Bulk Pressure Research Articles

Computation of bulk viscous pressure with observational constraints via scalar field in the General relativity and [formula omitted] gravity

The present article deals with the isotropic cosmological model of f(Q) gravity filled with bulk viscous fluid, where Q is the non-metricity term and it is responsible for the gravitational interaction. Aside from the tachyon and quintessence scalar fields, the modified Einstein’s field equations have been resolved through the application of the power law form of the expansion. In this model, the Markov chain Monte Carlo (MCMC) analysis method has been utilized to obtained the best-fit value of the model parameter and it confirms that the model satisfies the recent observational data. We have also examined the EoS parameter for bulk viscosity in these cosmological contexts and it has been determined that ωeff will be located in the phantom region. The correspondence between bulk pressure and the reconstructed ωrec,Q in f(Q) gravity has also been addressed. In the presence of holographic Ricci dark energy, the reconstructed f(Q) gravity yields a transition from the quintessence era into phantom era.

Thermal and hydrodynamic analysis inside a channel partially filled with porous material

AbstractIn this work, the transfer of heat and changes in fluid pressure within a rectangular channel fairly packed with porous material have been studied numerically for various Darcy numbers and dimensionless porous layers. The model was run with the following assumptions: laminar flow, forced convection, isotropic porous material, local thermodynamic equilibrium, constant wall temperature boundary condition, and no thermal dissipation. The study covers a broad range for the dimensionless porous layer, 0 ≤ hr < 1, and the Darcy number, 10−4 < Da < 10−2. The fully developed and developing flow over the channel is investigated in numerical study. It was observed that the inertia effect may be disregarded when Da < 10−4. The local dimensionless bulk temperature distribution, pressure drop, and velocity profiles were all shown to be impacted by the Darcy number and dimensionless porous layers, according to the numerical analysis results. The maximum heat transfer rate was attained when the ratio of the porous layer inside the channel was 0.8, and the pressure gradient was the highest. Partial packing of the channel with a porous material has two advantages: it increases the rate heat transmission rate and results in a much smaller pressure drop than a filled porous medium.

Stenosis to stented: decrease in flow disturbances following stent implantation of a diseased arteriovenous fistula.

The arteriovenous fistula (AVF) is commonly faced with stenosis at the juxta-anastomotic (JXA) region of the vein. Implantation of a flexible nitinol stent across the stenosed JXA has led to the retention of functioning AVFs leading to the resulting AVF geometry being distinctly altered, thereby affecting the haemodynamic environment within it. In this study, large eddy simulations of the flow field within a patient-specific AVF geometry before and after stent implantation were conducted to detail the change in flow features. Although the diseased AVF had much lower flow rates, adverse flow features, such as recirculation zones and swirling flow at the anastomosis, and jet flow at the stenosis site were present. Larger velocity fluctuations (leading to higher turbulent kinetic energy) stemming from these flow features were apparent in the diseased AVF compared to the stented AVF. The unsteadiness at the stenosis created large regions of wall shear stress (WSS) fluctuations downstream of the stenosis site that were not as apparent in the stented AVF geometry. The larger pressure drop across the diseased vein, compared to the stented vein, was primarily caused by the constriction at the stenosis, potentially causing the lower flow rate. Furthermore, the WSS fluctuations in the diseased AVF could lead to further disease progression downstream of the stenosis. The change in bulk flow unsteadiness, pressure drop, and WSS behaviour confirms that the haemodynamic environment of the diseased AVF has substantially improved following the flexible stent implantation.

Application of supervised learning algorithms for temperature prediction in nucleate flow boiling

This work investigates the use of supervised learning algorithms to predict temperatures in an experimental test bench, which was initially designed for studying nucleate boiling phenomena with ethylene glycol/water mixtures. The proposed predictive model consists of three stages of machine learning. In the first one, a supervised algorithm block is employed to determine whether the critical heat flux (CHF) will be reached within the test bench limits. This classification relies on input parameters including bulk temperature, tilt angle, pressure, and inlet velocity. Once the CHF condition is established, another machine learning algorithm predicts the specific heat flux at which CHF will occur. Subsequently, based on the classification generated by the first block, the evolution of temperature in response to increases in heat flux is predicted using either the previously estimated heat flux or the physical limits of the experimental facility as the stopping criterion. To accomplish all these predictions, the study compares the performance of various algorithms including artificial neural networks, random forest, support vector machine, AdaBoost, and XGBoost. These algorithms were specifically trained using cross-validation and grid search methods to optimize their effectiveness. Results for the CHF classification purpose demonstrate that the support vector machine algorithm performs the best, achieving an F1-score of 0.872 on the testing dataset, while the boosting methods (AdaBoost and XGBoost) exhibit signs of overfitting. In predicting the CHF value, the artificial neural network achieved the lower nMAE on the testing dataset (6.18%). Finally, the validation of the temperature forecasting models, trained on a dataset composed of 314,476 samples, reveals similar performances across all methods, with R2 values greater than 0.95.

Abstract 12955: Shear-Dependent Changes in Blood Viscosity Negatively Affect Energetic Efficiency in Patient-Specific Models of the Fontan Circulation

Introduction: Since blood flow through the lungs is passive in patients with the Fontan circulation, cardiac output is sensitive to power loss through the pulmonary vasculature. Prior small in vitro studies of how changes in blood viscosity at venous shear rates (“non-Newtonian behavior”) affect power loss have yielded conflicting results. The purpose of this study was to evaluate the extent to which non-Newtonian behavior contributes to power loss in a large number of pediatric patient-specific models of the Fontan circulation. Hypothesis: Shear-dependent changes in blood viscosity cause increased power loss in the low-shear Fontan circulation. Methods: Pulmonary vascular geometry was segmented from clinical cardiac MRI images. Pressure and velocity fields were then calculated using computational fluid dynamic simulations. Cardiac index was set at 2 L/min/m2 to simulate a low cardiac output state. A constant viscosity model (“Newtonian”) was compared to a shear-dependent viscosity model (“non-Newtonian”). Power loss was calculated by simplified power loss (using bulk flow and pressure changes) and viscous dissipation rate (using element-wise shear rates and viscosities). Both metrics were indexed to adjust for cardiac output and body surface area. Wilcoxon signed-rank test was used to compare differences between the two viscosity models. Results: Twenty patients (mean 10.9 years, 35% female) were included. Mean viscosity was nearly 10-fold higher in the non-Newtonian vs. Newtonian simulations (mean non-Newtonian to Newtonian viscosity ratio 9.7, SD 0.6). Accordingly, power loss by both metrics was significantly larger for the non-Newtonian simulations ( Figure ). Conclusions: Shear-dependent increases in blood viscosity cause increased power loss in computational models of the Fontan circulation under low cardiac output conditions. Further studies are needed to evaluate the degree to which this correlates with clinical outcomes.

Synergy principle and its application of endwall loss analyses in the turbine stator

The endwall flow significantly impacts the turbine performance behavior, and it is necessary to investigate the development of endwall secondary vortices and relevant losses. Inspired by the field synergy principle, the synergy between the velocity and the pressure gradient established by the three-dimensional mechanical energy conservation equation is innovatively applied to the endwall loss analysis in the stator of an axial-inflow turbine. In terms of the synergy equation, the loss is not only related to the viscous dissipation, but also the included angle (or the synergy angle) between the velocity vector and the pressure gradient vector. The physical content of the synergy angle suggests that the larger synergy angle is (i.e., the worse synergy), the higher losses should be. This conclusion has been validated by present numerical results, and an apparent positive correlation between the synergy angle and the losses could be perceived under time-averaged and transient conditions. The worse synergy could be observed at the passage rear part and the wake, where complex passage vortices and local separation exist. In these regions, the local velocity vector is not aligned with the bulk pressure gradient of the mainstream. Hence, the synergy angle has a marked rise, corresponding to the local high losses.

Numerical study of the steady core-annular flow in a focusing geometry in the presence of soluble surfactants

The steady state core annular flow of two immiscible fluids in the presence of soluble surfactants is investigated numerically in an axisymmetric tubular or flow focusing geometry. The two immiscible fluids form an interface onto which surfactants can be adsorbed/desorbed from the annular fluid where they are dissolved. The finite element method is employed to discretize the governing equations using the spine method to create the computational mesh, follow the shape of the interface and the axisymmetric wall and capture the flow domain. An unknown pressure distribution is imposed along the interface that quantifies the pressure discontinuity, while allowing the bulk pressure to remain continuous in each phase. GMRES is employed to accommodate the nearly singular nature of the jacobian matrix. The bulk surfactant concentration varies only inside a thin boundary layer of thickness δ∼Pe−1, which justifies omission of the advection-diffusion equation in the annulus bulk. Simulations reveal that the onset of Marangoni stresses tends to decelerate the velocity at the interface which is seen to translate towards the wall/axis of symmetry when the more viscous fluid is the inner/outer fluid, respectively. Finally, simulations in flow-focusing nozzles capture the development of spatially growing instabilities in the downstream region of the interface, with wavelength on the order of the nozzle radius, especially when the outer fluid is more viscous or when the surfactant concentration increases. Raising the volumetric flow rate in the annular region suppresses growth of waves due to interfacial friction. Spatial stability analysis verifies downstream growth of waves and recovers the relevant wavelength. Possible repercussions on the onset of jetting instabilities in devices with liquid focusing are discussed.

Statistical ensembles and logarithmic corrections to black hole entropy

In this paper, we consider general statistical ensembles and compute logarithmic corrections to the microcanonical entropy resulting due to thermodynamic fluctuations which are controlled by the boundary conditions, i.e. due to choice of ensemble. The framework is applied to the case of non-extremal black holes to give certain logarithmic corrections to the Bekenstein–Hawking entropy. We argue that within the framework of black hole chemistry, where the cosmological constant is identified with bulk pressure, the isoenthalpic-isobaric entropy rather than microcanonical entropy carries a more natural and consistent thermodynamic interpretation as black hole entropy. Logarithmic corrections to both microcanonical and isoenthalpic-isobaric entropies of black holes are computed, and we show that the latter set of corrections in black hole chemistry are of the same form as corrections to the microcanonical entropy in theories where the cosmological constant is not interpreted as a thermodynamic pressure. Finally, we compute logarithmic corrections to entropy in the framework of holographic black hole chemistry. We emphasize upon the choice of statistical ensemble, both in the bulk and on the boundary, in order to have a consistent comparison between them. The corrections studied in this paper are distinct from those obtained from Euclidean quantum gravity.

Influence of thermal conductivity on transient mixed convection in a vented cavity with a hollow cylinder and filled with CNT-water nanofluid

This study is aimed to perform a numerical time-dependent investigation thermal conductivity effect of the annular cylinder within a vented cavity using CNT based-water nanofluid. For demonstrating the effect of thermal conductivity, four distinct hollow cylinder materials such as Ks = 0.5(Plastic tiles), Ks = 0.84(Clay tiles), Ks = 1.1(Concrete tiles), and Ks = 2(Slate tiles) are introduced together with a suitable variation of dimensionless time (0 ≤ τ ≤ 1). The governing equations of the model with associated boundary conditions is solved using finite element based Galerkin's weighted residual method. Different contour plots for thermal and flow field transformation and mean Nusselt number, mean fluid temperature, bulk convective field temperature, temperature gradient, pressure gradient, vortices, and fluid velocity magnitude are presented for qualitative and quantitative thermal performance analysis. With the decrease of solid thermal conductivity, 27.3% thermal transport enhancement is noted from the heated surface of the cylinder. However, a 16.3% increase in the bulk fluid temperature has been recorded with the increase in cylinder conductivity. The numerical outcomes from this investigation propose a better thermo-fluid efficiency compared to the existing methodology which can be suggestive to engineers and researchers for designing heat exchangers, heat pipes, and other thermal systems.

A limitation to determine heat transfer of water at supercritical pressure: The repeatability issue

• Published data indicates non-reproducibility of the heat transfer in the pseudo-critical region. • Non-reproducibility appears at conditions for which DHT is possible. • Experimental results at the Technical University of Munich confirm this hypothesis. • Origin of the non-reproducibility may be linked to variations of the properties close to the pseudo-critical points. As the heat transfer is based on physical processes which are expected to be deterministic, a major anticipation is that the experimental data should show good repeatability, if not reproducibility. In other words, it is expected that repetition of experiments with very similar inputs (bulk pressure and enthalpy, mass flow rate, heat flux …), should result in consistent and very similar outputs (heat transfer coefficient or inside wall temperature). However, comparable experiments with supercritical water in the literature, although rather scarce, as well as the experimental work conducted at the Chair of Energy Systems at the Technical University of Munich tend to show that repeatable or reproducible data are difficult to obtain. This is the case particularly within the pseudo-critical region where DHT is likely to occur. This may impair the ability to assess accurately the overheating of the piping in critical cases, and therefore the safety of the concerned plants. This paper aims to focus on this problem from an experimental perspective, trying to identify both the magnitude of the phenomenon as well as at which parameters non-reproducibility occurs. Ultimately, the authors try to answer the following question: Is it possible to reach reproducibility?

Free and enclosed inertial active gas.

In this work, the free expansion of an inertial active gas in three dimensions made of spherical non-interactive active Brownian particles with both translational and rotational inertia (IABPs) is studied. After elucidating the active particles' orientational correlation in three dimensions by employing a Fokker-Planck formalism, the diffusion, mean-square speed, persistence length, reorientation time, Swim and Reynolds pressures and total pressure of this system, are obtained theoretically and corroborated by performing Langevin dynamics simulations. Afterwards, a numerical study on particles' distribution and the mechanical pressure exerted by the active gas enclosed in a cubic box and its dependence on inertia is also carried out. This experiment highlights two important observations: first, as inertia in the system grows while fixing activity, a more uniform particle distribution within the box is achieved. In other words, the classical accumulation of active particles at the walls is seen to be suppressed by inertia. Second, an active gas with translational and rotational inertiae and made of spherical particles still has a state equation which is offered here. This is supported by the fact that both the mechanical pressure definition and the bulk pressure definition as the trace of the swim and Reynolds stress tensors, coincide in the thermodynamic limit.

Bulk modulus of CaO under high pressure up to 65 GPa

In the present study we used some equilibrium experimental data reported by Mammon et al. (Geophysical Research Letters, Vol. 8, No. 2, pp. 140-142) to investigate the effect of high pressure up to 65 GPa on the bulk modulus of calcium oxide (CaO) material. We used the Vinet’s equation of state (EOS) model. The fit of the bulk modulus as a function of pressure p obeys the 2nd order polynomial expression: B = 115.37 + 3.84 p - 7.25 x 10-3 p2 (where both B and p are expressed in GPa). Our results are analyzed and compared with other theoretical data of the literature. Similar behavior for the bulk modulus versus pressure was observed for some other materials with different crystallographic structures from the literature, which commonly increases with increasing pressure. We estimate also the Debye temperature θD of our material of interest using the experiential lattice parameter and elastic constants measured at normal conditions by Speziale et al. (Journal of Geophysical Research, Vol. 111, (2006), pp. B02203 (12 pages)). Our obtained value (670.1 K) is in good agreement with other data of the literature.

On the contact conditions for the density and charge profiles in the theory of electrical double layer: From planar to spherical and cylindrical geometry

In this paper, starting from the Bogoliubov-Born–Green–Yvon equations of the liquid-state theory, we formulate two equivalent approaches for the calculation of the total density profile and of the charge density profile of ionic fluids near nonplanar charged surfaces. In the framework of these approaches, we establish exact conditions, that a particular point of these profiles should satisfy, in the form of contact theorems. These contact theorems for the total density profile and the charge density profile are obtained by direct integration of a system of equations derived from the Bogoliubov-Born–Green–Yvon equations. The contact theorems for both profiles have nonlocal character. It is shown that the contact value of the total density profile for uncharged surfaces is characterized by the bulk pressure and the surface tension. The contact theorems are applied to the cases of spherical and cylindrical surfaces. It is shown that the contact theorem for the total density profile coincides with the recent results obtained by W. Silvester-Alcantara, D. Henderson and L.B. Bhuiyan (Mol. Phys., 113, 3403, 2015).

Nonextensive hydrodynamics of boost-invariant plasmas

We use quasiparticle anisotropic hydrodynamics to study the non-conformal and non-extensive dynamics of a system undergoing boost-invariant Bjorken expansion. To introduce nonextensivity, we use an underlying Tsallis distribution with a time-dependent nonextensivity parameter q. By taking moments of the quasiparticle Boltzmann equation in the relaxation-time approximation, we obtain dynamical equations which allow us to determine the time evolution of all microscopic parameters including q. We compare numerical solutions for bulk observables obtained using the nonextensive evolution with results obtained using quasiparticle anisotropic hydrodynamics with a Boltzmann distribution function (q rightarrow 1). We show that the evolution of the temperature, pressure ratio, and scaled energy density, are quite insensitive to which distribution function is assumed. However, we find significant differences in the early-time evolution of the bulk pressure which are observed for even small deviations from the Boltzmann distribution function. Finally, we discuss the existence of non-conformal hydrodynamic attractors for the longitudinal and transverse pressures, the bulk and shear viscous corrections, and the nonextensivity parameter q.

Topology Optimization Design of Scramjet Structures With Forced Convective Heat Transfer on Unstructured Meshes

Abstract A topology optimization strategy with coupled fluid–solid interaction was proposed to maximize the cooling efficiency of a kind of structure applied for scramjets. The Galerkin finite element method (FEM) is used to solve the forced convective heat transfer, and the rational approximation of material properties (RAMP) method combined with the globally convergent method of moving asymptotes (GCMMA) method are used to solve the topological optimization models with different boundary conditions and objective functions. Examples are provided to demonstrate the validity and effectiveness of the optimization strategy. The optimal flow passages of scramjet structures are achieved successfully. Compared with a baseline structure with rectangular straight passages, the optimized flow passages significantly reduce the averaged bulk temperature and pressure loss, and the bulk temperature is more uniform to avoid the occurrence of concentrated high-temperature areas. With the Reynolds number changing from 1000 to 1750, the heat transfer performance of the three-dimensional topology-optimized structure increases by 16.79% to 20.82%.

Heat transfer analysis of a pulse-heated microwire in CO2 at supercritical pressures

This paper analyzes the characteristics of convective heat transfer of a pulse-heated platinum microwire cooling in CO2 under supercritical pressures based on experimental data. The microwire undergoes a rapid temperature rise of around 664 K within 0.35 ms. An inverse problem is formulated and numerically solved to extract heat transfer data from experimental measurements. In addition, a predictive model for the convective heat transfer coefficient is developed to fully close the equation set. Results are interpreted based on the bulk pressure from 7.38 to 9 MPa and bulk temperature from 295 to 325 K. The convective heat flux of CO2 generally decreases with time, and in the medium-term, the reduction is slightly decelerated owing to buoyancy-driven flows. This demonstrates that high-pressure and low-temperature bulk states generally exert larger convective heat flux to cool the microwire. During the early 10 ms, the time-averaged convective heat flux is of the order of 1 MW/m2, resulting in rapid cooling. This value shows a weak critical enhancement upon crossing the Widom line. During the remaining time, the time-averaged convective heat flux drops to the order of 0.1 MW/m2. Such a drop in heat flux is more obvious in low-bulk-density cases, leading to a relatively long time for sufficient W cooling.

Automated generation of 0D and 1D reduced-order models of patient-specific blood flow.

Three-dimensional (3D) cardiovascular fluid dynamics simulations typically require hours to days of computing time on a high-performance computing cluster. One-dimensional (1D) and lumped-parameter zero-dimensional (0D) models show great promise for accurately predicting blood bulk flow and pressure waveforms with only a fraction of the cost. They can also accelerate uncertainty quantification, optimization, and design parameterization studies. Despite several prior studies generating 1D and 0D models and comparing them to 3D solutions, these were typically limited to either 1D or 0D and a singular category of vascular anatomies. This work proposes a fully automated and openly available framework to generate and simulate 1D and 0D models from 3D patient-specific geometries, automatically detecting vessel junctions and stenosis segments. Our only input is the 3D geometry; we do not use any prior knowledge from 3D simulations. All computational tools presented in this work are implemented in the open-source software platform SimVascular. We demonstrate the reduced-order approximation quality against rigid-wall 3D solutions in a comprehensive comparison with N=72 publicly available models from various anatomies, vessel types, and disease conditions. Relative average approximation errors of flows and pressures typically ranged from 1% to 10% for both 1D and 0D models, measured at the outlets of terminal vessel branches. In general, 0D model errors were only slightly higher than 1D model errors despite requiring only a third of the 1D runtime. Automatically generated ROMs can significantly speed up model development and shift the computational load from high-performance machines to personal computers.

Intermolecular forces at ice and water interfaces: Premelting, surface freezing, and regelation.

Using Lifshitz theory, we assess the role of van der Waals forces at interfaces of ice and water. The results are combined with measured structural forces from computer simulations to develop a quantitative model of the surface free energy of premelting films. This input is employed within the framework of wetting theory and allows us to predict qualitatively the behavior of quasi-liquid layer thickness as a function of ambient conditions. Our results emphasize the significance of vapor pressure. The ice-vapor interface is shown to exhibit only incomplete premelting, but the situation can shift to a state of complete surface melting above water saturation. The results obtained serve also to assess the role of subsurface freezing at the water-vapor interface, and we show that intermolecular forces favor subsurface ice nucleation only in conditions of water undersaturation. We show that ice regelation at ambient pressure may be explained as a process of capillary freezing, without the need to invoke the action of bulk pressure melting. Our results for van der Waals forces are exploited in order to gauge dispersion interactions in empirical point charge models of water.

Force density functional theory in- and out-of-equilibrium.

When a fluid is subject to an external field, as is the case near an interface or under spatial confinement, then the density becomes spatially inhomogeneous. Although the one-body density provides much useful information, a higher level of resolution is provided by the two-body correlations. These give a statistical description of the internal microstructure of the fluid and enable calculation of the average interparticle force, which plays an essential role in determining both the equilibrium and dynamic properties of interacting fluids. We present a theoretical framework for the description of inhomogeneous (classical) many-body systems, based explicitly on the two-body correlation functions. By consideration of local Noether-invariance against spatial distortion of the system we demonstrate the fundamental status of the Yvon-Born-Green (YBG) equationas a local force-balance within the fluid. Using the inhomogeneous Ornstein-Zernike equationwe show that the two-body correlations are density functionals and, thus, that the average interparticle force entering the YBG equationis also a functional of the one-body density. The force-based theory we develop provides an alternative to standard density functional theory for the study of inhomogeneous systems both in- and out-of-equilibrium. We compare force-based density profiles to the results of the standard potential-based (dynamical) density functional theory. In-equilibrium, we confirm both analytically and numerically that the standard approach yields profiles that are consistent with the compressibility pressure, whereas the force-density functional gives profiles consistent with the virial pressure. For both approaches we explicitly prove the hard-wall contact theorem that connects the value of the density profile at the hard-wall with the bulk pressure. The structure of the theory offers deep insights into the nature of correlation in dense and inhomogeneous systems.

Variance of the Casimir force in an ideal Bose gas

We consider an ideal Bose gas enclosed in a d-dimensional slab of thickness D. Using the grand canonical ensemble we calculate the variance of the thermal Casimir force acting on the slab’s walls. The variance evaluated per unit wall area is shown to decay like Δvar/D for large D. The amplitude Δvar is a non-universal function of two scaling variables λ/ξ and D/ξ, where λ is the thermal de Broglie wavelength and ξ is the bulk correlation length. It can be expressed via the bulk pressure, the Casimir force per unit wall area, and its derivative with respect to chemical potential. For thermodynamic states corresponding to the presence of the Bose–Einstein condensate the amplitude Δvar retains its non-universal character while the ratio of the mean standard deviation and the Casimir force takes the scaling form , where L is the linear size of the wall.