OVERLAPPING GRIDS FOR THE SIMULATION OF PARTICLE INTERACTION AT THE MICRO SCALE

This paper describes the use of overlapping grids for the calculation of flow around single and multiple-particle configurations at the micro scale. The basic equations for calculation are those for conservation of mass and momentum which are solved using a common Finite-Volume formulation. The hydrodynamic particle-particle and particle-wall interaction can be calculated by using an overlapping or Chimera grid scheme. With the grid structuring procedure it is possible to use simple and structured grids around the particles and the overall main grid geometry. The particle grids are lapped over the main grid such that they can move independently after each time step without remeshing the whole geometry. The paper gives results for the validation of the code developed for general test cases, for a rotating ellipsoid in simple shear flow, the flow around particles attached to a wall, the motion of a particle in the vicinity of a wall and some results for the flow through a packed bed configuration.

An overlapping grid spectral collocation analysis on a newly developed hybrid nanofluid flow model

The present study investigates the axisymmetric stagnation point radiativeflow of a Cu-Al2O3/water hybrid nanofluid over a radially stretched/shrunk disk.In this paper,a new mathematical model has been developed by taking into consideration the concept of different nanoparticles concentration in a hybrid nanofluid,which are Brownian motion and thermophor-esis of nanoparticles.A new model for entropy generation has also been provided in the present study.The non-dimensional governing equations of the developed mathematical model are solved using newly developed and efficient overlapping grid spectral collocation method.Numerical stability and residual error test are provided here to show the accuracy of the numer-ical method in this mathematical model.The outcomes offluidflow,temperature,and two different types of concentration profiles are depicted,and described in graphical and tabular forms.For the limiting instances,comparison shows excellent agreement among current and results established in the literature.Increasing the strength of magneticfield is seen to increase the radial component offluid velocity as well as the entropy generated within the system.Two different nanofluid concentration profiles are increasing and decreasing with rising thermophor-esis and Brownian motion parameters,respectively,from a particular height above the disk because of the revised nanofluid boundary condition.Temperature profile increases here with increasing Biot number,and increasing Brinkman number causes higher entropy generation number for both stretching and shrinking disks.The enhanced thermal characteristics of the hybrid nanofluid over the single particle nanofluid has been observed.ª2024 The Authors.Publishing services by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

The effect of phase angle and wing spacing on tandem flapping wings

In a tandem wing configuration, the hindwing of- ten operates in the wake of the forewing and, hence, its per- formance is affected by the vortices shed by the forewing. Changes in the phase angle between the flapping motions of the fore and the hind wings, as well as the spacing between them, can affect the resulting vortex/wing and vortex/vortex interactions. This study uses 2D numerical simulations to in- vestigate how these changes affect the leading dege vortexes （LEV） generated by the hindwing and the resulting effect on the lift and thrust coefficients as well as the efficiencies. The tandem wing configuration was simulated using an incom- pressible Navier-Stokes solver at a chord-based Reynolds number of 5 000. A harmonic single frequency sinusoidal oscillation consisting of a combined pitch and plunge motion was used for the flapping wing kinematics at a Strouhal num- ber of 0.3. Four different spacings ranging from 0.1 chords to 1 chord were tested at three different phase angles, 0°, 90° and 180°. It was found that changes in the spacing and phase angle affected the timing of the interaction between the vor- tex shed from the forewing and the hindwing. Such an inter- action affects the LEV formation on the hindwing and results in changes in aerodynamic force production and efficiencies of the hindwing. It is also observed that changing the phase angle has a similar effect as changing the spacing. The re- suits further show that at different spacings the peak force generation occurs at different phase angles, as do the peak efficiencies.

Effect of Cambering on the Aerodynamic Performance of Heaving Airfoils

In the present work,a parametric numerical study is conducted in order to assess the effect of airfoil cambering on the aerodynamic performance of rigid heaving airfoils.The incompressible Navier-Stokes equations are solved in their velocity-pressure formulation using a second-order accurate in space and time finite-difference scheme.To tackle the problem of moving boundaries,the governing equations are solved on overlapping structured grids.The numerical simulations are performed at a Reynolds number of Re=1100 and at different values of Strouhal number and reduced frequency.The results obtained show that the airfoil cambering geometric parameter has a strong influence on the average lift coefficient,while it has a smaller impact on the average thrust coefficient and propulsive efficiency of heaving airfoils.

A Three-dimensional Heterogeneous Mass Transfer Model for the Absorption of Gas into Multiphase System

A three-dimensional heterogeneous mass transfer model was proposed to investigate the enhancement of dispersed particles on gas absorption. The strategy to calculate local and overall enhancement factors is proposed. Instead of a global grid, the composite overlapping grid is adopted, which simplifies the setup and solution of the three-dimensional model equations. It is found that dispersed particle hold-up, particle size, liquid-solid partition coefficient of transported component, characteristic contact time, and the shortest distance between particles and gas-liquid interface have major influence on absorption enhancement factor. The particle-particle interaction on gas absorption and the lateral diffusion of transported component in liquid film were studied with the multi-particle simulation. The proposed model predicted the experimental data from the literature reasonably well.

Numerical Simulation of Self-Excited and Forced Vibration of Circular Cylinders in Current

Numerical simulations of a low-mass-damping circular cylinder which can oscillate freely at transverse and stream- wise directions are presented in this work. The Navier-Stokes equations are solved with finite volume method, and large eddy simulation of vortex is also performed in the calculation. In order to implement dynamic mesh, overlapping grids are generated to lessen the computation for mesh field itself. Self-excited vibrations are firstly calculated to obtain the average amplitudes and frequencies of the target circular cylinder in the current flow situation, and then forced oscillations are implemented with parameters obtained in vortex-induced vibrations previously. With slight amplitude modulation, time series of displacements in vortex-induced vibrations are essentially harmonic. Regarding the fluid force, which are larger in forced oscillations than those in corresponding self-excited cases because the fluid subtracts energy from the forced cylinders. The phase angles between forces and displacements are 0° and 180° for self-excited ease and forced case respectively. In vortex-induced vibrations, the interactions between fluid and structure produce some weakly energetic vortices which induce the modulations of amplitude and frequency.

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice podded propulsor interaction.

Aerodynamic Characteristics of Projectile with Exotic Wraparound Wings Configuration

A projectile with exotic wraparound wings( WAW) configuration is designed to improve the finstabilized projectile shooting quality. Two fin-stabilized projectiles with the same body with and without exotic WAW configuration are simulated numerically by applying the Roe scheme. The shear-stress transport turbulence models and the lower-upper symmetric Gauss-Seidel implicit method are used to solve 3D Reynoldsaveraged Navier—Stokes equations. The differences in aerodynamic coefficients and aerodynamic characteristics of the projectiles when the Mach number varies from 0. 35 to 0. 95 are obtained,and the cause of these differences is analyzed. The calculation results indicate that the lift-to-drag ratio of the projectile significantly increases,the rolling moment decreases,and the position of the pressure center of the projectile shows relatively small changes when the exotic WAW configuration is used. Therefore,this projectile can obviously reduce rolling effect,enlarge range and improve flying stability.

Development process of muzzle flows including a gun-launched missile

Numerical investigations on the launch process of a gun-launched missile from the muz- zle of a cannon to the free-flight stage have been performed in this paper. The dynamic overlapped grids approach are applied to dealing with the problems of a moving gun-launched missile. The high-resolution upwind scheme （AUSMPW ＋） and the detailed reaction kinetics model are adopted to solve the chemical non-equilibrium Euler equations for dynamic grids. The development process and flow field structure of muzzle flows including a gun-launched missile are discussed in detail. This present numerical study confirms that complicated transient phenomena exist in the shortly launching stages when the gun-launched missile moves from the muzzle of a cannon to the free- flight stage. The propellant gas flows, the initial environmental ambient air flows and the moving missile mutually couple and interact. A complete structure of flow field is formed at the launching stages, including the blast wave, base shock, reflected shock, incident shock, shear layer, primary vortex ring and triple point.

Numerical Simulation of the Tip Aerodynamics and Acoustics Test

The application of an efficient flow control system on helicopter rotor blades may lead to improved aerodynamic performance. Recently, our invention of Rod Vortex Generators(RVGs) has been analyzed for helicopter rotor blades in hover with success. As a step forward, the study has been extended to forward flight conditions. For this reason, a validation of the numerical modelling for a reference helicopter rotor(without flow control) is needed. The article presents a study of the flow-field of the AH-1G helicopter rotor in low-, medium- and high-speed forward flight. The CFD code FLOWer from DLR has proven to be a suitable tool for the aerodynamic analysis of the two-bladed rotor without any artificial wake modelling. It solves the URANS equations with LEA(Linear Explicit Algebraic stress) k-ω model using the chimera overlapping grids technique. Validation of the numerical model uses comparison with the detailed flight test data gathered by Cross J. L. and Watts M. E. during the Tip Aerodynamics and Acoustics Test(TAAT) conducted at NASA in 1981. Satisfactory agreements for all speed regimes and a presence of significant flow separation in high-speed forward flight suggest a possible benefit from the future implementation of RVGs. The numerical results based on the URANS approach are presented not only for a popular, low-speed case commonly used in rotorcraft community for CFD codes validation but preferably for medium- and high-speed test conditions that have not been published to date.