Journal articles on the topic 'Flow gradients'
To see the other types of publications on this topic, follow the link: Flow gradients.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 journal articles for your research on the topic 'Flow gradients.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse journal articles on a wide variety of disciplines and organise your bibliography correctly.
1
B N, Shobha, Govind R. Kadambi, S. R. Shankapal, and Yuri Vershinim. "Effect of variation in colour gradient information for optic flow computations." International Journal of Engineering & Technology 3, no. 4 (September 17, 2014): 445. http://dx.doi.org/10.14419/ijet.v3i4.2722.
Abstract:
Optic flow algorithms provide mapping of 3D velocities on 2D image space. Optic flow is computed on the pair of images which are in sequence and is normally gray scale images. Optic flow computation using Horn and Schunck assumes that brightness consistency is maintained. Colour optic flow has the advantage that optic flow vectors are obtained even when there is a variation of brightness in the input images. The use of colour bands for optic flow is investigated by considering gradients of colour bands and component gradients. Results of applying these two types of gradients to three colour models are presented and analyzed. Decision logic is proposed to select the best colour model for colour optic flow computation based on gradient analysis. Keywords: Activity Measure. Colour Bands, Component Gradients, Decision Logic, Optic Flow Computation.
2
Xu, Wenrui, and James M. Stone. "Bondi–Hoyle–Lyttleton accretion in supergiant X-ray binaries: stability and disc formation." Monthly Notices of the Royal Astronomical Society 488, no. 4 (July 25, 2019): 5162–84. http://dx.doi.org/10.1093/mnras/stz2002.
Abstract:
Abstract We use 2D (axisymmetric) and 3D hydrodynamic simulations to study Bondi–Hoyle–Lyttleton accretion with and without transverse upstream gradients. We mainly focus on the regime of high (upstream) Mach number, weak upstream gradients, and small accretor size, which is relevant to neutron star accretion in wind-fed supergiant X-ray binaries (SgXBs). We present a systematic exploration of the flow in this regime. When there are no upstream gradients, the flow is always stable regardless of accretor size or Mach number. For finite upstream gradients, there are three main types of behaviour: stable flow (small upstream gradient), turbulent unstable flow without a disc (intermediate upstream gradient), and turbulent flow with a disc-like structure (relatively large upstream gradient). When the accretion flow is turbulent, the accretion rate decreases non-convergently as the accretor size decreases. The flow is more prone to instability and the disc is less likely to form than previously expected; the parameters of most observed SgXBs place them in the regime of a turbulent, disc-less accretion flow. Among the SgXBs with relatively well-determined parameters, we find OAO 1657−415 to be the only one that is likely to host a persistent disc (or disc-like structure); this finding is consistent with observations.
3
Alicia, Toh G. G., Chun Yang, Zhiping Wang, and Nam-Trung Nguyen. "Combinational concentration gradient confinement through stagnation flow." Lab on a Chip 16, no. 2 (2016): 368–76. http://dx.doi.org/10.1039/c5lc01137j.
4
Herbelin, Armando, and Jaromir Ruzicka. "Pulse Modulation - A Novel Approach to Gradient-Based Flow Injection Techniques." Collection of Czechoslovak Chemical Communications 66, no. 8 (2001): 1219–37. http://dx.doi.org/10.1135/cccc20011219.
Abstract:
Development of a novel system for generation of gradients in flow injection analysis by pulse modulation is described. These user-selectable gradients are created by computer-controlled mixing of two solutions with a total volume as low as 75 μl and can be delivered under incremental or continuous flow conditions. Applications such as automated, single-standard instrument calibration are expected to benefit from high-precision linear gradients (r2 = 0.99989, n = 55). Gradient methods in biochemisty and immunology such as kinetic measurement of biomolecular interactions will benefit from the small volume of these gradients, especially for analytes with limited availability.
5
Wright, Stephen P., Alexander R. Opotowsky, Tayler A. Buchan, Sam Esfandiari, John T. Granton, Jack M. Goodman, and Susanna Mak. "Flow-related right ventricular to pulmonary arterial pressure gradients during exercise." Cardiovascular Research 115, no. 1 (June 6, 2018): 222–29. http://dx.doi.org/10.1093/cvr/cvy138.
Abstract:
Abstract Aims The assumption of equivalence between right ventricular (RV) and pulmonary arterial systolic pressure is fundamental to several assessments of RV or pulmonary vascular haemodynamic function. Our aims were to (i) determine whether systolic pressure gradients develop across the RV outflow tract in healthy adults during exercise, (ii) examine the potential correlates of such gradients, and (iii) consider the effect of such gradients on calculated indices of RV function. Methods and results Healthy untrained and endurance-trained adult volunteers were studied using right-heart catheterization at rest and during submaximal cycle ergometry. RV and pulmonary artery (PA) pressures were simultaneously transduced, and the cardiac output was determined by thermodilution. Systolic pressures, peak and mean gradients, and indices of chamber, vascular, and valve function were analysed offline. Summary data are reported as mean ± standard deviation or median (interquartile range). No significant RV outflow tract gradients were observed at rest [mean gradient = 4 (3–5) mmHg], and the calculated effective orifice area was 3.6 ± 1.0 cm2. The increase in right ventricular systolic pressure during exercise was greater than the PA systolic pressure. Accordingly, mean gradients were developed during light exercise [8 (7–9) mmHg] and increased during moderate exercise [12 (9–14) mmHg, P < 0.001]. The magnitude of the mean gradient was linearly related to the cardiac output (r2 = 0.70, P < 0.001). Conclusions In healthy adults without pulmonic stenosis, systolic pressure gradients develop during exercise, and the magnitude is related to the blood flow rate.
6
Dai, Bo, Yan Long, Jiandong Wu, Shaoqi Huang, Yuan Zhao, Lulu Zheng, Chunxian Tao, et al. "Generation of flow and droplets with an ultra-long-range linear concentration gradient." Lab on a Chip 21, no. 22 (2021): 4390–400. http://dx.doi.org/10.1039/d1lc00749a.
Abstract:
A concentration gradient generator is demonstrated to generate ultra-long linear gradients, periodic gradients and droplets with various concentrations and produce drugs with different concentrations for drug screening applications.
7
Chittur K, Subramaniam, Aishwarya Chandran, Ashwini Khandelwal, and Sivakumar A. "Energy Conversion using electrolytic concentration gradients." MRS Proceedings 1774 (2015): 51–62. http://dx.doi.org/10.1557/opl.2015.758.
Abstract:
ABSTRACTSalinity gradient is an enormous source of clean energy. A process for potential generation from an ionic concentration gradient produced in single and multicell assembly is presented. The ionic gradient is created using a fuel cell type cell with a micro-porous ion exchange membrane, both anionic (AEM) and cationic (CEM). Various salinity gradients, Salt : Fresh, from 100 : 0 to 16000 : 0 was established using NaCl solution, in the electrode chambers. A potential of 20 mV/cm to 25 mV/cm can be realized at ambient temperatures and pressures for a bipolar AEM/CEM cell. The performance was optimized for various static and dynamic flow rates of the saline and fresh water. The cell performance can further be optimized for Membrane Electrode System (MES) morphology. A multicell unit was assembled and the results presented for various conditions like concentration gradients, flow rates and pressure. The thermodynamic and electrical efficiency needs to be evaluated for various gradients and flow rates. The relation with number of valance electrons/ ion and the potential generated changes for various dynamic condition of salinity. The higher the salinity gradient the larger is the potential generated. This is limited by the membrane characteristics. There exists a monotonic relation between the number of valence electron/ion/unit time and the potential generated up to about 16000 concentration. The membrane characteristics have been studied for optimal ion crossover for various gradients and flow. The graph between ln (gradient) versus Voltage provides insights into this process. This presents a very cost effective and clean process of energy conversion.
8
Williams, Ian, Sangyoon Lee, Azzurra Apriceno, Richard P. Sear, and Giuseppe Battaglia. "Diffusioosmotic and convective flows induced by a nonelectrolyte concentration gradient." Proceedings of the National Academy of Sciences 117, no. 41 (September 28, 2020): 25263–71. http://dx.doi.org/10.1073/pnas.2009072117.
Abstract:
Glucose is an important energy source in our bodies, and its consumption results in gradients over length scales ranging from the subcellular to entire organs. Concentration gradients can drive material transport through both diffusioosmosis and convection. Convection arises because concentration gradients are mass density gradients. Diffusioosmosis is fluid flow induced by the interaction between a solute and a solid surface. A concentration gradient parallel to a surface creates an osmotic pressure gradient near the surface, resulting in flow. Diffusioosmosis is well understood for electrolyte solutes, but is more poorly characterized for nonelectrolytes such as glucose. We measure fluid flow in glucose gradients formed in a millimeter-long thin channel and find that increasing the gradient causes a crossover from diffusioosmosis-dominated to convection-dominated flow. We cannot explain this with established theories of these phenomena which predict that both scale linearly. In our system, the convection speed is linear in the gradient, but the diffusioosmotic speed has a much weaker concentration dependence and is large even for dilute solutions. We develop existing models and show that a strong surface–solute interaction, a heterogeneous surface, and accounting for a concentration-dependent solution viscosity can explain our data. This demonstrates how sensitive nonelectrolyte diffusioosmosis is to surface and solution properties and to surface–solute interactions. A comprehensive understanding of this sensitivity is required to understand transport in biological systems on length scales from micrometers to millimeters where surfaces are invariably complex and heterogeneous.
9
Dixon, D. A., J. Graham, and M. N. Gray. "Hydraulic conductivity of clays in confined tests under low hydraulic gradients." Canadian Geotechnical Journal 36, no. 5 (November 23, 1999): 815–25. http://dx.doi.org/10.1139/t99-057.
Abstract:
Clay barriers normally function at hydraulic gradients much lower than are commonly used in laboratory tests for hydraulic conductivity. This paper describes low-gradient tests on compacted illite, Na-bentonite, and sand-bentonite specimens at a range of dry densities. The tests examined the effects of deionized or saline pore fluid, the initial degree of saturation at the time of compaction, and back-pressuring to achieve saturation during permeation. No "critical" or "threshold" gradients were observed. In low-density materials, "transitional" gradients defined two separate regions of Darcian flow. Low flow rates and low hydraulic conductivities were associated with hydraulic gradients below the transitional gradient. Higher conductivities were associated with gradients above the transitional gradient. Hydraulic conductivities decreased with increasing initial density. They appeared independent of initial degree of saturation or back-pressuring. Tests on Na-rich bentonite showed that deionized and saline permeants produced similar hydraulic conductivities. Measured hydraulic conductivities were in reasonable agreement with values computed using the Poiseuille and Kozeny-Carman equations. Key words: hydraulic conductivity, gradient, Darcian, illite, bentonite.
10
Cardin, Velia, and Andrew T. Smith. "Sensitivity of human visual cortical area V6 to stereoscopic depth gradients associated with self-motion." Journal of Neurophysiology 106, no. 3 (September 2011): 1240–49. http://dx.doi.org/10.1152/jn.01120.2010.
Abstract:
The principal visual cue to self-motion (egomotion) is optic flow, which is specified in terms of local 2D velocities in the retinal image without reference to depth cues. However, in general, points near the center of expansion of natural flow fields are distant, whereas those in the periphery are closer, creating gradients of horizontal binocular disparity. To assess whether the brain combines disparity gradients with optic flow when encoding egomotion, stereoscopic gradients were applied to expanding dot patterns presented to observers during functional MRI scanning. The gradients were radially symmetrical, disparity changing as a function of eccentricity. The depth cues were either consistent with egomotion (peripheral dots perceived as near and central dots perceived as far) or inconsistent (the reverse gradient, central dots near, peripheral dots far). The BOLD activity generated by these stimuli was compared in a range of predefined visual regions in 13 participants with good stereoacuity. Visual area V6, in the parieto-occipital sulcus, showed a unique pattern of results, responding well to all optic flow patterns but much more strongly when they were paired with consistent rather than inconsistent or zero-disparity gradients. Of the other areas examined, a region of the precuneus and parietoinsular vestibular cortex also differentiate between consistent and inconsistent gradients, but with weak or suppressive responses. V3A, V7, MT, and ventral intraparietal area responded more strongly in the presence of a depth gradient but were indifferent to its depth-flow congruence. The results suggest that depth and flow cues are integrated in V6 to improve estimation of egomotion.
11
FARRELL, M. D., and P. E. RICHES. "IONIC OSMOTIC EFFECTS INCREASE FLUID FLOW DURING PERMEATION TESTS." Journal of Mechanics in Medicine and Biology 12, no. 04 (September 2012): 1250063. http://dx.doi.org/10.1142/s0219519412004995.
Abstract:
Fluid flow is essential for the transport of metabolites to, from and within the intervertebral disc (IVD). By applying quadriphasic mixture theory experimentally, this study relates fluid flow within the bovine nucleus pulposus (NP) to the applied fluid pressure gradients and fixed charge and mobile ion concentration gradients. 24 plugs of NP tissue (diameter 10 mm, height 1020 ± 122 μm (mean ± SD)), orientated in the axial direction, were harvested from bovine tail discs. The plugs were permeated with either; 0 M NaCl , 0.15 M NaCl or 3 M NaCl solutions by subjecting them to 30, 45 and 60 kPa fluid pressure gradients applied in a random sequence. The hypertonic solution was assumed to render all non-fluid pressure gradients negligible, whilst the hypotonic solution was assumed to render the mobile ion concentration gradient negligible. The effects of these gradients on fluid flow were expressed as a percentage of the applied fluid pressure. Fluid velocity was significantly increased through the tissue in the isotonic case compared to the hypertonic case by up to 55% of the applied fluid pressure. The fixed charges accounted for between 26% and 43% and the mobile ion gradient responsible for 12–26% of this increased fluid flow. These results highlight the importance of using a constitutive equation for permeability that includes mobile ions and fixed charges as separate phases when modeling cartilaginous tissue in order to better describe fluid flow, and thus convective transfer of metabolites, within the tissue.
12
Stine, Caleb A., and Jennifer M. Munson. "Autologous Gradient Formation under Differential Interstitial Fluid Flow Environments." Biophysica 2, no. 1 (January 4, 2022): 16–33. http://dx.doi.org/10.3390/biophysica2010003.
Abstract:
Fluid flow and chemokine gradients play a large part in not only regulating homeostatic processes in the brain, but also in pathologic conditions by directing cell migration. Tumor cells in particular are superior at invading into the brain resulting in tumor recurrence. One mechanism that governs cellular invasion is autologous chemotaxis, whereby pericellular chemokine gradients form due to interstitial fluid flow (IFF) leading cells to migrate up the gradient. Glioma cells have been shown to specifically use CXCL12 to increase their invasion under heightened interstitial flow. Computational modeling of this gradient offers better insight into the extent of its development around single cells, yet very few conditions have been modelled. In this paper, a computational model is developed to investigate how a CXCL12 gradient may form around a tumor cell and what conditions are necessary to affect its formation. Through finite element analysis using COMSOL and coupled convection-diffusion/mass transport equations, we show that velocity (IFF magnitude) has the largest parametric effect on gradient formation, multidirectional fluid flow causes gradient formation in the direction of the resultant which is governed by IFF magnitude, common treatments and flow patterns have a spatiotemporal effect on pericellular gradients, exogenous background concentrations can abrogate the autologous effect depending on how close the cell is to the source, that there is a minimum distance away from the tumor border required for a single cell to establish an autologous gradient, and finally that the development of a gradient formation is highly dependent on specific cell morphology.
13
Grobler, Lindi, Ryno Laubscher, Johan van der Merwe, and Philip G. Herbst. "Evaluation of Aortic Valve Pressure Gradients for Increasing Severities of Rheumatic and Calcific Stenosis Using Empirical and Numerical Approaches." Mathematical and Computational Applications 29, no. 3 (April 28, 2024): 33. http://dx.doi.org/10.3390/mca29030033.
Abstract:
The evaluation and accurate diagnosis of the type and severity of aortic stenosis relies on the precision of medical imaging technology and clinical correlations and the expertise of medical professionals. The application of the clinical correlation to different aortic stenosis morphologies and severities is investigated. The manner in which numerical techniques can be used to simulate the blood flow through pathological aortic valves was analysed and compared to the ground-truth CFD model. Larger pressure gradients are estimated in all severities of rheumatic aortic valves compared to calcific aortic valves. The zero-dimensional morphology-insensitive model underpredicted the transvalvular pressure gradient with the greatest error. The 1D model underestimated the pressure gradient in rheumatic cases and overestimated the pressure gradient in calcific cases. The pressure gradients estimated by the clinical approach depends on the location of the flow vena contracta and is sensitive to the severity and type of valve lesion. Through the analysis of entropy generation within the flow domain, the dominant parameters and regions driving adverse pressure gradients were identified. It is concluded that sudden expansion is the dominant parameter leading to higher pressure gradients in rheumatic heart valves compared to calcific ones.
14
Sonmez, Utku M., Adam Wood, Kyle Justus, Weijian Jiang, Fatima Syed-Picard, Philip R. LeDuc, Pawel Kalinski, and Lance A. Davidson. "Chemotactic Responses of Jurkat Cells in Microfluidic Flow-Free Gradient Chambers." Micromachines 11, no. 4 (April 4, 2020): 384. http://dx.doi.org/10.3390/mi11040384.
Abstract:
Gradients of soluble molecules coordinate cellular communication in a diverse range of multicellular systems. Chemokine-driven chemotaxis is a key orchestrator of cell movement during organ development, immune response and cancer progression. Chemotaxis assays capable of examining cell responses to different chemokines in the context of various extracellular matrices will be crucial to characterize directed cell motion in conditions which mimic whole tissue conditions. Here, a microfluidic device which can generate different chemokine patterns in flow-free gradient chambers while controlling surface extracellular matrix (ECM) to study chemotaxis either at the population level or at the single cell level with high resolution imaging is presented. The device is produced by combining additive manufacturing (AM) and soft lithography. Generation of concentration gradients in the device were simulated and experimentally validated. Then, stable gradients were applied to modulate chemotaxis and chemokinetic response of Jurkat cells as a model for T lymphocyte motility. Live imaging of the gradient chambers allowed to track and quantify Jurkat cell migration patterns. Using this system, it has been found that the strength of the chemotactic response of Jurkat cells to CXCL12 gradient was reduced by increasing surface fibronectin in a dose-dependent manner. The chemotaxis of the Jurkat cells was also found to be governed not only by the CXCL12 gradient but also by the average CXCL12 concentration. Distinct migratory behaviors in response to chemokine gradients in different contexts may be physiologically relevant for shaping the host immune response and may serve to optimize the targeting and accumulation of immune cells to the inflammation site. Our approach demonstrates the feasibility of using a flow-free gradient chamber for evaluating cross-regulation of cell motility by multiple factors in different biologic processes.
15
Linge, Svein O., Kent-A. Mardal, Anders Helgeland, John D. Heiss, and Victor Haughton. "Effect of craniovertebral decompression on CSF dynamics in Chiari malformation Type I studied with computational fluid dynamics." Journal of Neurosurgery: Spine 21, no. 4 (October 2014): 559–64. http://dx.doi.org/10.3171/2014.6.spine13950.
Abstract:
Object The effect of craniovertebral decompression surgery on CSF flow dynamics in patients with Chiari malformation Type I (CM-I) has been incompletely characterized. The authors used computational fluid dynamics to calculate the effect of decompression surgery on CSF flow dynamics in the posterior fossa and upper cervical spinal canal. Methods Oscillatory flow was simulated in idealized 3D models of the normal adult and the CM-I subarachnoid spaces (both previously described) and in 3 models of CM-I post–craniovertebral decompressions. The 3 postoperative models were created from the CM model by virtually modifying the CM model subarachnoid space to simulate surgical decompressions of different magnitudes. Velocities and pressures were computed with the Navier-Stokes equations in Star-CD for multiple cycles of CSF flow oscillating at 80 cycles/min. Pressure gradients and velocities were compared for 8 levels extending from the posterior fossa to the C3–4 level. Relative pressures and peak velocities were plotted by level from the posterior fossa to C3–4. The heterogeneity of flow velocity distribution around the spinal cord was compared between models. Results Peak systolic velocities were generally lower in the postoperative models than in the preoperative CM model. With the 2 larger surgical defects, peak systolic velocities were brought closer to normal model velocities (equal values at C-3 and C-4) than with the smallest surgical defect. For the smallest defect, peak velocities were decreased, but not to levels in the normal model. In the postoperative models, heterogeneity in flow velocity distribution around the spinal cord increased from normal model levels as the degree of decompression increased. Pressures in the 5 models differed in magnitude and in pattern. Pressure gradients along the spinal canal in the normal and CM models were nonlinear, with steeper gradients below C3–4 than above. The CM model had a steeper pressure gradient than the normal model above C3–4 and the same gradient below. The postoperative models had lower pressure gradients than the CM model above C2–3. The most conservative decompression had lower pressure gradients than the normal model above C2–3. The two larger decompression defects had CSF pressure gradients below those in the normal model above C2–3. These 2 models had a less steep gradient above C-3 and a steeper gradient below. Conclusions In computer simulations, craniovertebral surgical defects generally diminished CSF velocities and CSF pressures.
16
Al-Hadhrami, Luai M., S. M. Shaahid, Lukman O. Tunde, and A. Al-Sarkhi. "Experimental Study on the Flow Regimes and Pressure Gradients of Air-Oil-Water Three-Phase Flow in Horizontal Pipes." Scientific World Journal 2014 (2014): 1–11. http://dx.doi.org/10.1155/2014/810527.
Abstract:
An experimental investigation has been carried out to study the flow regimes and pressure gradients of air-oil-water three-phase flows in 2.25 ID horizontal pipe at different flow conditions. The effects of water cuts, liquid and gas velocities on flow patterns and pressure gradients have been studied. The experiments have been conducted at 20°C using low viscosity Safrasol D80 oil, tap water and air. Superficial water and oil velocities were varied from 0.3 m/s to 3 m/s and air velocity varied from 0.29 m/s to 52.5 m/s to cover wide range of flow patterns. The experiments were performed for 10% to 90% water cuts. The flow patterns were observed and recorded using high speed video camera while the pressure drops were measured using pressure transducers and U-tube manometers. The flow patterns show strong dependence on water fraction, gas velocities, and liquid velocities. The observed flow patterns are stratified (smooth and wavy), elongated bubble, slug, dispersed bubble, and annular flow patterns. The pressure gradients have been found to increase with the increase in gas flow rates. Also, for a given superficial gas velocity, the pressure gradients increased with the increase in the superficial liquid velocity. The pressure gradient first increases and then decreases with increasing water cut. In general, phase inversion was observed with increase in the water cut. The experimental results have been compared with the existing unified Model and a good agreement has been noticed.
17
Nicolas Receveur, Dmitry Nechipurenko, Yannick Knapp, Aleksandra Yakusheva, Eric Maurer, Cécile V. Denis, François Lanza, Mikhail Panteleev, Christian Gachet, and Pierre H. Mangin. "Shear rate gradients promote a bi-phasic thrombus formation on weak adhesive proteins, such as fibrinogen in a VWF-dependent manner." Haematologica 105, no. 10 (November 14, 2019): 2471–83. http://dx.doi.org/10.3324/haematol.2019.235754.
Abstract:
Blood flow profoundly varies throughout the vascular tree due to its pulsatile nature and to the complex vessel geometry. While thrombus formation has been extensively studied in vitro under steady flow, and in vivo under normal blood flow conditions, the impact of complex hemodynamics such as flow acceleration found in stenosed arteries has gained increased appreciation. We investigated the effect of flow acceleration, characterized by shear rate gradients, on the function of platelets adhering to fibrinogen, a plasma protein which plays a key role in hemostais and thrombosis. While we confirmed that under steady flow, fibrinogen only supports single platelet adhesion, we observed that under shear rate gradients, this surface becomes highly thrombogenic, supporting efficient platelet aggregation leading to occlusive thrombus formation. This shear rate gradient-driven thrombosis is biphasic with an initial step of slow platelet recruitment supported by direct plasma VWF adsorption to immobilized fibrinogen and followed by a second phase of explosive thrombosis initiated by VWF fiber formation on platelet monolayers. In vivo experiments confirmed that shear rate gradients accelerate thrombosis in a VWF-dependent manner. Together, this study characterizes a process of plasma VWF-dependent accelerated thrombosis on immobilized fibrinogen in the presence of shear rate gradients.
18
Gao, Jianfeng, Yang Wu, Xiaojun Shao, Yanan Han, Bingang Guo, and Jianjun Liang. "Analysis of the inhomogeneous LPG-air flow field in a tube containing mixed obstructions." Journal of Physics: Conference Series 2584, no. 1 (September 1, 2023): 012018. http://dx.doi.org/10.1088/1742-6596/2584/1/012018.
Abstract:
Abstract Based on numerical simulation, this paper further investigates the flow field structure near the obstacle during the premixed gas deflagration. In the deflagration flow field, the pressure gradient variation and vortex structure intensity in the upper surface region of the obstacle are larger, indicating that the pressure gradient has a stronger effect on the vortex structure. The changes in density gradients and pressure gradients induced by the combined rectangular, flat barrier configuration will be more pronounced than in the model with only flat barriers. This change in turn acts back on the combustion field, which in turn has a strong perturbative effect on the flame.
19
Quick, Christopher M., Arun M. Venugopal, Anatoliy A. Gashev, David C. Zawieja, and Randolph H. Stewart. "Intrinsic pump-conduit behavior of lymphangions." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 292, no. 4 (April 2007): R1510—R1518. http://dx.doi.org/10.1152/ajpregu.00258.2006.
Abstract:
Lymphangions, segments of lymphatic vessels bounded by valves, have characteristics of both ventricles and arteries. They can act primarily like pumps when actively transporting lymph against a pressure gradient. They also can act as conduit vessels when passively transporting lymph down a pressure gradient. This duality has implications for clinical treatment of several types of edema, since the strategy to optimize lymph flow may depend on whether it is most beneficial for lymphangions to act as pumps or conduits. To address this duality, we employed a simple computational model of a contracting lymphangion, predicted the flows at both positive and negative axial pressure gradients, and validated the results with in vitro experiments on bovine mesenteric vessels. This model illustrates that contraction increases flow for normal axial pressure gradients. With edema, limb elevation, or external compression, however, the pressure gradient might reverse, and lymph may flow passively down a pressure gradient. In such cases, the valves may be forced open during the entire contraction cycle. The vessel thus acts as a conduit, and contraction has the effect of increasing resistance to passive flow, thus inhibiting flow rather than promoting it. This analysis may explain a possible physiological benefit of the observed flow-mediated inhibition of the lymphatic pump at high flow rates.
20
Ruffert, M. "Hydrodynamic Simulations of Wind-Accretion with Gradients." International Astronomical Union Colloquium 163 (1997): 215–19. http://dx.doi.org/10.1017/s0252921100042676.
Abstract:
AbstractI investigate the hydrodynamics of three-dimensional Bondi-Hoyle-Lyttleton accretion including velocity and density gradients in the incoming flow and determine how much angular momentum is accreted. A medium taken to be an ideal gas with an adiabatic index of 5/3 or 4/3 moves at supersonic speeds (Mach 3 and 10) past a totally absorbing sphere with a radius of 0.1 or 0.02 accretion radii. The velocity within the medium is given a gradient of 3% or 20% (over one accretion radius). I find that a substantial amount (0.1 to 0.7) of the specific angular momentum available within one accretion radius in the upstream flow actually is accreted. The amount is smaller for smaller accretor sizes. The flow is roughly just as unstable as in the previous models without gradients. The unstable flow is best seen in animated sequences.
21
Bruder, Enrico, Tilman Bohn, and Clemens Müller. "Properties of UFG HSLA Steel Profiles Produced by Linear Flow Splitting." Materials Science Forum 584-586 (June 2008): 661–66. http://dx.doi.org/10.4028/www.scientific.net/msf.584-586.661.
Abstract:
Linear flow splitting is a new cold forming process for the production of branched sheet metal structures. It induces severe plastic strain in the processing zone which results in the formation of an UFG microstructure and an increase in hardness and strength in the flanges. Inbuilt deformation gradients in the processing zone lead to steep gradients in the microstructure and mechanical properties. In the present paper the gradients in the UFG microstructure and the mechanical properties of a HSLA steel (ZStE 500) processed by linear flow splitting are presented, as well as a calculation of local strength from hardness measurements on the basis of the Ludwikequation. In order to investigate the thermal stability of the UFG microstructure heat treatments below the recrystallization temperature were chosen. The coarsening process and the development of the low angle to high angle grain boundary ratio in the gradient UFG microstructure were monitored by EBSD measurements. It is shown that heat treatment can lead to a grain refinement due to a strong fragmentation of elongated grains while only little coarsening in the transverse direction occurs. A smoothing of the gradients in the UFG microstructure as well as in the mechanical properties is observed.
22
Paduthol, Gauri, Teji Shenne Korma, Amit Agrawal, and Debjani Paul. "Dynamic generation of power function gradient profiles in a universal microfluidic gradient generator by controlling the inlet flow rates." Lab on a Chip 22, no. 3 (2022): 592–604. http://dx.doi.org/10.1039/d1lc00938a.
23
García-Sandoval, J., Fernando Bautista, Jorge Puig, and Octavio Manero. "Inhomogeneous Flow of Wormlike Micelles: Predictions of the Generalized BMP Model with Normal Stresses." Fluids 4, no. 1 (March 8, 2019): 45. http://dx.doi.org/10.3390/fluids4010045.
Abstract:
In this work, we examine the shear-banding flow in polymer-like micellar solutions with the generalized Bautista-Manero-Puig (BMP) model. The couplings between flow, structural parameters, and diffusion naturally arise in this model, derived from the extended irreversible thermodynamics (EIT) formalism. Full tensorial expressions derived from the constitutive equations of the model, in addition to the conservation equations, apply for the case of simple shear flow, in which gradients of the parameter representing the structure of the system and concentration vary in the velocity gradient direction. The model predicts shear-banding, concentration gradients, and jumps in the normal stresses across the interface in shear-banding flows.
24
Agudo, M., J. Marcos, A. Ríos, and M. Valcárcel. "Analytical potential of flow gradients in unsegmented flow systems." Analytica Chimica Acta 239 (1990): 211–20. http://dx.doi.org/10.1016/s0003-2670(00)83855-6.
25
Zbib, H. M., and E. C. Aifantis. "A Gradient-Dependent Flow Theory of Plasticity: Application to Metal and Soil Instabilities." Applied Mechanics Reviews 42, no. 11S (November 1, 1989): S295—S304. http://dx.doi.org/10.1115/1.3152403.
Abstract:
We propose a gradient-dependent flow theory of plasticity for metals and granular soils and apply it to the problems of shear banding and liquefaction. We incorporate higher order strain gradients either into the constitutive equation for the flow stress or into the dilantancy condition. We examine the effect of these gradients on the onset of instabilities in the form of shear banding in metals or shear banding and liquefaction in soils under both quasi-static and dynamic conditions. It is shown that the higher order gradients affect the critical conditions and allow for a wavelength selection analysis leading to estimates for the width or spacing of shear bands and liquefying strips. Finally, a nonlinear analysis is given for the evolution of shear bands in soils deformed in the post-localization regime.
26
Mafi, MD, Zhen Qin, Yuting Wu, Sung-Ki Lyu, and Chicheng Ma. "Research on the Interfacial Instability of Non-Newtonian Fluid Displacement Using Flow Geometry." Coatings 13, no. 11 (October 27, 2023): 1848. http://dx.doi.org/10.3390/coatings13111848.
Abstract:
The variation of the classical viscous fingering instability is studied numerically in this work. An investigation of the viscous fingering phenomenon of immiscible displacement in the Hele–Shaw cell (HSC), where the displaced fluid is a shear-thinning fluid, was carried out numerically using the volume of fluid (VOF) method by adding a minor depth gradient or altering the geometry of the top plate in the HSC. The findings demonstrate how the presence of depth gradients can change the stability of the interface and offer a chance to regulate and adapt the fingering instability in response to the viscous fingering properties of air driving non-Newtonian fluids under various depth gradients. The relative breadth will shrink under the influence of the depth gradient, and the negative consequences of the gradient will be increasingly noticeable. Specifically, under different power-law indices, we found that with the enhancement of shear-thinning characteristics (lower power-law exponent n) in both positive and negative depth gradients, the fingers that protrude from the viscous fingers become shorter and thicker, resulting in higher displacement efficiency. Additionally, several modifications were performed to the upper plate’s design, and the findings revealed that the shape had no effect on the viscous fingering and only had an impact on the longitudinal amplitude. Based on the aforementioned traits, we may alter the HSC’s form or depth gradient to provide high-quality and effective work.
27
Ma, Xiaotian, Shuangming Xu, Feifan Wang, Yaobang Zhao, Xiangchen Meng, Yuming Xie, Long Wan, and Yongxian Huang. "Effect of Temperature and Material Flow Gradients on Mechanical Performances of Friction Stir Welded AA6082-T6 Joints." Materials 15, no. 19 (September 22, 2022): 6579. http://dx.doi.org/10.3390/ma15196579.
Abstract:
The temperature and material flow gradients along the thick section of the weld seriously affect the welding efficiency of friction stir welding in medium-thick plates. Here, the effects of different gradients obtained by the two pins on the weld formation, microstructure, and mechanical properties were compared. The results indicated that the large-tip pin increases heat input and material flow at the bottom, reducing the gradient along the thickness. The large-tip pin increases the welding speed of defect-free joints from 100 mm/min to 500 mm/min compared to the small-tip pin. The ultimate tensile strength and elongation of the joint reached 247 MPa and 8.7%, equal to 80% and 65% of the base metal, respectively. Therefore, reducing the temperature and material flow gradients along the thickness by designing the pin structure is proved to be the key to improving the welding efficiency for thick plates.
28
Willatzen, M. "Temperature gradients and flow-meter performance." Ultrasonics 39, no. 5 (August 2001): 383–89. http://dx.doi.org/10.1016/s0041-624x(01)00063-4.
29
García, I. López, P. Viñas, N. Campillo, and M. Hernández Córdoba. "Linear flow gradients for automatic titrations." Analytica Chimica Acta 308, no. 1-3 (June 1995): 67–76. http://dx.doi.org/10.1016/0003-2670(94)00629-z.
30
Walton, J. H., and Mark S. Conradi. "Flow velocity measurement with ac gradients." Magnetic Resonance in Medicine 4, no. 3 (March 1987): 274–81. http://dx.doi.org/10.1002/mrm.1910040308.
31
Mendiburu, A. A., L. R. Carrocci, and J. A. Carvalho. "ANALYTICAL SOLUTION FOR TRANSIENT ONEDIMENSIONAL COUETTE FLOW CONSIDERING CONSTANT AND TIME-DEPENDENT PRESSURE GRADIENTS." Revista de Engenharia Térmica 8, no. 2 (December 31, 2009): 92. http://dx.doi.org/10.5380/reterm.v8i2.61921.
Abstract:
This paperaims to determine the velocity profile, in transient state, for a parallel incompressible flow known as Couette flow. The Navier-Stokes equations were applied upon this flow. Analytical solutions, based in Fourier series and integral transforms, were obtained for the one-dimensional transient Couette flow, taking into account constant and time-dependent pressure gradients acting on the fluid since the same instant when the plate starts it´s movement. Taking advantage of the orthogonality and superposition properties solutions were foundfor both considered cases. Considering a time-dependent pressure gradient, it was found a general solution for the Couette flow for a particular time function. It was found that the solution for a time-dependent pressure gradient includes the solutions for a zero pressure gradient and for a constant pressure gradient.
32
Burwash, I. G., A. D. Forbes, M. Sadahiro, E. D. Verrier, A. S. Pearlman, R. Thomas, C. Kraft, and C. M. Otto. "Echocardiographic volume flow and stenosis severity measures with changing flow rate in aortic stenosis." American Journal of Physiology-Heart and Circulatory Physiology 265, no. 5 (November 1, 1993): H1734—H1743. http://dx.doi.org/10.1152/ajpheart.1993.265.5.h1734.
Abstract:
The anatomy of degenerative valvular aortic stenosis has been poorly represented in animal models, limiting the evaluation of noninvasive echo-Doppler measures of transvalvular volume flow rate and stenosis severity during progressive disease evolution or under conditions of changing volume flow rates. To study these issues, chronic valvular aortic stenosis, characterized by stiff leaflets without commissural fusion, was created in nine adult mongrel dogs by suturing pericardial covered Teflon-felt pads into the sinuses of Valsalva below the coronary ostia during hypothermic cardiac arrest. In the eight surviving dogs, echo-Doppler examinations were performed weekly for up to 8 wk postoperatively. Simultaneous invasive micromanometer pressure data were collected at 2-wk intervals in all subjects, with simultaneous ascending aortic transit time-volume flow measurement in four subjects. Volume flow rates were altered with saline and dobutamine infusions during invasive studies for comparison of echo-Doppler and invasive pressure gradients, volume flow, and valve areas. Serial echo-Doppler follow-up (39 +/- 11 days) demonstrated that, from baseline to final study, mean transvalvular pressure gradient increased (4 +/- 1 to 38 +/- 7 mmHg, P = 0.001), continuity equation aortic valve area decreased (2.06 +/- 0.18 to 0.54 +/- 0.04 cm2, P < 0.0001), and progressive left ventricular hypertrophy developed (62 +/- 6 to 114 +/- 9 g, P = 0.0003). Echo-Doppler and invasive data correlated well for measures of transvalvular pressure gradients (n = 98, maximum instantaneous gradient r = 0.95, mean gradient r = 0.91), volume flow (n = 75, stroke volume r = 0.86, cardiac output r = 0.86), and valve area (n = 73, r = 0.73) despite acute alterations in volume flow and progressive disease evolution. This chronic canine model, with anatomy and hemodynamics similar to clinical degenerative valvular aortic stenosis, should provide a valuable tool for investigating clinically relevant new measures of stenosis severity with use of invasive or noninvasive techniques.
33
Mohammadpourfard, M., and F. Ghaderi. "Numerical study of biofluid flow over a backward-facing step: The hydro-thermal behavior in the presence of magnetic field effects." Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 231, no. 4 (April 19, 2016): 800–812. http://dx.doi.org/10.1177/0954408916637985.
Abstract:
In this paper, the results of adding nanoparticles and applying non-uniform magnetic fields on a biofluid (blood) flow through a two-dimensional horizontal channel with a step are reported. Two magnetic fields with positive and negative gradients were applied. The control volume technique and two-phase mixture model in the numerical approach have been used to illustrate the hydro-thermal behavior of flow. Simulation results reveal that nanoparticles can significantly increase the Nusselt number and wall shear stress. Also, the wall shear stress, Nu, and recirculation length in the presence of a magnetic field with different gradients can be externally controlled. Based on the results, the negative gradient magnetic field increases wall shear stress and Nu in the affected region, unlike the positive gradient.
34
Li, Lindong, and Changbo Fu. "Role of Longitudinal Temperature Gradients in Eliminating Interleaving Inclusions in Casting of Monocrystalline Silicon Ingots." Crystals 14, no. 5 (May 17, 2024): 471. http://dx.doi.org/10.3390/cryst14050471.
Abstract:
Infrared analysis reveals the presence of interwoven inclusions, primarily comprised of silicon nitride and silicon carbide, in the casting process of monocrystalline silicon ingots. This study investigates how the longitudinal temperature gradient affects the removal of inclusions during the casting of monocrystalline silicon ingots through simulations and comparative experiments. Two monocrystalline silicon ingots were cast, each using different longitudinal temperature gradients: one employing smaller gradients and the other conventional gradients. CGSim (Version Basic CGSim 23.1) simulation software was utilized to analyze the melt flow and temperature distribution during the growth process of quasi–monocrystalline silicon ingots. The findings indicate that smaller longitudinal temperature gradients lead to a more robust upward flow of molten silicon at the solid–liquid interface, effectively carrying impurities away from this interface and preventing their inclusion formation. Analysis of experimental photoluminescence and IR results reveals that although inclusions may not be observed, impurities persist but are gradually displaced to the top of the silicon melt through a stable growth process.
35
Hutchinson, J. W., and T. J. Lu. "Laminate Delamination Due to Thermal Gradients." Journal of Engineering Materials and Technology 117, no. 4 (October 1, 1995): 386–90. http://dx.doi.org/10.1115/1.2804730.
Abstract:
Flaw-induced delamination of orthotropic laminates subject to through-thickness temperature gradients is analyzed. A crack-like flaw impedes heat flow through the laminate, producing thermal stresses and crack tip stress intensities. The focus is on delamination cracks which propagate under steady-state conditions. The steady-state analysis becomes accurate for a crack whose length is about one laminate thickness. Moreover, the analysis provides realistic fail-safe criteria for excluding delamination.
36
Tian, Pei, Chengzhong Pan, Xinyi Xu, Tieniu Wu, Tiantian Yang, and Lujun Zhang. "A field investigation on rill development and flow hydrodynamics under different upslope inflow and slope gradient conditions." Hydrology Research 51, no. 5 (September 24, 2020): 1201–20. http://dx.doi.org/10.2166/nh.2020.168.
Abstract:
Abstract Few studies focus on the quantitative impact of upslope inflow rate and slope gradient on rill development and erosion processes. Field plot experiments under varying inflow rates (6–36 L min−1m−1) and slope gradients (26, 42 and 57%) were conducted to address this issue. The results showed soil loss rates significantly demonstrated temporal variability in relevance to the rill developing process. Rill erosion and its contribution to soil loss increased with increasing inflow rates and slope gradients by power functions. There was a threshold inflow discharge (12–24 L min−1m−1), under which, rill erosion became the dominant erosion pattern. At the initial stage, downcutting of rill bottom and headward erosion were obvious, whereas rill broadening was significant at the actively rill developing period. Rill density increased with slope gradient increasing from 26% to 42%, and then decreased. For the 57% slope under high inflow rates (24–36 L min−1m−1), gravity caused an increase in the collapse of rills. Mean rill width increased with increasing inflow rates but decreased as slope gradients increased, while mean rill depth increased with increasing inflow rates and slope gradients. Stream power and rill flow velocity were the best hydrodynamic parameter to simulate rill erosion and rill morphology, respectively.
37
Haidekker, Mark A., Charles R. White, and John A. Frangos. "Analysis of Temporal Shear Stress Gradients During the Onset Phase of Flow Over a Backward-Facing Step." Journal of Biomechanical Engineering 123, no. 5 (April 17, 2001): 455–63. http://dx.doi.org/10.1115/1.1389460.
Abstract:
Endothelial cells in blood vessels are exposed to blood flow and thus fluid shear stress. In arterial bifurcations and stenoses, disturbed flow causes zones of recirculation and stagnation, which are associated with both spatial and temporal gradients of shear stress. Such gradients have been linked to the generation of atherosclerotic plaques. For in-vitro studies of endothelial cell responses, the sudden-expansion flow chamber has been widely used and described. A two-dimensional numerical simulation of the onset phase of flow through the chamber was performed. The wall shear stress action on the bottom plate was computed as a function of time and distance from the sudden expansion. The results showed that depending on the time for the flow to be established, significant temporal gradients occurred close to the second stagnation point of flow. Slowly ramping the flow over 15 s instead of 200 ms reduces the temporal gradients by a factor of 300, while spatial gradients are reduced by 23 percent. Thus, the effects of spatial and temporal gradients can be observed separately. In experiments on endothelial cells, disturbed flow stimulated cell proliferation only when flow onset was sudden. The spatial patterns of proliferation rate match the exposure to temporal gradients. This study provides information on the dynamics of spatial and temporal gradients to which the cells are exposed in a sudden-expansion flow chamber and relates them to changes in the onset phase of flow.
38
Bauer, Wolfgang Rudolf. "Impact of Interparticle Interaction on Thermodynamics of Nano-Channel Transport of Two Species." Entropy 22, no. 4 (March 25, 2020): 376. http://dx.doi.org/10.3390/e22040376.
Abstract:
Understanding the function and control of channel transport is of paramount importance for cell physiology and nanotechnology. In particular, if several species are involved, the mechanisms of selectivity, competition, cooperation, pumping, and its modulation need to be understood. What lacks is a rigorous mathematical approach within the framework of stochastic thermodynamics, which explains the impact of interparticle in-channel interactions on the transport properties of the respective species. To achieve this, stochastic channel transport of two species is considered in a model, which different from mean field approaches, explicitly conserves the spatial correlation of the species within the channel by analysis of the stochastic dynamics within a state space, the elements of which are the channel’s spatial occupation states. The interparticle interactions determine the stochastic transitions between these states. Local flow and entropy production in this state space reveal the respective particle flows through the channel and the intensity of the Brownian ratchet like rectifying forces, which these species exert mutually on each other, together with its thermodynamic effectiveness and costs. Perfect coupling of transport of the two species is realized by an attractive empty channel and strong repulsive forces between particles of the same species. This confines the state space to a subspace with circular topology, in which the concentration gradients as thermodynamic driving forces act in series, and channel flow of both species becomes equivalent. For opposing concentration gradients, this makes the species with the stronger gradient the driving, positive entropy producing one; the other is driven and produces negative entropy. Gradients equal in magnitude make all flows vanish, and thermodynamic equilibrium occurs. A differential interparticle interaction with less repulsive forces within particles of one species but maintenance of this interaction for the other species adds a bypass path to this circular subspace. On this path, which is not involved in coupling of the two species, a leak flow of the species with less repulsive interparticle interaction emerges, which is directed parallel to its concentration gradient and, hence, produces positive entropy here. Different from the situation with perfect coupling, appropriate strong opposing concentration gradients may simultaneously parallelize the flow of their respective species, which makes each species produce positive entropy. The rectifying potential of the species with the bypass option is diminished. This implies the existence of a gradient of the other species, above which its flow and gradient are parallel for any gradient of the less coupled species. The opposite holds for the less coupled species. Its flow may always be rectified and turned anti-parallel to its gradient by a sufficiently strong opposing gradient of the other one.
39
Wu, H., S. Moreau, and R. D. Sandberg. "Effects of pressure gradient on the evolution of velocity-gradient tensor invariant dynamics on a controlled-diffusion aerofoil at." Journal of Fluid Mechanics 868 (April 17, 2019): 584–610. http://dx.doi.org/10.1017/jfm.2019.129.
Abstract:
A weakly compressible flow direct numerical simulation of a controlled-diffusion aerofoil at $8^{\circ }$ geometrical angle of attack, a chord-based Reynolds number of $Re_{c}=150\,000$ and a Mach number of $M=0.25$ based on the free-stream velocity relevant to many industrial applications was conducted to improve the understanding of the impact of the pressure gradient on the development of turbulent structures. The evolution equations for the two invariants $Q$ and $R$ of the velocity-gradient tensor have been studied at various locations along the aerofoil chord on its suction side. The shape of the mean evolution of the velocity-gradient tensor invariants were found to vary strongly when the flow encounters favourable, zero and adverse pressure gradients and as well for different wall-normal locations. The coupling between the pressure-Hessian tensor and the velocity-gradient tensor was found to be the major factor that causes these changes and is greatly influenced by the mean pressure-gradient condition and the wall-normal distance. Striking differences exist from the mean trajectories of this coupling at least in the log layer and outer layer subject to different mean pressure gradients. The nonlinearity and viscous diffusion effects keep their respective invariant characters regardless of the pressure-gradient effects and wall-normal locations. The wall and the mean adverse pressure gradient were both found to suppress the vortical stretching features of the flow. These features are of great importance for the development of future turbulence models on wall-bounded flows, especially on surfaces with significant curvature such as cambered aerofoils and blades for which significant mean pressure gradients exist.
40
Penfold, J., E. Staples, I. Tucker, G. J. T. Tiddy, and A. Khan Lodhi. "Shear-Induced Structures in Concentrated Surfactant Micellar Phases." Journal of Applied Crystallography 30, no. 5 (October 1, 1997): 744–49. http://dx.doi.org/10.1107/s0021889897001623.
Abstract:
Constant and oscillatory Couette shear flow have been used in combination with small-angle neutron scattering to observe the shear-induced ordering in concentrated surfactant micellar phases. For the lamellar phase of hexaethylene glycol monohexadecyl ether, C16 E 6, two distinct lamellae orientations have been identified. At low shear gradients the lamellae are ordered parallel to the flow–vorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow-shear gradient plane, corresponding to a rotation through 90° of the axis of orientation. At intermediate values of constant shear and for oscillatory shear, both lamellae orientations are simultaneously observed for the first time in a surfactant lamellar phase. For the lamellar phase, a dispersion of the binary surfactant mixtures of dioleyl cationic and 2-ethyl hexaglycerol monoether surfactants, a high degree of alignment, in the direction parallel to the flow–vorticity plane, is observed at zero and low shear. With time, during the application of a shear gradient of 25 s−1, the lamellar phase transforms to a highly ordered solution of monodisperse multilamellar vesicles.
41
Olsen, Harold W. "Osmosis: a cause of apparent deviations from Darcy's law." Canadian Geotechnical Journal 22, no. 2 (May 1, 1985): 238–41. http://dx.doi.org/10.1139/t85-032.
Abstract:
Recent evidence for deviations from Darcy's law at very low gradients provides a reminder that the origin of similar deviations reported during the last three decades has not been fully clarified. In most of these studies, the potential significance of osmosis was not considered. This review of the existing evidence shows that osmosis causes intercepts in flow rate versus hydraulic gradient relationships that are consistent with the observed deviations from Darcy's law at very low gradients. Moreover, it is suggested that a natural cause of osmosis in laboratory samples could be chemical reactions such as those involved in aging effects. This hypothesis is analogous to the previously proposed occurrence of electroosmosis in nature generated by geochemical weathering reactions. Key words: Darcy's law, non-Darcy flow, hydraulic intercept, threshold gradient, osmosis.
42
Qu, Liqin, Tingwu Lei, Chenyan Zhou, and Xiusheng Yang. "Measuring Sediment Transport Capacity of Concentrated Flow with Erosion Feeding Method." Land 12, no. 2 (February 3, 2023): 411. http://dx.doi.org/10.3390/land12020411.
Abstract:
Sediment transport capacity in rills is an important parameter for erosion modeling on hillslopes. It is difficult to measure, especially at gentle slopes with limited rill length. In this study, a special flume with variable slope gradients in upper and lower sections was implemented to measure the sediment transport capacity. The upper flume section with a higher slope gradient generates faster water flow that could scout more sediment to feed the water flow in the rill. The rest of the flume is set at the designated slopes to measure the transport capacity in different slope and runoff conditions. A series of flume experiments were conducted with silt-loam soil to verify the method. The sediment transport capacity was measured under slope gradients of 5°, 10°, 15°, 20°, and 25° and a flow rate of 2, 4, 8, and 16 L min−1. The measured sediment transport capacity values were compared with reference measurements from other rill erosion experiments with similar materials and setups. At high slope gradients of 15°, 20°, and 25°, the newly suggested method produced almost the same transport capacity values. Under the low slope gradients of 5° and 10°, the maximum sediment concentrations from the 8 m long flume with the uniform gradients in the previous experiments, rill erosion with an 8 m long flume produced were about 36% lower than the values measured with the new method, which is insufficient to make the flow reach sediment transport capacity. The sediment transport capacities at lower slopes measured with the new method followed the same trend as those at higher slopes. The new method can supply enough sediments to ensure the flow approach transport capacity measurement and, therefore, provides a feasible approach for estimating sediment transport capacity for conditions with relatively gentle slopes.
43
Vicker, M. G. "The regulation of chemotaxis and chemokinesis in Dictyostelium amoebae by temporal signals and spatial gradients of cyclic AMP." Journal of Cell Science 107, no. 2 (February 1, 1994): 659–67. http://dx.doi.org/10.1242/jcs.107.2.659.
Abstract:
The tactic and kinetic locomotion of Dictyostelium discoideum amoebae were examined in cyclic AMP (cAMP) spatial gradient and temporal signal fields. The distributions of migrating cells were examined within 150 microns-thick micropore filters after incubation with different cAMP concentrations, [cAMP], applied in three ways across the fields: as positively or negatively developing gradients, generated either by increasing or decreasing the [cAMP] on one side of the filter, respectively, or as static, linear gradients after negative development. Chemotaxis was only induced by oriented, temporally increasing [cAMP]. Pulses propagated by molecular diffusion or mechanical flow were equally effective. Negatively developing cAMP gradients had no initial effect on cell accumulation. However, if the subsequent static spatial gradient was maintained by an infusion system, some gradients also induced cell accumulation, whose degree and direction depended on the gradient [cAMP]. The basis of this new effect was examined by tracking individual cells by computer-assisted videomicroscopy during locomotion in different [cAMP]. Cells produced a triphasic [cAMP]-dependent response, with optimal cell motility induced by 10–30 nM. The results demonstrate that cell accumulation either up-field or down-field in spatial gradients is governed by the field locations of the attractant concentrations that induce the relative locomotory maxima and minima in the gradient field. Cells perceive the ambient [cAMP], but cannot read the spatial gradient orientation in static or yet steeper regions of developing gradients. Accumulation in static spatial gradients is a function of klino- and orthokinesis, but chemotaxis requires an oriented cAMP pulse or impulse.(ABSTRACT TRUNCATED AT 250 WORDS)
44
Shen, Nan, Zhanli Wang, Qingwei Zhang, Hao Chen, and Bing Wu. "Modelling soil detachment capacity by rill flow with hydraulic variables on a simulated steep loessial hillslope." Hydrology Research 50, no. 1 (August 23, 2018): 85–98. http://dx.doi.org/10.2166/nh.2018.037.
Abstract:
Abstract Modelling soil detachment capacity by rill flow with hydraulic variables is essential to understanding the rill erosion process and developing physically based rill erosion models. A rill flume experiment with non-erodible flume bed and small soil samples was conducted. Seven flow discharges and six steep slope gradients were combined to produce various flow hydraulics. The soil detachment capacity increases with the increase in slope gradient and flow discharge. The critical slope gradients of 21.26 and 26.79% cause the detachment capacity to increase at a slow pace. The soil detachment capacity can be defined by a power function of flow discharges and slopes. The contribution rates of slope gradient and flow discharge to soil detachment capacity are 42 and 54%, respectively. The soil detachment capacity increases with shear stress, stream power and unit stream power; the increase rates of these parameters are greater under gentle slopes than steep slopes. Stream power is the superior hydrodynamic parameter describing soil detachment capacity. The linear model equation of stream power is stable and reliable, which can accurately predict soil detachment capacity by rill flow on steep loessial hillslopes. This study can help to sufficiently clarify the dynamic mechanism of soil detachment and accurately predict soil detachment capacity for steep loessial hillslopes.
45
White, Charles R., Hazel Y. Stevens, Mark Haidekker, and John A. Frangos. "Temporal gradients in shear, but not spatial gradients, stimulate ERK1/2 activation in human endothelial cells." American Journal of Physiology-Heart and Circulatory Physiology 289, no. 6 (December 2005): H2350—H2355. http://dx.doi.org/10.1152/ajpheart.01229.2004.
Abstract:
We have previously demonstrated temporal gradients in shear stress stimulate endothelial cell proliferation, whereas spatial gradients do not. In the present study, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway was investigated as a possible mediator for the promitogenic effect of temporal gradients. The sudden expansion flow chamber (SEFC) model was used to differentiate the effect of temporal gradients in shear from that of spatial gradients on ERK1/2 activation in human umbilical vein endothelial cells (HUVEC). ERK1/2 activation in the SEFC was not significantly different from control when HUVEC were exposed to spatial gradients alone. When a single temporal impulse was superimposed on spatial gradients, ERK1/2 activation was stimulated 330% (relative to spatial alone) within the region of spatial gradients. Inhibition of the ERK1/2 pathway with U-0126 abolished all effects of temporal gradients. To further separate temporal and spatial gradients, a conventional parallel plate flow chamber was utilized. Acute exposure to oscillations in flow at a frequency of 1 Hz stimulated ERK1/2 activation 620 ± 88% relative to control, whereas a single impulse of flow increased ERK1/2 activation 166 ± 19%. Flow without the temporal component did not significantly activate ERK1/2. These results suggest that the ERK1/2 pathway directly mediates the promitogenic effects of temporal gradients in shear stress.
46
Nickeler, D. H., and T. Wiegelmann. "Thin current sheets caused by plasma flow gradients in space and astrophysical plasma." Annales Geophysicae 28, no. 8 (August 13, 2010): 1523–32. http://dx.doi.org/10.5194/angeo-28-1523-2010.
Abstract:
Abstract. Strong gradients in plasma flows play a major role in space and astrophysical plasmas. A typical situation is that a static plasma equilibrium is surrounded by a plasma flow, which can lead to strong plasma flow gradients at the separatrices between field lines with different magnetic topologies, e.g., planetary magnetospheres, helmet streamers in the solar corona, or at the boundary between the heliosphere and interstellar medium. Within this work we make a first step to understand the influence of these flows towards the occurrence of current sheets in a stationary state situation. We concentrate here on incompressible plasma flows and 2-D equilibria, which allow us to find analytic solutions of the stationary magnetohydrodynamics equations (SMHD). First we solve the magnetohydrostatic (MHS) equations with the help of a Grad-Shafranov equation and then we transform these static equilibria into a stationary state with plasma flow. We are in particular interested to study SMHD-equilibria with strong plasma flow gradients perpendicular to separatrices. We find that induced thin current sheets occur naturally in such situations. The strength of the induced currents depend on the Alfvén Mach number and its gradient, and on the magnetic field.
47
Zhang, Kai, Wang Xuan, Bai Yikui, and Xu Xiuquan. "Prediction of sediment transport capacity based on slope gradients and flow discharge." PLOS ONE 16, no. 9 (September 7, 2021): e0256827. http://dx.doi.org/10.1371/journal.pone.0256827.
Abstract:
Sediment transport capacity (Tc) is an essential parameter in the establishment of the slope soil erosion model. Slope type is an important crucial factor affecting sediment transport capacity of overland flow, and vegetation can effectively inhibit soil loss. Two new formulae of sediment transport capacity (Tc) are proposed of brown soil slope and vegetation slope in this study and evaluate the influence of slope gradient (S) and flow discharge (Q) on sediment transport capacity of different slope types. Laboratory experiments conducted using four flow discharges (0.35, 0.45, 0.55, and 0.65 L s-1), four slope gradients (3, 6, 9, and 12°), and two kinds of underlying surface (Brown soil slope, Vegetation slope). The soil particle size range is 0.05–0.5mm. The vegetation stems were 2mm in diameter and randomly arranged. The results show that the sediment transport capacity was positively correlated with the flow discharge and slope gradient. The vegetation slope’s average sediment transport capacity is 11.80% higher than the brown soil slope that same discharge and slope gradient conditions. The sensitivity of sediment transport capacity to flow discharge on brown soil slope is higher than that of slope gradient. The sensitivity of sediment transport capacity of vegetation slope to slope gradient is more heightened than flow discharge. The sediment transport capacity was well predicted by discharge and slope gradient on brown soil slope (R2 = 0.982) and vegetation slope (R2 = 0.993). This method is helpful to promote the study of the sediment transport process on overland flow.
48
Neubauer, Vanessa J., Florian Hüter, Johannes Wittmann, Vanessa T. Trossmann, Claudia Kleinschrodt, Bettina Alber-Laukant, Frank Rieg, and Thomas Scheibel. "Flow Simulation and Gradient Printing of Fluorapatite- and Cell-Loaded Recombinant Spider Silk Hydrogels." Biomolecules 12, no. 10 (October 3, 2022): 1413. http://dx.doi.org/10.3390/biom12101413.
Abstract:
Hierarchical structures are abundant in almost all tissues of the human body. Therefore, it is highly important for tissue engineering approaches to mimic such structures if a gain of function of the new tissue is intended. Here, the hierarchical structures of the so-called enthesis, a gradient tissue located between tendon and bone, were in focus. Bridging the mechanical properties from soft to hard secures a perfect force transmission from the muscle to the skeleton upon locomotion. This study aimed at a novel method of bioprinting to generate gradient biomaterial constructs with a focus on the evaluation of the gradient printing process. First, a numerical approach was used to simulate gradient formation by computational flow as a prerequisite for experimental bioprinting of gradients. Then, hydrogels were printed in a single cartridge printing set-up to transfer the findings to biomedically relevant materials. First, composites of recombinant spider silk hydrogels with fluorapatite rods were used to generate mineralized gradients. Then, fibroblasts were encapsulated in the recombinant spider silk-fluorapatite hydrogels and gradually printed using unloaded spider silk hydrogels as the second component. Thereby, adjustable gradient features were achieved, and multimaterial constructs were generated. The process is suitable for the generation of gradient materials, e.g., for tissue engineering applications such as at the tendon/bone interface.
49
Blanckaert, K. "Flow separation at convex banks in open channels." Journal of Fluid Mechanics 779 (August 17, 2015): 432–67. http://dx.doi.org/10.1017/jfm.2015.397.
Abstract:
Laboratory experiments in an open channel bend provide insight into the physics of convex bank flow separation occurring in a variety of channel configurations, including confluences and bifurcations. The edge of the zone of flow separation is characterized by a shear layer, enhanced velocity gradients, tke, turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity. The latter result from turbulence-induced secondary flow near the convex bank. When bankline curvature abruptly increases, flow tends to move away from the convex bank along a straight path, as represented by the inertial forces – including the centrifugal force – in the transverse momentum equation written in curvilinear coordinates. Mass accumulation at the opposite bank leads to a transverse tilting of the water surface, and a pressure gradient towards the convex bank that causes the flow to change direction. The pressure gradient force lags spatially behind the inertial forces, which promotes flow separation. Flow separation typically occurs downstream of the location of maximum change in the bankline curvature, because an abrupt increase in bankline curvature also leads to water surface gradients that cause local flow redistribution towards the convex bank that opposes flow separation. The zone of convex bank flow separation is shaped by the secondary flow induced by streamline curvature and turbulence. The latter is conditioned by the production rate of tke, which crucially depends on the accurate description of the Reynolds stresses. Hydrodynamic, geometric and sedimentologic control parameters of convex bank flow separation are identified and discussed.
50
Cloitre, M., and E. Guyon. "Forced Rayleigh scattering in turbulent plane Poiseuille flows. Part 1. Study of the transverse velocity-gradient component." Journal of Fluid Mechanics 164 (March 1986): 217–36. http://dx.doi.org/10.1017/s0022112086002537.
Abstract:
In this article, we describe an optical set-up designed to measure directly velocity gradients (strophometry). This strophometer is based on the analysis of the distortions of a fringe pattern ‘written’ instantaneously in a flow field. We apply it to study the transverse velocity-gradient component ∂u/∂y in a plane Poiseuille flow at moderate Reynolds numbers. Mean values and different moments of the fluctuating gradient distribution related to viscous dissipation, vorticity dynamics and intermittency are obtained. These results are interpreted in terms of the large-scale structures which are present in the flow.