Modeling rarefied gas-solid surface interactions for Couette flow with different wall temperatures using an unsupervised machine learning technique.

In rarefied gas flows, discontinuity phenomena such as velocity slip and temperature jump commonly appear in the gas layer adjacent to a solid boundary. Due to the physical complexity of the interactions at the gas-solid interface, particularly in the case of systems with local nonequilibrium state, boundary models with limited number of parameters cannot completely describe the reflection of gas molecules at the boundary. In this work, the Gaussian mixture (GM) model, which is an unsupervised machine learning technique, is employed to construct a statistical gas-solid surface scattering model based on the collisional data obtained from molecular dynamics (MD) simulations. The GM model is applied to study Couette flow for different inert gases (Ar and He) confined between two parallel infinite gold walls at different temperatures. A direct comparison between the results obtained from the GM model and the Cercignani-Lampis-Lord (CLL) scattering kernel against the MD collisional data in terms of the distribution of the predicted postcollisional velocities, and accommodation coefficients has shown that the results from the GM model are an excellent match with the MD results outperforming the CLL scattering kernel. As an example, for He gas, while the predicted energy accommodation coefficient by the CLL model is more than two times higher than the MD predictions, the value computed by the GM model is in excellent agreement with the MD results. This superior performance of the GM model confirms its high potential to derive a generalized boundary condition in systems encountered with highly nonequilibrium and complex gas flow conditions.

The Influence of Gas-Wall and Gas-Gas Interactions on the Accommodation Coefficients for Rarefied Gases: A Molecular Dynamics Study.

Molecular dynamics (MD) simulations are conducted to determine energy and momentum accommodation coefficients at the interface between rarefied gas and solid walls. The MD simulation setup consists of two parallel walls, and of inert gas confined between them. Different mixing rules, as well as existing ab-initio computations combined with interatomic Lennard-Jones potentials were employed in MD simulations to investigate the corresponding effects of gas-surface interaction strength on accommodation coefficients for Argon and Helium gases on a gold surface. Comparing the obtained MD results for accommodation coefficients with empirical and numerical values in the literature revealed that the interaction potential based on ab-initio calculations is the most reliable one for computing accommodation coefficients. Finally, it is shown that gas-gas interactions in the two parallel walls approach led to an enhancement in computed accommodation coefficients compared to the molecular beam approach. The values for the two parallel walls approach are also closer to the experimental values.

Publisher Correction: The Scales Project, a cross-national dataset on the interpretation of thermal perception scales.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Scales Project, a cross-national dataset on the interpretation of thermal perception scales.

Thermal discomfort is one of the main triggers for occupants' interactions with components of the built environment such as adjustments of thermostats and/or opening windows and strongly related to the energy use in buildings. Understanding causes for thermal (dis-)comfort is crucial for design and operation of any type of building. The assessment of human thermal perception through rating scales, for example in post-occupancy studies, has been applied for several decades; however, long-existing assumptions related to these rating scales had been questioned by several researchers. The aim of this study was to gain deeper knowledge on contextual influences on the interpretation of thermal perception scales and their verbal anchors by survey participants. A questionnaire was designed and consequently applied in 21 language versions. These surveys were conducted in 57 cities in 30 countries resulting in a dataset containing responses from 8225 participants. The database offers potential for further analysis in the areas of building design and operation, psycho-physical relationships between human perception and the built environment, and linguistic analyses.

Effect of local skin blood flow during light and medium activities on local skin temperature predictions.

The quality of local skin temperature prediction by thermophysiological models depends on the local skin blood flow (SBF) control functions. These equations were derived for low activity levels (0.8-1met) and mostly in sitting or supine position. This study validates and discusses the prediction of foot SBF during activities of 1-3met in male and females, and the effect on the foot skin temperature prediction (ΔTskin,foot) using the thermophysiological simulation model ThermoSEM. The SBF at the foot was measured for ten male and ten female human subjects at baseline and during three activities (sitting, walking at 1km/h, preferred walking around 3km/h). Additional measurements included the energy expenditure, local skin temperatures (Tskin,loc), environmental conditions and body composition. Measured, normalized foot SBF is 2-8 times higher than the simulated SBF during walking sessions. Also, SBF increases are significantly higher in females vs. males (preferred walking: 4.8±1.5 versus 2.7±1.4, P < 0.05). The quality of ΔTskin,foot using the simulated foot SBF is poor (median deviation is -4.8°C, maximumumdeviationis-6°C). Using the measured SBF in ThermoSEM results in an improved local skin temperature prediction (new maximum deviation is -3.3°C). From these data a new SBF model was developed that includes the walking activity level and gender, and improves SBF prediction and ΔTskin,foot of the thermophysiological model. Accurate SBF and local skin temperature predictions are beneficial in optimizing thermal comfort simulations in the built environment, and might also be applied in sport science or patient's temperature management.

Local clothing thermal properties of typical office ensembles under realistic static and dynamic conditions.

An accurate local thermal sensation model is indispensable for the effective development of personalized conditioning systems in office environments. The output of such a model relies on the accurate prediction of local skin temperatures, which in turn depend on reliable input data of the local clothing thermal resistance and clothing area factor. However, for typical office clothing ensembles, only few local datasets are available in the literature. In this study, the dry thermal resistance was measured for 23 typical office clothing ensembles according to EN-ISO 15831 on a sweating agile manikin. For 6 ensembles, the effects of different air speeds and body movement were also included. Local clothing area factors were estimated based on 3D scans. Local differences can be found between the measured local insulation values and local area factors of this study and the data of other studies. These differences are likely due to the garment fit on the manikin and reveal the necessity of reporting clothing fit parameters (e.g., ease allowance) in the publications. The increased air speed and added body movement mostly decreased the local clothing insulation. However, the reduction is different for all body parts, and therefore cannot be generalized. This study also provides a correlation between the local clothing insulation and the ease allowance for body parts covered with a single layer of clothing. In conclusion, the need for well-documented measurements is emphasized to get reproducible results and to choose accurate clothing parameters for thermo-physiological and thermal sensation modeling.

Microfluidic magnetic bead conveyor belt.

Magnetic beads play an important role in the miniaturization of clinical diagnostics systems. In lab-on-chip platforms, beads can be made to link to a target species and can then be used for the manipulation and detection of this species. Current bead actuation systems utilize complex on-chip coil systems that offer low field strengths and little versatility. We demonstrate a novel system based on an external rotating magnetic field and on-chip soft-magnetic structures to focus the field locally. These structures were designed and optimized using finite element simulations in order to create a number of local flux density maxima. These maxima, to which the magnetic beads are attracted, move over the chip surface in a continuous way together with the rotation of the external field, resulting in a mechanism similar to that of a conveyor belt. A prototype was fabricated using PDMS molding techniques mixed with iron powder for the magnetic structures. In the subsequent experiments, a quadrupole electromagnet was used to create the rotating external field. We observed that beads formed agglomerates that rolled over the chip surface, just above the magnetic structures. Field rotation frequencies between 0.1-50 Hz were tested resulting in magnetic bead speeds of over 1 mm s-1 for the highest frequency. With this, we have shown that our novel concept works, combining a simple design and simple operation with a powerful and versatile method for bead actuation. This makes it a promising method for further research and utilization in lab-on-chip systems.

The impact of morning light intensity and environmental temperature on body temperatures and alertness.

Indoor temperature and light exposure are known to affect body temperature, productivity and alertness of building occupants. However, not much is known about the interaction between light and temperature exposure and the relationship between morning light induced alertness and its effect on body temperature. Light intensity and room temperature during morning office hours were investigated under strictly controlled conditions. In a randomized crossover study, two white light conditions (4000K, either bright 1200lx or dim 5lx) under three different room temperatures (26, 29 and 32°C) were investigated. A lower room temperature increased the core body temperature (CBT) and lowered skin temperature and the distal-proximal temperature gradient (DPG). Moreover, a lower room temperature reduced the subjective sleepiness and reaction time on an auditory psychomotor vigilance task (PVT), irrespective of the light condition. Interestingly, the morning bright light exposure did affect thermophysiological parameters, i.e. it decreased plasma cortisol, CBT and proximal skin temperature and increased the DPG, irrespective of the room temperature. During the bright light session, subjective sleepiness decreased irrespective of the room temperature. However, the change in sleepiness due to the light exposure was not related to these physiological changes.

On-line monitoring of electrolytes in hemodialysis: on the road towards individualizing treatment.

INTRODUCTION: End-stage renal disease (ESRD) patients depend on dialysis for removal of toxic waste products, fluid overload relief and maintenance of electrolyte balance. Dialysis prolongs millions of lives. To some extent, ESRD has become a manageable disease with a steadily growing dialysis population of increasing average age and associated comorbidity. During 7 decades many technical refinements have been developed e.g. sodium profiling, blood volume, ultrafiltration variation based on blood pressure measurement, urea kinetics etc. Despite its large potentials, in-line electrolyte monitoring lags behind in dialysis treatment. Areas covered: In this paper, we review the state of technologies available for in-line monitoring of the electrolytes sodium, potassium and calcium during hemodialysis. Expert commentary: We concluded that individual optimization of dialysate composition should be able to improve hard medical outcomes, but practical clinical implementation stands/falls with reliable and affordable in-line ion-selective sensing technology. Optical ion-selective microsensors and microsystems form a promising pathway for individualizing the dialysis treatment.

Molecular simulation of water vapor outgassing from silica nanopores.

The outgassing problem is solved numerically by molecular dynamics. A slit-shaped nanopore consisting of cavity and channel is built with an implicit tabulated wall potential that describes the water-silicon/silica interaction. A flexible three-point water model is used for the simulation. The effects of varying the system temperature, outlet pressure, geometry, and materials of the nanopore on the outgassing rate are investigated. The results show that the temperature plays an important role in the outgassing rate, while the effect of the outlet pressure is negligible as long as it is in the high to medium vacuum range. The geometry of the channel also has an influence on the outgassing rate, but not as much as the surface material. Three different types of silica materials are tested: silicon, silica-cristobalite (hydrophilic material), and silica-quartz (super hydrophilic material). The fastest outgassing rate is found for a silicon nanopore. It is also found that a thin water film is formed on the surface of the silica-quartz nanopore. This material shows hardly any outgassing of water.

A microfluidic device based on an evaporation-driven micropump.

In this paper we introduce a microfluidic device ultimately to be applied as a wearable sweat sensor. We show proof-of-principle of the microfluidic functions of the device, namely fluid collection and continuous fluid flow pumping. A filter-paper based layer, that eventually will form the interface between the device and the skin, is used to collect the fluid (e.g., sweat) and enter this into the microfluidic device. A controllable evaporation driven pump is used to drive a continuous fluid flow through a microfluidic channel and over a sensing area. The key element of the pump is a micro-porous membrane mounted at the channel outlet, such that a pore array with a regular hexagonal arrangement is realized through which the fluid evaporates, which drives the flow within the channel. The system is completely fabricated on flexible polyethylene terephthalate (PET) foils, which can be the backbone material for flexible electronics applications, such that it is compatible with volume production approaches like Roll-to-Roll technology. The evaporation rate can be controlled by varying the outlet geometry and the temperature. The generated flows are analyzed experimentally using Particle Tracking Velocimetry (PTV). Typical results show that with 1 to 61 pores (diameter = 250 µm, pitch = 500 µm) flow rates of 7.3 × 10(-3) to 1.2 × 10(-1) µL/min are achieved. When the surface temperature is increased by 9.4°C, the flow rate is increased by 130 %. The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor. The theoretical and experimental results are in good agreement.

Beyond the classic thermoneutral zone: Including thermal comfort.

The thermoneutral zone is defined as the range of ambient temperatures where the body can maintain its core temperature solely through regulating dry heat loss, i.e., skin blood flow. A living body can only maintain its core temperature when heat production and heat loss are balanced. That means that heat transport from body core to skin must equal heat transport from skin to the environment. This study focuses on what combinations of core and skin temperature satisfy the biophysical requirements of being in the thermoneutral zone for humans. Moreover, consequences are considered of changes in insulation and adding restrictions such as thermal comfort (i.e. driver for thermal behavior). A biophysical model was developed that calculates heat transport within a body, taking into account metabolic heat production, tissue insulation, and heat distribution by blood flow and equates that to heat loss to the environment, considering skin temperature, ambient temperature and other physical parameters. The biophysical analysis shows that the steady-state ambient temperature range associated with the thermoneutral zone does not guarantee that the body is in thermal balance at basal metabolic rate per se. Instead, depending on the combination of core temperature, mean skin temperature and ambient temperature, the body may require significant increases in heat production or heat loss to maintain stable core temperature. Therefore, the definition of the thermoneutral zone might need to be reformulated. Furthermore, after adding restrictions on skin temperature for thermal comfort, the ambient temperature range associated with thermal comfort is smaller than the thermoneutral zone. This, assuming animals seek thermal comfort, suggests that thermal behavior may be initiated already before the boundaries of the thermoneutral zone are reached.

Mathematical modeling of thermal and circulatory effects during hemodialysis.

Intradialytic hypotension (IDH) is one of the most common complications of hemodialysis (HD) treatment. The initiating factor of IDH is a decrease in blood volume, which is related to an imbalance between ultrafiltration (UF) and refilling rate. Impaired reactivity of resistance and capacitance vessels in reaction to hypovolemia plays possibly a major role in the occurrence of IDH. These vessels also fulfill an important function in body temperature regulation. UF-induced cutaneous vasoconstriction would result in a reduced surface heat loss and an increase in core temperature. To release body heat, skin blood flow is increased at a later stage of the HD treatment, whereby possibly IDH can occur. The aim of the study is to develop a mathematical model that can provide insight into the impact of thermoregulatory processes on the cardiovascular (CV) system during HD treatment. The mathematical procedure has been created by coupling a thermo-physiological model with a CV model to study regulation mechanisms in the human body during HD + UF. Model simulations for isothermal versus thermoneutral HD + UF were compared with measurement data of patients on chronic intermittent HD (n = 13). Core temperature during simulated HD + UF sessions increased within the range of measurement data (0.23°C vs. 0.32 ± 0.41°C). The model showed a decline in mean arterial pressure of -7% for thermoneutral HD + UF versus -4% for isothermal HD + UF after 200 min during which relative blood volume changed by -13%. In conclusion, simulation results of the combined model show possibilities for predicting circulatory and thermal responses during HD + UF.

The thermoneutral zone: implications for metabolic studies.

A thermoneutral environment is important for many human physiological studies. The thermoneutral zone (TNZ) is defined as the range of ambient temperatures without regulatory changes in metabolic heat production or evaporative heat loss. Many factors influence the thermoneutral zone, such as body composition, clothing, energy expenditure, age and gender. These factors have the potential to introduce bias in study results and therefore need to be taken into consideration in many metabolic studies or studies on obesity, medical conditions, thermal comfort or vigilance. Given new developments on the TNZ combined with historical views the aim of this review is to 1) provide insight in how the human TNZ is affected by internal and external factors, 2) indicate how skin blood flow characteristics could be used as an objective criterion for determining whether someone is in the thermoneutral zone, 3) explain implications of the TNZ on metabolic studies and 4) indicate future directions to enhance understanding of the TNZ, especially for the elderly and obese.

Physiological modeling for technical, clinical and research applications.

Various and disparate technical disciplines have identified a growing need for tools to predict human thermal and thermoregulatory responses to environmental heating and cooling and other thermal challenges such as anesthesia and non-ionizing radiation. In this contribution, a dynamic simulation model is presented and used to predict human thermophysiological and perceptual responses for different applications and situations. The multi-segmental, multi-layered mathematical model predicts body temperatures, thermoregulatory responses, and components of the environmental heat exchange in cold, moderate, as well as hot stress conditions. The incorporated comfort model uses physiological states of the human body to predict thermal sensation responses to steady state and transient conditions. Different validation studies involving climate-chamber physiological and thermal comfort experiments, exposures to uncontrolled outdoor weather conditions, extreme climatic and radiation asymmetry scenarios revealed the model to predict physiological and perceptual responses typically within the standard deviation of the experimental observations. Applications of the model in biometeorology, clothing research, the car industry, clinical and safety applications are presented and discussed.

Measurement of model coefficients of skin sympathetic vasoconstriction.

Many researchers have already attempted to model vasoconstriction responses, commonly using the mathematical representation proposed by Stolwijk (1971 NASA Contractor Report CR-1855 (Washington, DC: NASA)). Model makers based the parameter values in this formulation either on estimations or by attributing the difference between their passive models and measurement data fully to thermoregulation. These methods are very sensitive to errors. This study aims to present a reliable method for determining physiological values in the vasoconstriction formulation. An experimental protocol was developed that enabled us to derive the local proportional amplification coefficients of the toe, leg and arm and the transient vasoconstrictor tone. Ten subjects participated in a cooling experiment. During the experiment, core temperature, skin temperature, skin perfusion, forearm blood flow and heart rate variability were measured. The contributions to the normalized amplification coefficient for vasoconstriction of the toe, leg and arm were 84%, 11% and 5%, respectively. Comparison with relative values in the literature showed that the estimated values of Stolwijk and the values mentioned by Tanabe et al (2002 Energy Build. 34 637-46) were comparable with our measured values, but the values of Gordon (1974 The response of a human temperature regulatory system model in the cold PhD Thesis University of California, Santa Barbara) and Fiala et al (2001 Int. J. Biometeorol. 45 143159) differed significantly. With the help of regression analysis a relation was formulated between the error signal of the standardized core temperature and the vasoconstrictor tone. This relation was formulated in a general applicable way, which means that it can be used for situations where vasoconstriction thresholds are shifted, like under anesthesia or during motion sickness.

Effect of forced-air heaters on perfusion and temperature distribution during and after open-heart surgery.

OBJECTIVES: After cardiopulmonary bypass, patients often show redistribution hypothermia, also called afterdrop. Forced-air blankets help to reduce afterdrop. This study explores the effect of forced-air blankets on temperature distribution and peripheral perfusion. The blood perfusion data is used to explain the observed temperature effects and the reduction of the afterdrop. METHODS: Fifteen patients were enrolled in a randomised study. In the test group (n=8), forced-air warmers were used. In the control group (n=7), only passive insulation was used. Core and skin temperatures and thigh temperatures at 0, 8, 18 and 38 mm depth were measured. Laser Doppler flowmetry (LDF) was used to record skin perfusion from the big toe. Blood flow through the femoral artery was determined with ultrasound. RESULTS: Afterdrop in the test group was smaller than in the control group (1.2+/-0.2 degrees C vs 1.8+/-0.7 degrees C: P=0.04) whilst no significant difference in mean tissue thigh temperature was found between the groups. Local skin temperature was 2.5-3.0 degrees C higher when using forced-air heaters. However, skin perfusion was unaffected. Ultrasound measurements revealed that leg blood flow during the first hours after surgery was reduced to approximately 70% of pre- and peri-operative values. CONCLUSIONS: Forced-air blankets reduce afterdrop. However, they do not lead to clinical relevant changes in deep thigh temperature. LDF measurements show that forced-air heating does not improve toe perfusion. The extra heat especially favours core temperature. This is underlined by the decrease in postoperative leg blood flow, suggesting that the majority of the warmed blood leaving the heart flows to core organs and not to the periphery.

Validation of an individualised model of human thermoregulation for predicting responses to cold air.

Most computer models of human thermoregulation are population based. Here, we individualised the Fiala model [Fiala et al. (2001) Int J Biometeorol 45:143-159] with respect to anthropometrics, body fat, and metabolic rate. The predictions of the adapted multisegmental thermoregulatory model were compared with measured skin temperatures of individuals. Data from two experiments, in which reclining subjects were suddenly exposed to mild to moderate cold environmental conditions, were used to study the effect on dynamic skin temperature responses. Body fat was measured by the three-compartment method combining underwater weighing and deuterium dilution. Metabolic rate was determined by indirect calorimetry. In experiment 1, the bias (mean difference) between predicted and measured mean skin temperature decreased from 1.8 degrees C to -0.15 degrees C during cold exposure. The standard deviation of the mean difference remained of the same magnitude (from 0.7 degrees C to 0.9 degrees C). In experiment 2 the bias of the skin temperature changed from 2.0+/-1.09 degrees C using the standard model to 1.3+/-0.93 degrees C using individual characteristics in the model. The inclusion of individual characteristics thus improved the predictions for an individual and led to a significantly smaller systematic error. However, a large part of the discrepancies in individual response to cold remained unexplained. Possible further improvements to the model accomplished by inclusion of more subject characteristics (i.e. body fat distribution, body shape) and model refinements on the level of (skin) blood perfusion, and control functions, are discussed.