Presence Of Side Walls Research Articles

Dense, steady, fully developed fluid-particle flows over inclined, erodible beds

We phrase boundary-value problems for a dense, steady, fully developed, gravitational flow of identical inelastic spheres and water over an inclined erodible bed in the presence of sidewalls. We obtain numerical solutions for the profiles of the solid volume fraction, the strength of the particle velocity fluctuations, and the mean velocities of the particles and fluid. We compare these with those measured in experiments.

Experimental Study on Propagation of Particle-Bearing Jets in a Confined Geometry

Abstract The spreading characteristics of particle-bearing jets down a gentle slope in a uniform ambient with confined side-walls are reported through a series of laboratory experiments. A round water jet is mixed with solid particles having particle volume fraction ranging between 0% and 0.4%. The jet Reynolds number is varied between Re = 3000 and 8000. We document that the jet front position varies with time t as xf∝t2/3. The jet front propagation in the presence of side-walls is found to be higher than the unconfined case and is attributed to the recirculation due to the confinement that increases the flow inertia and accelerates the flow. The jet propagation is found to be self-similar and is unaffected by the variations in the volume fraction and Re. The sediment pattern near the source, formed by the settling of the particles, exhibits a similar tear-drop shape, which is well-predicted using the unconfined jet theory that assumes a Gaussian velocity profile. The results have implication in engineering and environmental flows, where higher jet propagation rate for confined jets should be modeled accurately without modifying the sedimentation dynamics.

Explicit Algebraic Reynolds-Stress Modeling of Pressure-Induced Separating Flows in the Presence of Sidewalls

AbstractReynolds-averaged Navier–Stokes (RANS) approach is used to simulate the steady-state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the baseline explicit algebraic Reynolds stress model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer are generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for cases LB and MB. For case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k−ω shear-stress transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the “simplest” case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

Cellular vortex shedding from a cylinder at low Reynolds number

Abstract

Large-scale unsteadiness in a compression ramp flow confined by sidewalls

The complexity of a shock-wave/turbulent boundary layer interaction (SWTBLI) in the presence of sidewalls is investigated both qualitatively and quantitively in this work. Additional spanwise no-slip conditions led to the occurrence of secondary flows, which were not observed in the quasi-two-dimensional cases. Statistical analysis revealed asymmetric back-and-forth motion of the interaction across the midspan, possibly caused by the variation in the instantaneous size of the sidewall separated zones. Coupling between corner and centerline interactions was also observed as a result of higher confinement ratio.

Analysis of Equivalent Diaphragm Vault Structures in Masonry Construction under Horizontal Forces

In seismic areas, masonry construction is prone to brittle failures due to the mechanical behavior of the constituent materials and to the low capacity of force redistributions. The redistribution capacity is mainly due to the presence of horizontal connections upon the walls and to the stiffness of the roof, which is typically a vaulted structure. The modeling of the global behavior of a masonry building, taking into account the accurate stiffness of the vaults, is a major issue in seismic design and assessment. The complex geometry of the vaults can be considered as an equivalent plate, able to replicate the stiffness behavior and the force redistribution capacity of the real vault. In this study, the efforts of the authors are addressed to the definition of a plate, able to replace the vaulted surfaces in a global numerical model. The ideal diaphragm is considered as a generally orthotropic plate with the same footprint and the same thickness of the original vault. An extended parametric study was conducted in which the mechanical and geometrical parameters were varied, such as the vault thickness, its dimensions, the constraint conditions, and the possible presence of side walls. The results are presented and discussed herein, with the aim of providing useful information to the researchers and practitioners involved in seismic analyses of historical masonry construction.

Investigation of sidewall height effect on the burning rate and flame tilt characteristics of pool fire in cross wind

In the current study the combining effect of sidewall height and cross air flow on the mass burning rate, flame height and flame tilt behavior of pool fire has been experimentally investigated. The mass burning rate increases as the cross wind velocity increases. The presence of sidewalls also influences the mass burning rate, as it is increased at lower cross wind velocities and decreased as this velocity further increases. A dimensionless parameter is introduced to illustrate the sidewall height effect on the mass burning rate. The effect of sidewall height on burning rate, flame height, and flame tilt angle of pool fire is further investigated. The flame height is an exponential function of a new dimensionless parameter Fr˜. Based on the experimental results, new empirical correlations are proposed to predict flame height and flame tilt angle α in relation to the combining effects of sidewall height and cross wind.

Effect of Variable Sidewall Temperatures on the Combined Surface Radiation—Convection in a Discretely Heated Enclosure

This paper reports the changes made in the flow and heat transfer characteristics of a closed enclosure in the presence of sidewalls with symmetrical linear heating. The flow inside the enclosure is primarily driven by a centrally placed discrete heater with thermal radiation included at all surfaces involved. Finite volume method-based computational results corresponding to the resulting steady-state were obtained. The factors causing augmentation and suppression of heat transfer are discussed for two types of sidewall heating. Moreover, it is found that the role of radiation is well stronger than convection in determining the total heat transfer rate when the sidewall heating is decreasing with height.

Characterization of Plasma Induced Damage and Strain on InP Patterns and Their Impact on Luminescence

ABSTRACTThe effect of damage induced by plasma etching on the cathodoluminescence intensity of micron-size InP features is studied. At the etched bottom, it is found that the hard mask stripping process is sufficient to recover the luminescence. Within features, the presence of sidewalls reduces luminescence intensity due to additional non-radiative surface recombinations. For a n-doped sample, a carrier diffusion length of 0.84 μm and a reduced nonradiative surface recombination velocity of 2.58 are calculated. Hydrostatic strain within the etched features is measured using the peak shift of the luminescence signal, while in plane strain anisotropy is obtained from its degree of polarization, both with a resolution of about 100 nm.

Vertical temperature profile of fire-induced facade thermal plume ejected from a fire compartment window with two adjacent side walls

This work investigates the vertical temperature profiles of fire-induced facade thermal plumes ejected from a compartment window with two adjacent side walls. A small-scale cubic compartment (1:8) with its window attached by a vertical facade wall is employed. Two side walls are positioned symmetrically at the two sides of the window of the compartment. The compartment ventilation and side wall constraint conditions are varied by changing the window dimensions and the side wall separation distances during the experiments. K-type thermocouples are installed along the facade wall to measure the vertical temperature profile. Results show that the temperature at a given height increases with the decrease in side wall separation distance for a “(half) axisymmetric plume”, as the air entrainment into the plume is reduced with the presence of side walls. A global model is further developed, based on the change of the air entrainment (in relation to the separation distance of side walls, the characteristic length scales of the window and dimensionless excess heat release rate), to account for the side wall constraint effect on vertical temperature profile upon the facade wall. Experimental data for various window dimensions and side wall separation distances are well correlated by the proposed model.

Wave power extraction of a heaving floating oscillating water column in a wave channel

The performance evaluation of a wave energy converter in wave channel is influenced by the hydrodynamic effects caused by the near presence of the side walls. Since this phenomenon is not observed in the open ocean, it is important to assess the walls influence in the converter dynamics when analysing experimental results. This paper studies the dynamics and power extraction of an axisymmetric floating oscillating water column (OWC) device, the Spar-buoy OWC, using experimental data obtained in a wave channel. A two heaving body model (spar-buoy and OWC) based on linear forces is formulated in the frequency domain. Linear hydrodynamic coefficients are obtained from a boundary integral equation method. The presence of the channel side walls is simulated approximately by a periodic array of devices, and alternatively by two finite-length walls. Linearized drag forces are derived from small-scale model tests. Power extraction results are presented for regular and irregular waves. The numerical simulations show that the wall effect may amplify the power capture up to a maximum of 15% for regular waves and 10% for irregular wave conditions, for a channel-width-to-device-diameter ratio equal to 5.25.

Numerical investigation on window ejected facade flames

A numerical simulation was performed to assess the ability of the used code to model a compartment fire with external flames. Thereafter, numerical investigations were performed in order to study the influence of geometrical parameters such as opening dimensions, opening position and presence of sidewalls at the opening on window ejected flames. The influence of the heat release rate was also discussed. These studies highlight the different phenomena in each configuration and their influence on the risk of vertical fire spread. Moreover, to study the influence of sidewalls at the opening, numerical results were compared to Lu's correlation (Lu et al., 2013, 2014) [1,2]).

A coupled finite volume immersed boundary method for simulating 3D viscoelastic flows in complex geometries

We report on simulations of an unsteady three dimensional viscoelastic fluid flow through a model porous medium, employing a finite volume methodology (FVM) with a staggered grid. Boundary conditions at the walls of the porous structures are imposed using a second order immersed boundary method (IBM), allowing for accurate simulations using a relatively coarse grid. We compare the viscoelastic stresses obtained using this new IBM technique with those published in literature and find good correspondence. Next, we applied this methodology to model viscoelastic fluids with a FENE-P constitutive model flowing through closely spaced cylinders. Using periodic boundary conditions, we modeled the flow behavior for Newtonian and viscoelastic fluids for successive contractions and expansions. We observe the presence of counter-rotating vortices in between the closely spaced cylinders. The viscoelastic flow structure is symmetric for lower Deborah (De) number, but onset of an asymmetry occurs after a critical De for an infinite array of cylinders. In the presence of side walls, we observe that the onset of flow asymmetry happens at a much lower De, which can be related to higher viscoelastic stresses normal to the flow direction and larger extensional viscosities which affect the curved streamlines. The three-dimensional flow characteristics for viscoelastic flow at higher De number are quite different in comparison with Newtonian flow behavior.

Assessment of porous media instead of slatted floor for modelling the airflow and ammonia emission in the pit headspace

In order to reduce the emission, proper understanding of the transportation behaviour of gaseous ammonia inside the slurry pit is required. Numerical simulation by the aid of computational fluid dynamics (CFD) technique can be used for this purpose. However, direct modelling of slatted floors is complicated and may be replaced by the porous media model (PMM) as shown in earlier studies. The objective of our study is to improve the quality of simulation results by PMM, and to assess the effects of air velocity above the slatted floor (as affected by wind), pit headspace height (as affected by amount of slurry in the pit) and sidewall height (as affected by the dairy house sidewall) on the airflow features inside the pit and ammonia emission from the pit. Three different CFD models of a slatted floor were developed to evaluate whether porous media is capable to represent a slatted floor for modelling the airflow inside and ammonia emission from the slurry pit, and to study the effect of turbulence treatment in the porous media on the modelling results: a slatted floor model (SFM) which models the slatted floor as it is, a turbulent porous media model (PMM-T) and a laminar porous media model (PMM-L). Both PMM-T and PMM-L represent the slatted floor by porous media, the PMM-T assumes turbulent airflow and the PMM-L assumes laminar airflow in the porous media. The SFM was verified for a dataset acquired from a 1:8 scale wind tunnel model of the slurry pit. Results showed that the PMM (PMM-T and PMM-L) were able to predict both the airflow features inside the slurry pit and the ammonia emission from the slurry pit if the resistance parameters and flow regime of the porous media were properly set. In comparison to the SFM, the PMM-T predicted the flow pattern better, but overestimated the turbulence intensity and the consequent emission rate. PMM-L performed better in predicting the ammonia emission rate because of the relatively accurate prediction of turbulence intensity. Simulation results also showed that the ammonia emission rate increased with a higher mean airflow velocity, a smaller headspace height and the presence of sidewalls.

Long-range wall perturbations in dense granular flows

AbstractWe explore how the rheology of dense granular flows is affected by the presence of sidewalls. The study is based on discrete element method simulations of plane-shear flows between two rough walls, prescribing both the normal stress and the shear rate. Results confirm previous observations for different systems: large layers near the walls develop where the local viscosity is not constant, but decreases when approaching the walls. The size of these layers can reach several dozen grain diameters, and is found to increase when the flow decelerates, as a power law of the inertial number. Two non-local models are found to adequately explain such features, namely the kinetic elasto-plastic fluidity (KEP) model and the eddy viscosity model (EV). The analysis of the internal kinematics further shows that the vorticity and its associated length scale may be a key component of these non-local behaviours.

INTERMITTENCY AND CHAOS NEAR HOPF BIFURCATION WITH BROKEN 𝕆(2) × 𝕆(2) SYMMETRY

The eight-dimensional normal form for a Hopf bifurcation with 𝕆(2) × 𝕆(2) symmetry is perturbed by imperfection terms that break a continuous translation symmetry. The parameters of the fully symmetric normal form are fixed to values for which all basic periodic solutions residing in two-dimensional fixed point subspaces are unstable, and the dynamics is attracted by a chaotic attractor resulting from a period doubling cascade of periodic orbits. By using symmetry-adapted variables, the dimension of the phase space of the normal form is reduced to four and the dimension of the perturbed normal form is reduced to five. In the reduced phase space, periodic solutions are revealed as fixed points, and quasiperiodic solutions as periodic orbits. For the perturbed normal form, parameter regimes with different types of chaotic dynamics are identified when the imperfection parameter is varied. The characteristics of this complex dynamics are symmetry breaking and increasing, various period doubling cascades, intermittency and crises, and switching between symmetry-conjugated chaotic saddles. In particular, the perturbed system serves as a low dimensional model for the complicated switching dynamics found in simulations of the globally coupled system of Ginzburg–Landau equations extending the 𝕆(2) × 𝕆(2)-symmetric normal form to account for spatial modulations. In addition, this system can be considered as a low dimensional model for the dynamics of perturbed waves in anisotropic systems with imperfect geometries due to the presence of sidewalls.

Experimental investigation of side wave formation in forest covers

Experiments on propagation of very-high-frequency and ultrahigh-frequency waves over short distances between antennas immersed in the forest environment with different structure and species composition are discussed. It is shown that the biometric parameters of the forest vegetation and the radiation frequency play a decisive role in the formation of side walls propagation along the top layer of the forest. It is found that the conventional approach to describing the attenuation property of the forest environment using the notion of the attenuation per unit length is incorrect in the presence of side walls.

Experimental investigation of the effect of endplates and sidewalls on the near field development of a smooth contraction rectangular jet

The effects of different boundary conditions on the near field development of a turbulent rectangular jet issuing from a smooth contraction nozzle of aspect ratio 15 have been studied experimentally with x -wire Hot-Wire Anemometry (HWA). Four different configurations resulting from the presence or absence of endplates–configuring a jet out of a wall–and sidewalls, confining the jet at the planes of the short side walls of the exit nozzle, under identical inlet conditions have been considered. The mean streamwise and lateral velocity and associated turbulent characteristics up to an axial distance of 35 nozzle widths, H , have been measured. Results comprise centreline data for three Reynolds numbers, based on mean exit velocity and nozzle width, Re = 10,000 , 20,000 and 30,000 as well as lateral distributions at 13 downstream locations for Re = 20,000 . In the absence of sidewalls the presence or absence of an endplate has little effect on the development of the flow field. Measurements made in the presence of sidewalls indicate that the presence of the endplate results in a jet with higher lateral outward velocities within the jet’s shear layers and smaller inward lateral velocities outside the jet’s edges, followed by higher turbulent intensities of both streamwise and lateral components in the early stages of development. Furthermore, in configurations comprising sidewalls, the presence of an endplate results in a faster spreading and decaying jet whereas a reduction of the momentum flow in the far field is also observed.

Patterned heteroepitaxial processing applied to ZnSe and ZnS0.02Se0.98 on GaAs (001)

We have demonstrated the patterned heteroepitaxial processing (PHP) approach for the removal of threading dislocations (TDs) from ZnSe and ZnS0.02Se0.98 on GaAs (001). PHP involves the growth of a continuous heteroepitaxial layer followed by postgrowth patterning and annealing. We found that the basic mechanism of TD removal by PHP is thermally activated dislocation motion in the presence of sidewalls. By studying the temperature dependence we showed that the activation energy for the annealing process (∼0.7 eV in ZnSe on GaAs) is consistent with dislocation motion by glide. We showed that there is a minimum mesa thickness required for the complete removal of TDs by PHP (∼3000 Å for 70 μm×70 μm mesas of ZnSe on GaAs). This is because the lateral forces acting on TDs are proportional to the mesa thickness. We also conducted a preliminary study of the mismatch dependence of PHP. Our results suggest that PHP removes TDs more effectively in the higher lattice mismatch system ZnSe/GaAs (001) than in the lower lattice mismatch system ZnS0.02Se0.98/GaAs (001). This is expected based on the mismatch dependence of the line tension forces in the misfit segments of dislocations.

Three-dimensionality of grooved channel flows at intermediate Reynolds numbers

This paper describes the three-dimensional flow structure in grooved channels with different cavity lengths at intermediate Reynolds numbers. For steady flow, the three-dimensional effects are dominant near the side walls of the channel. However, after the onset of self-sustained oscillatory flow due to Tollmien–Schlichting waves as the primary instability, a secondary instability produces a three-dimensional flow with Taylor–Geortler-like vortical structure, at the bottom of the groove. This trend becomes more significant as the cavity length increases. Furthermore, the reason for three-dimensional flow is discussed using additional numerical analysis, and it is confirmed that the source of three-dimensional instability is the groove vortices due to the presence of side walls, rather than the channel traveling wave.