Progress in Computational Fluid Dynamics, An International Journal (32 papers in press)
Application of different k-e turbulence models on combustion process modelling in small-scale pellet stoves for household heating
by Zagorka Brat, Nebojša Manić, Dragoslava Stojiljković, Marta Trninić
Abstract: Constant demand for development and construction improvement of small-scale pellet stoves and boilers (nominal power up to 50 kWth) for household heating leads to conclusion that appropriate upgrading should be done on that field. Optimization of stove construction should be performed based on results of experimental tests, as well as results of numerical analysis and mathematical modelling of combustion process. This is compulsory, because stove construction shall comply with demands for energy and environmental characteristics defined in appropriate quality standards. Besides, stove should operate with pellets of different quality, produced from various biomass raw materials. Considering that numerical analysis is less time consuming process than experimental tests, nowadays it is more often used for development and improvement of combustion appliance construction. As the turbulence modelling is one of the key parts in the eligible numerical analysis, the aim of the research presented in this paper is to define the influence of different k-e turbulence models and their application on defined CFD mathematical model of combustion process in small-scale pellet stove for household heating.
Keywords: small-scale pellet stove; turbulence model; combustion; CFD.
By-pass transition control with a DBD plasma actuator model coupled with a laminar kinetic energy turbulence model
by Zinon Vlahostergios, Pavlos Kaparos, Kyros Yakinthos
Abstract: The effect of a DBD plasma actuator on the by-pass boundary layer transition control is numerically investigated. A two-equation DBD plasma actuator model is coupled with a three-equation eddy-viscosity turbulence model, which adopts the laminar kinetic energy concept. The investigated test-cases concern zero and variable freestream pressure gradient transitional flows on a flat plate with a sharp leading edge, which belong to the ERCOFTAC experimental database. The experimental data describe the transition characteristics with no plasma actuation and hence, they are used to assess the turbulence model behavior and as a reference point in order to quantify the coupling effect of the plasma actuator model with the laminar kinetic energy concept. The investigation is focused on the effect of the varying plasma actuator voltage on transitional and turbulent boundary layer characteristics, such as the Reynolds-stresses, the turbulent dissipation and the laminar kinetic energy distributions. The results show that with the DBD actuator activated, the turbulent quantities are suppressed, the transition onset location moves downstream and a boundary layer transition delay is observed.
Keywords: plasma actuator; by-pass transition control; laminar kinetic energy; plasma and turbulence interaction.
A new approach to solve mixture Multi-phase Flow model using Time Splitting Projection Method
by Farhang Behrangi, Mohammad Ali Banihashemi, Masoud Montazeri Namin, Asghar Bohluly
Abstract: This paper was aimed at introducing a new approach to numerical simulation of two-phase flows by solving the mixture model equations. In this approach, an effective numerical algorithm was developed based on the time splitting projection method to solve the mixture model equations for an incompressible two-phase flow. The mixture model contained a set of momentum equations for the mixture phase, a mixture continuity equation, a second phase continuity equation, and the algebraic equation for the relative velocity. In this study, a new technique was developed to solve the mixture continuity equation based on the time splitting projection method. One of the advantages of the mixture model is that, in addition to determining the velocities of each phase, it solves only one set of momentum equations and calculates the velocity differences between the phases by solving the relative velocity equation. Accordingly, an extended finite volume vertical two-dimensional numerical model was used to determine the pressure distribution, velocity field, and volume fraction of each phase at each time step. The model has been used in various tests to simulate unsteady flow problems. Comparison between numerical results, analytical solutions and experimental data demonstrated a satisfactory performance.
Keywords: time splitting; projection method; mixture model; multi-phase flow; slip velocity; computational fluid dynamics; phase interpenetrate; numerical methods; numerical modelling;numerical solution.
Numerical study on the engine room cooling performance of a medium size excavator
by HyungRak Kim, Young Min Seo, Man Yeong Ha, JaeSeok Lee, Pan Young Kim
Abstract: The temperatures at the inlet and outlet of the heat exchanger located inside an engine room are an important design variable which has much effect on the engine room cooling performance of a vehicle and heavy machinery. This study carried out the numerical simulation to investigate three-dimensional thermal and flow characteristics inside the engine room and also to predict the temperatures at the inlet and outlet of the heat exchanger located inside of the engine room that can evaluate the engine room cooling performance of a medium size excavator. The effects of the change in distance between the fan located inside the engine room and heat exchanger, the shroud tip clearance to the fan, and the fan immersion depths of box type shroud and bell mouth type shroud on three-dimensional thermal flow inside the engine room and cooling performance are presented in this paper.
Keywords: Heat exchanger; Cooling performance; Excavator; CFD.
Benchmark simulations of flow past rigid bodies using an open-source, sharp interface immersed boundary method
by Utku Senturk, Daniel Brunner, Hrvoje Jasak, Nicoleta Herzog, Clarence W. Rowley, Alexander J. Smits
Abstract: This study reports benchmark results for a new immersed boundary method based finite-volume solver within the framework of the open-source toolbox foam-extend 3.2. The immersed boundary formulation uses a discrete forcing approach based on a weighted least squares approximation that preserves the sharpness of the boundary. Five test cases with increasing complexity are used. Results are also presented for the flow past a low-aspect-ratio plate that pitches about its leading edge at a Reynolds number of 2000. Force coefficient results are compared with available experimental and computational data. The results show that foam-extend 3.2 appears to be a promising open-source tool for solving flows with steady and unsteady immersed boundaries.
Keywords: Computational fluid dynamics; open source; OpenFOAM; immersed boundary method; discrete.
Investigation of numerical simulation of non-Newtonian flows dam break and dam breach in open channels using modified VOF method
by Esmail Lakzian, Mostafa Niazi
Abstract: This paper concerns modeling dam break flows of non-Newtonian Herschel-Bulkley (HB) fluid by using the volume of fluid (VOF) method and high resolution advection schemes. The VOF method is based on the fact that two or more fluids (or phases) are not interpenetrating. In the present study, a modified approach is used to overcome this problem. The OpenFoam software is employed and inter-Foam solver is used in two and three-dimensional model with non-Newtonian fluid. The numerical results of the present study are compared with analytical, numerical, and experimental results found in literature. The numerical code which is used showed better agreement with the experimental data than those using shallow water equations and PLIC method.
Keywords: Volume of Fluid Method (VOF); Dam Break flows; non-Newtonian Model.
Use of nanofluids as coolants in buoyancy-driven thermal management of embedded heating components of small-scale devices
by Massimo Corcione, Alessandro Quintino, Elisa Ricci, Andrea Vallati, Paweł Ocłoń
Abstract: A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in square cavities partially heated at the bottom wall and cooled at both sides, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles, is solved through a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on literature experimental data. The pressure-velocity coupling is handled by the way of the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity width, the heated fraction of the bottom wall, the average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the cooperation between the solutal and thermal buoyancy forces results in a significant enhancement of the heat transfer performance of the nanofluid compared with the pure base liquid. Moreover, the impact of the nanoparticle dispersion into the base liquid is found to increase notably with increasing the average temperature, whereas, by contrast, the other controlling parameters have moderate or negligible effects.
Keywords: Nanofluid; Natural convection; Square enclosure; Partially-heated bottom wall; Twophase modeling; Enhanced heat transfer.
Colocated Pressure-Velocity Coupling in Finite Difference Methods
by Mikael Ersson
Abstract: A simple method to be used for colocated pressure-velocity coupling in incompressible flows is presented with a full derivation. A number of standard test cases are shown that demonstrate the ability of the method to produce accurate results. The method avoids spurious pressure oscillations while keeping the pressure Poisson equation stencil compact. This is obtained by discretizing the continuity and pressure derivatives with first order differences with opposite directions, i.e. backward difference for continuity and forward difference for pressure (BCFP). The equations are also approximated using a forward difference for continuity and a backward difference for pressure (FCBP). In order to obtain a second order approximation the mean between BCFP and FCBP is used, i.e. a central difference. The paper gives a useful alternative to existing methods for pressure-velocity coupling in Finite Difference Methods in which a staggered arrangement is not desirable.
Keywords: Finite Difference Method; FDM; Spurious Oscillations; Colocated Variable Arrangement; Collocated Variable Arrangement.
INVESTIGATION OF PEMFC PERFORMANCE WITH VARIOUS CONFIGURATIONS OF SERPENTINE AND INTERDIGITATED FLOW CHANNEL
by LAKSHMINARAYANAN VARADHARAJAN, Karthikeyan Palaniswamy
Abstract: The Proton Exchange Membrane Fuel Cell (PEMFC) performance is influenced by design and operating parameters like rib to channel width ratio (R: C), the depth of the channel, the number of passes on the flow channel, operating pressure, temperature, relative humidity and the inlet gases stoichiometric ratio. In this paper, the PEMFC performance in an effective area of 25 cm2 with various ribs to channel width ratios (R: C - 1:1, 1:2, 2:1 and 2:2) on serpentine and interdigitated flow channels was investigated numerically and experimentally. The numerical and experimental maximum power density deviation of serpentine and interdigitated flow channel results with rib to channel width 1:2 were found to be 12.16 % and 12.2 % respectively.
Keywords: Rib to channel width ratio; CFD; Operating parameters; Stoichiometric ratio; Design parameters; Serpentine flow channel; interdigitated flow channel.
Effects of magnetohydrodynamics on temperature and shock standoff distance in a supersonic flow over a blunt body
by Sanjiv Paudel, Saleen Bhattarai, Sudip Bhattrai
Abstract: Magnetohydrodynamics has been one of the promising ways of increasing efficiency of supersonic as well as hypersonic propulsion systems, mostly by applying magnetic fields over shockwaves. This paper presents the effects of magnetic fields with different strengths both along and transverse to an electrically conducting flow at Mach 2.94 over a blunt body using a density-based solver implemented with MHD equations in OpenFOAM. The solver utilizes Riemann method with an AUSM+ flux splitting technique along with limited linear interpolation. The effects of imposed magnetic field on temperature and shock standoff distance are observed. Movement of the shockwave due to the application of magnetic field on the geometry, along with a consequent change in shock standoff distance is presented in the paper. The influence of a magnetic fields direction on the Mach number of the flow is also shown. Likewise, the stagnation temperature of the blunt body is demonstrated to be independent to the direction of applied magnetic field.
Keywords: bow shock; flow control; supersonic; re-entry; OpenFOAM.
Numerical study of the effect of suction at a compressible and high Reynolds number flow to control the flow separation over Naca 2415 airfoil
by Esmaeel Fatahian, Ali Lohrasbi Nichkoohi, Hossein Fatahian
Abstract: This study focused on delaying and controlling the flow separation over Naca 2415 airfoil by finding the best slot location with the most effective suction velocity ratio and suction angle to apply suction at a compressible and high Reynolds number flow using computational fluid dynamics method. The results were obtained with two dimensional compressible Reynolds-averaged Navier-Stokes equations, and the turbulence was simulated with k-ԑ RNG turbulence model. The results indicated that the most effective slot locations for applying suction were between 0.3 to 0.6 of the airfoil chord length. Also, it was found that the maximum value of the lift coefficient was obtained at an angle of attack of 16
Keywords: Flow control; Suction; Lift coefficient; Drag coefficient; Flow separation; Boundary layer.
Choice of Model Parameters in Turbulent Gas-Solid Flow Simulations of Particle Classifiers
by Arun Appadurai, Vasudevan Raghavan
Abstract: Comprehensive numerical simulations of particle laden gas flows are complex in general and its complexity increases dramatically when real world equipment are modelled. In this study, several model parameters required to economically simulate such a flow in a lab-scale classifier are tested. Initially, numerical results obtained from a single-phase (gas alone) model of a static classifier are validated against experimental results from literature. For flows characterised with dominant swirl, Reynolds Stress Model is capable of predicting the measured gas velocities with reasonable accuracy. The single-phase model is further extended to two-phase by the addition of particle phase in a static separator through Lagrangian formulation. Several parameters such as turbulence dispersion, particle rough wall model and wall restitution coefficients are investigated. On comparing the numerical results against experimental data from literature, it is found that the accuracy of particle separation is improved with the inclusion of turbulence dispersion. Particle rough wall model and wall restitution coefficients are found to affect the flow of coarse particles only. The validated single-phase model is further used to simulate single-phase flow in a dynamic classifier with a rotor. The influence of rotor rotation is captured by using a multiple frames of reference model. The predicted results in terms of pressure drop across the classifier from the model are validated against the experimental results from literature. Physics involved in the flow inside a dynamic separator are explained using the simulated results.
Keywords: Gas-solid flow; swirl flow; particle separation; turbulence models; particle rough wall model; turbulence particle dispersion; multiple reference frame.
A local and fast interpolation method for mesh deformation
by Jing Tang, Mingsheng Ma, Bin Li, Pengcheng Cui
Abstract: In this study, a local interpolation approach is developed to deal with the deformation of a mesh containing structured and stretched viscous elements. A new strategy is proposed to compute the local influence distance at each movable mesh point. This strategy not only helps to exclude unnecessary donor points from the computation of the new position of the mesh point, but also gets those viscous elements stiffened appropriately such that they are no longer inverted easily. Further improvements on the efficiency of the proposed algorithm are achieved by parallelizing the algorithm and developing a bounding box technique to speed up the procedure of searching donor points. Meanwhile, the robustness of the proposed algorithm in the case of large boundary movement is enhanced by incorporating a state-of-the-art multi-step technique. Various numerical experiments are conducted, and the results show that the proposed algorithm could treat cases involving complex boundary movements at a low level of computing consumptions.
Keywords: mesh deformation; local interpolation; acceleration method; parallel algorithm.
Comparative RANS turbulence modelling of lost salt core viability in high pressure die casting
by Sebastian Kohlstädt, Michael Vynnycky, Alexander Neubauer, Andreas Gebauer-Teichmann
Abstract: In this work, the implementation of three turbulence models inside the open-rnsource C++ computational fluid dynamics (CFD) library OpenFOAM were tested in 2Drnand 3D to determine the viability of salt cores in high pressure die casting. A finite-volume and volume of fluid approach was used to model the two-phase flow of molten metal and air, with the latter being treated as compressible. Encouragingly, it is found that, although the choice of turbulence model seems to affect the dispersion of the two-phase interface, the force acting at the surface of the salt core depends only very weakly on the turbulence model used. The results were also compared against those obtained using the commercially available and widely-used casting software MAGMA5.
Keywords: turbulence; RANS; volume-of-fluid method; OpenFOAM; high-pressure die casting; aluminum; lost cores; salt core viability.
NUMERICAL SIMULATION OF LIQUID MASS COLLISION WITH A WALL
by Alexander Aganin, Tatiana Guseva
Abstract: Numerical simulation of high-speed liquid mass collision with dry or wetted rigid walls by the CIP-CUP method on dynamically adaptive Soroban grids is studied. A number of specially selected test problems is used for validating the resolution of the main features of the collision separately: 1D uniform liquid flow impact onto a wall, 1D plane discontinuity breakdown at the gas-liquid interface, 2D cylindrical discontinuity breakdown in liquid, blunt liquid cone and plane semi-infinite liquid flow impacts onto a dry wall in the attached shock wave mode, sharp liquid cone impact onto a dry wall in the shockless mode with laterally spreading liquid flow. The results of their calculations are in good correlation with the corresponding analytical and numerical solutions. A problem of hemispherically-ended high-speed jet collision with a wall covered by a thin liquid layer is considered as a benchmark to illustrate the capabilities of the CIP-CUP/Soroban-grid approach to resolving the main features of the high-speed liquid collision together. Its numerical solution is in good agreement with the known similar results.
Keywords: liquid jet impact; shock waves; large deformations of gas-liquid interface; Euler equations; CIP-CUP method; Soroban grid; artificial viscosity; interface capturing.
Experimental and computational study of optically-driven electrothermal vortex
by Xudong Pan, Jae-Sung Kwon, Jian Wei Khor, Steven T. Wereley
Abstract: Optically-driven electrothermal vortex flow is one of the most effective ways to induce convection in microscale domains. In this work, side view microscopic imaging is used for qualitative flow visualization as well as quantitative μPIV measurements of the vortex pattern in microfluidic chips between simple parallel-plate electrodes. The uniform electric field is generated from parallel-plate, indium tin oxide (ITO) electrodes separated by 400 μm. Alternating current (AC) electric fields at a range of frequencies (9kHz-100kHz) and voltages (5V-40V) are applied to the electrodes to drive the flow. Simultaneously, a near-infrared (1064 nm) laser beam at various intensities is focused on the bottom electrode surface of the microchannel. Strong electrothermal vortices are observed in the flow visualization experiments and measured by μPIV. A realistic computational model featuring a laser heat source and an applied electrical field is constructed. Computational simulations are performed under the same conditions as the experiments. The simulations and the experiments show the same qualitative behaviors and also agree quantitatively when comparing the maximum velocities.
Keywords: electrothermal vortex flow; qualitative flow; μPIV,optically-driven; microfluidics.
A High Order MOOD Method For Compressible Navier-Stokes Equations : Application To Hypersonic Viscous Flows
by Rodolphe Turpault, Tanh-Ha Nguyen-Bui
Abstract: A very high-order finite volumes numerical method is designed for the\r\nsimulation of Navier-Stokes equations on 2D unstructured meshes. This scheme is based on the MOOD methods described for Eulers equations, its an interesting alternative in the design of a scheme adapted to accurate simulations of flows with discontinuities, in all the domain. The main originality of our method is to include the diffusion and viscosity terms of Navier-Stokes equations. These terms are discretized with the same accuracy that convection terms. It allowss to treat the hypersonic viscous interactions with high\r\naccuracy. Numerical experiments are conducted to demonstrate the performance of the proposed method.
Keywords: MOOD Methods; High-Order; Viscous Fluids; Supersonic; Discontinuities; Simulations.
Simulating High Rayleigh Number Natural Convection in High Aspect Ratio Horizontal Rectangular Enclosures Using Code Saturne Platform
by Fahad Sarfraz Butt, Abdul Waheed Badar, Abdullah Zafar, Ajaz Bashir Janjua
Abstract: In the present study, buoyancy driven flows of water and air in very high aspect ratio (12-48) horizontal rectangular enclosures are studied numerically for Rayleigh numbers ranging from 103 to 107 using open-source CFD tools to obtain natural convection heat transfer coefficients which are then compared with established empirical correlations based on experimental results. This study has been motivated by the finding that hardly any CFD based literature on very high aspect ratio rectangular enclosures exist and also by the fact that such enclosures have numerous applications in solar thermal systems such as solar collectors, solar passive heating/cooling systems, and solar stills etc. It has been reported in the literature that at high aspect ratios, the numerical solution suffers from convergence issues. In the present study, using Code_Saturne as flow solver, the convergence issues have not risen and the error between numerical and experimental results increases with an increase in Rayleigh number bounded by maximum relative error of approximately 8%.
Keywords: CFD; Heat-Transfer; Convection; Enclosures; Code saturne.
A numerical approach to solve fluid-solid two-phase flows using time splitting projection method with a pressure correction technique
by Kambiz Farahi Moghadam, Mohammad Ali Banihashemi, Peyman Badiei, Ali Shirkavand
Abstract: This study used a vertical 2D model to simulate the sediment transport pattern in open channels by applying the EulerEuler two-phase flow model or the complete two-fluid model based on the Favre averaging method. In this study, a two-phase hydrodynamic method was developed to simulate the dynamics of water flow and sediment. The projection method with a pressure correction technique, along with the time splitting algorithm was used for the first time to solve the EulerEuler complete two-phase equations of fluid-sediment. Moreover, the conservation of mass and momentum of both phases was assured generally and partially in the equation discretization stage by providing a novel method developed from integration of the momentum and continuity equations of each phase. In addition, the horizontal and vertical velocities of both phases were discretized independently, without multiplying them by additional parameters, such as the phase concentration. At the end, the models performance in simulating the sediment flow process in a direction perpendicular to the flow was assessed. This assessment for the first time used a Favre averaging-based two-phase model to investigate longitudinal changes in the sediment profile. Moreover, the sediment concentration profile was evaluated in a specific type of flow, known as the sheet flow. Assessment of the model showed its high capability in dynamic simulation of water and sediment flows either at the flow depth or along the channel.
Keywords: complete two fluid model; Favre averaging; numerical modelling; pressure correction technique; sediment transport; timesplitting; two-phase flow; vertical 2-D model.
Computational investigation and optimization study on system performance of heat sink using perforated pin fins mounted at different angles
by Ambarish Maji, Dipankar Bhanja, Promod K. Patowari
Abstract: The current work investigates the performance of heat sink using perforated pin fins of different shapes with various perforation geometries like circular, diamond shaped and elliptical type. Three dimensional CFD simulations have been performed to explore the effects of fin shape, perforation geometry and perforation size on system performance. This work also analyzes the variations in heat transfer enhancement through elliptical and square shaped staggered pin fins with elliptical perforations mounted at different angular position on the path of the flow. Multi Objective Optimization under Ratio Analysis is applied to select the best heat sink configuration. Exergy efficiencies, entropy generations and irreversibilities are also calculated for all the fin shapes having various sizes of perforations. It is observed that heat transfer rate of perforated fins up to certain perforation number and size are always greater than their solid counterparts and with the change in perforation geometry and fin shape heat dissipation rate enhances significantly.
Keywords: Heat Transfer; Perforated fin; System performance; Angular position; Optimization; Computational fluid dynamics.
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;.
CFD-modelling of free surface flows in closed conduits
by Katharina Teuber, Tabea Broecker, Arnau Bayon, Gunnar Nützmann, Reinhard Hinkelmann
Abstract: Computational Fluid Dynamics (CFD) is gaining an increasing importance in the
field of hydraulic engineering. This publication presents different application examples of a two-phase approach as implemented in the open source software OpenFOAM. The chosen approach is based on the volume of fluid method focusing on the simulation of flow in closed conduits. Three examples are presented: single-phase flow over a ground sill and free surface flow over a hill as well as complex free surface flow in a sewer model. The first example compares the results of different RANS turbulence models to experimental results. The results of the second example are compared to an analytical solution. In the last example the behaviour of the free surface flow is compared with the results of a model test and existing simulations using a simplified, open channel geometry for the closed conduit. For the examples analysed, the two-phase approach provides stable and reliable results.
Keywords: Computational Fluid Dynamics (CFD); Three-dimensional Models; Turbulence Simulation and Modeling; Volume of Fluid.
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.
Development of a model for unsteady conjugate heat transfer simulations
by Stefan Voigt, Berthold Noll, Manfred Aigner
Abstract: Recent developments in CFD simulations go towards higher accuracy by using time-resolved simulations instead of steady RANS simulations and adding solid structures to the simulation instead of assuming constant wall temperatures or heat fluxes at wall boundaries. This means a very large number of time steps are necessary to both accurately resolve the turbulent fluctuations in the fluid parts and have a converged temperature field in the solid parts of the computational domain. Another problem in near-wall heat transfer is the often used assumption of a constant turbulent Prandtl number, which is not correct for many flow configurations. The present work therefore focuses on the development of a methodology for accelerated unsteady conjugated heat transfer simulations. Furthermore, the implementation of a near-wall heat transfer model that does not rely on a constant turbulent Prandtl number was evaluated. The models are implemented in the CFD solver ANSYS CFX. The validation of the turbulent heat transfer model shows improved agreement with the experimental results compared to a constant turbulent Prandtl number approach. The improved unsteady conjugate heat transfer methodology gives the same accuracy of the results as the implicit unsteady conjugate heat transfer model but with considerable reduction of the computational time.
Keywords: CFD; conjugate heat transfer; unsteady CHT simulation; turbulent heat transfer modelling.
Fictitious domain method combined with the DEM for studying particle-particle/particle-wall collision in fluid
by Shengli Ma, Zhengying Wei, Xueli Chen, Qiyin Lin
Abstract: The interaction between particles and a fluid is very important in many fields. The hydrodynamic characteristics of both spheroidal particles and non-spherical particles sediment in the fluid and their rebound dynamics were investigated by using the fictitious domain method-discrete element method (FDM-DEM) in this paper. The novelty lies in the combination of the Monte Carlo scheme with FDM to improve the accuracy of hydrodynamic force. A soft-sphere scheme of DEM is used to model the collision of particles. The hydrodynamic force on the particle is fully resolved by the FDM. The numerical results of spherical particle are verified by comparing the previous numerical and experimental results implemented by Cate. The results of non-spherical particles sediment show that the path instability occurs. Those results are in good agreement with the corresponding published data. The method presented in this paper can be used in practical applications.
Keywords: fictitious domain method; FDM; particle-wall collision; particle-laden flow; discrete element method; DEM.
Airfoil noise reduction using boundary layer control
by Dawei Li, Yansen Liu, Guijuan Li, Lixun Xie, Lin Sun
Abstract: The flow field and sound radiation of a three component high lift configuration in free-flight was investigated through computational fluid dynamics simulations in conjunction with Ffowcs Williams-Hawkings acoustics solver. The boundary layer blow and suction control method on the inner slat surface has been used to suppress the broadband noise that generated by the high lift configuration. Studies have shown that the free shear layer which originated from the leading edge of the slat can be stabilised and the secondary separation fluids which located on the inner slat surface can be weakened with the boundary layer control. When the suitable boundary layer blow control parameters have been chosen, the computation results show that the fluctuating of the velocity and pressure, turbulence kinetic energy, vorticity and Lamb vector in the slat cove are suppressed by the boundary layer control. The significant reduction of noise level in far-field and the stabilisation of flow field in the slat cove both demonstrate that the boundary layer control is an effective way to control the noise of the high lift configuration.
Keywords: computational fluid dynamics; CFD; turbulent kinetic energy; TKE; blow and suction control; overall sound pressure level.
Control of self-sustained oscillations of a three-dimensional water jet in a slender channel
by Nora Bensider, Amina Mataoui, Mohamed Aksouh
Abstract: This work focuses on control of an oscillatory main jet submerged in a slender channel of small thickness to width ratio, by two lateral opposite and normal injections arranged at the same height, on the end sidewalls of the channel right above the main jet exit. The objective of this work is about the control of the jet deviation and its oscillations frequency, according to the flow rate of the two lateral injections. The validation confirms that second-order turbulence models predict more accurately such flows than first-order models. Due to Coanda effect, jet oscillations have been highlighted for several Flow rate ratios (β). Two flow behaviours of time averaged flow fields corresponding to two ranges of β are detected. It appears that the key question of this flow is also the two oscillatory motions of the main and the lateral jets.
Keywords: submerged jet; self-sustained oscillations; turbulence; slender channel; thrust vectoring; Coanda effect.
Computational and experimental study of swirl flow within SI engine with modified shrouded intake valve
by Bidesh Roy, Rahul Dev Misra, Krishna Murari Pandey, Abhijit Sinha, Bachu Deb
Abstract: Swirling flow can be used in a spark-ignition engine to increase turbulence intensity of the working fluid in the engine by using a shrouded intake valve. However, on using shrouded intake valve, a greater restriction to the incoming fluid is offered. In this backdrop, a computational and experimental study of swirl flow within the spark-ignition engine with a modified shrouded intake valve has been carried out and the same is then compared with 100°, 120° and 180° shrouded intake valves. The results show that the fluid flow patterns generated by the intake valves within the engine cylinder are similar in nature. The engine with the modified shrouded intake valve generates a substantial amount of swirl with comparatively lesser restriction to the incoming fluid. Whereas, the engine using other types of intake valves, either generates higher swirl ratio with lower mean flow coefficient or lower swirl ratio with higher mean flow coefficient.
Keywords: computational fluid dynamics; CFD; mean flow coefficient; modified shrouded intake valve; spark ignition engine; steady flow test; swirl ratio.