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Статті в журналах з теми "Pseudo-density operator"
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XU, YE-JUN, JUN SONG, HONG-CHUN YUAN, HONG-YI FAN, and QIU-YU LIU. "QUANTIZATION SCHEME FOR FERMIONIC SYSTEM AND s-ORDERED OPERATOR EXPANSION FORMULA OF FERMIONIC DENSITY OPERATORS." Modern Physics Letters A 26, no. 11 (April 10, 2011): 833–42. http://dx.doi.org/10.1142/s0217732311035213.
Повний текст джерелаАнотація:
We introduce the generalized fermionic Wigner operator with an s parameter. Based on its remarkable properties, we establish one-to-one mapping between fermion operators and their s-parametrized pseudo-classical correspondence, which may involve fermionic Weyl pseudo-classical correspondence, P-representation and Q-representation in a unified way. Furthermore, starting with the projector of the fermionic coherent state, we obtain the s-ordered operator expansion formula of fermionic density operators, which includes normally ordered, antinormally ordered and Weyl ordered product of operators for different values of s. Applications in calculating some Fermi operators' s-ordered expansions are presented.
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Liu, Junfeng, and Litan Yan. "On a nonlinear stochastic pseudo-differential equation driven by fractional noise." Stochastics and Dynamics 18, no. 01 (November 6, 2017): 1850002. http://dx.doi.org/10.1142/s0219493718500028.
Повний текст джерелаАнотація:
In this paper, we study the existence, uniqueness and Hölder regularity of the solution to a class of nonlinear stochastic pseudo-differential equation of the following form [Formula: see text] where [Formula: see text] is a pseudo-differential operator with negative definite symbol of variable order which generates a stable-like process with transition density, the coefficient [Formula: see text] is a measurable function, and [Formula: see text] is a double-parameter fractional noise. In addition, the existence and Gaussian type estimates for the density of the mild solution are proved via Malliavin calculus.
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Hanyga, Andrzej, and Richard L. Magin. "A new anisotropic fractional model of diffusion suitable for applications of diffusion tensor imaging in biological tissues." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 470, no. 2170 (October 8, 2014): 20140319. http://dx.doi.org/10.1098/rspa.2014.0319.
Повний текст джерелаАнотація:
An anomalous anisotropic diffusion equation is constructed in which the order of the spatial pseudo-differential operator is generalized to be distributed with a directionally dependent distribution. A time fractional version of this equation is also considered. First, it is proved that the equation is positivity-preserving and properly normalized. Second, the existence of a smooth Green's function solution is proved. Finally, an expression for the diffusive flux density for this new fractional order process is calculated. This approach may find utility in modelling diffusion tensor imaging data in the white matter of the human brain where both the apparent diffusion coefficient and the order of the pseudo-differential operator are anisotropic.
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Atallah-Baraket, Amel, and Maryem Trabelsi. "Analysis of the energy decay of a viscoelasticity type equation." Analele Universitatii "Ovidius" Constanta - Seria Matematica 24, no. 3 (November 1, 2016): 21–45. http://dx.doi.org/10.1515/auom-2016-0046.
Повний текст джерелаАнотація:
AbstractIn this paper, we study the evolution of the energy density of a sequence of solutions of a problem related to a viscoelasticity model where the viscosity term is a pseudo-differential operator of order 2α with α ∈ (0, 1). We calculate the weak limit of the energy density in terms of microlocal defect measures and under special assumption we prove that the viscosity term prevents propagation of concentration and oscillation effects contrary to what happens in the wave equation.
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Allen, Blake J., Edward R. Mansell, David C. Dowell, and Wiebke Deierling. "Assimilation of Pseudo-GLM Data Using the Ensemble Kalman Filter." Monthly Weather Review 144, no. 9 (September 2016): 3465–86. http://dx.doi.org/10.1175/mwr-d-16-0117.1.
Повний текст джерелаАнотація:
Total lightning observations that will be available from the GOES-R Geostationary Lightning Mapper (GLM) have the potential to be useful in the initialization of convection-resolving numerical weather models, particularly in areas where other types of convective-scale observations are sparse or nonexistent. This study used the ensemble Kalman filter (EnKF) to assimilate real-data pseudo-GLM flash extent density (FED) observations at convection-resolving scale for a nonsevere multicell storm case (6 June 2000) and a tornadic supercell case (8 May 2003). For each case, pseudo-GLM FED observations were generated from ground-based lightning mapping array data with a spacing approximately equal to the nadir pixel width of the GLM, and tests were done to examine different FED observation operators and the utility of temporally averaging observations to smooth rapid variations in flash rates. The best results were obtained when assimilating 1-min temporal resolution data using any of three observation operators that utilized graupel mass or graupel volume. Each of these three observation operators performed well for both the weak, disorganized convection of the multicell case and the much more intense convection of the supercell case. An observation operator using the noninductive charging rate performed poorly compared to the graupel mass and graupel volume operators, a result that appears likely to be due to the inability of the noninductive charging rate to account for advection of space charge after charge separation occurs.
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Mamalyha, Kh V., and M. M. Osypchuk. "On single-layer potentials for a class of pseudo-differential equations related to linear transformations of a symmetric $\alpha$-stable stochastic process." Carpathian Mathematical Publications 11, no. 2 (December 31, 2019): 350–60. http://dx.doi.org/10.15330/cmp.11.2.350-360.
Повний текст джерелаАнотація:
In this article an arbitrary invertible linear transformations of a symmetric $\alpha$-stable stochastic process in $d$-dimensional Euclidean space $\mathbb{R}^d$ are investigated. The result of such transformation is a Markov process in $\mathbb{R}^d$ whose generator is the pseudo-differential operator defined by its symbol $(-(Q\xi,\xi)^{\alpha/2})_{\xi\in\mathbb{R}^d}$ with some symmetric positive definite $d\times d$-matrix $Q$ and fixed exponent $\alpha\in(1,2)$. The transition probability density of this process is the fundamental solution of some parabolic pseudo-differential equation. The notion of a single-layer potential for that equation is introduced and its properties are investigated. In particular, an operator is constructed whose role in our consideration is analogous to that the gradient in the classical theory. An analogy to the classical theorem on the jump of the co-normal derivative of the single-layer potential is proved. This result can be applied for solving some boundary-value problems for the parabolic pseudo-differential equations under consideration. For $\alpha=2$, the process under consideration is a linear transformation of Brownian motion, and all the investigated properties of the single-layer potential are well known.
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Mamalyha, Kh V., and M. M. Osypchuk. "Properties of single layer potentials for a pseudo- differential equation related to a linear transformation of a rotationally invariant stable stochastic process." Matematychni Studii 55, no. 1 (March 4, 2021): 94–106. http://dx.doi.org/10.30970/ms.55.1.94-106.
Повний текст джерелаАнотація:
This article is aimed at determining existence conditions of single layer potentials for pseudo-differential equations related to some linear transformations of a rotationally invariant stable stochastic process in a multidimensional Euclidean space and investigating their properties as well. The carrier surface of the potential is smooth enough. In this article, we consider two main cases: the first, when this surface is bounded and closed; the second, when it is unbounded, but could be presented by an explicit equation in some coordinate system. The density of this potential is a continuous function. It is bounded with respect to the spatial variable and, probably, has an integrable singularity with respect to the time variable at zero. Classic properties of this potential, including a jump theorem of the action result of some operator (an analog of the co-normal differential) at its surface points, considered.
A rotationally invariant $\alpha$-stable stochastic process in $\mathbb{R}^d$ is a L\'{e}vy process with the characte\-ristic function of its value in the moment of time $t>0$ defined by the expression $\exp\{-tc|\xi|^\alpha\}$, $\xi\in\mathbb{R}^d$, where $\alpha\in(0,2]$, $c>0$ are some constants. If $\alpha=2$ and $c=1/2$, we get Brownian motion and classic theory of potential. There are many different results in this case. The situation of $\alpha\in(1,2)$ is considered in this paper. We study constant and invertible linear transformations of the rotationally invariant $\alpha$-stable stochastic process. The related pseudo-differential equation is the parabolic equation of the order $\alpha$ of the ``heat'' type in which the operator with respect to the spatial variable is the process generator. The single layer potential is constructed in the same way as the single layer potential for the heat equation in the classical theory of potentials. That is, we use the fundamental solution of the equation, which is the transition probability density of the related process. In our theory, the role of the gradient operator is performed by some vector pseudo-differential operator of the order $\alpha-1$. We have already studied the following main properties of the single layer potentials: the single layer potential is a solution of the relating equation outside of the carrier surface and the jump theorem is held. These properties can be useful to solving initial boundary value problems for the considered equations.
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Wang, Yuwei, Liangguo Dong, Yuzhu Liu, and Jizhong Yang. "2D frequency-domain elastic full-waveform inversion using the block-diagonal pseudo-Hessian approximation." GEOPHYSICS 81, no. 5 (September 2016): R247—R259. http://dx.doi.org/10.1190/geo2015-0678.1.
Повний текст джерелаАнотація:
Elastic full-waveform inversion (EFWI) of multicomponent seismic data is a powerful tool for estimating the subsurface elastic parameters with high accuracy. However, the trade-offs between multiple parameters increase the nonlinearity of EFWI. Although the conventional diagonal-approximate Hessian matrix describes the illumination and limited bandwidth effects, it ignores the trade-off effects and decreases the convergence rate of EFWI. We have developed a block-diagonal pseudo-Hessian operator for 2D frequency-domain EFWI to take into account the approximate trade-offs among the P-wave (compressional-wave) velocity, S-wave (shear-wave) velocity, and density without extra computational costs on forward simulations. The Hessian matrix tends toward a block-diagonal matrix as the frequency grows to infinity; thus, the proposed block-diagonal pseudo-Hessian matrix is more accurate at higher frequencies. The inverse of the block-diagonal pseudo-Hessian matrix is used as a preconditioner for the nonlinear conjugate-gradient method to simultaneously reconstruct P- and S-wave velocities and density. This approach effectively mitigates the crosstalk artifacts by correcting the gradients from the trade-off effects and produces more rapid inversion convergence, which becomes more significant at higher frequencies. Synthetic experiments on an inclusion model and the elastic Marmousi2 model demonstrate its feasibility and validity in EFWI.
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Murguía-Romero, Gabriela, Angel Sánchez, and Ricardo Zavaleta-Madrid. "Electromagnetic Properties in Multilayer Graphene within the Ritus Formalism: Transverse Electrical Conductivity." Advances in Science and Technology 98 (October 2016): 125–30. http://dx.doi.org/10.4028/www.scientific.net/ast.98.125.
Повний текст джерелаАнотація:
Within the framework of the Quantum Field Theory, we discuss how to study electromagnetic properties of a multilayer graphene sample in the presence of electric and magnetic fields, both perpendicular to the graphene planes. We deal with the multilayer system by taking into account the quantum mechanical supersymmetric property of the monolayer Hamiltonian. We solve the Dirac equation for the graphene charge carriers by using the Ritus formalism. This formalism consists in the diagonalization of the operator with and the Dirac gamma matrices which contain information about the pseudo-spin. We calculate the charge carrier propagator for the monolayer case, and we obtain the photon polarization operator, the leading quantum correction to the classical Lagrangian density that encodes the electromagnetic properties of the system through the constitutive equations. With the quantum supersymmetrical properties of both, the monolayer and the multilayer graphene Hamiltonians, it is possible to extend our results to obtain the charge carrier propagator for the multilayer case.
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Lee, Hyun Geun. "Numerical Simulation of Pattern Formation on Surfaces Using an Efficient Linear Second-Order Method." Symmetry 11, no. 8 (August 5, 2019): 1010. http://dx.doi.org/10.3390/sym11081010.
Повний текст джерелаАнотація:
We present an efficient linear second-order method for a Swift–Hohenberg (SH) type of a partial differential equation having quadratic-cubic nonlinearity on surfaces to simulate pattern formation on surfaces numerically. The equation is symmetric under a change of sign of the density field if there is no quadratic nonlinearity. We introduce a narrow band neighborhood of a surface and extend the equation on the surface to the narrow band domain. By applying a pseudo-Neumann boundary condition through the closest point, the Laplace–Beltrami operator can be replaced by the standard Laplacian operator. The equation on the narrow band domain is split into one linear and two nonlinear subequations, where the nonlinear subequations are independent of spatial derivatives and thus are ordinary differential equations and have closed-form solutions. Therefore, we only solve the linear subequation on the narrow band domain using the Crank–Nicolson method. Numerical experiments on various surfaces are given verifying the accuracy and efficiency of the proposed method.
Дисертації з теми "Pseudo-density operator"
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Hughes, David J. J. "The integrated density of states for periodic elliptic pseudo-differential operators in dimension one." Thesis, University of Sussex, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.410366.
Повний текст джерелаАнотація:
In this thesis, we study an elliptic, one dimensional, pseudo-differential operator, with homogeneous symbol, which is perturbed by a lower order, symmetric, pseudo-differential operator, whose symbol is periodic, magnified by a real coupling constant. The main goal is to prove that such an operator generates a complete asymptotic expansion of the integrated density of states for large energies and an arbitrary large coupling constant. The Floquet theory, which may be viewed as the foundation of the study of periodic operators, is rigorously developed for pseudo-differential operators in arbitrary dimension. We prove the existence, through the use of a developed calculus, of a "Gauge transformation" which is a unitary operator, transforming the original operator into an operator whose symbol, up to some controllable perturbation and remainder, has constant coefficients. This operator with constant coefficients is shown to admit a complete asymptotic expansion. Finally, we show, by the use of a suitable perturbation argument, that these asymptotics coincide with that of the original operator
Тези доповідей конференцій з теми "Pseudo-density operator"
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Farshad, M., and H. Chauris. "Sparsity-Promoting Pseudo-Inverse Born Operator in the Presence of Density Variations: An Efficient MultiParameter Imaging Tool." In 82nd EAGE Annual Conference & Exhibition. European Association of Geoscientists & Engineers, 2020. http://dx.doi.org/10.3997/2214-4609.202011366.
Повний текст джерела
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Saha, Sankhajit, Prajit Chakrabarti, Johannes Vossen, Sourav Mitra, and Tuhin Podder. "Proactive Decision-Making Through Real-Time Geomechanical Support Leveraging Drilling a Long Horizontal Section Through a Tight Unconventional Reservoir of an Oil and Gas Field: Middle East." In SPE Middle East Oil & Gas Show and Conference. SPE, 2021. http://dx.doi.org/10.2118/204776-ms.
Повний текст джерелаАнотація:
Abstract This paper discusses the Integrated Role of Geomechanics and Drilling Fluids Design for drilling a well oriented towards the minimum horizontal stress direction in a depleted, yet highly stressed and complex clastic reservoir. There are multiple challenges related to such a well that need to be addressed during the planning phase. In this case, the well needs to be drilled towards the minimum horizontal stress direction (Shmin) to benefit multi-stage hydraulic fracturing. At the same time, the most prominent challenge is that this well orientation is more prone to wellbore failure and requires a maximum mud weight, due to the present strike slip stress environment. Well planning challenges in such an environment include (a) the determination of formation characteristics and rock properties, (b) the anticipation of higher formation collapse pressure during the course of drilling the lateral section within the reservoir, (c) the determination of the upper bound mud weight to prevent lost circulation due to a low fracture gradient against depleted sections, or due to the presence of pre-existing natural fractures, d) mitigating the higher risk of differential sticking against depleted porous layers, and determining appropriate bridging in the drilling fluids, (e) recognizing the prolonged exposure time of the formation due to the length of the lateral and the lower rate of penetration against the tight highly dense formations. For successful drilling, and to mitigate the above risks, the first step is to prepare a predrill GeoMechanical model along with adequate fluid design and drillers action plans to be considered during drilling. Offset well petrophysical logs and core data are considered for the preparation of the predrill GeoMechanical model, along with the drilling experiences in the offset locations. Based on the above, a predrill GeoMechanical model is prepared, a risk matrix is being established, and a representative mud weight window is recommended (Wellbore Stability Analysis). In most cases, the offset well locations considered are vertical- or inclined-, or lateral wells of different trajectory azimuth than the target well location and the predrill GeoMechanical model can incorporate such variations easily; however, any Geology uncertainty, leading to a different rock property- and stress set-up (or even different pore pressure than expected), at the actual well location will be part of the uncertainty of the predrill GeoMechanical model and Wellbore Stability Analysis. This is where the real time monitoring is playing out its full potential: giving an updated model and wellbore stability analysis during drilling. While drilling the lateral section, the wellbore condition is being monitored using LWD (logging while drilling) tools, e.g. Gamma Ray, Density, Neutron, Acoustic Caliper, Azimuthal density image and ECD (equivalent circulating density). While gamma ray helps in determining the lithology, density logs help to understand the formation hardness, and they can be used to generate a calibrated pseudo acoustic log. Based on this pseudo acoustic log, the rock strength and other rock mechanical properties of the pre- GeoMechanical model can be updated as soon as they become available. This gives insight into the model differences and helps to understand model variations and adjust Wellbore Stability recommendations accordingly. While the neutron log helps to determine the zones of high porosity, and thus potential risk zones for differential sticking, the azimuthal density image clearly indicates the breakout zones caused by the shear failure of the wellbore. The presence of wellbore failure (breakout) is further confirmed by acoustic caliper data, and accordingly wellbore stability related recommendations are communicated to the operator, for an increase in the specific gravity of the mud, and thus, to balance the wellbore. From a mud rheology perspective, high performance OBM (oil-based mud) parameters are maintained consistent with the formation properties, to minimize fluid loss, optimize wellbore strengthening characteristics and minimize at the same time solids concentrations in order to avoid excessive ECD (equivalent circulating density) which may open pre-existing natural fractures resulting in downhole losses and in consequence might lead to differential sticking. In the case study presented herein, the proactive implementation of GeoMechanics and its Wellbore Stability application as well as the integration of drilling fluids services, resulted in the smooth and successful drilling of the lateral section, and also in the delivery of an in gauge hole necessary for multi-stage fracturing (MSF) completion optimization.
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Luo, Zhen, Eing Yee Yeoh, Xiaosong Chen, Linsen Li, and Mian Xing. "Fuel Management Strategy of LBE-Cooled Fast Reactor BLESS-D." In 2022 29th International Conference on Nuclear Engineering. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/icone29-90443.
Повний текст джерелаАнотація:
Abstract As one of the six nuclear reactor candidates selected by the Generation IV International Forum (GIF), lead-cooled fast reactor (LFR) has become one of the most promising concepts and attracted more attention from the institutes. A lead-bismuth eutectic (LBE) cooled fast reactor called BLESS-D has been proposed by State Power Investment Corporation Research Institute of China. The fuel management is critical to reactor design because it affects reactor economics. Although the research on fuel management of PWR has matured, there are few studies on fuel management for LFR. Fuel management design includes the entire process from initial cycle to transition cycle, and to equilibrium cycle. To obtain a better refueling scheme and optimize the key parameters of equilibrium cycle, a new refueling scheme is first proposed, and then traditionally, the simulation covering the entire refueling operation is required cycle by cycle, resulting in consuming a massive computing resource. If the scheme fails to meet expectations, expenditure will multiply until the modified scheme meets the safety design criteria. Fuel management design using “pseudo-equilibrium cycle” instead of traditional method is carried out in this work. The “pseudo-equilibrium cycle” method can directly solve the core key parameters of equilibrium cycle by replacing fuel model with approximate nuclides densities estimated from initial core arrangement and refueling scheme. In this paper, a two-batch refueling scheme is proposed with “pseudo-equilibrium cycle” method and then transition cycle are designed to ensure the feasibility that the fresh core successfully transition to the “pseudo-equilibrium cycle” state. Afterwards, neutronics parameters are solved for each cycle from the fresh core and results show that when the burnup calculation comes to the 5th cycle, the reactor key parameters, including assembly peaking factor, linear power density, delayed neutron fraction, and prompt neutron lifetime are in good agreement with “pseudo-equilibrium cycle”, which proves that the “pseudo-equilibrium cycle” method can be used accurately and efficiently to design the refueling scheme.
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Xu, Yuting, Zhifang Wu, and Qiang Wang. "A Study on Radiation Imaging Mechanism and Characteristics in Different Inspection Systems." In 2021 28th International Conference on Nuclear Engineering. American Society of Mechanical Engineers, 2021. http://dx.doi.org/10.1115/icone28-66127.
Повний текст джерелаАнотація:
Abstract Radiation imaging, as a key issue in nuclear technology, has received considerable attention in the industry. It is widely used in nuclear medicine, Customs supervision, and many other areas. The objective of this investigation is to get insight into the principle, operation characteristics and image characteristics of radiation imaging. In this paper, an investigation on radiation imaging is conducted on three main inspection systems for Customs supervision, including small X-ray inspection machine, CT baggage inspection system, and large container inspection system. The principle, operation characteristics, evaluation indexes, pseudo-color processing and image characteristics are discussed in detail. The results indicate that the spatial resolution of small X-ray inspection machine is much higher than that of CT baggage/goods inspection system and large container/vehicle inspection system. It is a challenge to identify substances and specific shapes in the case of overlapping for small X-ray inspection system. Moreover, the mechanism of X-ray images is discussed as well. The radiation images are divided into three types, including two-dimensional, pseudo-color, high spatial resolution; two-dimensional, gray, high spatial resolution; three-dimensional, pseudo-color, high density resolution. The further investigation on machine inspection images is suggested to focus on the application environment. For some objects with specific characteristics, such as amorphous, explosive, the CT baggage inspection has much better performance than other systems. The research in this paper reveals the mechanism, parametric effect and imaging characteristics. It could provide a necessary foundation for the follow-up intelligent processing, detection, identification and annotation for radiation imaging in nuclear area. The research on inspection devices could lend strong experience to medical treatment, industry and many other fields.
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Yang, Dong, Zhi Shen, Xin Nie, Wanyu Liu, Fengjun Wang, and Ying Huang. "Hydraulic Resistance of Subcritical and Supercritical Water Flowing in a Rifled Tube." In ASME 2016 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/imece2016-65597.
Повний текст джерелаАнотація:
Large capacity supercritical boiler is at the leading edge of efficiency boost for thermal power plant. Water wall design is a key issue for a supercritical boiler. To ensure successful design and safe operation of water wall, studying hydraulic resistance of water is significant. Considerable work on frictional pressure drop of gas-liquid two-phase flow in tubes has been done and various correlations have been proposed to predict it. However, these correlations are restricted to particular rib geometries and flow conditions. Because of significant variations in thermo physical properties near the critical and pseudo-critical points, pressure drop at supercritical pressures is different from that at subcritical pressures. However, limited studies have been devoted to estimate hydraulic resistance of supercritical water. More work need be conducted to develop prediction method for pressure drop at supercritical pressures. Therefore, to accumulate fundamental experimental data for the design of a supercritical boiler, an experiment on hydraulic resistance of water was performed in a vertical upward rifled tube. The experiment was carried out in the high-temperature and high-pressure steam-water test loop at Xi’an Jiaotong University. Based on the experimental data, the two-phase frictional multiplier was calculated to analyze the two-phase frictional pressure drop. At low to moderate vapor quality, the two-phase frictional multiplier increases rapidly and reaches a peak. When the vapor quality exceeds a certain value, the two-phase frictional multiplier starts to decrease with increasing vapor quality. It is because the tube wall is covered by liquid film at low to moderate vapor quality. Within the high vapor quality region, the high-speed vapor tears the liquid film and the flow pattern turns to mist flow with lower frictional pressure drop. Increasing pressure decreases the two-phase frictional multiplier and when the pressure approaches the critical pressure, the multiplier is close to 1. The effect of mass flux on the multiplier is so weak that it can be neglected. At supercritical pressures, the pressure drops due to frictional resistance and flow acceleration both increase with bulk fluid enthalpy. Increasing pressure decreases the frictional pressure drop. This result is mainly attributed to pressure approaching the critical point. Frictional pressure drop is significantly affected by fluid property variations; in particular, severe density decreases with increasing bulk fluid enthalpy. Acceleration pressure drop increases with decreasing pressure and increasing heat flux. When heat flux increases, the density difference between the inlet and the outlet increases with the same mass flux, which results in a considerable acceleration pressure drop. Decreasing pressure results in a similar acceleration pressure drop variation because of the same reason. The frictional resistance coefficient was calculated to analyze the supercritical frictional pressure drop. In the large specific heat region, the frictional resistance coefficient peaks at a certain enthalpy in the vicinity of the pseudo-critical point, and increasing mass flux reduces the magnitude of the peak value.
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Amalfi, Raffaele L., Cong H. Hoang, Ryan Enright, Filippo Cataldo, Jackson B. Marcinichen, and John R. Thome. "Operational Map and Thermal Performance of a Thermosyphon Cooling System for Compact Servers." In ASME 2021 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems. American Society of Mechanical Engineers, 2021. http://dx.doi.org/10.1115/ipack2021-72612.
Повний текст джерелаАнотація:
Abstract This paper advances the state-of-the-art in novel passive two-phase systems for more efficient cooling of datacenters and telecom central offices compared to the traditional air-based cooling solutions (e.g. aisle-based containment systems). The proposed passive two-phase technology uses numerous server-level thermosyphons to dissipate the heat generated by critical components, such as central processing units, accelerators, etc., with the flexibility of selecting the rack-level and room-level cooling elements depending on the deployment scenarios. The main goal of this paper is to experimentally investigate the thermal performance and maximum heat removal capability of a server-level thermosyphon for cooling compact servers. The experimental apparatus, built at Nokia Bell Labs, incorporates a single 7-cm high liquid-cooled thermosyphon that fits within a 2U server (smaller form factors can be achieved by a proper design that would further reduce the thermosyphon’s height). The heat source is represented by a pseudo-chip, composed of six parallel cartridge heaters installed in a copper block that incorporates local temperature measurements and is able of dissipating a total power of ≈ 500 W over a footprint area of 3.5 cm × 3.5 cm (corresponding heat flux of ≈ 41 W/cm2). Steady-state experiments were carried out at various heat loads up to 240 W (corresponding heat flux of ≈ 20 W/cm2), filling ratios and secondary side inlet conditions (coolant temperatures and mass flow rates), using R1234ze(E) and deionized water as the working fluids on the primary and secondary side, respectively. Test results demonstrate high heat transfer performance of the server-level thermosyphon over a wide range of conditions, and operating points are identified and classified into an operational map. Thermosyphon-based cooling systems across multiple length scales can significantly improve operation in terms of lowering energy consumption, allowing for higher hardware density, increased processing speed and reliability.
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Collison, Michael J., Peter X. L. Harley, and Domenico di Cugno. "Experimental and Numerical Investigation of Transition Effects on a Low Reynolds Number Airfoil." In ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/gt2017-63294.
Повний текст джерелаАнотація:
Low speed, small scale turbomachinery operates at low Reynolds number with transition phenomena occurring. In small consumer product applications, high efficiency and low noise are key performance metrics. Transition behaviour will partly determine the state of the boundary layer at the trailing edge; whether it is laminar, turbulent or separated impacts aerodynamic and acoustic performance. This study aimed to evaluate a commercially available CFD transition model on a low Reynolds number Eppler E387 airfoil and identify whether it was able to correctly model the boundary layer transition, and at what expense. CFD was carried out utilising the ANSYS Shear Stress Transport (SST) k-ω γ-Reθ transition model. The CFD progressed from 2D in Fluent v150, through to single cell thickness 3D (pseudo 2D) in CFX v172. An Eppler E387 low Reynolds number airfoil, for which experimental data was readily available from literature at Re = 200,000 was used as the validation case for the CFD, with results computed at numerous incidence angles and mesh densities. Additionally, experimental surface oil flow visualisation was undertaken in a wind tunnel using a scaled E387 airfoil for the zero incidence case at Re = 50,000. The flow visualisation exhibited the expected key features of transition in the breakdown of the boundary layer from laminar to turbulent, and was used as a validation case for the CFD transition model. The comparison between the results from the CFD transition model and the experimental data from literature suggested varying levels of agreement based on the mesh density and CFD solver in the starting location of the laminar separation bubble, with higher disparity for the position of the reattachment point. Whether 2D or 3D, the prediction accuracy was seen to worsen at high incidence angles. Finally, the location of the laminar separation bubble between CFD and oil flow visualisation had good agreement and a set of guidelines on the mesh parameters which can be applied to low Reynolds number turbomachinery simulations was determined.
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Voigt, Torsten, Peter Habisreuther, and Nikolaos Zarzalis. "Simulation of Vorticity Driven Flame Instability Using a Flame Surface Density Approach Including Markstein Number Effects." In ASME Turbo Expo 2009: Power for Land, Sea, and Air. ASMEDC, 2009. http://dx.doi.org/10.1115/gt2009-59331.
Повний текст джерелаАнотація:
The combustion induced and vorticity driven instability behaviour of a lean flame within a premixed combustion system under typical gas turbine conditions is investigated numerically. Over a wide range of operation conditions, the flame is stabilised by the well known mechanisms of premixed confined combustion systems interacting with a turbulent swirled flow field. However, the behaviour of the flame changes drastically just as a critical equivalence ratio is reached: the flame moves upstream close to the rotational axis at positions of originally highest downstream velocities. This flashback behaviour is known as combustion induced vortex breakdown (CIVB) [1] since the upstream propagation results from a local breakdown of the swirled flow field caused by the heat release. A Reynolds-Stress turbulence approach is used as closure model for the three dimensional uRANS simulations to capture the highly turbulent and exceedingly anisotropic flow field. In addition, a flame surface density (FSD) formulation is utilised for combustion modelling. Hence, the essential process of flame vortex interaction is considered in particular. This process is expressed by the important effects of flame strain and curvature and is directly taken into account by means of solving the Favre averaged area of flame surface with the aid of a transport equation. The considered stretch effects do not only influence the production and destruction of the flame surface but moreover modify the local heat release by altering the pseudo-chemical parameter of flame speed. This effect is captured through the Markstein number that is determined by preliminary flamelet calculations. The numerical results lead to the conclusion that the strained and re-orientated vorticity filaments are mainly responsible for the formation of locally negative velocities close to the rotational axis. The re-orientation and straining of vortex filaments is in turn controlled by the local heat release. Finally, the overall occurrence of the simulated flashback is in good accordance with the experiments but does also demonstrate the importance of strain effects on global flame behaviour.