Optical Hilbert Diagnostics of the Vortical Structures Induced by the Pressure Front on an Aperture

2010 ◽  
Vol 437 ◽  
pp. 208-211
Author(s):  
Vitaly A. Arbuzov ◽  
Yury N. Dubnishchev ◽  
Nikolay A. Dvornikov ◽  
Viktor G. Nechaev
Keyword(s):  
Optical Density ◽  
Elevated Pressure ◽  
Vortex Rings ◽  
Cylindrical Chamber ◽  
Vortical Structures ◽  
Hilbert Optics

The methods of Hilbert optics widely use in tasks of visualization and measurement of phase optical density in fluids. In this work, the problem of vortex rings generation upon sudden opening of a cylindrical chamber containing a gas at elevated pressure is considered. At diffraction of pressure front on an aperture in a wall the vortical rings propagated in the opposite directions from the wall have been detected by optical Hilbert methods.

The behaviour of circular synthetic jets in a laminar boundary layer

2005 ◽  
Vol 109 (1100) ◽  
pp. 461-470 ◽  
Author(s):  
S. Zhong ◽  
F. Millet ◽  
N. J. Wood
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Vortex Rings ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
Flow Separation Control ◽  
Wall Flow ◽  
Near Wall

Abstract Dye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0·05 and 0·1ms–1. The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (VR ) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and VR , the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and VR , these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and VR , the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and VR are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

scholarly journals About bubbles and vortex rings

2015 ◽  
Vol 780 ◽  
pp. 1-4 ◽  
Author(s):  
C. Martínez-Bazán
Keyword(s):  
Vortex Rings ◽  
Complex Mechanism ◽  
Complex Interplay ◽  
Single Vortex ◽  
Vortical Structures ◽  
Turbulent Eddies ◽  
Deformation Dynamics ◽  
Bubble Interaction ◽  
Elongation Process ◽  
Shed Light

Bubble interaction with turbulence has a number of applications in engineering processes and nature. The complex interplay between the vortical structures present in a turbulent flow and the bubbles drives their deformation dynamics, which may lead to bubble rupture under the appropriate conditions. Such a process includes nonlinear interaction among the turbulent eddies and between the eddies and the bubbles. Thus, the coupled evolution of a single vortex ring with a bubble represents an idealized scenario that can provide a framework to shed light on understanding such a common and complex mechanism. Jha & Govardhan (J. Fluid Mech., vol. 773, 2015, pp. 460–497) have performed elegant experiments generating controlled vortex rings and injecting bubbles of known volume. They have reported interesting results on the elongation process of the bubble and its impact on vortex dynamics.

Dynamics and Instability of a Vortex Ring Impinging on a Wall

2015 ◽  
Vol 18 (4) ◽  
pp. 1122-1146 ◽  
Author(s):  
Heng Ren ◽  
Xi-Yun Lu
Keyword(s):  
Vortex Ring ◽  
Axial Flow ◽  
Core Region ◽  
Vortical Flow ◽  
Vortex Rings ◽  
Flow Transition ◽  
Azimuthal Component ◽  
Vortical Structures ◽  
The Core ◽  
Eddy Simulation

AbstractDynamics and instability of a vortex ring impinging on a wall were investigated by means of large eddy simulation for two vortex core thicknesses corresponding to thin and thick vortex rings. Various fundamental mechanisms dictating the flow behaviors, such as evolution of vortical structures, formation of vortices wrapping around vortex rings, instability and breakdown of vortex rings, and transition from laminar to turbulent state, have been studied systematically. The evolution of vortical structures is elucidated and the formation of the loop-like and hair-pin vortices wrapping around the vortex rings (called briefly wrapping vortices) is clarified. Analysis of the enstrophy of wrapping vortices and turbulent kinetic energy (TKE) in flow field indicates that the formation and evolution of wrapping vortices are closely associated with the flow transition to turbulent state. It is found that the temporal development of wrapping vortices and the growth rate of axial flow generated around the circumference of the core region for the thin ring are faster than those for the thick ring. The azimuthal instabilities of primary and secondary vortex rings are analyzed and the development of modal energies is investigated to reveal the flow transition to turbulent state. The modal energy decay follows a characteristic –5/3 power law, indicating that the vortical flow has become turbulence. Moreover, it is identified that the TKE with a major contribution of the azimuthal component is mainly distributed in the core region of vortex rings. The results obtained in this study provide physical insight of the mechanisms relevant to the vortical flow evolution from laminar to turbulent state.

On the structure of vortex rings from inclined nozzles

2011 ◽  
Vol 686 ◽  
pp. 451-483 ◽  
Author(s):  
Trung Bao Le ◽  
Iman Borazjani ◽  
Seokkoo Kang ◽  
Fotis Sotiropoulos
Keyword(s):  
Vortex Ring ◽  
Vortex Sheet ◽  
Vortex Rings ◽  
Cylinder Wall ◽  
Primary Vortex ◽  
Vortical Structures ◽  
Ambient Flow ◽  
Piston Cylinder ◽  
Axial Stretching ◽  
Vortex Tubes

AbstractWe carry out numerical simulations to investigate the vortex dynamics of laminar, impulsively driven flows through inclined nozzles in a piston–cylinder apparatus. Our simulations are motivated by the need to provide a complete description of the intricate vortical structures and governing mechanisms emerging in such flows as documented in the experiments of Webster & Longmire (Phys. Fluids, vol. 10, 1998, pp. 400–416) and Troolin & Longmire (Exp. Fluids, vol. 48, 2010, pp. 409–420). We show that the flow is dominated by the interaction of two main vortical structures: the primary inclined vortex ring at the nozzle exit and the secondary stopping ring that arises due to the entrainment of the flow into the cylinder when the piston stops moving. These two structures are connected together with pairs of vortex tubes, which evolve from the continuous vortex sheet initially connecting the primary vortex ring with the interior cylinder wall. In the exterior of the nozzle the key mechanism responsible for the breakup of the vortical structure is the interaction of the stronger inclined primary ring with the weaker stopping ring near the longest lip of the nozzle. In the interior of the nozzle the dynamics is governed by the axial stretching of the secondary ring and the ultimate impingement of this ring on the cylinder wall. Our simulations also clarify the kinematics of the azimuthal flow along the core of the primary vortex ring documented in the experiments by Lim (Phys. Fluids, vol. 10, 1998, pp. 1666–1671). We show that the azimuthal flow is characterized by a pair of two spiral saddle foci at the long and short lips of the nozzle through which ambient flow enters and exits the primary vortex core.

Optically Accessible Multisector Combustor: Application and Challenges of Laser Techniques at Realistic Operating Conditions

2015 ◽  
Author(s):  
U. Meier ◽  
J. Heinze ◽  
E. Magens ◽  
M. Schroll ◽  
C. Hassa ◽  
...  
Keyword(s):  
Optical Density ◽  
Dynamic Range ◽  
Soot Formation ◽  
Laser Doppler Anemometry ◽  
Imaging Techniques ◽  
Measurement Techniques ◽  
Optical Path Length ◽  
Elevated Pressure ◽  
Operating Conditions ◽  
Camera System

For their application in a multisector combustor, several laser-based measurement techniques underwent further development to generate useful results in the demanding environment of highly luminous flames under elevated pressures. The techniques were applied to two burner configurations and the results were used to explain their respective behavior. Multisector combustors at elevated pressure present formidable difficulties to the operation of laser based techniques, as the optical path length is longer than for a single sector while the optical density of the flowing medium can be quite high. Hence, the techniques have to be set up to perform under low signal to noise levels. Nevertheless for a validation exercise geared at multidimensional simulation, quantitative results are requested. Here the modification of standard Laser Induced Incandescence as a means to measure soot concentrations with higher dynamic range is described. For situations where the optical density is too high for the application of imaging techniques, laser absorption was used and its application in the multisector combustor is presented. Since combustion and soot formation is closely coupled to flowfield and mixing, velocity measurements are highly desired for comparison with computed flowfields. Although with Laser-Doppler Anemometry a well-established technique is at hand, the high operating costs of a multisector combustor cannot be supported for the needed time of operation. Therefore an effort was made to make the Particle Imaging Velocimetry technique operable in highly luminous flames by using a second camera. The two-camera system and its operation are described in the paper. Finally the application on two different burner configurations is reported together with chemiluminescence as a tracer for heat release, and differences in soot production are related to the measured flow field.

Oscillating structures in a stretched–compressed vortex

2002 ◽  
Vol 450 ◽  
pp. 207-233 ◽  
Author(s):  
MALEK ABID ◽  
BRUNO ANDREOTTI ◽  
STÉPHANE DOUADY ◽  
CAROLINE NORE
Keyword(s):  
Strain Field ◽  
Linear Analysis ◽  
Kelvin Waves ◽  
Periodic Array ◽  
Vortex Rings ◽  
Two Dimensional ◽  
Pure Strain ◽  
Vortical Structures ◽  
Localized Structures ◽  
Strong Contrast

The dynamics of a vortex subject to a localized stretching is numerically investigated. The structure of the flow is analysed in the case of an initially two-dimensional vortex surrounded by a periodic array of vortex rings localized far from its core. Amplified oscillations of both the axial vorticity and the stretching are found, in strong contrast with Burgers-like vortices. The resulting dynamics is the appearance, around the vortex, of successive vortical structures of smaller and smaller radius and alternate sign embedded in the previous vortical rings. The frequency scaling of the oscillations is recovered by linear analysis (Kelvin modes) but not the amplification nor the shape of the successive tori. An inviscid model based on structures is presented, which compares better with the numerical computations. These results suggest that the formalism of Kelvin waves is not sufficient to describe the full dynamics, which is instead related to the feedback of rotation on stretching and more conveniently described in terms of localized structures. We finally discuss the relative timescales of vortex stretching and of vortex reaction. The Burgers-like vortices, where there is no such reaction, turn out to correspond to a nearly pure strain field, slightly disturbed by rotation.

TRACKING OF VORTICAL STRUCTURES IN THREE-DIMENSIONAL DECAYING HOMOGENEOUS ISOTROPIC TURBULENCE

2011 ◽  
Vol 22 (12) ◽  
pp. 1373-1391 ◽  
Author(s):  
WALEED ABDEL-KAREEM
Keyword(s):  
Velocity Field ◽  
Flow Field ◽  
Isotropic Turbulence ◽  
Three Dimensional ◽  
Statistical Characteristics ◽  
Vortex Rings ◽  
Homogeneous Isotropic Turbulence ◽  
Vortical Structures ◽  
Decaying Turbulence ◽  
Boltzmann Method

Direct numerical simulation of three-dimensional decaying homogeneous isotropic turbulence with a resolution of 1283 is carried out using the Lattice Boltzmann Method (LBM). The aim of this paper is to investigate the statistical characteristics of the obtained turbulent flow field and the behavior of the vortical structures. Most of the LBM simulations of decaying turbulence were mainly focused on the statistical results of the velocity field, however the characteristics of the coherent vortices and their time evolution are ignored. In this paper, the statistical properties of the velocity field as well as the extraction and tracking processes of individual vortices are considered. Results show that the present simulation could recover important features of turbulence such as isotropy, skewness, energy spectrum and elongated vortical structures. A vortex ring is identified in the flow field which can be considered as a sign for the existence of vortex rings in homogeneous isotropic turbulence. Forward and reverse tracking of individual vortical structures shows that vortex rings can be generated from the interaction and the overlapping of vortex tubes.

Simulation of the Pressure Field Due to a Submerged Oscillating Jet Impacting on a Solid Wall

1984 ◽  
Vol 106 (4) ◽  
pp. 491-495 ◽  
Author(s):  
Ph. F. Genoux ◽  
G. L. Chahine
Keyword(s):  
Pressure Field ◽  
Solid Wall ◽  
Vortex Rings ◽  
Constant Frequency ◽  
Vortical Structures ◽  
Oscillating Jet ◽  
Induced Velocity ◽  
High Shearing

This paper presents some results obtained with the simulation of a submerged oscillating jet impacting on a solid wall. The oscillating jet which organizes into large vortical structures is simulated by the emission of vortex rings at a constant frequency. Outside of the cores of the rings the fluid is assumed to be inviscid and irrotational. The positions of the tori are obtained by combining for each torus its self-induced velocity with the velocity induced by all other rings and ring-images in the wall. The tangential velocities and the pressures in the fluid at the wall are then computed. The high shearing and suction forces found at the wall may explain the enhanced erosivity and cleaning action of oscillating jets.

Analysis of photographic emulsions for High-Voltage Electron Microscopy

1993 ◽  
Vol 51 ◽  
pp. 452-453 ◽  
Author(s):  
James R. Kremer ◽  
Paul S. Furcinitti ◽  
Eileen O’Toole ◽  
J. Richard McIntosh
Keyword(s):  
Electron Microscope ◽  
Optical Density ◽  
High Voltage ◽  
Noise Power ◽  
Limited Information ◽  
Modulation Transfer ◽  
Transmission Microscopy ◽  
High Voltage Electron Microscopy ◽  
Voltage Range ◽  
Voltage Electron

Characteristics of electron microscope film emulsions, such as the speed, the modulation transfer function, and the exposure dependence of the noise power spectrum, have been studied for electron energies (80-100keV) used in conventional transmission microscopy. However, limited information is available for electron energies in the intermediate to high voltage range, 300-1000keV. Furthermore, emulsion characteristics, such as optical density versus exposure, for new or improved emulsions are usually only quoted by film manufacturers for 80keV electrons. The need for further film emulsion studies at higher voltages becomes apparent when searching for a film to record low dose images of radiation sensitive biological specimens in the frozen hydrated state. Here, we report the optical density, speed and relative resolution of a few of the more popular electron microscope films after exposure to 1MeV electrons.Three electron microscope films, Kodak S0-163, Kodak 4489, and Agfa Scientia 23D56 were tested with a JEOLJEM-1000 electron microscope operating at an accelerating voltage of 1000keV.

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