bluuebunny/crossref_metadata_2025 huggingface dataset:
Yield Strength Asymmetry Predictions From Polycrystal Elastoplasticity
Authors: Barton N. , Dawson P. , Miller M.
Published: February 2008

Since the 1960s, it has been known that elastoplastic polycrystal models predict asymmetries in the yield strength for polycrystals that have been prestrained. After prestraining in tension, a model polycrystal exhibits Bauschinger-like behavior in that it yields in compression at a lower stress magnitude than in tension. Furthermore, the knee of the reloading stress-strain curve is more gradual for compression than for tension. The origins of these behaviors reside in the assumption that links the macroscopic deformation to the deformations in individual crystals. More precisely, the reloading response is biased by the residual stress field which is induced with plastic straining by the anisotropy of the single crystal yield surface. While the earlier work pointed to the polycrystalline origins of the asymmetry, it did not resolve the degree to which the particular linking assumption affects the amount of asymmetry. However, due to the strong influence of the linking assumption on the crystal stresses, the sensitivity of the asymmetry to the linking assumption is expected to be appreciable. In this paper we examine the influence of the linking assumption on the magnitude of the computed yield strength asymmetry of prestrained polycrystals. Elastoplastic polycrystal simulations based on upper bound (Taylor) and lower bound (equilibrium-based) linking assumptions are compared to finite element computations in which elements constitute individual crystals. The finite element model maintains compatibility while satisfying equilibrium in a weak sense and treats the influence of neighboring crystals explicitly. The strength of the predicted Bauschinger effect does depend on the linking assumption, with ‘compatibility first’ models developing stronger yield strength asymmetries.
Dynamic Indentation Hardness and Rate Sensitivity in Metals
Authors: Subhash G. , Koeppel B. J. , Chandra A.
Published: February 2008

An experimental technique for determining the dynamic indentation hardness of materials is described. Unlike the traditional static hardness measurements, the dynamic hardness measurements can capture the inherent rate dependent material response that is germane to high strain rate processes such as high speed machining and impact. The dynamic Vickers hardness (DHV) of several commonly used engineering materials is found to be greater than the static Vickers hardness (HV). The relationship between the hardness and yield stress under static conditions, i.e., HV =3σy, is also found to be valid under dynamic conditions. It is suggested that this simpler technique can be used to assess the rate sensitive nature of engineering materials at moderate strain rates in the range of around 2000/s.
Characterization of a Water Peening Process
Authors: Daniewicz S. R. , Cummings S. D.
Published: February 2008

A traversing water jet was used to impact the surface of 1100 series aluminum specimens in an effort to generate compressive residual stresses on the surface. Stresses induced by the water peening operation were measured using X-ray diffraction, and compressive stress increases as large as 60 percent of the monotonic yield strength resulted. Surface roughness and hardness were also measured. Finite element modeling of a stationary water jet impinging on an elastic-plastic half-space was performed to characterize the water peening process. Surface residual stresses were found to be a result of sub-surface plastic deformations.
Effect of Wrought Product Thickness and Stress Ratio on Fatigue Crack Growth Behavior of a Quaternary Aluminium-Lithium Alloy
Authors: Saxena Vikas Kumar , Radhakrishnan V. M.
Published: February 2008

Effect of thickness on fatigue crack growth (FCG) behavior has been investigated at varying stress ratio in a quaternary Al-Li alloy. The thickness has been observed to influence FCG behavior in two opposite ways, depending on stress ratio. The crack closure concept has been invoked to rationalize the observed behavior. The roughness-induced crack closure is the main governing mechanism in the alloy under investigation. Factors such as fracture surface asperity size and severity of plane strain condition have been argued to affect the extent of crack closure, and consequently, fatigue crack growth behavior.
Fracture Behavior of Kaolin-Reinforced High Density Polyethylene
Authors: Wetherhold Robert C. , Mouzakis Dionysios E.
Published: February 2008

The addition of the low-cost mineral filler kaolin to high-density polyethylene (HDPE) creates a composite with both improved stiffness and toughness properties. This study focuses on two aspects of the toughness of these composites: the fracture toughness increment produced by work at the fracture surface and the directionality induced by the injection molding fabrication process. The Essential Work of Fracture (EWF) method gives results which show that a higher volume fraction of kaolin produces more surface work, consistent with earlier work using Compact Tension (CT) tests. The EWF method also demonstrates that a lower volume fraction can produce a higher overall plastic work and apparent toughness. A heat treatment that removes the orientation of the matrix but not that of the particles was applied to study the effect of matrix crystallinity. The results indicate that the matrix supramolecular structure (crystallinity and skin-core effect) is responsible for the directionality of toughness, and that a heat treatment can be used to produce high toughness behavior in both major directions.
SMA Pseudoelastic Finite Strains. Theory and Numerical Application
Authors: Boubakar M. L. , Moyne S. , Lexcellent C. , Boisse Ph.
Published: February 2008

A tridimensional finite element model of isothermal pseudoelastic deformations is proposed in order to analyze and predict the behavior of shape memory alloys (SMA). To perform finite strain calculations, the mechanical modeling is based on a formulation in a non material rotating frame. The constitutive equations in a particular configuration are closed form of those developed with small strain assumption. This leads naturally to a numerical scheme composed of an elastic prediction and a possible pseudoelastic correction. To insure the convergence of the equilibrium equation solving method, a consistent tangent operator with the stress calculation algorithm is defined. In the case of thin structures, the proposed model is consistent with the zero normal stress condition within a three node shell element based on a mixed interpolation in order to avoid transverse shear locking. Numerical results are presented to show the accuracy of the proposed approach.
Simulation of Cyclic Displacement by Counterpoised Shape Memory Elements
Authors: Wu Xiaochuang , Grummon David S. , Pence Thomas J.
Published: February 2008

The most common shape memory actuation procedure involves a shape memory element that works against a conventional elastic biasing element. Actuation is provided by the movement of the contact interface separating the two elements. The interface then occupies one location at high temperature and another location at low temperature. Certain applications call for switching between two separate interface locations, both of which are stable at a common ambient temperature. A shape memory element working against a conventional elastic biasing element cannot generally give such capability. However, two shape memory elements working against each other does give this capability, provided that the ambient temperature supports stress-free martensite, and provided as well that the contact interface thermally isolates the elements during short duration heating spikes which trigger such a switch. There is an obvious operational benchmark involving one element with fully oriented martensite and the other element with random martensite. The benchmark operation exchanges these states between the two elements, in which case the device stroke follows immediately from the material’s transformation strain. This benchmark behavior is not achievable due to elastic strain effects. Here we analyze the achievable behavior of such a device using a recent model for shape memory behavior. This model tracks the state of the shape memory material in terms of austenite and two variant martensite. This enables the determination of the device’s shakedown behavior in terms of repeatable per-stroke strain delivery, and also gives the temperature excursions necessary for the associated full displacement.
An Experimental Data Base for the Computational Fluid Dynamics of Reacting and Nonreacting Methanol Sprays
Authors: McDonell V. G. , Samuelsen G. S.
Published: February 2008

The present data set consists of detailed measurements obtained within methanol sprays produced by a research atomizer which is operated with three atomizing air modes: none, non-swirling, and swirling. In addition, the cases with nonswirling and swirling atomizing air are characterized under reacting conditions. In each case, state-of-the-art diagnostics are applied. Measurements of the gas phase velocities in both the single and two-phase cases, droplet size distributions, and vapor concentration are obtained. The data are reported in a standardized format to ensure usefulness as modeling challenges. The results obtained reveal the presence of significant interaction between phases and significant changes in spray structure as a result of altering the atomizing air characteristics. Efforts have been directed toward delineation of errors and comparison with existing data sets where possible. The results is a comprehensive data base for vaporizing sprays under reacting and non-reacting conditions which permit a systematic variation in aerodynamic effects to be explored.
Meta-Analysis of Gas Flow Resistance Measurements Through Packed Beds
Authors: Taylor Malcolm S. , Zoltani Csaba K.
Published: February 2008

Measurements of the resistance to flow through packed beds of inert spheres have been reported by a number of authors through relations expressing the coefficient of drag as a function of Reynolds number. A meta-analysis of the data using improved statistical methods is undertaken to aggregate the available experimental results. For Reynolds number in excess of 103 the relation log Fv = 0.49 + 0.90 log Re′ is shown to be a highly effective representation of all available data.
Effects of Initial Acceleration on the Flow Field Development Around Rapidly Pitching Airfoils
Authors: Gendrich C. P. , Koochesfahani M. M. , Visbal M. R.
Published: February 2008

Computational results are presented to show how the acceleration period at the start of nominally constant pitch rate trajectories affects the dynamic stall process. Large amplitude motions of an NACA 0012 airfoil pitching about the quarter-chord axis were studied using constant (ON/OFF) acceleration profiles with nondimensional acceleration periods ranging between 0.039 to 0.6, and large pitch rates (0.1 ≤ Ω* ≡ α˙ c/2U∞≤0.4). The initial acceleration is observed to affect the integrated loads, surface pressure distribution, and the evolution of reversed flow regions just above the surface of the airfoil only during the acceleration period and for a relatively short time δτ ≤ 0.25 afterwards; τ≡ tU∞/c. After that time, all of these quantities only depend on the instantaneous angle of attack for a given pitch rate. These results are consistent with and explain previous experimental flow visualization observations. The onset of leading edge separation at high and low pitch rates is shown to be characterized by different processes. At low pitch rates leading edge separation occurs after the reversed flow front originating at the trailing edge has reached the leading edge. At higher pitch rates leading edge separation and the upstream progression of the (trailing edge) reversed flow front develop independently.
Mixing of a Generalized Newtonian Fluid in a Cavity
Authors: Ling F. H. , Zhang X.
Published: February 2008

Mixing of a generalized Newtonian fluid in a corotating discontinuous cavity flow is studied. The flow period and the rheological coefficients are used as parameters. Using geometrical construction of periodic orbits and bifurcation analysis we find mixing windows in the parameter space. A system within a mixing window is capable of producing uniform mixtures. Some bifurcation values have also been determined. A system whose parameters are in the neighborhood of these values will mostly not produce a uniform mixture. The main conclusion is that shear thinning degrades mixing in the cavity flow.
A Calculation Method for Developing Turbulent Flow in Rectangular Ducts of Arbitrary Aspect Ratio
Authors: Naimi M. , Gessner F. B.
Published: February 2008

This paper describes a full Reynolds stress transport equation model for predicting developing turbulent flow in rectangular ducts. The pressure-strain component of the model is based on a modified form of the Launder, Reece and Rodi pressure-strain model and the use of a linear wall damping function. Predictions based on this model are compared with predictions referred to high Reynolds number and low Reynolds number k–ε transport equation models and with experimental data taken in square and rectangular ducts. The results indicate that the proposed model yields improved predictions of primary flow development and Reynolds stress behavior in a square duct. The proposed model also yields Reynolds stress anisotropy and secondary flow levels that are compatible and agree well with experiment, without recourse to a quadratic damping function to model near-wall pressure-strain behavior.
Quantitative Visualization of a Submerged Pseudoplastic Jet Using Particle Image Velocimetry
Authors: Shekarriz A. , Phillips J. R. , Weir T. D.
Published: February 2008

A preliminary experimental study of a pseudoplastic jet flow is reported in this paper. The velocity field was measured using Particle Image Velocimetry. Unlike a Newtonian jet, the pseudoplastic jet was observed to experience a sudden drop in its velocity at a reproducible position downstream of the nozzle for the range of velocities examined. This position moved downstream with an increase in the nozzle exit velocity. The center-line streamwise velocity decayed as X–15 to X–30 within the terminating region of the jet for three different nozzle exit velocities of 2.43, 3.17, and 5.42 m/s. This decay is in contrast to X–1 decay for a turbulent or laminar Newtonian jet. The location of the terminating region did not appear to scale with Reynolds number, Plasticity number, or Hedstrom number. At Reynolds numbers of 3000 and 6400, the instantaneous streamwise velocity maps indicated that the flow was fairly laminar, with a sinuous instability appearing at the higher Reynolds number condition. Close observation of the jet indicated that local turbulence could exist within regions of high shear rate. Further detailed study is required to confirm this observation.
Structural Parameters and Prediction of Adverse Pressure Gradient Turbulent Flows: An Improved k-ε Model
Authors: Chukkapalli G. , Turan O¨. F.
Published: February 2008

A modified k-ε model is proposed to predict complex, adverse pressure gradient, turbulent diffuser flows. The need for an eddy viscosity is eliminated by using three structural parameters. A fuller treatment of the rate of kinetic diffusion terms is incorporated with a Reynolds stress model representation. A thorough evaluation is given of the three structural parameters in three decreasing and one increasing adverse pressure gradient diffuser flows leading to a three-layer representation. The results indicate the need for better modeling of the ε-equation.
Calculation of Numerical Uncertainty Using Richardson Extrapolation: Application to Some Simple Turbulent Flow Calculations
Authors: Celik Ismail , Zhang Wei-Ming
Published: February 2008

Richardson extrapolation has been applied to turbulent pipe flow and turbulent flow past a backward facing step. A commercial CFD code is used for this purpose. It is found that the application of the method is not straightforward and some aspects need careful consideration. Some of the problems are elucidated. The particular code used for the present application employs a hybrid scheme, and it does not give monotonic convergence for all the variables in all regions as the grid is refined. The flow regions and the variables which converge monotonically in these regions should be identified first before the method is applied. When this is done Richardson extrapolation gives good results in calculating the apparent order of the numerical procedure used, as well as obtaining grid independent results with which discretization error bounds can be calculated as measures of numerical uncertainty. Even in cases where it does not work, the method can be used as an error indicator for some obscured user mistakes. This paper also demonstrates several shortcomings of using commercial CFD codes. The present findings should help the users of CFD software in general, to quantify discretization errors in their calculations.
Measurement and Analysis of Static Pressure Field in a Torque Converter Turbine
Authors: By R. R. , Lakshminarayana B.
Published: February 2008

In this paper, the static pressure field and performance parameters of a torque converter turbine are measured, analyzed, and interpreted under three speed ratio conditions (0, 0.6, and 0.8). A proven measurement technique was developed for the steady-state measurement of static pressures in the turbine. Results show that: 1) the static pressure field is generally poor at the core section; 2) centrifugal force has the dominant effect on the static pressure drop in the turbine at SR = 0.6 and SR = 0.8; and 3) the static pressure loss due to viscous effects and due to the diffusion of the relative velocity is very pronounced at SR = 0.
An Experimental Investigation of the Flow Through an Axial-Flow Pump
Authors: Zierke W. C. , Straka W. A. , Taylor P. D.
Published: February 2008

The high Reynolds number pump (HIREP) facility at ARL Penn State has been used to perform a low-speed, large-scale experiment of the incompressible flow of water through a two-blade-row turbomachine. The objectives of this experiment were to provide a database for comparison with three-dimensional, turbulent flow computations, to evaluate engineering models, and to improve our physical understanding of many of the phenomena involved in this complex flow field. This summary paper briefly describes the experimental facility, as well as the experimental techniques—such as flow visualization, static-pressure measurements, laser Doppler velocimetry, and both slow- and fast-response pressure probes. Then, proceeding from the inlet to the exit of the pump, the paper presents highlights of experimental measurements and data analysis, giving examples of measured physical phenomena such as endwall boundary layers, separation regions, wakes, and secondary vortical structures. In conclusion, this paper provides a synopsis of a well-controlled, larger scope experiment that should prove helpful to those who wish to use the database.
The Effect of Constriction Size on the Pulsatile Flow in a Channel
Authors: Rosenfeld Moshe , Einav Shmuel
Published: February 2008

The effect of the constriction size on the pulsatile flow in a channel is studied by solving the time-dependent incompressible Navier-Stokes equations. A pulsating incoming flow is specified at the upstream boundary and the flow is investigated for several constriction sizes. Large flow structures are developed downstream of the constriction even for very small constriction size. The flow structures consist of several vortices that are created in each cycle and propagate downstream until they are washed away with the acceleration of the incoming flow. Additional vortices are created by a vortex multiplication process. The strength and total number of vortices generated in each cycle increase with the severity of the constriction. The maximal size of the vortices as well as their propagation speed are independent of the constriction size. These findings may be used for devising noninvasive methods for detecting the severity of stenoses in blood vessels and the potential damage to blood elements and thrombus formation caused by vortices.
Theoretical and Experimental Study of Development of Two-Dimensional Steady and Unsteady Wakes Within Curved Channels
Authors: Schobeiri M. T. , Pappu K. , John J.
Published: February 2008

Development of steady and periodic unsteady wake flows downstream of stationary and rotating cylindrical rods within a curved channel under zero longitudinal pressure gradient is theoretically and experimentally investigated. Wake quantities such as the mean velocity and turbulent fluctuations in longitudinal and lateral directions, as well as the turbulent shear stress, are measured. For the nondimensionalized velocity defect, affine profiles are observed throughout the flow regime. Based on these observations and using the transformed equations of motion and continuity, a theoretical frame work is established that generally describes the two-dimensional curvilinear wake flow. To confirm the theory, development of steady and periodic unsteady wakes in the above curved channel are experimentally investigated. The detailed comparison between the measurement and the theory indicates that the complex steady and unsteady wake flows are very well predicted.
Phase-Resolved Surface Pressure and Heat-Transfer Measurements on the Blade of a Two-Stage Turbine
Authors: Dunn M. G. , Haldeman C. W.
Published: February 2008

Phase-resolved surface pressure, and unsteady pressure measurements are reported for the first-stage blade row of the Space Shuttle Main Engine two-stage fuel-side turbine. Measurements were made at 10, 50, and 90 percent span on both the pressure and suction surfaces of the blade. Phase-resolved and unsteady heat-flux measurements are also reported.
Capsule Transport in Coal Slurry Medium
Authors: Seaba J. , Xu G.
Published: February 2008

The use of coal slurry instead of water in a coal log pipeline (CLP) is investigated for the first time. This investigation reveals significant differences and possible benefits by using coal slurry instead of water in CLPs. The lift-off velocity, and capsule and total pressure gradients are presented for a 51 mm pipeline using two capsule geometries. The fluid velocity was tested from 1 to 3 m/s, which includes the lift-off velocity of the capsule train. The diameter ratio (k) and specific gravity (S) are held constant at 0.75 and 1.3, respectively. Two capsule lengths were studied corresponding to aspect ratios (a) of 2 and 4. Aluminum-Plexiglas capsules are used to simulate the coal logs. The coal slurry significantly lowered the lift-off velocity, and transported more coal per total pressure gradient than coal logs in water. The capsule pressure gradient was nearly constant over the velocity range investigated. This indicates that the optimal operation velocity range may be much larger for coal slurry compared to water. Further tests and abrasion studies are required to fully assess the potential benefits of using coal slurry.
Simulation of Paint Transfer in an Air Spray Process
Authors: Hicks P. G. , Senser D. W.
Published: February 2008

A methodology for simulating drop transport and deposition in air-spray, paint-application processes is presented. Simulation of the complex physical phenomena involved is made possible through a number of key assumptions based on measurements of typical air paint sprays. The significant advance is the inclusion of the direct effects of turbulent air velocity fluctuations on the trajectories of paint drops via a stochastic separated flow approach. The model accurately predicts the mean air velocity field, paint transfer efficiency, and drop transfer efficiency. Owing to increased inertia, the mechanisms controlling drop transport shift with increasing drop size.
On the Roughness Reynolds Number Transition Criterion
Authors: Masad J. A.
Published: February 2008

The theoretically validated Fage’s transition criterion for predicting transition in flow over a single roughness element is used to show that in the widely used roughness Reynolds number (Rek) transition criterion, the critical value of Rek depends strongly on the length of the roughness element. At roughness length where the criterion is valid, the variation of the roughness height with free-stream Reynolds number is quantified.
The Turbulent Incompressible Jet in a Curved Coflow
Authors: O¨tu¨gen M. V. , Girlea F. , Sforza P. M.
Published: February 2008

The effects of small streamline curvature on the growth and axial flow development of a turbulent incompressible jet in a curved coflow was investigated experimentally. The jet streamline curvature was achieved by introducing the initially round jet tangentially into a stream flowing through a curved channel of square cross-section. The jet issued from a straight pipe and had a fully developed velocity profile at the exit plane. The jet Reynolds number and the coflow-to-jet-velocity ratio were 4300 and 0.11, respectively. A single component laser Doppler anemometer was used to measure the streamwise velocity. Axial mean velocity and turbulence intensity profiles were measured at various streamwise locations in both the plane of curvature and the surface perpendicular to the plane of curvature. The results indicate that the jet growth and turbulence intensity are influenced by the small streamline curvature. The growth rate of the curved jet in the plane of curvature is slightly increased compared to that of a straight jet. However, the growth of the same curved jet is suppressed in the plane perpendicular to the plane of curvature. In the plane of curvature, the inner jet half-width is larger than the outer jet half-width. The mean velocity profiles in this plane are nearly Gaussian when the lateral distance is normalized by the respective inner and outer side jet half-widths. The axial turbulence intensity profiles show asymmetry in the plane of curvature with a pronounced peak on the outer side of the jet.
Sources of Error in Eigenvalue Spectra for Pipe-Poiseuille Flows
Authors: Abbot A. H. , Moss E. A.
Published: February 2008

An operational Chebyshev Tau approach was used to establish temporal eigenvalue stability spectra for pipe-Poiseuille flows, using the Sexl equation which was transformed to remove the singularity at the origin. Substantial deviations were noted between the long-standing results of Davey and Drazin (1969) which are scattered and display branching patterns, and those of the present work which manifest a more orderly progression of the stable high-order eigenvalues. Formal distinctions are made between errors arising from round-off, basis truncation, and poor choice of basis function: the inaccuracies in Davey and Drazin’s data are thought to have arisen from the latter two effects.
Flow Behavior Around Stayvanes and Guidevanes of a Francis Turbine
Authors: Suzuki Toshiaki , Nagafuji Tomotatsu , Komiya Hiroshi , Shimada Takako , Kobayashi Toshio , Taniguchi Nobuyuki
Published: February 2008

The three-dimensional computation of steady and incompressible internal flows is of interest in numerical simulations of turbomachinery, and such simulations are currently under investigation, from inviscid to viscous flow analyses. First, surface pressure distributions have been measured for the stayvanes and the guidevanes of a Francis turbine. They are presented to verify the numerical results. Second, both inviscid and viscous three-dimensional flow analyses have been made, so as to predict the flow behavior in the same domain. Comparison of the measured pressure distributions to the predicted pressure distributions has been made to study the usefulness of the present simulations. It can be pointed out that a global analysis which includes a runner flow passage, except runner blades, is necessary to predict the three-dimensional flow characteristics and that inviscid flow analysis has the capability of good prediction for flow without separation. Viscous flow analysis gives similar results, though it is necessary to investigate further the improvement of prediction accuracy. Flow characteristics around the stayvanes and the guidevanes are also discussed.
Enhanced Axial Dispersion in Oscillating Pipe Flow With Different Solute Concentrations at Its Ends
Authors: Zhang J. G. , Zegel W. C. , Kurzweg U. H.
Published: February 2008

The problem of the enhanced axial contaminant dispersion by sinusoidal oscillation of fluid in a cylindrical pipe connected to reservoirs filled with contaminated groundwater involving different concentrations of species has been examined. It is found that the axial contaminant transfer along the pipe can be greatly enhanced by the fluid oscillation, which leads to a rapid removal of contaminants from groundwater and develops a relative low concentration region in the neighborhood of the subsurface collector. The enhanced dispersion coefficients, Deff, of several heavy metal ions and organic compounds with water as carrying fluid were computed, and the results show they are about four to six orders of magnitude larger than those in the absence of fluid oscillation. The operational energy cost is low. For example, by operating the presented U-type enhanced mass pump, whose total cross-section area in one leg is 0.0154 m2, it takes 15 days to extract and remove acetone from a contaminated groundwater body that is 12 m below grade and 10 m3 in volume, such that the acetone concentration can be decreased from 650 mg/l to 325 mg/l (i.e:, 50 percent lower than its original level) and the averaged daily (24 hours) operational energy cost is only 12.6 kJ (1.56 Btu).
Effect of Branch Orientation on Annular Two-Phase Flow in T-Junctions
Authors: Peng F. , Shoukri M. , Chan A. M. C.
Published: February 2008

Experimental data on dividing steam-water two-phase annular flow in T-junctions with horizontal inlet and downwardly inclined branches were obtained. The experiments were performed under experimental conditions which have not been examined before. The branch orientation was found to be a significant parameter affecting phase redistribution in the junction. Increasing the downward inclination of the branch was found to reduce the degree of phase separation in the junction under the present test conditions. This is caused by the nonuniform distribution of the liquid film thickness associated with the horizontal inlet annular flow. The phase redistribution data were compared with available models. The need for additional modelling studies was evident. The pressure changes of two-phase flow in the junction were closely correlated with the phase redistribution phenomenon. The data on pressure changes in the junction were correlated using simple models based on momentum and mechanical energy balances.
Elements of an Approach to the Assessment of Random Uncertainty in Transient Measurements
Authors: Maciejewski P. K.
Published: February 2008

This paper presents an approach to the assessment of random uncertainty in transient measurements based on the modeling of a time series that is representative of the variable of interest. The recommended approach employs conventional regression techniques to estimate all parameters that appear in a model for a given time series. Assessments of random uncertainty are based on the residual deviations associated with an adequate model for a given time series. The approach also incorporates a treatment of the dynamic response of a measurement system as it pertains to random uncertainty in transient measurements.
3-D Microscopic Measurement and Analysis of Chemical Flow and Transport in Porous Media
Authors: Rashidi Mehdi , Tompson Andrew , Kulp Tom , Peurrung Loni
Published: February 2008

Chemical flow and transport have been studied at the pore-scale in an experimental porous medium. Measurements have been taken using a novel nonintrusive fluorescence imaging technique. The experimental setup consists of a cylindrical column carved out of a clear plastic block, packed with clear beads of the same material. A refractive index-matched fluid was pumped under laminar, slow-flow conditions through the column. The fluid was seeded with tracer particles or a solute organic dye for flow and chemical transport measurements, respectively. The system is automated to image through the porous medium for collecting microscopic values of velocity, concentration, and pore geometry at high-accuracy and high-resolution. Various geometric, flow, and transport quantities have been obtained in a full three-dimensional volume within the porous medium. These include microscopic (pore-scale) medium geometry, velocity and concentration fields, dispersive solute fluxes, and reasonable estimates of a representative elementary volume (REV) for the porous medium. The results indicate that the range of allowable REV sizes, as measured from averaged velocity, concentration, and pore volume data, varies among the different quantities, however, a common overlapping range, valid for all quantities, can be determined. For our system, this common REV has been estimated to be about two orders of magnitude larger than the medium’s particle volume. Furthermore, correlation results show an increase in correlation of mean-removed velocity and concentration values near the concentration front in our experiments. These results have been confirmed via 3-D plots of concentration, velocity, pore geometry, and microscopic flux distributions in these regions.
Velocity Perturbations Induced by the Longitudinal Vortices in a Cylinder Wake
Authors: Wu J. , Sheridan J. , Welsh M. C.
Published: February 2008

This paper presents data showing the three-dimensional vortical structures in the near wake region of circular cylinders. The in-plane velocity field was measured using a digital Particle Image Velocimetry (PIV) technique. The vortical structures are found to include inclined counter-rotating longitudinal vortices in the braids joining consecutive Ka´rma´n vortices. A simple vortex-pair model is proposed to estimate velocity perturbation induced by the longitudinal vortices in the near wake region. The perturbation resulting from the longitudinal vortices is shown to induce spanwise velocity modulation and a velocity spike of a nominally two-dimensional vortex street.
A Nonlinear Calculation of Rotating Cavitation in Inducers
Authors: Tsujimoto Y. , Watanabe S. , Kamijo K. , Yoshida Y.
Published: February 2008

In the previous linear analysis (Tsujimoto et al., 1993) it was found that there can be a backward rotating cavitation as well as a forward mode which rotates faster than impeller. Although some shaft vibration has been observed, which might be caused by the backward mode, experimental evidence has been obtained only for the forward mode. The purpose of the present study is to find out the factors which determine the amplitude of each mode of rotating cavitation by taking into account several nonlinearities. A time marching nonlinear 2-D flow analysis was carried out for this purpose. It was found that the increase of cavitation compliance at lower inlet pressure can be a factor which limits the amplitude. The mode selectivity is mainly dependent on the stability limit obtained by a linear analysis for which the phase delay of cavity has a most important effect.
Dynamic Response of Ducted Bubbly Flows to Turbomachinery-Induced Perturbations
Authors: d’Auria Fabrizio , d’Agostino Luca , Brennen Christopher E.
Published: February 2008

The present work investigates the dynamics of the three-dimensional, unsteady flow of a bubbly mixture in a cylindrical duct subject to a periodic pressure excitation at one end. One of the purposes is to investigate the bubbly or cavitating flow at inlet to or discharge from a pump whose blade motions would provide such excitation. The flow displays various regimes with radically different wave propagation characteristics. The dynamics effects due to the bubble response may radically alter the fluid behavior depending on the void fraction of the bubbly mixture, the mean bubble size, the pipe diameter, the angular speed of the turbomachine and the mean flow Mach number. This simple linearized analysis illustrates the importance of the complex interactions of the dynamics of the bubbles with the average flow, and provides information on the propagation and growth of the turbopump-induced disturbances in the feed lines operating with bubbly or cavitating liquids. Examples are presented to illustrate the influence of the relevant flow parameters. Finally, the limitations of the theory are outlined.
Contaminant Transport Through Single Fracture With Porous Obstructions
Authors: Lage J. L.
Published: February 2008

Transport of contaminant through a single fracture, modeled as a closely spaced parallel plates aperture, is analyzed theoretically and numerically. Permeable contact regions between the plates are modeled as fixed packs of homogeneous, isotropic, and inert porous material. A nondimensional theoretical expression for estimating the equivalent global permeability of the aperture is presented in terms of the relative volume occupied by the contact regions. Transient transport of contaminant (solute) through this heterogeneous system is analyzed considering injection of fluid with uniform concentration at the inlet of the fracture. For natural systems, it is verified that relative volume and distribution of contact regions affect clean-up time, defined as the time necessary for the complete removal of contaminant, only indirectly by varying the equivalent permeability of the system, otherwise their effect is negligible. The clean-up time of a clear (of contact regions) fracture, is correlated with the Peclet number for 10−1 ≤ Pe ≤ 106.
Fluid Flow Through Randomly Packed Monodisperse Fibers: The Kozeny-Carman Parameter Analysis
Authors: Rahli O. , Tadrist L. , Miscevic M. , Santini R.
Published: February 2008

Experimental studies have been carried out on fluid flow through porous media made up of randomly packed monodisperse fibers. The permeability and the Kozeny-Carman parameter kk are deduced from experimental results. The variations of the permeability increase exponentially with the porosity. The parameter kk is a decreasing function of the porosity ε and tends asymptotically to a value close to that deduced from a modified Ergun relation. The important decrease, observed for small aspect ratios, is certainly an effect of the cut sections of fibers. The results in terms of parameter kk are systematically compared to those deduced from various theoretical models. The variation laws of the parameter kk, deduced from different models, present important discrepancies with our experimental results.
Strongly-Coupled Multigrid Method for 3-D Incompressible Flows Using Near-Wall Turbulence Closures
Authors: Lin F. B. , Sotiropoulos F.
Published: February 2008

An efficient artificial compressibility algorithm is developed for solving the three-dimensional Reynolds-averaged Navier-Stokes equations in conjunction with the low-Reynolds number k-ω turbulence model (Wilcox, 1994). Two second-order accurate central-differencing schemes, with scalar and matrix-valued artificial dissipation, respectively, and a third-order accurate flux-difference splitting upwind scheme are implemented for discretizing the convective terms. The discrete equations are integrated in time using a Runge-Kutta algorithm enhanced with local time stepping, implicit residual smoothing, and V-cycle multigrid acceleration with full- and semi-coarsening capabilities. Both loosely and strongly-coupled strategies for solving the turbulence closure equations are developed and their relative efficiency is evaluated. Calculations are carried out for turbulent flow through a strongly-curved 180 deg pipe bend discretized with fine, highly-stretched and skewed meshes. It is shown that the strongly-coupled multigrid algorithm, with semi-coarsening in the transverse plane, is an efficient approach for simulating flows of practical interest with advanced near-wall turbulence closures.
Simulation of Chaotic Particle Motion in Particle-Laden Jetflow and Application to Abrasive Waterjet Machining
Authors: Yong Z. , Kovacevic R.
Published: February 2008

A novel method is presented for modeling the abrasive waterjet machining process. Particle motion on the cross section of particle-laden jetflow is simulated in order to quantify the erosion contributions of millions of particles having different kinematic behaviors. The simulation is performed using fractal point sets with chaotic behavior for the cases of circular and noncircular (elliptical, triangular) jets. The jetflow constructed can be endowed with any desired velocity profile and/or particle flowrate. In association with a classical constitutive equation for estimating the penetration ability of a particle, the drilling and cutting operations are simulated and verified by experiment for titanium, glass, and other materials. Roughness and waviness of the cutting surfaces are also simulated and there is good consistency between theory and experiment. Triangular and elliptical jetflows are utilized to explore potential applications of noncircular abrasive waterjets.
Application of Large Eddy Simulation to an Oscillating Flow Past a Circular Cylinder
Authors: Lu Xiyun , Dalton Charles , Zhang Jianfeng
Published: February 2008

Three-dimensional sinusoidally oscillating flows around a circular cylinder are investigated by using a viscous flow method (VFM) and a large eddy simulation (LES). A second-order accurate in time fractional step method and a combined finite-difference/spectral approximation are employed to solve the filtered incompressible Navier-Stokes equations. To demonstrate the viability and accuracy of the method, we calculate two cases of steady approach, flows at Reynolds numbers Re = 100 using VFM and Re = 104 using LES. For sinusoidally oscillating flows at β = 1035, the flow is 2D for KC< 0.5, 3D for 0.5 < KC < 2, and turbulent for KC > 2. For KC = 0.5, 0.8 and 1, the flow is calculated using VFM. For KC = 2, 3, 4, 5, 8 and 10, we have simulated the flow using LES with the Smagorinsky subgrid scale model. The drag and inertia coefficients are calculated from the in-line force acting on the cylinder and are in very good agreement with experimental data.
Air Entrainment in the Developing Flow Region of Plunging Jets—Part 1: Theoretical Development
Authors: Cummings P. D. , Chanson H.
Published: February 2008

Air-water bubbly flows are encountered in many engineering applications. One type of air-water shear flows is the developing flow region of a plunging jet. The mechanisms of air entrainment by plunging liquid jets are discussed in the light of new experimental evidence. Then the air bubble diffusion is analyzed analytically in the near-flow field of both circular and two-dimensional plunging jets. The theoretical developments are compared with existing circular plunging jet data and new experiments performed with a two-dimensional vertical supported jet. The study highlights two mechanisms of air entrainment at the plunge point depending upon the jet impact velocity and results suggest that the dispersion of air bubbles within the shear layer is primarily an advective diffusion process.
Theoretical Study of Pressure Fluctuations Downstream of a Diffuser Pump Impeller—Part 1: Fundamental Analysis on Rotor-Stator Interaction
Authors: Qin W. , Tsukamoto H.
Published: February 2008

A theoretical method was developed to calculate the unsteady flow caused by the interaction between impeller and diffuser vanes in a diffuser pump by using the singularity method. The unsteady flow in the diffuser vane is assumed to be induced by three kinds of unsteady vortices: bound vortices distributed on the impeller blades and diffuser vanes, and free vortices shed from the trailing edge of diffuser vanes. In order to make clear the contribution of each harmonic component of unsteady vortices to unsteady pressure, all the unsteady vortices are expressed in the form of Fourier series. The calculated unsteady pressures downstream of impeller agree well with the corresponding measured ones. Moreover, it was shown that impulsive pressure plays a predominant role for unsteady pressures.
On Oscillatory Instability of Convective Flows at Low Prandtl Number
Authors: Gelfgat A. Yu. , Bar-Yoseph P. Z. , Yarin A. L.
Published: February 2008

Numerical investigation of the oscillatory instability of convective flows in laterally heated rectangular cavities is presented. Cavities with no-slip isothermal vertical boundaries, no-slip adiabatic lower boundary, and stress-free adiabatic upper boundary are considered. Dependence of the critical Grashof number and the critical frequency of oscillations on the aspect ratio (A = length/height) of the cavity are investigated. The stability diagrams were obtained for the whole interval of the aspect ratio 1 ≤ A ≤ 10. The study was carried out for two values of the Prandtl number, Pr = 0 and 0.015. It was shown that the oscillatory instability sets in as a result of the Hopf bifurcation. It was found that at two different values of the Prandtl number considered the instability is caused by different infinitely small dominant perturbations, which means that the convective heat transfer strongly affects stability of the flow even for cases having small Prandtl number. No asymptotic behavior for large aspect ratios was found up to A = 10. Slightly supercritical oscillatory flows were approximated asymptotically by means of the weakly nonlinear analysis of the calculated bifurcation.
Numerical Simulation of Channel Flows by a One-Equation Turbulence Model
Authors: Vasiliev V. I. , Volkov D. V. , Zaitsev S. A. , Lyubimov D. A.
Published: February 2008

A one-equation model for turbulent viscosity, previously developed and tested for parabolic flows, is implemented in elliptic cases. The incompressible 2-D and axisymmetric flows in channel with back step as well as the incompressible and compressible 2-D flows in turbine blade cascades are calculated. The CFD procedures, developed for both incompressible and compressible turbulent flows simulation, are described. The results of calculations are compared with known experimental and numerical data.
Multigrid Computation of Incompressible Flows Using Two-Equation Turbulence Models: Part I—Numerical Method
Authors: Zheng X. , Liao C. , Liu C. , Sung C. H. , Huang T. T.
Published: February 2008

A highly efficient numerical approach based on multigrid and preconditioning methods is developed for modeling 3-D incompressible turbulent flows. The incompressible Reynolds-averaged Navier-Stokes equations are written in pseudo-compressibility from, then a preconditioning method is used to reduce the wave speed disparity. The k-ω and k-ε turbulence models are used to estimate the effects of turbulence. The model equations are solved together with the N-S equations in a strongly-coupled way, and all the acceleration techniques originally developed for N-S equations are also used for the turbulence model equations. A point-implicit technique is developed to improve the efficiency of the solution of the turbulence model equations.
Multigrid Computation of Incompressible Flows Using Two-Equation Turbulence Models: Part II—Applications
Authors: Zheng X. , Liao C. , Liu C. , Sung C. H. , Huang T. T.
Published: February 2008

In this paper, computational results are presented for three-dimensional high-Reynolds number turbulent flows over a simplified submarine model. The simulation is based on the solution of Reynolds-Averaged Navier-Stokes equations and two-equation turbulence models by using a preconditioned time-stepping approach. A multiblock method, in which the block loop is placed in the inner cycle of a multi-grid algorithm, is used to obtain versatility and efficiency. It was found that the calculated body drag, lift, side force coefficients and moments at various angles of attack or angles of drift are in excellent agreement with experimental data. Fast convergence has been achieved for all the cases with large angles of attack and with modest drift angles.