of physical exercises the mobility of the patients surveyed were improving. It was found, that the improvement of mobility was the same when using both „MTD-balance“platform and a mirror 2. The static balance improved using both „MTD-balance“platform and a mirror. However, using both „MTD-balance“platform as more effective in improving static balance when standing with closed eyes (p < 0.05). 3. Better results of dynamic balance are achieved when balance is trained using the „MTD-balance“platform (p < 0.05).Keywords: the stroke, balance, feedback

scholarly journals Bayesian performance evaluation of evapotranspiration models based on eddy covariance systems in an arid region

2019 ◽  
Vol 23 (7) ◽  
pp. 2877-2895 ◽  
Author(s):  
Guoxiao Wei ◽  
Xiaoying Zhang ◽  
Ming Ye ◽  
Ning Yue ◽  
Fei Kan
Keyword(s):  
Extinction Coefficient ◽  
Land Surface ◽  
Arid Region ◽  
Hydrological Cycle ◽  
Model Performance ◽  
Integration Algorithm ◽  
Energy Imbalance ◽  
Physical Mechanisms ◽  
Available Energy ◽  
Spring Maize

Abstract. Evapotranspiration (ET) is a major component of the land surface process involved in energy fluxes and energy balance, especially in the hydrological cycle of agricultural ecosystems. While many models have been developed as powerful tools to simulate ET, there is no agreement on which model best describes the loss of water to the atmosphere. This study focuses on two aspects, evaluating the performance of four widely used ET models and identifying parameters, and the physical mechanisms that have significant impacts on the model performance. The four tested models are the Shuttleworth–Wallace (SW) model, Penman–Monteith (PM) model, Priestley–Taylor and Flint–Childs (PT–FC) model, and advection–aridity (AA) model. By incorporating the mathematically rigorous thermodynamic integration algorithm, the Bayesian model evidence (BME) approach is adopted to select the optimal model with half-hourly ET observations obtained at a spring maize field in an arid region. Our results reveal that the SW model has the best performance, and the extinction coefficient is not merely partitioning the total available energy into the canopy and surface but also including the energy imbalance correction. The extinction coefficient is well constrained in the SW model and poorly constrained in the PM model but not considered in PT–FC and AA models. This is one of the main reasons that the SW model outperforms the other models. Meanwhile, the good fitting of SW model to observations can counterbalance its higher complexity. In addition, the detailed analysis of the discrepancies between observations and model simulations during the crop growth season indicate that explicit treatment of energy imbalance and energy interaction will be the primary way of further improving ET model performance.

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