scholarly journals Experiments on the elliptic instability in vortex pairs with axial core flow

2011 ◽  
Vol 677 ◽  
pp. 383-416 ◽  
Author(s):  
CLÉMENT ROY ◽  
THOMAS LEWEKE ◽  
MARK C. THOMPSON ◽  
KERRY HOURIGAN
Keyword(s):  
Growth Rate ◽  
Axial Velocity ◽  
Water Channel ◽  
Three Dimensional ◽  
Axial Flow ◽  
Short Wave ◽  
Measured Velocity ◽  
Vortex Pairs ◽  
Theoretical Predictions ◽  
Good Agreement

Results are presented from an experimental study on the dynamics of pairs of vortices, in which the axial velocity within each core differs from that of the surrounding fluid. Co- and counter-rotating vortex pairs at moderate Reynolds numbers were generated in a water channel from the tips of two rectangular wings. Measurement of the three-dimensional velocity field was accomplished using stereoscopic particle image velocimetry, revealing significant axial velocity deficits in the cores. For counter-rotating pairs, the long-wavelength Crow instability, involving symmetric wavy displacements of the vortices, could be clearly observed using dye visualisation. Measurements of both the axial wavelength and the growth rate of the unstable perturbation field were found to be in good agreement with theoretical predictions based on the full experimentally measured velocity profile of the vortices, including the axial flow. The dye visualisations further revealed the existence of a short-wavelength core instability. Proper orthogonal decomposition of the time series of images from high-speed video recordings allowed a precise characterisation of the instability mode, which involves an interaction of waves with azimuthal wavenumbers m = 2 and m = 0. This combination of waves fulfils the resonance condition for the elliptic instability mechanism acting in strained vortical flows. A numerical three-dimensional stability analysis of the experimental vortex pair revealed the same unstable mode, and a comparison of the wavelength and growth rate with the values obtained experimentally from dye visualisations shows good agreement. Pairs of co-rotating vortices evolve in the form of a double helix in the water channel. For flow configurations that do not lead to merging of the two vortices over the length of the test section, the same type of short-wave perturbations were observed. As for the counter-rotating case, quantitative measurements of the wavelength and growth rate, and comparison with previous theoretical predictions, again identify the instability as due to the elliptic mechanism. Importantly, the spatial character of the short-wave instability for vortex pairs with axial flow is different from that previously found in pairs without axial flow, which exhibit an azimuthal variation with wavenumber m = 1.

scholarly journals Steady flow separation patterns in a 45 degree junction

2000 ◽  
Vol 411 ◽  
pp. 1-38 ◽  
Author(s):  
C. ROSS ETHIER ◽  
SUJATA PRAKASH ◽  
DAVID A. STEINMAN ◽  
RICHARD L. LEASK ◽  
GREGORY G. COUCH ◽  
...  
Keyword(s):  
Flow Separation ◽  
Stokes Equations ◽  
Bypass Graft ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Measured Velocity ◽  
Curved Tube

Numerical and experimental techniques were used to study the physics of flow separation for steady internal flow in a 45° junction geometry, such as that observed between two pipes or between the downstream end of a bypass graft and an artery. The three-dimensional Navier–Stokes equations were solved using a validated finite element code, and complementary experiments were performed using the photochromic dye tracer technique. Inlet Reynolds numbers in the range 250 to 1650 were considered. An adaptive mesh refinement approach was adopted to ensure grid-independent solutions. Good agreement was observed between the numerical results and the experimentally measured velocity fields; however, the wall shear stress agreement was less satisfactory. Just distal to the ‘toe’ of the junction, axial flow separation was observed for all Reynolds numbers greater than 250. Further downstream (approximately 1.3 diameters from the toe), the axial flow again separated for Re [ges ] 450. The location and structure of axial flow separation in this geometry is controlled by secondary flows, which at sufficiently high Re create free stagnation points on the model symmetry plane. In fact, separation in this flow is best explained by a secondary flow boundary layer collision model, analogous to that proposed for flow in the entry region of a curved tube. Novel features of this flow include axial flow separation at modest Re (as compared to flow in a curved tube, where separation occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.

A Simple Statistical Theory for Grain Growth in Materials.

1990 ◽  
Vol 209 ◽  
Author(s):  
P. Mulheran ◽  
J.H. Harding
Keyword(s):  
Growth Rate ◽  
Monte Carlo ◽  
Statistical Theory ◽  
Stochastic Model ◽  
Grain Growth ◽  
Three Dimensional ◽  
Monte Carlo Procedure ◽  
Short Time ◽  
2D And 3D ◽  
Good Agreement

A Monte Carlo procedure has been used to study the ordering of both two and three dimensional (2d and 3d) Potts Hamiltonians, further to the work of Anderson et al. For the 3d lattice, the short time growth rate is found to be much slower than previously reported, though the simulated microstructure is in agreement with the earlier studies. We propose a new stochastic model that gives good agreement with the simulations.

Studies on the Three-Dimensional Temperature Transients in the Canine Prostate During Transurethral Microwave Thermal Therapy

2000 ◽  
Vol 122 (4) ◽  
pp. 372-379 ◽  
Author(s):  
Jing Liu ◽  
Liang Zhu ◽  
Lisa X. Xu
Keyword(s):  
Three Dimensional ◽  
Blood Perfusion ◽  
Thermal Therapy ◽  
Bioheat Transfer ◽  
Target Tissue ◽  
Urethral Wall ◽  
Theoretical Predictions ◽  
Closed Form Analytical Solution ◽  
Good Agreement

Thermal therapy of benign prostatic hyperplasia requires accurate prediction of the temperature distribution induced by the heating within the prostatic tissue. In this study, the Pennes bioheat transfer equation was used to model the transient heat transfer inside the canine prostate during transurethral microwave thermal therapy. Incorporating the specific absorption rate of microwave energy in tissue, a closed-form analytical solution was obtained. Good agreement was found between the theoretical predictions and in-vivo experimental results. Effects of blood perfusion and the cooling at the urethral wall on the temperature rise were investigated within the prostate during heating. The peak intraprostatic temperatures attained by application of 5, 10, or 15 W microwave power were predicted to be 38°C,41°C, and 44°C. Results from this study will help optimize the thermal dose that can be applied to target tissue during the therapy. [S0148-0731(00)01004-9]

Backward Extrusion of Internally Shaped Tubes From Round Billets

1984 ◽  
Vol 106 (2) ◽  
pp. 143-149 ◽  
Author(s):  
D. Y. Yang ◽  
C. H. Han
Keyword(s):  
Pure Aluminum ◽  
Three Dimensional ◽  
Mapping Function ◽  
Velocity Transformation ◽  
Backward Extrusion ◽  
Commercially Pure ◽  
Theoretical Predictions ◽  
Experimental Values ◽  
Punch Pressure ◽  
Good Agreement

An analytical method is proposed for estimating the steady-state punch pressure for three-dimensional backward extrusion (or piercing) of complicated internally shaped tubes from circular billets. A kinematically admissible velocity field is derived to formulate an upper-bound solution using velocity transformation and mapping function. The configuration of deforming boundary surfaces are determined by minimizing the extrusion power with respect to some chosen parameters. Experiments are carried out with commercially pure aluminum billets for internally shaped tubes at various reductions of area by using different sizes of shaped punches, such as square and regular hexagons. It is shown that the theoretical predictions for extrusion load are in good agreement with the experimental values.

Turbulence Modeling of Axial Flow in a Bare Rod Bundle

1979 ◽  
Vol 101 (4) ◽  
pp. 628-634 ◽  
Author(s):  
J. G. Bartzis ◽  
N. E. Todreas
Keyword(s):  
Secondary Flow ◽  
Turbulent Flows ◽  
Axial Velocity ◽  
Nuclear Reactor ◽  
Turbulence Modeling ◽  
Three Dimensional ◽  
Axial Flow ◽  
Reynolds Stresses ◽  
Rod Bundle ◽  
Aspect Ratios

Temperature distribution within the rod bundle of a nuclear reactor is of major importance in nuclear reactor design. However temperature information presupposes knowledge of the hydrodynamic behavior of the coolant which is the most difficult part of the problem due to the complexity of the turbulence phenomena. In the present work a two equation turbulence model—a strong candidate for analyzing actual three dimensional turbulent flows—has been used to predict fully developed flow of infinite bare rod bundle of various aspect ratios (P/D). The model has been modified to take into account anisotropic effects of eddy viscosity. Secondary flow calculations have been also performed although the model seems to be too rough to predict the secondary flow correctly. Heat transfer calculations have been performed to confirm the importance of anisotropic viscosity in temperature predictions. Experimental measurements of the distribution of axial velocity, turbulent axial velocity, turbulent kinetic energy and radial Reynolds stresses were performed in the developing and fully developed regions. A two channel Laser Doppler Anemometer working in the reference mode with forward scattering was used to perform the measurements in a simulated interior subchannel of a triangular rod array with P/D = 1.124. Comparisons between the analytical results and the results of this experiment as well as other experimental data in rod bundle arrays available in the literature are presented. The predictions are in good agreement with the results for high Reynolds numbers.

Experimental Study of Propeller-Induced Vibratory Pressures on Simple Surfaces and Correlation With Theoretical Predictions

1964 ◽  
Vol 8 (05) ◽  
pp. 15-28
Author(s):  
J. P. Breslin ◽  
T. Kowalski
Keyword(s):  
Free Space ◽  
Three Dimensional ◽  
Theoretical Treatment ◽  
Plate Boundaries ◽  
Practical Application ◽  
Finite Area ◽  
Experimental Procedure ◽  
Theoretical Predictions ◽  
Pressure Changes ◽  
Good Agreement

Vibratory pressures exerted on cylindrical and flat-plate boundaries due to a model propeller were measured at three advance coefficients. A number of "free-space" measurements also were made. All measurements were made by driving a propeller past fixed pressure gages. This method yielded curves of pressure changes which are entirely free from background noise. The magnitudes of the free-space pressures were found to be larger than one half the corresponding magnitudes measured by gages mounted flush in a large plate at equal clearances from the propeller. By postulating that the finite area of the gage diaphragm produces a partial image of the propeller (and hence a larger pressure than that in free space) an experimental procedure was devised for correcting for this finite-area effect yielding results in good agreement with theory. A theoretical treatment of this effect of finite gage size is given in Appendix 2. The decay of maximum amplitudes of vibrating pressures is shown by means of three-dimensional plots. The pressures were found to become vanishingly small within approximately one propeller diameter fore and aft of the center of the propeller. The comparison with theoretically calculated pressures and forces gives very close agreement for free-space pressures and reasonable agreement for forces on a cylindrical surface. The agreement of both pressures and forces with theory is excellent for operation near the design advance ratio. A strong plea is made for further experiments with ship models in an effort to develop design criteria for practical application.

Effect of a Finite Boundary Junction Mobility on the Growth Rate of Two and Three Dimensional Grains

2012 ◽  
Vol 715-716 ◽  
pp. 574-578 ◽  
Author(s):  
Luis A. Barrales Mora
Keyword(s):  
Growth Rate ◽  
Three Dimensional ◽  
Granular System ◽  
Present Contribution ◽  
Von Neumann ◽  
Model Simulations ◽  
Theoretical Approaches ◽  
Dimensional Network ◽  
Zero Growth ◽  
Good Agreement

t has been shown by computer simulations that the MacPherson-Srolovitz relation predicts accurately the growth rate of a grain undergoing ideal grain growth. However, since a finite mobility of the boundary junctions (triple lines and quadruple junctions) affects the evolution of a granular system, it is necessary to modify this equation in order to take into account their effect. In the present contribution, an equation which allows considering these factors is presented and used to modify the von Neumann-Mullins and MacPherson-Srolovitz equations. In order to corroborate these equations two and three dimensional network model simulations were performed. The results showed a very good agreement with the theoretical approaches for both dimensions and all topological classes except those near the classes of zero growth rate in 3D. The reason is that the proposed function is very sensitive to small changes of the finite mobility of the junctions.

The observation of the simultaneous development of a long- and a short-wave instability mode on a vortex pair

1994 ◽  
Vol 265 ◽  
pp. 289-302 ◽  
Author(s):  
P. J. Thomas ◽  
D. Auerbach
Keyword(s):  
Vortex Pair ◽  
Short Wave ◽  
Wave Mode ◽  
Core Diameter ◽  
Vortex Pairs ◽  
Long Wave ◽  
Instability Modes ◽  
Bending Mode ◽  
Theoretical Predictions ◽  
The Stability

Experiments on the stability of vortex pairs are described. The vortices (ratio of length to core diameter L/c of up to 300) were generated at the edge of a flat plate rotating about a horizontal axis in water. The vortex pairs were found to be unstable, displaying two distinct modes of instability. For the first time, as far as it is known to the authors, a long-wave as well as a short-wave mode of instability were observed to develop simultaneously on such a vortex pair. Experiments involving single vortices show that these do not develop any instability whatsoever. The wavelengths of the developing instability modes on the investigated vortex pairs are compared to theoretical predictions. Observed long wavelengths are in good agreement with the classic symmetric long-wave bending mode identified by Crow (1970). The developing short waves, on the other hand, appear to be less accurately described by the theoretical results predicted, for example, by Windnall, Bliss & Tsai (1974).

scholarly journals Ion-channel laser growth rate and beam quality requirements

2018 ◽  
Vol 84 (3) ◽  
Author(s):  
X. Davoine ◽  
F. Fiúza ◽  
R. A. Fonseca ◽  
W. B. Mori ◽  
L. O. Silva
Keyword(s):  
Growth Rate ◽  
Electron Beam ◽  
Ion Channel ◽  
Beam Quality ◽  
Three Dimensional ◽  
Quality Requirements ◽  
Particle In Cell ◽  
Theoretical Predictions ◽  
First Time ◽  
Wakefield Accelerator

In this paper, we determine the growth rate of the exponential radiation amplification in the ion-channel laser, where a relativistic electron beam wiggles in a focusing ion channel that can be created in a wakefield accelerator. For the first time the radiation diffraction, which can limit the amplification, is taken into account. The electron beam quality requirements to obtain this amplification are also presented. It is shown that both the beam energy and wiggler parameter spreads should be limited. Two-dimensional and three-dimensional particle-in-cell simulations of the self-consistent ion-channel laser confirm our theoretical predictions.

A Simplified Method of Using Four-Hole Probes to Measure Three-Dimensional Flow Fields

1985 ◽  
Vol 107 (1) ◽  
pp. 31-35 ◽  
Author(s):  
N. Sitaram ◽  
A. L. Treaster
Keyword(s):  
Flat Plate ◽  
Three Dimensional ◽  
Flow Fields ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Application Procedure ◽  
Simplified Method ◽  
Theoretical Predictions ◽  
Gradient Effects ◽  
Good Agreement

A simplified method of using four-hole probes to measure three-dimensional flow-fields is presented. This method is similar to an existing calibration and application procedure used for five-hole probes. The new method is demonstrated for two four-hole probes of different geometry. These four-hole probes and a five-hole probe are used to measure the turbulent boundary layer on a flat plate. The results from the three probes are in good agreement with theoretical predictions. The major discrepancies occur near the surface of the flat plate and are attributed to wall vicinity and velocity gradient effects.

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