Mean Temperature Profiles Research Articles

TWO-PHASE MODELLING OF THERMAL DISSIPATION IN A NATURAL BASIN

The state of two‐phase flow ‘liquid‐gas’ has been modeled numerically by the three‐dimensional method of complex research of heat and mass transfer. This allows examining the interaction of some transfer processes in a natural cooling basin (the Drūkšiai lake): the wind power and direction, variable water density, the coefficient of heat conduction and heat transfer of the water‐air interface. Combined effect of these natural actions determines the heat amount that the basin is able to dissipate to the surrounding atmospheric media in thermal equilibrium (without changes in the mean water temperature). This paper presents a number of the most widely used expressions for the coefficients of vertical and horizontal heat transfer. On the basis of stream velocity and mean temperature profiles measured in the cooling pond as well as on that of their time variations suggestions are made that the mixing rate at the water surface is caused by natural space ‐ time variation of the wind, and can be described by the value of eddy viscosity coefficient ‐ 1 m2/s (numerical modeling with 0,9–1,3 m2/s). The wind influences the surface of the lake according to the experimental data, i e 1–3 % of the mean wind velocity. The model applies to the weakly wind, approximately 1–5 m/s of the mean wind velocity. Comparison of experimental and numerical results showed a qualitative agreement. For a better quantitative approximation, it is necessary to have more boundary conditions variable with time and to solve unsteady set equations for transfer processes.

Influence of indoor microclimate and diet on survival of Anopheles gambiae s.s. (Diptera: Culicidae) in village house conditions in western Kenya

The survival of female Anopheles gambiae s.s. mosquitoes inside two village house types (grass-thatched and iron-roofed) was studied in relation to diet and ambient indoor microclimatic conditions. Two batches of 20–30, 1-day-old laboratory-bred mosquitoes were maintained inside cages in the grass-thatched (n = 2) and iron-roofed (n = 2) houses and fed daily, one group on 10% glucose and the other on human blood. Throughout the experiments, indoor temperature and relative humidity of the houses were recorded, and mortality of mosquitoes monitored daily until all had died. The experiments were replicated thrice. There was no significant variation in the overall mean temperature (P = 0.93) or relative humidity profiles (P = 0.099) between the two house types, although the iron-roofed houses recorded higher temperature peaks. A Kaplan-Meier survival analysis showed that the mean survival times of mosquitoes were 8 and 10 days in the two grass-thatched huts and 7 and 10 days in the two iron-roof houses for mosquitoes feeding on blood and sugar meals, respectively. The mean survival times of mosquitoes maintained inside similar house types differed only due to diet. In the proportionality of hazards model (Cox regression), the dietary regimes significantly influenced the probability of survival (P = 0.0001), with mosquitoes surviving longer on sugar meals than on blood. Microclimatic factors inside houses also significantly influenced mosquito survival. Although higher peak temperatures were recorded in corrugated iron-roofed houses, the survival of the mosquitoes resting in them did not differ significantly from that in grass-thatched houses. However, the impact of these temperatures on the development of malaria parasites inside the vector needs to be investigated.

Large eddy simulation of turbulent forced gas flows in vertical pipes with high heat transfer rates

Large eddy simulation (LES) of vertical turbulent pipe flows with significant property variations has been performed to investigate the effects of high heat fluxes on the turbulent structures and transport. The Cartesian-based, compressible filtered Navier–Stokes equations were solved using a second-order accurate finite volume method. Low Mach number preconditioning was used to enable the compressible code to perform efficiently at low Mach numbers. A dynamic subgrid-scale stress model accounted for the subgrid-scale turbulence. In this study, the simulations were designed to simulate the experiments of Shehata and McEligot with three different near-constant heat fluxes. Step-periodic boundary conditions based on a quasi-developed assumption were used. The predicted integral parameters and mean velocity and temperature profiles agreed well with the experimental data. The fluid structures have been distorted due to high heat fluxes leading to significant property variations in the near wall region. The results showed that strong heating resulted in remarkable reductions of turbulent intensities, shear stresses, and turbulent heat flux. Apparent “laminarization” of the flow has been observed.

SIMULATION OF CONDUCTIVE‐CONVECTIVE HEAT TRANSFER IN A NATURAL BASIN

The state of water flow has been modelled numerically by a three‐dimensional method of a complex investigation of heat and mass transfer. This allows examining interaction of some transfer processes in a natural cooling basin (the Drûkšiai lake): variable density of water, heat conduction in water‐air interface, direct and diffuse solar radiation. Combined effect of these natural actions determines the heat amount that the basin is able to dissipate to the surrounding atmospheric media in thermal equilibrium (no change in the mean water temperature). Mathematical model is simplified ignoring transient derivatives in transfer equations, and time as a variable is included in relaxation coefficients for outer iterations. Full time of calculations using this simulation mechanism for this variable is about 6 h. Basing on water stream velocity and mean temperature profiles measured in the cooling pond as well as on their time variations, suggestions are made that the mixing rate in water is caused by natural and forced convection. Comparison of experimental and numerical results showed a qualitative agreement. For a better quantitative approximation, it is necessary to have more boundary conditions variable with time and to solve unsteady set equations for transfer processes.

A note on temperature profiles of rich clusters of galaxies

We derive here the mean temperature profile for a sample of hot, medium distant clusters recently observed with XMM-Newton, whose profiles are available from the literature, and compare it with the mean temperature profile found from BeppoSAX data. The XMM-Newton and BeppoSAX profiles are in good agreement between 0.05 and 0.25 r_180. From 0.25 to about 0.5 r_180 both profiles decline, however the BeppoSAX profile does so much more rapidly than the XMM-Newton profile.

Numerical Analysis of Compressible Turbulent Channel Flow with Wall-Temperature Difference

The objective of this study is to clarify the effect of different thermal wall boundary conditions on turbulence statistics and structures in the compressible turbulent flow. We performed direct numerical simulation (DNS) of the compressible turbulent channel flow between isothermal walls with temperature difference (Case C1). The DNS data was compared with that of the compressible turbulent channel flow between adiabatic and isothermal walls (Case C2), and that of the incompressible turbulent channel flow between isothermal walls with temperature difference (Case I). The main results are as follows : (1) The mean velocity in the logarithmic region and the maximum of the RMS streamwise velocity fluctuation in wall units are smaller than those of Case I near the high temperature isothermal wall, while they are larger than those of Case I near the adiabatic wall. (2) Near the high temperature isothermal wall, the mean temperature profile has an additional maximum due to the friction work, and the RMS temperature fluctuation has the corresponding maximum. This is consistent with the fact the production term of temperature variance has the additional maximum in the region very close to the high temperature isothermal wall. (3) The correlation coefficient between streamwise velocity and temperature fluctuations of Case C1 is significantly different from those of Cases C2 and I in the region very close to the high temperature wall. (4) The direction of energy transfer due to pressure work near the high temperature isothermal wall is the same as that of near the adiabatic wall. (5) The compressible terms of the temperature variance transport equation are not negligible near the low temperature isothermal wall.

On the Prandtl or Schmidt number dependence of the turbulent heat or mass transfer coefficient

Numerical experiments using a direct numerical simulation (DNS) of turbulent flow between two parallel plates in conjunction with Lagrangian scalar tracking (LST) of trajectories of thermal markers in the flow field are conducted for Prandtl or Schmidt numbers between 0.01 and 50,000. The LST methodology is used to generate mean temperature profiles as a function of the entry distance in the case of a step change in heat or mass flux at the walls of the channel. The heat transfer coefficient and the Nusselt number ratio, Nu( x)/ Nu( x→∞), downstream from the step change in the wall flux are determined for the range of Pr or Sc fluids examined. Relations between the heat or mass transfer coefficient at the fully developed part of the channel and Pr or Sc are proposed for low and high Pr or Sc cases. Finally, unified correlations, which provide the heat or mass transfer coefficient for all Pr or Sc, in the Reynolds number range examined, are proposed. Also, the exponent of the asymptotic dependence of the eddy diffusivity close to the wall is obtained.

Laser optical investigation of turbulent transport of temperature ahead of the preheat zone in a premixed flame

This experimental study investigates temperature profiles upstream of localized thin reaction zones in a turbulent premixed flame stabilized on a low-swirl-burner. A simultaneous dual-sheet Rayleigh/OH-LIPF measurement technique was applied using two parallel laser light sheets to measure the temperature in the preheat zone for three lean (ϕ=0.7) turbulent methane/air flames. The ratio of the turbulence intensity to the laminar burning velocity v′/sL was varied from 3.5 to 9.2 and to 18.7. The first of these flames lies on the borderline between the “corrugated flamelets” regime and the “thin reaction zones” regime, while the other two are in the “thin reaction zones” regime. The results confirm the occurrence of strong temperature fluctuations ahead of the preheat zone for flames in the “thin reaction zones” regime. Single temperature images show a significant temperature increase ahead of the preheat zones of up to 700 K for flames at the highest turbulence intensity. For statistical analysis conditional mean temperature profiles and probability density functions conditioned on the distance from the flame contour were calculated from the experimental data. Only those portions of the flame front were included which were found to be approximately normal to the two laser sheets. The resulting probability density functions show that the effect of temperature rise ahead of the preheat zone becomes significant only for flames in the “thin reaction zones” regime. The mean temperature profiles show a much smaller temperature rise which, however, increases with increasing velocity ratios v′/sL.

Multisensor analysis of integrated atmospheric water vapor over Canada and Alaska

Atmospheric water vapor is a key parameter for the analysis of climatic systems (greenhouse gas effect), in particular over high latitudes where water vapor displays an important seasonal variability. The sparse spatial and temporal sampling of atmospheric water vapor observations across Canada needs to be improved. A series of instruments and methods including a 940‐nm solar absorption band radiometer (R) and radiosonde (S) analysis from a numerical weather prediction model and a ground‐based bi‐frequency Global Positioning System (GPS) were used to evaluate the integrated atmospheric water vapor (IWV) at various sites in Canada and Alaska from a multiyear database. The IWV‐R measurements were collected within the framework of the North American Sun Radiometry network (AERONET/AEROCAN). Intercomparisons between [IWV‐GPS and IWV‐S], [IWV‐R and IWV‐GPS], and [IWV‐R and IWV‐S] show root mean square (RMS) differences of 1.8, 1.9, and 2.2 kg m−2, respectively. GPS meteorology appears to be the easiest approach to calibrate the solar radiometer water vapor band owing to its flexibility, and it allows us to overcome the Sun radiometry limitation in high‐latitude areas like the Arctic. The sensitivity of the GPS retrieval to various parameters like GPS satellite constellation and meteorological data are discussed. The classical linear relationship between the surface temperature and the integrated weighted mean temperature profile needed for IWV‐GPS retrieval may be significantly different for Arctic air masses compared with midlatitude air masses in the case of tropospheric temperature profile inversion. An ever‐expanding multiyear (1994–2001) North American summer water vapor climatology, derived from AERONET/Canadian Sun Radiometer Network, is presented and analyzed, showing a mean value of 19.8 ± 6.1 kg m−2 and variations from 17 kg m−2 in Alaska to 23 kg m−2 in southeastern Canada. The results in Bonanza Creek, Alaska, show significant interannual variations with a peak in 1997, which may be linked to an El Niño event that occurred in the same year. Such a database may also be useful for climate model validation as shown for the Canadian Global Environmental Model (RMS difference of 3.4 kg m−2). In the end we show that, even if data are selected only for cloud‐free atmospheres, there are no significant differences as compared with radiosonde climatology at Canadian Northwestern sites (≤3% relatively to Bonanza Creek summer mean value).

Bursting Frequency Predictions for Compressible Turbulent Boundary Layers

A computational method for the prediction of the bursting frequency associated with the coherent streamwise structures in high-speed compressible turbulent boundary layers is presented. The structures are described as wavelike disturbances of the turbulent mean flow. A direct resonance theory is used to determine the frequency of bursting. The resulting hydrodynamic linear stability equations are discretized by using a Chebyshev collocation method. A global numerical method capable of resolving the entire eigenvalue spectrum is used. Realistic turbulent mean velocity and temperature profiles are applied. For all of the compressible turbulent boundary layers calculated, the results show at least one frequency that satisfies the resonance condition. A second frequency can be identified for cases with high Reynolds numbers. An estimate is also made for the profile distribution of the temperature disturbance.

Tropical mesopause climatology over the Arecibo Observatory

We report on the first annual cycle of mesopause‐region (82–105 km) temperature profiles at a tropical latitude site, the Arecibo Observatory (18.35°N, 66.75°W), measured by potassium resonance lidar. Based on 74 night‐time temperature profiles of observing periods greater than 5 hours we observe a mean temperature profile with a mesopause altitude of 98 km and temperature of 193 K. The annual mean nighttime temperature profile is similar to that seen at mid‐latitudes, but there are striking differences in seasonal structure. The mesopause region mean seasonal structure at Arecibo experiences a temperature minimum of 176 K at 100 km during mid‐summer. The winter period minimum of about 190 K is found mostly in a broad flat distribution between 90 and 95 km, and less frequently near 100 km.

Transport properties for turbulent dispersion from wall sources

AbstractThe dispersion of a passive contaminant by turbulence is not only intrinsically interesting, but also is fundamental to practical heat‐ and mass‐transfer problems. The mechanism of turbulent dispersion from the wall and effects of the molecular Prandtl number are examined. Transport properties for scalar dispersion from an instantaneous line source at the wall of a parallel‐plate channel in which fully developed turbulent flow occurs, as well as from a continuous line source, are studied. Numerical experiments using a direct numerical simulation (DNS) of the flow, combined with Lagrangian scalar tracking (LST) of trajectories of thermal markers in the flow field, are conducted. The method is applied for a wide range of molecular Prandtl (Pr) or Schmidt (Sc) number fluids (0.1⩽Pr⩽50,000, which corresponds to liquid metals, gases, refrigerants, lubricants and electrochemical fluids). Results from the DNS/LST method are compared with experimental data, which are available for low Pr or Sc fluids. The turbulent diffusivity and turbulent Prandtl number for the plume that results from a line source are calculated, and an appropriate scaling is proposed. Finally, a model is developed for the prediction of the mean temperature or mean concentration profile as a function of Pr or Sc.

Spatial variations of the mean and statistical quantities in the thermal boundary layers of turbulent convection

An experimental study of Rayleigh-Benard turbulent convection in a cubic cell is presented. We measure the mean temperature profiles, the root mean square (RMS) temperature profiles and the thermal boundary layer thickness for various values of the Rayleigh number Ra and at various positions x along the direction of large-scale circulation (LSC), using water as the working fluid. The scaled mean temperature profiles measured at the same Ra but different x are found to be self-similar, but those measured at different Ra do not show a universal form. In contrast, RMS temperature profiles measured at the same position but different Ra appear to show an invariant form, whereas those with different x but of the same Ra could not be scaled onto a single curve. Irrespective of the measuring positions and Ra, the RMS profiles show a peak value around the boundary layer thickness, and their gradients at bottom surface increase monotonically in the direction of LSC and with increasing Ra.

A Large-Eddy Simulation Study of the Convective Boundary Layer over Philadelphia during the 1999 Summer NE-OPS Campaign

This study presents a large-eddy simulation (LES) study of the convective boundary layer on August 1, 1999 over Philadelphia, PA during a summer ozone episode. The study is an evaluation of the Colorado State University's Regional Atmospheric Modeling System Version 4.3 (RAMS4.3) with the LES option using Northeast Oxidant and Particulate Study (NE-OPS) data. Simulations were performed with different imposed sensible heat fluxes at the ground surface. The model was initialized with the atmospheric sounding data collected at Philadelphia at 1230 UTC and model integrations continued till 2130 UTC. The resulting mean profiles of temperature and humidity obtained from the LES model were compared with atmospheric soundings, tethered balloon and aircraft data collected during the NE-OPS 1999 field campaign. Also the model-derived vertical profiles of virtual temperature were compared with NE-OPS Radio Acoustic Sounder System (RASS) data while the humidity profiles were compared with NE-OPS lidar data. The comparison of the radiosonde data with the LES model predictions suggests that the growth of the mixing layer is reasonably well simulated by the model. Overall, the agreement of temperature predictions of the LES model with the radiosonde observations is good. The model appears to underestimate humidity values for the case of higher imposed sensible heat flux. However, the humidity values in the mixing layer agree quite well with radiosonde observations for the case of lower imposed sensible heat flux. The model-predicted temperature and humidity profiles are in reasonable agreement with the tethered balloon data except for some small overestimation of temperature at lower layers and some underestimation of humidity values. However, the humidity profiles as simulated by the model agree quite well with the tethered balloon data for the case of lower imposed sensible heat flux. The model-predicted virtual temperature profile is also in better agreement with RASS data for the case of lower imposed sensible heat flux. The model-predicted temperature profile further agrees quite well with aircraft data for the case of lower imposed heat flux. However, the relative humidity values predicted by the model are lower compared with the aircraft data. The model-predicted humidity profiles are only in partial agreement with the lidar data. The results of this study suggest that the explicitly resolved energetic eddies seem to provide the correct forcing necessary to produce good agreement with observations for the case of an imposed sensible heat flux of 0.1 K m s−1 at the surface.

A Comparative Study of Three Warming Interventions to Determine the Most Effective in Maintaining Perioperative Normothermia

A Comparative Study of Three Warming Interventions to Determine the Most Effective in Maintaining Perioperative Normothermia

Origin of Lapse Rate Changes in the Upper Tropical Troposphere

Vertical motions in clouds arise from a variety of thermodynamic processes, including latent heat release, evaporative cooling, melting, and cloud radiative heating. In the Tropics, the net upward vertical mass flux from convective systems should approximately balance subsidence in clear sky regions associated with radiative cooling, provided the exchange of mass with midlatitudes can be assumed small. Tropical climatologies of temperature, water vapor, and ozone are used to calculate the clear sky radiative mass flux, and the derivative of this mass flux with respect to potential temperature, dMr(θ)/dθ, is used as a proxy for net convective outflow. Convective outflow increases rapidly at 345 K (∼11.3 km). This corresponds to the pseudoequivalent potential temperature θe at which air parcels near the surface first attain positive convective available potential energy (CAPE). The rate at which dMr(θ)/dθ decreases above 345 K is similar to the rate at which the near surface θe probability distribution function (PDF) decreases. This behavior is referred to as “scaling.” It suggests that the timescale for removal of an air parcel from the convective boundary layer is independent of θe (once it has positive CAPE), and that the residual vertical mass flux from convective clouds can be described as if air parcels detrain near their level of neutral buoyancy (LNB). It is also suggested that the mean tropical temperature profile above 345 K is controlled, not by mixing, but by the need for the vertical variation in net convective outflow to be consistent with the near-surface θe PDF, and that this accounts for the fact that the mean temperature profile above 345 K increasingly deviates from a moist adiabat. It is also argued that there are sufficient high θe air parcels near the surface to sustain the Brewer–Dobson circulation by detrainment at the LNB followed by radiative ascent into the stratosphere.

Modulation of upper mesospheric temperature inversions due to tidal-gravity Wave interactions

Two nights of coincident measurements with the University of Western Ontario's MF radar and Purple Crow Lidar have been extensively analyzed to illustrate the possible effects due to tidal-gravity wave interactions on upper mesospheric inversion layers. On both nights an inversion layer in the upper mesosphere disappears as the westward component of the tide increases. Inversions similar in height and magnitude to these two nights, as seen in the nightly mean temperature profile, occur in 11% of the measurements made by the Purple Crow Lidar since 1994. Of these inversions, 76% show similar behavior to the two nights considered here in detail. The statistical results suggest the upper mesospheric inversion occur in two types. The first type, illustrated by the individual nights considered in this study, are associated with increased gravity wave variance as the magnitude of the westward tide decreases. These inversions are persistence over many hours. The second type of inversion has a similar nightly mean structure, but is due to shorter (on the order of <2 h ) periods of heating with temperature changes much greater than that expected from tidal dissipation.

On the displacement in origin for turbulent boundary layers subjected to sudden changes in wall temperature and roughness

The present work studies the behavior of flows that develop over surfaces that present a sudden change in surface temperature and roughness. A particular interest of this study is to investigate any existing relationship between the error in origin for both the velocity and the temperature profiles, so that any analogy between the logarithmic laws for the velocity and the temperature profiles can be assessed. Three different types of surfaces are considered and the flow is made to pass from a cold smooth surface to a hot rough surface. Measurements are presented for the mean velocity and temperature profiles.

Refracted arrival waves in a zone of silence from a finite thickness mixing layer.

Refracted arrival waves which propagate in the zone of silence of a finite thickness mixing layer are analyzed using geometrical acoustics in two dimensions. Here, two simplifying assumptions are made: (i) the mean flow field is transversely sheared, and (ii) the mean velocity and temperature profiles approach the free-stream conditions exponentially. Under these assumptions, ray trajectories are analytically solved, and a formula for acoustic pressure amplitude in the far field is derived in the high-frequency limit. This formula is compared with the existing theory based on a vortex sheet corresponding to the low-frequency limit. The analysis covers the dependence on the Mach number as well as on the temperature ratio. The results show that both limits have some qualitative similarities, but the amplitude in the zone of silence at high frequencies is proportional to omega(-1/2), while that at low frequencies is proportional to omega(-3/2), omega being the angular frequency of the source.

Numerical Analysis of Compressible Turbulent Channel Flow between Adiabatic and Isothermal Walls. 1st Report. Mean Velocity and Temperature Profiles.

The effects of the different thermal wall boundary conditions on the mean velocity and temperature profiles in the compressible turbulent channel flows are investigated. Dimensional analysis says that the mean velocity and temperature profiles depend on the non-dimensional heat flux Bq and the friction Mach number Mτ, when the Prandtl number and the ratio of specific heats are constants. The effects of Bq and Mτ on the mean velocity and temperature profiles are examined using direct numerical simulation (DNS). Two cases of DNS are carried out. The first case is the compressible turbulent flow between isothermal walls, which is the same as that performed by Coleman et al. The second case is one between adiabatic and isothermal walls. In the DNS, the Mach number based on the bulk velocity and sound speed at the isothermal wall is 1.5 and the Reynolds number based on the bulk density, bulk velocity, channel half-width, and viscosity at the isothermal wall is 3000. The main results are as follows : (1) The mean velocity profile with Van Driest transformation is independent of Bq and Mτ, while the untransformed one increases with an increase of -Bq and decreases with an increase of Mτ. (2) The mean temperature increases with an increase of -Bq and decreases with an increase of Mτ. (3) The similarity law between the mean velocity and temperature is not satisfied when the effect of friction work is large. As a result, logarithmic law of the mean temperature is not obtained.