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Mechanical Engineering, Aerospace Engineering, Physics, Mechanics, Convection, and 11 moreTurbulence, Natural Convection, Thermal, Buoyancy, Planetary boundary layer, Convection Rayleigh Benard, Interdisciplinary Engineering, Boundary Layer, Thermal Convection, Rayleigh Number, and Temperature field
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... 505 Page 5. RJ Adrian and R J Goldstein perform the ensuing analysis for the simple case in which r= (ut, yo, ZO). ... where the Doppler frequency WD = ~ KU Sin 8 (=2?TVD) (16) is identical to equation (l), and A is the photocathode... more
... 505 Page 5. RJ Adrian and R J Goldstein perform the ensuing analysis for the simple case in which r= (ut, yo, ZO). ... where the Doppler frequency WD = ~ KU Sin 8 (=2?TVD) (16) is identical to equation (l), and A is the photocathode area illuminated by the scattered light waves. ...
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ABSTRACT
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ABSTRACT
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ABSTRACT High-Speed Multi-Frame Laser Schlieren is used for visualization of a range of explosive and non-explosive events. Schlieren is a well-known technique for visualizing shock phenomena in transparent media. Laser backlighting and a... more
ABSTRACT High-Speed Multi-Frame Laser Schlieren is used for visualization of a range of explosive and non-explosive events. Schlieren is a well-known technique for visualizing shock phenomena in transparent media. Laser backlighting and a framing camera allow for Schlieren images with very short (down to 5 ns) exposure times, band pass filtering to block out explosive self-light, and 14 frames of a single explosive event. This diagnostic has been applied to several explosive initiation events, such as exploding bridgewires (EBW), Exploding Foil Initiators (EFI) (or slappers), Direct Optical Initiation (DOI), and ElectroStatic Discharge (ESD). Additionally, a series of tests have been performed on "cut-back" detonators with varying initial pressing (IP) heights. We have also used this Diagnostic to visualize a range of EBW, EFI, and DOI full-up detonators. The setup has also been used to visualize a range of other explosive events, such as explosively driven metal shock experiments and explosively driven microjets. Future applications to other explosive events such as boosters and IHE booster evaluation will be discussed. Finite element codes (EPIC, CTH) have been used to analyze the schlieren images to determine likely boundary or initial conditions to determine the temporal-spatial pressure profile across the output face of the detonator. These experiments are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to detonation to full detonation to transition to booster and booster detonation.
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Characteristics of the Karhunen–Loéve expansion of a strongly inhomogeneous random process possessing small viscous length scales and a large outer scale have been investigated in relation to the application of the expansion to turbulent... more
Characteristics of the Karhunen–Loéve expansion of a strongly inhomogeneous random process possessing small viscous length scales and a large outer scale have been investigated in relation to the application of the expansion to turbulent flow fields. Monte Carlo simulations of a randomly forced Burgers’ equation with zero velocity boundary conditions generate the random process numerically and the Karhunen–Loéve (KL) eigenfunctions and the eigenvalue spectra are computed for different Reynolds numbers. The eigenfunctions possess thin viscous boundary layers at the walls and are independent of Reynolds number in the core, where the random process is quasihomogeneous. The eigenfunctions and eigenvalues of the outer, large scale motions obey a principle of Reynolds number similarity. Eigenvalue spectra contain much of the energy in the first few modes, but they are as broad as ordinary trigonometric power spectra. The rate at which the expansion converges to within 90% of the total ene...
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Physics, Turbulence, Monte Carlo Simulation, Statistical Physics, Turbulent Flow, and 14 moreRandom Processes, Eigenvalues, Length scale, Boundary Layer, Boundary Value Problems, Large Scale, Scaling Law, Boundary Condition, Velocity Field, Numerical Solution, Physics of Fluids, Reynolds Number, Burgers equation, and Monte Carlo Method
Instrumentation Developments * Turbulent Shear Flows * Aerodynamics * Mixers and Rotating Flows * Combustion and Engines * Two-Phase Flows.
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Conditional averages of turbulent flow quantities can be approximated in terms of unconditional correlation data by means of stochastic estimation. The validity and accuracy of this procedure are investigated by comparing stochastic... more
Conditional averages of turbulent flow quantities can be approximated in terms of unconditional correlation data by means of stochastic estimation. The validity and accuracy of this procedure are investigated by comparing stochastic estimates to conditional averages measured in four turbulent flows: grid turbulence, the axisymmetric shear layer of a round jet, a plane shear layer, and pipe flow. Comparisons are made for quantities that are separated from the conditional data in time or space, and for turbulent pressures, as well as turbulent velocities. In each case, the linear estimate accurately represents large scale structure. Nonlinear quadratic estimation shows little improvement over linear estimation, because the second-order terms are small for probable values of the turbulent fluctuations.
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Given their small size, microchannels are susceptible to being blocked by small amounts of solid matter. The lifetime of certain microfluidic devices depends on their ability to maintain flow without interruption, and certain microfluidic... more
Given their small size, microchannels are susceptible to being blocked by small amounts of solid matter. The lifetime of certain microfluidic devices depends on their ability to maintain flow without interruption, and certain microfluidic applications require devices that include the transport of either liquids that may contain impurities or liquids that intentionally contain particles. Under the conditions studied in the
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... schlieren photography, by particle image velocimetry (PIV) in air, and by PIV in transparent, polymeric witness plates. ... The visible light emitted from the aluminum chamber following each laser pulse results from laser-induced... more
... schlieren photography, by particle image velocimetry (PIV) in air, and by PIV in transparent, polymeric witness plates. ... The visible light emitted from the aluminum chamber following each laser pulse results from laser-induced argon gas breakdown through multi-photon ...
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The analysis of Reeks (1977) for particle dispersion in isotropic turbulence is extended so as to include a nonlinear drag law. The principal issue is the evaluation of the inertial time constants, βα−1, and the mean slip. Unlike what is... more
The analysis of Reeks (1977) for particle dispersion in isotropic turbulence is extended so as to include a nonlinear drag law. The principal issue is the evaluation of the inertial time constants, βα−1, and the mean slip. Unlike what is found for the Stokesian drag, the time constants are functions of the slip velocity and are anisotropic. For settling velocity, VT, much larger than root-mean-square of the fluid velocity fluctuations, u0, the mean slip is given by VT. For VT→0, the mean slip is related to turbulent velocity fluctuation by assuming that fluctuations in βα are small compared to the mean value. An interpolation formula is used to evaluate βα and VT in regions intermediate between conditions of VT→0 and VT≫ u0. The limitations of the analysis are explored by carrying out a Monte-Carlo simulation for particle motion in a pseudo turbulence described by a Gaussian distribution and Kraichnan’s (1970) energy spectrum.
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... Experiments in Fluids 9 Springer-Verlag 1989 Measurement of velocity vectors with orthogonal and non-orthogonal triple-sensor probes IC Lekakis* Dept. ... Page 2. IC Lekakis et al.: Measurement of velocity vectors with orthogonal and... more
... Experiments in Fluids 9 Springer-Verlag 1989 Measurement of velocity vectors with orthogonal and non-orthogonal triple-sensor probes IC Lekakis* Dept. ... Page 2. IC Lekakis et al.: Measurement of velocity vectors with orthogonal and non-orthogonal triple-sensor probes 229 ...
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The structure of turbulent motions was measured in a region surrounding the blade tips of a Rushton turbine mixer. The tip vortices responsible for micromixing were created in this region, as was the radial jet that drives the large‐scale... more
The structure of turbulent motions was measured in a region surrounding the blade tips of a Rushton turbine mixer. The tip vortices responsible for micromixing were created in this region, as was the radial jet that drives the large‐scale circulation responsible for macromixing. Measurements of instantaneous velocity fields in axial–radial planes showed large cycle‐to‐cycle variability relative to the mean. The average measurements of the azimuthal vorticity and the Reynolds stresses in the radial–axial plane showed the growth and motion of the tip vortices as they drift away from the blades. Radial and axial spatial derivatives were measured to assess various methods of computing the turbulent viscous dissipation. The small scales were significantly anisotropic at the Reynolds numbers studied.
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Spectral similarity laws are derived for the power spectra of turbulent static pressure fluctuations by application of dimensional analysis in the limit of large turbulent Reynolds number. The theory predicts that pressure spectra are... more
Spectral similarity laws are derived for the power spectra of turbulent static pressure fluctuations by application of dimensional analysis in the limit of large turbulent Reynolds number. The theory predicts that pressure spectra are generated by three distinct types of interaction in the velocity fields: a fourth order interaction between fluctuating velocities, an interaction between the first order mean shear and the third order velocity fluctuations, and an interaction between the second order mean shear rate and the second order fluctuating velocity. Measurements of one-dimensional power spectra of the turbulent static pressure fluctuations in the driven mixing layer of a subsonic, circular jet are presented, and the spectra are examined for evidence of spectral similarity. Spectral similarity is found for the low wavenumber range when the large scale flow on the centerline of the mixing layer is self-preserving. The data are also consistent with the existence of universal inertial subranges for the spectra of each interaction mode.
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1: Department of Mechanical Engineering, Monash University, Melbourne, Australia, elham.tolouei@eng.monash.edu.au 2: The Australian Center for Blood Diseases, Monash University, Melbourne, Australia, warwick.nesbitt@med.monash.edu.au 3:... more
1: Department of Mechanical Engineering, Monash University, Melbourne, Australia, elham.tolouei@eng.monash.edu.au 2: The Australian Center for Blood Diseases, Monash University, Melbourne, Australia, warwick.nesbitt@med.monash.edu.au 3: Division of ...
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Tomographic PIV has become a widely used 3-D flow measurement technique. It utilizes projected images recorded by cameras from different views and iterative algebraic reconstruction technique to reconstruct the intensity distribution of a... more
Tomographic PIV has become a widely used 3-D flow measurement technique. It utilizes projected images recorded by cameras from different views and iterative algebraic reconstruction technique to reconstruct the intensity distribution of a measured volume, and computes the 3D3C velocity field by 3-D cross-correlation. Surface segmentation technique was proposed by Adrian (2011) accelerates the reconstruction process by extracts from the cloud of particle images only those images originating from particles that lay on a mathematically prescribed surface(s). 2-D cross-correlations are performed on orthogonal stacks of surfaces to compute the 3D3C velocity field, which further reduce the computational cost compared to 3-D crosscorrelation. The method is not as effective as the MART techniqueIn this study, we investigate the reconstruction of adaptive surfaces. Those surfaces are chosen so that the particle out-of-plane motion is minimized and 2-D interrogation works well. Numerical asse...
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The physical mechanisms that need to be involved in sediment transport modelling are indissolubly associated to the evolution of coherent structures in a rough boundary layer. In literature many attempt have been found to relate burst... more
The physical mechanisms that need to be involved in sediment transport modelling are indissolubly associated to the evolution of coherent structures in a rough boundary layer. In literature many attempt have been found to relate burst frequency and intensity with sediment entrainment or saltation phenomena. Several questions remained partially unanswered concerning the feed back effect of sediment particles on hydrodynamics, but even a deep knowledge of roughness effects on coherent structures has to be involved. The question that may still rise is related to the undisturbed spatial scales of the structures responsible for sediment entrainment. Those structures are the very same structures triggered by the vast regime of roughness elements (from sand grains to cobbles or even stones) lying in the bottom of a river. Turbulent flows in a smooth wall boundary layer have been studied extensively over the past decades. Since the early work of Kline (1967), experimental investigations on the detailed structure of the turbulent field in boundary layers have revealed the existence of distinctly organized motions. The present research and the recent investigation of Tomkins (2001) are focused mainly on the effect of a roughness transition over turbulent structures, with the purpose of shedding more light on the variation that may occur on bursting intensity, pattern of dominant structures and representative length scales. In literature, Grass (1993) outlined intense ejection event able to disrupt vortices organization with the consequent reduction of the streamwise integral length scale.
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The dilatational model of noise generated by turbulent motions is extended to include subsonic motion of the reference frame with respect to a uniform acoustic medium and arbitrary motion of the observer. The application of particular... more
The dilatational model of noise generated by turbulent motions is extended to include subsonic motion of the reference frame with respect to a uniform acoustic medium and arbitrary motion of the observer. The application of particular interest is the fly-by of a noise producing jet over a stationary observer. The model neglects refraction by mean shear, but it allows the
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It is shown that conditional averages in the form of expected values of functions of the velocity at an arbitrary point given the velocities at a finite number of distinct points, appear naturally in certain types of turbulence theories... more
It is shown that conditional averages in the form of expected values of functions of the velocity at an arbitrary point given the velocities at a finite number of distinct points, appear naturally in certain types of turbulence theories and that the closure problems in such theories ultimately reduce to the approximation of these averages. Two exemplary theories are considered. The first is characteristic of turbulence models formulated in terms of probability density functions whereas the second is related to the derivation of optimal algorithms for the numerical integration of the turbulent Navier-Stokes equations at large Reynolds numbers. Some mathematical properties of conditional expected values, including relations between conditional and unconditional second order tensor moments and results for the special case of isotropic turbulence are also presented.
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Chemical Engineering, Mathematics, Algorithms, Turbulence, Statistical Physics, and 13 moreIntegral Equations, Average, Turbulent Flow, Numerical Integration, Second Order, Conditional Expectation, PROBABILITY DENSITY, Flow Velocity, Integral Equation, Probability Density Function, Reynolds Number, closures, and Probability Density Functions
PIV observations of Rayleigh convection at Ra between 10^7 and 10^9 indicate a coupling between the largest scales of motion and the smallest scales. The largest scales, having roll-like cross-sections, sweep the small scales, having... more
PIV observations of Rayleigh convection at Ra between 10^7 and 10^9 indicate a coupling between the largest scales of motion and the smallest scales. The largest scales, having roll-like cross-sections, sweep the small scales, having mushroom-like cross-sections, horizontally towards stagnation lines on the boundaries. At these lines buoyant small-scales are swept up and merged to form thin, buoyant sheets that shunt heat across the entire layer depth, and drive the large-scale circulation. Analysis by proper orthogonal decomposition shows that sixty per-cent of the total energy resides in the large scales in the core of the flow. In the wall layers the small scales dominate the vertical kinetic energy, but the large scales continue to carry most of the kinetic energy of the horizontal component. Measurement over hundreds of hours show that the large-scale horizontal flows are, ultimately, axisymmetric with respect to the gravitational acceleration vector.
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Two methods are developed for the measurement of fluid velocity gradients using a laser-Doppler anemometer. In the first method the velocity gradient is found by a single measurement of the bandwidth of the laser-Doppler signal spectrum.... more
Two methods are developed for the measurement of fluid velocity gradients using a laser-Doppler anemometer. In the first method the velocity gradient is found by a single measurement of the bandwidth of the laser-Doppler signal spectrum. In the second, velocity gradients very close to a wall are calculated from the amount the signal spectrum shifts in frequency for a small change in the position at which the spectrum is measured. Both methods are examined theoretically and experimentally.
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Engineering, Materials Science, Monte Carlo Simulation, Monte Carlo, Particle Image Velocimetry, and 9 moreSuper resolution, Measurement Science and Technology, Physical sciences, Turbulent Flow, Single Particle Tracking, Correlation Analysis, Spatial resolution, Measurement Science, and Particle Tracking Velocimetry
The structure of velocity in the outer region of turbulent channel flow (y+ [gsim ] 100) is examined statistically to determine the average flow field associated with spanwise vortical motions. Particle image velocimetry measurements of... more
The structure of velocity in the outer region of turbulent channel flow (y+ [gsim ] 100) is examined statistically to determine the average flow field associated with spanwise vortical motions. Particle image velocimetry measurements of the streamwise and wall-normal velocity components are correlated with a vortex marker (swirling strength) in the streamwise–wall-normal plane, and linear stochastic estimation is used to estimate the conditional average of the two-dimensional velocity field associated with a swirling motion. The mean structure consists of a series of swirling motions located along a line inclined at 12°–13° from the wall. The pattern is consistent with the observations of outer-layer wall turbulence in which groups of hairpin vortices occur aligned in the streamwise direction. While the observational evidence for the aforementioned model was based upon both experimental and computational visualization of instantaneous structures, the present results show that, on av...
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Fully developed turbulent pipe flow at low Re-number is studied by means of direct numerical simulation (DNS). In contrast to many previous DNS's of turbulent flows in rectangular geometries, the present DNS code,... more
Fully developed turbulent pipe flow at low Re-number is studied by means of direct numerical simulation (DNS). In contrast to many previous DNS's of turbulent flows in rectangular geometries, the present DNS code, developed for a cylindrical geometry, is based on the finite volume technique rather than being based on a spectral method. The statistical results are compared with experimental
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... Z.-C. Liu, CC Landreth, RJ Adrian* and TJ Hanratty University of Illinois, Urbana, Illinois 61801, USA Abstract. ... One method for doing this is particle image velocimetry (PIV), cf, Merzkirch (1987), Adrian (1986a), Hesselink... more
... Z.-C. Liu, CC Landreth, RJ Adrian* and TJ Hanratty University of Illinois, Urbana, Illinois 61801, USA Abstract. ... One method for doing this is particle image velocimetry (PIV), cf, Merzkirch (1987), Adrian (1986a), Hesselink (1988), Dudderar et al. (1988). ...
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Measurements of the streamwise velocity over solid sinusoidal waves with height to wavelength ratios of 2 a/lambda=0.05, 0.125 and with dimensionless wave numbers alpha+=0.00624, 0.00135 have been made. For these conditions the... more
Measurements of the streamwise velocity over solid sinusoidal waves with height to wavelength ratios of 2 a/lambda=0.05, 0.125 and with dimensionless wave numbers alpha+=0.00624, 0.00135 have been made. For these conditions the instantaneous flow reverses direction, but the time-averaged flow is non-separated. Many features of the flow are similar to those reported in previous papers for a time-averaged flow that is separated. Approximate agreement is obtained from an eddy viscosity model derived for flow over small amplitude waves. However, the differences are more interesting than the agreement in that they point out shortcomings of present Reynolds stress models. Comparison with other measurements in the literature shows how increasing Reynolds number decreases the size of the separated region. Measurements of pressure profiles and of drag are interpreted in terms of measured flow patterns.
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Normal and streamwise components of the velocity fields of turbulent flow in a channel at low Reynolds numbers have been measured with laser-Doppler techniques. The experiments duplicate the conditions used in current direct numerical... more
Normal and streamwise components of the velocity fields of turbulent flow in a channel at low Reynolds numbers have been measured with laser-Doppler techniques. The experiments duplicate the conditions used in current direct numerical simulations of channel flow, and good, but not exact, agreement is found for single-point moments through fourth order. In order to eliminate LDV velocity bias and
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This letter discusses experimentation with optically induced phase change in nanoparticle liquid suspensions—commonly termed nanofluids. Four different types of nanofluids at five concentrations were exposed to a ∼120 mW, 532 nm laser... more
This letter discusses experimentation with optically induced phase change in nanoparticle liquid suspensions—commonly termed nanofluids. Four different types of nanofluids at five concentrations were exposed to a ∼120 mW, 532 nm laser beam to determine the minimum laser flux needed to create vapor. Laser irradiance was varied between 0–770 W cm−2. While the experiments were simple, they involved many complex, interrelated physical phenomena, including: subcooled boiling, thermal driven particle/bubble motion, nanoparticle radiative absorption/scattering, and nanoparticle clumping. Such phenomena could enable novel solar collectors in which the working fluid directly absorbs energy and undergoes phase change in a single step.