Wall Gradient Research Articles

Nonlinear Mixed Convection Flow of Nanofluid Past a Moving Vertical Slender Cylinder

In this article, the nonlinear mixed convection flow of nanofluid past a slender cylinder, which is vertically moving with constant velocity, with viscous dissipation effects is examined. The governing equations of the flow are in the form of dimensional nonlinear partial differential equations (NPDEs), which are brought to non-dimensional form, along with the boundary conditions, by employing non-similar transformations. The resultant non-dimensional NPDEs are solved by adopting quasilinearization technique and implicit finite difference method. The numerical study focuses analysis of various non-dimensional parameters, such as Brownian diffusion Nb, nonlinear mixed convection $$\gamma$$, Richardson number Ri, Lewis number Le, Eckert number or viscous dissipation Ec, thermophoresis Nt, nanoparticle buoyancy ratio Nr, velocity ratio $$\varepsilon$$, on profiles as well as gradients in detail. The numerical results unveil that the increasing values of Ec increase temperature of the fluid. Further, wall gradients for energy transfer and also nanoparticle mass transfer are studied in the presence as well as in the absence of nonlinear mixed convection effects. At $$\xi = 1$$ as Nt increases from 0.05 to 0.15, the friction between the wall and the fluid increases by about 15.23% and 22.41% for Nb = 0.15 and Nb = 0.3, respectively.

Effect of Varying Viscosity on Two-Layer Model of Pulsatile Flow through Blood Vessels with Porous Region near Walls

The present work concerns the effect of hematocrit-dependent viscosity on pulsatile flow of blood through narrow tube with porous walls. Two-fluid model of blood is assumed to be consisting of a core region (Casson fluid) and a plasma region (Newtonian fluid). No slip condition is assumed on wall and pressure gradient has been considered as periodic function of time. The wall of the blood vessel composed of a thin porous (Brinkman) layer. The stress jump condition has been employed at the fluid–porous interface in the plasma region. Up to first order, approximate solutions of governing equations are obtained using perturbation approach. A comparative analysis for relative change in resistance offered against the flow between our model and previously studied single and two-fluid models without porous walls has also been done. Mathematical expressions for velocity, rate of flow and resistance offered against the flow have been obtained analytically for different regions and influence of plasma layer thickness, varying viscosity, stress jump parameter, permeability and viscosity ratio parameter on above quantities are pictorially discussed. It is perceived that the values of flow rate for two-fluid model with porous region near walls are higher in comparison with two-fluid model without porous region near walls. Dependency of hematocrit (Ht) on the porosity parameters is graphically discussed. The study reveals a significant impact of various parameters on hematocrit (Ht). A novel observation is that a slight increase in pressure wave amplitude leads to significant fluctuation in hematocrit (Ht) which also indicates how systole and diastole (which controls the pressure gradient amplitudes) leads to changes on blood hematocrit (Ht).

High‐fidelity Modeling of Blood Flow in Physiologically Realistic Microvascular Networks

High-fidelity in silico modeling of blood flow in microvascular networks continues to be a challenge. Blood in small vessels behaves as a concentrated suspension of red blood cells (RBC), which are extremely deformable. Computational modeling must resolve the dynamics and deformation of individual RBC while simultaneously extending to a dense suspension. Most cellular-scale in silico models to date have considered blood flow in simple geometries, such as straight tubes or channels, which do not represent physiological conditions, since the architecture of vascular networks in vivo is characterized by bifurcating, merging, and winding vessels. Such geometrical differences result in significant deviations in hemodynamics between that achieved in a straight conduit versus a vascular network. We have recently developed a high-fidelity, multiscale, 3D computational model of flow of deformable RBCs in complex microvascular networks resembling in vivo-like architectures. The vascular networks are designed following in vivo images and data, and are comprised of bifurcating, merging, and tortuous vessels. Our model accurately resolves the large deformation and dynamics of each individual RBC, while simultaneously retaining complex geometric details of the vascular architecture. The model is versatile, and can consider networks irrespective of topological/geometrical complexities. Flow can be driven by either pressure or flow boundary conditions of physiological values. Cell distributions naturally develop in the networks from an initial random distribution, eventually resulting in a heterogeneous cell distribution as observed in vivo. Extreme deformation and a wide range of RBC shapes are observed that are in agreement with in vivo observations. Quantitative comparisons were made with in vivo data using hemodynamic quantities such as flow resistance, wall shear stress, cell-free layer, etc., and good agreement was observed. The model can also consider white blood cells and platelets, and receptor-mediated adhesion between vascular wall and cells. The model provides a full 3D quantification of hemodynamic quantities with unprecedented details that are not available in experimental measurements. For example, the model allows full 3D characterizations of the near-wall cell-free layer (CFL) and wall shear stress (WSS) and gradients (WSSG). It predicts that CFL becomes more asymmetric along vessel lengths with increasing vessel tortuosity. While WSS itself decreases from arterial to venous sides, the model predicts that WSS in presence of RBCs increases more than that without RBCs on the venous sides and at capillary bifurcations. This implies that the effect of RBCs on WSS is more enhanced at specific locations in a microvascular network. WSSG is also predicted to be much higher in presence of the RBCs, and increase at capillary bifurcations. The model is used further to study transport and deposition of nanoparticle drug carriers in the microvasculature. Results show that nanoparticle accumulation is highly nonuniform across the vasculature with the highest accumulation occurring in bifurcations and the lowest in venules. Support or Funding Information Supported by the National Science Foundation In silico model of flow of blood cells in a microvascular network. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Dynamic progressive buckling of thin-wall grooved conical tubes under impact loading

In the present study, a new analytical model is employed to investigate thin-wall grooved conical tubes under impact loading. In fact, by considering the groove that made on the wall structure, the effects of groove on the process of crushing and absorption ability have been investigated. Crushing of non-grooved cone has been compared with one grooved and then by considering different numbers of grooves and thickness, and variable gradient for cone’s wall is investigated under the axial impact loading. The number of grooves, thickness and cone’s wall gradient are the main variables in this paper. The results of current new model are compared with the finite element simulation results obtained by the ABAQUS code. A good agreement is observed between the analytical and numerical model results. Finally, absorption ability and more control of the process of crushing in conical energy absorbers are optimised by executing grooves on the cone’s wall. The initial peak load and mean axial crushing load are the main factors of the study, which showed that by increasing tension concentration at grooves, the initial peak load and the amount of absorbed energy will decrease, but the decrease in absorbed energy compared to reduction in initial peak load and control and more uniformity of absorber’s crushing can be ignored.

Effect of varying viscosity on two-fluid model of pulsatile blood flow through porous blood vessels: A comparative study

Present work concerns the pulsatile blood flow of two-fluid model through porous blood vessels under the effect of radially varying viscosity. Blood is modeled as two-phase fluid model consisting a core region by non-Newtonian (Herschel-Bulkley) fluid and a plasma region modeled as Newtonian fluid. No slip condition has been used on wall and pressure gradient is taken as periodic function of time. Up to first order approximate solutions of governing equations are obtained using perturbation approach. A comparative analysis for relative change in flow resistance between our model and previously studied single and two-fluid models without porous layer near wall has also been done. The wall of the blood vessel is composed by a thin Brinkman (porous) layer. The stress jump condition has been imposed on fluid-porous interface. Analytical expressions for the velocity profile, flow rate, wall shear stress and flow resistance have been obtained for different regions and the effect of plasma layer thickness, varying viscosity, yield stress, permeability and viscosity ratio parameter on the flow variables are pictorially discussed. It is perceived that values of flow rate for two-fluid model with porous region near wall is higher in comparison to two-fluid model without porous region near wall. Present study reveals a significant impact of glycocalyx layer on blood flow through blood vessels with a porous layer near wall.

Analysis of Buoyancy-Driven Flow in the Rocom Test Facility

In emergency core cooling (ECC) situations after a loss of coolant accident, cold ECC water is injected into the hot water of the cold leg and downcomer. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses.Numerous experiments were realized in the test facility ROCOM of the Helmholz Zentrum Dresden- Rossendorf, Dresden, Germany to investigate the effects of density differences between the primary loop inventory and the ECC water on the mixing in the downcomer. In 2005, an experiment with 5% flow rate in loop 1 and 10% density difference between ECC and loop water were compared to CFD calculations with Trio_U. In this paper, the same experiment is analyzed here with the TrioCFD code, taking into account 10 years of code and hardware development. The comparison of the LES to the experiment is globally satisfying. However, the calculation shows important turbulent fluctuations overestimating the measurements at the upper and lower downcomer sensors. Besides this, the resistance of the perforated drum seems to be under predicted. For further studies, a key issue seems to be an improved finer meshing of the perforated drum and of the wall near region of the downcomer.

Bioconvective electromagnetic nanofluid transport from a wedge geometry: Simulation of smart electro‐conductive bio‐nanopolymer processing

AbstractA mathematical model is presented for steady, two‐dimensional, stagnation‐point flow, heat, mass, and micro‐organism transfer in a viscous, incompressible, bioconvective, electromagnetic nanofluid along a wedge with Stefan blowing effects, hydrodynamic slip, and multiple convective boundary conditions. Gyrotactic micro‐organisms are present in the nanofluid and bioconvection arises, characterized by micro‐organisms swimming under a competing torque. Similarity transformations are used to render the system of governing partial differential equations into a system of coupled similarity equations. The transformed equations are solved numerically with the BVP5C method. The impact of emerging parameters on dimensionless velocity, temperature, magnetic induction function, nanoparticle volume fraction, and density of motile micro‐organisms is studied graphically. Furthermore, the responses of the local skin friction, local Nusselt number, local Sherwood number, and the wall gradient of density of motile micro‐organism number to variation in these parameters are elaborated. Validation of solutions with previous studies based on special cases of the general model is included. The simulations are relevant to the processing of biological, electro‐conductive nanomaterials and industrial hygienic coating systems exploiting combined electromagnetics, nanosystems, and microscopic, bio‐propulsion mechanisms.

1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion & Power

1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion & Power

Large eddy simulation of turbulent vertical wall fires supplied with gaseous fuel through porous burners

This paper presents a large eddy simulation (LES) study of vertical turbulent wall fires, which aims at bringing fundamental insight into the near-wall flame structure and heat transfer characteristics. The LES simulations are wall-resolved, i.e., the simulations are performed with sufficient grid resolution to capture the wall gradients and do not require a wall heat transfer model. The wall fires considered herein correspond to a simplified configuration in which gaseous fuel is supplied from an array of vertical porous burners, featuring a series of meter-scale wall flames with different fuel mass flow rates and different fuel types. Four fuels (methane, ethane, ethylene and propylene) are studied with prescribed fuel flow rates. Simulations are performed using an LES solver called FireFOAM. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for turbulence modeling. The Eddy Dissipation Concept (EDC) model is used for combustion modeling. The thermal radiation model uses the discrete ordinate method with the simplifying assumption of an optically-thin medium characterized by a fixed radiant fraction. In the fuel blowing region, grid convergence is achieved using a 3 mm computational grid; while in the downstream flame preheating region, a 1.5 mm near-wall grid spacing is required to fully resolve the wall convective heat transfer. The change in grid requirement is due to the presence or absence of fuel blowing which affects the thickness of the wall viscous sub-layer. The simulations are in good qualitative and quantitative agreement with experimental data in the turbulent flame region; in particular: the radiative heat flux increases with elevation and with the fuel mass loss rate, while the convective heat flux remains approximately constant with elevation and decreases with the fuel mass loss rate. The present wall-resolved simulations are considered a first step on the route to developing accurate wall models needed for engineering simulations of wall fires.

The fertile physics of chemical gardens

Many a child has enjoyed watching the gardens grow; many a physicist has puzzled over the transformation of self-organized, nonequilibrium patterns into permanent structures.

Asymptotic accelerated boundary layer over the permeable wall

The paper reports on numerical and analytical investigation of the laminar boundary layer with a favorable pressure gradient over a permeable wall. Researchers have obtained an analytical solution of boundary layer equations for asymptotic flow conditions. This solution allows proposing a relative asymptotic skin-friction function, which determines a degree of influence of flow acceleration and permeable wall on the flow. There are ranges of this function, where effects of permeable wall and streamwise pressure gradient have to be considered only in combination. Numerical simulation has showed that such combined influence of favorable pressure gradient and permeable wall extends the asymptotic flow. The study of a strong gas blowing into the accelerated flow has revealed that favorable pressure gradient impedes separation of the boundary layer. At that asymptotic flow starts from the point, where the separation would occur at gas injection into the zero-pressure gradient flow.

Morphology of submarine canyons along the continental margin of the Potiguar Basin, NE Brazil

New insights into equatorial slope morphology were acquired through analysis of the continental margin of the Potiguar Basin (NE Brazil). In this paper, we present the first full data coverage of the seafloor between the upper and middle continental slopes (100–1300 m) adjacent to the Brazilian equatorial margin, developed using multibeam bathymetric data. Some of the submarine canyons mapped in this study have wall gradients greater than 35°. Wide (∼1700 km) and deep (∼250 m) incisions are present on the continental slope and can be linked to incised valleys that are underfilled or incised only on the outer shelf at depths up to 60 m. Two different types of canyons were identified. Canyons of one type are characterized by heads that indent the shelf edge, association with incised valleys and large fluvial systems, high sinuosity, ‘V’ shapes, and terraces along margins, in addition to erosive features such as landslides and gullies. These characteristics suggest that canyons of this type are associated with the deposition of submarine fan systems, which are considered permeable hydrocarbon reservoirs, on the base of the continental slope. The presence of gullies and sediment waves illustrates the role of bottom currents in the shaping of the slope. The enlargement of the canyons in the study area and the changes in their courses where they cross an important fault suggest that tectonic activity has probably also influenced the morphology of the deep-water environments of the Potiguar Basin. The results of this study constitute initial steps in describing and understanding submarine canyons as part of the equatorial continental Brazilian margin.

Abstract W MP44: Investigations Using a Combination of Computational Fluid Dynamics Technique and an Animal Model of Experimentally Induced Cerebral Aneurysms Suggest Important Roles of Wall Shear Stress on the Cerebral Aneurysm Development

We have a series of reports suggesting that an increase in hemodynamic factors, especially wall shear stress, plays a central role in cerebral aneurysm development. We show here the effect of wall shear stress on the cerebral aneurysm growth, using computational fluid dynamics (CFD) analyses and an animal model of experimentally induced cerebral aneurysms. 3-dimentional CT angiographic images of 34 human cerebral aneurysms were used for CFD analyses. Measured flow rates derived from physiological data of individual patients, but not theoretically assumed rates, were used as inlet boundary conditions because there exists a significant influence of inlet boundary conditions between measured and theoretically assumed values on hemodynamics in human cerebral aneurysms as we reported previously. CFD analyses in human cerebral aneurysms frequently indicated an increase in wall shear stress at the aneurysm orifice. Moreover, if the arterial geometry around the aneurysm is virtually reconstructed just before the aneurysm induction by artificial removal of the aneurysm, the enhanced magnitude of wall shear stress and gradient oscillatory number was observed near the apex of arterial bifurcation where the aneurysm was removed. In animal experiments, we examined the effect of blockage in the P2X4 purinoceptor, the vascular endothelial shear-sensor, on the frequency of aneurysm induction after aneurysm-inducing surgery, using P2X4 knockout mice (n=19) and wild type mice (n=21). The incidence of induced aneurysm formation in P2X4 knockout mice was significantly lower than in wild types (p<0.02). Immunohistochemical examinations indicated that the immunoreactivity of MCP-1, which is closely associated with inflammatory responses during aneurysm formation, was weaker in P2X4 knockout mice than in wild types. These data suggest that during aneurysm formation, vascular endothelial cells might sense excessively high levels of wall shear stress over physiological limit, which may initiate damage to vascular components, leading to aneurysm development. A combination of CFD simulation in the human cerebral aneurysm and an animal model of experimentally induced cerebral aneurysms may be a good tool for the study of cerebral aneurysm formation.

Influence of deflection hole angle on effusion cooling in a real combustion chamber condition

Fluid-solid coupling simulation is conducted to investigate the performance of effusion cooling in the real combustion chamber condition of strong rotation and primary holes. The wall temperature and film cooling effectiveness of different deflection angle is analyzed. From the results, it is concluded that the performance of effusion is better than conventional film cooling. The wall temperature and gradient is lower, the cooling efficiency is higher and the coolant is reduced by 20%, but pressure loss is slightly increased. The cooling effectiveness decreases behind primary holes because of local combustion. Comparison with the effect of deflection angle, the cooling performance of 60 deg deflection angle is best. The coolant is better attached to the wall downstream when the deflection angle is same as the rotating mainstream. In addition, the effect of deflection angle is not so significant on the coolant flow rate, but a large negative impact on the pressure loss. Although the cooling effectiveness of 60 deg deflection angle is highest, the total pressure recovery coefficient is lower. The maximum temperature drops about 70K and the outlet temperature distribution trends more consistent. So various factors should be taken into consideration when designing of deflection angle.

Optimization of functionally graded foam-filled conical tubes under axial impact loading

Metallic foams as a filler in thin-walled structures can improve their crashworthiness characteristics. In this article, nonlinear parametric finite element simulations of FGF foam-filled conical tube are developed and the effect of various design parameters such as density grading, number of grading layers and the total mass of FGF tube on resulting mode shapes, specific energy absorption and initial peak load is investigated. Multi design optimization (MDO) technique and the geometrical average method, both are based on FE model are applied to maximize the specific energy absorption and minimize the impact peak force by estimating the best wall thickness and gradient exponential parameter “m” that controls the variation of foam density. The results obtained from the optimizations indicated that functionally graded foam material, with graded density, is a suitable candidate for enhancing the crashworthiness characteristics of the structure compared to uniform density foam.

동해 울릉분지 남서부 해저지형 및 표층퇴적물 분포

Multi-beam echosounder data and grain size analysis data of surface sediment were acquired and analyzed in order to investigate the shelf-to-slope morphology, geological character, and their geological controlling factors in the southwestern margin of the Ulleung Basin. According to the morphological character, the continental shelf can be divided into two parts: (1) shallow (~100 m) and steep (0.5o ) inner shelf, (2) deep (100-300 m) and gentle (0.2 o ) outer shelf. The continental slope is featured with eight distinct topographic depressions of various spatial dimension (~121 km 2 in area) and head wall gradient (~24.3 o ). They are developed adjacent to each other and presumably formed by submarine landslides which have recurred under the strong influences of earthquakes and eustatic sea-level change. The inner continental shelf and the continental slope are dominated by fine-grained sediment, whereas the outer continental shelf is dominated by coarse-grained sediment. The surface sediment distribution seems dominantly influenced by eustatic sea-level change. The outer continental shelf is mostly covered by coarse relict sediment deposited during lowstand sea-level, while the inner shelf is covered with recent sediment during highstand sea-level. The surface of the continental slope is covered with fine-grained sediments which were supplied by hemipelagic advection process.

Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100m wide, 5–10m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only subtle modification by Holocene processes active during the present sea-level high-stand.

Research on Geogrid Strain of Reinforced Earth Retaining Wall by Numerical Simulation

Numerical simulation was used to study the relationship between reinforced earth retaining wall gradient and geogrid strain. Research results show that when the load on the wall top is small, the function of geogrids in the upper section of retaining wall is not brought to full play, the angle between potential rupture surface and horizontal plane is small and resembles Rankines active soil pressure rupture surface, and it is far from wall face. When the load on wall top increases, potential rupture surface becomes steep, the distance between the surface and wall face is about one third of wall height. Inclined wall face also displays this property. The gentler the reinforced earth retaining wall gradient is, the smaller the geogrid stress is, and the safer the wall is. However, when gradient drops below 1:0.4, safety increase becomes vague. When wall gradient is 1:0.3, geogrid stress is most uniformly distributed, and thus has the highest use efficiency and the wall is the most cost-effective.

Direct numerical simulation of top-down and bottom-up diffusion in the convective boundary layer

AbstractA direct numerical simulation (DNS) of an unstably stratified convective boundary layer with system rotation was performed to study top-down and bottom-up diffusion processes. In order to better understand near-wall dynamics associated with scalar diffusion in the absence of surface roughness, direct simulation is utilized to numerically integrate the governing equations that model the atmospheric boundary layer. The ratio of the inversion height to Obukhov length scale, ${z}_{i} / L= - 49. 1$, indicates moderately strong heating for the case studied. Two passive scalars were initialized in the flow field: the first with a zero gradient at the wall (${q}_{t} $, top-down diffusion), and the second with a non-zero wall gradient and a close-to-zero gradient at the height of the temperature inversion (${q}_{b} $, bottom-up diffusion). Scalar flux, variance and covariance profiles show good agreement between the DNS and rough-wall large-eddy simulation (LES). The top-down gradient function displays a slight increase in amplitude, indicating reduced mixing efficiency for the smooth-wall, low-Reynolds-number convective boundary layer. For the bottom-up process, the gradient matches other rough-wall simulations. The only notable difference between the smooth-wall DNS data and other rough-wall simulations is an increase in the gradient function near the wall. This indicates that the bottom-up gradient functions for a rough wall and a smooth wall are nearly identical except as the viscous sublayer is approached. Finally, a new empirical model for the scalar variance of a bottom-up scalar is proposed: here, a single function replaces two piecewise relationships to accurately capture the DNS results up to the viscous sublayer. The scalar covariance between top-down and bottom-up processes agrees with rough-wall and tree-canopy LES results; this indicates that the scalar covariance is independent of both Reynolds number and surface friction.

Investigating the Influence of Haemodynamic Stimuli on Intracranial Aneurysm Inception

We propose a novel method to reconstruct the hypothetical geometry of the healthy vasculature prior to intracranial aneurysm (IA) formation: a Frenet frame is calculated along the skeletonization of the arterial geometry; upstream and downstream boundaries of the aneurysmal segment are expressed in terms of the local Frenet frame basis vectors; the hypothetical healthy geometry is then reconstructed by propagating a closed curve along the skeleton using the local Frenet frames so that the upstream boundary is smoothly morphed into the downstream boundary. This methodology takes into account the tortuosity of the arterial vasculature and requires minimal user subjectivity. The method is applied to 22 clinical cases depicting IAs. Computational fluid dynamic simulations of the vasculature without IA are performed and the haemodynamic stimuli in the location of IA formation are examined. We observe that locally elevated wall shear stress (WSS) and gradient oscillatory number (GON) are highly correlated (20/22 for WSS and 19/22 for GON) with regions susceptible to sidewall IA formation whilst haemodynamic indices associated with the oscillation of the WSS vectors have much lower correlations.