Showing papers in "European Journal of Mechanics B-fluids in 2012"
TL;DR: In this article, the authors focused on numerical modeling of steady laminar mixed convection flow in single and double-lid square cavities filled with a water-Al2O3 nanofluid.
Abstract: This work is focused on the numerical modeling of steady laminar mixed convection flow in single and double-lid square cavities filled with a water–Al2O3 nanofluid. Two viscosity models are used to approximate nanofluid viscosity, namely, the Brinkman model and the Pak and Cho correlation. The developed equations are given in terms of the stream function–vorticity formulation and are non-dimensionalized and then solved numerically by a second-order accurate finite-volume method. Comparisons with previously published work are performed and found to be in good agreement. A parametric study is conducted and a selective set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles and the Richardson number on the flow and heat transfer characteristics in both cavity configurations and to compare the predictions obtained by the two different nanofluid models. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by increasing the nanoparticle volume fractions at moderate and large Richardson numbers using both nanofluid models for both single- and double-lid cavity configurations. However, for small Richardson number, the Pak and Cho model predicts that the presence of nanoparticle causes reductions in the average Nusselt number in the single-lid cavity configuration.
179 citations
TL;DR: In this paper, a 2D Smoothed Particle Hydrodynamics (SPH) model is used to simulate a broad range of open-channel flows, and an appropriate algorithm is developed to enforce different upstream and downstream flow conditions and simulate uniform, nonuniform and unsteady flows.
Abstract: The present work deals with the development and application of a 2D Smoothed Particle Hydrodynamics (SPH) model to simulate a broad range of open-channel flows. Although in the last decades the SPH modelling has been widely used to simulate free-surface flows, few applications have been performed for free-surface channels. For this reason, an appropriate algorithm is developed to enforce different upstream and downstream flow conditions and simulate uniform, non-uniform and unsteady flows. First, the proposed algorithm is validated for a viscous laminar flow in open channel characterized by Reynolds numbers of order O ( 10 2 ) . The second test case deals with a hydraulic jump for which different upstream and downstream conditions are needed. Varying the Froude number, several types of jumps are investigated with specific focus on the velocity field, pressure forces, water depths and location of the jump. Comparisons between numerical results, theory and experimental data are provided. Finally, the interaction between a flash flood generated by an unsteady inflow condition and a bridge is shown as an example of an engineering application.
148 citations
TL;DR: In this article, a 3D Numerical Wave Tank (NWT) based on a nonlinear model using the High-Order Spectral (HOS) method, which exhibits high level of accuracy as well as efficiency properties provided by a Fast Fourier Transform (FFT) solution.
Abstract: This paper presents the recent development on the nonlinear directional wave generation process in a 3D Numerical Wave Tank (NWT). The NWT is based on a nonlinear model using the High-Order Spectral (HOS) method, which exhibits high level of accuracy as well as efficiency properties provided by a Fast Fourier Transform (FFT) solution. The wavemaker modeling appears to be a key point in the simulation and it is carefully detailed. Different levels of approximation of the wave generation (up to third-order in nonlinearity) are studied. The properties of the numerical scheme in terms of convergence, stability and accuracy are discussed. This NWT features all the characteristics of the real wave tank (directional wavemaker, absorbing zone, perfectly reflective side walls). Furthermore, several validation results and practical applications where numerical simulations are successfully compared to experiments on 2D and 3D wave fields are presented.
117 citations
TL;DR: In this paper, the structural response of a rectangular cantilevered flexible hydrofoil submitted to various flow regimes is analyzed through an original experiment carried out in a hydrodynamic tunnel at a Reynolds number of 0.75 × 10 6.
Abstract: The structural response of a rectangular cantilevered flexible hydrofoil submitted to various flow regimes is analyzed through an original experiment carried out in a hydrodynamic tunnel at a Reynolds number of 0.75 × 10 6 . The experiment considers static and transient regimes. The latter consists of transient pitching motions at low and fast pitching velocities. The experiments are also performed for cavitating flow. The structural response is analyzed through the measurement of the free foil tip section displacement using a high speed video camera and surface velocity vibrations using a laser doppler vibrometer. In non cavitating flows, it is shown that the structural response is linked to the hydrodynamic loading, which is governed by viscous effects such as laminar to turbulent transition induced by Laminar Separation Bubble (LSB), and stall. It is also observed that the foil elastic displacement depends strongly on the pitching velocity. Large overshoots and hysteresis effect are observed as the pitching velocity increases. Cavitation induces a large increase of the vibration level due to hydrodynamic loading unsteadiness and change of modal response for specific frequencies. The experimental results presented in this paper will help to develop high fidelity fluid–structure interaction models in naval applications.
88 citations
TL;DR: Alfredsson et al. as mentioned in this paper proposed a diagnostic plot-based analysis of the streamwise velocity turbulence intensity in wall-bounded turbulent boundary layers, and showed that the deviation from the previously established linear region appears at a given wall distance in viscous units (around 120) for all three canonical flows.
Abstract: The distribution of the streamwise velocity turbulence intensity has recently been discussed in several papers both from the viewpoint of new experimental results as well as attempts to model its behavior. In the present paper numerical and experimental data from zero pressure-gradient turbulent boundary layers, channel and pipe flows over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & Orlu [P.H. Alfredsson, R. Orlu, The diagnostic plot–a litmus test for wall bounded turbulence data, Eur. J. Mech. B Fluids 29 (2010) 403–406]. In the diagnostic plot the local turbulence intensity is plotted as function of the local mean velocity normalized with a reference velocity scale. Alfredsson et al. [P.H. Alfredsson, A. Segalini, R. Orlu, A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the outer peak, Phys. Fluids 23 (2011) 041702] observed that in the outer region of the boundary layer a universal linear decay of the turbulence intensity independent of the Reynolds number exists. This approach has been generalized for channel and pipe flows as well, and it has been found that the deviation from the previously established linear region appears at a given wall distance in viscous units (around 120) for all three canonical flows. Based on these results, new empirical fits for the streamwise velocity turbulence intensity distribution of each canonical flow are proposed. Coupled with a mean streamwise velocity profile description the model provides a composite profile for the streamwise variance profile that agrees nicely with existing numerical and experimental data. Extrapolation of the proposed scaling to high Reynolds numbers predicts the emergence of a second peak of the streamwise variance profile that at even higher Reynolds numbers overtakes the inner one.
70 citations
TL;DR: It is shown that the structure of the mitral vortex ring consists of the main vortex ring and trailing vortex tubes, which originate at the heart wall, and that the vortex ring impinges on the wall and the intraventricular flow transitions to a weak turbulent state.
Abstract: We study the formation of the mitral vortex ring during early diastolic filling in a patient-specific left ventricle using direct numerical simulation. The geometry of the left ventricle is reconstructed from Magnetic Resonance Imaging (MRI). The heart wall motion is modeled by a cell-based activation methodology, which yields physiologic kinematics with heart rate equal to 52 beats per minute. We show that the structure of the mitral vortex ring consists of the main vortex ring and trailing vortex tubes, which originate at the heart wall. The trailing vortex tubes play an important role in exciting twisting circumferential instability modes of the mitral vortex ring. At the end of diastole, the vortex ring impinges on the wall and the intraventricular flow transitions to a weak turbulent state. Our results can be used to help interpret and analyze three-dimensional in-vivo flow measurements obtained with MRI.
67 citations
TL;DR: Several different measures for assessment of cardiac fluid dynamics of heart valves are described using the novel experimental system that is particularly designed and developed for in vitro investigation of intracardiac flow.
Abstract: Abnormality in cardiac fluid dynamics is highly correlated with several heart conditions. This is particularly true in valvular heart diseases and congenital heart defects where changes in flow-field accompany significant variations in chambers’ pressure gradients. Particle Image Velocimetry (PIV) is a convenient technique in assessing cardiac fluid dynamics in vitro. With PIV, it is possible to quantitatively differentiate between normal and abnormal intracardiac flow fields in transparent models of cardiac chambers. Understanding the flow-field inside the heart chambers is challenging due to the fast pace of the flow, three dimensionality of the events, and complex deformability of the heart chambers that highly depends on compliance. Defining standard test-phantoms for particular performance studies ensure accuracy of the tests and reproducibility of the data for implantable devices, regardless of who performs the tests. In this work, we have described several different measures for assessment of cardiac fluid dynamics of heart valves using our novel experimental system that is particularly designed and developed for in vitro investigation of intracardiac flow.
65 citations
TL;DR: Computational modeling is used to study intracardiac flows in normal and diseased left-ventricles, examining characteristic features of these various conditions including vortex dynamics, ‘virtual’ color M-mode cardiography as well as mixing and transport of blood through the left-ventions during the entire cardiac cycle.
Abstract: Computational modeling is used to study intracardiac flows in normal and diseased left-ventricles. The left-ventricle is modeled as a semi-prolate-spheroid, and the wall motion is driven by a prescribed ventricular volume-change that consists of five stages: early (E) filling, diastasis, atrial (A) filling, isovolumetric contraction (ISVC) and systole. Simulations are carried out with a parallelized immersed-boundary flow solver that allows us to simulate this flow on a stationary Cartesian grid. The ventricular flow behavior is analyzed to reveal blood flow patterns during both filling and ejection for normal ventricles, as well as ventricles with diastolic and systolic dysfunctions. Impaired relaxation associated with early-stage diastolic dysfunction is modeled by a reduced E/A ratio, and the systolic dysfunction addressed here is obstructive hypertrophic cardiomyopathy (HOCM), where the thickened ventricular septum in the basal region obstructs the outflow tract. Simulations are also performed to study the effect of septal myectomy on the ventricular flow. We examine the characteristic features of these various conditions including vortex dynamics, ‘virtual’ color M-mode cardiography as well as mixing and transport of blood through the left-ventricle during the entire cardiac cycle.
61 citations
TL;DR: An in vivo assessment of the normal left ventricular flow is presented in order to create a reference ground for the assessment of changes in presence of pathology and improve the understanding of the mechanisms involved in reducing the efficiency of LV pump.
Abstract: The intraventricular fluid dynamics is considered a potential novel indicator of cardiac health. This study present an in vivo assessment of the normal left ventricular flow in order to create a reference ground for the assessment of changes in presence of pathology. The systematic analysis is performed by Echographic PIV technique. Normal patients presented small differences in their overall flow features and parameters. The intraventricular vortex flow features a vortex region extending over most of the ventricular length and smoothly accompanies the flow from the inlet toward the outflow tract. The normal intraventricular pressure gradient features a base-to-apex alignment that properly matches with the left ventricular geometry that is adapt to properly sustain forces directed along its axis. The spatial analysis of cardiac flow provides novel information that integrate tissue deformation analysis and improve the understanding of the mechanisms involved in reducing the efficiency of LV pump.
60 citations
TL;DR: In this paper, Masson et al. construct a BGK operator for gas mixtures starting from the true Navier-Stokes equations, which is the one with transport coefficients given by the hydrodynamic limit of the Boltzmann equation(s).
Abstract: The aim of this article is to construct a BGK operator for gas mixtures starting from the true Navier-Stokes equations. That is the ones with transport coefficients given by the hydrodynamic limit of the Boltzmann equation(s). Here the same hydrodynamic limit is obtained by introducing relaxation coefficients on certain moments of the distribution functions. Next the whole model is set by using entropy minimization under moment constraints as in Brull and Schneider (2008. 2009) [23,24]. In our case the BGK operator allows to recover the exact Fick and Newton laws and satisfy the classical properties of the Boltzmann equations for inert gas mixtures. (C) 2012 Elsevier Masson SAS. All rights reserved.
56 citations
TL;DR: In this paper, a modified version of the GSPH method using the HLLC (Harten Lax and van Leer-Contact) Riemann solver is presented.
Abstract: Free-surface flows are of significant interest in Computational Fluid Dynamics (CFD). However, modelling them especially when the free-surface breaks or impacts on solid walls can be challenging for many CFD techniques. Smoothed Particle Hydrodynamics (SPH) has been reported as a robust and stable method when applied to these problems. In modelling incompressible flows using the SPH method an equation of state with a large sound speed is typically used. This weakly compressible approach (WCSPH) results in a stiff set of equations with a noisy pressure field and stability issues at high Reynolds number. As a remedy, the incompressible SPH (ISPH) technique was introduced, which uses a pressure projection technique to model incompressibility. Although the pressure field calculated by ISPH is smooth, the stability of the technique is still an open discussion. An alternative approach is to use an acoustic Riemann solver and replace the particle velocities and pressures by pressures and velocities determined from a Riemann solver. This technique is equivalent to the Godunov method in Eulerian techniques and so will be called the Godunov SPH method (GSPH). However, since the acoustic Riemann solver is a first order approximation of the Riemann solution, it is highly dissipative and cannot be employed in energetic free-surface flows without modification. In this paper, the GSPH method is modified by using the HLLC (Harten Lax and van Leer-Contact) Riemann solver. The accuracy of the modified GSPH technique is further improved by utilising the MUSCL (Monotone Upstream-centred Schemes for Conservation Laws) scheme with Slope–Limiter. This modified GSPH method along with the WCSPH and ISPH techniques are used to study non-linear sloshing flow. The accuracy, stability and efficiency of the techniques are assessed and the results are compared with experimental data.
TL;DR: In this paper, the authors provided the experimental results of the flow pattern around two-circular piers positioned in side-by-side arrangement, and the results of power-spectra analysis are presented inside and outside the scour hole.
Abstract: The present study provides the experimental results of the flow pattern around two-circular piers positioned in side-by-side arrangement. The experiments were performed for two bed configurations (with and without a scour hole). Velocities were measured by an Acoustic Doppler Velocimeter (ADV). Flat bed and scour hole were frozen by synthetic glue to facilitate the performance of the experiments. The contours and distributions of the time-averaged velocity components, turbulence intensities, turbulence kinetic energy, and Reynolds stresses at different horizontal and vertical planes are presented. Streamlines and velocity vectors obtained from time-averaged velocity fields are used to show further flow features. Bed shear stresses at specific points around the piers are given. The results of power-spectra analysis are presented inside and outside the scour hole. It is shown that the horseshoe vortex is elongated further to the downstream of the gap between the two piers. The flow between the two piers is accelerated into the scour hole so that it influences the vertical and transverse deflections of the flow around and especially between the two piers. The maximum downflow was inside the scour hole near the base of the pier. Between the two piers, the magnitude of downflow and vertical turbulence intensity as well as turbulence kinetic energy are greater than that at the outer sides of the two piers. Bed shear stress has substantially large values between the two piers, as much as two times in comparison to the other sides of the piers. The flow pattern including the contracted flow and interference between the horseshoe vortices play an important role in the creation and formation of the greater scour depth between the two piers. The presence of scour hole changes the behavior of vortex shedding considerably. The present detailed measurements can also be used for the verification of numerical models.
TL;DR: Developing an efficient volume meshing method to obtain accurate results on three-dimensional complex geometries in the shortest time compatible with the computational resources available and quantitatively support the clinical experience that suggests to perform the distal access instead of the proximal one during the PSB approach.
Abstract: Nowadays the provisional side branch (PSB) approach is the preferred coronary bifurcation stenting technique. It is usually concluded by the final kissing balloon (FKB) procedure which consists in the simultaneous expansion of two balloons in both the bifurcation branches. Several kinds of accesses to the side branch (SB) can be used to perform the FKB procedure resulting in different final geometrical configurations of both the artery and the implanted stent and, consequently, altered hemodynamic scenarios. Computational fluid dynamic investigations have been frequently used to study the influence of stent implantation on blood flow. However, due to the complexity of the geometry of stented arteries, the high computational cost required for this kind of simulation has strongly limited their use in both the clinical and the industrial field. Hence, the present study firstly focuses on the development of an efficient volume meshing method, which led us to obtain accurate results on three-dimensional complex geometries in the shortest time compatible with the computational resources available. A hybrid meshing strategy was chosen, using both tetrahedral and hexahedral elements. Then, this discretization method was applied on a stented coronary bifurcation to quantitatively examine the different hemodynamic scenarios provoked by a FKB inflation performed with a proximal or a distal access to the SB. Transient fluid dynamic simulations were performed to analyse both near-wall variables like the wall shear stresses acting on the arterial wall and bulk flow quantities such as velocity magnitude and helicity fields. The results prove that the percentage of area characterised by wall shear stress smaller than 0.5 Pa is lower in the case of the distal access (84.7 % versus 88.0 %). The velocity and helicity contour maps resulted to be better with this type of access, too. In conclusion, fluid dynamic simulations provided a valid tool to quantitatively support the clinical experience that suggests to perform the distal access instead of the proximal one during the PSB approach.
TL;DR: This study validate the technique in vitro using contrast-enhanced imaging and demonstrates that data processing needs to be done with care to avoid biased measurements, especially relevant for wall shear stress measurements.
Abstract: Ultrasound Imaging Velocimetry, or particle image velocimetry applied to ultrasound data, has received an increasing interest in recent years. In particular, it shows great promise to obtain non-invasive, in vivo hemodynamic information. Before the technique can be used in a clinical setting, the limitations and accuracy need to be addressed. In this study we validate the technique in vitro using contrast-enhanced imaging and demonstrate that data processing needs to be done with care to avoid biased measurements. In particular, bias toward integer displacements is much more prominent than in conventional particle image velocimetry. This is especially relevant for wall shear stress measurements, as velocities near the wall will be underestimated. A remedy is provided by using an alternative displacement estimator. Finally, preliminary results for the in vivo measurement in a rabbit abdominal aorta are presented.
TL;DR: In this article, an ensemble average (EA) technique based upon 20 repeated experiments, a variable interval time average (VITA) method based upon a single experiment, and the variable interval-time-averaged method averaged over 20 experimental runs were analyzed.
Abstract: In an open channel, a sudden rise in free-surface elevation is associated with the development of a bore. The bore front is a hydrodynamic shock with a sharp discontinuity in terms of water depth and velocity field. In this study, some turbulent velocity measurements were conducted in breaking bores. The unsteady turbulent properties were analysed using three methods: an ensemble average (EA) technique based upon 20 repeated experiments, a variable interval time average (VITA) method based upon a single experiment, and the variable interval time average (VITA) method averaged over 20 experimental runs. The instantaneous free-surface and velocity measurements showed a marked effect of the bore front passage. The longitudinal velocity components were always characterised by a rapid flow deceleration at all vertical elevations, and some large fluctuations of all velocity components were recorded beneath the surge. The EA and VITA methods showed some comparable long-term trends superposed to some rapid turbulent fluctuations, as well as close results in terms of the turbulent Reynolds stress components. The VITA data based upon a single run presented some differences with the EA median results, but all methods exhibited comparable long-term trends superposed to rapid turbulent fluctuations.
TL;DR: In this article, a fluid-structure interaction method was introduced to define the interaction between the water and the cylindrical shell, and finite element models were built according to the experimental models, and the calculated results were compared with the experimental data.
Abstract: The propagation of the shock wave and the bubble pulse of an underwater explosion and the dynamic response of a cylindrical shell were examined in a water pool. Numerical simulations of the experimental model were performed using the MSC.DYTRAN software, which included a developed subroutine that defined the initial conditions of the fluid field. A fluid-structure interaction method was introduced to define the interaction between the water and the cylindrical shell. The finite element models were built according to the experimental models, and the calculated results were compared with the experimental data. It was found that the artificial bulk viscosity had a significant effect on the peak pressure of the shock wave. The peak pressure of the shock wave, the period of the bubble pulse and the deformation displacement of the cylinder were consistent between the experiment and the finite element analysis. The effects of the length-to-diameter ratio and the angle to the peak pressure of the shock wave for a cylindrical explosive were discussed. Different plastic deformations were measured at different standoff distances, obtaining generalising curves.
TL;DR: In this paper, a ray-tracer is used to calculate the focal position and the measured velocities inside a Taylor?Couette apparatus, where the curvature of the outer cylinder can be altered by curved surfaces.
Abstract: In the present work it will be shown how the curvature of the outer cylinder affects laser Doppler anemometry measurements inside a Taylor?Couette apparatus. The measurement position and the measured velocity are altered by curved surfaces. Conventional methods for curvature correction are not applicable to our setup, and it will be shown how a ray-tracer can be used to solve this complication.
By using a ray-tracer the focal position can be calculated, and the velocity can be corrected. The results of the ray-tracer are verified by measuring an a priori known velocity field, and after applying refractive corrections good agreement with theoretical predictions are found. The methods described in this paper are applied to measure the azimuthal velocity profiles in high Reynolds number Taylor?Couette flow for the case of outer cylinder rotation.
TL;DR: In this article, the effects of the Rayleigh number, the aspect ratio of the annulus, and the volume fraction of the nanoparticles on the fluid flow and heat transfer in two differentially-heated square ducts filled with TiO2-water nanofluid are investigated numerically.
Abstract: The natural convection fluid flow and heat transfer in the annuli of two differentially-heated square ducts filled with the TiO2-water nanofluid are investigated numerically. The outer duct is maintained at a constant temperature T c while the inner duct is kept at a differentially higher constant temperature T h . The governing equations written in terms of the primitive variables are solved using the finite volume method and the SIMPLER algorithm. Through a parametric study conducted, the effects of the Rayleigh number, the aspect ratio of the annulus, and the volume fraction of the nanoparticles on the fluid flow and heat transfer are investigated. To verify the numerical procedure, two different natural convection simulations are conducted using the proposed code, and the results are found to be in good agreement with the existing results already available in the literature. The numerical outcome of the present study shows that, by increasing the width of the gap between the ducts and also the Rayleigh number, multiple eddies are developed in the gap between the top walls of the square ducts. The eddies formed demonstrate the characteristics of the Rayleigh–Benard convective type. Moreover, it is observed from the results that, the average Nusselt number increases by increasing the volume fraction of the nanoparticles.
TL;DR: The new regenerative pump design is considered with a comparable duty centrifugal pump, proving that for many high head low flowrate applications the regenerateative pump is a better choice.
Abstract: Regenerative pumps are low cost, compact and able to deliver high heads at low flowrates Furthermore with stable performance characteristics they can operate with very small NPSH The complexity of the flowfield is a serious challenge for any kind of mathematical modelling This paper compares an analytical and numerical technique of resolving the performance for a new regenerative pump design The performance characteristics computed by a CFD approach and a new one-dimensional model are compared and matched to experimental test results The approaches of both modelling techniques are assessed as potential design tools The approaches are shown to not only successfully resolve the complex flowfield within the pump; the CFD is also capable of resolving local flow properties to conduct further refinements The flow field is represented by the CFD as it has never been before A new design process is suggested The new regenerative pump design is considered with a comparable duty centrifugal pump, proving that for many high head low flowrate applications the regenerative pump is a better choice
TL;DR: In this article, the effects of viscosity differential on buoyancy-induced interpenetration of two immiscible fluids in a tilted channel using a two-phase lattice Boltzmann method implemented on a graphics processing unit are studied.
Abstract: We study the effects of viscosity differential on buoyancy-induced interpenetration of two immiscible fluids in a tilted channel using a two-phase lattice Boltzmann method implemented on a graphics processing unit. The effects of viscosity differential on the flow structures, average density profiles and front velocities are studied. Relatively stable fingers are observed for high viscosity ratios. The intensity of the interfacial instabilities and the transverse interpenetration of the fluids are seen to increase with decreasing viscosity differential of the fluids.
TL;DR: In this article, a model wing based on the geometry of the wing of a barn owl was designed, in which the feather structure of the barn-owl wing is approximated by a velvet-like surface.
Abstract: A model wing based on the geometry of the wing of a barn owl was designed, in which the feather structure of the barn-owl wing is approximated by a velvet-like surface. The first objective of this paper is to investigate the impact of artificial surface filaments on the overall flow field of a quasi-2D configuration of the model 3D wing. Two velvet-like surfaces are used and the velocity field is measured by particle-image velocimetry in a chord-length based Reynolds number range 20 , 000 ≤ R e c ≤ 60 , 000 at angles of attack 0 ° ≤ α ≤ 6 ° . An explanation of the mechanism that leads to the change in the near-wall flow field due to the surface structures is given. The second objective of the paper is the comparison of the 2D and the 3D results and the analysis of the impact of the three-dimensionality on the flow field. The first surface structure (“velvet 1”) mimics the length and density of the hairs and the softness of the natural owl-wing surface. It diminishes the size of the separation bubble or completely prevents separation. However, at three-dimensional flow the effect of the “velvet 1” surface is clearly reduced. The “velvet 2” surface consists of longer and thinner filaments than the “velvet 1” surface. At the lower Reynolds numbers ( R e c ≤ 40 , 000 ), the “velvet 2” surface structure does not alter the near-wall flow field significantly. However, at R e c > 40 , 000 the “velvet 2” surface structure serves as a distributed field of moving roughness elements such that the size of the separation bubble is reduced and becomes nearly independent of the angle of attack. When the three-dimensional flow field at the highest Reynolds number ( R e c = 60 , 000 ) is considered it is evident that the “velvet 2” surface yields the aerodynamically more stable flow field.
TL;DR: In this article, a canonical transformation is applied to a water wave equation to remove cubic nonlinear terms and to simplify fourth-order terms in the Hamiltonian, which is very suitable for analytical studies and numerical simulations.
Abstract: We apply a canonical transformation to a water wave equation to remove cubic nonlinear terms and to drastically simplify fourth-order terms in the Hamiltonian. This transformation explicitly uses the vanishing exact four-wave interaction for water gravity waves for a 2D potential fluid. After transformation, the well-known but cumbersome Zakharov equation is drastically simplified and can be written in X -space in a compact form. This new equation is very suitable for analytical studies and numerical simulations.
TL;DR: It is found that stress-based models predict higher levels of blood damage than the strain-based one, indicating that the adopted mechanical valve is primary risk factor for hemolysis.
Abstract: The paper reports the prediction of mechanical hemolysis by three different models for the case of blood flow through aortic valved prostheses. Two of the adopted models are based on the action of instantaneous shear stress on the blood cells (stress-based), while the third accounts for the finite response time of cell deformation and relaxation (strain-based). Two aortic Dacron grafts commonly adopted in clinical practice are considered, and both are equipped with a bileaflet mechanical valve. One of the grafts reproduces the three sinuses of Valsalva, while the other is a straight tube. A direct numerical simulation approach is utilized to solve the complex fluid-structure-interaction problem and obtain detailed information of the flow patterns. To evaluate hemolysis, a large number of Lagrangian tracer particles were released at the inlet of the computational domain (upstream of the valve), and blood damage was evaluated along each trajectory for each model. We found that stress-based models predict higher levels of blood damage than the strain-based one. The same level of blood damage is observed in the two geometric configurations we considered, indicating that the adopted mechanical valve is primary risk factor for hemolysis.
TL;DR: In this article, the effect of the vessel elasticity on the flow is analyzed by the velocity distribution, the vessel dilatation, and the static pressure inside an elastic, axisymmetric stenosed, transparent vessel at pulsatile flow in a range of Reynolds numbers 310 ≤ R e D ≤ 690 and Womersley numbers 7.5 ≤ W o ≤ 13.
Abstract: Stenosis strongly alters the flow field inside a blood vessel not only through geometric changes but also due to changing the mechanical properties of the vessel. Since the flow field has via, e.g., the oscillating wall-shear stress (WSS), a significant effect on the pathogenesis of atherosclerosis a better understanding of the unsteady velocity field and the fluid-structure interaction is necessary. Time-resolved particle-image velocimetry (TRPIV) is combined with simultaneous measurements of the static pressure inside an elastic, axisymmetric stenosed, transparent vessel at pulsatile flow in a range of Reynolds numbers 310 ≤ R e D ≤ 690 and Womersley numbers 7.5 ≤ W o ≤ 13 . The location of the elastic wall and the near-wall velocity distribution is highly resolved to determine the unsteady WSS. The effect of the vessel elasticity on the flow is analyzed by the velocity distribution, the vessel dilatation, and the static pressure. At dilatations of the vessel model up to 5% the measurement quantities exhibit phase lags and deviations from the temporal bulk flow distribution which depend on the exciting frequency. Below the eigenfrequency local dilatation and static pressure are in phase. Above the eigenfrequency a higher mode oscillation superimposed onto the bulk flow in the throat region of the stenosis exits which is caused by a phase shift in the dilatation on both sides of the stenosis. The TRPIV measurements show the stenosis to induce a jet through the throat together with complex flow structures like ring vortices. The unsteady footprints of these structures are evidenced by oscillating WSS distributions which also aggravate stenosis.
TL;DR: In this article, a series of experiments were conducted to quantify and compare the shear stress distribution in the bottom boundary layer (BBL) of saline and particle-laden gravity currents. And the bulk drag coefficients were defined for both flows using three methods, (i) a log-fit method based on the law of the wall, (ii) the observed maximum total stress and (iii) direct measurements of turbulent velocities.
Abstract: The internal stress distribution within weakly depositional turbidity currents has often been assumed to be similar to saline gravity currents. This assumption is investigated by analyzing a series of experiments to quantify and compare the shear stress distribution in the bottom boundary layer (BBL) of saline and particle-laden gravity currents. Vertical profiles of Reynolds stresses, viscous stresses and turbulent kinetic energy (TKE) were obtained from the mean downstream velocity profiles and turbulent velocity fluctuations, and were broadly similar in both flow types, suggesting that saline gravity currents are a good analogue to turbidity currents. Maximum positive Reynolds stresses occur where the velocity gradient is largest in the BBL but below this maximum, the Reynolds stresses decrease significantly and are balanced by an increase of viscous stresses. The bulk drag coefficients C D is defined for both flows using three methods, (i) a log-fit method based on the law of the wall, (ii) the observed maximum total stress and (iii) direct measurements of turbulent velocities. The C D values of both flow types were broadly similar but each method led to C D values of different orders of magnitude. The log-fit method yielded the largest drag coefficients of O ( 1 0 − 2 ) whereas measurements of turbulent velocities gave relatively small values of O ( 1 0 − 4 ) . The best correlation with drag coefficients observed in field measurements of O ( 1 0 − 3 ) was obtained by using the maximum total stresses next to the wall. The variation of C D is discussed in relation to parameterization methods in experimental and numerical modeling.
TL;DR: In this paper, the authors used Lagrangian Coherent Structure time evolution and flow dispersion properties to verify the asymmetric structure of the ventricular filling in two orthogonal views.
Abstract: A laboratory model reproducing the main characteristics of the heart pump was used to investigate the flow in a ventricle model during a cardiac cycle. Velocity fields were measured both in Lagrangian and Eulerian frames of reference by using image analysis. In particular, by imaging the flow field from two orthogonal views, we were able to verify the asymmetric structure of the ventricular filling. The flow features were investigated considering Lagrangian based methodologies: Finite Time and Finite Size Lyapunov Exponents were evaluated to assess Lagrangian Coherent Structure time evolution and flow dispersion properties, respectively. These quantities have proven to be powerful tools to elucidate the complex dynamics associated with the intraventricular flow.
TL;DR: The results confirm that the presence of stenosis enhances the risk for flow-induced activation of platelets, and that helical flow is instrumental in moderating the burden of shear-induced activated platelets in stenosed carotid bifurcations.
Abstract: Vascular pathologies responsible for the narrowing (stenosis) of arterial lumen are a major healthcare problem in the Western world. The presence of stenosis, by further altering the yet disturbed hemodynamics within the vessel, could lead to the establishment of varying and abnormal shear stress levels which may induce activation and aggregation of platelets, thus enhancing the development of the pathology and increasing the risk for thromboembolic complications. In this study, we present a comprehensive analysis of the local hemodynamics within an image-based model of a 51% stenosed internal carotid artery, focused on the influence of the disturbed flow caused by the stenosis on transport and flow-induced activation of platelets. The flow field was resolved using computational fluid dynamics, and the flow-induced level of activation of transported platelets was predicted by adopting a consolidated Lagrangian-based blood damage model that takes into account the cumulative effect of the shear stress level and the time of exposure to it. Moreover, by adopting a Lagrangian-based bulk flow-descriptor, we investigated the influence that helical flow dynamics within the bifurcation has on platelet activation in order to assess whether a relationship exists between bulk flow structures and platelet activation levels. The results confirm that the presence of stenosis enhances the risk for flow-induced activation of platelets, and that helical flow is instrumental in moderating the burden of shear-induced activation of platelets in stenosed carotid bifurcations.
TL;DR: In this paper, the dispersion relation for linearized small-amplitude gravity waves for various choices of non-constant vorticity was derived, including polynomial, exponential, trigonometric and hyperbolic functions.
Abstract: We derive the dispersion relation for linearized small-amplitude gravity waves for various choices of non-constant vorticity. To the best of our knowledge, this relation is only known explicitly in the case of constant vorticity. We provide a wide range of examples including polynomial, exponential, trigonometric and hyperbolic vorticity functions.
TL;DR: Results showed that the BCPA model is able to realistically capture oscillations due to both cardiac and respiratory effects, when compared to the venous Doppler velocity tracings acquired preoperatively on the patient.
Abstract: Single ventricle malformations are complex congenital heart defects which require a three-stage surgical treatment, starting from the very first days of life, to separate the systemic and pulmonary circulations, and restore the serial circuit occurring in normal patients. The final surgery results in a total cavopulmonary connection (TCPC), where both the superior and the inferior vena cava are connected to the right pulmonary artery. Several clinical and computational studies have been done to optimize the geometry of the TCPC, with the aim of minimizing energy losses and improving surgical outcomes. To date, only few modeling studies have taken into account respiration and exercise as important factors to quantify the performance of a Fontan geometry. The objective of this work is to test the dependence of fluid dynamic variables and energy efficiency on respiration in patient-specific models of Fontan circulation, when subjected to exercise tests.|A closed-loop multiscale approach was used, including a simple respiration model that modulates the extravascular pressures in the thoracic and abdominal cavities, to generate physiologic time-varying flow conditions. A lumped parameter network (LPN) representing the whole circulation was coupled to a patient-specific 3D finite volume model of the preoperative bidirectional cavo-pulmonary anastomosis (BCPA) with detailed pulmonary anatomy. Subsequently, three virtual TCPC alternatives were coupled to the LPN and investigated in terms of both local and global hemodynamics. In particular, a T-junction of the venae cavae to the pulmonary arteries, a design with an offset between the venae cavae and a Y-graft design were compared under exercise conditions.|Results showed that the BCPA model is able to realistically capture oscillations due to both cardiac and respiratory effects, when compared to the venous Doppler velocity tracings acquired preoperatively on the patient.|The differences in hemodynamics between the three investigated TCPC options were minimal and similar to those obtained without inclusion of respiratory effects. Hence, the three surgical options result to be equivalent according to the analyzed parameters. Moreover, although the simulation of the Fontan circulation with a respiratory model requires a longer computational time, the developed framework allows for a more physiologic method to incorporate respiratory effects that was not possible using other methods.
TL;DR: In this article, the velocity and vorticity fields are determined experimentally using PIV technique for different resolutions in order to study the global flow around the plate and the formation and advection of vortices upstream and downstream of the plate.
Abstract: This paper presents the vortex dynamics generated by the interaction of a submerged horizontal plate, considered as a vortex generator, and a monochromatic wave The velocity and vorticity fields are determined experimentally using PIV technique for different resolutions in order to study the global flow around the plate and the formation and advection of vortices upstream and downstream of the plate The global flow around the plate shows great discrepancies with the potential flow solution: two recirculation cells are formed beneath the plate, the global flow is non-symmetric and the advection of vortices induces strong velocities not represented by the potential flow theory The formation of vortices at the edges of the plate is characterised At each period, one vortex is formed at the edge followed by the formation of an opposite sign vortex The upstream and downstream vortex pairs are then advected in front of the plate and toward the bottom respectively, over a distance of about one third the plate length The lifetime of vortices is about two wave periods This study will help us validate a numerical software to be used for analysing the influence of various parameters on the dynamics These results will be presented in the second part of this paper