Axial Gradient Research Articles

Stability of centrifugal convection in a rotating pipe

This numerical study reveals that the thermal convection, induced by the axial gradient of temperature in a rotating pipe, suffers from the shear-layer instability if the Prandtl number, Pr, is small. As Pr increases, this instability is suppressed by the stable density stratification in the field of centrifugal force. In an annular pipe, the thermal instability develops for large Pr if a temperature of walls is prescribed. For a narrow annulus, these features agree with the known results for a planar flow driven by gravity and a horizontal gradient of temperature. It is shown here that the thermal instability does not develop if the walls are adiabatic. The centrifugal and Marangoni convection of a liquid, partially filling the pipe, also suffers from the shear-layer instability for small Pr and has no thermal instability. These features agree with the experiments for the planar flow performed by Kirdyashkin. The obtained results are of fundamental interest and can be relevant for the development of centrifugal heat exchangers.

Gone in seconds: praxis, performance, and peculiarities of ultrafast chiral liquid chromatography with superficially porous particles.

A variety of brush-type chiral stationary phases (CSPs) were developed using superficially porous particles (SPPs). Given their high efficiencies and relatively low back pressures, columns containing these particles were particularly advantageous for ultrafast "chiral" separations in the 4-40 s range. Further, they were used in all mobile phase modes and with high flow rates and pressures to separate over 60 pairs of enantiomers. When operating under these conditions, both instrumentation and column packing must be modified or optimized so as not to limit separation performance and quality. Further, frictional heating results in axial thermal gradients of up to 16 °C and radial temperature gradients up to 8 °C, which can produce interesting secondary effects in enantiomeric separations. It is shown that the kinetic behavior of various CSPs can differ from one another as much as they differ from the well-studied C18 reversed phase media. Three additional interesting aspects of this work are (a) the first kinetic evidence of two different chiral recognition mechanisms, (b) a demonstration of increased efficiencies at higher flow rates for specific separations, and

European alternatives to design perforated thin-walled cold-formed beam–columns for steel storage systems

Summary Rack systems to store goods and products are one of the most important applications for thin-walled cold-formed steel members. Adjustable pallet racks are used widely worldwide and are characterized by regular systems of perforations along the uprights (i.e. vertical members) to connect easily pallet beams and diagonal members forming the skeleton frame. Design rules are based on the well-established approaches for the more traditional solid members, suitably extended to account for the presence of regular perforations via component tests accurately described in rack specifications. This paper focuses on the performance of isolated uprights under axial load and gradient moment. In particular, three European alternatives to design beam–column rack members have been discussed and applied: the first and the second are traditionally used for thin-walled cold-formed steel members while the latter, the so-called general method of Eurocode 3 part 1-1, allows to take directly into account the key features associated with upright response. The geometry of the cross-sections and of their perforation system, the member slenderness and the load condition has been assumed as parameters of this study. Research outcomes demonstrate that the predicted performances can result significantly different, reflecting directly on the weight and the cost of the whole storage system. Furthermore, numerical analyses and design calculations show that standard codes need urgent revisions, leading in few cases to design beam–columns from the unsafe side.

Observations of hysteresis in the annual exchange circulation of a large microtidal estuary

AbstractA nonlinear relation between the salinity field and the subtidal exchange circulation in the Tampa Bay estuary is demonstrated using observational data from 1999 to 2011. The data are averaged to form mean monthly climatological values of total freshwater discharge ( ), axial and vertical salinity gradients, and subtidal vertical shear. Well‐known steady state solutions indicate that the exchange circulation is linearly proportional to the horizontal salinity gradient, assuming a constant vertical eddy viscosity ( ). The exchange flow is found to be multivalued with respect to the horizontal salinity gradient, forming a hysteresis loop in parameter space that passes through three dynamical regimes. Regime I is relatively dry with weak salinity gradients and exchange circulation. Regime II is the wet season (June–September) in which all quantities rapidly increase. In regime III, the exchange flow persists even though and the axial salinity gradient are again low. Gradient Richardson numbers and Simpson numbers also form a loop in parameter space with remaining subcritical (turbulent) until the wet season when rises above criticality (weak vertical mixing) where it remains through the end of regime III. The Simpson number is in a narrow range around 0.2, indicating that the horizontal salinity gradient is always a driver of the exchange circulation. The , estimated from a parameterization of the Richardson number, decreases by almost an order of magnitude from regimes I to II. It remains low during III, indicating that the persistent stratification is insulating the exchange flow from destruction by tidal mixing during this time period.

Mechanisms of growth and patterns of gene expression in oxygen‐deprived rice coleoptiles

Coleoptiles of rice (Oryza sativa) seedlings grown under water commonly elongate by up to 1 mm h(-1) to reach the atmosphere. We initially analysed this highly specialized phenomenon by measuring epidermal cell lengths along the coleoptile axis to determine elongation rates. This revealed a cohort of cells in the basal zone that elongated rapidly following emergence from the embryo, reaching 200 μm within 12 h. After filming coleoptiles in vivo for a day, kinematic analysis was applied. Eight time-sliced 'segments' were defined by their emergence from the embryo at four-hourly intervals, revealing a mathematically simple growth model. Each segment entering the coleoptile from the embryo elongated at a constant velocity, resulting in accelerating growth for the entire organ. Consistent with the epidermal cell lengths, relative rates of elongation (mm mm(-1) h(-1)) were tenfold greater in the small, newly emerged basal segments than the older distal tip segments. This steep axial gradient defined two contrasting growth zones (bases versus tips) in which we measured ATP production and protein, RNA and DNA content, and analysed the global transcriptome under steady-state normoxia, hypoxia (3% O2) and anoxia. Determination of the transcriptome revealed tip-specific induction of genes encoding TCP [Teosinte Branched1 (Tb1) of maize, Cycloidea (Cyc), and Proliferating Cell Factor (Pcf)] transcription factors, RNA helicases, ribosomal proteins and proteins involved in protein folding, whilst expression of F-box domain-containing proteins in the ubiquitin E3-SCF complex (Skp, Cullin, F-box containing complex) was induced specifically in bases under low oxygen conditions. We ascribed the sustained elongation under hypoxia to hypoxia-specific responses such as controlled suppression of photosystem components and induction of RNA binding/splicing functions, indicating preferential allocation of energy to cell extension.

A Comparison Study of the Oxygen-Rich Side Blow Furnace and the Oxygen-Rich Bottom Blow Furnace for Liquid High Lead Slag Reduction

This work investigates the characteristics of the oxygen-rich side blow furnace (OSBF) and the oxygen-rich bottom blow furnace (OBBF) as the reductive smelting reactor for molten high lead slag. The slags were collected from different sampling points of these furnaces during a regular high lead slag reduction process and analyzed. It is disclosed that lead content of the melt in the OSBF shows dramatic fluctuations, while melt from different sampling points of the furnace behave similarly, exhibiting the characteristics of batch reactor. An obvious axial lead content gradient is detected in the OBBF, showing the characteristics of a plug flow reactor. The industrial performances of these furnaces are also compared. The results indicate that 1.38% higher lead recovery can be achieved by using the OSBF instead of the OBBF. Unit energy consumptions of the OBBF–OSBF and OBBF–OBBF processes can be reduced to 230 kgce/t crude lead, which is 70 kgce/t crude lead less than that of the tradition Shuikoushan (SKS) process.

Online tangential laser profiling of coarse-grained bronze-bonded diamond wheels

A novel online tangential laser profiling method was developed in this study. The system consisted of a pulsed fiber laser, a surface grinder, a motorized three-dimensional (3D) translation stage, laser displacement sensors and a laser power meter. Online tangential laser profiling was conducted on coarse-grained (120#) bronze-bonded diamond wheels. When the laser power exceeded 40 W, the laser beam simultaneously removed the diamond grains and the bronze bond to produce a smooth wheel surface. The mean circular runout error and the axial gradient error of the grinding wheel surface decreased from 203.4 and 67.6 μm to 9.5 and 0.8 μm, respectively, indicating that the wheel contour accuracy improved significantly after profiling. The diamond grains on the grinding wheel surface were graphitized during laser profiling. However, blowing a protective argon (Ar) stream or spraying water mist from the side during profiling decreased the extent of graphitization compared to that in air.

Comparison between heat transfer to supercritical water in a smooth tube and in an internally ribbed tube

Numerical studies on heat transfer to supercritical water in an internally ribbed tube were performed and compared with those in a smooth tube. The Shear–Stress Transport k–ω model was adopted and validated against experimental data from tubes with different geometries. Comparisons between upward and downward flows show that the Jackson buoyancy criterion, which was proposed based on analysis and experiments in smooth tubes, can be used to evaluate effect of buoyancy in internally ribbed tubes. Radial profiles of turbulence and property reveal that in both smooth tube and internally ribbed tube, forced convection heat transfer is mainly influenced by specific heat, and effects of thermal conductivity and viscosity cancel each other out. As heat flux and bulk temperature increased, changes in the integral effect of specific heat resulted in variation in the heat transfer coefficient. The Jackson Nusselt correlation for smooth tubes can also accurately predict heat transfer coefficients of forced convection in an internally ribbed tube. Under conditions of mixed convection, buoyancy had a weaker impact on heat transfer in the internally ribbed tube. Detailed velocities and turbulence explained this weaker effect: the sharp drops in axial velocity gradient and turbulent kinetic energy, which occurred in a heated smooth tube at a radial position similar to the law of the wall region for isothermal flow, did not occur in a heated internally ribbed tube. Moreover, friction factors for internally ribbed tubes were always higher than that for smooth tubes in both forced convection and mixed convection.

Improving the performance of structure-embedded acoustic lenses via gradient-index local inhomogeneities

We investigate the use of graded inhomogeneities in order to enhance the focusing and collimation performance of structure-embedded acoustic metamaterial lenses. The type of inhomogeneity exploited in this study consists in axial symmetric exponential-like gradients of either material or geometric properties that create gradient-index inclusions able to bend and redirect propagating waves. In particular, we exploit the concept of gradient index inclusions to achieve focusing and collimation of ultrasonic beams created by embedded drop-channel lenses in both bulk and thin-walled structures. In the latter, the implementation is possible thanks to geometric exponential tapers known as Acoustic Black Holes (ABH). ABH tapers allow accurate control of the characteristics of the acoustic beam emanating from the lens channel which in the conventional design is severely affected by diffraction. The concept of beam control via graded inclusions is numerically illustrated and validated by using a combination of methodologies including geometric acoustics, finite difference time domain, and finite element methods.

Radial fast‐neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards

AbstractCharacterizing the neutron‐irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J‐value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast‐neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J‐values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three‐dimensionally distributed metallic fast (Ni) and thermal‐ (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR‐15 reactor in Řež, Czech Republic. The 58Ni(nf,p)58Co activation reaction and γ‐spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast‐neutron fluence. The fast‐neutron fluences at known positions in the irradiation container correlate with the J‐values determined by mass‐spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR‐15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of high‐precision dating and gradient monitoring can be crucial.

Crack-Tip Stress Field of Cracked Pipe under Combined Tension and Thermal Load

In the presence of excessive plasticity, the fracture toughness depends on the size and geometry. For material under fully yielded conditions, the stresses near the crack tip are not unique, but depend on geometry. So Single-parameter; J-approach is limited to high-constraint crack geometry. J-Q theory has been proposed in order to decide crack geometry constraint. This approach assumes that the crack-tip fields have two degrees of freedom. In this paper, based on J-Q theory, crack-tip stress field of fully circumferential cracked pipe under combined load is investigated using FE analysis. Combined loads are tensile axial force and thermal gradient of radial direction. Q-stresses of a crack geometry and it‘s loading state are used to determine constraint effect, and give a characteristic order for crack-tip constraint.

A Fully Gradient Model for Euler-Bernoulli Nanobeams

A fully gradient elasticity model for bending of nanobeams is proposed by using a nonlocal thermodynamic approach. As a basic theoretical novelty, the proposed constitutive law is assumed to depend on the axial strain gradient, while existing gradient elasticity formulations for nanobeams contemplate only the derivative of the axial strain with respect to the axis of the structure. Variational equations governing the elastic equilibrium problem of bending of a fully gradient nanobeam and the corresponding differential and boundary conditions are thus provided. Analytical solutions for a nanocantilever are given and the results are compared with those predicted by other theories. As a relevant implication of applicative interest in the research field of nanobeams used in nanoelectromechanical systems (NEMS), it is shown that displacements obtained by the present model are quite different from those predicted by the known gradient elasticity treatments.

Radiologic Determination of Corpus Callosum Injury in Patients with Mild Traumatic Brain Injury and Associated Clinical Characteristics.

ObjectiveTo investigate the incidence of corpus callosum injury (CCI) in patients with mild traumatic brain injury (TBI) using brain MRI. We also performed a review of the clinical characteristics associated with this injury.MethodsA total of 356 patients in the study were diagnosed with TBI, with 94 patients classified as having mild TBI. We included patients with mild TBI for further evaluation if they had normal findings via brain computed tomography (CT) scans and also underwent brain MRI in the acute phase following trauma. As assessed by brain MRI, CCI was defined as a high-signal lesion in T2 sagittal images and a corresponding low-signal lesion as determined by axial gradient echo (GRE) imaging. Based on these criteria, we divided patients into two groups for further analysis : Group I (TBI patients with CCI) and Group II (TBI patients without CCI).ResultsA total of 56 patients were enrolled in this study (including 16 patients in Group I and 40 patients in Group II). Analysis of clinical symptoms revealed a significant difference in headache severity between groups. Over 50% of patients in Group I experienced prolonged neurological symptoms including dizziness and gait disturbance and were more common in Group I than Group II (dizziness : 37 and 12% in Groups I and II, respectively; gait disturbance : 12 and 0% in Groups I and II, respectively).ConclusionThe incidence of CCI in patients with mild TBI was approximately 29%. We suggest that brain MRI is a useful method to reveal the cause of persistent symptoms and predict clinical prognosis.

Assessment and comparison of left ventricular shear in normal and situs inversus totalis hearts by means of magnetic resonance tagging.

Situs inversus totalis (SIT) is characterized by complete mirroring of gross cardiac anatomy and position combined with an incompletely mirrored myofiber arrangement, being normal at the apex but inverted at the base of the left ventricle (LV). This study relates myocardial structure to mechanical function by analyzing and comparing myocardial deformation patterns of normal and SIT subjects, focusing especially on circumferential-radial shear. In nine control and nine SIT normotensive human subjects, myocardial deformation was assessed from magnetic resonance tagging (MRT) image sequences of five LV short-axis slices. During ejection, no significant difference in either circumferential shortening (εcc) or its axial gradient (Δεcc) is found between corresponding LV levels in control and SIT hearts. Circumferential-radial shear (εcr) has a clear linear trend from apex-to-base in controls, while in SIT it hovers close to zero at all levels. Torsion as well as axial change in εcr (Δεcr) is as in controls in apical sections of SIT hearts but deviates significantly towards the base, changing sign close to the LV equator. Interindividual variability in torsion and Δεcr values is higher in SIT than in controls. Apex-to-base trends of torsion and Δεcr in SIT, changing sign near the LV equator, further substantiate a structural transition in myofiber arrangement close to the LV equator itself. Invariance of εcc and Δεcc patterns between controls and SIT subjects shows that normal LV pump function is achieved in SIT despite partial mirroring of myocardial structure leading to torsional and shear patterns that are far from normality.

STUDY ON STRESS CONCENTRATION FACTOR OF BOLT SCREW

Stress concentration in the bolt screws of bolt-sphere joints is the main reason for bolt screw fatigue failure. These stress concentrations in bolt screws were studied through theoretical derivation and numerical analysis. Ignoring lead angle, the variation of axial force gradient was obtained according to the deformable coordination condition. In addition, thread displacement factor and thread force were determined by numerical computation. The stress concentration factor in the bolt screw was developed through iteration using local modeling. Compared with the calculation results from whole models, the method proposed in this paper has higher computing accuracy and efficiency.

Morphology development from rod-like to nanofibrillar structures of dispersed poly (lactic acid) phase in a binary blend with poly (vinyl alcohol) matrix along the spinline

The morphological evolution of thermoplastic poly (vinyl alcohol) (PVA) and poly (lactic acid) (PLA) blend filaments along the spinline is investigated for the first time. Emphasis of the study is given on considering the axial velocity gradient and tensile stress that influence the final state of deformation of the dispersed PLA phase. In this study, pieces of the PVA/PLA blend filaments at different locations along the spinline are collected by using a special self-constructed fiber-capturing device. This fiber-capturing device allows closer off-line study of the morphological properties of PVA/PLA blend filaments. The axial velocity gradient at different zones along the spinline is calculated from velocity data using Laser Doppler Velocimetry (LDV) technique. The tensile stress is determined from the momentum balance equation. It is observed that the fibrillation process takes place up to a certain distance from the die exit, where the polymeric PVA/PLA blend filament reaches its glass transition temperature. In this region, the axial velocity and tensile stress undergoes major changes. A phase inversion of dispersed PLA phase from rod-like structures to continuous long thin fibrils has been found at the point in which the axial velocity gradient reaches a maximum value.

Fluidized countercurrent solvent extraction of oil pollutants from contaminated soil. Part 1: Fluid mechanics

Liquid–solid countercurrent solvent extraction is a potential application way for the removal of oil pollutant from contaminated soil. This study focuses primarily on the fluidization performance of liquid–solid two-phase flow in an extraction column (2.1m×50mm diameter), with the solvent and solid flow rates ranging from 10 to 800Lh−1 and from 8 to 107kgh−1, respectively. Several key hydrodynamic parameters, including pressure drop, solid holdup, superficial velocity of the solvent and solid, and dimensionless slip velocity, were investigated experimentally. At a given solid flow rate, a uniform axial distribution of solid holdup was observed at low solvent flow rate, whereas the axial solid holdup gradient increased with the solvent flow rate. The appearance of a solid holdup gradient was mainly attributed to the presence of small particles. An operating curve for the soil remediation by countercurrent solvent extraction was obtained according to the critical flow rate of the solvent and solid for predicting the occurrence of flooding and the appearance of the solid holdup gradient.

CFD characterization of monolithic reactors for kinetic studies

A laboratory reactor for kinetic studies has been simulated using computational fluid dynamics (CFD). Analysis of temperature distribution within the system shows that adding an inert monolith upstream the catalyst enhances heat conduction and therefore significantly reduces radial temperature gradients. Adjustment of the heating coil plays an important role as well by allowing the gas phase to smooth out the radial temperature profile. Radiative heat transfer and its effects on both the heat losses from the catalyst and on the measurements with an unprotected thermocouple have been particularly investigated. In order to prevent both the falsifying effect of radiation and the influences from the ongoing reaction, thermocouples should be placed and shielded inside a clogged channel. An inert monolith that is placed downstream the catalyst effectively serves as a radiation shield and drastically reduces both axial and radial gradients. Studies of the dispersion in the system reveal that the FTIR's gas cell is the most important source of the overall broadening in the concentration signal, with the reactor tube being the next major source of the distortion. An algorithm based on the Tikhonov regularization method has been developed for calculating the deconvolution of the concentration data, which has refined the time‐resolution in transient experiments from 20 to 2 s.

Peripheral nerve stimulation measures in a composite gradient system

ABSTRACTPurpose: This work compared peripheral nerve stimulation (PNS) thresholds for the MRI scanner body gradients (BODY), a head/neck insert gradient (INSERT), and the combination of both gradient sets used simultaneously (COMPOSITE). Methods: For BODY, INSERT and COMPOSITE gradients, PNS thresholds were determined by exposing subject volunteers to stepped increases in gradient strength. For COMPOSITE mode, the INSERT was applied at equal (experiment 1) or double (experiment 2) the BODY gradient strength. Results: The locations and thresholds of peripheral nerve stimulation depended on the gradient system configuration, gradient axis, and gradient strength. Stimulation in the body occurred when using the BODY Y‐gradient axis either singly (110 T/m/s) or in COMPOSITE mode (315 T/m/s experiment 2) and adding the insert gradient had negligible effect on stimulation. Stimulation in the head/sinus area generally occurred when using the INSERT X‐gradient either singly (213 T/m/s) or in COMPOSITE mode (320 T/m/s) and adding the body gradient had negligible effect on stimulation. In the COMPOSITE mode, both the location of stimulation and the limiting gradient strength matched location and strength of the limiting component gradient. Conclusion: Stimulation, to a first‐order approximation, is independent for the two gradient systems. In COMPOSITE mode, PNS can be dominated by either of the individual gradient components, indicating that the contribution of each component can be increased until the threshold limit of each component gradient is reached. COMPOSITE gradients provide increased gradient performance with PNS thresholds higher than either component gradient system operating alone. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 66–74, 2015

Turbulent Schmidt number and eddy diffusivity change with a chemical reaction

AbstractWe provide empirical evidence that the eddy diffusivity$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}D_{{t}\alpha }$and the turbulent Schmidt number${\mathit{Sc}}_{{t}\alpha }$of species$\alpha $($\alpha =\mathrm{A}, \mathrm{B}$or$\mathrm{R}$) change with a second-order chemical reaction ($\mathrm{A} + \mathrm{B} \rightarrow \mathrm{R}$). In this study, concentrations of the reactive species and axial velocity are simultaneously measured in a planar liquid jet. Reactant A is premixed into the jet flow and reactant B is premixed into the ambient flow. An optical fibre probe based on light absorption spectrometry is combined with I-type hot-film anemometry to simultaneously measure concentration and velocity in the reactive flow. The eddy diffusivities and the turbulent Schmidt numbers are estimated from the simultaneous measurement results. The results show that the chemical reaction increases${\mathit{Sc}}_{t\mathrm{A}}$;${\mathit{Sc}}_{t\mathrm{B}}$is negative in the region where the mean concentration of reactant B decreases in the downstream direction, and is positive in the non-reactive flow in the entire region on the jet centreline. It is also shown that${\mathit{Sc}}_{t\mathrm{R}}$is positive in the upstream region whereas it is negative in the downstream region. The production terms of axial turbulent mass fluxes of reactant B and product R can produce axial turbulent mass fluxes opposite to the axial gradients of the mean concentrations. The changes in the production terms due to the chemical reaction result in the negative turbulent Schmidt number of these species. These results imply that the gradient diffusion model using a global constant turbulent Schmidt number poorly predicts turbulent mass fluxes in reactive flows.