Progress in Computational Fluid Dynamics, An International Journal (27 papers in press)
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.
Aerodynamics of symmetric permeable airfoils and wings: CFD Simulation
by Mohammed Aldheeb, Waqar Asrar, Afaq Altaf, Ashraf Omar, Erwin Sulaeman
Abstract: This paper presents a simple, and effective CFD modelling of a permeable thin symmetric airfoil and a wing based on the profile of NACA 0008 at a chord-based Reynolds number of 3.45
Keywords: Permeability; Permeable Airfoil; Permeable Wing; Aerodynamics; Porosity.
Evaluation of Dynamic Impact of Flow with Bridge Pier Using Smoothed Particle Hydrodynamics Method
by Tu Anh Do, Thuan Huu Nguyen, Hai Manh Nguyen
Abstract: This paper presents a numerical model for the hydrodynamic impact of water flow with a bridge pier placed on a group of piles using the Smoothed Particle Hydrodynamics (SPH) method. A new boundary processing technique using a combination of ghost particles and dynamic boundary particles is implemented in the SPH simulations of this study. The numerical probes are uniformly distributed over the vertical surface of the structure in order to obtain detailed measurements of the flow pressure on the structure over time. Using the SPH simulations, the velocity field of the unstable flow surrounding the piles and the equivalent hydrodynamic force acting on the pile cap are determined. The vulnerable impact locations on the structure surface are also predicted from the time-pressure histories. Additionally, the predicted hydrodynamic pressure values are compared with those proposed by AASHTO (2012) for bridge design considerations.
Keywords: smoothed particle hydrodynamics; flow impact load; bridge pile cap; pile/pier-flow interaction.
Optimisation of a Mixed Flow Fan with NACA Profiled Blades using Computational Fluid Dynamics
by Ahmet Aydin, Cemil Yigit, Tahsin Engin, Huseyin Akkoc
Abstract: In this study, the optimisation of a mixed flow fan, which is widely used in industrial applications has been made using computational fluid dynamics. In the optimisation process, the CAD file of the mixed flow fan, which can operate under the specific conditions, has been created and preliminary design has been realized. Blade angle, wall thickness and length parameters were taken, and the blades have been optimised by using Ansys/Response Surface Optimisation (RSO) tool. Fans were produced in preliminary and optimised design, and experimental studies were carried out in a laboratory. As a result of optimisation, the maximum shaft power of the design-improved mixed flow fan was reduced by 8.62%, while an increase of 8.70% in total pressure value for the best operating point was achieved. In addition, the yield of the mixed flow fan was increased by 16.42%, while the sound pressure level has been decreased by 1.52%.
Keywords: mixed flow fan; computational fluid dynamics; goal driven optimisation.
Numerical simulation on heat transfer of nanofluid in conical spiral heat exchanger
by Ali Davoudi, Saeed Daneshmand, Vahid Monfared
Abstract: In this research article, the performance of a conical spiral heat exchanger with rectangular cross sections is numerically investigated by using two different nanofluids; aluminum oxide/water (?Al?_2 O_3/Water) and copper oxide/water (CuO/Water) nanofluid. For this purpose, the effects of nanofluid concentration on the secondary flow, pressure drop, heat transfer and figure of merit (FOM) (the ratio of total heat transfer to the required fluid for pumping) are investigated. On the structured grid, the continuity, momentum, and energy equations are solved by employing a finite volume method. Results indicate that by enhancing the concentration of a nanofluid, the formed secondary flow gains more power. Based on the obtained results, the pressure drop increases with enhancing the nanofluid concentration along the tube. The heat transfer rate is slightly increased by adding nanoparticles to the base water fluid in very low concentrations, but with increasing the concentration of nanofluids, the heat transfer rate reduces. Moreover, FOM decreases with increasing nanofluid concentration. This variation is higher for copper oxide compared to alumina nanofluids at lower concentrations, while it is higher for alumina nanofluid at higher concentrations.
Keywords: Heat exchanger; Numerical simulation; Nanofluid; Heat transfer.
Aerodynamic design and computational fluid dynamic analysis of radial outflow turbines for steam Rankine cycle and supercritical carbon dioxide Brayton cycle
by Vijayaraj Kunniyoor, Karthik Nadella, Punit Singh
Abstract: The first part of this paper presents the design of a radial outflow steam turbine for a micro steam power pump block of 200 kW capacity based on a unique Ljungstrom turbine design methodology. Computational Fluid Dynamic (CFD) simulations were carried out for the 18-stage radial outflow steam turbine at design and off-design points, and results proved the validity of the undertaken design methodology. The design point CFD simulation showed a total to total efficiency of 74.4 % for the steam turbine. Specific speed and specific diameter values for the radial outflow steam turbine stages were calculated and superimposed on the Baljes specific speed-specific diameter chart, thus identifying a unique radial outflow turbine zone in the chart. The second part of this paper presents a new design methodology based on specific speed and specific diameter values for designing a supercritical carbon dioxide radial outflow turbine for a 1MW supercritical carbon dioxide (SCO2) Brayton cycle. CFD simulations were carried out at design and off-design points for the SCO2 turbine. The total to total efficiency from the CFD simulation at the design point for the SCO2 turbine is 84.6 %.
Keywords: Radial outflow turbine; steam; supercritical carbon dioxide; Specific speed-specific diameter chart.
Numerical and experimental investigation of sloshing in a water tank with a fully coupled fluid-structure interaction method
by Abdullah Demir, A. Ersin Dinçer, Sevki Öztürk, Ilker Kazaz
Abstract: In the present study, the harmonic movement of fluid flow and the behavior of elastic structure under this movement are investigated. Accordingly, a recently developed fluid-structure interaction method in which fluid and structure are simulated with smoothed particle hydrodynamics (SPH) and finite element method (FEM) is used. The interaction between fluid and the structure is satisfied with contact mechanics. In order to validate the numerical model under harmonic movement, different experiments are used. First, the structure is assumed to be rigid and the pressures calculated on the structure are compared with the experimental data available in literature. Similarly, free-surfaces are also validated with novel experiments carried out in the context of this study. In addition, the interaction between an elastic structure and fluid is investigated in the novel experiments in which a water tank having an elastic buffer in the middle is moved under harmonic horizontal movement and the deflection of the elastic buffer and free-surface profiles are measured. Comprehensive results are given for all validation cases. According to the results, the numerical method is successful and can be used in these type of problems.
Keywords: Smoothed particle hydrodynamics; contact mechanics; fluid-structure interaction; FSI,;sloshing; elastic buffer.
Effect of Design Parameters on Flow Characteristics of an Aerodynamic Swirl Nozzle
by Sharif M. Islam, Md. Tanvir Khan, Zahir Uddin Ahmed
Abstract: Achieving uniform flow for an incoming round jet with swirl is a challenge in turbulent flow, and is often difficult to predict the flow at the nozzle exit. Aerodynamic swirl jet had been found advantageous over geometrically generated swirl for its better control on the flow. The purpose of this study is to numerically investigate the flow characteristics for the effect of various nozzle design variables, such as tangential inlet number, length of the nozzle and nozzle shape of an incompressible turbulent swirl air jet. In this regard, axial-plus-tangential flow based swirling nozzle is considered for the simulation using finite volume method, where turbulence is approximated by Shear Stress Transport (SST) k-? model. The results show that introduction of swirl into the flow results in a creation of a forced vortex flow in which circumferential velocity increases as the radius increases and suddenly decays near the nozzle wall. The axial velocity and its peak increase with the number of ports, whereas the tangential velocity is the maximum after the tangential flow is imparted. The boundary layer at the nozzle exit is thin compared to non-swirling flow, the extent of which is being independent of the number of ports. The static pressure and turbulent kinetic energy are found to be significantly influenced, particularly up to the length of four nozzle diameters, by the number of tangential inlets. A thicker boundary layer is predicted for longer nozzle lengths, and relatively uniform flow at the nozzle exit plane is predicted for the cases with comparably shorter nozzles. As far as the geometry of the transition section between the inlet ports and the downstream straight nozzle is concerned, the most uniform turbulence is predicted for the tapered cone shape, while the strongest turbulence is observed for the curved shape with the expense of losing some uniformity near the nozzle wall.
Keywords: Turbulent; nozzle; swirl; aerodynamic; CFD; pressure.
Numerical Study on the Sloshing flows in a Prismatic Tank using Natural Frequency of the Prismatic Shapes
by Hyunjong Kim, Nanjundan Parthasarathy, Yeon-Won Lee
Abstract: In this numerical study, a 2D prismatic tank subjected under horizontal excitation is used to analyze the sloshing characteristics for a specific range of Reynolds number, from 2.5E4 to 2.0E5. Three models of geometric variable delta1, namely, delta1=50mm, delta1=150mm and delta1 =250mm are used to observe the effects of the lower chamfered shape of the prismatic tank, where the Reynolds number for each delta1 ranges from 2.5E4 to 2.0E5 (12 Cases). The Volume of Fluid (VOF) method is used for the multiphase flow analysis. The Fast Fourier Transforms (FFT) technique is used to analyse the frequency components of excitation force and the magnitude of the amplitude spectrum. The results show that the sloshing wave fluctuation becomes small when the geometric variable delta1 is larger. Also, the FFT technique shows that the resonance does not occur due to frequencies which are not integral multiple of the excitation frequency. Moreover, for the sloshing load analysis, the free surface length is an important parameter than the shapes of the lower section.
Keywords: Sloshing; Reynolds number; VOF method; FFT analysis; CFD; Prismatic tank; Sloshing impact pressure; Two-phase flow.
Numerical Simulation and Optimization of a Reinforced Steel Plate against Underwater Explosions
by Arman Jafari Valdani, Armen Adamian
Abstract: Optimum arrangement of reinforcing blades can increase the resistance of marine structures and reduce potential losses in the case of explosive events. In this research, to find the most appropriate arrangement, six different geometries were modeled in 3D
(1 base case and 5 cases with different arrangements of reinforcing blades). By using Lagrangian and Eulerian methods for grid generation, the effects of the explosion on these structures in the water environment were simulated, and the variation of the Von-Mises stress, damage parameter, strain, displacement, and displacement rate were predicted for all cases. The simulations were based on finite element-finite volume methods. The accuracy of the numerical approaches was confirmed by analyzing benchmark problems and comparing the obtained results with analytical and experimental data. It was found that maximum damage takes place in case A (plate without any blades) with maximum displacement and displacement rate of 521 mm and 151 mm/ms, respectively. Also, the results of parametric simulations revealed that case F with six perpendicular reinforcing blades was the best arrangement. In this case, the displacement and displacement rate of the plate relative to the base case (without any blades) were reduced by 40% and 64%, respectively.
Keywords: Numerical Simulation; Explosive Loads; Water Environment; Fluid-Structure Interaction; Reinforcing Blades.
INVESTIGATION OF HEAT TRANSFER PERFORMANCE USING AN OSCILLATING VERTICAL FLAT PLATE
by Selma Akcay, Unal Akdag
Abstract: In this study, the heat transfer performance of an oscillating vertical flat plate is experimentally and numerically investigated. Rayleigh number (Ra), Womersley number (Wo), and dimensionless oscillation amplitude (Ao) are changed, and the effects of these parameters on heat transfer performance are analyzed. The numerical simulations are performed using a finite volume method based Computational Fluid Dynamics procedure, and the numerical results are validated by comparisons with the experiments. To explain the heat transfer mechanism, the instantaneous velocity and temperature profiles on the plate surface are obtained. It has been observed that the periodic oscillations continuously renewed the velocity and thermal boundary layer, increased the convection effects, and improved the heat transfer. Consequently, the heat transfer performance was significantly affected by the oscillation parameters and increased as both Wo and Ao increased for all tested Rayleigh numbers (Ra). The maximum heat transfer improvement achieved was about 45% at a high amplitude (Ao = 1.4) and frequency (Wo = 146) at Rayleigh number (Ra = 1.17
Keywords: oscillating vertical plate; heat transfer performance; mixed convection.
The Galerkin Least Squares MLPG Method for Convection-dominated Problems
by Xuehong Wu
Abstract: Meshless method had been applied widely in the computational mechanics, materials science and computational heat transfer and fluid flow. It is difficult for meshless method to develop the high efficient method for convection term in the computational fluid dynamics. In this paper, a truly meshless method---MLPG (Meshless local Petrov-Galerkin)method is applied to solve convection-diffusion problem, and GLS(Galerkin Least Squares) approximation method is proposed and applied to overcome the influence of convection term, Some cases with benchmark solutions are used to validate the accuracy and efficiency of the present method. The computational results show that GLS method can very good to deal to convection-diffusion problems and has very high computational precise.
Keywords: Upwind Scheme; GLS method; SUPG method; MLPG method; Meshless method.
Role of aspiration to enhance MHD convection in protruded heater cavity
by Nirmalendu Biswas, Aparesh Datta, Nirmal Kumar Manna, Dipak Kumar Mandal, A.C. Benim
Abstract: The implementation of free aspiration of the working fluid can significantly intensify heat transfer in the absence of any fan/pump for low power thermal applications. To explore the effectiveness of aspiration, the present study is devoted to a classical protruded-heater thermal cavity along with the magnetic field. The heater is located centrally at the middle position of the bottom of the cavity. The aspiration openings are provided at the bottom of the sidewalls (for passive and partial suction of working fluid) and in the middle of the top wall (for partial venting). The efficacy of aspiration is assessed with respect to the no-aspiration condition. The magneto-thermal-hydrodynamics in the cavity is captured numerically considering different strengths of the magnetic field and buoyancy and heater sizes. The numerical simulations are performed using an in-house CFD code in dimensionless form for a broad range of Rayleigh number (Ra), Hartmann number (Ha), and heater aspect ratio (A). The evolved results indicate substantial augmentation of heat transfer (even with the absence of any fluid pumping means) obtained from free aspiration. The heat transfer augmentation here could be up to 46% based on the parametric conditions. On the other hand, the imposed magnetic field shows a prominent role in the control of temperature and convective flow in the cavity. The aspiration plays a positive role in heat transfer enhancement under natural convection with and without a magnetic field.
Keywords: Magneto-hydrodynamic (MHD) flow; Aspiration; Natural convection; protruded heater; Heat transfer augmentation.
Efficiency of particle search methods in Smoothed Particle Hydrodynamics:
A comparative study (Part I)
by Eric Plaza, Leoanrdo Di G. Sigalotti, Luis Perez, Jorge Troconis, Wilfredo Angulo, Maria Castro
Abstract: Nearest neighbor searching is a fundamental problem in all applications of Smoothed Particle Hydrodynamics (SPH). Here we compare the efficiency of most used searching methods in vectorized form such as the Brute Force (BF) or Direct Search (DS),
the cell-linked list (CLL) and the KD-tree (KDT) methods. The time duration of the calculations for the SPH scales as N^2.74 and N^2.45 with the BF and CLL methods, respectively, where N is the total number of particles. In contrast, the Vectorized Brute Force (VBF) method works efficiently only for N<=10000 and scales as N^13.83 for larger N. The results also indicate, as was shown in previous works, that the efficiency of the SPH calculations is greatly improved with the Vectorized cell-linked list (VCLL) and the Vectorized KD-tree (VKDT) methods. However, the VKDT approach is more efficient than the VCLL method for N<= 10^5 in two dimensions (2D) and N<=10^6 in three dimensions (3D). For larger N, the time duration of the SPH calculations with the VKDT method grows steeply (scaling as N^14.53 in 2D and N^6.42 in 3D), while a linear trend is maintained with the VCLL method. The trend of complexity here is measured not only for early events, but also close to the point of hardware limit. The measure of complexity in this limit does not correspond to the trend of early events. So, each method has a different behavior, which can be measured and compared using a power fit.
Keywords: Nearest neighbor search; Closest pair search; Data structures; Smoothed Particle Hydrodynamics.
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 turbulence energy at the stagnation point was investigated based on multiple turbulence models able to capture the transition phenomena. Next, the influence of turbulence modelling on thermal stresses was addressed. The stresses produced by experimental temperature field were compared with the modelled 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; stagnation point.
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 behaviour 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.
Studies on the evolution of shock wave in gas film clearance for aerostatic thrust bearing
by Shang-Han Gao, Sheng-Long Nong, Wu-Bin Xu
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® is adopted to discuss the flow field characteristics, such as, supersonic flow, negative pressure, load capacity of bearing and backflow area with large values of both gas film thickness and supply pressure. Under thick clearance and large supply pressure conditions, the supersonic flow leads to the appearance of shock wave and causes a negative pressure. If the multi-reflection phenomenon of shock wave occurs, the total load capacity of bearing will even drop to be negative. The above results provide a theoretical basis for people to understand the flow field characteristics of the gas film in the aerostatic thrust bearing under large gas film thickness and large supply pressure conditions, and help the researchers to find out the effective way to restrain the emergence of shock waves and improve the load capacity of aerostatic thrust bearings.
Keywords: aerostatic thrust bearing; flow field characteristics; shock wave; load capacity.
Wave making characteristics of submerged vehicle in a stratified ocean
by Dawei Li, Guijuan Li, Zhenshan Wang, Lin Sun, Lixun Xie
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-normalisation group theory k-ε turbulence model and volume of fluid method are used to simulate the fluid characteristics of the submerged body moving in a stratified ocean. Two ocean layers with finite depth and different densities are considered. Typical Froude numbers of flow vary between 0.05 and 2. The simulated results show that the wave making characteristics on the surface and interface contain contributions from two different modes: a surface-wave mode and an internal-wave mode. It is observed that there is a large effect of submerged body velocity on the wave systems, structures of free surface and pycnocline interface. The typical wavelength of the interface is predicted to increase with Froude number, which is in agreement with the theory.
Keywords: submerged vehicle; stratified ocean; interface wave mode; surface wave mode; computational fluid dynamics; CFD.
Reflection behaviour 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 modelling 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 seven 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.
The effect of orbital motion and eccentricity of drill pipe on pressure gradient in eccentric annulus flow with Newtonian and non-Newtonian fluids
by Hicham Ferroudji, Ahmed Hadjadj, Titus Ntow Ofei, Mohammad Azizur Rahman
Abstract: The correct prediction of the pressure gradient is the fundamental parameter to establish an effective hydraulics program, which enables an optimised drilling process. In the present work, the effect of the orbital motion of the drill pipe on the pressure drop in an eccentric annulus flow with Newtonian and non-Newtonian fluids is studied numerically for both laminar and turbulent regimes using finite volume method (FVM). Furthermore, the effect of eccentricity when the inner pipe makes an orbital motion is evaluated. Different behaviours are observed in laminar and turbulent regimes. In the laminar regime, the simulation results showed that an increase of the orbital motion speed causes a considerable increment of the pressure gradient for the Newtonian fluid. For the power-law, non-Newtonian fluid in the laminar regime, on the contrary, a decrease of the pressure gradient is observed due to the shear-thinning effect. In the turbulent regime the mentioned trends are predicted to be much weaker. As eccentricity increases, the pressure drop of the non-Newtonian fluid decreases with a more pronounced diminish in pressure drop when the drill pipe is in orbital motion for both laminar and turbulent flow regimes.
Keywords: computational fluid dynamics; CFD; pressure drop; orbital motion; laminar flow; turbulent flow.