Fluid Wall Research Articles

A model for upstream signalling in the control of capillary blood flow

Blood supply to tissues is controlled through vasoconstriction and vasodilation activated by components of the vascular wall and smooth muscle cells. Local oxygen saturation and wall shear stress play an important role in this process and wall shear stress has a role in capillary and venular flow as well as flow in arterioles and arteries. Local capillary networks can dilate to increase flow but there is little clear evidence of how local capillary networks act to increase arteriolar blood flow when needed. Pulsatile flow plays an important part in signalling wall shear stress within the vascular system and it has recently been shown that pulse pressure changes can be efficiently transmitted along soft walled fluid filled tubes such as blood vessels. This confirms that pulse pressure within the system can maintain downstream flow; it also raises the question of whether pulse pressure changes can signal upstream from local capillary beds to the supplying arteriole.

Effects of the launch parameters on trans-phase stability performance and mechanism for submarine-launched missiles model

The impact of missile head shape, launch angle, and Fr number on the stability of trans-phase missiles was investigated, providing both experimental and theoretical support for designing diverse ballistic trans-phase weapons and aircraft. Through water-to-air trans-phase experiments and numerical calculations of missiles, deflection angle, velocity, and trajectory are obtained, while statistical and flow-field analyses reveal associated stability mechanisms. The research results indicate a good agreement between the experimental and numerical results. As the Fr number increases for the same head shape, the variations in transient deflection angle decrease. The deflection angle shows notable variations for different head shapes under the same Fr number. Specifically, the flat head demonstrates the highest deflection angle, followed by the 120° conical head, the 90° conical head, and finally the round head shape. Different head shapes lead to varying water film deformations and turbulent kinetic energy dissipations, resulting in differences in shedding vortex intensity and frequency. Launch angles cause variable component wall shear forces and fluid resistances, while changing the Fr number affects initial kinetic energy and energy dissipation, leading to missile model stability differences.

A new insight into wormhole propagation prediction in carbonate matrix acidizing based on experimental approaches

Carbonate acidizing is a common practice for production/injection enhancement in damaged oil/gas wells by creating conductive flow tunnels called wormholes. Previous linear acid flood studies are mostly performed on core plugs limited to the Darcy scale with a strong dependency on lab results for upscaling techniques. In this study, a novel scale-free approach of wormhole propagation inside linear porous media for small to very large core diameters is introduced. The model associates the wormhole tip velocity and wall fluid loss to the injection rate using novel flow correlations in terms of core diameter and wormhole geometries (wormhole density, length and diameter). These correlations are constructed based on the results obtained from a large number of 3D-FEM (Finite Element Method) simulations of water flow utilizing the porous rock with predefined dominant cylindrical wormholes. The application of the model is easy and quick without complex or time-consuming calculations even for large rock diameters up to 28 in (71 cm) which simultaneously predicts the acid response curve by matching with the acid coreflood experiments on specific acid/rock systems. Also, the output matching parameters for each acid/rock system can be directly used for larger core diameters without extra-large scale acid flood experiments. In the experimental parts of this study, acid flooding is performed on various rock types of four Iranian carbonate reservoir layers to find their corresponding acid response curves and optimum flow conditions required to design stimulation programs. Besides, the collected data bank could help to enrich the data pool required to validate the developed model practically. The proposed model is validated by literature model results and also experimental data obtained in our laboratory and those presented in the literature and the results showed that good agreement is obtained in regression analysis with R2 = 0.953, MSE = 0.046452, RMSE = 0.215528 and MAPE = 24.11357.

Phase distribution in nanochannels of supercritical fluid with different fluid–wall interactions

Supercritical fluids (SFs) are classically regarded as single-phase fluids without bubbles or interfaces, but a recent study shows nanobubbles in SFs under unconfined conditions. The objective of this paper is to explore the phase distribution under confined conditions. Molecular dynamics simulations are performed for supercritical argon. Two walls containing the SF have equal fluid–wall interactions with equal and unequal wall temperatures. An external force is applied on the top wall to control the pressure at 1.5Pc, in which Pc is the critical pressure. Periodic boundary conditions are applied on the four side surfaces of the simulation box. The study indicates that the bulk fluid density is not only dependent on pressure and temperature, but also on fluid–wall interactions, this result deviates from the classical theory, where density depends on only pressure and temperature. For strong fluid–wall interactions, three- or five-layer structures are found, including liquid-like (LL) layers on the walls and two-phase-like (TPL) and gas-like (GL) layers (depending on bulk density) in the channel core. For weak fluid–wall interactions, the phase distribution becomes GL on the wall, and TPL and LL (depending on bulk density) in the channel core, which is inverse to those of strong fluid–wall interactions. Correspondingly, the phase distributions for strong and weak fluid–wall interactions can be analogous to annular or Leidenfrost patterns at subcritical pressures, respectively. The density profile is symmetric against the channel centerline at equal wall temperatures, but symmetry-breaking may exist when applying different wall temperatures. This work provides a phase-distribution link between subcritical and supercritical pressures, which is useful for the design and analysis of SF systems.

A platform for modular assembly and feeding of micro-organoids on standard Petri dishes.

Organoids grow in vitro to reproduce structures and functions of corresponding organs in vivo. As diffusion delivers nutrients over only ∼200 µm, refreshing flows through organoids are required to avoid necrosis at their cores; achieving this is a central challenge in the field. Our general aim is to develop a platform for culturing micro-organoids fed by appropriate flows that is accessible to bioscientists. As organs develop from layers of several cell types, our strategy is to seed different cells in thin modules (i.e. extra-cellular matrices in stronger scaffolds) in standard Petri dishes, stack modules in the required order, and overlay an immiscible fluorocarbon (FC40) to prevent evaporation. As FC40 is denser than medium, one might expect medium to float on FC40, but interfacial forces can be stronger than buoyancy ones; then, stacks remain attached to the bottom of dishes. After manually pipetting medium into the base of stacks, refreshing upward flows occur automatically (without the need for external pumps), driven mainly by differences in hydrostatic pressure. Proof-of-concept experiments show that such flows support clonal growth of human embryonic kidney cells at expected rates, even though cells may lie hundreds of microns away from surrounding fluid walls of the two immiscible liquids.

Effect of hot leg length on the steady-state performance of single-phase natural circulation based tube-in-tube thermosyphon heat transport device with heat sink directly above the heat source

Thermosyphon heat transport device (THTD) is a compact natural circulation loop to enhance the heat transport distance and the power rating compared to existing devices like heat pipe. Hence it finds application in indoor solar cooktops and diverse passive fuel cooling systems to eliminate Fukushima type accidents in advanced nuclear power plants. THTDs for the nuclear application have the heat sink directly above the heat source in the same vertical leg. Practically no study is reported, even for simple rectangular loops with heat sink directly above the heat source in the same vertical leg. All reported studies for rectangular loops with vertical heater vertical cooler configuration are for the heat source in the vertical up leg and heat sink in the other vertical down leg. The main issue with a heat sink located directly above the heat source in the same vertical leg is that it leads to instability with recirculation loops (bidirectional flow in the cooler with the heavy coolant coming down along the cooled wall and light fluid rising along the adiabatic wall) in the cooler. Earlier reported studies indicated that at higher powers, the instability is suppressed. In this paper, another method of eliminating this instability was experimentally studied, which involved increasing the hot leg length and, thereby the elevation difference. The main finding is that the instability can be avoided if the hot leg length (equivalent to increasing the height) is increased. The same finding may hold good even for the rectangular loops with heat sink directly above the heat source in the same vertical leg. Besides, the study also revealed that the same one dimensional steady-state dimensionless flow equation applicable to simple rectangular loops is found to be valid for the tube-in-tube type THTD considered in the present work.

Anicteric gall bladder rupture secondary to necrotising cholecystitis in a dog

AbstractA 4‐year‐old, female cocker spaniel was presented for investigation of acute onset of vomiting and lethargy. Pyrexia, abdominal discomfort and marked abdominal distension were detected during clinical examination. Clinicopathological findings revealed mild neutrophilia with band neutrophils. Biochemistry showed a mild hyperalbuminemia (40 g/L; reference: 23–35) and mild elevation of alanine aminotransferase (165 U/L; reference: 0–110). A computed tomography scan of the abdomen raised suspicion of gall bladder wall rupture secondary to necrotising cholecystitis, associated with marked peritoneal effusion and cholangitis with duodenitis. An exploratory laparotomy and cholecystectomy were performed, with intraoperative abdominal lavage. Histopathology revealed a necrotic gall bladder wall, and analysis and culture of the abdominal free fluid confirmed the presence of a septic exudate with growth of Escherichia coli. The dog was treated with potentiated amoxicillin and clinical signs resolved. Cases presenting with biliary tract rupture could occur without suggestive serum biochemistry findings.

Preliminary design and modeling of mini channels to enhance heat transfer in a millimetric catalytic combustor

Mini channel solution is used in devices that require a high density of transmitted thermal power as a very large-scale integration design in computer systems and compact exchangers. Furthermore, the mini channels are extensively investigated in the literature for turbulent and laminar regimes. In this project, different configurations of mini channels have been studied to enhance heat transfer, using simulations with a commercial multi-physics code. Thanks to the results of the models, more promising configurations with 3D printing technique may be built. The project challenge is improving convective thermal power extracted by the exhaust gases of a mini-catalytic combustor. The combustor feeds six modules for thermoelectric power production (TEMs). As the first step, three different mini channel geometries have been chosen; the first one with 19 channels with rectangular cross-section, the second one with 6 channels with a convergent profile, and the latter with 2 channels with a fractal branching geometry. Simulations started from studying fluid dynamic to investigate the velocity field at the exit of the mini channels. The analysis has been extended by adding the conjugate heat exchange between fluid and combustor wall. The results show an increase in heat exchange compared to the base case for all configurations, with a maximum value for the 19 mini channels configuration.

Optimal duct length of ducted fans for minimizing first blade-passing frequency noise

Fans generate tone noise and broadband noise. The first blade-passing frequency noise is a tone noise and is the most dominant; attaching a duct and a stator can reduce this noise. However, there has been no theoretical development in using structural characteristics to reduce the noise generated by ducted fans. In this study, noise predictions using Lighthill's equation and Green's function were performed to determine the optimal duct length for a ducted fan's noise reduction. New formulations for predicting the length were derived using a modal noise analysis. Noise sources were obtained from computational fluid dynamics data and wall pressure spectra. The formulations deduce a linear relationship between a duct's radius and length and can be used as a tool to indicate fan size at the design stage under space and weight constraints without high computational costs.

Optimised Electro-Osmotic Flow in Rectangular Microchannels with Smoothed Corners

Electro-osmotic flows are a means of circulating polar fluids through microchannels without resorting to mechanical pumping. The lack of moving parts, of noise and the ease of integration in silicon chips make them an interesting option for microchip cooling and miniaturized total analysis systems. This paper describes an optimization in terms of first- and second-law analysis of the cross-section of a microchannel subject to electro-osmotic flow. Starting from rectangular cross-sections of different aspect ratios, its corners are progressively smoothed and the resulting Poiseuille and Nusselt numbers computed. Performance evaluation criteria are then used to assess the change in, among others, heat transfer rate, temperature difference between wall and bulk fluid, and equivalent pumping power. The entropy number is also computed and the results commented. It is found that the latter criterion highlights a configuration of minimum entropy generation, whereas the trends of the heat transfer and temperature difference are opposite to that of the equivalent pumping power.

The research of vasoconstriction effect on the solitary waves in stenotic arterial vessel

Vascular stenosis is a common cardiovascular disease, and the in-depth study of its biomechanical mechanism will help to explore the occurrence mechanism and law of the disease, which is of great significance in the prevention and diagnosis of cardiovascular disease. Different from previous studies, radial and axial motions are considered in the realistic configuration of the wall, and the wall equation of the stenotic artery is established. On the basis of the wall equation and fluid equation, the KdV-Burgers equation is obtained by scale analysis and perturbation expansion. The effects of axial displacement and wall initial conditions on the propagation of solitary waves in stenotic arteries are discussed. It is shown that with the increase of the axial and radial tension ratios, the amplitude and width of the solitary wave increase, and the solitary wave becomes steeper and more sharp. The results of this study provide a theoretical value for detecting the shape change of solitary wave in blood vessel to predict vascular stenosis.

Reinforcement learning of control strategies for reducing skin friction drag in a fully developed turbulent channel flow

Reinforcement learning is applied to the development of control strategies in order to reduce skin friction drag in a fully developed turbulent channel flow at a low Reynolds number. Motivated by the so-called opposition control (Choi et al., J. Fluid Mech., vol. 253, 1993, pp. 509–543), in which a control input is applied so as to cancel the wall-normal velocity fluctuation on a detection plane at a certain distance from the wall, we consider wall blowing and suction as a control input, and its spatial distribution is determined by the instantaneous streamwise and wall-normal velocity fluctuations at distance 15 wall units above the wall. A deep neural network is used to express the nonlinear relationship between the sensing information and the control input, and it is trained so as to maximize the expected long-term reward, i.e. drag reduction. When only the wall-normal velocity fluctuation is measured and a linear network is used, the present framework reproduces successfully the optimal linear weight for the opposition control reported in a previous study (Chung & Talha, Phys. Fluids, vol. 23, 2011, 025102). In contrast, when a nonlinear network is used, more complex control strategies based on the instantaneous streamwise and wall-normal velocity fluctuations are obtained. Specifically, the obtained control strategies switch abruptly between strong wall blowing and suction for downwelling of a high-speed fluid towards the wall and upwelling of a low-speed fluid away from the wall, respectively. Extracting key features from the obtained policies allows us to develop novel control strategies leading to drag reduction rates as high as 37 %, which is higher than the 23 % achieved by the conventional opposition control at the same Reynolds number. Finding such an effective and nonlinear control policy is quite difficult by relying solely on human insights. The present results indicate that reinforcement learning can be a novel framework for the development of effective control strategies through systematic learning based on a large number of trials.

A one‐point second‐order Dirichlet boundary condition for the thermal lattice Boltzmann method

AbstractA one‐point second‐order Dirichlet boundary condition for convection‐diffusion equation based on the lattice Boltzmann method has been proposed. The unknown temperature distribution is interpolated from the distributions at the wall node and fluid node nearest to the wall in the direction of the lattice velocity. At the wall node, the unknown temperature distribution is expressed as a summation of equilibrium and nonequilibrium distributions. The equilibrium distribution is obtained using the wall temperature, while the nonequilibrium distribution is approximated from the nearest fluid node in the direction of the lattice velocity. Both asymptotic analysis and numerical simulations of heat conduction indicate that the Dirichlet boundary condition is second‐order accurate. Further comparisons demonstrate that the newly proposed boundary method is sufficiently accurate to simulate natural convection, convective and unsteady heat transfer involving straight and curved boundaries.

PD23-10 REAL-TIME MRI REVEALS A CHANGE IN PROSTATIC URETHRAL ANGLE DURING VOIDING

You have accessJournal of UrologyCME1 Apr 2023PD23-10 REAL-TIME MRI REVEALS A CHANGE IN PROSTATIC URETHRAL ANGLE DURING VOIDING Cody Johnson, Juan P. Gonzalez-Pereira, Labib Shahid, Maxwell Kounga, Shane Wells, Wade Bushman, and Alejandro Roldan-Alzate Cody JohnsonCody Johnson More articles by this author , Juan P. Gonzalez-PereiraJuan P. Gonzalez-Pereira More articles by this author , Labib ShahidLabib Shahid More articles by this author , Maxwell KoungaMaxwell Kounga More articles by this author , Shane WellsShane Wells More articles by this author , Wade BushmanWade Bushman More articles by this author , and Alejandro Roldan-AlzateAlejandro Roldan-Alzate More articles by this author View All Author Informationhttps://doi.org/10.1097/JU.0000000000003296.10AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookLinked InTwitterEmail Abstract INTRODUCTION AND OBJECTIVE: Dynamic changes in urethral anatomy during voiding have never been quantitatively assessed. We used real-time magnetic resonance imaging (MRI) to obtain dynamic, high-fidelity 3D images of the lower urinary tract during voiding. These were then used to create accurate real-time urethral reconstructions throughput the voiding event coupled with determinations of urine flow, pressure and wall shear stress by computational fluid dynamics (CFD). METHODS: MRI scans were performed on a healthy young man using a clinical 3T scanner according to an IRB-approved protocol to acquire a 3D image every 3.7 s during voiding. 3D models of the urethral lumen were created using MIMICS (Materialise, Leuven, Belgium). RESULTS: Diameters of the internal and external urethral sphincter (IUS, EUS) and prostatic urethral angle (PUA) were obtained throughout voiding. CFD, using computational software to model and simulate fluid behavior under specified constraints of geometry and flow, was used to determine wall shear stress (WSS), pressures and fluid velocities along the length of the urethra during maximum flow (Figure 1). IUS diameter remains unchanged throughout voiding whereas EUS diameter correlates positively with urine flow rate (data not shown here). Most notably, we noted a striking change in PUA during voiding. PUA becomes less acute as flow increases, being straightest at peak flow rate (Figure 2). CONCLUSIONS: Real-time MRI can be used to define the biomechanics of the urethra during voiding and reveals – for the first time – a change in prostatic urethral angle during voiding that correlates with urinary flow rate. Source of Funding: NIH: R01 DK126850-01 © 2023 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 209Issue Supplement 4April 2023Page: e675 Advertisement Copyright & Permissions© 2023 by American Urological Association Education and Research, Inc.MetricsAuthor Information Cody Johnson More articles by this author Juan P. Gonzalez-Pereira More articles by this author Labib Shahid More articles by this author Maxwell Kounga More articles by this author Shane Wells More articles by this author Wade Bushman More articles by this author Alejandro Roldan-Alzate More articles by this author Expand All Advertisement PDF downloadLoading ...

Research on process modeling and simulation of spent lead paste desulfurization enhanced reactor

In the reaction process of carbonate desulfurization lead paste, the produced PbCO3 is easily wrapped in the outer periphery of PbSO4 to form a product layer, hindering the mass transfer process. Therefore, it is necessary to break the PbCO3 product layer. In this work, the rotor stator-reinforced reactor was selected as the enhanced desulfurization reactor for the purpose of breaking the PbCO3 product layer and promoting mass transfer. The breakage process of the PbCO3 product layer generated during the PbSO4 desulfurization was modeled. Computational fluid dynamics simulation to the rotation conditions was carried out to theoretically analyze the fluid flow characteristics of PbSO4 slurry and the wall shear stress affecting the breakage of PbCO3 product layer. By optimizing the rotation conditions, the distribution ratio of effective rotor wall shear stress range achieved 96.1%, and the stator wall shear stress range reached 99.15% under a rotation of 2000 r·min−1. The research work provides a reference for analysis of the mechanism of product layer breakage in the PbSO4 desulfurization process, and gives a clear and intuitive systematic study on the fluid flow characteristics and wall shear stress of the desulfurization reactor.

Two‐scale conjugate heat transfer solution for micro‐structured surface

AbstractThe primary challenges for simulating a turbulent flow over a micro‐structured surface arise from the two hugely disparate spatial length scales. For fluid–solid coupled conjugate heat transfer (CHT), there is also a time‐scale disparity. The present work addresses the scale disparities based on a two‐scale framework. For the spatial scale disparity, a dual meshing is employed to couple a global coarse‐mesh domain with local fine‐mesh blocks around micro‐structures through source terms generated from the local fine‐mesh and propagated to the global coarse‐mesh domain. The convergence and robustness of the source terms driven coarse‐mesh solution is enhanced by a balanced eddy‐viscosity damping. In this work, the two‐scale method previously developed only for a fluid‐domain is extended to a solid domain so that thermal conduction around micro‐elements can now be resolved accurately and efficiently. The fluid–solid timescale disparity is dealt with by a frequency domain approach. The time‐averaged (zeroth harmonic) is effectively obtained in the same way as steady CHT. And remarkably, wall temperature unsteadiness can be simply obtained from the fluid temperature harmonics through a wall fluid–solid temperature harmonic transfer‐function at minimal computational cost. The developed CHT capability is validated for an experimental internal cooling channel with multiple surface rib‐elements. For a test configuration with 100 micro‐structures, the fluid domain‐only, the solid domain‐only and the fluid–solid coupled CHT solutions are analyzed respectively to examine and demonstrate the validity of the present framework and implementation methods. Some of the results also serve to illustrate the primary underlying working of the methodology.

On the non-Newtonian blood flow in the elliptical multi-stenosed inclined artery: Analytical approach

The analysis of blood flow in the stenotic artery is imperative as the existence and progression of stenosis effectively disturbs the blood flow in the artery, which may cause vascular disease. In this study, we considered the linearized Phan-Thien–Tanner (PTT) fluid model to study the non-Newtonian nature of blood flow in the stenosed artery inclined at an angle [Formula: see text]. The artery is considered to have multiple stenoses and a cross-section of elliptical shape. The PTT model is appropriate for this analysis as viscoelastic properties and shear-thinning characteristics characterize it. The elliptical cross-section of the artery raises the nonlinearity of the mathematical model and makes it effortful to solve the equations analytically. The mathematical equations of the model are processed to non-dimensional form under the assumptions of mild stenosis, which help us to get the exact solutions of the equations in the elliptical domain. The analytically acquired solutions are investigated in detail by their graphical interpretation. It is analyzed that progressive height of stenosis generates the more significant disorder in the narrowed part of the channel as the velocity reverses its behavior in that region. The advancing values of the Weissenberg number, Grashof number and extensional and heat source parameters cause to lessen the axial velocity and slow the fluid flow. The rising Grashof number has nearly no impact on the velocity in the center of the channel. The wall shear stress aggrandizes with the growth of stenosis height, and its behavior for other physical constraints is similar to the flow velocity. It also has a practical enhancement in the stenotic region of the conduit. It is observed that fluid velocity and wall shear stress have larger values for zero angles of inclination than the inclination angle of [Formula: see text]. The streamlined examination indicates the generation of the vortices in the stenotic part of the artery. Moreover, the flow velocity and wall shear stress have lower values for the nonuniform shape than for the uniform form of stenosis.

Development of a novel experimental technique for the measurement of residual wall layer thickness in water-oil displacement flows

The effective removal and displacement of fluids is important in many industrial and environmental applications, such as for operation and cleaning of process equipment, fluid injection in porous media for oil recovery or aquifer remediation, or for achieving subsurface zonal isolation in new or abandoned wells. The accurate measurement of the residual fluid wall film left behind after displacement by a cleaning fluid is a long-standing challenge, particularly so for very thin fluid films where the thickness can be of the order of micrometer. We focus on the characterization of oil films left on the wall of a horizontal pipe after the pipe has been displaced by water, and develop a novel, non-intrusive analytical technique that allows the use of relevant pipe materials. The oil that originally occupies the pipe is stained by a hydrophobic dye Nile red, and an intermediate organic solvent is used to collect the residual oil volume that remains after displacing the pipe with a known volume of water. Finally, ultraviolet-visible spectroscopy is used to measure the Nile red concentration in the collected fluid, which is proportional to the residual volume of oil in the pipe. We demonstrate the methodology by conducting experiments where the displacing fluid is injected at two different imposed velocities, and where the injected fluid volume is varied. As expected, we find a gradual thinning of the oil film with increasing injected fluid volume. We compare the measured film thicknesses to a displacement model based on the steady velocity profile in a pipe, and find that experiments consistently produce smaller film thicknesses. This developed technique allows quantification of displacement and cleaning mechanisms involved in immiscible displacements at laminar, transitional and turbulent regimes, for different non-Newtonian fluid pairs, and for different realistic pipe materials and surface roughnesses.

MR imaging of the airways.

The need for airway imaging is defined by the limited sensitivity of common clinical tests like spirometry, lung diffusion (DLCO) and blood gas analysis to early changes of peripheral airways and to inhomogeneous regional distribution of lung function deficits. Therefore, X-ray and computed tomography (CT) are frequently used to complement the standard tests.As an alternative, magnetic resonance imaging (MRI) offers radiation-free lung imaging, but at lower spatial resolution. Non-contrast enhanced MRI shows healthy airways down to the first subsegmental level/4th order (CT: eighth). Bronchiectasis can be identified by wall thickening and fluid accumulation. Smaller airways become visible, when altered by peribronchiolar inflammation or mucus retention (tree-in-bud sign).The strength of MRI is functional imaging. Dynamic, time-resolved MRI directly visualizes expiratory airway collapse down to the lobar level (CT: segmental level). Obstruction of even smaller airways becomes visible as air trapping on the expiratory scans. MRI with hyperpolarized noble gases (3He, 129Xe) directly shows the large airways and peripheral lung ventilation. Dynamic contrast-enhanced MRI (DCE MRI) indirectly shows airway dysfunction as perfusion deficits resulting from hypoxic vasoconstriction of the dependent lung volumes. Further promising scientific approaches such as non-contrast enhanced, ventilation-/perfusion-weighted MRI from periodic signal changes of respiration and blood flow are in development.In summary, MRI of the lungs and airways excels with its unique combination of morphologic and functional imaging capacities for research (e.g., in chronic obstructive lung disease or asthma) as well as for clinical imaging (e.g., in cystic fibrosis).

Temperature Field, Flow Field, and Temporal Fluctuations Thereof in Ammonothermal Growth of Bulk GaN-Transition from Dissolution Stage to Growth Stage Conditions.

With the ammonothermal method, one of the most promising technologies for scalable, cost-effective production of bulk single crystals of the wide bandgap semiconductor GaN is investigated. Specifically, etch-back and growth conditions, as well as the transition from the former to the latter, are studied using a 2D axis symmetrical numerical model. In addition, experimental crystal growth results are analyzed in terms of etch-back and crystal growth rates as a function of vertical seed position. The numerical results of internal process conditions are discussed. Variations along the vertical axis of the autoclave are analyzed using both numerical and experimental data. During the transition from quasi-stable conditions of the dissolution stage (etch-back process) to quasi-stable conditions of the growth stage, significant temperature differences of 20 K to 70 K (depending on vertical position) occur temporarily between the crystals and the surrounding fluid. These lead to maximum rates of seed temperature change of 2.5 K/min to 1.2 K/min depending on vertical position. Based on temperature differences between seeds, fluid, and autoclave wall upon the end of the set temperature inversion process, deposition of GaN is expected to be favored on the bottom seed. The temporarily observed differences between the mean temperature of each crystal and its fluid surrounding diminish about 2 h after reaching constant set temperatures imposed at the outer autoclave wall, whereas approximately quasi-stable conditions are reached about 3 h after reaching constant set temperatures. Short-term fluctuations in temperature are mostly due to fluctuations in velocity magnitude, usually with only minor variations in the flow direction.