Turbulent Temperature Research Articles

The characterization of turbulent heat and moisture transport during a gust-front event over the Indian peninsula

The ramifications of gust-front on atmospheric surface layer turbulence is a vexing issue, with nearly no information available over the Indian region where such events are not uncommon. Over the Indian peninsula, our recent field-experimental study has shown that a cold pool associated with the gust-front creates two distinct regimes in ASL turbulence, where temperature fluctuations display contrasting behavior. In this work, we use a multi-level observational dataset to evaluate the corresponding impacts on the moisture fluctuations and turbulent heat and moisture transport. We discover that the topology of the turbulent structures, which govern the temperature and moisture fluctuations, clearly exhibit a regime-wise distinction. In the first regime, the structures in temperature and moisture fluctuations are inclined in the vertical, while demonstrating a self-similarity in their time scales by being related through a power-law distribution. The inclination angle is inferred from the vertical shifts in the peak positions of the cross-correlation coefficients. However, in the second regime, the vertical inclination disappears for the temperature structures with hardly any change observed for the moisture. Moreover, the power-law exponents of the turbulent temperature time scales remain sensitive to the regimes, although no such effect is visible in the power-law character of the moisture time scales. Additionally, the dissimilarity in the heat and moisture transport is investigated through a novel polar-quadrant based approach that separates phases and amplitudes of the flux-transporting motions.

Plasma jet stream simulation for formation coating and powder manufacturing processes

To carry out a comparative analysis of the parameters of the plasma jet flow, the configurations of the plasmatrons that are most common in industrial practice have been selected. The values of temperature, velocity and intensity of turbulence at the characteristic point are determined depending on the shape of the components of the plasmatrons. Recommendations have been developed for the configuration of technological equipment used in the processes of coating and powder production.

The effect of static pressure-wind covariance on vertical carbon dioxide exchange at a windy subalpine forest site

Accounting for turbulent temperature and water vapor fluctuations on the vertical flux of CO2 measured by an open-path infrared gas analyzer are commonly known as the Webb–Pearman–Leuning (WPL) corrections. Static pressure fluctuations also affect air sample density, but the magnitude and effect of these changes on CO2 flux is not usually considered. In our study, the turbulent static pressure p and the vertical wind component w covariance w′p′¯ from just-above a Rocky Mountain subalpine forest are examined. The magnitude of w′p′¯ was highly correlated to the mean horizontal wind speed U with similar characteristics during the daytime and nighttime. The pressure term was calculated from w′p′¯ and compared to other terms in the equation for the vertical net ecosystem exchange of CO2 (NEE). We found the following: (1) for U≲6 m s−1, the pressure term was small, (2) as U increased beyond 6 m s−1, the pressure term became more and more important reaching a magnitude of ≈ 2 μmol m−2 s−1 at U of 12 m s−1 (for leaf area index LAI ≈ 2.6 m2 m−2), (3) for a more dense forest (LAI ≈ 4.8 m2 m−2), the magnitude of the pressure term at U of 12 m s−1 was ≈ 1 μmol m−2 s−1, or about half the open forest value, and (4) based on 14 years of measurements, the interannual mean and standard deviation of the yearly cumulative pressure and NEE terms were −32.8± 24.5 g C m−2 year−1 and −147.4± 231.7 g C m−2 year−1, respectively. Therefore, on average, including the pressure term in the NEE calculation reduces the annual carbon uptake by the GLEES forest from 147.4 g C m−2 year−1 to 114.6 g C m−2 year−1. This implies that carbon uptake by forests in windy locations that ignore the pressure term can be overestimated by as much as 20%. However, the year-to-year variability of both NEE and the pressure term was large due to changes in the forest structure and LAI from beetle attack and tree die-off. Finally, we present results from sensor-manipulation experiments examining the effect of tilting the quad-disk pressure ports on the turbulent static pressure.

Convection Theory and Relevant Problems in Stellar Structure, Evolution, and Pulsational Stability I Convection Theory and Structure of Convection Zone and Stellar Evolution

A non-local and time-dependent theory of convection was briefly described. This theory was used to calculate the structure of solar convection zones, the evolution of massive stars, lithium depletion in the atmosphere of the Sun and late-type dwarfs, and stellar oscillations (in Part Ⅱ). The results show that: 1) the theoretical turbulent velocity and temperature fields in the atmosphere and the thermal structure of the convective envelope of the Sun agree with the observations and inferences from helioseismic inversion very well. 2) The so-called semi-convection contradiction in the evolutionary calculations of massive stars was removed automatically, as predicted by us. The theoretical evolution tracks of massive stars run at higher luminosity and the main sequence band becomes noticeably wider in comparison with those calculated using the local mixing-length theory (MLT). This means that the evolutionary mass for a given luminosity was overestimated and the width of the main sequence band was underestimated by the local MLT, which may be part of the reason for the contradiction between the evolutionary and pulsational masses of Cepheid variables and the contradiction between theoretical and observed distributions of luminous stars in the H-R diagram. 3) The predicted lithium depletion, in general, agrees well with the observation of the Sun and Galactic open clusters of different ages. 4) Our theoretical results for non-adiabatic oscillations are in good agreement with the observed mode instability from classic variables of high-luminosity red giants. Almost all the instability strips of the classical pulsating variables (including the Cepheid, δ Scuti, γ Doradus, βCephei, and SPB strips) were reproduced (Part Ⅱ).

Feasibility study for a high-k temperature fluctuation diagnostic based on soft x-ray imaging

A new pseudolocal tomography algorithm is developed for soft X-ray(SXR) imaging measurements of the turbulent electron temperature fluctuations (δ Te) in tokamaks and stellarators. The algorithm overcomes the constraints of limited viewing ports on the vessel wall (viewing angle) and limited number of lines of sight (LOS). This is accomplished by increasing the number of LOS locally in a region of interest. Numerical modeling demonstrates that the wavenumber spectrum of the turbulence can be reliably reconstructed, with an acceptable number of viewing angles and LOS and suitable low SNR detectors. We conclude that a SXR imaging diagnostic for measurements of turbulent δ Te using a pseudolocal reconstruction algorithm is feasible.

Scaling patch analysis of turbulent planar plume

Proper scaling in turbulent planar plumes is investigated here using a scaling patch approach. Based on the scaled boundary conditions, a proper velocity scale for the mean axial flow is the plume centerline velocity Uref=Uctr, and a proper temperature scale for the temperature excess is Θref=Tctr−T∞, where Tctr is the plume centerline temperature and T∞ is the ambient fluid temperature. By seeking an admissible scaling, a key concept in the scaling patch approach, for the mean continuity, mean momentum, and mean energy equations, respectively, the following is found: (1) a proper scale for the mean transverse flow is Vref=(dδ/dx)Uctr, where dδ/dx is the growth rate of the plume width. (2) A proper scale for the Reynolds shear stress is Rvu,ref=UctrVref=(dδ/dx)Uctr2, a mix of the scales for the mean axial and transverse flows. (3) A proper scale for the turbulent heat flux is Rvθ,ref=VrefΘctr, a mix of the scales for the mean transverse flow and mean temperature excess. The mean transverse flow thus plays a critical role in the scaling of turbulent planar plumes. Approximate functions are developed for the scaled mean transverse flow, Reynolds shear stress, and turbulent temperature flux, and are found to agree favorably with experimental and numerical simulation data. The integral analysis of the mean momentum equation yields a Richardson number Ri, which remains invariant in the axial direction. The Richardson number is defined as Ri=defgβΘctrδt/(UctrVref)≈1/2, where g is the gravitational acceleration, β is the thermal expansion coefficient, and δt is the plume half-width based on the mean temperature profile. This Richardson number arises directly from the scaling patch analysis of the mean momentum equation, including both the streamwise and transverse velocity scales.

Modelling turbulent heat flux accounting for Turbulence-Radiation Interactions

The present work investigates the modeling of turbulent heat transfer in flows where radiative and convective heat transfer are coupled. In high temperature radiatively participating flows, radiation is the most relevant heat transfer mechanism and, due to its non-locality, it causes counter intuitive interactions with the turbulent temperature field. These so-called Turbulence-Radiation Interactions (TRI) largely affect the temperature field, modifying substantially the turbulent heat transfer. Therefore, in the context of modeling (RANS/LES), these interactions require a closure model. This work provides the inclusion of TRI in the modeling of the turbulent heat transfer by adopting a unique approach which consists in approximating the fluctuations of the radiative field with temperature fluctuations only. Based on this approximation, coefficients of proportionality are employed in order to close the unknown terms in the relevant model equations. A closed form of all radiation-temperature-velocity correlation is explicitly derived depending on the chosen turbulent heat transfer model. This model is applied to a standard two-equation turbulent heat transfer closure and used to reproduce results obtained with high-fidelity DNS simulations. While a standard approach (i.e., neglecting TRI) is not able to correctly predict the DNS data, the new model’s results shows exceptional agreement with the high-fidelity data. This clearly proves the validity (and the necessity) of the proposed model in non-reactive, radiative turbulent flows.

Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments

The formation and life-span of clouds as well as the associated unsteady processes concerning the micro-physics of the water phases they may contain are open questions in atmospheric physics.We here use three-dimensional direct numerical simulation to analyse the temporal evolution of a small portion of the top of a cloud. The Eulerian description of the turbulent velocity, temperature and vapor fields is combined with the Lagrangian description of two different ensembles of cloud droplets, that is, with a monodisperse and a polydisperse size distribution. A shear-free turbulent mixing layer is used to model the background air flow of the cloud top. This flow is considered appropriate because clouds cannot stand the presence of shear, which inevitably destroys them quickly. Luke-warm clouds are generally found at an altitude of 1000-2000 meters, live for a few hours or up to 1-2 days, continuously change shape, and have typical dimensions of some hundreds of meters. The global time-scale of these changes is recognized as being of the order of 100 seconds (Shaw (2003), Warhaft (2009)). From the formation phase to the dying out phase, clouds live under a continuous sequence of transients that are slightly different one from the other.In this study, we have tried to reduce the simplification level with respect to the real warm cloud situation as much as possible. We have included the same level of supersaturation of warm clouds, the same amount of liquid water content, and thus, the same numerical number of water droplets, and finally, a typical unstable perturbation of the density stratification and a typical kinetic energy cloud / clear air ratio (order of 10). We have considered an observation duration of the order of a few seconds (about 10 initial turnaround times). During this time, the kinetic energy decays throughout the system by 95%. It should be recalled that the kinetic energy inside the interfacial layer (the shear-free turbulent mixing layer that matches the cloud region to the ambient air region) also decays spatially, by nearly 85%. We observed, with respect to the cloud region, in the interfacial layer, a five times faster achievement of a common value of standard deviation for the probability density of both the monodisperse and poly-disperse populations. This acceleration of the dynamics is remarkable and is somewhat counterintuitive. It is closely correlated with the intermittency of the small scale of the air flow and of the supersaturation fluctuation. We give information on the size distribution of both the positive and negative droplet growth and on the drop size and the corresponding numerical concentration value of the distribution peak as time passes. Finally, we comment on the extension of the concept of the collision kernel for an unstable and inhomogeneous system in which turbulence decays faster than the time scales of the involved aqueous phases.

Effects of different simulation models on pyrolysis performance predictions for regenerative cooling applications

The one-dimensional plug flow reactor (PFR) model is commonly used in regenerative cooling simulations. However, the extreme operational conditions in regenerative cooling channels may affect the predictive accuracy of PFR, in which the effects of turbulent flow and heat transfer were not considered. In this study, we compared the chemical reaction predictions from PFR and Computational Fluid Dynamics (CFD) in regenerative cooling channels, and studied the effects of the main operating conditions on predictive accuracy, namely: pressure, flow rate, and heat flux. Experiments were performed to validate the CFD model and illustrate the predicative deviations in PFR under different operating conditions. Additional simulations with special operational conditions were also carried out to analyze the key factors. Results revealed that turbulence and radial temperature heterogeneity caused by turbulent flow and convective heat transfer influenced the pyrolysis predictions in different ways. The intensities of the two competitive effects changed with the operating conditions, which affect the accuracy of PFR model. Besides, the importance of heat transfer effects was emphasized in predicting the changing regularity of heat sink. This work is expected to provide novel understandings of reacting flow to engineering applications.

Characterization of temperature distributions in a swirled oxy-fuel coal combustor using tomographic absorption spectroscopy with fluctuation modelling

Oxy-fuel combustion promises efficient and inexpensive carbon dioxide sequestration and is therefore subject to active experimental research. However, the transfer of mature, non-intrusive diagnostic methods for temperature measurement like Coherent Anti-Stokes Raman Spectroscopy (CARS) from air-fed to oxy-fuel systems is challenging due to the deficiency in diatomic species suitable for thermometry. Although not limited to oxy-fuel atmospheres, we demonstrate the application of linear hyperspectral absorption tomography on water vapor in a swirled oxy-fuel coal combustor as a complementary diagnostic method, supplementing reference and validation data sets. Due to the burner design an axisymmetric reconstruction of the time-averaged temperature field is conducted. To compensate the temperature bias expected when evaluating time-averaged spectroscopic data we incorporate turbulent temperature fluctuations into our spectroscopic model, providing a fluctuation measure in addition to mean temperatures. The results quantitatively agree with vibrational O2-CARS measurements and qualitatively recreate spatial structures known from particle image velocimetry (PIV) flow fields.

New, improved analysis of correlation ECE data to accurately determine turbulent electron temperature spectra and magnitudes (invited).

Turbulent electron temperature fluctuation measurement using a correlation electron cyclotron emission (CECE) radiometer has become an important diagnostic for studying energy transport in fusion plasmas, and its use is widespread in tokamaks (DIII-D, ASDEX Upgrade, Alcator C-Mod, Tore Supra, EAST, TCV, HL-2A, etc.). The CECE diagnostic typically performs correlation analysis between two closely spaced (within the turbulent correlation length) ECE channels that are dominated by uncorrelated thermal noise emission. This allows electron temperature fluctuations embedded in the thermal noise to be revealed and fluctuation level and spectra determined. We have demonstrated a new, improved CECE coherency-based analysis for calculating the temperature fluctuation frequency spectrum and level, which has been verified both numerically through the simulation of synthetic ECE radiometer data and through analysis of experimental data from the CECE system on DIII-D. The new formulation places coherency-based analysis on a firm foundational footing and corrects some currently published methodologies. This new method accurately accounts for bias error in the coherence function and correctly calculates noise levels for a fixed data record length. It provides excellent accuracy in determining temperature fluctuation level (e.g., <10% error) even for a small realization number in the ensemble average. The method also has a smaller uncertainty (i.e., error bar) in the power spectrum when compared to the more standard cross-power method when evaluated at low coherency. Direct calculation of system noise level using correlation between randomized intermediate frequency signals is recommended.

Measurement and Comparison of Melt-Blowing Airflow Fields: Nozzle Modifications to Reduce Turbulence and Fibre Whipping.

In the melt-blowing process, micro/nanofibrous nonwovens are attenuated and formed through aerodynamic force in a turbulent airflow field. In this work, two types of airflow-directors were added under a common melt-blowing slot-die nozzle to obtain modified airflow fields. The effect of airflow-directors on time-averaged characteristics, turbulence intensity, and temperature fluctuation intensity are achieved through the simultaneous measurement of fluctuating velocity and fluctuating temperature using a two-wire probe hot-wire anemometer. Moreover, the influence of airflow-directors on fibre oscillations are also investigated through high-speed photography. The distribution of turbulence intensity and temperature fluctuation intensity reveals the characteristics of fluctuating airflow fields formed by different melt-blowing slot-die nozzles. Through the analyses of airflow characteristics and fibre oscillations, we can find that the arrangement of airflow-directors has a great impact on both turbulence distribution and fibre oscillation.

Numerical study on thermal performance of multirow helically coiled tube heat exchanger for surface water heat pump system

Owing to their good heat transfer advantage, multirow helically coiled tubes (MHCTs) have been widely used in industrial engineering, including surface water heat pump systems. To investigate the thermal performance of MHCTs at low surface-water velocities, a detailed numerical simulation is conducted based on a previous experimental system. The SST k–ω turbulence model and temperature dependent properties are used. The special feature in this study is that the heat convection inside and outside the tube is concerned instead of using constant heat flux or wall temperature. The results show that the variation characteristics of the Nusselt number and the outer wall temperature of the MHCT are consistent with the experimental results. Owing to the velocity attenuation of surface water flowing through the MHCT, the convective intensity in the negative area is lower than that in the positive area by more than 50%. When the natural convection is enhanced and the forced convection is weakened on the surface-water side, the outside Nusselt number decreases with the rows. Therefore, if the temperature of the surface water is not ideal or the velocity is small, the row number should not be extremely high. Moreover, the average inside Nusselt numbers corresponding to the average coil diameter are similar to the results of the MHCT. The effect of the flow direction of the medium on the thermal performance is insignificant and hence negligible.

Diapycnal mixing across the photic zone of the NE Atlantic

Abstract. Variable physical conditions such as vertical turbulent exchange, internal wave, and mesoscale eddy action affect the availability of light and nutrients for phytoplankton (unicellular algae) growth. It is hypothesized that changes in ocean temperature may affect ocean vertical density stratification, which may hamper vertical exchange. In order to quantify variations in physical conditions in the northeast Atlantic Ocean, we sampled a latitudinal transect along 17 ± 5∘ W between 30 and 63∘ N in summer. A shipborne conductivity–temperature–depth (CTD) instrumented package was used with a custom-made modification of the pump inlet to minimize detrimental effects of ship motions on its data. Thorpe-scale analysis was used to establish turbulence values for the upper 500 m from three to six profiles obtained in a short CTD yo-yo, 3 to 5 h after local sunrise. From south to north, average temperature decreased together with stratification while turbulence values weakly increased or remained constant. Vertical turbulent nutrient fluxes did not vary significantly with stratification and latitude. This apparent lack of correspondence between turbulent mixing and temperature is likely due to internal waves breaking (increased stratification can support more internal waves), acting as a potential feedback mechanism. As this feedback mechanism mediates potential physical environment changes in temperature, global surface ocean warming may not affect the vertical nutrient fluxes to a large degree. We urge modellers to test this deduction as it could imply that the future summer phytoplankton productivity in stratified oligotrophic waters would experience little alterations in nutrient input from deeper waters.

Seasonality and time scale dependence of the relationship between turbulent surface heat flux and SST

The present study examined the relationship between turbulent surface heat flux (SHF) and sea surface temperature (SST) variations using daily observational data. The SHF and SST relationship displays notable differences between winter and summer and prominent time-scale dependence in both seasons. In the mid-latitude SST frontal regions, SST has a larger role in driving SHF in winter than in summer. In the subtropical gyre regions, SHF plays a larger role in the SST change in summer than in winter. In winter, SHF has a larger effect on the SST change in the South China Sea than in the Arabian Sea and Bay of Bengal. In summer, the SST effect on SHF is dominant in the Arabian Sea, whereas the SHF impact on SST is dominant in the Philippine Sea. In the Gulf Stream, Kuroshio Extension and Agulhas Return Current, the SST effect extends up to 90-day time scales in winter, the SHF impact is limited to time scales below 20 days and the SST effect is dominant on time scales above 20 days in summer. In winter, the SHF effect extends up to 90-day time scales in the Bay of Bengal, South China Sea, and Philippine Sea, but is limited to time scales below 40 days in the Arabian Sea. In summer, the SST effect extends up to 90-day time scales in the Arabian Sea, whereas the SHF and SST effect is large on time scales shorter and longer than 40 days, respectively, in the Philippine Sea.

Temperature compensation design and experiment for a giant magnetostrictive actuator

Because the performance of giant magnetostrictive materials (GMMs) can vary at different temperatures, the positioning accuracy of a giant magnetostrictive actuator is affected by heat. In this work, a new simplified control strategy under compulsory water cooling is proposed to maintain a constant GMM temperature. Based on this strategy, a coupled turbulent flow field and temperature field finite element model is created for a GMM smart component. The model is simulated using COMSOL Multiphysics software version 5.3. Through simulations, the temperature field distribution of GMM smart components is analysed under different drive input currents and cooling water flow rates. Based on the obtained simulation results, a GMM intelligent component temperature control device is constructed. The experimental results are in good agreement with the simulation results; a thermostatic control effect is achieved in the thermostat of the giant magnetostrictive rod. Thus, the proposed temperature control strategy is proven effective via simulations and experiments.

Near-Ground Effects of Wind Turbines: Observations and Physical Mechanisms

AbstractWind turbines generate wakes, which can potentially influence the local microclimate near the ground. To verify and quantify such effects, the Vertical Enhanced Mixing (VERTEX) field campaign was conducted in late summer 2016 to measure near-surface turbulent fluxes, wind speed, temperature, and moisture under and outside of the wake of an operational wind turbine in Lewes, Delaware. We found that, in the presence of turbine wakes from a single wind turbine, friction velocity, turbulent kinetic energy, and wind speed were reduced near the ground under the wake, while turbulent heat flux was not significantly affected by the wake. The observed near-ground temperature changes were &lt;0.4°C in magnitude. Near-ground temperature changes due to the wake correlated well with the temperature lapse rate between hub height and the ground, with warming observed during stable and neutral conditions and cooling during unstable conditions. Of the two properties that define a wake (i.e., wind speed deficit and turbulence), the wind speed deficit dominates the surface response, while the wake turbulence remains aloft and hardly ever reaches the ground. We propose that the mechanism that drives changes in near-ground temperature in the presence of turbine wakes is the vertical convergence of turbulent heat flux below hub height. Above hub height, turbulence and turbulent heat flux are enhanced; near the ground, turbulence is reduced and turbulent heat flux is unchanged. These conditions cause an increase (during stable/neutral stability) or decrease (during unstable stability) in heat flux convergence, ultimately resulting in warming or cooling near the ground, respectively.

CFD analysis of fixed matrix with glass refractory particle regenerator

The following paper presents work on the Heat Recovery System (HRS) which have been used constructively in modern industries to recover heat from the flue gases. The HRS are analysed and outcomes of various factors such as its permeability, length, D/dp ratio, and porosity of the model are studied. Various CFD simulations are shown for the results of thermal trait of model. It is hence implied that for thermal and flow simulations the modelled geometry in fluent is to be used. The CFD analysis starts with heating cycle by allowing the flue gases to enter the regenerator at the higher temperature (200 ℃) through the inlet of the regenerator bed for a cycle time of 10 mins thus charging the glass particle bed. Once the heating cycle is completed then the cooling cycle starts with the entry of air at ambient temperature at 27 ℃ for the cycle time of 10 mins regenerator entering in the counter flow direction. The simulations deduce that with the aid of Fluent software, various contours of velocity, pressure, turbulent kinetic energy and temperature are studied for regenerators of fixed bed length. The thermal efficiency of heat recovery form the fixed matrix regenerator with glass particles and D/dp ratio of 10 is calculated as 15%. This 15% increase in the thermal efficiency will improve the overall efficiency of the plant where these type of regenerators are used by 5–8% and decrease the total required heat input by 10–15%.

On Termodynamics of Kolmogorov’s Scaling in Turbulence

The model balance equation of turbulence is represented in the form of the first law of thermodynamics. The forms for entropy are derived with an integrating factor. For the Kolmogorov–Obukhov law, the entropy has the same expression as for an ideal gas. A possible formula for the turbulence temperature is proposed on the basis of hydrodynamical instability processes.

Effect of Turbulence on the Wavefront of an Ultrahigh Intensity Laser Beam

Abstract Ultrahigh intensity lasers face thermal management issues that limit their repetition rates. The key challenge is to efficiently evacuate the heat deposited in the amplifier by the optical pumping without impacting the output laser beam quality. The amplifier can have a multislab geometry where the laser beam crosses successive amplifying slabs and the cooling channels that separate them. This work investigates numerically how a cryogenic cooling of the amplifier by turbulent channel flows may affect the wavefront of the laser beam. To this end, large eddy simulations (LESs) representative of the amplifier cooling are performed using TrioCFD, a code developed by the CEA. First, validation simulations are carried out for heated channel flows, allowing comparisons to direct numerical simulation (DNS) results from the literature. Then, LESs of an open turbulent channel flow cooling two slabs are conducted using conjugated heat transfer between the solid and the fluid. The phase distortions, mean and fluctuations, induced by the inhomogeneous and turbulent temperature field are computed directly from the LES. A moderate although non-negligible effect of the turbulence on the laser wavefront was found. This optical effect increases when the slab heating increases. A comparison to the Sutton model, widely used in aero-optic studies, was performed, and its applicability was found limited for this problem. For the first time, TrioCFD is used to address the question of the beam impact of the cooling of laser amplifiers, and it has proven to be a valuable tool for such application.