bluuebunny/crossref_metadata_2025 huggingface dataset:
Skin Friction Measurements in a Gas-Liquid Pipe Flow Via Optical Interferometry
Authors: Garrison T. J. , Manceau E. , Nikitopoulos D. E.
Published: February 2008

An instrument for the measurement of wall shear stress in two-phase flows is described. The device, termed a Laser Interferometer Skin Friction (LISF) meter, determines the wall shear by optically measuring the time rate of thinning of a thin oil film placed on the wall of the flow channel. The LISF meter has proven to be a valuable tool for measurement of wall shear in single-phase gaseous flows but, to date, had not been applied to liquid or gas-liquid flows. This paper describes modifications to the LISF meter developed to facilitate its use in two-phase flows. The instrument’s configuration, governing theory, and data reduction procedure are described. Additionally, results of validation experiments for a single-phase water flow are presented, which demonstrate the instrument’s ability to accurately measure wall shear. Measurements are also presented for two-phase, water-air flows in a duct of square cross section at various superficial gas and liquid velocities within the bubbly flow regime. Results of the measurements confirm previous observations that the addition of a very small amount of the gaseous phase increases the wall shear significantly over that in a single-phase water flow. The two-phase wall shear saturates to a maximum and then declines again as the superficial gas velocity is increased. The peak two-phase wall shear increases as the liquid superficial velocity is decreased. These trends are qualitatively in agreement with previous measurements obtained in pipes using an electrochemical technique.
Simulation of the Piston Driven Flow Inside a Cylinder With an Eccentric Port
Authors: Gharakhani Adrin , Ghoniem Ahmed F.
Published: February 2008

A grid-free Lagrangian approach is applied to simulate the high Reynolds number unsteady flow inside a three-dimensional domain with moving boundaries. For this purpose, the Navier-Stokes equations are expressed in terms of the vorticity transport formulation. The convection and stretch of vorticity are obtained using the Lagrangian vortex method, while diffusion is approximated by the random walk method. The boundary-element method is used to solve a potential flow problem formulated to impose the normal flux condition on the boundary of the domain. The no-slip condition is satisfied by a vortex tile generation mechanism at the solid boundary, which takes into account the time-varying boundary surfaces due to, e.g., a moving piston. The approach is entirely grid-free within the fluid domain, requiring only meshing of the surface boundary, and virtually free of numerical diffusion. The method is applied to study the evolution of the complex vortical structure forming inside the time-varying semi-confined geometry of a cylinder equipped with an eccentric inlet port and a harmonically driven piston. Results show that vortical structures resembling those observed experimentally in similar configurations dominate this unsteady flow. The roll-up of the incoming jet is responsible for the formation of eddies whose axes are nearly parallel to the cylinder axis. These eddies retain their coherence for most of the stroke length. Instabilities resembling conventional vortex ring azimuthal modes are found to be responsible for the breakup of these toroidal eddies near the end of the piston motion. The nondiffusive nature of the numerical approach allows the prediction of these essentially inviscid phenomena without resorting to a turbulence model or the need for extremely fine, adaptive volumetric meshes.
Prediction of Air-Water Two-Phase Flow Performance of a Centrifugal Pump Based on One-Dimensional Two-Fluid Model
Authors: Minemura Kiyoshi , Uchiyama Tomomi , Shoda Shinji , Egashira Kazuyuki
Published: February 2008

To predict the performance of centrifugal pumps under air-water two-phase flow conditions, a consistent one-dimensional two-fluid model with fluid viscosity and air-phase compressibility in a rotating impeller is proposed by considering energy changes in the transitional flow from the rotating impeller to the stationary volute casing. The two-fluid model is numerically solved for the case of a radial-flow pump after various constitutive equations are applied. The head and shaft power predicted are found to agree well with the measured values within ±20 percent of the rated flow capacity.
A New Approach to Evaluate the Cavitation Erosion Power
Authors: Fortes Patella Regiane , Reboud Jean-Luc
Published: February 2008

The dynamic response of various materials exposed to liquid jet and pressure wave impacts was simulated making use of an elastoplastic solid model. Numerical pit profiles were compared to experimental ones produced in test materials by the cavitation of water, mercury, or sodium. It was found that a high pressure wave emission was the main factor contributing to the observed cavitation damage. A parametric study concerning the pressure wave phenomenon was proposed and some similarity laws were obtained. By using these laws, the corresponding impact hydrodynamic parameters can be deduced from the pit geometric characteristics and the material properties. Based on that study, histograms concerning impact hydrodynamic parameters were plotted as a function of impact number and/or pit eroded volume. The influence of the mean flow velocity, the geometric scale, the fluid characteristics, and the material properties on the flow aggressivity could be evaluated during the cavitation incubation period.
Numerical Study of the Steady-State Tip Vortex Flow Over a Finite-Span Hydrofoil
Authors: Hsiao Chao-Tsung , Pauley Laura L.
Published: February 2008

The flow over a finite-span hydrofoil creating a tip vortex was numerically studied by computing the full Navier-Stokes equations. A good agreement in pressure distribution and oil flow pattern was achieved between the numerical solution and available experimental data. The steady-state roll-up process of the tip vortex was described in detail from the numerical results. The effect of the angle of attack, the Reynolds number, and the hydrofoil planform on the tip vortex was investigated. The axial and tangential velocities within the tip-vortex core in the near-field wake region were greatly influenced by the angle of attack. A jet-like profile in the axial velocity was found within the tip-vortex core at high angle of attack, while a wake-like profile in the axial velocity was found at low angle of attack. Increasing the Reynolds number was found to increase the maximum axial velocity, but only had a slight impact on the tangential velocity. Finally, a swept hydrofoil planform was found to attenuate the strength of the tip vortex due to the low-momentum boundary layer traveling into the tip vortex on the suction side.
Relaxation Models for Wave Phenomena in Liquid-Vapor Bubble Flow in Channels
Authors: Bilicki Z. , Kardas D. , Michaelides E. E.
Published: February 2008

We examine wave characteristics of a liquid-vapor mixture in order to investigate certain features of the homogeneous relaxation model. The model is described by one-dimensional averaged mass, momentum, energy equations, and a rate equation. Since the homogeneous relaxation model delivers a qualitative incompatibility of numerical and experiment results of large wave propagation, it is extended so as to take into account the heat conduction in the liquid surrounding vapor bubbles. With this extension, the effects of spreading and damping of the waves in the numerical solutions are similar to those observed in the experiment. Thus, a new model is created, the homogeneous relaxation-diffusion model which contains two physical quantities—the relaxation time and macroscopic heat conduction coefficient. Both quantities are determined based on experimental data. It seems that the results obtained from the new model agree well qualitatively with the experiments.
Effect of Trailing Edge Suction on Coherent Structures in Near Wake
Authors: Sharma S. D. , Sahoo R. K.
Published: February 2008

Experimental results, obtained from hot-wire measurements using a conditional sampling technique, demonstrate feasibility of controlling large-scale spanwise vortices (coherent structures) in the near wake region behind a rectangular base by means of suction through a slit at just one of the trailing edges. The suction thus employed, is found to influence the near wake topology with strong asymmetry and disturb the net vorticity balance. Moreover, a significant reduction in the base drag is achieved as a consequence of the trailing edge suction. The mechanism of the drag reduction is understood to lie in a marked change in the wake dynamics including attenuation in the size and strength of the coherent structures.
A Simple Analytical Theory for Interpreting Measured Total Pressure in Multiphase Flows
Authors: Guha Abhijit
Published: February 2008

This paper presents a simple, analytical theory for determining total pressure in multiphase flows, a subject of theoretical interest as well as of practical importance. It is shown here that the nonequilibrium processes occurring in the vicinity of a measuring device have a significant influence on the magnitude of flow velocity inferred from Pitot measurements. The present theory predicts that, depending on the size of the particles or droplets, the total pressure varies monotonically between the two limiting values: the frozen total pressure (when there is no interphase mass, momentum, and energy transfer in the decelerating flow toward the stagnation point) and the equilibrium total pressure (when the dispersed phase, either liquid droplets, or solid particles, is always at inertial and thermodynamic equilibrium with the continuous vapour phase). The presented analytical theory is a relation between nondimensional total pressure and Stokes number, representing particle size or inertia, and specifies the total pressure under different nonequilibrium conditions. One simple equation applies to diverse multiphase mixtures, solid particle laden gas as well as vapour-droplet mixtures, and at a wide range of flow conditions, both subsonic and supersonic. The associated issue of interpreting total temperature, and the relation between measured total pressure and entropy production in multiphase flows have been discussed at length by Guha (1998).
A Note on Particle Size Measurements in Two-Phase Flows
Authors: Balachandar R. , Mulpuru S. R. , Ungurian M. H.
Published: February 2008

This paper discusses the use of a phase-Doppler anemometer to obtain the size and velocity characteristics of droplets formed and transported in a flashing water jet. In particular, the validation and calibration procedures adopted to ensure correct acquisition of experimental data are discussed in detail. The methodology involved in choosing the magnitudes of the various operating parameters of a phase-Doppler anemometer such as the intensity of the transmitting beams, signal-to-noise ratio, sample size, and sensitivity of the photomultiplier tubes are described. The droplet size and velocity distributions were acquired at two axial stations covering the width and height of the jet cross-section. Attempts were made to identify the proper scaling variables to develop suitable nondimensional parameters to characterize a flashing jet.
High-Speed PIV Analysis Using Compressed Image Correlation
Authors: Hart Douglas P.
Published: February 2008

With the development of Holographic PIV (HPIV) and PIV Cinematography (PIVC), the need for a computationally efficient algorithm capable of processing images at video rates has emerged. This paper presents one such algorithm, sparse array image correlation. This algorithm is based on the sparse format of image data—a format well suited to the storage of highly segmented images. It utilizes an image compression scheme that retains pixel values in high intensity gradient areas eliminating low information background regions. The remaining pixels are stored in sparse format along with their relative locations encoded into 32 bit words. The result is a highly reduced image data set that retains the original correlation information of the image. Compression ratios of 30:1 using this method are typical. As a result, far fewer memory calls and data entry comparisons are required to accurately determine tracer particle movement. In addition, by utilizing an error correlation function, pixel comparisons are made through single integer calculations eliminating time consuming multiplication and floating point arithmetic. Thus, this algorithm typically results in much higher correlation speeds and lower memory requirements than spectral and image shifting correlation algorithms. This paper describes the methodology of sparse array correlation as well as the speed, accuracy, and limitations of this unique algorithm. While the study presented here focuses on the process of correlating images stored in sparse format, the details of an image compression algorithm based on intensity gradient thresholding is presented and its effect on image correlation is discussed to elucidate the limitations and applicability of compression based PIV processing.
Numerical Study of Oscillation Mechanism in Underexpanded Jet Impinging on Plate
Authors: Sakakibara Y. , Iwamoto J.
Published: February 2008

The mechanism of the oscillatory phenomena of an underexpanded jet impinging on a flat plate is studied numerically. Pressure changes generated in the flow field near the plate propagate radially in the surrounding region of the jet. The configuration of the jet boundary is changed by them and so, the waves forming the underexpanded jet are displaced when they are reflected from the jet boundary. And then, the pressure disturbances return to the region near the plate. Unsteady flow with repetition of growth and decay of the separation bubble on the plate is also found under certain conditions.
Thickness Variation of a Liquid Sheet Formed by Two Impinging Jets Using Holographic Interferometry
Authors: Shen Y.-B. , Poulikakos D.
Published: February 2008

In the work presented in this paper a real time holographic interferometry technique is developed to measure instantaneously and nonintrusively the thickness distribution of a liquid sheet formed by the impingement of two liquid jets. The experimental results are compared with earlier largely unverified analytical predictions. It is shown that the assumption that the sheet thickness is inversely proportional to the radial distance from the impingement point is in principle good. The dependence of the theoretically obtained proportionality constant on the azimuthal angle, however, while exhibiting the same trend it also shows some quantitative differences. Reasons are given in the context of the work. In addition, a weak effect of the jet velocity on the proportionality constant is found to exist. In the theories no such effect was modeled. Finally, comparisons between theoretical and experimental isothickness contours show differences. Overall, there appears to be a justification for improved theoretical studies including effects such as that of gravitation.
Flowfield Investigation of the Effect of Rib Open Area Ratio in a Rectangular Duct
Authors: Liou Tong-Miin , Kao Chih-Wen , Chen Shih-Hui
Published: February 2008

The spatially periodic turbulent fluid flows and friction in a rectangular passage of width-to-height ratio of 4:1 with perforated rectangular ribs mounted on one wall have been studied using laser Doppler velocimetry and pressure probing. The parameters fixed were rib height to duct hydraulic diameter ratio of 0.106, rib width-to-height ratio of 0.76, rib pitch-to-height ratio of 10, and Reynolds number of 2 × 104, while the main parameter investigated was the rib open-area ratio (β) with values of 0%, 10%, 22%, 38%, and 44%. Two critical ranges of β and three characteristic flow regimes were identified, which provides useful references of practical tests of computational models. The results also showed that the dominant fluid dynamic factors responsible for the reported peak values of local Nusselt number distribution could be recognized. Moreover, the secondary-flow mean velocity components were found to be one to two orders of magnitude smaller than the bulk mean velocity.
Numerical Analysis of the Flow in an Annular-Conical Passage
Authors: Balatka K. , Mochizuki S.
Published: February 2008

The purpose of this paper is to bring new insights into the flow phenomena in an annular-conical passage, which—as previously disclosed experimentally—forms a toroidal-vortex street. The two-dimensional, time dependent Navier-Stokes equations are solved with an explicit finite-difference scheme based on the Marker and Cell method. Solutions are obtained for four different cone apex angles (β = 60, 90, 120, and 180 deg). For each apex angle three cases of different passage spacing are computed. The Reynolds number ranges from Re = 100 ˜ 5000. The stress is put on the initiation and subsequent development of the toroidal-vortex street. Critical Reynolds number for several parameter settings is determined and compared with experimental results.
Steady and Unsteady Flow Field at Pump and Turbine Exits of a Torque Converter
Authors: Dong Y. , Lakshminarayana B. , Maddock D.
Published: February 2008

The steady and unsteady flow field at the pump and the turbine exit of a 245 mm diameter automotive torque converter was measured by a miniature high-frequency-response five-hole probe and a high-frequency-response total pressure Pitot probe in the stationary reference frame. The data were decomposed into blade periodic, blade aperiodic, and unresolved unsteady components. The periodic flow data shows that the pump exit flow has four major zones; the free-stream flow, the blade wake flow, the core-suction corner separation flow, and the mixing zone. The unsteady flow data shows that the unsteadiness in the free-stream is uniform, and the unsteadiness in the wake mixing flow zone is very high. The turbine exit flow is almost fully developed at the measurement plane, the flow field is uniform in the tangential direction, and only radial gradients in flow properties exist. A region of separated flow with high unsteadiness and high axial component of vorticity was observed at the measurement plane near the core.
A Design Method for High-Speed Propulsor Blades
Authors: Griffin Paul E. , Kinnas Spyros A.
Published: February 2008

This study uses a nonlinear optimization method coupled with a vortex lattice cavitating propeller analysis method to design efficient propeller blades. Different constraints are imposed to improve propeller design. Several advancements in the method are shown, including the option for quadratic skew, user specified skew distribution, and a constraint limiting the minimum pressure in wetted regions of the blade. Results for a series of fully wetted runs demonstrate the effectiveness of the constraint on minimum pressure in preventing the onset of bubble or mid-chord cavitation. A comparison of a design in uniform inflow with a design in non-axisymmetric inflow indicates that a propeller designed by the present method in non-axisymmetric inflow has more favorable cavitating flow characteristics than a propeller design assuming uniform inflow. Results are also shown for a series of runs utilizing the cavity constraints. These results indicate that the present method can be used to improve on propeller designs by imposing constraints on the cavity area and cavity volume velocity harmonics, as well as by using a quadratic skew distribution.
The TAR Model for Calculation of Droplet/Wall Impingement
Authors: Xu Haitao , Liu Yongchang , He Ping , Wang Haiqing
Published: February 2008

In this paper, we present a new model, called the TAR model, for calculation of droplet/wall impingement. Using this model, we find that the critical Weber number for rebound is not a constant. It varies with the droplet radius. For large drops, the critical Weber numbers and rebound velocities predicted by the TAR model agree with experimental results very well. Whereas, the predicted hydrodynamic behavior of small droplets is very different from that of large drops. This conclusion is significant for modeling engine spray/wall interaction.
Two-Phase Flow From a Stratified Region Through a Small Side Branch
Authors: Hassan I. G. , Soliman H. M. , Sims G. E. , Kowalski J. E.
Published: February 2008

Experimental data are presented for the mass flow rate and quality of two-phase (air-water) discharge through a small branch (6.35 mm i.d.) located on the side of a large reservoir under stratified conditions. These data correspond to different values of the interface level between the onsets of gas and liquid entrainments for test-section pressures ranging from 316 to 517 kPa, test-section-to-separator pressure differences ranging from 40 to 235 kPa, and different hydraulic resistances of the line connecting the test section and separator. Influences of these independent variables on the mass flow rate and quality are discussed and normalized plots are presented showing that the data can be collapsed for a wide range of conditions. Comparisons are made with previous investigations and new empirical correlations are formulated and shown to be capable of predicting the present data with good accuracy.
Transport of Heavy Particles in a Three-Dimensional Mixing Layer
Authors: Wang Qunzhen , Squires Kyle D.
Published: February 2008

Particle transport in a three-dimensional, temporally evolving mixing layer has been calculated using large eddy simulation of the incompressible Navier-Stokes equations. The initial fluid velocity field was obtained from a separate simulation of fully developed turbulent channel flow. The momentum thickness Reynolds number ranged from 710 in the initial field to 4460 at the end of the calculation. Following a short development period, the layer evolves nearly self-similarly. Fluid velocity statistics are in good agreement with both the direct numerical simulation results of Rogers and Moser (1994) and experimental measurements of Bell and Mehta (1990). Particles were treated in a Lagrangian manner by solving the equation of motion for an ensemble of 20,000 particles. The particles have the same material properties as in the experiments of Hishida et al. (1992), i.e., glass beads with diameters of 42, 72, and 135 μm. Particle motion is governed by drag and gravity, particle-particle collisions are neglected, and the coupling is from fluid to particles only. In general, the mean and fluctuating particle velocities are in reasonable agreement with the experimental measurements of Hishida et al. (1992). Consistent with previous studies, the Stokes number (St) corresponding to maximum dispersion increases as the flow evolves when defined using a fixed fluid timescale. Definition of the Stokes number using the time-dependent vorticity thickness, however, shows a maximum in dispersion throughout the simulation for St ≈ 1.
Particle Entrainment in a Bounded Rotating Flow With a Drain
Authors: Mang J. , Minkov E. , Schaflinger U. , Ungarish M.
Published: February 2008

A bathtub vortex is usually formed at the axis of a drain. In the presence of such a vortex, gravity separation of solid impurities lighter than the embedding fluid is modified by centrifugal separation and viscous resuspension. Both mechanisms are responsible for the agglomeration of impurities at the axis of the vortex. From there the impurities are easily sucked into the outlet. In the investigated case, a viscous fluid with a given initial rotation is spinning down in a container with endplates both at the bottom and the top. The amount of fluid withdrawn through a circular hole in the center of the vortex is constantly replaced by a radial influx. The resulting time-dependent flow was solved by means of a finite difference method taking into account the influence of Ekman layers at the bottom and the top. Subsequently, the process of centrifugal separation of particles lighter than the embedding fluid was studied in the aforementioned flow field. The results were compared with the particle motion in a classical Oseen vortex. For a simplified case an analytical solution was derived and compared with the corresponding numerical solution. Both results were found to be in good agreement.
The Interaction of Spinning Liquid Film With Swirling Gas in Cylindrical Vessel—Experiments and Numerical Simulations
Authors: Birk Avi , DeSpirito James
Published: February 2008

Experimental flow visualizations and numerical simulations of the interaction of a spinning liquid film with a swirling gas in a cylindrical vessel are reported. A gas/liquid flow that simulates the high-pressure conditions of combustion was successfully visualized in a transparent test chamber. The test chamber was a mockup of a liquid propellant gun ignition system component called the hydrodynamically-stabilized combustor. Water-glycerol mixtures were used for the liquid, and ballistically compressed helium-nitrogen was used for the gas. The liquid is injected tangentially along the cylindrical test chamber wall where it spreads as a spinning film. The gas is then injected tangentially and interacts with the liquid. The flows were insensitive to the tilt angle of the test chamber and only mildly sensitive to the liquid viscosity. Liquid entrainment by the gas and subsequent atomization occurs promptly (within 2 ms) after the onset of gas injection, and the flow in the test chamber vent passage is a swirling, transonic, two-phase flow. Two types of three-dimensional simulations of the liquid and gas injection into the test chamber were performed using the CRAFT Navier-Stokes code. The first type was of the initial liquid flow only. The second type was of the high-pressure gas injection into the chamber, with the liquid initialized in an annulus around the chamber surface with a swirl velocity. The numerical simulations were successful in capturing the primary characteristics of the flow phenomena observed in the experimental flow visualizations. This included yielding the observed liquid flow patterns before gas injection and capturing the cellular structure observed after gas injection.
A Three-Dimensional Analysis of Rotordynamic Forces on Whirling and Cavitating Helical Inducers
Authors: d’Agostino Luca , d’Auria Fabrizio , Brennen Christopher E.
Published: February 2008

This paper investigates the linearized dynamics of three-dimensional bubbly cavitating flows in helical inducers. The purpose is to understand the impact of the bubble response on the radial and tangential rotordynamic forces exerted by the fluid on the rotor and stator stages of whirling turbomachines under cavitating conditions. The flow in the inducer annulus is modeled as a homogeneous inviscid mixture, containing vapor bubbles with a small amount of noncondensable gas. The effects of several contributions to the damping of the bubble dynamics are included in the model. The governing equations of the inducer flow are written in “body-fitted” orthonormal helical Lagrangian coordinates, linearized for small-amplitude perturbations about the mean flow, and solved by modal decomposition. The whirl excitation generates finite-speed propagation and resonance phenomena in the two-phase flow within the inducer. These, in turn, lead to a complex dependence of the lateral rotordynamic fluid forces on the excitation frequency, the void fraction, the average size of the cavitation bubbles, and the turbopump operating conditions (including, rotational speed, geometry, flow coefficient and cavitation number). Under cavitating conditions the dynamic response of the bubbles induces major deviations from the noncavitating flow solutions, especially when the noncondensable gas content of the bubbles is small and thermal effects on the bubble dynamics are negligible. Then, the quadratic dependence of rotordynamic fluid forces on the whirl speed, typical of cavitation-free operation, is replaced by a more complex behavior characterized by the presence of different regimes where, depending on the whirl frequency, the fluid forces have either a stabilizing or a destabilizing effect on the inducer motion. Results are presented to illustrate the influence of the relevant flow parameters.
A New Calibration Method for Dynamically Loaded Transducers and Its Application to Cavitation Impact Measurement
Authors: Soyama Hitoshi , Lichtarowicz Andrzej , Momma Takahiro , Williams Edward J.
Published: February 2008

The erosion produced by cavitation is a serious problem in hydraulic machinery. During investigations of the dynamic loading generated by collapsing cavitation on a surface, a dynamic pressure transducer was developed. The piezoelectric polymer PVDF (Polyvinylidene fluoride) was used as the pressure sensitive material. A novel method of dynamic calibration has also been developed. The transducer is loaded through pencil lead by a beam supported at its other end on a knife edge and loaded at the center by weights. As the static load is increased, the pencil lead breaks and the load is released suddenly. The unloading time is faster than for any other conventional calibration method and is of the same order as cavitation loading. Descriptions of the developments of both the calibration method and the transducer are given. The principal advantages of the new method are the short pulse duration and the simplicity of the test procedure. The paper is an extension of the previously reported work by Momma and Lichtarowicz (1994), giving further information on the operating characteristics of the transducer in comparison with the traditional ball-dropping method.
Cavitation Nuclei Population and Event Rates
Authors: Liu Zhenhuan , Brennen Christopher E.
Published: February 2008

To model the processes of cavitation inception, noise and damage, it is necessary to generate a model of the cavitation event rate which can then be coupled with the consequences of the individual events to produce a complete synthesis of the phenomenon. In this paper we describe recent efforts to connect the observed event rates to the measured distributions of cavitation nuclei in the oncoming stream. Comparisons are made between the observed event rates and event rates calculated from measured nuclei distributions using an algorithm which includes the dynamics of the nuclei motion and growth. Various complications are explored including the effect of the boundary layer, the relative motion between the nucleus and the liquid, the observable bubble size effect, and the effect of bubble growth on neighboring nuclei. All of these are seen to have important influences on the event rate, and therefore, on cavitation inception and other macroscopic consequences. We demonstrate that it is possible to predict the correct order of magnitude of the event rate when an attempt is made to model the important flow complications.
Experimental Investigations of Free-Surface Aeration in the Developing Flow of Two-Dimensional Water Jets
Authors: Brattberg T. , Chanson H. , Toombes L.
Published: February 2008

Turbulent water jets discharging into the atmosphere are often characterized by a substantial amount of free-surface aeration. The effects can be detrimental or beneficial. In any case, the knowledge of the air entrainment mechanisms is essential for an optimum design. New experimental data are presented in the developing flow region of two-dimensional water jets discharging into air. The results indicate that the air diffusion takes place rapidly downstream of the nozzle and it is nearly independent of the momentum transfer process. Further, the distribution of air bubble frequency may be related to the air content distribution by a parabolic relationship.
Elements of an Approach to the Assessment of Systematic Uncertainty in Transient Measurements
Authors: Maciejewski Paul K.
Published: February 2008

Although there has been an increasing interest in experimental research investigating time-dependent fluid phenomena, accepted methods for assessing and reporting measurement uncertainty, i.e., those contained in ANSI/ASME PTC 1991-1998. do not consider issues pertaining specifically to the assessment of uncertainty in transient measurements. Complementing the author’s previous work which presented a method for assessing the random component of uncertainty in transient measurements, this paper presents a method for assessing the systematic component of uncertainty in transient measurements.
Some Explicit Analytical Solutions of Unsteady Compressible Flow
Authors: Cai Ruixian
Published: February 2008

The analytical solutions of different flows are meaningful in theory. In addition, they are very useful to CFD also as the standard solutions to check the numerical solutions and to develop numerical differencing schemes, grid generation methods and so forth. However, very few analytical solutions have been known for the compressible flow, especially the unsteady flow. Some explicit analytical solutions of unsteady compressible flow have been derived since the early nineties and are given in this paper, which include the 1-D isentropic unsteady flow, 1-D unsteady flow with friction, heat transfer as well as with shock, and 2-D isentropic unsteady flow.
Turbulence Measurements of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer
Authors: Zhang X.
Published: February 2008

A longitudinal vortex in a flat-plate turbulent boundary layer was examined in a wind tunnel experiment using Laser Doppler Anemometry. The vortex was produced by an inclined round jet (D = 14 mm) in the turbulent boundary layer (δ0.99 ≈ 25 mm). The jet nozzle was positioned at pitch and skew angles of 45 deg to the oncoming stream, and the jet speed ratios (jet speed/freestream flow speed) were 0.5, 1.0, and 1.5. The flow was characterized by embedded vortices, induced high turbulent kinetic energy peak, local areas of high primary shear stress, and negative shear stress. Two types of normal stress evolution were observed: (a) low normal stress beneath the vortex on the upwash side and high normal stress above the center of the vortex, caused by spanwise momentum transfer and local turbulent production; (b) high normal stress beneath the vortex on the upwash side and high normal stress coinciding with the center of the vortex, produced by spanwise and transverse momentum transfer of a vortex away from the wall with turbulent convection playing an important role. The study provided a database for numerical modeling effort.
Identification of Coherent Structure in Turbulent Shear Flow With Wavelet Correlation Analysis
Authors: Li Hui
Published: February 2008

In order to identify coherent structure of turbulent shear flow, a new combination of familiar techniques of signal processing, called wavelet correlation analysis, is developed based on the wavelet transform. The wavelet correlation analysis provides the unique capability for decomposing the correlation of arbitrary signals over a two-dimensional time delay-period plane. By analyzing two superposition functions implicating several pure frequencies, the correlation of periodic oscillations at several frequencies can well be separated and observed clearly. Coherent structures in the intermediate region of a plane turbulent jet are investigated using the wavelet correlation method. It is shown that the wavelet correlation analysis can extract the most essential scales governing features of eddy motions. The coherent structure information and apparent flapping behaviors are clearly revealed over a two-dimensional time-period plane.
Analysis of Fluid-Structure Interaction by Means of Dynamic Unstructured Meshes
Authors: Blom F. J. , Leyland P.
Published: February 2008

This paper presents a computational analysis on forced vibration and fluid-structure interaction in compressible flow regimes. A so-called staggered approach is pursued where the fluid and structure are integrated in time by distinct solvers. Their interaction is then taken into account by a coupling algorithm. The unsteady fluid motion is simulated by means of an explicit time-accurate solver. For the fluid-structure interaction problems which are considered here the effects due to the viscosity can be neglected. The fluid is hence modeled by the Euler equations for compressible inviscid flow. Unstructured grids are used to discretise the fluid domain. These grids are particularly suited to simulate unsteady flows over complex geometries by their capacity of being dynamically refined and unrefined. Dynamic mesh adaptation is used to enhance the computational precision with minimal CPU and memory constraints. Fluid-structure interaction involves moving boundaries. Therefore the Arbitrary Lagrange Euler method (ALE-method) is adopted to solve the Euler equations on a moving domain. The deformation of the mesh is controlled by means of a spring analogy in conjunction with a boundary correction to circumvent the principle of Saint Venant. To take advantage of the differences between fluid and structure time scales, the fluid calculation is subcycled within the structural time step. Numerical results are presented for large rotation, pitching oscillation and aeroelastic motion of the NACA0012 airfoil. The boundary deformation is validated by comparing the numerical solution for a flat plate under supersonic flow with the analytical solution.
Sea Trials of the Ducted Tip Propeller
Authors: Hordnes Ivar , Green Sheldon I.
Published: February 2008

Sea trials of a 45 ft seine boat, fitted first with a conventional tip, and subsequently with a ducted tip, 36 in. diameter propeller have been conducted. The trials consisted of careful propeller efficiency measurements at a number of advance ratios, and underwater video imaging of the different propellers under Bollard pull conditions. The trials have shown that flow-through ducts installed at the blade tips suppress the tip vortex roll-up, thus resulting in a substantial delay in the onset of tip vortex cavitation. The cavitation inception index for the ducted tip propeller is 50 percent lower than for the conventional propeller. This cavitation improvement comes without any efficiency penalty. In fact, the efficiency of the ducted tip propeller is up to 6 percent higher (at high advance ratios) than the efficiency of the same propeller fitted with conventional tips. The ducted tip propeller was originally thought to be useful only for boats with heavily loaded propellers, such as tug boats and trawlers. The current research has shown that the ducted tips may have much broader application.
Heterogeneous Drag Reduction Concepts and Consequences
Authors: Hoyer Klaus W. , Gyr Albert
Published: February 2008

This paper deals with the nature of the heterogeneous drag reduction which occurs in turbulent pipe flows when a concentrated polymer solution is injected into the pipe center. According to earlier concepts, the achieved drag reduction is due to a direct, large-scale interaction of the viscoelastic polymer thread with the turbulent flow field. The authors prove that the heterogeneous drag reduction originates exclusively from agglomerates of dissolved polymer molecules present in the flow.
Numerical Simulation of Concentrated Emulsion Flows
Authors: Loewenberg Michael
Published: February 2008

The macroscopic flow and detailed microphysics of a concentrated emulsion are described with three-dimensional numerical simulations. Numerical predictions for deformable drop interactions in shear-flow are in very close agreement to direct microscopic measurements. The results illustrate that drop deformation stabilizes drops against coalescence. Numerical simulations are used to describe an emulsion in shear flow at dispersed-phase volume fractions up to 30 percent. Shear-thinning viscosities and large normal stresses are found. The results are used to describe pressure-driven flow of a concentrated emulsion in a cylindrical tube. Blunted macroscopic velocity profiles and shear-thinning apparent viscosities are predicted. Our results suggest that some features of moderately concentrated emulsion flows can be predicted by an effective mean-field model.
Toward a Vortex Breakdown Condition for Swirling Annular Jets
Authors: Stein C. F.
Published: February 2008

Vortex breakdown is a significant phenomenon in science and technology. In spite of extensive research, the question of the underlying mechanisms for vortex breakdown still lacks a definite answer. The uncertainty of the governing principles for vortex breakdown is revealed by the common use of a variety of different parameters to describe the degree of swirl. In this paper, a theoretical discussion on the suitability of three kinds parameters was conducted, and it was found that one appears to be the natural one if the flow is primarily a swirling channel flow, but if the jet character of the flow is dominant, another one appears to be the most suitable. CFD simulations were performed for a channel with an annular inlet considerably smaller than the channel width. For this case the jet character of the flow should predominate and it was found that the parameter, which theoretically appeared to be best suited for jet flows, indeed was less dependent of the detailed inflow geometry than the others.
Pipe Flow With Radial Inflow: Experimental and Modeling Work
Authors: Schulkes R. M. S. M. , Rinde Trygve , Utvik Ole Harald
Published: February 2008

In this paper results are presented from experiments in which the pressure loss in single-phase pipe flow is studied when radial inflow occurs. Experiments have been carried out with pipes which have different perforation geometries so as to be able to investigate the effect of perforation geometry on the pressure loss. Data analysis of these experiments, as well as analysis of experiments carried out by other groups, yields a pressure loss model which accurately describes pressure losses in single-phase pipe flow with radial inflow through perforations in the pipe wall. The experimental data is subsequently used to establish a numerical value of a parameter which is used in a model description. This leads to the formulation of an effective friction factor for pipe flow with radial inflow.
Optimization of Trailing Edge Ejection Mixing Losses: A Theoretical and Experimental Study
Authors: Schobeiri M. T. , Pappu K.
Published: February 2008

The aerodynamic effects of trailing edge ejection on mixing losses downstream of cooled gas turbine blades were experimentally investigated and compared with an already existing one-dimensional theory by Schobeiri (1989). The significant parameters determining the mixing losses and, therefore, the efficiency of cooled blades, are the ejection velocity ratio, the cooling mass flow ratio, the temperature ratio, the slot thickness ratio, and the ejection flow angle. To cover a broad range of representative turbine blade geometry and flow deflections, a General Electric power generation gas turbine blade with a high flow deflection and a NASA-turbine blade with intermediate flow deflection and different thickness distributions were experimentally investigated and compared with the existing theory. Comprehensive experimental investigations show that for the ejection velocity ratio μ = 1, the trailing edge ejection reduces the mixing losses downstream of the cooled gas turbine blade to a minimum, which is in agreement with the theory. For the given cooling mass flow ratios that are dictated by the heat transfer requirements, optimum slot thickness to trailing edge thickness ratios are found, which correspond to the minimum mixing loss coefficients. The results allow the turbine aerodynamicist to minimize the mixing losses and to increase the efficiency of cooled gas turbine blades.
Effects of Increasing Particle Loading in an Axisymmetric, Vertical, Liquid-Solid Sudden Expansion Flow
Authors: Founti Maria , Achimastos Thomas , Klipfel Athinodoros
Published: February 2008

Measurements of particle and fluid velocities are reported for a turbulent, liquid-solid, sudden expansion flow flowing in the direction of gravity and laden with solid particles, at loadings equal to 1, 2, 3, 4, and 5 percent per volume. The measured two-phase flow velocities are compared to the characteristics of the corresponding single phase flow. Forces and flow mechanisms affecting particle dispersion in the various flow regimes are identified and it is indicated that there exist regions where the transverse Saffman lift force attains high values and controls particle dispersion. A consistent correlation between the mean reattachment point and the volumetric particle loading is indicated. All the two phase flows examined reattached upstream the corresponding mean reattachment location measured for the single phase flow. Increasing particle concentration affected locally the flow behaviour, with most obvious consequences within the recirculation zone and the near wall region.
Measurement in the Wake Region of Two Bubbles in Close Proximity by Combined Shadow-Image and PIV Techniques
Authors: Tokuhiro A. , Fujiwara A. , Hishida K. , Maeda M.
Published: February 2008

An experimental study on flow around two similarly-sized, adjacent air bubbles confined in a 1000 mm vertical, square channel (100 × 100 mm2) with downward flow of water was conducted. The bubbles were D = 11.7 mm in major diameter, ellipsoidal in shape (0.4 ml volume) and 12 mm apart. The Reynolds and Eo¨tvo¨s numbers were 1950 < ReD < 2250, 11 < Eo < 11.5 such that the bubbles oscillated. Velocity measurements were taken using Digital Particle Image Velocimetry, Complemented by Laser Induced Fluorescence. Simultaneously, a second CCD camera recorded the shadow image of the bubble pair’s motions. Visualization revealed that the bubbles move out of phase and do not collide nor coalesce. The velocity data revealed the dynamic interaction of two wake-flow velocity fields with a jet-like flow in-between. From the DPIV data, estimates of the vorticity, Reynolds-stress and turbulent kinetic energy (TKE) distributions confirmed the spatio-temporal nature of the flow. Details will be presented.
Concentration Profiles in Shallow Turbulent Wakes
Authors: Balachandar R. , Tachie M. F. , Chu V. H.
Published: February 2008

The present study deals with the noninvasive measurement of concentration in the intermediate shallow turbulent wake region using a video-imaging technique. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. On the basis of the observed behavior, the waves are classified as deep-shallow wakes and shallow-shallow wakes. The topology of the dye concentration distribution in the near and intermediate wake region indicates that the vortex structure tends to be preserved when the flow depth is relatively high and the dominant eddy structures are similar to that noticed in conventional two-dimensional wakes. In shallow-shallow wakes, the conventional Karman vortex street appears to be annihilated or intermittent. The lateral concentration distribution at several axial stations covering the first thirty body widths are considered for analysis. The instantaneous concentrations are observed to be several times higher than the corresponding mean values. Attempts are also made to determine the paths traversed by the vortex cores and the vortex core convection velocity. The axial variation of the wake half-width with depth of flow is also examined. A model is developed to predict the spread of the wake with downstream distance from the test body. A friction length scale is introduced in the model to account for the influence of depth and bed friction on the development of the wake.