Small Flow Velocities Research Articles

On the flow-facilitated ignition in a mixture with low Lewis number

The imposed laminar flow has been shown to facilitate ignition under certain conditions in our previous work (Chen et al. 2023). Such flow-facilitated ignition (FFI) was observed only for mixtures with sufficiently large effective Lewis numbers, i.e., Le>>1. It is not clear whether FFI can also occur in a mixture with Le<1 and Le∼1. This study will investigate the occurrence of FFI in a mixture with low Lewis number. We conduct multi-dimensional simulations of forced ignition by a pair of electrodes in both static and flowing hydrogen/air mixtures. A simplified ignition model is used in simulations. The laminar flow direction is perpendicular to the pair of electrodes. We consider different values for equivalence ratio (ϕ=0.4 and 1.0), pressure (P = 0.3–1 atm), electrode gap distance (dgap=0.64–3.88 mm) and flow velocity (Uin=0–10 m/s). The simulation results show that FFI can occur in a fuel-lean (ϕ=0.4) H2/air mixture with Le≈0.43 at sub-atmospheric pressure. This indicates that large Lewis number is not a necessary condition for FFI occurring in simulations considering a simplified ignition model. Here, the favorable conditions for FFI are large critical ignition radius at sub-atmospheric pressure, sufficiently small electrode gap distance and moderate flow velocity. Moreover, the reduction in heat loss to the electrodes by the imposed flow is found to be crucial for FFI. To further understand the boundary conditions of FFI, an ignition mode regime diagram in terms of flow velocity and normalized critical ignition radius (i.e. the ratio of critical ignition radius and electrode gap distance) is proposed. The present study provides insights on the effects of laminar flow on forced ignition of premixed flames.

Thermo-mechanical vibration and stability behaviors of bi-directional FG nano-pipe conveying fluid

This work focuses on the thermo-mechanical modeling of a size-dependent pipe made of bi-directional functionally graded (FG) materials and conveying fluid through its inner wall. The material properties of this FG nano-scale pipe along radial and axial directions are assumed to be separately varying according to the power-law and exponential distributions. The governing equations of the FG nano-scale pipe are derived according to nonlocal strain gradient theory and Hamilton’s principle, and interactions between the size-dependent pipe and its inner fluid are revealed via size-dependent constitutive relation. The frequency response of the pipe and its critical flow velocity are calculated via generalized differential quadrature method. After validation of the proposed model by comparing with previous works, a detailed parametric study is conducted to fully examine the coupling effect of material distribution and various parameters including small size, aspect ratio, boundary constraint, temperature change and flow velocity on the frequency response of the FG nano-pipe. Simultaneously, the critical flow velocity of inner fluid under which the divergence phenomenon appears is also predicted and physically explained in detail. Numerical results show that bi-directional FG design is especially beneficial to improve the stability of FG nano-pipe, which can provide a guide to design advanced micro/nanofluidic devices for bio-engineering applications.

Non-ideal MHD growth of current interchange tearing modes at plasma edge and response to externally-imposed flow

Current interchange tearing modes (CITMs) have been proposed to explain an intermittent eruption of electric current toward a divertor at an edge region of a tokamak device. Extended magnetohydrodynamic (MHD) simulations combined with the Scrape-Off-Layer diffusion model, which has been developed by the authors [Miura et al., Phys. Plasmas 24, 092111 (2017)], are carried out in order to study the growth of a CITM under influences of two-fluid and gyro-viscous effects, or non-ideal MHD effects. Numerical simulations both with and without an externally imposed azimuthal flow show that the growth of a CITM is observed for a relatively small flow velocity, and that a CITM can be a candidate mechanism for the intermittent eruption of the current in a tokamak, whether the growth is under influences of non-ideal MHD effects or not. The growth can be suppressed when an externally imposed azimuthal flow is sufficiently large to cause a finite radial displacement because such a displacement prevents a transition from an interchange to a tearing mode that is the key process of a CITM. Furthermore, it is also found that a stripe pattern similar to so-called streamers is formed in the course of the CITM growth. The basic nature of this streamer-like structure is also presented.

Velocity field estimation in transcranial small vessel using super-resolution ultrasound imaging velocimetry

Based on the diameter and position information of small vessels obtained by transcranial super-resolution imaging using 3 MHz low-frequency chirp plane waves, a Gaussian-like non-linear compression was adopted to compress the blood flow signals in spatiotemporal filtering (STF) data to a precise region, and then estimate the blood flow velocity field inside the region over the adjacent time intervals using ultrasound imaging velocimetry (UIV). Imaging parameters, such as the mechanical index (MI), frame rate, and microbubble (MB) concentration, are critical during the estimation of velocity fields over a short time at high MB contrast agent concentrations. These were optimized through experiments and algorithms, in which dividing the connected domain was proposed to calculate MB cluster spot centroid spacing (SCS) and the spot-to-flow area ratio (SFAR) to determine the suitable MB concentration. The results of the in vitro experiments showed that the estimation of the small vessel flow velocity field was consistent with the theoretical results; the velocity field resolution for vessels with diameters of 0.5 mm and 0.3 mm was 36 μm and 21 μm, and the error between the mean velocity and the theoretical value was 0.7 % and 0.67 %, respectively.

Vortex-induced vibrations of a top tensioned riser subjected to flows with spanwise varying directions

Vortex-induced vibrations (VIVs) of a top tensioned riser (TTR) subjected to flows with spanwise varying directions have been simulated and studied in this paper. The riser is simplified as an Euler–Bernoulli beam and its dynamics is described using the finite element method with the Absolute Nodal Coordinate Formulation (ANCF). The hydrodynamic forces due to vortex shedding are modelled with wake oscillators attached to nodes along the riser. The model has been firstly used to simulate an experiment and acceptable model results have been obtained in comparison with the measurements. Then, the VIVs of the TTR under two flow conditions, one with uniform and the other with linearly sheared velocity, have been simulated. In both cases, the direction of the flow varies linearly along the riser. Simulation results have shown different response characteristics at small and high flow velocities with respect to the frequency, amplitude and vibrating direction of the riser. In general, at a small flow velocity where the response of the riser is dominated by the standing wave, the variation in the flow direction has a major influence on the VIV of the riser. However, when the travelling wave is predominant at a high flow velocity, the effect of the spanwise varying flow direction is insignificant. The energy transfer between the structure and flow has been studied to reveal the possible mechanism that leads to these differences in the responses of the riser at small and high flow velocities. The results of the present paper suggest that the engineering simplification of ignoring the influence of flow direction variation on VIV may only be applicable at high flow velocities when the response of the riser is dominated by the travelling wave.

Effect of molten pool behaviors on welding defects in tandem NG-GMAW based on CFD simulation

The simulation of tandem narrow-gap GMAW based on computational fluid dynamics (CFD) was carried out to establish the relationship between the molten pool behaviors and weld bead formation under different wire feed rates (vf) and welding speed (vw). Inclined arcs were considered in the model by a novel rotated coordinate system. The variation of effective arc radius with welding current was also taken into consideration in the model. The experimental results illustrate that small vfor large vw easily leads to lack-of-fusion (LOF) defect. However, large vf also causes finger penetration. The simulation results indicate that a clockwise vortex in the right sidewall and an anticlockwise vertex in the left sidewall enhance the transverse transmission of fluid and heat, which is conducive to forming a concave weld bead. When vf is small or vw is large, due to low arc heat input, the groove sidewall cannot be melted and the transverse flow in the molten pool is weak. Furthermore, owing to the small flow velocity and large solidification rate of molten metal, the preferentially solidified metal further hinders the spreading of liquid metal to the sidewalls. As a result of high vf, the droplet that has high transition frequency and velocity transfers the arc heat and the high momentum to the molten pool bottom. The downward flow in the molten pool center is enhanced and its velocity is also increased, resulting in finger penetration.

Nonlinear evolution equation for modeling waves at the boundary of a stratified flow of viscous liquids in an inclined channel

The dynamics of perturbations of the interface of a two-layer Poiseuille flow in a flat closed inclined channel is studied. The velocity profiles of wave motion are analytically found neglecting dissipation, dispersion and pumping of perturbations. On the basis of the found solution, a nonlinear evolution integro-differential equation for plane moderately long perturbations of the interface of the liquids is derived. The coefficients of the equation are represented by integrals over the layer thicknesses from functions depending on the stationary flow and perturbation profiles. The equation takes into account viscous dissipation: one of the integrals in this equation corresponds to dissipation in lion-stationary boundary layers, and the other corresponds to the transfer of energy from the flow to the wave. For the case of small flow velocities, the coefficients of the equation are analytically calculated. The equation has also been generalized to the quasi-two-dimensional case when the gradients along the transversal coordinate are small.

Geometrically exact dynamics of cantilevered pipes conveying fluid

In this paper, the global dynamics of a hanging fluid conveying cantilevered pipe with a concentrated mass attached at the free end is investigated. The problem is of interest not only for engineering applications, but also because it displays interesting and often surprising dynamical behaviour. The widely used nonlinear models based on the transverse motion of the pipe are not able to accurately capture the dynamical behaviour of the system at very high flow velocities. Thus, a high-dimensional geometrically-exact model is developed for the first time, utilising Hamilton’s principle together with the Galerkin modal decomposition technique. Extensive numerical simulations are conducted to investigate the influence of key system parameters. It is shown that at sufficiently high flow velocities past the first instability (Hopf bifurcation), the system undergoes multiple bifurcations with extremely large oscillation amplitudes and rotations, beyond the validity of third-order nonlinear models proposed to-date. In the presence of an additional tip mass, quasi-periodic and chaotic motions are observed; additionally, it is shown that for such cases an exact model is absolutely essential for capturing the pipe dynamics even at relatively small flow velocities beyond the first instability.

Study on analytical solution model of heat transfer of ground heat exchanger in the protection engineering structure

In order to eliminate the disadvantages of the high cost of underground pipe drilling and space occupation, according to the relevant practices of existing energy underground structures, the idea of embedding the buried pipe of the ground source heat pump system into the tunnel lining of protection engineering was proposed. An analytical model of the heat transfer process of the ground heat exchanger was established, and the explicit expressions of the calculated parameters were obtained. The reliability of the model was verified by the existing experimental data. The model was used to analyze the influence of different air temperature and flow rate on the heat transfer performance of the ground heat exchanger. The study found that every time the air temperature rises by 1 °C, the heat transfer per unit passageway length was reduced by about 200W. The increase in air flow velocity has a limited impact on the heat transfer. The method of enhancing heat transfers by increasing the flow velocity can only be performed at a small flow velocity. In the total heat exchange per unit length, the proportion of the heat exchange on the air side and the heat exchange on the surrounding rock side affect the protection capability of the project in an isolated state. The greater the proportion of heat exchange on the side of the surrounding rock, the stronger the protection capability in the isolated state.

Moments of the Boltzmann collision operator for Coulomb interactions

Exact moments of the Boltzmann collision operator are calculated in the irreducible Hermitian moment expansion written in terms of the random-velocity variable of each species. The formulas are presented in closed, algebraic form and can be straightforwardly implemented in computer algebra systems. They are valid for two arbitrary masses, temperatures, and flow velocities, and hence include all other existing results derived for distribution functions expanded with respect to reference states of one temperature and flow velocity. In comparison, the Landau collisional moments are good approximations for large Coulomb logarithm and small relative flow velocity, but they fail to predict the correct behavior of most collisional moments for large relative flow even for weakly coupled plasmas.

Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics

AbstractUnderstanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand‐dominated layers (SDLs) and mud‐dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.

On the applicability of viscosity-based capillary bundle concepts to predict the penetration behaviour of polymer solutions into sand

The increasing use of polymer solutions as support fluids in pile drilling, diaphragm walling or tunnelling applications demands a more detailed discussion of their penetration behaviour and prediction thereof. In this context, the capillary bundle approach can be a useful tool. However, while it is widely discussed in the oil and gas application, the subject is currently addressed only scarcely with regard to support fluid penetration targeting stagnation, where small flow velocities and non-cohesive soil environments are of relevance. In these boundary conditions, the applicability of capillary bundle approaches is not yet sufficiently confirmed and substantiated. The current paper thus reviews current capillary bundle models based on Hagen–Poiseuille in combination with a power-law rheological model and discusses their applicability with respect to support fluid application in the context of experimental soil permeation tests for small gradients (ile 10). Two granular materials of similar grain size, but different angularity (glass beads and sand), and four polymer solutions varying in polymer chain length and concentration are investigated, and the impact of model assumptions and bulk material input variables is systematically discussed. The experimental results show that the theoretical models are generally able to predict the filter velocity qualitatively for values above {bar{v}}=5times 10^{-7} m/s and also quantitatively, when an empirical shift factor alpha ^* is introduced and water permeability values are determined experimentally. With respect to the influence of soil parameters, it was found that the soil particle roughness decreases the flow velocity of the polymer solution despite similar hydraulic conductivity in water. Polymer chain length and concentration were observed to control the degree of possible dilution (alpha ^*<1) in the porous system compared to bulk rheological characteristics. It can therefore be concluded that capillary bundle models can indeed be applied in the targeted fields even though they are unable to predict a complete stagnation for i>0. However, rather than specific model assumptions for tortuosity, taking into account the specific soil-polymer interaction has shown to be of primary importance to ensure no under- or over-prediction of penetration velocities solely based on bulk rheology.

Extended Lattice Boltzmann Model.

Conventional lattice Boltzmann models for the simulation of fluid dynamics are restricted by an error in the stress tensor that is negligible only for small flow velocity and at a singular value of the temperature. To that end, we propose a unified formulation that restores Galilean invariance and the isotropy of the stress tensor by introducing an extended equilibrium. This modification extends lattice Boltzmann models to simulations with higher values of the flow velocity and can be used at temperatures that are higher than the lattice reference temperature, which enhances computational efficiency by decreasing the number of required time steps. Furthermore, the extended model also remains valid for stretched lattices, which are useful when flow gradients are predominant in one direction. The model is validated by simulations of two- and three-dimensional benchmark problems, including the double shear layer flow, the decay of homogeneous isotropic turbulence, the laminar boundary layer over a flat plate and the turbulent channel flow.

Simulation of flow around a permeable dike using physical and 3D-CFD models

ABSTRACT In this research, complementary numerical and experimental physical models were conducted to investigate flow parameters around an isolated permeable spur dike using ANSYS FLUENT ver. 16.1 software. The volume of fluid method VOF with the help of CFD technique and the turbulence model was used to simulate the complex free-surface flow by applying a geometric reconstruction scheme. Two different mediums with different porosities were used, namely, gravel and glass beads. It was found that, for low flow rate values, there are no significant changes in the water profiles in both longitudinal and lateral directions for both media. Although for high flow rate values, the differences tend to be clearer. Results showed that there is a slight effect of the medium's properties if the dike width is less than 1/3 of width of the channel, in particular with low flow rate values. Good agreement was observed between the computational and the measured results for the water surface profiles. Also, there is an exact match between the lateral velocity distribution of gravels and glass beads. Downstream the dike, the lateral velocity profiles exhibit an S shape curve due to high velocity in the free flow and small flow velocity behind the porous dike. The results show good agreement between the CFD results and the experimental observations of the water surface profiles and velocity distribution according to the statistical analysis using some standard indexes error.

Pollutants removal efficiency assessment of constructed subsurface flow wetlands in lakes with numerical models

Ecological remediation with subsurface flow wetland is one of the most widely used methods for pollution abatement. Assessment on its implementation efficiency is necessary but difficult. To evaluate the pollutant removal effect of constructed subsurface flow wetlands, a wetland growth model coupling with a hydrodynamic water quality model has been put forward. The coupled model has been used to assess the impact of constructed subsurface flow wetlands on a polluted lake in Hubei Province, China. Four pieces of constructed subsurface flow wetlands composed of Reed and Smooth cordgrass with total area of 38.9 thousand square meters for pollutant removal have been designed for the lake. Results showed that reductions of chemical oxygen demand (COD), total phosphorus (TP) and total nitrogen (TN) were 246.38, 28.14 and 3.29 tons/year, respectively. Compared with the different pollutants, the removal efficiency of subsurface flow wetlands on TN (49%) is bigger than TP (34%), but less than COD (60%). Furthermore, the higher water temperature, smaller flow velocity and bigger water depth will improve the pollutants removal performance of subsurface flow wetlands.

Doppler profiles of the interacting contact binary W Corvi

ABSTRACT We use spectra from 2011 and 2012 to investigate the distribution of local effective temperature and non-orbital velocity over the surface of the common envelope of this peculiar contact binary. There seems to be a hot surface flow from the more massive to the less massive (secondary) component, possibly equatorial, which extends roughly one quarter-way around both sides of that secondary star, corresponding to the hotspot postulated to explain the star’s light-curve peculiarities. This feature is clear in the shape of the profiles of metallic lines, but it shows up in H α/H β profiles, as well. The profiles imply small flow velocities in contrast to those detected in some A-type W UMa systems, less than a few km s−1 for the primary but indeterminate for the secondary. We also classify the star’s spectrum (G1-2 V) and present more radial velocities confirming the Ruciński–Lu radial-velocity solution.

Blood Flow Imaging with Ultrafast Doppler.

The pulsed-Doppler effect is the main technique used in clinical echography to assess blood flow. Applied with conventional focused ultrasound Doppler modes, it has several limits. Firstly, a finely tuned signal filtering operation is needed to distinguish blood flows from surrounding moving tissues. Secondly, the operator must choose between localizing the blood flows or quantifying them. In the last two decades, ultrasound imaging has undergone a paradigm shift with the emergence of ultrafast ultrasound using unfocused waves. In addition to a hundredfold increase in framerate (up to 10000 Hz), this new technique also breaks the conventional quantification/localization trade-off, offering a complete blood flow mapping of the field of view and a simultaneous access to fine velocities measurements at the single-pixel level (down to 50 µm). This data continuity in both spatial and temporal dimensions strongly improves the tissue/blood filtering process, which results in an increase sensitivity to small blood flow velocities (down to 1 mm/s). In this method paper, we aim to introduce the concept of ultrafast Doppler as well as its main parameters. Firstly, we summarize the physical principles of unfocused wave imaging. Then, we present the Doppler signal processing main steps. Particularly, we explain the practical implementation of the critical tissue/blood flow separation algorithms and on the extraction of velocities from these filtered data. This theoretical description is supplemented by in vitro experiences. A tissue phantom embedding a canal with flowing blood-mimicking fluid is imaged with a research programmable ultrasound system. A blood flow image is obtained and the flow characteristics are displayed for several pixels in the canal. Finally, a review of in vivo applications is proposed, showing examples in several organs such as carotids, kidney, thyroid, brain and heart.

Investigation for Synergies of Ionic Strength and Flow Velocity on Colloidal-Sized Microplastic Transport and Deposition in Porous Media Using the Colloidal-AFM Probe.

Studies that explore the transport and retention behavior of colloidal-sized microplastic (MP) with focusing on the governing mechanisms for their attachment and detachment process using colloidal-atomic force microscopy (C-AFM) were still limited. In the present study, multiscale investigations ranging from pore-scale column test to microscale visualization and eventually to nanoscale interfacial and adhesive force measurement were conducted. Pore- and microscale tests were conducted at various flow velocity and over a broad range of IS values and found that IS and flow velocity could synergically impact the deposition of MPs during filtration, in particular under unfavorable condition at small flow velocity. The net difference between the highest and lowest deposition conditions became smaller while flow velocity was decreasing in porous media. However, the net difference between the high and low IS conditions in parallel plate chamber were not sensitive to the change of flow velocity. The measurement from C-AFM suggested that not only the interfacial force but also the adhesive forces changed while MP was approaching/retracting to the collector surface. Information related to the magnitude, location, and occurrence of interfacial/adhesive forces were analyzed. Comparisons of the interaction energy determined from the measured force and ones derived from surface energy components using DLVO theory were conducted to explain the synergies of IS and flow velocity on pathogenic size MPs transport and deposition during filtration.

Sediment consolidation in ephemeral river: the effect of applied loading on soil properties and dredging method selection

ABSTRACT Bengawan Solo River, a typical large ephemeral river in Indonesia, is facing stream problems induced by the erosion and deposition of sediment. Imbalance between these two phenomena on the riverbed can lead to excessive sedimentation, increased risk of flooding, and formation of sandbars. One proposed solution to address this problem is dredging. However, this process can have potential negative effects on the environment, such as removal or destruction of the biota in the dredged materials, increased turbidity, as well as coverage of the benthos in the vicinity. Also, different dredging methods are needed to suit sediments with different properties. Therefore, the knowledge of the deposited sediment properties is required so that appropriate methods can be selected. The sediment characteristics were obtained by performing laboratory tests on physical and consolidation properties and shear strength tests of a soil sample. The results indicate that the materials were broadly grouped into sandy sediment in downstream areas and clayey sediment in estuary areas. The distribution of sediment properties varies with depth because of subsequent erosion and deposition over time. Additionally, the distribution also shows that sand content decreases as the origin of sample approaches estuary areas. Erosion is predominant at locations with small hydraulic radius and high flow velocity, mostly at downstream areas, whereas deposition is predominant at locations with a large hydraulic radius and low flow velocity, specifically at estuary areas. The shear strength increases as the loads imposed by the self-weight of sediment layer increase. The analysis suggests that sediments cannot be eroded naturally as they have a shear strength that exceeds the small-capacity dredging method (); therefore, the removal of excess sediment in such areas requires dredging methods of larger capacities (such as grab dredger).

Free-molecular and near-free-molecular gas flows over backward facing steps

We consider the two-dimensional steady channel flow of a rarefied gas over a backward facing step in the limit of large Knudsen numbers. The free-molecular problem is solved analytically for both diffuse and specular-reflecting channel boundaries, and the solutions are validated through comparison with direct simulation Monte Carlo calculations. Prescribing the density and temperature differences between the inlet and outlet external equilibrium conditions, the results for the density- and temperature-drop-driven flows are analysed and contrasted, revealing higher flow velocities and mass flow rates in the former. While the flow rate is unaffected by the step geometry in the specular case, it increases with the step size in the diffuse-reflecting set-up. At conditions where small flow velocities occur, flow detachment is observed in the form of streamlines connecting the step edge stagnation points. Considering the problem at finite Knudsen numbers, the collisionless-flow regime breaks down at higher Knudsen numbers for lower gas speed flows, followed by the occurrence of step flow separation and recirculation.