Progress in Computational Fluid Dynamics, An International Journal (27 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.
Studies on the evolution of shock wave in gas film clearance for aerostatic thrust bearing
by Shang-han Gao, Sheng-Long Nong
Abstract: This paper researches the evolutions of shock waves in the gas film of aerostatic thrust bearing with a single air supply inlet. The ANSYS Fluent 18.0
Keywords: Aerostatic thrust bearing; Flow field characteristics; Shock wave; Load capacity.
Wall effects in flow past a rigid stationary sphere enclosed in a rectangular channel
by Asterios Pantokratoras
Abstract: The effect of finite boundaries on the drag coefficient applied in a sphere enclosed in a rectangular channel has been investigated numerically. The three-dimensional Navier-Stokes equations in Cartersian coordinates have been solved numerically. The investigation covers the Reynolds number range from 0.01 up to 300 and blockage ratio from very low value to unconfined case. It is found that when the sphere is close to channel walls the flow is unsteady and when the sphere lies away from the walls the flow becomes steady for Re≤200. For Re=300 the flow is unsteady for confined and unconfined cases. The drag coefficient has been calculated for different values of confinement plus the Strouhal number for Re=300.
Keywords: sphere; drag; rectangular; unsteady; steady.
Fluid-Structure Interaction Analysis of the Return Pipeline in the High-Pressure and Large-Flow-Rate Hydraulic Power System
by Yong Sang, Pengkun Liu, Xudong Wang, Weiqi Sun, Jianlong Zhao
Abstract: In order to investigate the static and dynamic characteristic of the return pipeline in the high-pressure and large-flow-rate hydraulic power system and avoid pipeline vibration, the geometry model of a simply supported pipeline connected to a sliding valve is established and a one-way coupling fluid-structure method is introduced to study the return pipeline vibration. First, the modal analyses with an empty pipeline and a pipeline filled with water are performed and compared. Pipeline resonance phenomenon is investigated with the prescribed fluctuating flow and the pipeline response frequency is achieved by FFT analysis. Then, the corresponding modal experiment is performed. The calculated and experimental results are proved to be consistent. In the end, the return pipeline dynamic transient response is simulated and the dynamic mesh and UDF are combined in Fluent when the sliding valve is open. The return pipeline vibration and water hammer phenomenon are observed. The analysis of the dynamic characteristics under the influence of the fluid velocity and the pipeline wall thickness is carried out. The results show that the flow induced pipeline vibration caused by the valve opening cant be lightened effectively by reducing the fluid inlet velocity, but can be greatly mitigated by increasing the pipeline wall thickness.
Keywords: Fluid-Structure interaction; pipeline; vibration; coupling; water hammer; sliding valve.
A switching ILU(0)-SGS preconditioner for matrix systems of incompressible flow and heat transfer using condition number estimates
by Krishna Chandran, Krishnamurthy Muralidhar
Abstract: Preconditioning strategies for pressure-velocity-temperature matrix systems obtained from an unstructured finite volume discretization of the three-dimensional incompressible flow and thermal energy equations are studied by reference to their condition numbers. The computational procedure for determining the exact condition number is expensive and is circumvented by providing Gershgorin-type theoretical bounds available for diagonally dominant matrices. The discretized system of linear algebraic equations of mass, momentum, and temperature are solved using the preconditioned-BiCGSTAB algorithm. Condition numbers of velocity and temperature matrices obtained from the Gershgorin-type estimates show these to be well-conditioned compared to pressure. As a result, for the well-conditioned matrices the symmetric Gauss Seidel (SGS) preconditioner performs well on single and multi-processor architectures when compared to ILU(0) that requires additional LU factorization. Based on numerical experiments, the present study proposes a preconditioning algorithm that switches from ILU(0) to SGS preconditioner for the well-conditioned matrices based on the Gershgorin-type condition number estimates. The composite algorithm shows a reduction in the overall simulation time for flow and mixed convection inside a 3D differentially heated cavity. The present study also shows that the Gershgorin based upper bounds for the largest singular value capture the transition details of flow dynamics in several applications. For developing flow of a power law fluid in a tube, the upper bound correlates with the power law index. For flow past a circular cylinder, the time-wise oscillation of the theoretical bound of the largest singular value predicts the onset of vortex shedding.
Keywords: Unstructured finite volume method; singular values; condition number; Gershgorin estimates; switching preconditioning.
A CFD Study on the Effect of Compression Ratio on Combustion Characteristics and Emissions in a Spark-Ignition Engine
by Sachin Kumar Gupta, Mayank Mittal
Abstract: The variable compression ratio is a viable technology to improve engine performance and to reduce greenhouse gas emissions. In the present study, numerical simulations were performed to quantify the effect of compression ratio (CR) on the characteristics of a spark-ignition (SI) engine. The simulations were carried out using Converge CFD with detailed chemical kinetics. The validation study for gasoline-driven SI engine, having bowl-in-piston and flat head type configuration, with CR of 8.5:1 showed that the numerical framework was accurate enough to predict the combustion characteristics of the engine. After verifying the model, a parametric study was conducted at different spark timings and CRs. Results showed that the indicated thermal efficiency increased by 3.4% and emission levels of carbon dioxide and carbon monoxide decreased by 11.8% and 8.1% when CR was increased from 8.5:1 to 12:1, respectively. Also, the flame velocity was found to be increased with the increase in CR.
Keywords: Computational fluid dynamics; Spark-ignition engine; Gasoline; CFD; Performance; Combustion; Emission; Detailed chemical kinetics.
Three-dimensional numerical modelling of the flow around a circular bridge pier: a scaling analysis
by Martin Cisternas, Olivier Skurtys
Abstract: Three-dimensional numerical simulations of water flow around a vertical circular pier using the Large Eddy Simulation'' method are presented. All simulations were performed using the solver interFoam'' supplied with OpenFOAM. The objective of this paper is to provide a better understanding of the influence of the free surface on the velocity and pressure fields. The influence of three parameters, the Froude number (Fr), the Reynolds number (Re) and the geometrical parameter G (ratio between the water depth and the pier diameter), on the behaviour of free surface is investigated. Qualitative observations and quantitative results are reported. It is shown that downstream the pier, the flow depends on Fr and G. Indeed, a dimensionless number combination of $Fr$ and G describes all the phenomena related with the free-surface deformation (the wavelength size, the free-surface deformation). On the contrary, for estimating the drag coefficient or for describing dynamic phenomena such as the separation of boundary layers in front of the pier a dimensionless number combination of Fr and Re must be used.
Keywords: Circular pier; Large Eddy Simulation; Scaling analysis; Turbulent flow; Two-phase volume of fluid.
Flow Instability of Two-Parallel Moving Walls in Cubical Cavity Induced by an Inner Cylindrical Shape at Different Radii Sizes
by Basma Souayeh, Fayçal Hammami
Abstract: A computational analysis has been performed to study the flow instability of two-parallel wall motions in a cubical cavity incorporated by a cylindrical shape under different radii sizes. A numerical methodology based on the Finite Volume Method and a full Multigrid acceleration is utilized in this paper. Left and right parallel walls of the cavity are maintained driven and all the other walls completing the domain are motionless. Different radii sizes (R=0.075, 0.1, 0.125, 0.15 and 0.175) are employed encompassing descriptive Reynolds numbers that range three orders of magnitude 100, 400 and 800 for the steady state. The obtained results show that positions R=0.15 and R=0.175 of the inner cylinder promote cell distortion. Also, when the radius equates to R=0.15, that may lead to the birth of tertiary cells at Re=400 which are more developed for Re=800. Thereafter, analysis of the flow evolution shows that with increasing Re beyond a certain critical value, the flow become unstable and undergoes a Hopf bifurcation. A non-uniform variation with the radius size of the inner cylinder is observed. Otherwise said, by elongating the radius of the cylinder, that leads to decrease the critical Reynolds number, hence, the acceleration of the unsteadiness. On the other hand, by further increasing Reynolds number more than the critical value from 1200 to 2100, we note that the kinetic energy is monotonously increasing with Reynolds number and a stronger motion in the velocity at the level of the rear wall of the cavity is observed. Furthermore, the symmetry of flow patterns observed in the steady state has been lost. Therefore, a systematic description of various effects illuminating the optimum geometrical parameters to achieve effective flow behavior in those systems has been successfully established through this paper.
Keywords: Computational analysis; Cylindrical shape; Lid-driven cavity; Two-parallel wall motions; Cylinder radius.
CFD validation of wind pressure distribution on a tall building under the influence of upstream terrain.
by Rajasekarababu KB, Vinayagamurthy Ganesan
Abstract: In order to predict the wind loads and behaviour of wind environment on and around buildings, it is essential to investigate the pressure distribution, aerodynamic coefficients, wind flow fields and recirculation lengths. Most of the investigations and international wind load standards have not highlighted the wind load and wind environment on and around structures under different terrain effects. The main objective of this investigation is to explore a practical numerical approach for the accurate estimation of wind loads and their impact on buildings under different upstream conditions. In this investigation, simulations are done in Improved Delayed Detached Eddy Simulation (IDDES) turbulence model along with the formulation of k-? Shear Stress Transport (SST) using Computational Fluid Dynamics (CFD). Mean pressure coefficients evidenced the influence of terrain effects as wind loads on building faces. The positive pressures on the windward face at a lower level greatly varied with upstream terrain due to the increase of ground roughness. The computed results are validated with the experimental data and error was estimated at various location. The CFD results over-predicted the experimental values in leeward and side faces by negative pressure in the edges due to strong shear turbulence and expanding wake vortices. Furthermore, in the cross-validation of the predicted mean drag coefficients by IDDES against wind tunnel measurements showed good agreement. Hence this investigation promotes the practice of IDDES in the wind-resistant design for tall buildings.
Keywords: Wind pressure distribution; upstream terrain; Open and suburban winds; Wake recirculation length; Aerodynamic coefficients; Rectangle building; Improved Detached Eddy Simulation; CFD.
Supercritical carbon dioxide turbomachinery development using scaling methodology, computational fluid dynamics and experimental testing in aeroloop
by Vijajaraj K, Punit Singh
Abstract: Supercritical carbon dioxide (SCO2) turbomachinery design experience is limited. This paper examines similarity-based scaling strategy to develop a radial inflow turbine and centrifugal compressor from existing proven designs for a 50kWe SCO2 Brayton cycle. The SCO2 turbine and compressor are developed from well-established NASA 1730 air turbine and NASA 4613 radial pump, respectively. Computational fluid dynamic (CFD) simulations with air and SCO2 and experimental testing in aeroloop are carried out for the developed turbomachinery. The results are compared with original NASA test data. For the turbine, the CFD simulation and experimental results were in good agreement with NASA data. For the compressor, CFD simulation results with SCO2 showed good conformance especially the, efficiency values, which were much lower for air. The compressor experimental results were well away from the NASA data when head rise coefficient was considered, but the flow coefficient zone coincided with that of simulation
Keywords: Supercritical carbon dioxide Brayton cycle; Scaling; CFD; Turbine; Compressor.