Showing papers on "Thermal published in 1998"

Frequency response of cantilever beams immersed in viscous fluids with applications to the atomic force microscope

Abstract: The vibrational characteristics of a cantilever beam are well known to strongly depend on the fluid in which the beam is immersed. In this paper, we present a detailed theoretical analysis of the frequency response of a cantilever beam, that is immersed in a viscous fluid and excited by an arbitrary driving force. Due to its practical importance in application to the atomic force microscope (AFM), we consider in detail the special case of a cantilever beam that is excited by a thermal driving force. This will incorporate the presentation of explicit analytical formulae and numerical results, which will be of value to the users and designers of AFM cantilever beams.

Simulations of Solar Granulation. I. General Properties

Abstract: Numerical simulations provide information on solar convection not available by direct observation. We present results of simulations of near surface solar convection with realistic physics: an equation of state including ionization and three-dimensional, LTE radiative transfer using a four-bin opacity distribution function. Solar convection is driven by radiative cooling in the surface thermal boundary layer, producing the familiar granulation pattern. In the interior of granules, warm plasma ascends with ≈ 10% ionized hydrogen. As it approaches and passes through the optical surface, the plasma cools, recombines, and loses entropy. It then turns over and converges into the dark intergranular lanes and further into the vertices between granulation cells. These vertices feed turbulent downdrafts below the solar surface, which are the sites of buoyancy work that drives the convection. Only a tiny fraction of the fluid ascending at depth reaches the surface to cool, lose entropy, and form the cores of these downdrafts. Granules evolve by pushing out against and being pushed in by their neighboring granules, and by being split by overlying fluid that cools and is pulled down by gravity. Convective energy transport properties that are closely related to integral constraints such as conservation of energy and mass are exceedingly robust. Other properties, which are less tightly constrained and/or involve higher order moments or derivatives, are found to depend more sensitively on the numerical resolution. At the highest numerical resolution, excellent agreement between simulated convection properties and observations is found. In interpreting observations it is crucial to remember that surfaces of constant optical depth are corrugated. The surface of unit optical depth in the continuum is higher above granules and lower in the intergranular lanes, while the surface of optical depth unity in a spectral line is corrugated in ways that are influenced by both thermal and Doppler effects.

Thermal Vibrational Convection

Abstract: Basic Equations: Mechanical "Quasi-equilibrium" and Its Stability. Plane-Parallel Flows and Their Stability. Non-linear Problems. Internal Heat Sources. Vibrations of Finite Frequencies. Thermovibrational Convection in the General Case of Arbitrary Vibrations. The Problem of Boundary Conditions. The Second-order Effects. Some Particular Problems. Index.

Thermal convection in a volumetrically heated, infinite Prandtl number fluid with strongly temperature‐dependent viscosity: Implications for planetary thermal evolution

Abstract: Parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere-convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales. Recent experimental results argue against this assumption. We have investigated both the definition of the lithosphere-convecting mantle boundary and the power law relation describing convecting heat transfer, based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature-dependent viscosity. Other recent studies have treated only the heating from below, but volumetric heating is likely to be the dominant mode of heating in planetary mantles, either as a consequence of radioactive heating or as a proxy for secular cooling. Convection can occur either in the whole box or be located under a stagnant lid. In the lid regime, convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature. This result, in agreement with experimental studies, indicates that boundary between the stagnant lid and the convecting layer (similar to the lithosphere-convecting mantle boundary) cannot be defined as a fixed isotherm. During thermal evolution of planets, the viscosity contrast in the convecting mantle remains constant, not the temperature at the bottom of the lithosphere. We present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere-convecting mantle boundary. For reasonable values of the activation energy for thermally activated creep, the temperature defining the lithosphere-convecting mantle boundary, the mantle temperature, and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm.

Galileo probe measurements of thermal and solar radiation fluxes in the Jovian atmosphere

Abstract: The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin-induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5-0.75 microns in radius) of optical depth 1.5-2 at 0.5 microns. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small-particle cloud, implying a heterogeneous cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3-4 times ground-based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5-bar level. The 5-microns thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 gm, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 microns, which might be due to unaccounted-for absorption by NH3 between 0.65 and 1.5 microns.

Enhanced Heat Transport in Turbulent Convection over a Rough Surface

Abstract: Turbulent flows over a rough surface are ubiquitous in nature. An example is convection in the atmosphere and oceans, where the underlying surfaces are almost always rough. The study of turbulence over a rough surface is of fundamental interest for understanding the structure and dynamics of turbulent boundary layers and is also relevant to many practical applications, ranging from effective heat transfer for a reentry vehicle to turbulent drag reduction of a commercial aircraft. Our current knowledge about the roughness effect on turbulent flows comes largely from experiments in wind tunnels and other open systems [1], where the disturbance flow produced by a rough wall is confined in the near-wall region and is quickly discharged to the downstream. Because of these reasons the surface roughness usually does not perturb the turbulent bulk region very much, and its effect can often be described by rescaling the relevant parameters with the surface roughness height k [2]. This situation is changed completely for flows in a closed cell, in which the disturbances produced by the boundaries are inevitably mixed into the turbulent bulk region. Turbulent Rayleigh-Benard convection is an example of such a closed system, which has attracted much attention in recent years [3]. Intermittent bursts of thermal plumes from the thermal boundary layers and the coherent largescale circulation, which modifies the boundary layer via its shear, are found to coexist in the convection cell. These salient features are directly related to the heat transport across the cell, and have been adopted in several theoretical models [3‐ 5] to explain the observed scaling laws in the heat flux and temperature statistics [4,6]. In this Letter, we describe a novel convection experiment in a closed cell with rough upper and lower surfaces. It is found that when k becomes much larger than the thermal boundary layer thickness d, the heat transport across the rough cell is increased by more than 76%. Flow visualization and near-wall temperature measurements reveal that the large-scale circulation interacts with the surface roughness and produces more thermal plumes from the tip of the rough elements. The striking effect of the surface roughness provides new insights into the nature of convective turbulence, and also has a myriad of applications in engineering, geography, and meteorology. The experiment is conducted in a cylindrical cell filled with water. Details about the apparatus have been described elsewhere [7], and here we mention only some key points. The rough upper and lower surfaces are made from identical brass plates and have woven V-shaped grooves on them. The grooves have a vertex angle of 90 ‐ and their spacing is such that a square lattice of pyramids is formed on the surface. The height of the pyramids is k › 9.0 mm and their spacing is 2k. The sidewall of the cell is a cylindrical ring made of transparent Plexiglas. Two cylindrical rings with the same inner diameter of 20 cm but having different heights of 20 and 40 cm, respectively, are used. The corresponding aspect ratios sA › diameteryheightd are 1.0 and 0.5. The upper plate is cooled by passing cold water through a cooling chamber fitted on the top of the plate. The lower plate is heated uniformly with an electric heating film attached on the back side of the plate. The temperature difference DT between the two plates is measured by two thermistors imbedded in the two plates. A small movable temperature probe is installed inside the cell in order to measure the local temperature T szd of the convecting fluid near the central axis of the cell as a function of distance z

Layered thermohaline convection in hypersaline geothermal systems

Abstract: Thermohaline convection occurs in hypersaline geothermal systems due to thermal and salinity effects on liquid density. Because of its importance in oceanography, thermohaline convection in viscous liquids has received more attention than thermohaline convection in porous media. The fingered and layered convection patterns observed in viscous liquid thermohaline convection have been hypothesized to occur also in porous media. However, the extension of convective dynamics from viscous liquid systems to porous media systems is complicated by the presence of the solid matrix in porous media. The solid grains cause thermal retardation, hydrodynamic dispersion, and permeability effects. We present simulations of thermohaline convection in model systems based on the Salton Sea Geothermal System, California, that serve to point out the general dynamics of porous media thermohaline convection in the diffusive regime, and the effects of porosity and permeability, in particular. We use the TOUGH2 simulator with residual formulation and fully coupled solution technique for solving the strongly coupled equations governing thermohaline convection in porous media. We incorporate a model for brine density that takes into account the effects of NaCl and CaCl2. Simulations show that in forced convection, the increased pore velocity and thermal retardation in low-porosity regions enhances brine transport relative to heat transport. In thermohaline convection, the heat and brine transport are strongly coupled and enhanced transport of brine over heat cannot occur because buoyancy caused by heat and brine together drive the flow. Random permeability heterogeneity has a limited effect if the scale of flow is much larger than the scale of permeability heterogeneity. For the system studied here, layered thermohaline convection persists for more than one million years for a variety of initial conditions. Our simulations suggest that layered thermohaline convection is possible in hypersaline geothermal systems provided the vertical permeability is smaller than the horizontal permeability, as is likely in sedimentary basins such as the Salton Trough. Layered thermohaline convection can explain many of the observations made at the Salton Sea Geothermal System over the years.

Thermal Management of an Avionics Module Using Solid-Liquid Phase-Change Materials

Abstract: A combined experimental and computational investigation of transient thermal control of an avionics module using phase-change material (PCM) is reported. The cone guration examined was a honeycomb core e lled with an organic PCM, n-triacontane, heated from the bottom. Experiments were conducted to evaluate the thermal performance of the PCM device by measuring temperatures at various locations as functions of time until the module temperature reached an acceptable maximum limit. An analysis of melting inside a single honeycomb cell, considering effects of natural convection, showed that, for the power levels and the cell geometry considered, the effect of natural convection on melting was negligible. A system-level analysis of the PCM-e lled device followed. Timewise variations of temperatures at various locations from the model were in good agreement with the experimental data. Times for complete melting, maximum temperature variations within the honeycomb, and evolution of melt shapes are presented as functions of power levels.

Field observations of thermals and thermal streets, and the theory of cross-country soaring flight

Abstract: The classical theory of cross-country soaring flight postulates climbs in discrete thermals, separated by glides through stationary air, and prescribes an "optimum" speed for the inter-thermal glides, based on the performance of the glider or bird, and the rate of climb in thermals. However, it is known that glider pilots usually fly slower than the theoretical optimum speed between thermals, and often achieve faster cross-country speeds than the maximum predicted by the theory. The reasons for this are analysed in relation to a survey of real thermals over the Serengeti National Park, Tanzania. About 1500 km was flown in straight lines, in a Schleicher ASK-14 motor glider, measuring the airmass lift (vertical velocity of the air) at 15-s intervals. It is shown how the bird or pilot can increase the mean rate of climb in the sample of thermals that are actually used, and so increase the cross-country speed, by selecting an inter-thermal speed below the supposed optimum. If the air between thermals is not stationary, the time lost in circling can be further reduced by variations of speed during the inter-thermal glides. Eventually, if thermals are organised into lines or "streets", circling can be eliminated altogether, and linear soaring becomes possible. Linear soaring is also sometimes possible along ridges and escarpments. Soaring in discrete thermals may be seen as one end of a spectrum of soaring techniques, with linear soaring at the other end, giving much higher cross-country speeds. In straight flight, soaring and flapping are end members of another spectrum, connected by various intermediate techniques. A bird migrating by flapping flight in a straight line can reduce its power requirements by flying through thermals or other forms of rising air.

Non‐local thermodynamic equilibrium in general circulation models of the Martian atmosphere 1. Effects of the local thermodynamic equilibrium approximation on thermal cooling and solar heating

Abstract: Calculations of CO2 thermal cooling and near-IR solar heating rates under non-local thermodynamic equilibrium (non-LTE) situations have been performed to understand and evaluate the effects of non-LTE on the energy balance of the upper atmosphere of Mars. We find that the 15-μm cooling rates can be in error if LTE is assumed above 80 km. In general, the correct non-LTE values are significantly smaller than the LTE values above about 85 km, but the magnitude and sign of the error depend on the temperature structure and the top altitude of the model and, to a lesser extent, on the collisions with atomic oxygen. A detailed analysis of the relevance of the upper boundary layer and a suggested buffer region are presented for both LTE and non-LTE. Based on general considerations of the thermal profile in the mesosphere and lower thermosphere, recommendations for general circulation models (GCM) are presented as a first guide for minimizing the LTE cooling rates inaccuracies. The error of assuming LTE on the CO2 near-IR solar heating rates is found to be about 20% at 85 km and increases strongly above this altitude. The dependences of this LTE-non-LTE difference on rate coefficients, thermal structure, surface pressure, and solar zenith angle (SZA) are studied. In contrast to the large effect of the SZA on the solar heating rate, we find it is not important for the LTE-non-LTE relative difference, which permits a simple tabulation of the non-LTE effect as a function of pressure only. A table of LTE correction factors for the solar heating rate is included for its potential use as a fast yet accurate operative scheme within GCMs.

Transient Thermal Effects of Radiant Energy in Translucent Materials

Abstract: When a solid or stationary fluid is translucent, energy can be transferred internally by radiation in addition to heat conduction. Since radiant propagation is very rapid, it can provide energy within a material more quickly than diffusion by heat conduction. Radiation emitted in a hot material can also be distributed rapidly in the interior. The result is that transient temperature responses including radiation can be significantly different from those by conduction alone. This is important for evaluating the thermal performance of translucent materials that are at elevated temperatures, are in high temperature surroundings, or are subjected to large incident radiation. Detailed transient solutions are necessary to examine heat transfer for forming and tempering of glass windows, evaluating ceramic components and thermal protection coatings, studying highly backscattering heat shields for atmospheric reentry, porous ceramic insulation systems, ignition and flame spread for translucent plastics, removal of ice layers, and other scientific and engineering applications involving heating and forming of optical materials. Radiation effects have been studied less for transients than for steady state because of the additional mathematical and computational complexities, but an appreciable literature has gradually developed. This paper will review the applications, types of conditions, and geometries that have been studied. Results from the literature are used to illustrate typical radiation effects on transient temperatures, and comparisons are made of transient measurements with numerical solutions.

Thermal effects and scaling in organic light-emitting flat-panel displays

Abstract: The temperature rise in flat-panel displays without forced air cooling has been both modeled and experimentally measured as a function of the display size. Both radiation and convection are important processes for the transfer of heat to the ambient. Because of much poorer convection and the lack of lateral heat transport at large dimensions, for a fixed power density large displays are expected to be substantially hotter than small displays. This could adversely impact the reliability of large displays based on organic light-emitting diode (OLED) technology.

Ultrasonic manipulation of particles in microgravity

Abstract: The manipulation of yeast cells and latex particles of diameters 1.3, 12 and in aqueous suspension in 1 and 3 MHz ultrasonic standing waves has been examined and compared in microgravity (0 g), 1 g and 1.8 g. The experiments were carried out during the 23rd ESA parabolic flight campaign. The suspended particles concentrated to form bands at half wavelength separation in the axial direction of a vertical tubular sample holder with a Bessel pressure amplitude distribution profile. At 1 g small ( latex) particles formed bands but these broke up within a few seconds. In contrast throughout 0 g bands of these particles formed and remained stable. The transition from 0 to 1.8 g during flight induced streaming which broke up the bands of latex. Bands formed with yeast cells were more stable at 1 g but, during transitions from 0 to 1.8 g, some bands broke up. Bands of the larger (12 and ) particles were stable at 0, 1 and 1.8 g and during all transitions between the fields. Thermal gradient convective flow rather than acoustic streaming was identified as the main source of flow in the sample holder at 1 g. The absence of thermal streaming at 0 g allowed manipulation of smaller particles in that situation. A frequency ramping technique appropriate for removal of particles from suspension in 1 g had a better performance in 0 g.

Convective contributions to the frequencies of solar oscillations

Abstract: Differences between observed and theoretical eigenfrequencies of the Sun have characteristics which identify them as arising predominantly from properties of the oscillations in the vicinity of the solar surface: in the super-adiabatic, convective boundary layer and above. These frequency differences may therefore provide useful information about the structure of these regions, precisely where the theory of solar structure is most uncertain. In the present work we use numerical simulations of the outer part of the Sun to quantify the influence of turbulent convection on solar oscillation frequencies. Separating the influence into effects on the mean model and effects on the physics of the modes, we find that the main model effects are due to the turbulent pressure that provides additional support against gravity, and thermal differences between average 3-D models and 1-D models. Surfaces of constant pressure in the visible photosphere are elevated by about 150 km, relative to a standard envelope model. As a result, the turning points of high-frequency modes are raised, while those of the low-frequency modes remain essentially unaffected. The corresponding gradual lowering of the mode frequencies accounts for most of the frequency difference between observations and standard solar models. Additional effects are expected to come primarily from changes in the physics of the modes, in particular from the modulation of the turbulent pressure by the oscillations.

Experimental investigation of air convection embankments for permafrost-resistant roadway design

Abstract: Air convection embankments have been proposed as a method for avoiding thaw-settlement of roadways in regions of warm permafrost. These embankments are constructed of poorly-graded open aggregate, resulting in a very high air permeability. Unstable air density gradients that develop within the embankment during winter result in buoyancy-induced pore air convection. This convection increases the heat flux out of the embankment and foundation material during winter months. During summer, the air density gradient is stable and convection does not occur. The net effect is an increase in winter cooling without a corresponding increase in summer warming and thawing is prevented in the permafrost layer beneath the embankment. The present study discusses thermal data collected from an experimental air convection embankment that was constructed at BrownOs Hill Quarry near Fairbanks, Alaska and monitored over a two-year period. The results show a large cooling influence due to air convection within the embankment during winter.

Turbulent convection and pulsational stability of variable stars. II. Oscillations of RR Lyrae and horizontal branch red variable stars

Abstract: Using a nonlocal time-dependent theory of convection, we have calculated the linear nonadiabatic oscillations of horizontal branch (HB) stars, carefully treating both the dynamic and thermodynamic coupling between convection and oscillations. Turbulent pressure and turbulent viscosity have been included consistently in our equations of nonadiabatic pulsation. When the coupling between convection and oscillations is ignored, for all models with T-e less than or equal to 7350 K, the fundamental through the second overtone are pulsationally unstable, while for T-e less than or equal to 6200 K all the models are unstable up to (at least) the 9th overtone. When the coupling between convection and oscillations is included, the RR Lyrae instability strip is very well predicted. Within the strip, most models are pulsationally unstable only for the fundamental and the first few low-order overtones. The turbulent viscosity is an important damping mechanism. Being exclusively distinct from the luminous red variables (long-period variables), the HE stars to the right of the RR strip are pulsationally stable for the fundamental and low-order overtones, but become unstable for some of the high-order overtones. This may provide a valuable clue to the short-period, low-amplitude red variables found outside the red edge of the RR strip on the H-R diagram of globular clusters. We also present a new radiation-modulated excitation mechanism functioning in a zone of radiation flux gradient. The effects of nonlocal convection and the dynamic coupling between convection and oscillations are discussed. The spatial oscillations of the thermal variables in the pulsational calculations have been effectively suppressed.

Double Diffusive Natural Convection in a Composite Fluid-Porous Layer

Abstract: This study deals with natural convection driven by combined thermal and solutal buoyancy forces in a binary fluid. The consequence of thermosolutal convection on heat and mass transfer is examined in a confined enclosure partially filled with a porous medium. The mathematical description of the problem makes use of one-domain formulation of the conservation equations. The presented numerical results quantitatively show the significant influence of the presence of a relatively thin porous layer on the flow structure and on heat and species transfer in the enclosure. The paper is dedicated to the analysis of the influence of the thickness and permeability of the porous layer in a range of governing parameters. For a low permeability porous layer, the numerical results are compared to an analysis based on simple scaling laws, which provide a good interpretation in terms of the wall transfer decrease with the porous layer thickness. The effect of permeability is investigated and it is shown that flow penetration in the porous layer induces a specific behavior of the flow structure and average heat transfer in the enclosure

High Rayleigh number thermo‐chemical models of a dense boundary layer in D″

Abstract: We simulate the eects of an intrinsically dense boundary layer at the base of the mantle on the thermal structure of D 00 and the dynamics of the lower mantle. Us- ing a nite element model of thermo-chemical convection in 2-D with a Rayleigh number of 10 7 ,w e investigate the dy- namic consequences of varying the density and of increasing thermal diusivity in the basal layer to simulate enrichment in metals. Convection may occur within a stable layer at the base of the model mantle, leading to short (400 km) wavelength temperature variations that contrast with the longer wavelength (1500 { 2000 km) variations above the dense layer. Strong small-scale convection within the layer is not observed if the layer is only marginally stable and easily deformed.

Compositional variation in hydrocarbon reservoirs with natural convection and diffusion

Abstract: The knowledge of horizontal compositional variation is of prime importance in hydrocarbon reservoir delineation. However, the effect of natural convection on this variation is largely unknown. This work examines the effect of natural convection and diffusion (thermal, pressure and Fickian) on a two-component, single-phase fluid occupying a horizontal cross-sectional reservoir in the presence of a prescribed linear temperature field. The behavior is investigated using a method of successive approximations, which iterates on solution of Poisson's equation. This behavior is then incorporated in a simplified perturbation solution (in the form of a cubic equation) which not only gives accurate values of horizontal compositional variation, but also clearly shows the interplay of reservoir/fluid properties. The perturbation and the full solutions indicate that a small amount of convection can cause the horizontal composition gradients to increase until a maximum is reached and then decay as 1/k. An alternative scenario is that the gradient asymptotes to a value where the horizontal density derivative approaches zero. Generalization of this perturbative solution to n-components apparently requires the simultaneous solution of n − 1 cubic equations.

Transient Thermal Response of a Rotating Cylindrical Silicon Nitride Workpiece Subjected to a Translating Laser Heat Source, Part II: Parametric Effects and Assessment of a Simplified Model

Abstract: In a companion paper (Rozzi et al., 1998), experimental validation was provided for a transient three-dimensional numerical model of the process by which a rotating workpiece is heated with a translating laser beam. In this paper, the model is used to elucidate the effect of operating parameters on thermal conditions within the workpiece and to assess the applicability of an approximate analysis which is better suited for on-line process control. From detailed numerical simulations, it was determined that the thickness of a surface thermal layer decreases with increasing workpiece rotational speed and that the influence of axial conduction on the workpiece temperature distribution increases with decreasing laser translational velocity. Temperatures increase throughout the workpiece with increasing laser power, while the influence of increasing beam diameter is confined to decreasing near-surface temperatures. Temperature-dependent thermophysical properties and forced convection heat transfer to the laser gas assist jet were found to significantly influence the maximum temperature beneath the laser spot, while radiation exchange with the surroundings and mixed convection to the ambient air were negligible. The approximate model yielded relations for calculatin g the radial temperature distribution within an r-o plane corresponding to the center of the laser source, and predictions were in reasonable agreement with results of the numerical simulation, particularly in a near-surface region corresponding to the depth of cut expected for laser-assisted machining.