Evolution Of Velocity Research Articles

Prediction on steady-oscillatory transition via Hopf bifurcation in a three-dimensional (3D) lid-driven cube

Massive computations of three-dimensional (3D) flow in subcritical flow state are carried out to investigate the transition from 3D steady flow to time-periodic oscillatory flow via Hopf bifurcation in a cube. Previous prediction of the steady-oscillatory transition in published works has been summarized. The transition point in lid-driven cavity has yet to come to consensus as the critical parameters are predicted in a wide range of values. The 3D unsteady governing equations are solved numerically using the spectral collocation method (SCM) in combination with the artificial compressibility method (ACM), SCM-ACM, a combined method with high accuracy that we developed. The temporal velocity evolution, amplitude evolution and dimensionless angular frequency are investigated. The accurate critical Reynolds number Recr is predicted by Richardson extrapolation. Results show that the initial conditions have significant effect to obtain the velocity which gradually approaches to steady state in the subcritical flow state, and appropriate initial conditions are given to efficiently obtain stable results by gradually increasing Re. Exponential decay of velocity amplitude becomes weaker with the increase of Re, and the decay rate is strictly linear against Re, while the dimensionless angular frequency remains constant. The Recr is affected by the number of nodes, which has also been observed by Feldman and Gelfgat [1] and Kuhlmann and Albensoeder [2], while the critical frequency is unaffected by the number of nodes in the present results. It also indicates that the Recr and the constant dimensionless angular frequency can be innovatively predicted with more accuracy and much fewer nodes. The improved point of steady-oscillatory transition in 3D lid-driven cube is Recr=1916.6 via Hopf bifurcation by Richardson extrapolation with dimensionless angular frequency ω=0.5752.

Numerical study of detonation wave propagation modes in annular channels

Modes of detonation wave propagation in annular channels were investigated numerically by using the adaptive mesh refinement technique. Two-dimensional, reactive Euler equations with a detailed hydrogen/oxygen reaction model were adopted in the computations to simulate the detonation dynamics in the annular geometry. Considering both the decoupling of the detonation wave front and the development of the Mach-stem in reflection, the propagation is divided into unstable and stable propagation modes with different Mach-stem evolutions, namely, a growing, steady, or decaying type. The numerical observations indicate that in the unstable propagation mode, velocity loss and oscillation occur near the inner wall, while the wave front shape and velocity evolution are steadier for the stable propagation mode. The overdriven degree near the outer wall increases as the Mach-stem strength attenuates. The propagation mode diagrams demonstrate that an increase in the initial pressure and wall curvature radius can extend the range of the stable propagation mode, and the Mach-stem is transformed from a growing to steady, and finally a decaying type with the increase in the initial pressure or the decrease in the wall curvature radius to channel width ratio. The limit of wall curvature radius separating the unstable and stable modes is independent of the channel width for the Mach-stem steady and decaying types, while they are positively correlated for the Mach-stem growing type. Finally, a qualitative procedure is proposed to help distinguish different propagation modes based on the formation mechanism of each propagation dynamics.

Kinetic and dynamic analysis of ozonolysis pre-treatment of empty fruit bunch in a well-mixed reactor for sugar production

Pre-treatment is the key step in biorefinery for enhancing cellulose accessibility from lignocellulosic biomass (LB) such as empty fruit bunch (EFB), which contains highly valuable cellulose. Ozonolysis pre-treatment appears as a promising green alternative for isolation of EFB to cellulose. This study develops the diffusion–reaction model of EFB ozonolysis inside a well-mixed novel OzBiONY® ozonolysis reactor at lab scale production. The mathematical model is numerically solved using COMSOL Multiphysics® software. Kinetic reaction parameters are computationally estimated via gradient-based Sparse Nonlinear Optimizer (SNOPT) method with sequential quadratic programming (SQP) algorithm. The evolution of ozone velocity and concentration profiles inside the reactor are simulated by computational fluid dynamics (CFD) method to study the effect of biomass particle sizes. The larger particle size consumes higher ozone (k1 = 2.23 m3/mol∙s) compared to the smaller particle size (k1= 0.09 m3/mol∙s). This reveals that larger biomass particle confers faster delignification rate and a much higher degradation of insoluble lignin. The simulation results demonstrate ozone velocity at the surface of larger particle is lower, but the surface concentration of ozone is higher. The diffusion–reaction model of EFB ozonolysis elucidates the plausible reaction pathway of lignin degradation via ozonolysis pre-treatment with minimum objective function (Z) of 1.41 × 10−3. From experimental results, the highest lignin degradation is 78 wt.% and the highest glucose yield is 12 wt.%. The outcomes of this research will contribute to the development of smart biorefinery technology and systems for sugar production in a sustainable and circular economy.

Stronger Intraseasonal Variability Observed Below the Seasonal Thermocline in the Kuroshio East of Taiwan During 2014 and 2015

AbstractIntraseasonal variability in the Kuroshio east of Taiwan was studied using moored observations at 23°N, 122°E from November 17, 2014 to September 5, 2015, high‐resolution ocean model data, and altimetry data. It is found for the first time that the observed meridional velocity has a peak signal at 50–60 days with a significant maximum variation at 200–600 m depth. Model outputs show that the 50–60‐day variability at the mooring station is related to meandering of the Kuroshio at 23°N. Both surface eddies, which can extend downward to ∼1,000 m depth, and coastal cold water upwelling associated with cyclonic eddies from deep layers can greatly change the isopycnal slope below ∼200 m depth at 23°N, 122°E, and their joint effect results in the maximum variation at 200–600 m depth. Ocean eddies southeast of Taiwan propagate northward along the current axis of the Kuroshio and have a large impact on intraseasonal variability. Case analysis indicates that the formation of ocean eddies or upwelling near the coast may be related to the Lüdao and Lanyu islands southeast of Taiwan or Kuroshio transport along the southern tip of Taiwan. In addition, the evolution of subsurface meridional velocity from mooring observations is inconsistent with that of the surface but is closely related to the evolution of local temperature below the seasonal thermocline. The maximum intraseasonal variability below the seasonal thermocline highlights the complex dynamics in the Kuroshio east of Taiwan.

Fault nucleation of tight sandstone by investigation of mechanical, acoustic, and hydraulic responses

To investigate the effect of water on the fracture mechanism and associated acoustic emission (AE) characteristics, triaxial compression experiments were performed on dry and water-saturated tight sandstone specimens sampled from the Sichuan Basin, China. The saturated specimen was precisely controlled by different drainage conditions at the two ends. The detailed spatiotemporal distribution of AE activities and the P-wave velocities were used to examine the initiation and propagation of microcracks, as well as fault nucleation process. There are significant differences in mechanical behavior, P-wave velocity, AE activities, and fault nucleation between the dry and saturated specimens. The mechanical behavior of the saturated specimen is characterized by water weakening, with 10% and 26% reductions in peak strength and elastic modulus, respectively, in comparison with those for the dry specimen. Water enhances inelastic axial deformation and strain softening. The evolution of P-wave velocity is associated with microcracking-induced dilatancy. The decrease in P-wave velocity is suppressed in the saturated specimen, especially at the drained end. Significant attenuation of AE energy is observed in the saturated specimen, resulting into a lower event rate compared with that for the dry specimen. It is shown that the saturated specimen has an extended fault nucleation duration and lower rupture velocity. The precursory phase of dynamic failure can be clearly mapped based on strain softening, accelerating AE event rate, and decreasing b-value, as well as decreasing pore pressure (in undrained condition) or increasing injection rate (in drained condition).

Quantitative measurements of flow dynamics in 3D hoppers using MRI

This work uses a recently developed MRI method to measure the solid fraction and velocity in three-dimensional (3D) hoppers quantitatively. We demonstrate that the measurements are quantitative by calculating the mass flow rate within the hopper using MRI and show good agreement with the mass flow rate measured gravimetrically. We study the velocity and solid fraction in hoppers with various angles and outlet sizes. We show that the solid fraction decreases smoothly from the bulk value above the outlet, indicating that the assumption of a “free-fall arch” used in some mass flow correlations is invalid. Furthermore, we show that the solid fraction, velocity and vertical evolution of the acceleration are all self-similar when normalised by the value at the centre of the outlet in a 3D hopper, in agreement with recent studies of 2D hoppers. Thus, these quantitative measurements enable evaluation of phenomenological models describing the flow rate from hoppers.

Sloshing dynamics in cylindrical tank with porous layer under harmonic and seismic excitations

The severe sloshing induced by earthquake in cylindrical liquefied petroleum gas (LPG) storage tank is of great concern for liquid storage design. For suppressing sloshing, porous material attached to inner periphery of the tank wall is proposed and studied numerically by using OpenFOAM. The model was validated against with the available data of sloshing in cylindrical tank without and with ring baffle, and flow through porous media in a U-tube and porous dam break. Fair agreements between them are obtained. Findings show that damping effects of porous material layer was significant in linear sloshing scenario compared with ring baffle. The porous material layer was also proved to be good pressure absorber and energy dissipator as evidenced in pressure distribution and evolution of velocity and kinetic energy. Under 1-D harmonic excitation, nonlinear swirling waves could be effectively changed into planar waves due to the effects of porous material layer inside cylindrical tank. As a demonstration, sloshing damping effects of porous material layer in a real scale upright cylindrical tank under EL-centro earthquake excitation were investigated numerically.

Study on Lifted Flame Stabilization Under Different Background Pressures

Abstract A numerical experimental investigation is presented for a steady methane lifted flame and a nonreaction jet flow in a co-flow of hot combustion products from lean premixed air-hydrogen combustion. The main objective has been to analyze the dependence of methane jet flame stability on the background pressure: a pressurized vitiated co-flow burner (PVCB) has been used to study the methane lifted flame and nonreaction jet flow under different background pressures (1–1.5 bars). The lifted flame is characterized by a liftoff height, which has been measured with a high-speed camera, and a central jet flow defined by the jet velocity, which has been measured by means of a high-sensitivity Schlieren imaging system. The experimental results show that the liftoff height decreases for an increment in the background pressure (from 1 to 1.5 bar at 1073 K) and in the co-flow temperature (from 1058 K to 1118 K at 1 bar). The standard deviation of the liftoff height also reduces for an increase in either the background pressure or the co-flow temperature, which indicates that the liftoff height is more stable at higher background pressures and co-flow temperatures. As far as the experimental tests on the nonreaction jet flow is concerned, the jet velocity becomes extinct faster as the background pressure rises, which is consistent with the decrease in the liftoff height as the background pressure grows. The evolution of the jet velocity has been proved to be another important factor that affects the liftoff height under different background pressures (physical factor), in addition to the fuel autoignition delay (chemical factor). The simulation data led with a Reynolds-averaged Navier–Stokes (RANS)/probability density function (PDF) model show that an increment in the background pressure makes the temperatures increase and induces a brighter yellow part of lifted flame, which leads to more soot production. This proves that the flame is not completely premixed. On the other hand, the Schlieren images of the non-reaction jet flow highlight that the flame is partially premixed, since the edge of the jet is not well defined, as the jet penetration increases with time. The liftoff height values of the flame in the numerical simulations were found to be generally higher than those measured in the corresponding experiments. This discrepancy was caused by an appreciable radiation heat loss at the thermocouple. A correlation was therefore developed for the thermocouple temperature measurement in order to correct the inaccuracy.

Investigation on the air-core vortex in a vertical hydraulic intake system

Air-core vortex occurred at the hydraulic intakes deteriorates the performance and operation stability of hydraulic machineries. To investigate the evolution process and generation mechanism of the air-core vortex formed at the vertical intake, large-eddy simulation with the coupled level-set and volume-of-fluid method is performed in a pump sump. This paper elucidates the evolution of velocity distribution caused by pump suction, and associations between vortices motions and velocity evolution and the accompanying free-surface deformations are clarified, which reveals the generation mechanism and evolution characteristics of air-core vortex. Vortices are created by the disturbances of small recirculation zone around the intake pipe. The recirculation is caused by the interaction of the wall of vertical pipe and converging flows with opposite directions driven by the pump suction. The enhancement of vortices due to vortex distortion dominates the air-core vortex formation, and the vertical stretching effect accounts for about 70%. The meandering characteristics of air-core vortex are characterized through analyses of meandering scope and velocity distribution, and the convection of mean flow and sump wall take the leading role in causing its meandering.

Mixed Eulerian–Lagrangian shell model for lateral run-off in a steel belt drive and its experimental validation

A non-material shell finite element model is developed and applied to the example problem of a slack steel belt moving on two rotating drums. For the first time in the open literature we demonstrate an approach for predicting the time evolution of the lateral run-off velocity of the belt in response to its geometric imperfection and angular drum misalignment. We adopt a novel Eulerian–Lagrangian kinematic description featuring a mixed parametrisation of the configurational space with a Eulerian circumferential coordinate and two Lagrangian coordinates for the transverse and lateral deflections. A nonlinear finite element approximation provides the necessary C1 inter-element continuity in this compound coordinate system. Using the model of elastic tangential contact, we account for the convective term in the local increments of the relative displacement between the contacting surfaces during the time integration. A thorough convergence study with respect to the mesh and time step sizes justifies the approach. Together with the successful validation against the results of a series of physical experiments, this makes the present contribution an important step towards a model-based controller design.

Sound pressure level spectrum analysis by combination of 4D PTV and ANFIS method around automotive side-view mirror models

This paper proposes a data augmentation method based on artificial intelligence (AI) to obtain sound level spectrum as predicting the spatial and temporal data of time-resolved three-dimensional Particle Tracking Velocimetry (4D PTV) data. A 4D PTV has used to measure flow characteristics of three side mirror models adopting the Shake-The-Box (STB) algorithm with four high-speed cameras on a robotic arm for measuring industrial scale. Helium filled soap bubbles are used as tracers in the wind tunnel experiment to characterize flow structures around automobile side mirror models. Full volumetric velocity fields and evolution of vortex structures are obtained and analyzed. Instantaneous pressure fields are deduced by solving a Poisson equation based on the 4D PTV data. To predict spatial and temporal data of velocity field, artificial intelligence (AI)-based data prediction method has applied. Adaptive Neural Fuzzy Inference System (ANFIS) based machine learning algorithm works well to find 4D missing data behind the automobile side mirror model. Using the ANFIS model, power spectrum of velocity fluctuations and sound level spectrum of pressure fluctuations are successfully obtained to assess flow and noise characteristics of three different side mirror models.

Coupled 3D numerical model for a landslide-induced impulse water wave: A case study of the Fuquan landslide

Large impulse waves are generated when a highly mobilized landslide impacts a water body along the runout path. On August 27, 2014, a massive landslide occurred in Fuquan, Guizhou, China and induced an impulse wave upon contact with a quarry lake. The landslide and its associated impulse wave were a multiphase and multicomponent mixture that collectively destroyed two villages. The present study provides insights into multiphase dynamic analysis by employing an existing 3D coupled discrete element modeling and computational fluid dynamics approach to model a case study of the Fuquan landslide and its impulse wave. EDEM and Fluent were two-way coupled by an application programming interface, which transferred data between the models to simulate subaerial movement and impulse wave generation. The simulated results indicate a maximum velocity of 31.7 m/s achieved by the subaerial movement and a runout duration of 39 s. The detached mass entered the quarry lake at a velocity of 24.3 m/s, at t = 7.1 s after the landslide initiation. Energy was transferred between the sliding mass and lake water, and the generated water wave rushed toward the opposite slope. The generated wave had a run-up of 28.4 m and traveled approximately 90 m in the lateral direction, with a maximum dynamic pressure of 3.03 MPa. Qualitative validation of the present model was performed using Fritz's experimental setup for wave generation, and the velocity evolution was physically verified. This study provides calibrated parameters for the forward simulation of similar cases of landslide-induced impulse water waves and related hazard zonation.

Progenitor mass constraints for the type Ib intermediate-luminosity SN 2015ap and the highly extinguished SN 2016bau

ABSTRACT Photometric and spectroscopic analyses of the intermediate-luminosity Type Ib supernova (SN) 2015ap and of the heavily reddened Type Ib SN 2016bau are discussed. Photometric properties of the two SNe, such as colour evolution, bolometric luminosity, photospheric radius, temperature, and velocity evolution, are also constrained. The ejecta mass, synthesized nickel mass, and kinetic energy of the ejecta are calculated from their light-curve analysis. We also model and compare the spectra of SN 2015ap and SN 2016bau at various stages of their evolution. The P Cygni profiles of various lines present in the spectra are used to determine the velocity evolution of the ejecta. To account for the observed photometric and spectroscopic properties of the two SNe, we have computed 12 M⊙ zero-age main-sequence (ZAMS) star models and evolved them until the onset of core-collapse using the publicly available stellar-evolution codeMESA. Synthetic explosions were produced using the public version of STELLA and another publicly available code, SNEC, utilizing the MESA models. SNEC and stella provide various observable properties such as the bolometric luminosity and velocity evolution. The parameters produced by SNEC/STELLA and our observations show close agreement with each other, thus supporting a 12 M⊙ ZAMS star as the possible progenitor for SN 2015ap, while the progenitor of SN 2016bau is slightly less massive, being close to the boundary between SN and non-SN as the final product.

Experimental study on the propagation characteristics of detonation wave in annular channels with convergent cross-sections

Methane (CH4) is an environmental-friendly fuel with a wide range of applications. The explosion and detonation hazards with respect to methane in tubes with different shapes should be paid in special attention. In this study, experiments were performed to investigate the propagation characteristics of detonation waves in annular channels with convergent cross-sections in stoichiometric methane-oxygen mixture at sub-atmospherical pressures (10 kPa-40 kPa). The annular channels with convergent cross-sections were assembled by inserting 0.5-m long cones into a 48-mm circular tube. Three conicities (dend/L) were employed, i.e., dend/L = 0.048, 0.072, 0.096, in which dend and L represent the bottom diameter and the length of the cone, respectively. The detonation velocity was obtained based on the time of arrival of the wave front. Smoked foils were used to register the detonation cellular evolution in the annular channels. The results show that the convergence of the channel has a great impact on the detonation velocity and cell sizes. Depending on the conicity, the detonation velocity and cellular structure exhibit different behaviors. For the smallest conicity, the continuous velocity deficit can be observed after detonation propagating into the annular tube. However, the characteristic of detonation velocity evolution shows distinct with the increase of the conicity, which is governed by the chemical scale (the detonation cell size) as well as the physical geometry (the channel width). The interaction between Mach reflection, boundary layer effect and the wall can be served as the dominating reason for the velocity behavior.

Dynamics of a Solidifying Icy Satellite Shell

AbstractOcean worlds have been identified as high‐priority astrobiology targets due to the link between life and liquid water. Young surface terrain on many icy bodies indicates that they support active geophysical cycles that may facilitate ocean‐surface transport that could provide observables for upcoming missions. Accurately interpreting spacecraft observations requires constraining the relationship between ice shell characteristics and interior dynamics. On Earth, the composition, physical characteristics, and bioburden of ocean‐derived ices are related to their formation history and parent fluid composition. In such systems, the ice–ocean interface, which exists as a multiphase mushy layer, dictates the overlying ice’s properties and evolution. Inclusion of the physics governing these boundaries is a novel strategy in modeling planetary ices and thus far has been limited to 1D approaches. Here, we present results from 2D simulations of an archetypal ice–ocean world. We track the evolution of temperature, salinity, porosity, and brine velocity within a thickening ice shell enabling us to place improved constraints on ice–ocean world properties, including the composition of planetary ice shells, the thickness and hydraulic connectivity of ice–ocean interfaces, and heterogeneous dynamics/structures in the interfacial mushy layer. We show that stable eutectic horizons are likely a common feature of ice–ocean worlds and that ocean composition plays an important role in governing the structure and dynamics of the interface, including the formation of chemical gradient‐rich regions within the mushy layer. We discuss the geophysical and astrobiological implications of our results and highlight how they can be validated by instrument‐specific measurements.

Mathematical models of supersonic and intersonic crack propagation in linear elastodynamics

This paper presents mathematical models of supersonic and intersonic crack propagation exhibiting Mach type of shock wave patterns that closely resemble the growing body of experimental and computational evidence reported in recent years. The models are developed in the form of weak discontinuous solutions of the equations of motion for isotropic linear elasticity in two dimensions. Instead of the classical second order elastodynamics equations in terms of the displacement field, equivalent first order equations in terms of the evolution of velocity and displacement gradient fields are used together with their associated jump conditions across solution discontinuities. The paper postulates supersonic and intersonic steady-state crack propagation solutions consisting of regions of constant deformation and velocity separated by pressure and shear shock waves converging at the crack tip and obtains the necessary requirements for their existence. It shows that such mathematical solutions exist for significant ranges of material properties both in plane stress and plane strain. Both mode I and mode II fracture configurations are considered. In line with the linear elasticity theory used, the solutions obtained satisfy exact energy conservation, which implies that strain energy in the unfractured material is converted in its entirety into kinetic energy as the crack propagates. This neglects dissipation phenomena both in the material and in the creation of the new crack surface. This leads to the conclusion that fast crack propagation beyond the classical limit of the Rayleigh wave speed is a phenomenon dominated by the transfer of strain energy into kinetic energy rather than by the transfer into surface energy, which is the basis of Griffiths theory.

Evolution of flow velocity from the leading edge of 2-D and 3-D submerged canopies

Abstract

Performance investigation of an original hybrid solar façade system used for HDH desalination and building natural ventilation

This work studies the flow, the thermal and the hygrometric performances of a novel solar hybrid ventilation-HDH desalination system. The proposed hybrid system as well as its operation is described in detail. The mathematical model of the flow and the heat transfer as well as the mass transfer is elaborated. The ventilation and the hygrometric performances of the proposed system are investigated within the solar façade (wall solar chimney) and the ventilated room. The effects of the principal operating conditions (solar irradiance, ambient temperature, chimney air gap dimension, room-chimney connector dimension and air initial relative humidity) on the evolution of air temperature, air velocity and air relative humidity within the chimney are investigated. The air flow and the air temperature distribution in the ventilated room are also simulated for usual operating conditions. The surface hourly water production is estimated, under moderate meteorological conditions, based on the difference of mass flux of water vapor between the inlet and the outlet of the solar façade system. All numerical results are quantified and discussed.

Numerical simulation on the nonaxisymmetry arc characteristics in narrow gap TIG welding: responses to welding parameters

Narrow gap TIG welding is a high efficiency and low-cost welding technique for heavy structures building. Due to the narrow groove’s constriction, the TIG arc characteristics are different from butt welding. Understanding the unique arc characteristics of narrow gap TIG welding is the foundation for investigating the heat and mass transfer, metallurgic process, as well as process design. This research conducted numerical simulation on the TIG arc plasma in the narrow groove. The effects of welding current and arc length on the arc characteristics are investigated. Results show that, with the welding current increasing, the global velocity magnitude of plasma rises. The evolution of axial velocity and radial velocity has different responses to the current changes. The arc pressure increases drastically, and the global temperature of arc plasma also goes up. With the arc length increasing, global axial velocity rises, but the axial velocity and its gradients decrease near the anode surface. Centripetal radial velocity near the cathode increases, while centrifugal radial velocity rises at the outside of arc plasma and drops near the central axis. The maximum arc pressure on the anode surface decreases. At the lower part of the arc, arc temperature decreases near the central axis and increases at the outside of the arc.

Response of Transitional Mixtures Retaining Memory of In-Situ Overburden Pressure Monitored Using Electromagnetic and Piezo Crystal Sensors.

The major and minor components in granular soil materials determine their properties and behavior. This study explores the transitional behavior within threshold fines fraction of soil mixtures based on the data from the literature and experiments. From the literature survey, the void ratio, shear wave velocity, compression index, and friction angle capture the transitional turning point between the low and data-adjusted high threshold fines fractions. In particular, there is a dramatic change in hydraulic conductivity below the low threshold fines fraction that highlights the critical role of small amounts of fines in the fluid flow (e.g., clogging). From an experimental study, the engineering properties of natural soil samples identified using deformation and elastic wave sensors show transitional trends within the Revised Soil Classification System framework. The evolution of compressibility and shear wave velocity indicate that either coarse, fine, or both particles are likely to contribute to large and small strain stiffnesses when the effective stress is below 400 kPa. Thereafter, both engineering properties indicate that the soil sample retains a memory of in-situ overburden pressure when the effective stress is around 400 kPa. There is a critical role of fines that are slightly higher than low threshold fines fraction on engineering properties that promote the application of Revised Soil Classification System RSCS to natural soils.