Local Mean Temperature Research Articles

Changes in the boundary-layer structure at the edge of the ultimate regime in vertical natural convection

In thermal convection for very large Rayleigh numbers ($Ra$), the thermal and viscous boundary layers are expected to undergo a transition from a classical state to an ultimate state. In the former state, the boundary-layer thicknesses follow a laminar-like Prandtl–Blasius–Polhausen scaling, whereas in the latter, the boundary layers are turbulent with logarithmic corrections in the sense of Prandtl and von Kármán. Here, we report evidence of this transition via changes in the boundary-layer structure of vertical natural convection (VC), which is a buoyancy-driven flow between differentially heated vertical walls. The numerical dataset spans $Ra$ values from $10^{5}$ to $10^{9}$ and a constant Prandtl number value of $0.709$. For this $Ra$ range, the VC flow has been previously found to exhibit classical state behaviour in a global sense. Yet, with increasing $Ra$, we observe that near-wall higher-shear patches occupy increasingly larger fractions of the wall areas, which suggest that the boundary layers are undergoing a transition from the classical state to the ultimate shear-dominated state. The presence of streaky structures – reminiscent of the near-wall streaks in canonical wall-bounded turbulence – further supports the notion of this transition. Within the higher-shear patches, conditionally averaged statistics yield a logarithmic variation in the local mean temperature profiles, in agreement with the log law of the wall for mean temperature, and an $Ra^{0.37}$ effective power-law scaling of the local Nusselt number. The scaling of the latter is consistent with the logarithmically corrected $1/2$ power-law scaling predicted for ultimate thermal convection for very large $Ra$. Collectively, the results from this study indicate that turbulent and laminar-like boundary layer coexist in VC at moderate to high $Ra$ and this transition from the classical state to the ultimate state manifests as increasingly larger shear-dominated patches, consistent with the findings reported for Rayleigh–Bénard convection and Taylor–Couette flows.

Thermophoretic migration of vesicles depends on mean temperature and head group chemistry

A number of colloidal systems, including polymers, proteins, micelles and hard spheres, have been studied in thermal gradients to observe and characterize their driven motion. Here we show experimentally the thermophoretic behaviour of unilamellar lipid vesicles, finding that mobility depends on the mean local temperature of the suspension and on the structure of the exposed polar lipid head groups. By tuning the temperature, vesicles can be directed towards hot or cold, forming a highly concentrated region. Binary mixtures of vesicles composed of different lipids can be segregated using thermophoresis, according to their head group. Our results demonstrate that thermophoresis enables robust and chemically specific directed motion of liposomes, which can be exploited in driven processes.

Low thermal fluctuations in a system heated out of equilibrium.

We study the mechanical fluctuations of a micrometer sized silicon cantilever subjected to a strong heat flow, thus having a highly nonuniform local temperature. In this nonequilibrium steady state, we show that fluctuations are equivalent to the thermal noise of a cantilever at equilibrium around room temperature, while its mean local temperature is several hundred degrees higher. Changing the mechanical dissipation by adding a coating to the cantilever, we recover the expected rise of fluctuations with the mean temperature. Our work demonstrates that inhomogeneous dissipation mechanisms can decouple the amplitude of thermal fluctuations from the average temperature. This property could be useful to understand out-of-equilibrium fluctuating systems, or to engineer low noise instruments.

The peak structure and future changes of the relationships between extreme precipitation and temperature

Extreme rainfall increases with temperature and this is predicted to continue in a future warmer climate. Whilst a downturn in rainfall scaling is observed at high temperatures in the present day, this does not imply an upper limit on rainfall extremes in the future. Theoretical models predict that, in the absence of moisture limitation, extreme precipitation intensity could exponentially increase with temperatures at a rate determined by the Clausius–Clapeyron (C–C) relationship1,2. Climate models project a continuous increase of precipitation extremes for the twenty-first century over most of the globe3,4,5. However, some station observations suggest a negative scaling of extreme precipitation with very high temperatures6,7,8,9, raising doubts about future increase of precipitation extremes. Here we show for the present-day climate over most of the globe, the curve relating daily precipitation extremes with local temperatures has a peak structure, increasing as expected at the low–medium range of temperature variations but decreasing at high temperatures. However, this peak-shaped relationship does not imply a potential upper limit for future precipitation extremes. Climate models project both the peak of extreme precipitation and the temperature at which it peaks (Tpeak) will increase with warming; the two increases generally conform to the C–C scaling rate in mid- and high-latitudes, and to a super C–C scaling in most of the tropics. Because projected increases of local mean temperature (Tmean) far exceed projected increases of Tpeak over land, the conventional approach of relating extreme precipitation to Tmean produces a misleading sub-C–C scaling rate.

Nonlinear relationship between extreme temperature and mortality in different temperature zones: A systematic study of 122 communities across the mainland of China.

Numerous previous studies have reported that human health risk is extremely sensitive to temperature. Very few studies, however, have characterized the relationship between temperature and mortality in different temperature zones due to the previous conclusions deduced from a regional or administrative division. A research covers different temperature zones was indispensable to have a comprehensive understanding of regional ambient temperature effect on public health. Based on the mortality dataset and meteorological variables of 122 communities in China from 2007 to 2012, a distributed lag nonlinear model (DLNM) was utilized to estimate the temperature effect on non-accidental mortality at the community level. Then, a meta-regression analysis was applied to pool the estimates of community-specific effects in various latitude-effected temperature zones. At the community level, the mean value of relative extreme cold risk (1.63) of all 122 communities was higher than that of extreme high temperature (1.15). At regional level, we found temperature-mortality relationship (e.g., U- or J-shaped) varied in different temperature zones. Meanwhile, the minimum-mortality temperature of each zone was near the 75th percentile of local mean temperature except the north subtropics (50th percentiles). Lag effect was also obvious, especially for cold effect. An interesting M-shaped curve for the relationship between cold risk and temperature was detected, while an inverted "U" shaped with a right tail for the heat effect. Such different responses might be attributed to the difference in social-economic status of temperature zones. The temperature-mortality relationship showed a distinct spatial heterogeneity along temperature zones across the Chinese mainland. Different characteristics of mortality responding to cold and heat stress highlighted the fact that, apart from the circumstance of temperature, the social-economic condition was also linked with health risk. Our findings suggest decision-makers should take more adaptive and effective measures to reduce health risks in China.

Structure and Lean Extinction of Premixed Flames Stabilized on Conductive Perforated Plates

The structure and lean extinction of premixed liquefied petroleum gas–air flames seated on conductive perforated plates were examined experimentally, with focus on the effects of plate material, thickness, and hole diameter. The lean extinction limit was determined by gradually reducing the fuel-flow rate for a given air-flow rate, until extinction occurred. Flame structure was quantified by mapping the local mean temperature and species concentrations and by imaging the average visual length of the flame plume. Pyrometer measurements of the temperature of the upper plate surface were made to estimate the heat transfer through the plate. It was found that the flames stabilized on plates with higher thermal conductivity were shorter and more stable (i.e., have lower lean extinction limits). This was attributed to preheating of fresh reactant mixture by greater heat transfer through the plate. Increasing the hole diameter (percentage open area) was found to enhance flame stability by reducing the reactant jet velocity for a given flow rate of reactant mixture. Heat transfer through the plate deteriorated with increasing hole size. However, the positive effect of smaller jet velocity on flame stability overpowered the negative effect of reduced heat transfer, and the net result was enhanced stability with larger hole sizes. Plate thickness, on the other hand, was found to have a weak effect on flame stability and structure. Thicker plates showed slightly better stability characteristics because of greater heat transfer through them. Nonetheless, plate heat transfer did not affect flame stability as significantly as jet velocity did.

Impact of climate variability on various Rabi crops over Northwest India

The Indian agriculture with its two prominent cropping seasons [summer (Kharif) and winter (Rabi)] is the mainstay of the rural economy. Northwest India (NWI) is an important region for the cultivation of Rabi crops grown during the period from October to April. In the present study, state wise impact analysis is carried out to ascertain the influence of climate indices Nino3.4 region Sea Surface Temperature (SST), Southern Oscillation Index (SOI), Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and local precipitation, soil moisture, minimum (Tmin), maximum (Tmax) and mean (Tmean) temperatures on different Rabi crops (wheat, gram, rapeseed–mustard, oilseeds, and total Rabi food grains) over NWI during the years 1966–2011.

Geographical variability in northern European sheep wool isotopic composition (δ(13) C, δ(15) N, δ(2) H values).

Light stable isotopic analysis of herbivore proteinaceous tissues (hair, muscle, milk) is critical for authenticating the point of origin of finished agricultural or industrial products in both ancient and modern economies. This study examined the distribution of light stable isotopes in herbivores in northern Europe (Iceland to Finland), which is expected to depend on regional-level environmental inputs (precipitation, temperature) and local variables (vegetation type, fodder type, soil type). Sheep wool was obtained from animals managed using traditional methods and located across a gradient of northern European environments. Defatted whole-year samples were analysed by isotope ratio mass spectrometry (IRMS) for carbon (δ(13) C values), nitrogen (δ(15) N values) and un-exchangeable hydrogen (δ(2) H values) isotopic composition. Wool δ(13) C, δ(15) N and δ(2) H values showed the same correlations to local mean annual precipitation and temperature as were expected for graze plants. Wool δ(2) H values were correlated with local modelled meteoric water δ(2) H values, mediated by plant solid tissue and leaf water fractionations. Cluster analysis distinguished wool from Sweden and the Baltic region from more western material. Local variation in vegetation or soil type did not disrupt dependence on climatic variables but did affect geospatial discrimination. Wool isotopic composition in northern Europe is controlled by the effects of local precipitation and temperature on graze plant inputs, and is only weakly affected by pasture type. Copyright © 2016 John Wiley & Sons, Ltd.

Migration of Vesicles and their Domains in a Thermal Gradient

Domains of coexisting liquid phases form on the surface of giant unilamellar vesicles of DiPhyPC/DPPC/cholestanol below the transition temperature. The domains are imaged by fluorescence microscopy and migrate towards higher temperatures when a thermal gradient is applied vertically across the vesicle. The migration is towards the hot side irrespective of whether the domains are formed from the Lo or Ld phase and regardless of the proximity to chamber boundaries. The arrangement of domains on each hemisphere is explored for both “normal” and “trapped” coarsening (for which long range repulsions keep domains apart). For “trapped” coarsening, the reduction in the coalescence rate enables clearer study of the spacing between domains. Domains crowd on the hotter side of the vesicle, depleting the cold side. Potential migration mechanisms are considered, including temperature preferences for each lipid, and favoured curvature for each phase. Observations are consistent with migration minimising line tension energy, due to the lower line interface energy closer to mixing. The motion of entire vesicles (here, sub-micrometer large unilamellar vesicles) composed of one lipid is also investigated in a temperature gradient. Thermophoresis accumulates vesicles towards the cold or hot, depending on the local mean temperature and the vesicle surface chemistry. A high degree of vesicle concentration is possible with a small temperature difference (chot/ccold∼0.35 for ΔT=5°C). The magnitude and sign of the Soret coefficient (characterising the migration) depends on the chemical structure of the lipid headgroup. We use thermophoresis to separate vesicles by headgroup composition and explore the behaviour of ternary vesicles (formed from two lipids with different headgroups and cholestanol). The deviation of vesicles from a spherical shape below the transition temperature is observed to enhance the Soret coefficient.

Enhanced heat transfer by exothermic reactions in laminar flow capillary reactors

Conversion and temperature distributions in a tubular reactor with an inner diameter of 1mm were numerically calculated for a second order exothermic reaction in laminar flow of homogeneous liquid. Based on the resulting radial temperature profiles, the local Nusselt numbers and bulk mean temperatures were determined along the reactor tube. Strong effects of the homogeneous reaction on the heat transfer can be observed in the entrance region of the reactor, where a hot spot emerges. Due to the large radial temperature gradients in vicinity of the reactor wall, heat transfer coefficients are significantly higher compared to a non-reactive system.The consequences of this effect on the design and control of exothermic reactions in reactor/heat exchangers are demonstrated by comparison with a simple one-dimensional plug flow model. In the simplified model, neglecting thermal influence of the exothermic reaction results in a significant underestimation of the required reactor length for defined conversion. Accordingly, numerical simulation of both axial and radial transport in the hot spot region can be essential to precisely predict the bulk temperatures and conversion rates in the reaction mixture, even with the small length scales of milli- and micro-structured reactors.

Experimental thermal analysis on air cooling for closed-cathode Polymer Electrolyte Membrane fuel cells

This work explores the strength and limits of using separate air cooling for closed-cathode Polymer Electrolyte Membrane (PEM) stacks. Evaluating the thermal behavior of the designs based on stack temperature profiles alone would lead to inaccuracy as the initial temperatures and the stack thermal powers are different. Thus, the thermal behavior of the cooling modes was qualitatively analyzed via heat transfer analyses. An experimental approach is reported here using three stacks with varied cooling channel geometry and aspect ratio. Two stacks were designed on parallel multi channel (20 and 40 channels) straight flow configuration. The third stack applied the concept of non-linear laminar flow trajectory for the cooling channels. The 3-cell stacks were constructed with an active area of 240 cm2. The cooling mode applied a cooling fan coupling of positive and negative pressure flows. Air flows were between Reynolds number of 200 and 400 while the humidity varied at 50% and 90%. The analytical methodology converted the first-order temperature profiles into second-order heat transfer profiles. The steady-state parameters studied were temperature uniformity, cooling response, average cooling rate, cooling effectiveness, cooling flux, the heat transfer coefficient and the mean local temperature difference. The width of analysis has successfully identified the dynamic capabilities of the individual cooling plate designs for further practical considerations.

Combining ecosystem service relationships and DPSIR framework to manage multiple ecosystem services.

Ecosystem service (ES) relationship occurs due to two types of mechanisms: (1) interact directly or (2) interact through the impact of a shared factor. Identifying such mechanisms behind ES relationship within a single land-use/land-cover category and combining it with a system thinking framework is especially necessary for effective decision-making to manage multiple ESs generated by this land-use/land-cover. In this study, we use tea plantations in China to investigate mechanisms behind ES relationships. We find that tea production is positively correlated with four regulating services (i.e., carbon sequestration, soil N protection, soil P protection, and water conservation). Several regulating services, such as carbon sequestration and soil N, P, and K protection, have positive correlations with each other. Tea production, carbon sequestration, and soil retention are significantly correlated with local annual mean temperature and precipitation. We then establish driver-pressure-state-impact-response (DPSIR) framework for tea plantations, which has been widely used for environmental management issues. Integrating our findings of ES relationship into DPSIR framework, we can estimate how ES change is responding to two types of responses: response to control drivers and response to maintain or restore state. Scenario analysis showed that the responses to control drivers have a larger impact on ES. We discuss that DPSIR would favor managing multiple ES because it enables a more precise understanding of how ES interacts through the effects of factors from various hierarchies. Finally, we suggest integrating ES direct interaction into DPSIR framework. We think such integration could improve the ability of DPSIR framework to support decision-making in multiple ES management, specifically in at least three aspects: (1) favor to identify all possible response alternatives, (2) enable us to evaluate ES which cannot be assessed if without such combining, and (3) help to identify ecological leverage points where small management investment can yield substantial benefits.

Temperature and geographic attribution of change in the Taraxacum mongolicum growing season from 1990 to 2009 in eastern China's temperate zone.

Using leaf unfolding and leaf coloration data of a widely distributed herbaceous species, Taraxacum mongolicum, we detected linear trend and temperature response of the growing season at 52 stations from 1990 to 2009. Across the research region, the mean growing season beginning date marginal significantly advanced at a rate of -2.1 days per decade, while the mean growing season end date was significantly delayed at a rate of 3.1 days per decade. The mean growing season length was significantly prolonged at a rate of 5.1 days per decade. Over the 52 stations, linear trends of the beginning date correlate negatively with linear trends of spring temperature, whereas linear trends of the end date and length correlate positively with linear trends of autumn temperature and annual mean temperature. Moreover, the growing season linear trends are also closely related to the growing season responses to temperature and geographic coordinates plus elevation. Regarding growing season responses to temperature, a 1 °C increase in regional mean spring temperature results in an advancement of 2.1 days in regional mean growing season beginning date, and a 1 °C increase in regional mean autumn temperature causes a delay of 2.3 days in regional mean growing season end date. A 1 °C increase in regional annual mean temperature induces an extension of 8.7 days in regional mean growing season length. Over the 52 stations, response of the beginning date to spring temperature depends mainly on local annual mean temperature and geographic coordinates plus elevation. Namely, a 1 °C increase in spring temperature induces a larger advancement of the beginning date at warmer locations with lower latitudes and further west longitudes than at colder locations with higher latitudes and further east longitudes, while a 1 °C increase in spring temperature causes a larger advancement of the beginning date at higher than at lower elevations.

The Two-Dimensional Heat Transfer Analysis in Arrayed Fins with the Thermal Dissipation Substrate

The aim of the present study is to investigate the two-dimensional heat transfer analysis in arrayed fins with thermal dissipation substrate. The governing equations for the fins and the substrate are expressed with Laplace equations, and the boundary conditions around the fins and substrate are Robin conditions. The present investigation first aims to provide a solution with regard to the geometry models by a series truncation method. Then the research will compare the results of the series truncation method with the point-matching method. Furthermore, the present study will also discuss the effects of dimension and Biot number of the fins on local dimensionless temperature, mean temperature, and heat transfer rate.

Modeling non-stationarity in intensity, duration and frequency of extreme rainfall over India

Significant changes are reported in extreme rainfall characteristics over India in recent studies though there are disagreements on the spatial uniformity and causes of trends. Based on recent theoretical advancements in the Extreme Value Theory (EVT), we analyze changes in extreme rainfall characteristics over India using a high-resolution daily gridded (1 degrees latitude x 1 degrees longitude) dataset. Intensity, duration and frequency of excess rain over a high threshold in the summer monsoon season are modeled by non-stationary distributions whose parameters vary with physical covariates like the El-Nino Southern Oscillation index (ENSO-index) which is an indicator of large-scale natural variability, global average temperature which is an indicator of human-induced global warming and local mean temperatures which possibly indicate more localized changes. Each non-stationary model considers one physical covariate and the best chosen statistical model at each rainfall grid gives the most significant physical driver for each extreme rainfall characteristic at that grid. Intensity, duration and frequency of extreme rainfall exhibit non-stationarity due to different drivers and no spatially uniform pattern is observed in the changes in them across the country. At most of the locations, duration of extreme rainfall spells is found to be stationary, while non-stationary associations between intensity and frequency and local changes in temperature are detected at a large number of locations. This study presents the first application of nonstationary statistical modeling of intensity, duration and frequency of extreme rainfall over India. The developed models are further used for rainfall frequency analysis to show changes in the 100-year extreme rainfall event. Our findings indicate the varying nature of each extreme rainfall characteristic and their drivers and emphasize the necessity of a comprehensive framework to assess resulting risks of precipitation induced flooding. (C) 2014 Elsevier B.V. All rights reserved.

Latitudinal gradients in the productivity of European migrant warblers have not shifted northwards during a period of climate change

AbstractAimAs global temperatures have increased, many species distributions have exhibited polewards shifts, a trend that is predicted to continue in future decades. However, the mechanisms underlying such shifts are not well understood. Here we quantify the impact of large‐scale variation in temperature on reproductive output within a group of migratory birds to assess the potential for future range changes.LocationWestern Europe.MethodsWe use data from captures of 350,000 individual birds, collected under constant effort ringing protocols from 1994–2006, to estimate productivity (percentage of juveniles caught) for seven species of migrant warblers (family Sylviidae) breeding in Europe in relation to spring temperature and latitude by fitting generalized linear mixed models.ResultsProductivity was highest at mid‐latitudes for six of our seven study species and did not change significantly over the study period. Only one species (reed warbler, Acrocephalus scirpaceus) showed increased productivity at northern sites as expected. Six of the seven species also showed evidence for local adaptation, with productivity decreasing as temperatures diverged from the local mean. However, for three of these species the ‘optimum’ temperature was greater than the local mean temperature at the majority of sites.Main conclusionsThese results indicate that latitudinal gradients in productivity are likely to influence large‐scale abundance patterns, but that adaptation to local climate conditions has the potential to constrain the rate of northward range shifts in many species.

Evaluation of the convection-resolving regional climate modeling approach in decade-long simulations

The uncertainties in current global and regional climate model integrations are partly related to the representation of clouds, moist convection, and complex topography, thus motivating the use of convection-resolving models. On climate time scales, convection-resolving methods have been used for process studies, but application to long-term scenario simulations has been very limited. Here we present a convection-resolving simulation for a 10 yearlong period (1998–2007) integrated with the Consortium for Small-Scale Modeling in Climate Mode model. Two one-way nested grids are used with horizontal resolutions of 2.2 km for a convection-resolving model (CRM2) on an extended Alpine domain (1100 km × 1100 km) and 12 km for a convection-parametrizing model (CPM12) covering Europe. CPM12 is driven by lateral boundary conditions from the ERA-Interim reanalysis. Validation is conducted against high-resolution surface data. The CRM2 model strongly improves the simulation of the diurnal cycles of precipitation and temperature, despite an enhanced warm bias and a tendency for the overestimation of precipitation over the Alps. The CPM12 model has a poor diurnal cycle associated with the use of parameterized convection. The assessment of extreme precipitation events reveals that both models adequately represent the frequency-intensity distributions for daylong events in summer, but large differences occur for hourly precipitation. The CPM12 model underestimates the frequency of heavy hourly events, while CRM2 shows good agreement with observations in the summer season. We also present results on the scaling of precipitation extremes with local daily mean temperatures. In accordance with observations, CRM2 exhibits adiabatic scaling for intermediate hourly events (90th percentile) and superadiabatic scaling for extreme hourly events (99th and 99.9th percentiles) during the summer season. The CPM12 model partly reproduces this scaling as well. The excellent performance of CRM2 in representing hourly precipitation events in terms of intensity and scaling is highly encouraging, as this addresses a previously untested (and untuned) model capability.

The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence

There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations—as they are, e.g. encountered in non-isothermal flows—on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces—despite a local mean temperature compensation of the velocity reading—significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.

Evidence for adaptive divergence of thermal responses among Bemisia tabaci populations from tropical Colombia following a recent invasion.

There is an increasing evidence that populations of ectotherms can diverge genetically in response to different climatic conditions, both within their native range and (in the case of invasive species) in their new range. Here, we test for such divergence in invasive whitefly Bemisia tabaci populations in tropical Colombia, by considering heritable variation within and between populations in survival and fecundity under temperature stress, and by comparing population differences with patterns established from putatively neutral microsatellite markers. We detected significant differences among populations linked to mean temperature (for survival) and temperature variation (for fecundity) in local environments. A QST -FST analysis indicated that phenotypic divergence was often larger than neutral expectations (QST >FST ). Particularly, for survival after a sublethal heat shock, this divergence remained linked to the local mean temperature after controlling for neutral divergence. These findings point to rapid adaptation in invasive whitefly likely to contribute to its success as a pest species. Ongoing evolutionary divergence also provides challenges in predicting the likely impact of Bemisia in invaded regions.

Melting behaviour of differently-sized micro-particles in a pipe flow under constant heat flux

The melting behaviour of differently-sized micro-particles in a pipe flow under constant heat flux is analysed by using a full two-phase micro/nanomelting model of a sphere. The size effect on the melting point depression of micro/nanoparticles is modelled at the liquid–solid interface of the melting model. The boundary conditions of a single particle are constructed based on the heat flux, flow rate and axial position of the particle in slurry. The melting time and temperature evolution of liquid–solid two phase within particles are presented. The results are seen to agree with the group melting behaviour of particles, as observed via the local bulk mean temperature of the slurry in the pipe. The results can be used for design and optimisation of a heat transfer channel while utilising the latent heat effect of phase change particles in the slurry.