Progress in Computational Fluid Dynamics, An International Journal (18 papers in press)
A Numerical Investigation of the Compressible Flow in the Ejector of a Vapor Ejector Refrigeration System
by Arumugam Megalingam, Viswanath Babu
Abstract: Numerical simulations of the flow in a vapor ejector have been carried out. Real gas effects are accounted for in the calculations. Both ejection as well as flow-through studies have been performed. Effects of the generator and evaporator temperatures and position of the primary nozzle have been investigated. Predicted values of the suction pressure and COP havernbeen compared with experimental values reported in the literature. In addition, secondary flow area has also been evaluated and correlated with the COP. By tracking the sonic line and the edge of the primary stream and flow separation, insights on the gas dynamic and fluid dynamic aspects of the flow field and how they influence the entrainment of the secondary stream and consequently the COP are brought out. The study reveals that, in addition to the choking of thernsecondary stream, the expansion of the primary stream and the area available for the secondary stream also play a key role in affecting the performance of the vapor ejector.
Keywords: Numerical simulation;Ejector; Real gas effects; Solar Refrigeration.
FEM Solution of MHD Flow in an Array of Electromagnetically Coupled Rectangular Ducts
by Munevver TEZER-SEZGIN, Selcuk Han AYDIN
Abstract: The magnetohydrodynamic (MHD) flow of an electrically conducting fluid is studied in an array of identical parallel ducts stacked in the direction of external magnetic field and are separated by conducting walls of arbitrary thickness. Such arrangement of electromagnetically coupled ducts arises in fusion blanket applications in which a liquid metal is used both as coolant and tritium generation. The finite element method (FEM) with SUPG stabilization is used for solving the set of coupled MHD equations. Numerical results show that, there is a significant effect of coupling the ducts with conducting walls of varying thickness, on the flow and induced current behaviors especially near the walls and for increasing values of Hartmann number. The results are presented for one, two and three coupled ducts in both co- and counter- flow configurations which induce reversal and counter-current flows.
Keywords: MHD; rectangular duct flow; stabilized FEM;.
Immersed boundary method for a permeable sedimenting circular particle between two parallel rigid walls
by Sudeshna Ghosh
Abstract: This paper demonstrates settling of an isolated permeable circular particle in 2D, which corresponds to infinite cylinder in 3D, in a viscous, incompressible fluid contained within a two-dimensional channel. The fluid-structure interaction problem considered here is simulated numerically using the immersed boundary method wherein,the added mass is formulated using Boussinesqs approximation. This has been possible since density of the particle is slightly greater than the density of the fluid. The paper has demonstrated both analytically and numerically that settling velocity of a permeable cylinder is greater than that of an impermeable cylinder and it increases with increasing permeability. The effect of channel walls, wall effect, on a permeable particle, located initially along the centreline, is investigated. It is found that wall-effect on the setting velocity decreases as the permeability of a particle increases. Furthermore, for a given permeability wall effect on terminal settling velocity was found to decrease with decreasing fluid viscosity. In addition to such, the settling dynamics of a particle initially located at an off-centre location has been studied for different viscosity and permeability values. It is shown that numerical results are able to reproduce the expected behaviour qualitatively.
Keywords: immersed boundary method; particle suspension; sedimentation; settling velocity; fluid-structure interaction; permeable; porous.
Development of Targeted Compact Nonlinear Scheme with Increasingly High Order of Accuracy
by Kaveh Fardipour, Kamyar Mansour
Abstract: In this paper we present general formulation of targeted compact nonlinear scheme (TCNS) with increasing order of accuracy. This new scheme introduces the incremental-size and ENO-like stencil selection of targeted essentially non-oscillatory (TENO) scheme to weighted compact nonlinear scheme (WCNS). Other aim of the paper is construction of new global smoothness indicators (GSI) for TCNS. We used various benchmark problems of scalar hyperbolic equations and system of Euler equations for investigation of performance of TCNS and new GSIs.
Keywords: High-order numerical method; Weighted compact nonlinear scheme; Targeted compact nonlinear scheme; Targeted interpolation; Shock-capturing.
Theoretical Analysis and Experimental Research of Non-cavitation noise on Underwater Counter-rotation Propellers
by Zeng Sai
Abstract: The non-cavitation noise of counter-rotation propeller is a key factor for detection and recognition of underwater high-speed vehicles. This paper presents a theoretical analysis and a hybrid numerical simulation method of underwater counter-rotation propeller non-cavitation noise. An experimental verification is also included. The sound pressure spectrum model is presented to describe the non-cavitation noise of a counter-rotation propeller with application of generalized acoustic analogy method. The counter-rotation propeller noise is predicted using the frequency-domain acoustic analogy. The flow field is analysed by computational fluid dynamics based on the viscous flow theory. The flow field pressure is used for the acoustic spectrum prediction. The numerical simulation results are verified by a cavitation-tunnel experiment. Throughout the study, the counter-rotation propeller non-cavitation noise is analysed. The overall results indicate that the numerical approach is reliable for predicting non-cavitation noise of counter-rotation propeller, which is beneficial for characteristics extraction and identification of underwater high-speed vehicles.
Keywords: counter-rotation propeller; non-cavitation noise; CFD simulation; generalized acoustic analogy method; cavitation-tunnel experiment.
AN IMMERSED BOUNDARY METHOD FOR SIMULATIONS OF FLOW AND MIXING IN MICRO-CHANNELS WITH ELECTRO KINETIC EFFECTS
by Ahamed Saleel C, Irfan Anjum Badruddin
Abstract: Immersed boundary method (IBM) is a methodology in which any real stationary or moving bodies in flow or heat transfer or mixing field is treated by distinct forcing functions introduced in the respective governing equations, thereby its presence is dealt with much easiness in the computational domain. In addition, the method eases the handling of complex geometry with the use of Cartesian grids that usually do not coincide with the surface of the body. The present work proposes an extension of an IBM based on discrete forcing approach to simulate pressure driven electro osmotic flow and mixing in constricted micro channels. The fractional-step based temporal discretization and finite-volume based spatial discretization on a staggered mesh are used to solve the governing equations. Velocity fields are corrected by a pseudo-pressure term to satisfy the continuity in each computational time step. Constriction is introduced with the help of triangular blocks to enhance the mixing efficiency. Concentration forcing function is a parameter, which is newly defined in this model to satisfy the no-flux boundary condition for species concentration on the blocks (immersed boundary) and is introduced in convection-diffusion equation. The present computational model is validated with the published experimental and numerical results in the literature and finally some sample results generated by the present model is also exemplified.
Keywords: CFD; Immersed Boundary method; Forcing Functions; Finite Volume Method; Navier Stokes Equation; Fractional Step Method; Staggered Mesh; Convection-Diffusion Equation etc.
Suppressing artificial equilibrium states caused by spurious currents in droplet spreading simulations with dynamic contact angle model
by Thomas Antritter, Martin Mayer, Peter Hachmann, Martin Worner
Abstract: Accurate methods for numerical simulation of dynamic wetting and spreading phenomena are a valuable tool to support the advancement of related technological processes such as inkjet-printing. Here, it is demonstrated that numerical methods employing dynamic contact angle models are prone to artificial equilibrium states caused by
spurious (parasitic) currents. The capability of different approaches in reducing spurious currents for sessile and spreading droplets with low equilibrium contact angle is evaluated. To minimize the influence of spurious currents on dynamic contact angle models, a smoothing step in the evaluation of the contact line velocity is introduced in this paper. The benefit and performance of this new approach is demonstrated by algebraic volume-of-fluid simulations of spreading and receding droplets with the Kistler dynamic contact angle model.
Keywords: droplet; wetting; spreading; inkjet printing; dynamic contact angle; spurious currents; parasitic currents; numerical simulation; volume-of-fluid method; OpenFOAM.
A numerical study on the influence of liquid properties on gas-focused micro-jets
by Rizwan Zahoor, Rok Regvar, Saša Bajt, Božidar Šarler
Abstract: In this paper we present a numerical study on the influence of liquid properties on gas-focused micro-jets, such as used for sample delivery in serial femtosecond crystallography. The study is based on solving mixture formulation of Newtonian, compressible two-phase model with the finite volume method and algebraic volume of fluid for treatment of the phase-interface. The density, viscosity and surface tension of the focused fluid span around the material properties of pure water in the range of ±30%, thus representing a large range of possible sample delivery fluids. Fixed liquid and helium gas flow rates are used for jet focusing and the length, diameter, velocity and temperature of the jet are assessed as a function of material properties of the liquid. A thicker and slower jet is observed in case of increased density and surface tension of the focused fluid, while change in liquid viscosity has no effect on flow characteristics. This study expands previous work on experimental validation of the model: influence of operating parameters (Zahoor et al., 2018a), nozzle geometry (Zahoor et al. 2018b) and types of focusing gas (Zahoor et al. 2018c) on the liquid jet behaviour and thus provides a complete computational fluid dynamics insight into jet production.
Keywords: gas dynamic virtual nozzle; focusing gas; liquid properties; micro-jet; compressible multiphase flow; finite volume method; volume of fluid; jetting; dripping.
CFD Modeling of Hydrate Slurry Flow in a Pipeline Based on Euler-Euler Approach
by Abdallah S. Berrouk, Jiang Peng
Abstract: The presence and agglomeration of hydrates particle in oil and gas transportation pipeline can pose a major threat for the flow assurance. Avoidance of hydrate formation by injecting thermodynamic inhibitors is a common but an expensive practice. For this reason, low dosage hydrate inhibitors (LDHIs) of anti-agglomerates are being considered which allows the transport of hydrates slurries directly with minimum risk of pipeline blockage. Understanding the hydrate-containing flow characteristics is of essence to efficiently manage and transport hydrate slurries. In this work, a three- dimensional CFD model of hydrates slurry flow in pipeline was built using Eulerian-Eulerian solid-liquid multiphase approach. RANS RSM model) was used to capture the turbulence. User defined functions (UDFs) of hydrate particle size model and hydrates shear viscosity model which are derived from a correlation of experimental data were developed and integrated into the CFD model. CFD model predictions on pressure gradients at different inlet velocities and different hydrates volume fractions were compared with the experimental data. It was found that very good matching of the experimental pressure gradients was obtained for the case where Camargo and Brinkman were used. The developed numerical model was then used to study the distributions of hydrates velocity magnitude and hydrates volume fraction for different flow conditions. In addition, hydrates deposition characteristics were investigated and the hydrates deposition bed heights were determined for low inlet velocities. This study should help provide valuable insight into hydrate- laden flow properties in pipelines that might help redesign them for better flow assurance.
Keywords: Flow assurance; Hydrate slurry flow; Computational Fluid Dynamics; Euler-Euler approach; hydrate deposition.
Numerical simulation of air flow in needle-to-cylinder electrohydrodynamic device
by Rafal Galek, Joanna Wilk
Abstract: The paper presents the methodology and results of numerical simulation of the flow induced by corona discharge obtained with open-source environment MOOSE (Multiphysics Object-Oriented Simulation Environment) Framework released by Idaho National Laboratory. Coupled system of governing partial differential equations is solved for the values of electric potential and space charge density. The solution is used to calculate the spatial distribution of body force acting on the fluid. The body force is subsequently introduced as a source term in Navier-Stokes equations solved in MOOSE for the value of fluid velocity. The results are presented as velocity profiles at the outlet of the flow generator and integrated to yield volumetric flow rate and mechanical power of the flow. The latter is used along with the value of the electric power to obtain the efficiency of the device. Comparison of the results with data available in literature reference for similar configuration shows satisfactory agreement.
Keywords: electrohydrodynamics; corona discharge; fluid flow; numerical simulation; MOOSE.
Hypersonic flow past a spherically blunted nose cone: A computational study
by Ashish Narayan, S. Narayanan, Rakesh Kumar, Chintoo Kumar, Jagadeesh Gopalan
Abstract: The present study focuses in investigating the aerodynamic characteristics of spherically blunted nose cones at a hypersonic Mach number of 5.8, numerically. The studies are conducted for different combinations of bluntness ratios and semi-cone angles at zero angle of attack in order to ascertain the nose cone geometry which provides minimum aerodynamic drag coefficient. The velocity vector shows the deceleration near the nose followed by a re-acceleration through the sides as well the formation of recirculation zones. The Mach contour depicts bow shock formed ahead of the nose as well as the shock detachment distance. It is observed that the shock detachment distance follows linearly increasing behaviour with increase in bluntness ratios for all the semi-cone angles studied. An empirical correlation is developed for the shock detachment distance using the regression analysis, which reveals that it is primarily a function of the bluntness ratio. It is observed that the aerodynamic drag coefficient attains a minimum value for smaller bluntness ratios and semi-cone angles. Further, the aerodynamic drag coefficient is observed to be a strong function of the bluntness ratios for smaller semi-cone angles. Thus, the current study sufficiently demonstrates that the spherically blunted nose cones with smaller, bluntness ratios and semi-cone angles could act as the finest passive control geometries for achieving lower aerodynamic drag coefficient in hypervelocity vehicles.
Keywords: spherically blunted nose cone; hypersonic flow; bluntness ratio; semi-cone angle; shock detachment distance; aerodynamic drag coefficient; wake profile.
Numerical Study on the Effect of Rheological Parameters on the Droplet Deformation Process in Newtonian and non-Newtonian Two-Phase Systems Using Extended Finite Element Method
by Mohammad Ali Moeeni, Mahdi Salami Hosseini, Mir Karim Razavi Aghjeh, Mehdi Mostafaiyan
Abstract: In the present study, attempts were made to study the effect of rheological parameters on the drop deformation process. For this purpose, both Newtonian and non-Newtonian (Carreau-Yasuda model) were considered and extended Finite Element Method (XFEM) along with Level-Set Method (LSM) were used to simulate the process. The result showed that in Newtonian-Newtonian systems, there was no shear stress overshoot (maximum) during the deformation process and the shear stress increased monotonically until it reached a steady-state, whereas, it exhibited an overshoot (maximum) for non-Newtonian systems. Results also showed that the increase of the wall confinement parameter (R/H) would increase the droplet deformation monotonically for studied viscosity ratios. It was further observed that the steady-state deformation parameter (Dss) was increased as Ca increased from 0.2 to 0.8 for viscosity ratio (λ) between 0.5 and 2.5.
Keywords: Extended finite element method; droplet deformation; viscosity ratio; Newtonian fluid; non-Newtonian fluid.
Penalty and Characteristic-based Operator Splitting with Multistep Scheme Finite Element Method for Unsteady Incompressible Viscous Flows
by Shui Qingxiang
Abstract: The penalty and characteristic-based operator splitting with multistep scheme (penalty-MCBOS) has been developed for the unsteady incompressible N-S equations. In each time step, the N-S equations are split into the diffusive part, the convective part by adopting the operator splitting method and the pressure part by applying the penalty method with low penalty parameters. For the diffusive part, the temporal discretization is based on backward difference method and is solved by PCG method. For the convective part, the temporal discretization is performed by characteristic-Galerkin method. It is solved explicitly and the multistep technique is introduced. The pressure can be solved from the pressure part and has no need to solve the pressure Poisson equation. The plane Poisseuille flow, the lid-driven cavity flow, the lid-driven triangular cavity flow and the backward-facing step flow are adopted to validate the present model. It is show that the numerical results are in good with previous published data, and the present model has high efficiency. efficiency. In particularly, for the backward-facing stepflow at Re=3000, the flow becomes periodical in time and dynamic evolution processof vortex can be simulated.
Keywords: unsteady incompressible N-S equations; characteristic-Galerkin method; penalty method; finite element method; multistep scheme.
FLOW AND WALL SHEAR RATE ANALYSIS FOR A CRUCIFORM JET IMPACTING ON A PLATE AT SHORT DISTANCE
by Florin Bode, Amina Meslem, Claudiu Patrascu, Ilinca Nastase
Abstract: There are numerous turbulence models that have been developed in the past years, many of them being used in predicting flows, turbulence, mass and/or heat transfer. The particular case of an impinging jet implies all of the above. In this study, the performance of eight highly used Reynolds Averaged Navier-Stokes (RANS) turbulence models, is examined in simulating a very sheared lobed impinging jet. The study is based on the investigation of an orthogonally lobed jet, impinging on a flat surface, that flows out from a nozzle having a cruciform cross-section at a Reynolds number of 5620. Two experimental methods were implied for the comparison with numerical results in order to evaluate the capability of these eight turbulence models. For the measurement of the radial distribution of the wall shear rate ? an electrodiffusion method was employed. The velocity flow fields in two characteristic planes (a major plane passing through two lobes, and a minor plane passing through two troughs of the orifice) were captured using a Particle Image Velocimetry technique. Steady state RANS numerical simulations are conducted in combination with several turbulence models in order to provide closure. The relative strengths and drawbacks of the SST k-?, k-? standard, TransSST, k-? realizable, RNG k-?, k-? standard, k-kl-? and RSM turbulence models are compared. The study shows that the centreline velocity decay, seen in the free wall region, cannot be predicted by none of the turbulence models, although, except k-? standard, all models predict close values to experimental data in the region that is being affected by the presence of the wall. Near the target wall, the TransSST model is the only one that can predict streamwise and radial velocity profiles in both major and minor planes. The maximum value of the wall shear rate is well predicted by SST k-?, whereas the best radial distribution of wall shear rate was obtained by TransSST.
Keywords: Impinging jet; Numerical simulation; PIV; Electrodiffusion method; Turbulence models comparison; Wall-shear rate.
Wave making characteristics of submerged vehicle in a stratified ocean
by Li Dawei, Xie Lixuan, Sun Lin
Abstract: The numerical methods are used to obtain the wave making characteristics of pycnocline interface and free surface in a stratified ocean. The disturbance source is a submerged vehicle (SUBOFF). The unsteady viscous numerical method based on 3-D incompressible RANS equations, re-normalization group k−ε turbulence model and VOF method are designed to simulate the fluid characteristics of the submerged body moving in a stratified ocean. Both ocean layers that have difference density are finite depth. Typical Froude numbers of flow varied between 0.05 and 2. The simulated results show that the wave making characteristics on the surface and interface contain contributions in two different modes: a surface-wave mode and an internal-wave mode. Meanwhile, the velocity of submerged body has a larger effect on the wave systems and structures of free surface and pycnocline interface. The typical wavelength of the interface fluid field is increased with Froude number, as theory predicts.
Keywords: Submerged Vehicle; Stratified Ocean; Interface wave mode; Surface wave mode.
Second-order slip condition considering Langmuir isothermal adsorption for rarefied gas microflows
by Nam T.P. Le, Thoai N. Tran, Minh H. Dang
Abstract: Effect of the slip boundary condition on rarefied gas flow simulations plays an important role to understand the behavior of gas microflows in MEMS. Several second-order slip conditions were proposed by the models of the kinetic theory of gases to simulate the rarefied gas microflows, in which the so-called classical second-order slip condition was derived from the Karniadakis et al. model. In this paper, a new second-order slip condition is proposed to employ with the Navier-Stokes-Fourier equations for simulating the rarefied gas flows in microchannels. It is derived by combining the Langmuir isothermal adsorption and the Karniadakis et al. model, with the aim of achieving a more realistic physical model. The pressure-driven back-forward-step, the Couette and pressure-driven Poiseulle rarefied gas flows in microchannels are investigated to validate our new second-order slip condition. Slip velocities using our new second-order slip condition are better than those using the conventional Maxwell and the so-called classical second-order slip conditions, and are in very good agreement with the DSMC data for all cases considered.
Keywords: Langmuir isothermal adsorption; new second-order slip condition; slip velocity; rarefied gas flows.
The requirements for the turbulence models in application to heat transfer analysis of turbine parts: The Thermal Stresses Point-of-View
by Kamil Banas
Abstract: This paper investigates the requirements for turbulence models, in application to heat transfer simulation of turbine part, from a thermal stresses point-of-view. The investigation was based on thermal-fluid-structure interaction (thermal-FSI) analysis of the convectively-cooled turbine vane C3X, for which experimental heat transfer data is available and a laminar-turbulent transition occurs in the boundary layer. First, the production of turbulent energy at the stagnation point was investigated based on multiple turbulence models able to capture the transition phenomena. Next, the influence of turbulence modeling on thermal stresses was addressed. The stresses produced by experimental temperature field were compared with the modeled temperature-induced stresses. The presented results show that even relatively small discrepancies in temperature in an area where gradients of temperature are high can lead to large discrepancies in thermal stresses.
Keywords: Requirements for turbulence models; turbulence modelling for turbine; thermal stresses; conjugate heat transfer analysis of turbine; laminar-turbulent transition; turbulence kinetic energy at a stagnation point.
Reflection behavior of the porous tube boundary condition for FSI simulations of the truncated vascular network
by Seyed Hamidreza Attaran, Hanieh Niroomand-Oscuii
Abstract: Among the several aspects of numerical simulations, the boundary condition is one of the most important issues to deal with in simulations of the cardiovascular network. Previously we introduced a new method for modeling the downstream of the truncated artery by means of porous media theory. In the present work, we aim to investigate the reflection characteristics of this new model and find the relation between the permeability and the reflection ratio. Numerical simulations have been performed and 7 different cases have been tested. The results show a strong dependence of reflection on the permeability magnitude and it was found that the porous interface behaves like an intermediate situation between closed-end and open-end tubes.
Keywords: boundary condition; wave reflection; fluid-structure interaction; numerical simulation.