Homogeneous Velocity Research Articles

A revisit of Rayleigh capillary jet breakup at low Ohnesorge number

The average Rayleigh capillary breakup length of a cylindrical Newtonian viscous liquid jet moving with homogeneous velocity must be determined by the selection of normal modes with time-independent amplitude and wavelength. Invariant modes (IMs) with both positive and negative group and phase velocities exist in ample ranges of the parameter domain (Weber and Ohnesorge), which explains (i) the self-sustained average breakup length (long-term resonance) and (ii) the governing role on breakup of both spatial growth rate and wave number of the dominant positive group velocity IM. Published experimental results at low Ohnesorge numbers confirm our proposal.

Design and validation of a suction device to reduce cross-contamination in multi-material laser-based powder bed fusion

Powder suction modules for additive manufacturing systems in the field of multi-material laser powder bed fusion (PBF-LB/M) have a critical influence on the quality of the manufactured components. In particular, metal powder cross-contamination originating from insufficient removal of foreign particles during multi-material processing can lead to undesired changes in material properties. In this paper, a suction device was simulated, designed, and validated to reduce cross-contamination. Requirements and necessary characteristic values of the multi-material PBF-LB/M system, the powder suction module, and the materials used were determined. Furthermore, an iterative simulative method and procedure for the design of suction devices was applied. For this purpose, a flattened bell-shaped geometry with flow-optimized fins was presented. This design led to both a high flow velocity and a homogenous velocity distribution at the nozzle inlet for full-surface multi-material PBF-LB/M suction modules. The developed method was validated by applying it to the material combination of tool steel 1.2709 and copper alloy CW106C. The aim was to reduce the cross-contamination generated during the in-process on an SLM 280HL multi-material PBF-LB/M system. Multi-material samples were built using the original nozzle design as well as the newly developed improved powder suction module and investigated in terms of resulting cross-contamination using energy-dispersive X-ray spectroscopy. The improved suction module had a significantly higher flow velocity and a more homogenous velocity distribution at the nozzle inlet compared to the original suction module. Cross-contamination of the manufactured multi-material samples was reduced by up to 93 % due to the improved suction model.

An Algorithm of Acoustic Emission Location for Complex Composite Structure

Acoustic emission (AE) is widely used in engineering and rock mechanics. The algorithm of AE location based on homogeneous medium or single velocity structure is confronted with lower accuracy when it is applied to the actual working conditions that are prevailing complicated and heterogeneous. In this paper, an AE location algorithm based on complex composite structure (CCS) is proposed via carrying out the following studies: (1) A new travel time calculation scheme suitable for CCS with step-like velocity change is proposed based on an optimized shortest path algorithm. By doing this, a reasonable ray path that is only deflected at the interface is obtained to improve the travel time accuracy. The time complexity of the new algorithm is O(nlog2n). (2) The availability of the new algorithm is verified via a theoretical analysis under a one-dimensional velocity structure as well as an AE experiment using a complex structure under artificial excitation. (3) The AE location during the failure of a simulated CCS in the laboratory indicates that the new algorithm can effectively calculate the travel time and ray path of the sample.

Effect of external fields on pattern formation of charged grains in plasma

AbstractTwo‐dimensional cluster and pattern formation by charged grains in plasma under external harmonic force and transverse magnetic field is investigated using molecular dynamics simulation. It is found that, as a result of the Yukawa‐like grain‐grain interaction and the external fields, the asymptotic quasistationary state of an initially homogeneous and Maxwellian velocity distributed grain system can be a disk cluster with the grain trajectories having flowery zonal patterns.

Slugging in horizontal channel flow

Abstract The stratified flow of gas–liquid mixtures is important in many industries, in particular oil and gas, where the formation of liquid slugs is usually undesirable. In this paper, we analyse the hydraulic model for turbulent, two-layer, gas–liquid flow in a horizontal channel developed by Needham et al. (2008, The development of slugging in two-layer hydraulic flows, IMA J. Appl. Math., 73, 274–322) who considered only the thin layer limit, for liquid layers of arbitrary thickness. This allows us to consider the effect of the prescribed inlet velocity ratio and the interaction of the gas layer with the upper wall of the channel on the formation of slug formation. We show that there is a critical Froude number above which small-amplitude disturbances to the uniform flow are unstable and grow to form non-linear roll waves. We investigate the existence of periodic travelling wave solutions and find that these are strongly influenced by the existence of two equilibrium solutions that are not present when the liquid layer is thin. The system of ordinary differential equations that describe travelling wave solutions has a complicated phase portrait which we study in some typical cases. We find that periodic solutions that have the channel close to filled with liquid can exist, and we identify them as slugs. We also study an initial value problem driven by noise at the inlet of the channel and find that structures locally similar to these periodic travelling waves are generated. We provide numerical evidence for the formation of slugs in cases where the initial liquid to homogeneous velocity ratio is greater than 0.5.

Bioclogging alleviation for constructed wetland based on the interaction among biofilm growth and hydrodynamics.

Bioclogging is the most crucial operation problem of the constructed wetlands, which reduce its removal efficiency and life span. A strategy through properly increasing hydraulic loading is proposed in this study to alleviate the bioclogging for CWs. The two-dimensional porous media flow cell (2D PMFC) test indicated that a quadratic correlation was found between local biofilms growth rate and the near-wall Reynolds number (r > 0.765, p < 0.05). The biofilm growth rate declined with the flowrate when Re exceeded about 6.0. It was also found that the higher flowrate (6mL/min) lead to the homogeneous biofilm and velocity distribution in the PMFC. The column test indicated that the highest hydraulic loading (9.2cm/h) produced the smallest decrease in hydraulic conductivity, which was 80 times more than that of low hydraulic load (3.0cm/h) at the end (40days) of experiment. Moreover, the relatively homogenized distribution of biofilm was found along the column with the highest hydraulic loading, which confirmed that the proper increase in hydraulic loading can alleviate bioclogging.

Simulation-based investigations of geometrical design effects on the efficiency of the intake manifold of a six-cylinder diesel engine

Design studies of heavy-duty diesel engines are very important due to the high cost of production and the difficulties of experimental performance tests. This paper deals with finding the optimum geometric design parameters of the intake manifold of a locomotive diesel engine using the Computational Fluid Dynamics (CFD) methods and designing a suitable model. The designed models with different shapes and diameters are compared with the existing intake manifold in terms of pressure, velocity distributions, and pressure loss. Since the location of the intake manifold on the engine is limited, the pressure and the velocity distributions of the models are evaluated when changing the diameters and geometries of intake manifolds of the same lengths. These modifications of the manifold (shapes and diameters) are made by arranging them to fit in the engine packaging space. After the plenum geometry is determined based on the analyses of the five models, the models are also analysed for different turning radii (45 mm, 50 mm, 65 mm) of the runners at the entrance to the combustion chamber. As a result of all the analyses, the model that obtained the lowest pressure loss and best homogeneous velocity distributions is determined as the optimum design.

Interacting particles in an activity landscape

We study interacting active Brownian particles (ABPs) with a space-dependent swim velocity via simulation and theory. We find that, although an equation of state exists, a mechanical equilibrium does not apply to ABPs in activity landscapes. The pressure imbalance originates in the flux of polar order and the gradient of swim velocity across the interface between regions of different activity. An active–passive patch system is mainly controlled by the smallest global density for which the passive patch can be close packed. Below this density a critical point does not exist and the system splits continuously into a dense passive and a dilute active phase with increasing activity. Above this density and for sufficiently high activity the active phase may start to phase separate into a gas and a liquid phase caused by the same mechanism as motility-induced phase separation of ABPs with a homogeneous swim velocity.

Sensitivity analysis to near-surface velocity errors of the Kirchhoff single-stack redatuming of multi-coverage seismic data

We present a study of a Kirchhoff-type redatuming approach for irregular multi-coverage land seismic data to remove the disturbing influence of rugged topography and a considerable near-surface velocity inhomogeneity, like a thick weathering layer, on the subsurface reflections. The redatuming method is physically more justified than the commonly used field-static correction methods that provide - in the indicated situation - non-physical temporal adjustments (field statics) by shifting traces in time, which lead to sub-optimal imaging and inversion of the reflections. The redatuming procedure studied here is different: It transforms the multi-coverage seismic reflection data on the irregular measurement surface to a regular measurement configuration on a pre-specified datum, below the inhomogeneous velocity overburden. This is done by stacking/summing the input traces with certain operators or traveltime curves which are calculated by ray tracing. In this way, incoherent subsurface reflections in the input data become coherent and thus recognizable in the output data. To have this happen, the redatuming operation requires a good approximation of the overburden velocity model and a reasonable one of the sub-burden homogeneous velocity model (below the datum). Then the multi-coverage output data on the new flat datum can be imaged and inverted much better than with standard data processing, i.e., with field-static correction. This work is dedicated to explaining the single-stack Kirchhoff-type redatuming approach kinematically using a synthetic model and data based on a real geological situation. However, our main aim is to study the sensitivity of this redatuming method to errors of the overburden velocity model, which is determined from multi-coverage seismic data using first arrivals traveltime tomography commonly used in the oil industry. We apply the redatuming process to a multi-coverage synthetic seismic dataset generated from the models that represent the real geological situation with irregular measurement surfaces and inhomogeneous overburden or near-surface velocity. To assess accuracy, the redatumed data was migrated in depth-domain and the results showed the correct positioning of the reflectors. The results revealed that this redatuming method produces reliable and accurate redatumed data even using approximate velocity models, which can be useful for prestack time and depth imaging.

A Differential Subgrid Stress Model and Its Assessment in Large Eddy Simulations of Non-Premixed Turbulent Combustion

We present a new subgrid stress model for the large eddy simulation of turbulent flows based on the solution of transport equations for stress tensor components. The model was a priori term-by-term calibrated against an open DNS database on forced isotropic turbulence (Johns Hopkins University database). After that, it was applied in a large eddy simulation of non-premixed turbulent combustion. To demonstrate the impact of the new subgrid stress model on scalar fields, we excluded the backward effect of heat release on the subgrid stresses, considering an isothermal reaction (i.e., diluted mixture; the density variations associated with chemical heat release can be neglected) and a Burke–Schumann reaction sheet approximation. A periodic box filled with a homogeneous turbulent velocity field and a three-layer top-hat mixture fraction field was studied. Four simulations were performed in which a fixed model for mixture fraction and its variance was combined with either the proposed subgrid stress model or one of the standard models, including Smagorinsky, dynamic Smagorinsky and WALE. Qualitatively correct backscatter was observed in a simulation with the new model. The differences in the statistics of the mixture fraction and reactive component fields caused by the new subgrid stress model were analyzed and discussed. The importance of using an advanced subgrid stress model was highlighted.

Image Human Thorax Using Ultrasound Traveltime Tomography with Supervised Descent Method

The change of acoustic velocity in the human thorax reflects the functional status of the respiratory system. Imaging the thorax’s acoustic velocity distribution can be used to monitor the respiratory system. In this paper, the feasibility of imaging the human thorax using ultrasound traveltime tomography with a supervised descent method (SDM) is studied. The forward modeling is computed using the shortest path ray tracing (SPR) method. The training model is composed of homogeneous acoustic velocity background and a high-velocity rectangular block moving in the domain of interest (DoI). The average descent direction is learned from the training set. Numerical experiments are conducted to verify the method’s feasibility. Normal thorax model experiment proves that SDM traveltime tomography can efficiently reconstruct thorax acoustic velocity distribution. Numerical experiments based on synthetic thorax model of pleural effusion and pneumothorax show that SDM traveltime tomography has good generalization ability and can detect the change of acoustic velocity in human thorax. This method might be helpful for the diagnosis and evaluation of respiratory diseases.

Convolutional neural network-based moment tensor inversion using domain adaptation for microseismicity monitoring

Microseismic monitoring is widely used to analyze the locations and growth directions of fractures formed at sites of hydraulic fracturing treatment and CO2 geologic sequestration. Because moment tensors can provide focal mechanisms, moment tensor inversion has received considerable attention in microseismic monitoring; the real-time processing of moment tensor inversion is important for rapid decision-making. Pre-trained machine learning (ML) models can make nearly instantaneous predictions in the application stage and thus present an attractive alternative to real-time processing. However, prior information regarding the velocity model at the target site is a prerequisite for generating the dataset used to train the ML model that is applied in moment tensor inversion. In addition, it is difficult to create the training dataset because it requires three-dimensional numerical modelling when the velocity model is complex; numerous simulations must be executed for sources with various locations and moment tensors. To overcome these limitations, we applied the domain adaptation technique to the convolutional neural network (CNN)-based moment tensor inversion method, which uses peak amplitudes and arrival times of P- and S-waves as input features. The CNN model was pre-trained with the dataset generated from a homogeneous velocity model. Then, in the domain adaptation stage, the pre-trained model was fine-tuned along with the target dataset. To validate the performance of the domain adaptation, moment tensors from both horizontal and tilted three-layer models were predicted. In each case, the domain-adapted model performance was similar to the performances of the CNN-based models that had been trained using the dataset generated with the exact target velocity models.

Hydrodynamics of a bubble-induced inverse fluidized bed reactor with a nanobubble tray

This paper reports on the hydrodynamics of a bubble-induced inverse fluidized bed reactor, using a nanobubble tray gas distributor, where solid particles are fluidized only by an upward gas flow. Increasing the gas velocity, the fixed layer of particles initially packed at the top of the liquid starts to move downwards, due to the rise of bubbles in this system, and then gradually expands downwards until fully suspended. The axial local pressure drops and standard deviation were examined to delineate the flow regime comprehensively under different superficial gas velocities. Four flow regimes (fixed bed regime, initial fluidization regime, expanded regime, and post-homogeneous regime) were observed and three transitional gas velocities (the initial fluidization velocity, minimum fluidization velocity, and homogeneous fluidization velocity) were identified to demarcate the flow regime. Three correlations were developed for the three transitional velocities. As the fine bubbles generated from the nanobubble tray gas distributor are well distributed in the entire column, the bed expansion process of the particles is relatively steady.

Numerical investigations on temperature and flow field performance of octagonal culture tank under different physical parameters for fish growth based on computational fluid dynamics

• Both velocity and temperature fields within octagonal rearing tank jointly solved by theory and CFD. • Effects of seven different physical parameters of rearing tank numerically studied. • Average and STD of temperature and velocity for seven horizontal planes used quantitatively to evaluate effects. • Completely mixing flow field & obvious secondary flow existing with increasing inlet velocity by 0.5 m/s. Both velocity and temperature field performance of fish octagonal rearing tank played an important role in the recirculating aquaculture system (RAS). To maintain the water temperature and velocity distributions in the optimal growth conditions for fish, particularly juvenile fish, is a great challenge. To solve this problem, computational fluid dynamics (CFD) was employed to investigate the dynamic temperature and velocity fields in the juvenile fish rearing tank. In this study, a full-scale CFD benchmark model of octagonal rearing tank based on real situation has been firstly validated by comparing on-site measurement data with simulation results. Following that, the effects of several different physical parameters of rearing tank, including inlet flow temperature and velocity, inlet position on the culturing tank wall, water flow inlet diameter and ambient air temperature around RAS on both velocity and temperature field performance were numerically studied, respectively. Meanwhile, average temperature and velocity as well as standard deviation (STD) of temperature and velocity for seven different horizontal plane have been employed to quantitatively evaluate the effects of aforementioned physical parameters on velocity and temperature field performance. Some key findings are found that 1) Benchmark scenario with inlet temperature of 25 °C and inlet velocity of 0.1 m/s could establish dynamic homogeneous temperature and velocity fields, which is suitable for fish growth; 2) Increasing inlet temperature will increase STD of horizontal plane temperature and achieve a maximum approximately 55% at the height of inlet position with inlet temperature of 28 °C; 3) As the inlet diameter increases up to 101.6 mm with inlet velocity 0.1 m/s, which is not only beneficial to clean the solid waste; but also could provide a suitable juvenile fish growth conditions.

Optical and near-infrared data and modelling of nova V5668 Sgr

ABSTRACT We present Hubble Space Telescope optical images, Keck-OSIRIS near-infrared (NIR) integral field spectroscopy data cubes and Keck-Near InfraRed Camera-2 (NIRC2) NIR images of nova V5668 Sgr from 2016 to 2019. The observations indicate enhanced emission at the polar caps and equatorial torus for low-ionization lines, and enhanced high-ionization emission lines only at the polar caps. The radial velocities are compatible with a homogeneous expansion velocity of v = 590 km s−1 and a system inclination angle of 24°. These values were used to estimate an expansion parallax distance of 1200 ± 400 pc. The NIRC2 data indicate the presence of dust in 2016 and 2017, but no dust emission could be detected in 2019. The observational data were used for assembling 3D photoionization models of the ejecta. The model results indicate that the central source has a temperature of 1.88 × 105 K and a luminosity of 1.6 × 1035 erg s−1 in August of 2017 (2.4 yr post eruption), and that the shell has a mass of 6.3 × 10−5 M⊙. The models also suggest anisotropy of the ionizing flux, possibly by the contribution from a luminous accretion disc.

Anomalous diffusion in a circular comb with external velocity field

Transport of particles confined in a branched circular medium under influence external force is a common phenomenon in nature and science. In this paper, the diffusion of particles, under an external homogeneous and inhomogeneous velocity field, in a circular comb has been studied and the impact of the geometry on the process has been discussed. The process is a circular motion under external velocity for fixed radius r=R and is interspersed with a radial motion inward and outward of the circle. The diffusion-advection equation that describes this process has been derived and solved numerically. The results show an enhance in the diffusion process in the circular direction according to the external force and the radius of the circle, where a transition between sub and normal diffusion in the case of homogeneous velocity, and a transition between sub and fast diffusion in the case of in-homogeneous velocity. However, due to the finite region of the structure, the process tends to saturate the structure with time. On the other hand, the speed of enhancement in the diffusion process and its saturation decreases as the radius of the circle increases. This model can be served in the analysis of the restricted diffusion of spin-bearing particles in circular layers in in-homogeneous magnetic fields.

Seismic source location using the shortest path method based on boundary discretisation scheme for microseismic monitoring in underground mines

In seismic monitoring at underground mines, the velocity models can be highly complex, particularly in presence of major geological structures . In practice, homogeneous velocity models have been widely used for the purposes of seismic event location in mine seismology . Although heterogeneous velocity models can be incorporated, ray tracing methods need be adopted in these velocity models to compute the travel times for accurately locating seismic events. These methods require each geological unit to be discretised to grids (corner-node or center-node discretisation scheme), which requires large computational capacity. In this research, the Shortest Path Method (SPM) (Moser, 1991) based on a novel Boundary Discretisation Scheme (BDS) is proposed to compute the travel times for seismic event location in heterogeneous velocity models. Only the boundaries of each geological unit are discretised to nodes. The graphical network is constructed by establishing ray paths between discretised nodes on the boundaries of geological units. Dijkstra’s algorithm (Dijkstra et al., 1959) using heap sorting method (Cao et al., 1993) is applied to search for the shortest ray paths with the first arrivals for seismic event location. The accuracy and efficiency of this method are validated using a 2D synthetic example. The seismic events recorded during the field monitoring of a dyke-roadway intersected area are located with the proposed BDS-SPM method. The results demonstrate that, compared with the conventional location method using homogeneous velocity models, the proposed method can provide more accurate seismic event locations for microseismic monitoring in underground mines.

Wave propagation in a doubly tapered shear beam: Model and application to a pyramid‐shaped skyscraper

AbstractOne dimensional wave propagation is studied in a cantilever conical structure deforming in shear. The structure has rectangular cross‐section both dimensions of which decrease linearly along the polar axis (hence, doubly tapered). The equation motion is derived and solved in terms of spherical Bessel functions, which can be expressed exactly as finite algebraic expressions in terms of powers and sine and cosine functions of the polar coordinate. The transfer‐matrix for a truncated pyramid element is then derived and generalized to a chain of elements. The model is used to represent a 48‐story pyramid‐shaped steel‐frame skyscraper, the Transamerica Tower in San Francisco, California, in which the Loma Prieta, 1989 earthquake (, epicentral distance, km) was recorded by an array of accelerometers. The building is modeled by homogeneous and layered truncated pyramids. Wave propagation through the building is studied by analysis of impulse response functions computed from the observed earthquake accelerations. In addition, its equivalent homogeneous beam shear‐wave velocity is identified solely from its geometry and observed fundamental frequency of vibration. Further, the variation of wave velocity along is height is identified by least squares fit of a layered pyramid model in the observed impulse response functions. The results reveal equivalent homogeneous pyramid wave velocity of about 150–160 m/s in both directions, which is similar to other steel‐frame structures. The identified wave velocity profiles are consistent with the structural design. The identified parameters can be used as reference in future structural health monitoring of this structure.

Assessment of a Non-Conservative Four-Equation Multiphase System with Phase Transition

This work focuses on the formulation of a four-equation model for simulating unsteady two-phase mixtures with phase transition and strong discontinuities. The main assumption consists in a homogeneous temperature, pressure and velocity fields between the two phases. Specifically, we present the extension of a residual distribution scheme to solve a four-equation two-phase system with phase transition written in a non-conservative form, i.e. in terms of internal energy instead of the classical total energy approach. This non-conservative formulation allows avoiding the classical oscillations obtained by many approaches, that might appear for the pressure profile across contact discontinuities. The proposed method relies on a finite element based residual distribution scheme which is designed for an explicit second-order time stepping. We test the non-conservative residual distribution scheme on several benchmark problems and assess the results via a cross-validation with the approximated solution obtained via a conservative approach, based on a HLLC scheme. Furthermore, we check both methods for mesh convergence and show the effective robustness on very severe test cases, that involve both problems with and without phase transition.

Approximations to a generalized real ray-tracing method for heterogeneous anisotropic viscoelastic media

The real ray-tracing approach leads to an effective solution in the real space domain using a homogeneous ray velocity vector. However, it fails to yield solutions for quasi-S-waves, which suffer from triplications of the wavefronts. To address this challenging problem, a new real ray-tracing method and its two approximations are developed to solve the complex ray equation. The numerical results indicate that the new real ray-tracing method is superior to the common real ray-tracing method in the presence of triplications of the quasi-S-waves in the computation of ray velocity, ray attenuation, and ray quality factors, as well as the reflection and transmission coefficients in viscoelastic anisotropic media. Based on the assumptions of the real slowness direction and real polarization vectors, two new approximations of the new real ray-tracing method are developed for directly computing the homogeneous complex ray velocity vectors of three wave modes (qP, qS1, and qS2). These approximations significantly improve computational efficiency by avoiding the iterative process required by the new real ray-tracing method that is inherited from the common real ray-tracing method. The computational accuracies are verified through transversely isotropic and orthorhombic models with different strengths of attenuation and anisotropy. Incorporation of the new approximation into the shortest-path method turned out to be efficient and accurate for seismic ray tracing in heterogeneous viscoelastic and transversely isotropic media with a vertical axis of symmetry, even in the presence of strong attenuation and anisotropy.