Measurements Of Temperature Fluctuations Research Articles

Simultaneous measurements of temperature and velocity fluctuations in a double stream-twin jet air curtain for heat confinement in case of tunnel fire

This paper briefly describes laboratory experiments performed to investigate the ability of air curtain arrangements to prevent heat propagation in case of fire in a corridor-like geometry. The arrangement considered in this study is a system based on two twin plane jets. Simultaneous measurements of temperature and velocity were performed with fine thermocouples and a Laser Doppler Anemometer (LDA), respectively. The jet Reynolds number based on the average discharge velocity U 0 and the total width 2 e of the investigated twin-jet was about 1000. Information about the spatial distributions of the one-point correlations u θ ¯ and v θ ¯ is provided. Results show that it is the longitudinal turbulent heat flux that predominantly contributes to turbulent transfers. Transfer rates increase on approaching the impingement region. The jet can be split into three regions: for x/2 e < 5 one can still distinguish the separate influence of the two initial jets composing the curtain: for 5 < x/2 e < 8.5, a transition zone exists where the two initial jets merge. In this region, transfers are not necessarily much higher. They are expected to intensify closer to the wall due to strong mixing in the impingement region.

Fluid Flow and Heat Transfer in a Horizontal Channel With Divergent Top Wall and Heated From Below

Secondary flow structure and its enhancement on the heat transfer in a horizontal divergent channel have been studied. The bottom wall is horizontal and is heated uniformly while the opposite top wall is insulated and inclined with respect to the horizontal plane so as to create a divergent angle of 3 deg. At the entrance of the channel, the aspect ratios for the width to the height and the channel length to the height are 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000 and the buoyancy parameter Gr/Re2 from 0 to 405. Both flow visualization and temperature fluctuation measurements at different locations are made to indicate the flow structure and oscillation of the secondary flow. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier initiation of secondary flow. Interaction between neighboring vortices and plumes becomes more severe and highly unstable. This precludes the formation of steady two-dimensional longitudinal vortex rolls in the downstream and leads to an earlier and larger enhancement of the heat transfer than the case of the parallel-plate channel. The effects of the buoyancy parameter and the divergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed.

Acoustic temperature measurement in a rocket noise field

A 1 μm diameter platinum wire resistance thermometer has been used to measure temperature fluctuations generated during a static GEM-60 rocket motor test. Exact and small-signal relationships between acoustic pressure and acoustic temperature are derived in order to compare the temperature probe output with that of a 3.18 mm diameter condenser microphone. After preliminary plane wave tests yielded good agreement between the transducers within the temperature probe's ∼2 kHz bandwidth, comparison between the temperature probe and microphone data during the motor firing show that the ±∼3 K acoustic temperature fluctuations are a significant contributor to the total temperature variations.

Using Temperature Fluctuation Measurements to Estimate Meteorological Inputs for Modelling Dispersion During Convective Conditions in Urban Areas

We examine the performance of several methods to estimate meteorological inputs for modelling dispersion in urban areas during convective conditions. Sensible heat flux, surface friction velocity and turbulent velocities are estimated from measurements of mean wind speed and the standard deviation of temperature fluctuations at a single level on a tower at two suburban sites and at one urban site in Riverside, California. These estimates are compared with observations made at these sites during a field study conducted in 2007. The sensible heat flux is overestimated in the urban area, while it is underestimated at a suburban site when temperature fluctuations are used in the free convection formulation to estimate heat flux. The bias in heat flux estimates can be reduced through a correction that depends on stability. It turns out that the bias in heat flux estimates has a minor effect on the prediction of surface friction velocity and turbulent velocities. Estimates of sensible heat flux, surface friction velocity and turbulent velocities are sensitive to estimates of aerodynamic roughness length, and we suggest estimating the aerodynamic roughness length through detailed micrometeorological measurements made during a limited field study. An examination of the impact of the uncertainty in estimating surface micrometeorology on concentrations indicates that, at small distances from a surface release, ground-level concentrations computed using estimates of heat flux and surface friction compare well with the those based on observed values: the bias is small and the 95% confidence interval of the ratio of the two concentrations is 1.7. However, at distances much larger than the Obukhov length, this confidence interval is close to 2.3 because errors in both friction velocity and heat flux affect plume spread. Finally, we show that using measurements of temperature fluctuations in estimating heat flux is an improvement on that based on the surface energy balance, even when net radiation measurements are available.

Simultaneous measurement of core electron temperature and density fluctuations during electron cyclotron heating on DIII-D

New measurements show that long-wavelength (kθρs&amp;lt;0.5) electron temperature fluctuations can play an important role in determining electron thermal transport in low-confinement mode (L-mode) tokamak plasmas. In neutral beam-heated L-mode tokamak plasmas, electron thermal transport and the amplitude of long-wavelength electron temperature fluctuations both increase in cases where local electron cyclotron heating (ECH) is used to modify the plasma profiles. In contrast, the amplitude of simultaneously measured long-wavelength density fluctuations does not significantly increase. Linear stability analysis indicates that the ratio of the trapped electron mode (TEM) to ion temperature gradient (ITG) mode growth rates increases in the cases with ECH. The increased importance of the TEM drive relative to the ITG mode drive in the cases with ECH may be associated with the increases in electron thermal transport and electron temperature fluctuations.

Primordial Non‐Gaussianity and Bispectrum Measurements in the Cosmic Microwave Background and Large‐Scale Structure

The most direct probe of non‐Gaussian initial conditions has come from bispectrum measurements of temperature fluctuations in the Cosmic Microwave Background and of the matter and galaxy distribution at large scales. Such bispectrum estimators are expected to continue to provide the best constraints on the non‐Gaussian parameters in future observations. We review and compare the theoretical and observational problems, current results, and future prospects for the detection of a nonvanishing primordial component in the bispectrum of the Cosmic Microwave Background and large‐scale structure, and the relation to specific predictions from different inflationary models.

Measurements of mean and fluctuating temperature in an underexpanded jet using electrostrictive laser-induced gratings

Instantaneous temperature measurements were obtained in an underexpanded jet using electrostrictive laser-induced gratings. Evaluation of the technique under static, low-pressure conditions provided a baseline uncertainty or precision for single-shot temperature measurements of 4.4% of the local mean temperature, which represents the minimum detectable temperature fluctuation. The underexpanded jet was operated at a nozzle pressure ratio of 2.39 and a fully expanded jet Mach number of 1.19. Data were acquired along the centerline and over two radial traverses through the shear layer. Mean temperature data agree well with expectations, describing the shock-cell structure and the compressible shear layer. The growth in shear-layer width with downstream distance can be identified in the mean and fluctuating temperature measurements. Temperature fluctuations are near the baseline detection limit in the jet core and surrounding ambient air, and reach a maximum in the shear layer. The temperature fluctuation measurements compare well with previous computational and experimental work, confirming the application of the technique to a turbulent, supersonic flow.

Acoustic temperature measurement in a full‐scale rocket noise field.

Acoustic pressure measurements in high‐amplitude sound fields are common, but acoustic temperature measurements are relatively rare. This is primarily because thermometers exhibit thermal lag and, consequently, a limited frequency range. In this study, a 1 μ diam platinum wire resistance thermometer was used to measure temperature fluctuations generated by propagating noise produced by a horizontally fired, static GEM‐60 solid rocket motor. The data are compared with those calculated from the pressure data of a nearby 3.18 mm condenser microphone by assuming an ideal adiabatic process.

Mixing Measurements on an Equatorial Ocean Mooring

Abstract Vaned, internally recording instruments that measure temperature fluctuations using FP07 thermistors, including fluctuations in the turbulence wavenumber band, have been built, tested, and deployed on a Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These were supplemented with motion packages that measure linear accelerations, from which an assessment of cable displacement and speed was made. Motions due to vortex-induced vibrations caused by interaction of the mean flow with the cable are small (rms &amp;lt; 0.15 cable diameters) and unlikely to affect estimates of the temperature variance dissipation rate χT. Surface wave–induced cable motions are significant, commonly resulting in vertical displacements of ±1 m and vertical speeds of ±0.5 m s−1 on 2–10-s periods. These motions produce an enhancement to the measurement of temperature gradient in the surface wave band herein that is equal to the product of the vertical cable speed and the vertical temperature gradient (i.e., dT/dt ∼ wcdT/dz). However, the temperature gradient spectrum is largely unaltered at higher and lower frequencies; in particular, there exists a clear scale separation between frequencies contaminated by surface waves and the turbulence subrange. The effect of cable motions on spectral estimates of χT is evaluated and determined to result in acceptably small uncertainties (&amp;lt; a factor of two 95% of the time, based on 60-s averages). Time series of χT and the inferred turbulent kinetic energy dissipation rate ε are consistent with historical data from the same equatorial location.

The Szekeres Swiss Cheese model and the CMB observations

This paper presents the application of the Szekeres Swiss Cheese model to the analysis of observations of the cosmic microwave background (CMB) radiation. The impact of inhomogeneous matter distribution on the CMB observations is in most cases studied within the linear perturbations of the Friedmann model. However, since the density contrast and the Weyl curvature within the cosmic structures are large, this issue is worth studying using another approach. The Szekeres model is an inhomogeneous, non-symmetrical and exact solution of the Einstein equations. In this model, light propagation and matter evolution can be exactly calculated, without such approximations as small amplitude of the density contrast. This allows to examine in more realistic manner the contribution of the light propagation effect to the measured CMB temperature fluctuations. The results of such analysis show that small-scale, non-linear inhomogeneities induce, via Rees-Sciama effect, temperature fluctuations of amplitude 10−7–10−5 on angular scale ϑ < 0.24° (ℓ > 750). This is still much smaller than the measured temperature fluctuations on this angular scale. However, local and uncompensated inhomogeneities can induce temperature fluctuations of amplitude as large as 10−3, and thus can be responsible the low multipoles anomalies observed in the angular CMB power spectrum.

A hemispherical power asymmetry from inflation

Measurements of cosmic microwave background temperature fluctuations by the Wilkinson Microwave Anisotropy Probe indicate that the fluctuation amplitude in one half of the sky differs from the amplitude in the other half. We show that such an asymmetry cannot be generated during single-field slow-roll inflation without violating constraints to the homogeneity of the Universe. In contrast, a multifield inflationary theory, the curvaton model, can produce this power asymmetry without violating the homogeneity constraint. The mechanism requires the introduction of a large-amplitude superhorizon perturbation to the curvaton field, possibly a preinflationary remnant or a superhorizon curvaton-web structure. The model makes several predictions, including non-Gaussianity and modifications to the inflationary consistency relation, that will be tested with forthcoming cosmic microwave background experiments.

Calibration of a Thomson scattering diagnostic for fluctuation measurements

Detailed calibrations of the Madison Symmetric Torus polychromator Thomson scattering system have been made suitable for electron temperature fluctuation measurements. All calibrations have taken place focusing on accuracy, ease of use and repeatability, and in situ measurements wherever possible. Novel calibration processes have been made possible with an insertable integrating sphere (ISIS), using an avalanche photodiode (APD) as a reference detector and optical parametric oscillator (OPO). Discussed are a novel in situ spatial calibration with the use of the ISIS, the use of an APD as a reference detector to streamline the APD calibration process, a standard dc spectral calibration, and in situ pulsed spectral calibration made possible with a combination of an OPO as a light source, the ISIS, and an APD used as a reference detector. In addition a relative quantum efficiency curve for the APDs is obtained to aid in uncertainty analysis.

Measurements of core electron temperature and density fluctuations in DIII-D and comparison to nonlinear gyrokinetic simulations

For the first time, profiles (0.3&amp;lt;ρ&amp;lt;0.9) of electron temperature and density fluctuations in a tokamak have been measured simultaneously and the results compared to nonlinear gyrokinetic simulations. Electron temperature and density fluctuations measured in neutral beam-heated, sawtooth-free low confinement mode (L-mode) plasmas in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] are found to be similar in frequency and normalized amplitude, with amplitude increasing with radius. The measured radial profile of two fluctuation fields allows for a new and rigorous comparison with gyrokinetic results. Nonlinear gyrokinetic flux-tube simulations predict that electron temperature and density fluctuations have similar normalized amplitudes in L-mode. At ρ=0.5, simulation results match experimental heat diffusivities and density fluctuation amplitude, but overestimate electron temperature fluctuation amplitude and particle diffusivity. In contrast, simulations at ρ=0.75 do not match either the experimentally derived transport properties or the measured fluctuation levels.

Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-level Jet

The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ricr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ricr during bursting events.

An analytical model for the design of optimized heat exchangers for thermoacoustic systems

A 1-D nonlinear model for the heat transport from the edge of a thermoacoustic stack to a heat exchanger is developed. This model is an extension of the previous study by [Gusev et al., J. of Sound and Vibration 235, (2000)] to the case of a finite dimension heat exchanger with a temperature difference between the stack and the heat exchanger plates. The model is based on a relaxation-time approximation for transverse heat transfer that allows for the calculation of the heat flux through an adiabatic gap, separating the stack and the heat exchangers. It shows the generation of temperature harmonics close to the edge of the plates that leads to nonlinear heat transport through the gap. The heat flux extracted by the hot side heat exchanger is calculated, taking into account viscous dissipation along the plates and reverse heat conduction toward the stack. An optimal set of geometrical and relaxation parameters for the heat exchanger is obtained. Results are compared with experimental measurements of temperature fluctuations behind the stack plates, and with the results of numerical simulations from the literature. [This work is supported by ANR (project MicroThermoAc NT05142101).]

Measurement of the fractal order of wall void fraction during nucleate boiling

The fractal nature of hydrodynamic features during nucleate boiling is illustrated in this study. Earlier reports in the literature explored the fractal nature of the thermal features in pool boiling. In these studies typically the time-delay technique was used to obtain chaos quantifiers in boiling from wall temperature fluctuation measurements. In this study experiments were performed at different wall superheats to obtain the images of voids (bubbles) on micro-heaters during nucleate boiling. This study quantifies boiling chaos based on the geometrical features of the voids (e.g., area and perimeter). The fractal order of the geometrical features was obtained by digital image processing. The fractal order (correlation dimension) was computed using the box-counting technique. The experimental apparatus consisted of an array of individually controlled micro-heaters which were used in conjunction with a total internal reflection technique to visualize the wetted area during boiling of FC-72. Images of the voids (bubbles) were captured with a high speed camera. The resulting images were analyzed using the image processing toolbox in MATLABR ®. Temporal fluctuations observed in the values of the fractal order (for different images in a sequence) were found to remain within distinct bounds with only marginal overlap between the values computed from experimental data obtained at different wall superheats. The fractal order of the voids (based on area, D c2) was found to increase with the wall superheat. This implies that the complexity of the boiling system (represented by the fractal order of the system) increases with wall superheat in nucleate boiling. The fractal order based on perimeter (edge lengths, D c1) of the voids was found to reach a maxima with wall superheat. Furthermore, the plot of D c2 versus D c1 was found to mimic the conventional boiling curve (wall heat flux versus wall superheat). This shows that D c2 and D c1 are intrinsically related to wall superheat and heat flux, respectively. Further, it implies that the liquid–vapor contact line plays a major role in modifying pool boiling heat flux compared to the size of the voids (bubbles).

A Review of ECE Correlation Radiometry Techniques for Detection of Core Electron Temperature Fluctuations

Over the past 15 years, correlation radiometry techniques have been developed to detect the small-amplitude, high-frequency electron temperature fluctuations to which standard electron cyclotron emission measurements are insensitive. In this paper we review the principles of these techniques and summarize recent core electron temperature fluctuation measurements and their impact on understanding turbulent transport.

Cosmological perturbations from inflation

Matter is distributed very homogeneously and isotropically on scales larger than a few hundred Mpc. The measurements of the microwave background temperature fluctuations show that at recombination the universe was extremely homogeneous and isotropic (with accuracy ∼10−4) on all scales up to the present horizon (Spergel et al 2006 Preprint astro-ph/0603449; MacTavish et al 2005 Preprint astro-ph/0507503). On the other hand, there is a large scale structure in the observable universe and one of the central issues of contemporary cosmology is the explanation of the origin of primordial inhomogeneities, which serve as the seeds for structure formation. Before the advent of inflationary cosmology the initial perturbations were postulated and their spectrum was designed to fit observational data. In this way practically any observation could be ‘explained’, or more accurately described, by arranging the appropriate initial conditions. In contrast, inflationary cosmology truly explains the origin of primordial inhomogeneities and predicts their spectrum (Mukhanov and Chibisov 1981 JETP Lett. 33 532; Mukhanov and Chibisov 1982 Sov. Phys.—JETP 56 258). Thus it becomes possible to test this theory by comparing its predictions with observations. According to cosmic inflation, primordial perturbations originated from quantum fluctuations. These fluctuations have substantial amplitudes only on scales close to the Planckian length, but during the inflationary stage they are stretched to galactic scales with nearly unchanged amplitudes. Thus, inflation links the large-scale structure of the universe to its microphysics. The resulting spectrum of inhomogeneities is not very sensitive to the details of any particular inflationary scenario and has nearly universal shape. This leads to concrete predictions for the spectrum of cosmic microwave background anisotropies.

Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array

Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of +/-100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.

Fluctuating temperature measurement by a fine-wire thermocouple probe: influences of physical properties and insulation coating on the frequency response

Theoretical analysis of the frequency response of a fine-wire thermocouple probe has been further developed to investigate influences of the difference in physical properties between the two wires composing a butt-welded fine-wire thermocouple. The solution obtained demonstrates complex behaviour of the frequency response of the copper–constantan (type-T) thermocouple whose physical properties differ greatly between the two constitutive wires. It was also shown that, in an air flow, the frequency response is almost uniform in the cross-section of the thermocouple wire, but, in a water flow, it becomes remarkably nonuniform. The present theoretical analysis provides useful information for improving the reliability of fluctuating temperature measurement in high-temperature turbulent flows.