Simulation Of Two-dimensional Flow Research Articles

Lift and wakes of flying snakes

Flying snakes use a unique method of aerial locomotion: they jump from tree branches, flatten their bodies, and undulate through the air to produce a glide. The shape of their body cross-section during the glide plays an important role in generating lift. This paper presents a computational investigation of the aerodynamics of the cross-sectional shape. Two-dimensional simulations of incompressible flow past the anatomically correct cross-section of the species Chrysopelea paradisi show that a significant enhancement in lift appears at a 35° angle of attack, above Reynolds numbers 2000. Previous experiments on physical models also obtained an increased lift, at the same angle of attack. The flow is inherently three-dimensional in physical experiments, due to fluid instabilities, and it is thus intriguing that the enhanced lift also appears in the two-dimensional simulations. The simulations point to the lift enhancement arising from the early separation of the boundary layer on the dorsal surface of the snake profile, without stall. The separated shear layer rolls up and interacts with secondary vorticity in the near-wake, inducing the primary vortex to remain closer to the body and thus cause enhanced suction, resulting in higher lift.

Numerical simulation of flows past flat plates using volume penalization

We present numerical simulations of two-dimensional viscous incompressible flows past flat plates having different kind of wedges: one tip of the plate is rectangular, while the other tip is either a wedge with an angle of 30∘30∘ or a round shape. We study the shear layer instability of the flow considering different scenarios, either an impulsively started plate or an uniformly accelerated plate, for Reynolds number Re=9500Re=9500 . The volume penalization method, with either a Fourier spectral or a wavelet discretization, is used to model the plate geometry with no-slip boundary conditions, where the geometry of the plate is simply described by a mask function. On both tips, we observe the formation of thin shear layers which are rolling up into spirals and form two primary vortices. The self-similar scaling of the spirals corresponds to the theoretical predictions of Saffman for the inviscid case. At later times, these vortices are advected downstream and the free shear layers undergo a secondary instability. We show that their formation and subsequent dynamics is highly sensitive to the shape of the tips. Finally, we also check the influence of a small riblet, added on the back of the plate on the flow evolution.

Simulation of Two-Dimensional Taylor-Couette Flow using the Lattice Boltzmann Method Combined with Smoothed Profile Method

Simulation of Two-Dimensional Taylor-Couette Flow using the Lattice Boltzmann Method Combined with Smoothed Profile Method

Impact of surface discontinuities on flowfield structure of a micronozzle array

This work carries out two-dimensional numerical simulations of rarefied gas flow in convergent–divergent micronozzles. Such a device is considered part of a micronozzle array. Although previous studies have investigated micronozzles with continuous divergent surfaces, the present work evaluates the impact of the surface smoothness on the flowfield structure by including discontinuities on the divergent contour. Such a scenario was not previously investigated. As a secondary goal, detailed physical explanations are given for those flow features reported but not extensively discussed in previous micronozzle studies. A direct simulation Monte Carlo method is employed to describe these flows due to the moderate rarefaction degree typically observed in microdevices. The results pointed out the sensitivity of the macroscopic properties—velocity, pressure, and temperature—due to the geometric variations on the divergent surface. In addition, Knudsen number distributions are computed to map nonequilibrium flow regions. The results indicated that the proposed surface discontinuities do not play the main role in the investigated micronozzle flows. This finding was not previously reported for micronozzle flows and contradicts the expected behavior in macroflows, where shock waves and/or expansion waves arise in supersonic flows experiencing abrupt changes in the flow direction.

Groundwater Management in the Esna City, Upper Egypt: An Application of Remote Sensing and Numerical Modeling

The major issues concerning water development and management include water conservation and the efficiency of water use, cost recovery, social and environmental factors. So continuous monitoring is essential to evaluate the reclamation impacts on the groundwater potentiality such as Esna Area. The main objective of this work is to determine the new development areas in Esna during the period from 1984 to 2011 using remote sensing technique. The impacts of the present and future development have been evaluated by using the two-dimensional numerical groundwater flow Simulation Package (visual modflow 4.2). The package is used to construct and calibrate a numerical model that can be used to simulate the response of the aquifer in the study area under implementing different management alternatives in the form of changes in piezometric levels and salinity. From land use maps of the study area in the year 2011, a lot of changes in development areas especially out the border of the old land were observed. The change in land reclaimed area was estimated with 44% in the period from year 1984 to 2011. This development accompanied with draw down about 2.5 meters through this period of time. The authors recommend applying different kinds of change detection technique on the study area. Comparing between results, continuous monitoring of the development area is highly recommended.

A simulation-optimization method for ecohydraulic design of fish habitat in a canal

A decision-support method for ecohydraulic design of a canal in rural engineering is developed using a GA (Genetic Algorithm)-based simulation-optimization technique. In order to demonstrate the ability of the method, a simple problem is presented aiming at searching for best configuration of blocks located for creating valuable fish habitat in a canal. The habitat condition is evaluated using a habitat suitability index (HSI) considering hydraulic factors such as water depth and velocity. The study area, a section of canal, is discretized into triangular finite elements with nodes, and the two-dimensional simulation of flow is conducted by the finite element method applied to the steady-state shallow water equations. The computed water depth and velocity at each node are then used to evaluate the weighted average HSI (AHSI) for the canal. In order to develop a robust method insensitive to uncertainty in these hydraulic parameters, a scenario-based stochastic optimization framework is adopted. The simulation part of the developed method comprises the numerical computation of canal flow at every scenario aiming at evaluation of the expected AHSI, whereas the optimization part includes rearrangement of blocks conducted by GA operators. The simulation and optimization process is iterated to maximize the expected AHSI in the canal section. The performance of the presented simulation-optimization method is demonstrated for a hypothetical ecohydraulic design of spawning area of Ayu (Plecoglossus altivelis) in a canal. The demonstrative example shows that the presented method can successfully specify the best block configuration in the canal section with the largest expected AHSI value.

A numerical method for two-phase flows of dense granular mixtures

In this paper, a novel algorithm for incompressible flows of two-phase granular mixtures is proposed. The algorithm constitutes a generalization of projection-type methods to multi-phase flows and employs a predictor–corrector time-marching scheme. For the computation of the phasial pressures, a Poisson equation and a second-order elliptic PDE with variable coefficients are solved at both the prediction and the correction stage. Further, the algorithm is combined with a regularization scheme for the numerical treatment of material interfaces which are a common characteristic of the flows of interest. The efficiency and robustness of the proposed algorithm are illustrated via numerical simulations of unsteady two-dimensional shear flows for a reference mixture of beach sand and water.

Two-dimensional simulations of resin flow in dual-scale fibrous porous medium under constant pressure

In liquid composite molding (LCM), the resin is filled into the stationary fiber to impregnate all the empty space. Because the use of fiber features the double-scale porous property, partially saturated region phenomenon will occur during the process of resin flow. Therefore, a non-zero sink-function appears on the right side of the equation of mass conservation in fluid dynamics. In this paper, the control volume/finite element method is applied to simulate the flowing characteristics of resin in two-dimensional mold under constant pressure. A corresponding algorithm is designed, and a two-dimensional program for finite element algorithm based on both dual-scale and single-scale is compiled. Compared with the flowing characteristics of resin in single-scale fibrous porous medium under the same conditions, it can be concluded that there is an unsaturated flow front in the resin flowing of the dual-scale fibrous porous medium according to the unsaturated region. The pressure distribution and filling time make huge difference when they reach the same flow front. Changing the initial pressure of resin in dual-scale fibrous porous medium will greatly affect the distribution of pressure and fill time.

저 레이놀즈수에 적용 가능한 가상경계기법

We develop a novel immersed boundary (IB) method based on implicit direct forcing scheme for incompressible flows. The proposed IB method is based on an iterative procedure for calculating the direct forcing coupled with the momentum equations in order to satisfy no-slip boundary conditions on IB surfaces. We perform simulations of two-dimensional flows over a circular cylinder for low and moderate Reynolds numbers. The present method shows that the errors for estimated velocities on IB surfaces are significantly reduced even for low Reynolds number with a fairly large time step while the previous methods based on direct forcing failed to provide no-slip boundary conditions on IB surfaces.

Two-dimensional surface water flow simulation of basin irrigation with anisotropic roughness

Simulating basin surface water flow with anisotropic roughness has practical significance. Based on the complete hydrodynamic model, a two-dimensional surface water flow model of basin irrigation with anisotropic roughness was developed in this study by constructing an anisotropic roughness model in the source terms of the governing equation. Then, the simulation performance of the proposed model was analyzed and compared based on typical experiments of basin irrigation. The results show that with basin surface anisotropic roughness, the proposed model can successfully simulate water flow in basin irrigation, and exhibits better simulation performance than the model with isotropic roughness. Three basin geometries and two kinds of inflow geometries were selected for the application of the proposed model. Applied results show that the anisotropic roughness can improve irrigation performance. When the basin width becomes narrow, the physical effect of the rotation angle in the anisotropic roughness model weakens, even becomes smooth. The two-dimensional surface water flow model of basin irrigation with anisotropic roughness provides a good numerical simulation tool for designing and evaluating the performance of basin irrigation system.

Hydraulic validation of two-dimensional simulations of braided river flow with spatially continuous aDcp data

[1] Gravel-bed braided rivers are characterized by shallow, branching flow across low relief, complex, and mobile bed topography. These conditions present a major challenge for the application of higher dimensional hydraulic models, the predictions of which are nevertheless vital to inform flood risk and ecosystem management. This paper demonstrates how high-resolution topographic survey and hydraulic monitoring at a density commensurate with model discretization can be used to advance hydrodynamic simulations in braided rivers. Specifically, we detail applications of the shallow water model, Delft3d, to the Rees River, New Zealand, at two nested scales: a 300 m braid bar unit and a 2.5 km reach. In each case, terrestrial laser scanning was used to parameterize the topographic boundary condition at hitherto unprecedented resolution and accuracy. Dense observations of depth and velocity acquired from a mobile acoustic Doppler current profiler (aDcp), along with low-altitude aerial photography, were then used to create a data-rich framework for model calibration and testing at a range of discharges. Calibration focused on the estimation of spatially uniform roughness and horizontal eddy viscosity, νH, through comparison of predictions with distributed hydraulic data. Results revealed strong sensitivity to νH, which influenced cross-channel velocity and localization of high shear zones. The high-resolution bed topography partially accounts for form resistance, and the recovered roughness was found to scale by 1.2–1.4 D84 grain diameter. Model performance was good for a range of flows, with minimal bias and tight error distributions, suggesting that acceptable predictions can be achieved with spatially uniform roughness and νH.

Simulating Free Boundaries in 2D FEM Flow Simulation with Direct Time Integration of the Surface Velocities

We have derived a novel approach to simulate free surfaces. For two-dimensional finite-element flow simulations of the incompressible Navier-Stokes equations we integrate out the velocities on the surface obtained from the FEM-simulations with integrators for ordinary differential equations. The advance of the fluid front does not need additional data structures or interpolated mesh points as in the front tracking for finite difference methods, the adaptive mesh of the FEM-simulation is sufficient. As we perform the time-integration of the FEM-code, the second-order Adams-Bashforth turns out to be the most suitable integrator for the surface motion. For the speed of the wavefronts, we get excellent agreement for large viscosity with the lubrication approximation by Huppert, and for small viscosity, we get very good agreement with the experimental data for water by Martin and Moyce.

Clumpy Outflows from Supercritical Accretion Flow

Abstract A significant amount of matter in supercritical (or super-Eddington) accretion flow is blown away by radiation force, thus forming outflows; however, the properties of such radiation-driven outflows have been poorly understood. We have performed global two-dimensional radiaion-magnetohydrodynamic simulations of supercritical accretion flow onto a black hole with 10 or 10$^{8} M_{\odot}$ in a large simulation box of 514 $r_{\rm S} \times 514 r_{\rm S}$ (with $r_{\rm S}$ being the Schwarzschild radius). We confirm that uncollimated outflows with velocities of 10 percent of the speed of light emerge from the innermost part of the accretion flow at a wide angle of 10$^{\circ}$ –50$^{\circ}$ from the disk rotation axis. Importantly, the outflows exhibit clumpy structures above heights of $\sim 250 r_{\rm S}$. The typical size of the clumps is $\sim 10 r_{\rm S}$, which corresponds to one optical depth, and their shapes are slightly elongated along the outflow direction. Since clumps start to form in the layer above which the (upward) radiation is superior in force to the (downward) gravity, the Rayleigh–Taylor instability seems to be a primary cause. In addition, a radiation-hydrodynamic instability, which arises when radiation funnels through a radiation-pressure-supported atmosphere, may also help to form clumps of one optical depth. A magnetic photon bubble instability does not seem to be essential, since a similar clumpy outflow structure is obtained in nonmagnetic radiation-hydrodynamic simulations. Since the spatial covering factor of the clumps is estimated to be $\sim$ 0.3, and since they are marginally optically thick, they will explain at least some of the rapid light variations of active galactic nuclei. We further discuss a possibility of producing broad-line region clouds by the clumpy outflow.

Numerical Simulation of Two-Dimensional Regular Wave Overtopping Flows over the Crest of a Trapezoidal Smooth Impermeable Sea Dike

AbstractThe objectives of the current study are to develop a two-phase flow solver and to investigate the two-dimensional regular wave overtopping flow field over a sea dike. The solver is based on the Navier-Stokes equations and the classical Smagorinsky eddy-viscosity model. A coupled approach of wave generation and absorption is accomplished in the numerical wave flume, and the wave reflection from the work zone can be eliminated efficiently. Simulations of the wave run-up and breaking were performed to validate the effectiveness of the numerical wave flume. The influences of turbulent eddy viscosity and grid refinement on the simulations of the breaking waves are discussed. The flow features of wave overtopping over a sea dike are investigated systematically for both nonbreaking and breaking waves. The numerical results show that the layer thicknesses along the dike crest decay following an exponential rule, whereas the maximum velocities along the dike crest exhibit a mild increasing trend. Furthermo...

On the role of initial and boundary conditions in numerical simulations of accretion flows

We study the effects of initial and boundary conditions, taking two-dimensional hydrodynamical numerical simulations of hot accretion flow as an example. The initial conditions considered include a rotating torus, a solution expanded from the one-dimensional global solution of hot accretion flows, injected gas with various angular momentum distributions, and the gas from a large-scale numerical simulation. Special attention is paid to the radial profiles of the mass accretion rate and density. Both can be described by a power-law function, Ṁ∝ rs and ρ ∝ r−p. We find that if the angular momentum is not very low, the value of s is not sensitive to the initial condition and lies within a narrow range, 0.47 ≲ s ≲ 0.55. However, the value of p is more sensitive to the initial condition and lies in the range 0.48 ≲ p ≲ 0.8. The diversity of the density profile is because different initial conditions give different radial profiles of radial velocity due to the different angular momentum of the initial conditions. When the angular momentum of the accretion flow is very low, the inflow rate is constant with radius. Taking the torus model as an example, we have also investigated the effects of inner and outer boundary conditions by considering the widely adopted ‘outflow’ boundary condition and the ‘mass flux conservation’ condition. We find that the results are not sensitive to these two boundary conditions.

Numerical simulation of a plasma actuator based on ion transport

Two-dimensional numerical simulation of ion transport and flow around a single dielectric barrier discharge plasma actuator (PA) is performed. Spatial distributions of ions and electrons as well as their time evolution are obtained by solving the transport equations of monovalent positive ions, monovalent negative ions, and electrons. Voltage and frequency of the driving alternating-current signal are assumed to be 8 kV and 5 kHz, respectively. Special focus is laid upon the effect of voltage gradient dV/dt on the magnitude of the body force. The validity of steady force models often used in flow simulation is also examined. The simulation results show that the magnitude of the body force induced by the PA increases as the voltage gradient dV/dt increases and its increase rate becomes milder at higher voltage. The mechanism of body force generation is explained from the time evolution of number density fields of ions and electrons. A comparison between flow simulations using a time-resolved body force and its time-averaged counterpart demonstrates that the time-averaged model gives sufficiently accurate results when the time scale of the flow is more than 30 times greater than that of the PA.

Numerical study of wind forces on parabolic solar collectors

Concentrated solar power is an established technology for megawatt-scale power generation. Recent advances in lower-cost and lighter materials have made kilowatt-scale concentrated solar power applications feasible. Despite the importance of designing support structures for solar collectors, very little reliable work has been done to investigate the forces of the wind on parabolic solar collectors. In the present study, two-dimensional numerical turbulent flow simulations around parabolic solar collectors are performed. Force and torque data are presented for several collector orientations and for wind speeds 8.9 m/s (20 mph) ≤V≤ 49.2 m/s (110 mph). The aperture width of the parabola, which corresponds to aperture widths used for kilowatt-scale as well as some megawatt-scale applications, is also varied from 1.2 m (4 ft) ≤a≤3.7 m (12 ft).

Mechanical energy transfer and flow parameter effects for a cylinder in transverse oscillation

This paper deals with the two-dimensional numerical simulation of low-Reynolds number flow past a circular cylinder forced to oscillate transverse to the main stream. The study concentrates on the investigation of mechanical energy transfer E between the fluid and a transversely oscillating cylinder. When E is negative the fluid works to dampen the cylinder oscillation. When E is positive, work is done on the cylinder by the fluid and can be a source of vortex-induced vibration (VIV) for free vibration cases. One object of this paper is to identify domains of Reynolds number, oscillation amplitude and frequency ratio (Re, Ay, f/St0) where the mechanical energy transfer is positive. Other objectives are to learn more about the behaviour of force coefficients in the phase space and to roughly determine the lock-in boundaries. Computations are carried out using an in-house code based on the finite difference method. The three-dimensional domain of (Re, Ay, f/St0) is computed for Re=100–180, Ay=0.1–1.0 and frequency ratios between 0.6 and 1.2. A comparison is made with results in the literature for mechanical energy transfer contours within the lock-in domain, with good agreement obtained.

Study on numerical methods for transient flow induced by speed-changing impeller of fluid machinery

In order to establish a reliable numerical method for solving the transient rotating flow induced by a speed-changing impeller, two numerical methods based on finite volume method (FVM) were presented and analyzed in this study. Two-dimensional numerical simulations of incompressible transient unsteady flow induced by an impeller during starting process were carried out respectively by using DM and DSR methods. The accuracy and adaptability of the two methods were evaluated by comprehensively comparing the calculation results. Moreover, an intensive study on the application of DSR method was conducted subsequently. The results showed that transient flow structure evolution and transient characteristics of the starting impeller are obviously affected by the starting process. The transient flow can be captured by both two methods, and the DSR method shows a higher computational efficiency. As an application example, the starting process of a mixed-flow pump was simulated by using DSR method. The calculation results were analyzed by comparing with the experiment data.

Dynamic Characteristics of Swing Check Valve Based on Dynamic Mesh and UDF

In view of the transient dynamic analysis in opening swing check valve, the motion model of valve clack is built, two-dimensional dynamic numerical simulation of the internal unsteady flow is performed by using dynamic mesh and UDF in opening two kinds of swing check valve at the same working condition, the visualized results are obtained. The results show that internal flow field of the modified model become more stable, the noise, impact and the local energy loss are decreased, the service life is improved obviously; an important reference for swing check valve design and analysis of dynamic characteristics are provided by the research results.