Solid transport measurements through image processing

The paper describes a novel technique which enables evaluation of sediment fluxes on the upper layer of a granular bed by means of separate measurements of concentration and velocity of the moving particles. Specific elaboration techniques based on digital image processing were applied to films of the solid fluxes, allowing for automatic measurement. Sediment concentration was measured via a technique based on image subtraction and following proper filtering procedures, while grain velocity was measured by Particle Image Velocimetry. The method was applied in laboratory experiments of one dimensional bed load; the solid discharges measured by the proposed image processing technique were compared to those obtained by manual count of the grains passing over a fixed plate used as a sight. After calibration of the automatic technique, dimensionless solid discharges ranging from 1.0 × 10⁻³ to 1.2 × 10⁻¹ were measured with a maximum error as large as 25%. The technique proposed also enables measurement of the time variation of the quantities and the two dimensional direction of sediment motion, for a complete characterization of grain kinematics in solid transport processes.     

Extrusion of pastes with a piston extruder for the determination of the local solid and fluid concentration, the local porosity and saturation and displacement profiles by means of NMR imaging

 Nuclear Magnetic Resonance (NMR) was used to investigate the extrusion behaviour of PTFE pastes in a ram extruder. By means of ¹H-NMR imaging (MRI) it is possible to determine the local proton density and therefore, the local fluid concentration. The ¹⁹F-MRI provides the local solid concentration. Thus the local saturation and the local porosity can be calculated with the information of the local fluid and solid concentration. Furthermore displacement profiles can be derived from NMR images by means of correlation techniques without any preparation or marking of the pastes. Received: 8 May 2000   Accepted: 1 May 2001     

Mathematical modeling of two problems of wave dynamics in heterogeneous media

Two problems of heterogeneous media mechanics are investigated in the paper. The first one, concerned with the shock wave/dust layer interaction, is solved within the framework of the equilibrium model of heterogeneous media mechanics. The second problem deals with the simulation of a Riemann problem for a stratified layer of solid particles.This paper is based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31–August 5, 2005     

The “Arch of equilibration” of Charles Hutton (1772)

This paper is a historical re-visitation of the work of C. Hutton, a mathematician of the XVIII century, who wrote an innovative essay on the shape of arches, whose intrados he identifies with the curve of pressure, that he names “arch of equilibration”. His solution was mostly ignored by his contemporaries and after the middle of the XIX century forgotten. But Hutton’s problem is important, also because it offers the first example of an inverse problem in structural mechanics.     

A double-front structure of detonation wave as the result of phase transitions

Using thermochemical code calculations, we show that the nanographite–nanodiamond phase transition, which may occur in the detonation products of a number of carbon containing explosives, can affect the detonation properties and can cause a specific detonation regime with some unusual peculiarities. Among them, we first note the failure of the Chapman–Jouguet condition and the presence of the sonic plane, where the Mach number is equal to unity, in a detonation product expansion wave at a lower pressure than that at the Chapman–Jouguet point. The peculiarities of this detonation regime are demonstrated by the example of TNT, HNS, and RDX. The computed detonation velocities are in excellent agreement with experiments over a wide range of initial charge densities for all of the investigated explosives. The results of this work allow one to explain, e.g., contradictory experimental data on the detonation pressure and on the length of the reaction zone for TNT. We believe that some other solid–solid, solid–liquid, and liquid–liquid phase transformations in the detonation products may also cause a detonation regime with the same features as shown here for the nanographite–nanodiamond transition. We suggest a computational study that should facilitate proposing detonation experiments strongly arguing in favor of the model presented. PACS 47.40.-x; 47.40.Rs; 64.70.-p; 64.70.Kb; 05.70.-a; 05.70-.CeThis paper was based on the work that was presented at the 19th International Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27–August 1, 2003.     

Mass flow-rate control unit to calibrate hot-wire sensors

Hot-wire anemometry is a measuring technique that is widely employed in fluid mechanics research to study the velocity fields of gas flows. It is general practice to calibrate hot-wire sensors against velocity. Calibrations are usually carried out under atmospheric pressure conditions and these suggest that the wire is sensitive to the instantaneous local volume flow rate. It is pointed out, however, that hot wires are sensitive to the instantaneous local mass flow rate and, of course, also to the gas heat conductivity. To calibrate hot wires with respect to mass flow rates per unit area, i.e., with respect to (ρU), requires special calibration test rigs. Such a device is described and its application is summarized within the (ρU) range 0.1–25 kg/m² s. Calibrations are shown to yield the same hot-wire response curves for density variations in the range 1–7 kg/m³. The application of the calibrated wires to measure pulsating mass flows is demonstrated, and suggestions are made for carrying out extensive calibrations to yield the (ρU) wire response as a basis for advanced fluid mechanics research on (ρU) data in density-varying flows.     

Effect of rounding side-corners on aerodynamic forces and turbulent wake of a cube placed on a ground plane

This paper describes the experimental study of a flow past a cube with rounded side-corners placed in a ground plane under the condition of δ/D < 1, where δ is the thickness of the upstream boundary layer. The experiment was carried out in an N.P.L. type wind-tunnel having a working section of 500 mm×500 mm × 2,000 mm at a Reynolds number 4.74×10⁴. The suface-pressure distribution on the cube was measured, and the drag coefficient was determined from the surface-pressure distribution. Furthermore, two kinds of vortices generated around the cube were observed. The distribution of velocities and turbulent intensities in the turbulent wake behind the cube with rounded side corners were measured, and compared with those of a two-dimensional cylinder. As a result, it was found that the drag coefficient decreases rapidly in the range of 0 ≦ 2R/D ≦ 0.3, and the Strouhal number for the arch-vortex shedding increases as the radius of the corner increases. A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, 17–19 October 1988     

Detonation diffraction from circular tubes to cones

An experimental study of the detonation diffraction from 26- and 52-mm inner diameter tubes to cones of various angles α in stoichiometric acetylene/oxygen mixture allowed us to determine critical conditions for diffraction and to detail the mechanisms involved. All soot-foil records show that critical transmission is due to super-detonation propagating transversally in shocked gas before the decoupled flame front. However, at large cone angles (α > 40^∘), super-detonation originates at the axis of the flow and propagates tangentially to the cone wall (this situation is close to detonation transmission to a space and a half-space). At smaller angles (i.e. α < 40^∘), on the opposite, super-detonation originates at the cone wall and propagates toward the axis. In addition the soot plates often give some evidence that, during escape of detonation products from the tube, a Mach disk is formed at a distance of about one tube diameter from the tube exit. Numerical two-dimensional simulations of detonation diffraction favorably agree with the observations. PACS 47.40.-x This paper was based on work that was presented at the 19th Inter-national Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003     

A Micromechanics Study of Lamellar TiAl

Microdeformation patterns of lamellar TiAl specimens with various grain sizes under uniaxial tension are mapped using the micro/nano experimental mechanics technique called SIEM (Speckle Interferometry w ith Electron Microscopy). The stress–strain relationships were obtained from deformations within decreasing areas ranging from mm² to μm². We found that the stress–strain relationship of the material depends on the size of strain measuring area in relation to the grain size. The stiffness at a grain boundary can be as large as 7–10 times more than that of the grain itself. From the data obtained so far, it seems that the traditional way of using PST (polysynthetically twinned) single crystal to predict polycrystalline behavior may not be appropriate.     

Radial filtration during constant-force squeeze flow of soft solids

Various soft solid suspensions were squeezed at constant force between polished and roughened circular glass plates and the time-dependence of the interplate separation was measured. The filterability of suspensions was quantified by their desorptivity S obtained from measurements of capillary suction time. The squeeze flow (SF) of suspensions for which S < 2 μm s^−1/2 was largely consistent with rheological theory, which neglected radial filtration: the relative motion between the liquid and solid phases of the suspension in the radial pressure gradient. Suspensions having S > 2 μm s^−1/2 showed SF behavior that was consistent with the presence of radial filtration.     

Uniaxial deformation of a random packing of particles

Summary Mechanical behavior of dense packing spheres with small irregularities is investigated in this paper. A generalization of the hertzian contact model for surfaces of the form x ^k yields a normal contact force F _n , which is proportional to ζ^{1+1/ k} , with the normal displacement ζ. For oblique forces, the frictional force can be calculated, [10]. Different load cases are explained in detail. It is shown that the stress-strain curve during initial loading of the packing is identical with the force-displacement relation at the contact point, using an appropriate constant. The results for uniaxial loading, unloading and reloading are illustrated. As experimentally observed, the axial pressure in unloading is smaller than during loading, while the lateral pressure increases. The stress-strain relation is compared with well-known empirical relations of rock and soil mechanics, and the wave velocity for spherical irregularities agrees with earlier geomechanical theories for random packing of smooth spheres. Received 19 July 1998; accepted for publication 19 October 1998     

Rosetta Stone Experiments

Dynamic failure events such as armor penetration and explosive fragmentation are too complex to be treated by classical single-crack continuum fracture mechanics. In such cases deformation and fracture result from multiple cracks, voids, and shear bands acting simultaneously and influencing one another’s evolution. An alternative “meso” fracture mechanics is needed that treats microfailure activity while permitting fast and inexpensive predictive computations. This paper discusses the approach and experiments that elucidate and quantify failure physics on the micron level. “Rosetta Stone” experiments that isolate a damage mode, produce statistical distributions of damage features, and “freeze in” damage at various stages of development are described and illustrated. The observations and data lead to equations describing nucleation and growth of cracks, voids, and shear bands. The resulting mesomechanical material failure models link the microworld with the macroworld and can be used in continuum hydrocodes for fast, efficient simulations of dynamic fracture scenarios.

G. N. Savin and the Lviv school of mechanics

Fragments of the development of the Lviv school of mechanics in the early postwar years (1945–1952) and the role of Academician G. N. Savin in its establishment are highlighted. Much attention is given to the scientific achievements of Savin in solid mechanics, including solutions to fundamental and applied problems of stress concentration around holes in plates and shells. Mention is also made of original solutions to some problems in mechanics that stimulated solution of new fundamental and applied problems in solid mechanics and engineering and establishment and development of the Lviv school of mechanics __________ Translated from Prikladnaya Mekhanika, Vol. 43, No. 1, pp. 16–36, January 2007.     

Turbulence beneath finite amplitude water waves

Babanin and Haus (J Phys Oceanogr 39:2675–2679, 2009) recently presented evidence of near-surface turbulence generated below steep non-breaking deep-water waves. They proposed a threshold wave parameter a ²ω/ν = 3,000 for the spontaneous occurrence of turbulence beneath surface waves. This is in contrast to conventional understanding that irrotational wave theories provide a good approximation of non-wind-forced wave behaviour as validated by classical experiments. Many laboratory wave experiments were carried out in the early 1960s (e.g. Wiegel 1964). In those experiments, no evidence of turbulence was reported, and steep waves behaved as predicted by the high-order irrotational wave theories within the accuracy of the theories and experimental techniques at the time. This contribution describes flow visualisation experiments for steep non-breaking waves using conventional dye techniques in the wave boundary layer extending above the wave trough level. The measurements showed no evidence of turbulent mixing up to a value of a ²ω/ν = 7,000 at which breaking commenced in these experiments. These present findings are in accord with the conventional understandings of wave behaviour.     

An investigation of ball-on-ball impact

The Bureau of Mines, U.S. Department of the Interior, has used a shock accelerometer to measure deceleration in ball-on-ball impact. The accelerometer signal was analyzed by a fast-Fourier-transform (FFT) technique, and signal components were identified from the accelerometer characteristics, solid mechanics, and finite-element modal and spectral analysis. Following frequency-domain digital filtering to remove extraneous frequencies, the remaining signal was reconstituted by an inverse FFT. Supporting Hertzian quasistatic and elastic and elastic-plastic dynamic finite-element calculations were carried out. All calculations predicted similar peak decelerations and contact times. The former agreed well with experiment. However, stress-wave effects make it difficult to use deceleration measurements to define contact times, even at an impact velocity of 8 ms⁻¹. Such an impact must be analyzed as dynamic rather than static.     

High speed digital imaging of cavitating vortices

 Researchers at the Cavitation and Multiphase Flow Laboratory of the University of Michigan worked in conjunction with Princeton Scientific Instruments (PSI) engineers to employ a new digital imaging system in the study of partial attached cavitation. The new high speed solid state system, the Princeton Scientific Ultra Fast Framing Camera (UFFC), was designed for cavitation studies where framing rates of 10⁵–10⁶ frames/s are required to image the detailed mechanisms of cavitating flows. The UFFC, which uses a PSI patented Charge Coupled Device (CCD) array image sensor, was designed to capture 30 frames at a maximum framing rate of 1 million frames/second. In these experiments, a maximum framing rate of 125000 frames per second (8 μs/frame) was used to examine cavitating vortices in the closure region of a partial attached cavity. The vortical structures in the closure region of the attached cavity were imaged, and the evolution and collapse of these flow structures were examined. Relationships between the cavitating vortices size, strength, and collapse time were observed. Received: 15 November 1996/Accepted: 1 December 1997     

Effect of porous disc receiver configurations on performance of solar parabolic trough concentrator

In this article, heat transfer enhancement of line focus solar collector with porous disc receiver is studied with water and therminol oil. A three dimensional (3-D) numerical simulation of porous disc enhanced receiver is carried out using commercial CFD software Fluent 6.3 to evolve the optimum configuration. The 3-D numerical model is solved by renormalization-group based k-ε turbulent model associated with standard wall function. The effect of porous disc receiver configurations (solid disc at bottom; porous disc at bottom; porous disc at top; and alternative porous disc) on performance of the trough concentrator is investigated. The effect of porous disc geometric parameters (φ, θ, W, H and t) and fluid parameters (Pr and m) on heat transfer enhancement of the receiver is also studied. The numerical simulation results show that the flow pattern around the solid and porous discs are entirely different and it significantly influences the local heat transfer coefficient. The porous disc receiver experiences low pressure drop as compared to that of solid disc receiver due to less obstruction. The optimum configuration of porous disc receiver enhances the heat transfer rate of 221 W m⁻¹ and 13.5% with pumping penalty of 0.014 W m⁻¹ for water and for therminol oil-55, heat transfer rate enhances of 575 W m⁻¹ and 31.4% with pumping penalty of 0.074 W m⁻¹ as compared to that of tubular receiver at the mass flow rate of 0.5 kg s⁻¹. The Nusselt number and friction factor correlations are proposed for porous disc receiver to calculate heat transfer characteristics. The porous disc receiver can be used to increase the performance of solar parabolic trough concentrator.     

Shear-strain-rate effects in a high-strength aluminum alloy

Methods of investigating the plastic flow of materials at high rates are reviewed, and experiments are described in which thin-walled tubular specimens were subjected to dynamic torsional loading. These experiments were performed using a modified version of a torsional Hopkinsonbar apparatus used in earlier work. The results show that, at strain rates of order 10³ sec⁻¹, the stress-strain curve of the alloy tested does not differ significantly from that found at 10⁻³ sec⁻¹. In tests involving the propagation of a stress increment, however, it was found that the speed of propagation was that of elastic shear waves, indicating that the initial response of the material is essentially rate dependent.     

Curing effects on the acoustical properties of epoxy polymers

This paper presents an experimental method for measuring the attenuation and the velocity of longitudinal ultrasonic waves propagating through flat epoxy polymer samples. The study takes place in the first phase of epoxy polymer's polymerization, where these materials pass slowly from liquid state to the solid state. For this purpose an experimental setup was introduced, suitable for the accurate evaluation of the acoustic properties Δα andc _ℓ ^e , when the epoxy polymers are in their first phase of polymerization, while they are cured for 24 hours at room temperature (20°C). The ultrasonic method used is the pulse echo-through transmission technique. From the variation ofc _ℓ ^e and Δα during the first phase of epoxy polymers curing, the three characteristic states: liquid, semi-solid and solid, are clearly determined. It is also observed that plasticizer reduces the testability and the semi-solid state shows greater attenuation than either the liquid or the solid state.     

Dislocation based high-rate plasticity model and its application to plate-impact and ultra short electron irradiation simulations

This paper focuses on the development of a plasticity model to describe high rate deformations of metals. Modeling of target mechanical response is performed in frames of continuum mechanics. Plastic flow is described as the result of an over barrier dislocation sliding in specific slip planes. Computations of shock wave propagation in fcc, bcc and hcp metals modeling in comparison with shock wave experiments are performed to verify the model. The model predicts yield strength increase on elastic precursor in aluminum monocrystal and titanium of high purity at high temperatures.The action on a copper target of the electron beams with energy density (the total energy incident on an unit area during an irradiation pulse) 8.6 J cm⁻² and varied pulse duration has been investigated. At the considered irradiation regime the target remains in a solid state (maximal temperature is 710 K) and shear stresses can reach values of about 0.72 GPa. Depth distribution of dislocation density after irradiation has a maximum that is localized on a distance of 10 μm from the irradiated surface and the maximum dislocation density is about 6 × 10⁹ cm⁻² in the target. The shortening of the exposure time to 1 ns leads to the increase of the dislocation density. Further reduction of exposure time has a weak effect on the dislocation density because the shear stresses reach a limit.