Turbulence manipulation in air–water flows on a stepped chute: An experimental study

In a stepped channel operating with large flow rates, the flow skims over the pseudo-bottom formed by the step edges as a coherent stream. Intense three-dimensional recirculation is maintained by shear stress transmission from the mainstream to the step cavities, while significant free-surface aeration takes place. The interactions between free-surface aeration and cavity recirculation are investigated herein with seven step cavity configurations. The experiments were conducted in a large stepped channel operating at large Reynolds numbers. For some experiments, triangular vanes, or longitudinal ribs, were placed across the step cavities to manipulate the flow turbulence to enhance the interactions between the mainstream flow and the cavity recirculation region. The results showed a strong influence of the vanes on the air–water flow properties in both free-stream and cavity flows. The findings demonstrate some passive turbulence manipulation in highly turbulent air–water flows.     

An experimental investigation on the characteristics of fluid–structure interactions of a wind turbine model sited in microburst-like winds

An experimental investigation is performed to assess the characteristics of the fluid–structure interactions and microburst-induced wind loads acting on a wind turbine model sited in microburst-liked winds. The experiment study was conducted with a scaled wind turbine model placed in microburst-like winds generated by using an impinging-jet-typed microburst simulator. In addition to quantifying complex flow features of microburst-like winds, the resultant wind loads acting on the turbine model were measured by using a high-sensitive force–moment sensor as the turbine model was mounted at different radial locations and with different orientation angles with respect to the oncoming microburst-like winds. The measurement results reveal clearly that, the microburst-induced wind loads acting on the turbine model were distinctly different from those in a conventional atmospheric boundary layer (ABL) wind. With the scales of the wind turbine model and the microburst-like wind used in the present study, the dynamic wind loadings acting on the turbine model were found to be significantly higher (i.e., up to 4 times higher for the mean loads, and up to 10 times higher for the fluctuation amplitudes) than those with the same turbine model sited in ABL winds. Both the mean values and fluctuation amplitudes of the microburst-induced wind loads were found to vary significantly with the changes of the mounted site of the turbine model, the operating status (i.e., with the turbine blades stationary or freely rotating), and the orientation angle of the turbine model with respect to the oncoming microburst-like wind. The dynamic wind load measurements were correlated to the flow characteristics of the microburst-like winds to elucidate underlying physics. The findings of the present study are helpful to gain further insight into the potential damage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of the wind turbines in thunderstorm-prone areas.     

Void nucleation and growth in mineral-filled PVC – An experimental and numerical study

The nucleation and growth of voids in mineral-filled PVC have been investigated through experimental and numerical studies. Uniaxial tensile specimens were deformed in tension to different elongation levels and then unloaded. The macroscopic strain fields were recorded by use of digital image correlation. After the test, the microstructure of the deformed specimens was investigated in a scanning electron microscope. It was found that the observed volume strain on the macroscale is related to void growth on the microscale. In addition, finite element simulations were performed on unit cell models representing the microstructure of the material in a simplified manner. The numerical simulations demonstrate macroscopic dilation as a result of void growth. Moreover, the numerical simulations indicate that the experimentally observed stress-softening response of the PVC composite material may result from matrix-particle debonding.     

An experimental study of circular sand–water wall jets

Laboratory experiments were carried out to study the effects of sand particles on circular sand–water wall jets. Mean and turbulence characteristics of sand particles in the sand–water wall jets were measured for different sand concentrations c_o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand–water wall jets with a higher concentration (c_o = 1.5–2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.     

An experimental study on effects of solid concentration and ζ-potential on pseudoplastic flow of suspensions

The pseudoplastic flow of suspensions, alumina or styrene-acrylamide copolymer particles in water or an aqueous solution of glycerin has been studied by the step-shear-rate method. The relation between the shear rate,D, and the shear stress,, in the step-shear-rate measurements, where the state of dispersion was considered to be constant, was expressed as = AD ^1/2 +CD. The effective solid volume fraction,ø _F, andA were dependent on the shear rate and expressed byø _F =aD ^b andA = D . Combining the above relations, the steady flow curve was expressed by = D ^{1/2 +} + ₀ (1 – a D ^b/0.74)^–1.85 D, where ₀ is the viscosity of the medium.With an increase in solid volume fraction and a decreases in the absolute value of the-potential, the flow behavior of the suspensions changed from Newtonian ( = = b = 0), slightly pseudoplastic ( = b = 0), pseudoplastic ( = 0) to a Bingham-like behavior.The change in viscosity of the medium had an effect on the change in the effective volume fraction.     

An ESPI experimental study on the phenomenon of fracture in glass. Is it brittle or plastic?

The crack opening displacement (COD) in annealed soda-lime (float) glass has been measured with an electronic speckle pattern interferometry (ESPI) apparatus using coherent laser light. Specimens, naturally pre-cracked with a particular technique, were loaded under strain-driven bending until crack propagated; at regular intervals loading was paused to let the crack reach subcritical equilibrium and the COD measured. By using a post-processing algorithm comparing four images lighted with phase-shifted laser beams, surface displacements could be measured at a resolution of .Glass transparency has allowed to see through that the propagating crack front is not sharp but curved, jagged and merged in an opaque neighborhood. Numerical simulations show that the measured CODs cannot be reproduced if cohesive surface forces à la Barenblatt-Dugdale bridge the crack lips; instead a plastic-like region must form in a bulk neighborhood of the tip, where inelastic strains are associated with volume increase rather than deviatoric distortion. For this, a Gurson-Tvergaard model of porous plasticity, accounting for the formation of microvoids/microcracks, has been found more efficient than classical von Mises plasticity. This study confirms the formation at the crack tip of a process zone, whose occurrence in brittle materials like glass is still a subject of controversy.     

On the forward and backward in time problems in the Kelvin–Voigt thermoviscoelastic materials

This paper studies the uniqueness of solutions to the forward and backward in time boundary value problems associated with the Kelvin–Voigt viscoelastic model of the thermoelastic materials. For thermoviscoelastic materials with a center of symmetry, it is shown the uniqueness of solutions to the forward in time boundary value problems without any assumptions upon the thermoviscoelastic constitutive coefficients other than the symmetry properties and those induced by the dissipation inequality. While for the final boundary value problems two uniqueness theorems are presented: the first one is essentially based on the assumption that the specific heat is of negative definite sign, while the second is established in the class of displacement–temperature variation fields whose dissipation energy has a temporal behavior lower than an appropriate growing exponential.     

An experimental study on SC-CNN based canonical Chua’s circuit

In this paper, a State Controlled Cellular Neural Network (SC-CNN) based autonomous canonical Chua’s circuit is presented. The proposed system is modeled by using suitable connection of three simple state controlled generalized CNN cells. The stability of the circuit is studied by determining the eigenvalues of the stability matrices, while the dynamics as well as onset of chaos have been studied through real time experiments and numerical analysis of the generalized SC-CNN equations. The experimental results such as phase portraits and power spectra are in good agreement with those of numerical computations.     

Grain–boundary interactions and orientation effects on crack behavior in polycrystalline aggregates

A dislocation-density grain–boundary interaction scheme has been developed to account for the interrelated dislocation-density interactions of emission, absorption and transmission in GB regions. The GB scheme is based on slip-system compatibility, local resolved shear stresses, and immobile and mobile dislocation-density accumulation at critical GB locations. To accurately represent dislocation-density evolution, a conservation law for dislocation-densities is used to balance dislocation-density absorption, transmission and emission from the GB. The behavior of f.c.c. polycrystalline copper, with different random low and high angle GBs, are investigated for different crack lengths. For aggregates with random low angle GBs, dislocation-density transmission dominates at the GBs, which can indicate that the low angle GB will not significantly change crack growth directions. For aggregates with random high angle GBs, extensive dislocation-density absorption and pile-ups occur. The high stresses associated with this behavior, along the GBs, can result in intergranular crack growth due to potential crack nucleation sites in the GB.     

A computational study of the wing–wing and wing–body interactions of a model insect

The aerodynamic interaction between the contralateral wings and between the body and wings of a model insect are studied, by using the method of numerically solving the Navier-Stokes equations over moving overset grids, under typical hovering and forward flight conditions. Both the interaction between the contralateral wings and the interaction between the body and wings are very weak, e.g. at hovering, changes in aerodynamic forces of a wing due to the present of the other wing are less than 3% and changes in aerodynamic forces of the wings due to presence of the body are less than 2%. The reason for this is as following. During each down- or up-stroke, a wing produces a vortex ring, which induces a relatively large jet-like flow inside the ring but very small flow outside the ring. The vortex rings of the left and right wings are on the two sides of the body. Thus one wing is outside vortex ring of the other wing and the body is outside the vortex rings of the left and right wings, resulting in the weak interactions.     

Fully resonant soliton interactions in the Whitham–Broer–Kaup system based on the double Wronskian solutions

With the aim of exploring whether the (1+1)-dimensional coupled nonlinear evolution equations admit abundant soliton interactions, like the cases in the Kadomtsev–Petviashvili II equation, we in this paper study the double Wronskian solutions to the Whitham–Broer–Kaup (WBK) system. We give the parametric condition for two double Wronskians to generate the non-singular, non-trivial and irreducible soliton solutions. Via the asymptotic analysis of two double Wronskians, we show that the soliton solutions of the WBK system is in general linearly combined of fully resonant (M,N)- and (M?1,N+1)-soliton configurations. It turns out that the WBK system can exhibit various complex soliton structures which are different pairwise combinations of elastic, confluent and divergent interactions. From a combinatorial viewpoint, we also explain that the asymptotic solitons of a [(M,N),(M?1,N+1)]-soliton solution are identified by a pair of Grassmannian permutations.     

An experimental and numerical investigation of the behaviour of AA5083 aluminium alloy in presence of the Portevin–Le Chatelier effect

An experimental investigation of the Portevin–Le Chatelier (PLC) effect in the aluminium alloy AA5083-H116 is undertaken in this study through five different tests involving round, prismatic and flat notched specimen geometries. Measurements based on strain gages and digital image correlation (DIC) are used to capture and characterize the spatio-temporal features of the PLC behaviour. Inhomogeneous deformation with various localization bands caused by the PLC effect is observed in all tests, and the band characteristics are measured. The McCormick elastic–viscoplastic constitutive relation, developed for metals exhibiting this type of dynamic strain aging, is then described in detail, before the various parameters required by the model are determined based on available material tests. The model is finally used in full-scale 3D numerical simulations of the physical tests using the explicit solver of the non-linear finite element code LS-DYNA. It will be shown that the numerical results are able to reproduce most of the experimentally observed phenomena with reasonable accuracy. However, if the model is used to study the micromechanical mechanisms controlling the macromechanical behaviour of materials exhibiting PLC effects (such as the band morphology), more advanced constitutive relations may be required.     

Air–water flow in a vertical pipe: experimental study of air bubbles in the vicinity of the wall

This study deals with the influence of bubbles on a vertical air–water pipe flow, for gas-lift applications. The effect of changing the bubble size is of particular interest as it has been shown to affect the pressure drop over the pipe. Local measurements on the bubbles characteristics in the wall region were performed, using standard techniques, such as high-speed video recording and optical fibre probe, and more specific techniques, such as two-phase hot film anemometry for the wall shear stress and conductivity measurement for the thickness of the liquid film at the wall. The injection of macroscopic air bubbles in a pipe flow was shown to increase the wall shear stress. Bubbles travelling close to the wall create a periodic perturbation. The injection of small bubbles amplifies this effect, because they tend to move in the wall region; hence, more bubbles are travelling close to the wall. A simple analysis based on a two-fluid set of equations emphasised the importance of the local gas fraction fluctuations on the wall shear stress.     

An improvement to the Kunz preconditioner and numerical investigation of hydrofoil interactions in tandem

ABSTRACT

Numerical simulations of the tandem-system flow are widely conducted because of the complex interactions of the configuration. The cavitation phenomenon is an important factor that affects the hydrofoil in tandem. In this paper, we developed a new parameter for the Kunz preconditioner based on the local cavitation volume fraction. To assess this parameter, a type of Fourier footprint analysis and numerical test of the hydrofoil are adopted. The preconditioning method is also conducted in hydrofoil turbulent cavitation flows with RANS turbulence models, to prove that this method has good stability and convergence. Based on this, a tandem Clark-Y hydrofoil configuration is investigated. The results show that the distance of components has a strong influence on the cavitation and lift coefficient of the tandem hydrofoils. Therefore, the purpose of this study is to provide guidance on the design of marine vehicles that have tandem configurations.     

Non-uniform distribution of gas–solid flow through six parallel cyclones in a CFB system: An experimental study

In large-scale circulating fluidized bed (CFB) boilers, it is common to use multiple cyclones in parallel for the capture of solids, assuming that gas–solid flow to be the same in the cyclones. This article presents a study investigating gas–solid flow through six parallel cyclones in a CFB cold test rig. The six cyclones were located asymmetrically on the left and right walls of the riser. Solid volume fraction and particle velocity profiles at the riser outlets and in the horizontal ducts were measured using a fiber optical probe. Cyclone pressure drop and solid circulating rate were measured for each individual cyclone. Measurements showed good agreement as to the non-uniform distribution of the gas–solid flow, which occurred mainly across the three cyclones on one side: the middle cyclones on both sides had higher particle velocities. Conversely, the solid volume fractions, solid fluxes and solid circulating rates of the middle cyclones were lower than those of the other four cyclones. The apparent reason for the flow non-uniformity among the cyclones is the significant flow non-uniformity at the riser outlets. Under typical operating conditions, the solid volume fractions at the riser outlets had a deviation of up to 26% whereas the solid circulating rates at the stand pipes, 7%. These results are consistent with most other studies in the literature.     

Two-phase flow phenomena along an adiabatic riser – An experimental study at the test-facility GENEVA

Long adiabatic riser geometries with low system pressures are present in a lot of energy and petrochemical processes. Natural-circulation systems are an appropriate solution to save operating and maintenance costs. Under certain circumstances natural-circulation systems tend to unstable mass flow, especially in the riser section. The pressure gradients can stress the construction materials and affect the heat transfer. This paper focuses on GENEVA test-facility and its natural-circulation circuit with heat input by steam condensation. The GENEVA test-facility is explained in detail with focus on the local void fraction measurement system in the adiabatic riser section. The differences to former natural circulation test-facilities are particular emphasized. Therefore a transient experiment is presented and analysed. Moreover the influence of flow restrictions at the downcomer outlet is explained and an experimental method is presented, to determine the maximum natural circulation mass flow of the natural-circulation circuit. Besides, a comparison between the two different riser inner diameters, which were used during the experiments, is presented. The convective heat transfer is analysed by taking into account different dimensionless numbers. A variety of experiments were performed up to 100 kW_el input power from the evaporators. Flashing and geysering as two types of occurred instabilities are stated and discussed in comparison to former test-facilities. Further phenomena like water hammer and counter current liquid flow are investigated. Based on these analyses constructive solutions can be derived, to stabilize the presented natural-circulation two-phase flow system.     

Experimental study on the effect of drag reducing polymer on flow patterns and drag reduction in a horizontal oil–water flow

In this study, a HMW anionic co-polymer of 40:60 wt/wt NaAMPS/acrylamide was used as a drag reducing polymer (DRP) for oil–water flow in a horizontal 25.4 mm ID acrylic pipe. The effect of polymer concentration in the master solution and after injection in the main water stream, oil and water velocities, and pipe length on drag reduction (DR) was investigated. The injected polymer had a noticeable effect on flow patterns and their transitions. Stratified and dual continuous flows extended to higher superficial oil velocities while annular flow changed to dual continuous flow. The results showed that as low as 2 ppm polymer concentration was sufficient to create a significant drag reduction across the pipe. DR was found to increase with polymer concentration increased and reached maximum plateau value at around 10 ppm. The results showed that the drag reduction effect tends to increase as superficial water velocity increased and eventually reached a plateau at U_sw of around 1.3 m/s. At U_sw > 1.0 m/s, the drag reduction decreased as U_so increased while at lower water velocities, drag reduction is fluctuating with respect to U_so. A maximum DR of about 60% was achieved at U_so = 0.14 m/s while only 45% was obtained at U_so = 0.52 m/s. The effectiveness of the DRP was found to be independent of the polymer concentration in the master solution and to some extent pipe length. The friction factor correlation proposed by Al-Sarkhi et al. (2011) for horizontal flow of oil–water using DRPs was found to underpredict the present experimental pressure gradient data.