Axial-radial Plane Research Articles

Reversing and non-reversing Taylor vortex flows in modulated Taylor–Couette flow with counter-oscillating cylinders

Two classes of time-periodic Taylor vortex flows, called reversing and non-reversing flows, were discovered by Youd et al. (2003) under the effect of a zero mean inner cylinder’s angular velocity modulation with fixed outer cylinder. In the non-reversing/reversing flow, which takes place at relatively high/low frequency, the direction of the Taylor vortices in the radial axial-plane is conserved/ not conserved and does not change/change every half-cycle/cycle. This dynamic has been explained on the basis of the time period (frequency) provided by the driving force whether it is sufficiently enough for such flow reversal to be produced. In this paper, we show, using Floquet theory, that the counter-oscillation of the outer cylinder leads to a non-reversing flow suppression in some frequency ranges. In this case, this latter type of Taylor vortex flow could occur only in the very high frequency limit. Taking into account existing results concerning the case of co-oscillating cylinders, it is concluded that the mechanism governing the apparition of such flow reversal in the system does not depends solely to the oscillation frequency but strongly depends on the oscillation of the outer cylinder also.On the other hand, the reported numerical stability diagrams related to this flow configuration (counter-oscillating cylinders) reproduce quite well the existing experimental findings carried-out by Tennakoon et al. (1997). Explanation of discontinuities and jumps pointed-out by these authors in their numerical and experimental stability diagrams is also presented in the framework of the present investigation where we show that these phenomena are due to a switch in the instability of the system between the reversing and non-reversing flows.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.

Mechanical role of intraluminal thrombus in aneurysm growth: A computational study.

Models that seek to improve our current understanding of biochemical processes and predict disease progression have been increasingly in use over the last decades. Recently, we proposed a finite element implementation of arterial wall growth and remodeling with application to abdominal aortic aneurysms (AAAs). The study focused on changes within the aortic wall and did not include the complex role of intraluminal thrombus (ILT) during the AAA evolution. Thus, in this work, we extend the model with a gradual deposition of ILT and its mechanical influence on AAA growth. Despite neglecting the increased biochemical activity due to the presence of a proteolytically active luminal layer of ILT, and thus underestimating rupture risk potential, we show that ILT helps to slow down the growth of the aneurysm in the axial direction by redirecting blood pressure loading from the axial-radial plane to predominately radial direction. This very likely lowers rupture potential. We also show that the ratio of ILT volume to volume sac is an important factor in AAA stabilization and that fully thrombosed aneurysms could stabilize quicker and at smaller maximum diameters compared to partially thrombosed ones. Furthermore, we show that ILT formation and the associated mural stress decrease negatively impact the wall constituent production and thickness. Although further studies that include increased biochemical degradation of the wall after the formation of ILT and ILT deposition based on hemodynamics are needed, the present findings highlight the dual role an ILT plays during AAA progression.

Development and Validation of Independent Dual-Focusing Transducer for Internal Inspection of Tubes.

Controllable and focused ultrasonic beams in 3-D cylindrical space are essential when inspecting tubes or tubular objects. In this article, we develop an independent dual-focusing (IDF) array ultrasonic transducer composed of 64 prefocused elements and validate the feasibility of this design. Using k-Wave toolbox based on the k-space pseudospectral method, we simulate the acoustic pressure fields in the time domain and analyze beam profiles in circumferential-radial plane, axial-radial plane and 3-D space. The simulation results prove that the structure of IDF array transducer with the designed delay laws is capable of forming focused beam. A four-element aperture with 15 mm element radius shows narrow beams on both the circumferential and axial directions. On the basis of the simulation results, we fabricate a prototype transducer, test its electrical and acoustical performance, and implement internal inspection of a tube with an inner diameter of 13 mm. The echoes of four-element aperture show a higher amplitude than others, and this is consistent with the simulation. The eight longitudinal grooves and eight ring grooves of no more than 0.5 mm width are applied in inspection experiments, and the results demonstrate that the detection sensitivity of the IDF transducer can reach 0.2 mm in both the circumferential and axial directions. The results demonstrate that forming regulable and focused beam in 3-D cylindrical space is feasible using the IDF array which can be applied when inspecting tubes.

Meso-modelling study of the mechanical response and texture evolution of magnesium alloy during hot compression

The mechanical response and texture evolution of magnesium alloy during hot deformation are closely related to the heterogeneous meso-deformation. The improved Voce hardening law was developed and introduced into the crystal plasticity (CP) theory, and a reconstruction method of textured representative polycrystal was proposed. Three-dimensional crystal plasticity finite element model (3D-CPFEM) was established to meso-simulate the hot compression deformation of ZK61 Mg alloy. The influence of the temperature and strain rate on mechanical response was investigated, and the corresponding texture evolution and the texture transformation mechanism was revealed through meso-simulation. The results show that the 3D-CPFEM with the improved Voce hardening law and the initial texture characteristic can well predict the mechanical response and texture evolution under different temperatures and strain rates during hot compression of ZK61 Mg alloy. The basal plane texture of the ZK61 tubular blank is transformed into the basal fiber texture after hot compression, where the basal plane of grains is deflected gradually from the radial-axial plane to the radial-tangential plane. The texture transformation is attributed to two reasons, i.e., the orientation deflection of the grains with larger basal slip systems Schmid factor (SF) and the orientation retention of the grains whose c-axis tends to be parallel to the axial direction (AD) of the tubular blank. The change of the grain orientation corresponding to the basal plane texture component of the tubular blank has little contribution to the formed basal fiber texture.

Numerical investigation of heat transfer from a plane surface due to turbulent annular swirling jet impingement

The heat transfer characteristic of an annular turbulent swirling jet impingement on a heated plane surface is investigated numerically. The annular jet configuration causes instabilities and fluctuations in the flow. Depending on the combinations of different parameters, the annular jet impingement may have positive or negative effects on the heat transfer from the impingement surface. Swirl, on the other hand, introduces vorticity and fluid mixing in an impinging jet, which is desirable in some applications. The axisymmetric two-dimensional flow domain on a radial-axial plane is considered in the numerical model. Both non-swirling and swirling jet impingement is studied. The numerical computations are performed using the ANSYS Fluent CFD code. The Realizable k-ε turbulence model with enhanced wall treatment is used in the computation. The computational process is validated against other published data on similar flow configuration for non-swirling annular impinging jets. The flow and geometric parameters are the jet exit Reynolds number, Re (5000 to 25,000), the swirl strength Sw (0–0.77), the jet exit to the impingement surface distance, H (0.5–4.0), and the moderate blockage ratio of the annular jet, BR (0.4–0.6). The thermal-hydraulic field in the domain is computed for various combinations of these parameters. The effects of these parameters on the Nusselt number distribution on the impingement surface are analyzed. For short separation distance (H = 0.5), the swirling motion positively affects the overall heat transfer, and the average Nu is increased as high as 8% for certain combinations of Re, Sw, and BR; compared to the non-swirling annular jet impingement. For higher separation distances, the average and peak Nusselt number is initially reduced and then increased with increasing swirl strength.

A weakly nonlinear analysis of the precessing vortex core oscillation in a variable swirl turbulent round jet

We study the emergence of precessing vortex core (PVC) oscillations in a swirling jet experiment. We vary the swirl intensity while keeping the net mass flow rate fixed using a radial-entry swirler with movable blades upstream of the jet exit. The swirl intensity is quantified in terms of a swirl number . Time-resolved velocity measurements in a radial–axial plane anchored at the jet exit for various values are obtained using stereoscopic particle image velocimetry. Spectral proper orthogonal decomposition and spatial cross-spectral analysis reveal the simultaneous emergence of a bubble-type vortex breakdown and a strong helical limit-cycle oscillation in the flow for where . The oscillation frequency, , and the square of the flow oscillation amplitudes vary linearly with . A solution for the coherent unsteady field accurate up to ( ) is determined from the nonlinear Navier–Stokes equations, using the method of multiple scales. We show that onset of bubble type vortex breakdown at , results in a marginally stable, helical linear global hydrodynamic mode. This results in the stable limit-cycle precession of the breakdown bubble. The variation of with is determined from the Stuart–Landau equation associated with the PVC. Reasonable agreement with the corresponding experimental result is observed, despite the highly turbulent nature of the flow in the present experiment. Further, amplitude saturation results from the time-averaged distortion imposed on the flow by the PVC, suggesting that linear stability analysis may predict PVC characteristics for .

Anisotropic crack propagation and deformation in dentin observed by four-dimensional X-ray nano-computed tomography

Understanding the cracking behaviour of biological composite materials is of practical importance. This paper presents the first study to track the interplay between crack initiation, microfracture and plastic deformation in three dimensions (3D) as a function of tubule and collagen fibril arrangement in elephant dentin using in situ X-ray nano-computed tomography (nano-CT). A nano-indenter with a conical tip has been used to incrementally indent three test-pieces oriented at 0°, 45° and 70° to the long axis of the tubules (i.e. radial to the tusk). For the 0° sample two significant cracks formed, one of which linked up with microcracks in the axial-radial plane of the tusk originating from the tubules and the other one occurred as a consequence of shear deformation at the tubules. The 70° test-piece was able to bear the greatest loads despite many small cracks forming around the indenter. These were diverted by the microstructure and did not propagate significantly. The 45° test-piece showed intermediate behaviour. In all cases strains obtained by digital volume correlation were well in excess of the yield strain (0.9%), indeed some plastic deformation could even be seen through bending of the tubules. The hoop strains around the conical indenter were anisotropic with the smallest strains correlating with the primary collagen orientation (axial to the tusk) and the largest strains aligned with the hoop direction of the tusk. Statement of SignificanceThis paper presents the first comprehensive study of the anisotropic nature of microfracture, crack propagation and deformation in elephant dentin using time-lapse X-ray nano-computed tomography. To unravel the interplay of collagen fibrils and local deformation, digital volume correlation (DVC) has been applied to map the local strain field while the crack initiation and propagation is tracked in real time. Our results highlight the intrinsic and extrinsic shielding mechanisms and correlate the crack growth behavior in nature to the service requirement of dentin to resist catastrophic fracture. This is of wide interest not just in terms of understanding dentin fracture but also can extend beyond dentin to other anisotropic structural composite biomaterials such as bone, antler and chitin.

Anisotropic Crack Propagation and Deformation in Dentin Observed by Four-Dimensional X-ray Nano-Computed Tomography

Understanding the cracking behaviour of biological compositematerials is of practical importance. This paper presents the first studyto track the interplay between crack initiation, microfracture andplastic deformation in three dimensions as a function of tubule andcollagen fibrils arrangement in elephant dentin using in situ X-ray nanocomputed tomography (nano-CT). A nano-indenter with a conical tip hasbeen used to incrementally indent three test-pieces oriented at 0°, 45°and 70° to the long axis of the tubules (i.e. radial to the tusk). Forthe 0° sample two significant cracks formed, one of which linked up withmicrocracks in the axial-radial plane of the tusk originating from thetubules and the other one occurred as a consequence of shear deformationat the tubules. The 70° test-piece was able to bear the greatest loadsdespite many small cracks forming around the indenter. These werediverted by the microstructure and did not propagate significantly. The45° test-piece showed intermediate behaviour. In all cases strainsobtained by digital volume correlation were well in excess of the yieldstrain (0.9%), indeed some plastic deformation could even be seen throughbending of the tubules. The hoop strains around the conical indenter wereanisotropic with the smallest strains correlating with the primarycollagen orientation (axial to the tusk) and the largest strains alignedwith the hoop direction of the tusk.

Proteoglycans contribute locally to swelling, but globally to compressive mechanics, in intact cervine medial meniscus

Loss of charged proteoglycans in the knee meniscus, which aid in the support of compressive loads by entraining water, is an effect of degeneration and is often associated with osteoarthritis. In healthy menisci, proteoglycan content is highest in the inner white zone and decreases towards the peripheral red zone. We hypothesized that loss of proteoglycans would reduce both osmotic swelling and compressive stiffness, spatially localized to the avascular white zone of the meniscus. This hypothesis was tested by targeted enzymatic digestion of proteoglycans using hyaluronidase in intact cervine medial menisci. Mechanics were quantified by creep indentation on the femoral surface. Osmotic swelling changes were assessed by measuring collagen fiber crimp period in the radial-axial plane in the lamellar layer along both the tibial and femoral contacting surfaces. All measurements were made in the inner, middle, and outer zones of the anterior, central, and posterior regions. Mechanical measurements showed variation in creep behavior with anatomical location, along with spatially uniform decreases in viscosity (average of 21%) and creep stiffness (average of 15%) with hyaluronidase treatment. Lamellar collagen crimp period was significantly decreased (average of 27%) by hyaluronidase, indicating a decrease in osmotic swelling, with the largest decreases seen in locations with the highest proteoglycan content. Taken together, these results suggest that while proteoglycans have localized effects on meniscus swelling, the resulting effect on compressive properties is distributed throughout the tissue.

Ultrahigh Speed Images of Hall Thruster Azimuthal Instabilities

Visible emission of the azimuthal plasma instabilities were observed within the plume of a low-power Hall thruster utilizing ultrahigh speed imaging. The captured images provide time-dependent and spatial information on the azimuthal instability in the axial-radial plane of the thruster, which have not been well characterized. These images are then compared with observations made based on their probe measurements of azimuthal instabilities termed spoke instabilities. Data from the images show that the spoke instabilities are more complex than originally described and have additional features not detectable with probe measurements in a single plane.

Assessment of a Complex Aerospace Design through Optical Techniques

This paper reports the design assessment carried out on a subassembly of an advanced rocket using experimental techniques. The design was very complex and critical, since a cylindrical shell had a square cutout on the axial-hoop plane interacting with a notch in the axial-radial plane. Herein, two optical techniques have been employed for assessing the interaction between the notch and the cutout, and their individual contributions to the strain-field. Initially, a photoelastic model was studied to estimate the stresses at the notch tip. Subsequently, DIC was employed for measuring strains at the notch during the ground based testing of the actual component. The outcomes of these two experiments showed that the effect of the cutout to the strain concentration was negligible due to the extra stiffness provided by other assemblies.

Direct numerical simulation of spiral turbulence

In this paper, we present results of three-dimensional direct numerical simulations of the spiral turbulence phenomenon in a range of moderate Reynolds numbers, in which alternating intertwined helical bands of turbulent and laminar fluids co-exist and propagate between two counter-rotating concentric cylinders. We show that the turbulent spiral is comprised of numerous small-scale azimuthally elongated vortices, which align into and collectively form the barber-pole-like pattern. The domain occupied by such vortices in a plane normal to the cylinder axis resembles a ‘crescent moon’, a shape made well known by Van Atta with his experiments in the 1960s. The time-averaged mean velocity of spiral turbulence is characterized in the radial–axial plane by two layers of axial flows of opposite directions. We also observe that, as the Reynolds number increases, the transition from spiral turbulence to featureless turbulence does not occur simultaneously in the whole domain, but progresses in succession from the inner cylinder towards the outer cylinder. Certain aspects pertaining to the dynamics and statistics of spiral turbulence and issues pertaining to the simulation are discussed.

Measurement of Rates of Delayed Hydride Cracking (DHC) in Zr-2.5Nb Alloys—An IAEA Coordinated Research Project

Abstract Values of DHC rates in Zr alloys are sensitive to measurement procedures. A standard method has been developed at the laboratories of AECL and evaluated in a round-robin involving ten IAEA member states. Two test materials were used—Zr-2.5Nb pressure tubes in the cold-worked (CANDU™) and heat-treated (RBMK) conditions. Cracks were grown from fatigued starter cracks in the axial direction on the axial-radial plane of the original tubes. To obtain the maximum value of Vc, specimens were heated to dissolve all their hydrogen, then cooled at 1 to 3°C/min to the test temperature before loading at 15 MPa√m. Although the start of cracking was detected by potential drop, the extent of cracking was measured directly on the crack faces. The values of incubation time to the start of cracking were highly variable but Vc was well behaved. The values of Vc were normally distributed with a range varying from a factor of 1.2 to 5.2. At 250°C the mean value of Vc from 80 specimens of cold-worked material was 8.9(±1.12)×10-8 m/s and from 41 specimens of heat-treated material the mean value of Vc was 3.3(±0.64)×10-8 m/s. Tests were also done at six other temperatures between 144 and 283°C, using up to 22 specimens at each temperature. Both materials had an Arrhenius-type temperature dependence, Vc= A exp(-Q/RT). The use of strictly defined and coordinated experimental procedures gave a consistent set of Vc values, allowing effective comparison of results obtained in different national laboratories and resulting in good correlations between the DHC velocity values and differences in strength, crystallographic texture, and distribution of β-phase in the test materials.

An efficient semi-coarsening multigrid method for variable diffusion problems in cylindrical coordinates

In this paper, we present an efficient multigrid (MG) algorithm for solving the three-dimensional variable coefficient diffusion equation in cylindrical coordinates. The multigrid V-cycle combines a semi-coarsening in azimuthal direction with the red-black Gauss–Seidel plane (radial-axial plane) relaxation. On each plane relaxation, we further semi-coarsen the axial direction with red-black line relaxation in the radial direction. We also prove the convergence of two-level MG with plane Jacobi relaxation. Numerical results show that the present multigrid method indeed is scalable with the mesh size.

Comparison of hybrid Hall thruster model to experimental measurements

A two-dimensional hybrid particle-in-cell numerical model has been constructed in the radial-axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi-one-dimensional fluid and the ions are treated as collisionless, unmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements of electron mobility in the Stanford Hall Thruster. While an experimental mobility results in improved simulation of electron temperature and electric potential relative to a Bohm-type model, results suggest that energy losses due to electron wall interactions may also be an important factor in accurately simulating plasma properties. Using a simplified electron wall damping model modified to produce general agreement with experimental measurements, an evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. Although background gas results in two populations of neutrals, the increased neutral density has little effect on other plasma properties. Diffuse neutral wall interactions are in better agreement with experimental measurements than specular scattering. Also, charge exchange collisions result in an increase in average neutral velocity of 11% and a decrease in average ion velocity of 4% near the exit plane. The momentum exchange that occurs during charge exchange collisions is found to be negligible.

On Flow Development in Jet-Driven Vortex Chambers

This work presents the study of the flow in a jet-driven vortex chamber over a wide range of Reynolds numbers, contraction ratios, inlet angles, area and aspect ratios. Dimensional analysis furnishes the general functional relationships between the fundamental dimensionless quantities. Application of the integral equations of continuity and energy over the control volume, along with the minimum-pressure-drop or maximum flow rate postulate, provide the required analytical means to relate the predominant non-dimensional parameters such as the chamber geometry, the core size, pressure drop, Reynolds number, and viscous losses. Both the n = 2 vortex model, with reverse and non-reverse flow, and the free vortex model have been used at the vortex chamber exit plane. The theoretical results are found to successfully capture most of the salient properties of the flow. The influence of vortex chamber geometry, such as contraction ratio, inlet angle, area ratio, aspect ratio, and Reynolds number, on the flow field has been analyzed and compared with the present experimental data. A parametric study explores how the pressure coefficient and the core size vary with the different dimensionless properties. The observations show the pressure drop to decrease with the length. At first this appears to be counterintuitive since one habitually expects the pressure drop to be larger for longer pipes. A closer examination however, reveals that in addition to the radial-axial plane flow there is also a substantial centrifugal force, which decays with the length, thus shaping the development of the overall flow-field. The pressure drop across the vortex chamber differs from that in pipe flow, due to the mechanism of swirl flow. It depends mainly on intensity of tangential velocity. If the chamber length is increased, the vortex decay factor decreases, which leads to less pressure drop. The current theory confirms that the previous published models are only applicable for high Reynolds numbers where the inertia dominates the viscous forces. Based on the present theory, a new approach to determine the tangential velocity and radial pressure profiles inside the vortex chamber is developed and compared with the available experimental data. The n = 2 vortex model with reverse flow gives better results for strongly swirling flow.

Influence of Hydride Re-orientation on BWR Cladding Rupture under Accidental Conditions

Hydride precipitation along the radial-axial plane increases in high burn-up boiling water reactor (BWR) fuel claddings. The radially-oriented hydrides may have an important role during fuel behavior in a reactivity-initiated accident and may reduce ductility of the cladding under pellet-cladding mechanical interaction (PCMI) conditions. In order to promote a better understanding of the influence of the radial hydrides on cladding failure behavior under the PCMI conditions, tube burst tests were conducted for unirradiated BWR claddings charged with 200 to 650 ppm of hydrogen. About 20 to 30% of hydrides were re-oriented and precipitated along the radial-axial plane. The claddings exhibited large rupture openings with an axial crack at room temperature and 373 K. The crack penetrated through cladding wall preferentially along the radial hydrides, and radial cross section showed cladding failure in a brittle manner. However, reduction in residual hoop strain by precipitation of the radial hydrides was very small. It is accordingly expected that ductility of high burn-up BWR cladding is significantly reduced not only by precipitation of radial hydrides as far as hydrogen concentration and radial hydride fraction range in the present study.

Flaws in large 7050 aluminum ring rolled forgings

Defects found in large-diameter 7050-T7451 aluminum alloy ring forgings were characterized with multiple destructive and nondestructive methods. The approximate length, width, and thickness of the flaws were estimated. The orientation of the flaws was found to be consistent with the grain flow within the forging, both in the primary (tangential) direction as well as in the secondary grain flow in the axial-radial plane. Several theories for the continued presence of the defects are supportable based on the data collected in the investigation. The flaws encountered appear to be the result of unhealed porosity. This porosity, resulting from the ingot casting process, is normally closed by the forging process in other forged product. The precise mechanism resisting closure has not been determined, but is speculated to be hydrogen gas.

Angular anisotropy of electron energy distributions in inductively coupled plasmas

The noncollisional electron transport that is typical of low-pressure (<10 mTorr) and low-frequency (<10 MHz) inductively coupled plasmas (ICPs) has the potential to produce highly anisotropic angle-dependent electron energy distributions (AEEDs). The properties of AEEDs in axially symmetric ICPs were investigated using a Monte Carlo simulation (MCS) embedded in a two-dimensional plasma equipment model. A method was developed to directly compute the coefficients for a Legendre polynomial expansion of the angular dependence of the distributions during advancement of the trajectories of pseudoelectrons in the MCS. We found significant anisotropy in the AEEDs for transport in the azimuthal–radial plane for a wide range of pressures and frequencies, and attributed this behavior to the superposition of both linear and nonlinear forces. The angular anisotropy of AEEDs in the radial–axial plane in the bulk plasma was found to be significant only when the skin layer was anomalous and nonlinear Lorentz forces are large.