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The reactor coolant pump (RCP) rotor seizure accident is defined as a short-time seizure of the RCP rotor. This event typically leads to an abrupt flow decrease in the corresponding loop and an ensuing reactor and turbine trip. The significant reduction of core coolant flow while the reactor is being operated at full load can have very negative consequences. This potentially dangerous event is typically characterized by a complex transient behavior in terms of flow conditions and energy transformation, which need to be analyzed and understood. This study constructed transient flow and rotational speed mathematical models under various degrees of rotor seizure using the test data collected from a dedicated transient rotor seizure test system. Then, bidirectional fluid-solid coupling simulations were conducted to investigate the flow evolution mechanism. It is found that the influence of the impeller structure size and transient braking acceleration on the unsteady head (Hu) is dominant in rotor seizure accident events. Moreover, the present results also show that the rotational acceleration additional head (Hu1) is much higher than the instantaneous head (Hu2). [ABSTRACT FROM AUTHOR]

This study explores the implementation of computed tomography (CT) reconstruction and simulation techniques for patient-specific valves, aiming to dissect the mechanical attributes of calcified valves within transcatheter heart valve replacement (TAVR) procedures. In order to facilitate this exploration, it derives pertinent formulas for 3D multi-material isogeometric hyperelastic analysis based on Hounsfield unit (HU) values, thereby unlocking foundational capabilities for isogeometric analysis in calcified aortic valves. A series of uniaxial and biaxial tensile tests is executed to obtain an accurate constitutive model for calcified active valves. To mitigate discretization errors, methodologies for reconstructing volumetric parametric models, integrating both geometric and material attributes, are introduced. Applying these analytical formulas, constitutive models, and precise analytical models to isogeometric analyses of calcified valves, the research ascertains their close alignment with experimental results through the close fit in displacement-stress curves, compellingly validating the accuracy and reliability of the method. This study presents a step-by-step approach to analyzing themechanical characteristics of patient-specific valves obtained fromCT images, holding significant clinical implications and assisting in the selection of treatment strategies and surgical intervention approaches in TAVR procedures. [ABSTRACT FROM AUTHOR]

This computational study concerns approximate solutions of singularly perturbed one-dimensional boundary-value problems having perturbed convection and diffusion terms. Such kinds of problems take different stands depending on the perturbation parameters. Typically, when the problem is convection-dominated, classical discretization methods suffer from numerical instability issues. Therefore, standard methods require special treatment in convection dominance. To this end, in this work, the standard Galerkin finite element method (GFEM) is stabilized with the streamlineupwind/Petrov-Galerkin (SUPG) formulation. Beyond that, an asymptotic approach, called the successive complementary expansion method (SCEM), is also proposed. Two test examples are provided to evaluate and compare the proposed methods' performances for various values of the convection and diffusion parameters. [ABSTRACT FROM AUTHOR]

The inflation process of a special parachute with slots and a fixed payload is investigated using a multimaterial arbitrary Lagrangian-Euler-coupled numerical approach, which considers fluid-structure interaction within the LS-DYNA nonlinear analysis code. The transient dynamic solver is set up using a Lagrangian-Euler penalty method, and a numerical simulation of the slot parachute design that considers permeability is performed. The inflation characteristics of a slot parachute at different initial velocities are analyzed. The threedimensional simulation results for the inflation are validated by comparing with the airdrop test results. Finally, the incompressible fluid dynamics and the evolution of vortexes during the opening process are analyzed. The results demonstrate the following. This slot parachute can rapidly attain a steady state after fully inflating without any obvious canopy breathing. The stress distribution near the slots is obviously higher than the average level across the canopy surface. A symmetrically counterrotated vortex couple appears at the top of the parachute, which then extrudes to asymmetry until the couple separates and is brushed off by the airflow. [ABSTRACT FROM AUTHOR]

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In the present work, important aspects of time-dependent nonlinear 3D finite element (FE) models for deep tunnel advance by the New Austrian Tunneling Method (NATM), characterized by repeated sequences of excavation, securing, and idle periods, are discussed on the example of a 3D finite element model of a stretch of the Brenner Base Tunnel, which is currently constructed between Austria and Italy. Nonlinear material models are utilized for representing the surrounding rock mass and the shotcrete shell. Based on the finite element model, strategies for the efficient implementation into a parallel distributed memory numerical code are proposed. They are essential to achieve reasonable computation times for numerical simulations of tunneling based on large 3D FE models. In particular, the implementation of the construction procedure, parallel computing and communication specific details, and efficient linear solvers for the global equation system within the incremental-iterative Newton-Raphson scheme are addressed. Furthermore, possible extensions of the material models for rock mass and shotcrete, used in the 3D FE model, are presented. They concern (i) a gradient-enhanced model for transversely isotropic rock and rock mass, taking into account hardening and softening behavior and (ii) the extension of the shotcrete model to nonlinear creep and damage due to creep. The possible benefits of the model extensions in numerical simulations of tunneling by the NATM are discussed. [ABSTRACT FROM AUTHOR]

The article presents a study of a two-fluid turbulence model in the Comsol Multiphysics software package for the problem of a subsonic flow around the DSMA661 and NACA 4412 airfoils with angles of attack of 0 and 13.87 degrees, respectively. In this paper, the finite element method is used for the numerical implementation of the turbulence equations. To stabilize the discretized equations, stabilization by the Galerkin least squares method was used. The results obtained are compared with the results of other RANS, LES, DES models and experimental data. It is shown that in the case of continuous flow around the DSMA661 airfoil, the results of the two-fluid model are very close to the SST results and are in good agreement with the experimental data. When flowing around the NACA 4412 airfoil, flow separation occurs and a recirculation zone appears. It is shown that in such cases the two-fluid model gives more accurate results than other turbulence models. Implementation of the Comsol Multiphysics software package showed good convergence, stability, and high accuracy of the two-fluid turbulence model. [ABSTRACT FROM AUTHOR]

In this work, transient free surface flows of a viscous incompressible fluid are numerically solved through parallel computation. Transient free surface flows are boundary-value problems of the moving type that involve geometrical nonlinearities, hi contrast to more conventional computational fluid dynamics problems, the computational flow domain is partially bounded by a free surface which is not known a priori, since its shape must be computed as part of the solution. In steady flow the free surface is obtained by an iterative process, but when the free surface evolves with time the problem is more difficult as it generates large distortions in the computational flow domain. The incompressible Navier-Stokes numerical solver is based on the finite element method with equal order elements for pressure and velocity (linear elements), and it uses a streamline upwind/ Petrov-Galerkin (SUPG) scheme (Hughes, T. J. R., and Brooks, A. N., 1979, "A Multidimensional Upwind Scheme With no Crosswind Diffusion," in Finite Element Methods for Convection Dominated Flows, ASME ed., 34. AMD, New York, pp. 19-35, and Brooks, A. N., and Hughes, T J. R., 1982, "Streamline Upwind/Petrov--Galerkin Formulations for Convection Dominated Flows With Particular Emphasis on the Incompressible Navier-Stokes Equations," Comput. Methods Appl. Mech. Eng., 32, pp. 199-259) combined with a Pressure-Stabilizing/Petrov-Galerkin (PSPG) one (Tezduyar, T. E., 1992, "Stablized Finite Element Formulations for Incompressible Flow Computations," Adv. Appl. Mech., 28, pp. 1-44, and Tezduyar, T. E., Mittal, S., Ray, S. E., and Shih, R., 1992, "Incompressible Flow Computations With Stabilized Bilinear and Lihear Equal Order Interpolation Velocity-Pressure Elements," Comput. Methods Appl. Mech. Eng., 95, pp. 221-242). At each time step, the fluid equations are solved with constant pressure and null viscous traction conditions at the free surface and the velocities obtained in this way are used for updating the positions of the surface nodes. Then, a pseudo elastic problem is solved in the fluid domain in order to relocate the interior nodes so as to keep mesh distortion controlled. This has been implemented in the PETSc-FEM code (PETSc-FEM: a general purpose, parallel, multi-physics FEM program. GNU general public license (GPL), http://www.cimec.org.ar/petscfem) by running two parallel instances of the code and exchanging information between them. Some numerical examples are presented. [ABSTRACT FROM AUTHOR]

Stroke remains a global health concern, necessitating early prediction for effective management. Atherosclerosis-induced internal carotid and intra cranial stenosis contributes significantly to stroke risk. This study explores the relationship between blood pressure and stroke prediction, focusing on internal carotid artery (ICA) branches: middle cerebral artery (MCA), anterior cerebral artery (ACA), and their role in hemodynamics. Computational fluid dynamics (CFD) informed by the Windkessel model were employed to simulate patient-specific ICA models with introduced stenosis. Central to our investigation is the impact of stenosis on blood pressure, flow velocity, and flow rate across these branches, incorporating Fractional Flow Reserve (FFR) analysis. Results highlight differential sensitivities to blood pressure variations, with M1 branch showing high sensitivity, ACA moderate, and M2 minimal. Comparing blood pressure fluctuations between ICA and MCA revealed heightened sensitivity to potential reverse flow compared to ICA and ACA comparisons, emphasizing MCA's role. Blood flow adjustments due to stenosis demonstrated intricate compensatory mechanisms. FFR emerged as a robust predictor of stenosis severity, particularly in the M2 branch. In conclusion, this study provides comprehensive insights into hemodynamic complexities within major intracranial arteries, elucidating the significance of blood pressure variations, flow attributes, and FFR in stenosis contexts. Subject-specific data integration enhances model reliability, aiding stroke risk assessment and advancing cerebrovascular disease understanding. [ABSTRACT FROM AUTHOR]

Parachute design is challenging to achieve innovative progress if the dominant role of testing continues, as it will be an increasingly expensive and time-consuming work. The aim of this study is to establish a reliable and efficient design tool using existing advanced numerical modeling methods. This paper presents a numerical method based on two-way coupled fluid-structure interaction (FSI) strategies for predicting aerodynamic and flight performance for parafoil design optimization. The nonlinear finite element method was used for the canopy fabric model and flow field, and the fluid dynamics were solved by Reynolds-averaged Navier-Stokes with the Spalart-Allmaras turbulence model. The FSI simulations are performed to assess the aerodynamic performance and structural deformations of full-scale parafoil canopies. The equilibrium shape of the parafoil canopy under steady gliding states and the relevant flow field were analyzed to enhance confidence and understanding in the performance prediction of new parachutes. Three-dimensional FSI simulation results of parafoils show that the inflation caused flexible bulges of canopy cells, and the maximum lift coefficient increased more than 16% with a higher stall angle of attack than that of the rigid body model. A parafoil with a smaller leading edge inlet or a scaling down area can improve the aerodynamic performance, mainly manifested in a higher lift-to-drag ratio and better anti-stall performance. Finally, the prediction results of parafoil glide performance were verified by flight test data, and the prediction accuracy of the flexible model is more than 10% higher than that of the rigid model. This work makes the simulation tools a step closer to practical application. [ABSTRACT FROM AUTHOR]

This study endeavours to introduce an inventive hierarchical coupling methodology for evaluating the flight capabilities of polymer micromachined flapping-wing nano air vehicles (FWNAVs) using advanced computational tools. Furthermore, our objective is to provide a practical demonstration of FWNAVs in both tethered and airborne scenarios. These dual objectives represent the distinctive and pioneering aspects of this research. Such FWNAVs, which are insect-inspired robots, have a 2.5-dimensional structure. An FWNAV comprises a micro piezoelectric drive system and a pair of micro thin flexible wings. The drive system includes a micro transmission and a piezoelectric bimorph actuator. The flight performance of the designed FWNAV is evaluated using a hierarchical coupling approach, where the whole system is decomposed into a micro wing and a micro piezoelectric drive system. The coupling between these parts is modelled as one-way coupling and that between the micro wings and the surrounding air is modelled as strong coupling. In the one-way coupling analysis, nonlinear structural dynamic analysis is conducted for the micro piezoelectric drive system, where the dynamic response is transmitted to the micro wings via the Dirichlet boundary condition. In the strong coupling analysis, strongly coupled fluid-structure interaction analysis is conducted for the micro wings and the surrounding air to consider their strong coupling. The optimisation of flight performance is conducted using fluid-structure interaction analysis to achieve sufficient lift force to support the weight of an FWNAV. The design of a tethered and flyable FWNAV with a size of 10mm or smaller is demonstrated. This FWNAV can be easily fabricated using polymer micromachining. [ABSTRACT FROM AUTHOR]

Laminar to transitional wakes occur in slow, quasi-steady flows past cylinders at low cylinder Reynolds numbers (R ed ≤ 250). Inviscid numerical solvers of the depth-averaged shallow water equations (SWE) introduce numerical dissipation that, depending on Red , may imitate the mechanisms of viscous turbulent models. However, the numerical dissipation rate in a second-order finite volume (FV2) SWE solver is so large at a practical resolution that this can instead hide these mechanisms. The extra numerical complexity of the second-order discontinuous Galerkin (DG2) SWE solver results in a lower dissipation rate, making it a potential alternative to the FV2 solver to reproduce cylinder wakes. This paper compares the DG2 and FV2 solvers, initially for wake formation behind one cylinder. The findings confirm that DG2 can reproduce the expected wake formations, which FV2 fails to capture, even at a 10-fold finer resolution. It is further demonstrated that DG2 is capable of reproducing key features of the flow fields observed in a laboratory random cylinder array. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the effect of magnetic field on two-dimensional flow of a viscous, incompressible fluid through composite porous channel using non-primitive boundary element method (BEM). We consider a rectangular channel consisting of two packings that are filled with fully saturated porous medium. It is assumed that both the porous regions are homogenous and isotropic with different permeabilities. Brinkman equation governs the fluid flow through porous media. We analyze the effect of Hartman number, stress-jump coefficient, Darcy number, thickness parameter, electrical conductivity ratio, and viscosity ratio on fluid mechanics. We present the effect of stress-jump coefficients on the interfacial velocity of the fluid against the thickness parameter and observe that the interfacial velocity increases with increasing stress-jump coefficients. We notice that for a fixed value of thickness parameter, the magnitude of vorticity (at lower and upper walls) increases with increasing Darcy number. Moreover, we observe that the magnitude of vorticity at the lower wall decreases and increases at the upper wall with increasing thickness parameter. We compute the Brinkman layer thickness near the interface of the composite porous channel in terms of several flow parameters and observe that the Brinkman layer thickness is strongly depend on the Hartman number, Darcy number, viscosity ratios, and stress-jump coefficient, respectively. [ABSTRACT FROM AUTHOR]

This paper analyzes the stability and convergence rate of a six-level time-split Leap-frog/ Crank–Nicolson method in the approximate solutions of two-dimensional nonlinear time-dependent convection-diffusion-reaction equations subjects to appropriate initial and boundary conditions. The computational time of the proposed algorithm is greatly improved thanks to the form of the splitting. Under a suitable time-step restriction, both theoretical and numerical results provided by the new approach are deeply analyzed in L m (0 , T ; L 2) -norm (m = 1 , 2 , ∞). A broad range of numerical examples suggest that the considered model is fast, temporal second-order accurate and spatial fourth-order convergent. This shows the utility and efficiency of the new formulation. [ABSTRACT FROM AUTHOR]

Oscillatory flows past two identical circular cylinders are investigated by two-dimensional direct numerical simulations in the parameter space of gap ratio (0.5 ≤ G ≤ 4.0), angle of flow incidence (0° ≤ α ≤ 90°) and Keulegan–Carpenter number (4 ≤ KC ≤ 12) with a constant Reynolds number Re = 150, where G = L/D, KC = UmT/D and Re = UmD/υ with D being the dimeter of the identical cylinders, L the shortest surface-to-surface distance between the two cylinders, Um and T being the velocity amplitude and period of the sinusoidal oscillatory flow, respectively, and α is defined as the angle between the flow direction to the line connecting the centers of the two cylinders. Comparing with the tandem or side-by-side arrangements of twin circular cylinders in oscillatory flows, the staggered twin cylinders (0° < α < 90°) involve more diverse flow regimes, including the periodic, quasi-periodic and chaotic flow states, due to the inherent asymmetric flow interference around the cylinder pair. In addition to introducing four flow regimes for the tandem and side-by-side arrangements, this study newly identifies 11 flow regimes for the staggered twin cylinders. The newly reported flow regimes in this work are collaboratively identified through the flow visualizations, steady streaming, frequency spectra of fluid forces and Lissajous phase diagrams, as well as the temporal-spatial symmetry features of the wake flows. Connecting with the previous work by Zhao and Cheng ["Two-dimensional numerical study of vortex shedding regimes of oscillatory flow past two circular cylinders in side-by-side and tandem arrangements at low Reynolds numbers," J. Fluid Mech. 751, 1–37 (2014)], this study presents overall regime maps in the KC-α plane for varied gap ratios. It is found that the flow regimes previously and presently identified for the tandem and side-by-side arrangements may also appear for the staggered twin cylinders. The present numerical results suggest the sensitive dependence of the flow regimes on the parameters of KC, α, and G. It is also found that a specific flow regime with narrow parameter bands may appear within another flow regime, forming the abnormal regime hole in the regime map. To understand the profound influence of the control parameters on the flow regime transition, and the relevant temporal-spatial symmetry breaking, the linear Floquet stability analysis is conducted in this work. It was confirmed that the variation of the KC number may result in the Ky symmetry breaking over several periodic flow regimes, while the change of the angle of flow incidence may account for the H2 symmetry covering various periodic and quasi-periodic flow regimes. The stability analysis also explains the temporal flow transition and the abnormal occurrence of the regime holes within either quasi-periodic or chaotic flow regimes. [ABSTRACT FROM AUTHOR]

The fluid-structure interaction (FSI) between the canopy and flow field on the inflating and inflated conditions is investigated based on the arbitrary Lagrange-Euler (ALE) method, in both a single-and double-cruciform parachute systems. The projection area of canopy is calculated in the inflation process. The flow field characteristics and the interaction between canopies are analyzed. Results showed that, with free stream velocity of 50m/s, overinflation phenomenon would not occur during the inflation process of the double-cruciform-parachute system, because the collision and extrusion of the two canopies during inflation obstructed the oscillation of the inner gores. Concurrently, compared with the single-cruciform parachute, the vortex motion in the wake of double-cruciform-parachute is more intense. Thus the double-cruciform parachute system oscillated at a velocity of 50 m/s with an angle of less than 6. 8°. By comparison, the oscillation angle of the single-cruciform parachute was within 3. 5° at the velocity of 50m/s. The results are consistent with those of the wind tunnel test. [ABSTRACT FROM AUTHOR]

In this work, a new global-in-time solution strategy for incompressible flow problems is presented, which highly exploits the pressure Schur complement (PSC) approach for the construction of a space–time multigrid algorithm. For linear problems like the incompressible Stokes equations discretized in space using an inf-sup-stable finite element pair, the fundamental idea is to block the linear systems of equations associated with individual time steps into a single all-at-once saddle point problem for all velocity and pressure unknowns. Then the pressure Schur complement can be used to eliminate the velocity fields and set up an implicitly defined linear system for all pressure variables only. This algebraic manipulation allows the construction of parallel-in-time preconditioners for the corresponding all-at-once Picard iteration by extending frequently used sequential PSC preconditioners in a straightforward manner. For the construction of efficient solution strategies, the so defined preconditioners are employed in a GMRES method and then embedded as a smoother into a space–time multigrid algorithm, where the computational complexity of the coarse grid problem highly depends on the coarsening strategy in space and/or time. While commonly used finite element intergrid transfer operators are used in space, tailor-made prolongation and restriction matrices in time are required due to a special treatment of the pressure variable in the underlying time discretization. The so defined all-at-once multigrid solver is extended to the solution of the nonlinear Navier–Stokes equations by using Newton's method for linearization of the global-in-time problem. In summary, the presented multigrid solution strategy only requires the efficient solution of time-dependent linear convection–diffusion–reaction equations and several independent Poisson-like problems. In order to demonstrate the potential of the proposed solution strategy for viscous fluid simulations with large time intervals, the convergence behavior is examined for various linear and nonlinear test cases. [ABSTRACT FROM AUTHOR]

The use of flexible materials for primary mover and power takeoff of wave energy converters (WECs) has attracted considerable attention in recent years, owing to their potential to enhance the reliability, survivability, and wave energy conversion efficiency. Although some reduced order models have been used to study the fluid–structure interaction (FSI) responses of flexible wave energy converters (fWECs), they are somehow inappropriate due to their limited accuracy and applicability span. To gain a deeper understanding of the physical mechanisms in fWECs, a high-fidelity approach is required. In this work, we build up a fluid–structure interaction analysis framework based on computational fluid dynamics and a finite element analysis method. The incompressible viscous flow is resolved by solving three-dimensional unsteady Navier–Stokes equations with a finite volume approach. The structure dynamics are solved by a finite element method, taking the nonlinear behavior of flexible material into consideration. A strong coupling strategy is utilized to enhance the numerical stability and convergence of the iterative process. We demonstrate the present FSI tool is able to provide rich flow field information and structural response details, such as the velocity, pressure, and structural stress distribution. This is illustrated through several case studies, including two types of fWECs. The unsteady wave–structure-interaction and the associated nonlinear phenomena are also accurately captured by this tool. [ABSTRACT FROM AUTHOR]

In this paper, a numerical estimation of wall shear stress (WSS) and study on hemodynamic pattern in carotid artery (CA) near the bifurcation region with dynamic growth of aneurysm using computational fluid dynamics (CFD) for making early clinical decision is presented. Aneurysm in the carotid artery affects the blood supply to brain and if it is untreated at early stage may lead to sudden death. Computed tomography images of four different cases of stroke subjects scanned of 600 slices with 1 mm resolution with neighbouring layers from neck to head are considered for this study. Numerically a CA with bifurcation region is developed from these images and aneurysm is and allowed to grow dynamically from 10 to 20 mm. WSS and hemodynamic pattern is estimated numerically using Ansys platform at various region of interest in both rigid and compliant wall conditions. The arterial wall thinning was analytically estimated using thick cylinder theory to estimate the increase in aneurysm under various stress conditions (rest and exercise conditions). The findings show that WSS is found to reduce at the aneurysm region with a corresponding strong vortex pattern. Thus, the vortex pattern could be the cause of tissue damage and thinning rather than WSS. The corresponding increase in velocity gradient at the bifurcation region is also captured. This high gradient is the cause for higher WSS at the bifurcation region which is a possible cause for the formation of plaque and arteriosclerosis. It is interesting to note that the WSS did not change drastically for 10 and 15 mm aneurysm but a large change was seen from normal to 10 and 15–20 mm respectively. This study provides a better clinical insight on the effect of aneurysm in CA bifurcation region using a systematic approach to numerical modelling compared to traditional imaging modalities. It can be used as an adjunct tool for physicians and surgeons for planning necessary clinical interventions. [ABSTRACT FROM AUTHOR]

The landing phase of an airdrop process is prone to accidents, and thus, it is important to assess the landing reliability for an airdrop system. However, full field tests to assess the reliability are unacceptable due to their cost and the time required. As such, it is necessary to estimate the reliability in the design stage. To address this problem, a method based on vine-Bayesian Network (vine-BN) is proposed to assess the landing reliability by fusing multisource information. First, the network structure is determined by the relationship between data of simulation or ground tests and failure modes. Then, nodes are defined as random variables on [0, 1] based on the definition of the performance metric. Finally, the dependence between nodes is quantified by expert opinions. To illustrate the effectiveness of the method, a particular ground test or simulation is chosen to establish a network for a typical heavy cargo airdrop system (HCADS). Forward and backward propagation is carried out on the network. The forward analysis predicts the landing reliability in the design stage through multisource information fusion. Beta distribution is applied to fit the fusion result, so Bayesian inference is made to perform field test times decision-making. The backward analysis works to identify the key performance metrics related to landing reliability. The results and analysis manifest that vine-BN is feasible for fusing multisource information. Through the network, the reliability of the current design can be predicted effectively, and the field test times can be remarkably reduced. This method plays a crucial role in airdrop system design and reducing test time and labor. [ABSTRACT FROM AUTHOR]

The proposed work focusses on the 2D design and modelling of integrated microcantilever in microchannel for the analysis of fluid pressure. In order to compute the microfluidic pressure in microchannel at various angles, fluid structure interaction is analyzed using the finite element method. With a fluid flow rate of 4.33 cm/s, a 2D integrated microcantilever can optimize both fluid pressure and microcantilever deflection. The novelty of the microfluidic pressure sensing mechanism allows pressure of fluid to be sensed in a microchannel without connecting any electrical method such as piezoresistor or piezoelectric approach. The objective of the research is to integrate a microcantilever into a microchannel to reduce setup complexity and procedure cost. Maximum deflection of the 2D T-microcantilever achieved 10.30µm at, pressure at the tip of T-microcantilever 10.89 Pa, fluid velocity 0.00309 m/sec, and Reynolds number is 1.22 [ABSTRACT FROM AUTHOR]

The enhancement of flow induced vibration of a circular cylinder by a rotating control rod is investigated through two-dimensional numerical simulations. The Reynolds number, diameter ratio, and gap ratio are 150, 0.2, and 0.2, respectively. Simulations are conducted for two rod position angles of β = 90° and 135°, rotation rates ranging from 0 to 6, and reduced velocities ranging between 1 and 20. The response of the cylinder–rod system at the rotation rates 0 and 1 has a lock-in regime where the vibration amplitude is high and the vibration frequency stops increasing with the increase in reduced velocity linearly. For rotation rates exceeding 2, the response amplitude increases with the increase in reduced velocity and enters the lock-in regime at the lower boundary reduced velocity. It remains high until the largest studied reduced velocity of 20; as a result, the higher boundary reduced velocity of the lock-in regime cannot be determined. The vibration with large amplitudes and large rotation rates repeats cyclically after every two or more vibration periods. As a result, two combined wake modes are found: 2S/P + S and 2P/P + S. In a combined mode, the vibration changes from one mode to another within each cycle. The cylinder receives power from the fluid, and the rotating rod gives power to the fluid although the net power exchange between the whole system and the fluid is zero. [ABSTRACT FROM AUTHOR]

Hydrodynamic forces caused by fluid-rotor interaction in fluid-rotor-stator systems can directly influence the occurrence of rubbing fault between the rotor and the stator, thereby affecting the stability of the rotating machinery. Considering the rotor-stator dynamics, the dry friction, and the flexibility on the contact surfaces as well as the fluid-rotor interaction, a 4D fluid-rotor-stator rubbing system was established to determine the global response characteristics exhibited in the general rotor/stator rubbing system with hydrodynamic forces. Aided by the features of orbits and full spectra, three types of rubbing responses were identified through experimental observation. The critical rotating speed of no-rub motion was analytically derived and confirmed by numerical simulation. Then, the global response characteristics on the parameter plane of the friction coefficient and rotating speed were numerically obtained. Results show that the critical rotating speed of no-rub motion decreases from 0.716 to 0.589 because of the hydrodynamic forces, and the coexistence of the partial rub with other responses is evident. From the influences of the system parameters, the decrease of the rotor mass has the same effect as increasing the rotor stiffness to mitigate rubbing responses. On the other hand, the increase of the rotor radius has the same effect as decreasing the clearance to increase the amplitudes of the rubbing responses. This study provides deeper insights into the highly detailed response characteristics of the general fluid-rotor-stator rubbing system. [ABSTRACT FROM AUTHOR]

This work focuses on a numerical analysis of fluid-structure thermal coupling in a heating slab. The latter consists of a rectangular cross-section duct located in the concrete slab of a discredited habitat. We modeled the thermal transfers of fluid flow in the pipe. In fact, the Navier-Stokes equations that govern this flow have been solved numerically. These equations were by an implicit method of finite differences. The systems of algebraic equations thus obtained were solved by the algorithms of Gauss and Thomas. The equation of conduction in the concrete slab was solved using the same methodology as that of flow. In this work, we based on an algorithm that interacts non stationary solid medium with a fluid medium consisting of permanent a state by ensuring equal flows and temperatures on the common interface between the two mediums at every moment. The numerical simulation of heat transfers and the thermal behavior of the heating slab were analyzed for various parameters influencing thermal diffusion. The results obtained show that the numerical methodology adopted for the control of fluid-structure coupling is acceptable in comparison with the literature. [ABSTRACT FROM AUTHOR]

The 2D shallow water equations are a common tool for the simulation of free surface fluid dynamics in civil engineering. However, the nonlinear structures of the equations' straightforward implementations lead to numerical problems, such as spurious oscillations and unphysical diffusion. Therefore, this research compared several strategies to overcome these problems, using various finite element formulations and combinations of stabilization methods and mesh options. The accuracy and performance of numerous approaches are examined on models of dam-break in one and two space dimensions. The analytical solution checks the numerical, derived shock wave heights and velocities for the 1D classical benchmark. The result showed that streamlined diffusion and shock capturing stabilization deal with the classical problems of spurious oscillations and numerical diffusion but still indicate similar problems locally in the vicinity of steep fronts and shock waves when used on fixed meshes. As adaptive meshing is the most promising method to deal with such situations, several concerned options are examined in detail. It is important to fine-tune the method to the model's needs, i.e. to adapt the maximum number of mesh refinements, the indicator functions, and the starting mesh. The use of adaptive meshing techniques leads to accurate solutions for the usual parameter range in 1D and 2D, requiring less computational resources than simulations on fixed meshes. Meanwhile, meshing reduces the model size of the 2D dam-break model adaptive by almost one order of magnitude and the execution time by a factor of 20. [ABSTRACT FROM AUTHOR]

We investigated differences in haemodynamic forces between carotid arteries that underwent primary closure (PC) or patch angioplasty (PA) using computational fluid dynamics (CFD). A total of 30 subjects were enrolled in this study, consisting of 10 subjects who underwent PC, 10 who underwent PA and 10 healthy subjects. Three-dimensional models of carotid arteries were reconstructed using patient-specific computed tomography angiography images. The conventional Navier-Stokes, continuity equation and constitutive stress-strain law with a stabilized Petrov-Galerkin scheme were solved with Newtonian and incompressible assumptions. The boundary conditions employed patient-specific velocity profiles as the inflow and lumped parameters of the three-element Windkessel model as the outflow with a rigid wall assumption. Thus, the CFD results exhibited good agreement with measurements from the subjects (r =.78). The carotid arteries of the PC group were exposed to abnormal haemodynamic forces related to building atherosclerosis in a smaller (p.05) to healthy arteries. The morphological characteristics of the carotid artery were significantly associated with the area exposed to abnormal haemodynamic forces. We identified that abnormal haemodynamic forces could be avoided by selecting appropriate surgical techniques that produce less bifurcation expansion. [ABSTRACT FROM AUTHOR]

Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI) are increasingly used to predict the hemodynamic and structural behaviors of cerebral aneurysms (CAs) whilst a less-pursued method is the static structural analysis (SSA) using the finite element method. In this paper, hemodynamic parameters including the flow velocity and wall shear stress predicted by CFD and FSI are compared whilst structural parameters including the wall displacement and wall stress predicted by SSA and FSI are compared for four patient-specific CA models under different systolic/diastolic pressures. The predicted distribution patterns of the same parameters for the same CA model under different pressures are similar. However, the percentage differences of the maximum hemodynamic parameters increase with increasing pressure. Conversely, the percentage differences of the maximum structural parameters decrease from a few to less than 1.5% when the systolic/diastolic pressure changes from 120/80 to 180/110 mmHg. The ratio of the computation times for CFD, SSA and FSI is typically 75:1:165. If the maximum wall stress under either ongoing or temporary hypertension is the most critical factor for immediate rupture and, thus, clinical treatment of CAs, SSA can provide a low cost and efficient predictions close to those of FSI for assessing the rupture risk. [ABSTRACT FROM AUTHOR]

The current work is intended to give an overview of issues related to the numerical simulation of adhesive spreading for liquid to semi-liquid adhesives. The advantages and limitations are presented in order to guide the choice of the suitable approach depending on the case under consideration. It is shown that methods are of two categories, whether they are grid-based or meshless. In the first, the movement of the matter is directly dependent on the mesh size and distribution. Contrariwise, in the meshfree methods, the particles are free to move and each carries its properties. Besides, cases of application are presented to provide a database for calculating adhesive spreading with the particulate SPH method. It is shown that it is possible to use simple behaviour laws to win this case. [ABSTRACT FROM AUTHOR]

With the advantages of biosafety and efficiency, increasing attention has been paid to the devices for gene and macromolecular drug delivery based on mechanoporation. The transient pore formation on the cell membrane allows cargo transportation when the membrane areal strain is beyond the critical pore value and below the lysis tension threshold. Based on this principle, we propose a method to apply the proper fluid stress on cells moving in a microchannel under the action of zero-net-mass-flux (ZNMF) jets. In this study, an immersed finite element method (IFEM) is adopted to simulate the interaction between the cells and the fluid fields so as to investigate the cell movement and deformation in this mechanoporation system. To evaluate the efficiency of the cargo delivery, a pore integral is defined as the mean pore rate when the cell passes through the jet region. By analyzing the effects of the parameters, including the pressure gradient along the microchannel, the jet amplitude, and the jet frequency, on the pore integrals, a group of optimized parameters for cargo delivery efficiency are obtained. Additionally, the stability and safety of this system are analyzed in detail. These results are helpful in designing the mechanoporation devices and improving their efficiency of drug delivery. [ABSTRACT FROM AUTHOR]

In this work, we propose and validate a new stabilized compressible flow finite element framework for the simulation of aerospace applications. The framework is comprised of the streamline upwind/Petrov–Galerkin (SUPG)-based Navier–Stokes equations for compressible flows, the weakly enforced essential boundary conditions that act as a wall function, and the entropy-based discontinuity-capturing equation that acts as a shock-capturing operator. The accuracy and robustness of the framework is tested for various Mach numbers ranging from low-subsonic to transonic flow regimes. The aerodynamic simulations are carried out for 2D and 3D validation cases of flow around the NACA 0012 airfoil, RAE 2822 airfoil, ONERA M6 wing, and NASA Common Research Model (CRM) aircraft. The pressure coefficients obtained from the simulations of all cases are compared with experimental data. The computational results show good agreement with the experimental findings and demonstrate the accuracy and effectiveness of the finite element framework presented in this work for the simulation of aircraft aerodynamics. [ABSTRACT FROM AUTHOR]

We formulate and solve a generalized inverse Navier--Stokes problem for the joint velocity field reconstruction and boundary segmentation of noisy flow velocity images. To regularize the problem, we use a Bayesian framework with Gaussian random fields. This allows us to estimate the uncertainties of the unknowns by approximating their posterior covariance with a quasi-Newton method. We first test the method for synthetic noisy images of two-dimensional (2-D) flows and observe that the method successfully reconstructs and segments the noisy synthetic images with a signal-to-noise ratio (SNR) of three. Then we conduct a magnetic resonance velocimetry (MRV) experiment to acquire images of an axisymmetric flow for low (≃6) and high (>30) SNRs. We show that the method is capable of reconstructing and segmenting the low SNR images, producing noiseless velocity fields and a smooth segmentation, with negligible errors compared with the high SNR images. This amounts to a reduction of the total scanning time by a factor of 27. At the same time, the method provides additional knowledge about the physics of the flow (e.g. pressure) and addresses the shortcomings of MRV (i.e. low spatial resolution and partial volume effects) that otherwise hinder the accurate estimation of wall shear stresses. Although the implementation of the method is restricted to 2-D steady planar and axisymmetric flows, the formulation applies immediately to three-dimensional (3-D) steady flows and naturally extends to 3-D periodic and unsteady flows. [ABSTRACT FROM AUTHOR]