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FLUID MECHANICS

 Volume 4, 2009
Print ISSN: 1790-5087
E-ISSN: 2224-347X

 
 

 

 

 

 

 

 


Issue 1, Volume 4, January 2009


Title of the Paper: Laboratory Investigations and Theoretical Analysis of Axial Thrust Problem in High Rotational Speed Pumps

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Authors: Andrzej Wilk

Abstract: Theoretical analysis of the influence of the increase of the rotational speed of the rotodynamic pump on axial thrust is made in the paper. The algorithm of the designing of the axial thrust relieving system by means of the relieving holes and relieving blades is presented. The calculation examples for pump working at low and high rotational speed are included. The conclusions concerning with both axial thrust relieving systems for high rotational speed are presented. The laboratory tests of rotodynamic pump working at rotational speed 6000 rpm are presented. The purpose of the tests was to determine the pump operation characteristics and pressure distribution around the impeller.

Keywords: Pump, high rotational speed, axial thrust, open impeller, radial blades, pressure distribution


Title of the Paper: Numerical Modeling of Extended Mild Slope Equation with Modified Mac Cormack Method

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Authors: Asu Inan, Lale Balas

Abstract: The transformation of waves is one of the important subjects in coastal engineering studies. Refraction, diffraction, shoaling, reflection can be analysed with the mild slope equation over mild sloped topographies. But the extended mild slope equation can be applied to the rapidly varying topographies since it includes higher order bottom effects such as square of bottom slope and bottom curvature. In this study, extended mild slope equation has been solved with finite difference method using Mac Cormack and Point Gauss Seidel Methods together. The nonlinear wave celerity and group velocity have been used. The numerical model has been tested on elliptic shoaling area and compared with the physical experiment measurements given in literature. The predictions are in consistency with the measurements. As a result of error analysis, it is decided that the numerical can be used for the simulation of wave propagation. Numerical model has been applied to a coastal area in the Kocaeli Bay in the Marmara Sea in Turkey.

Keywords: Extended mild slope equation, Mac Cormack Method, Point Gauss Seidel Method, wave refraction, diffraction, finite difference method, CFD


Title of the Paper: An Unstructured Grid Generation Approach for Inviscid Flow Solutions

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Authors: Bahrainian Seyed Saied, Daneh Dezfuli Alireza

Abstract: The unsteady and inviscid compressible flow around a circular cylinder and airfoil has been solved using a cell-centered finite volume method. A new algorithm for the generation of unstructured triangular grids was used to discretize the computational flow domain. Automatic grid generation, cell size distribution, and geometry treatment are some of the grid algorithm capabilities. Euler flow solutions have been obtained for subsonic and transonic flow conditions. The subsonic solution of flow around a circular cylinder grid verified the unstructured grid techniques, and was compared with an analytical solution. Transonic solution over the same grid fully converged after only 2791 iterations. The grid quality resulted in smooth pressure contours. Transonic flow solutions were also obtained for airfoil at free stream Mach number of 0.8 and attack angle of 1.25. The true position of the shock is illustrated in the pressure coefficient plots, and shows the shock capturing capability of the solution method. The convergence history obtained, points to the accuracy of the numerical solution.

Keywords: Unsteady, Inviscid, Geometry Movement, Unstructured Grid, Edge Swapping


Issue 2, Volume 4, April 2009


Title of the Paper: Hot-Film and LDV Investigation of the Boundary Layer Transition on a Turbine Profile at Different Reynolds Numbers

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Authors: Daniele Simoni, Marina Ubaldi, Pietro Zunino

Abstract: An experimental investigation of the suction side boundary layer of a large scale turbine cascade has been performed to study the effect of Reynolds number on the boundary layer transition process at large (1.6x106) and moderate (5.9x105) Reynolds numbers. The boundary layer development has been investigated by means of a two-component laser Doppler velocimeter. Time traces of the instantaneous velocities and boundary layer velocity and turbulence fields provide a physical insight of the different transition processes for the two Reynolds numbers. Surface mounted hot-film gauges were used to measure the quantity qτw, which is proportional to the wall shear stress. Statistical analysis of the instantaneous qτw data, including statistical moments, probability density function and intermittency function distributions, provides quantitative information on the state of advancement of the transition process at the two different Reynolds numbers.

Keywords: Boundary layer transition, Turbine profile, Hot-film probes, Wall shear stress, Intermittency detection technique, LDV measurements


Title of the Paper: Comparison of Different 2nd Order Formulations for the Solution of the 2D Groundwater Flow Problem over Irregular Triangular Meshes

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Authors: Costanza Arico, Tullio Tucciarelli

Abstract: Mixed and Mixed Hybrid Finite Elements (MHFE) methods have been widely used in the last decade for simulation of groundwater flow problem, petroleum reservoir problems, potential flow problems, etc. The main advantage of these methods is that, unlike the classical Galerkin approach, they guarantee local and global mass balance, as well the flux continuity between inter-element sides. The simple shape of the control volume, where the mass conservation is satisfied, makes also easier to couple this technique with a Finite Volume technique in the time splitting approach for the solution of advection-dispersion problems. In the present paper, a new MHFE formulation is proposed for the solution of the 2D linear groundwater flow problem over domain discretized by means of triangular irregular meshes. The numerical results of the modified MHFE procedure are compared with the results of a modified 2nd spatial approximation order Finite Volume (FV2) formulation [2], as well as with the results given by the standard MHFE method. The FV2 approach is equivalent to the standard MHFE approach in the case of isotropic medium and regular or mildly irregular mesh, but has a smaller number of unknowns and better matrix properties. In the case of irregular mesh, an approximation is proposed to maintain the superior matrix properties of the FV2 approach, with the consequent introduction of a small error in the computed solution. The modified MHFE formulation is equivalent to the standard MHFE approach in both isotropic and heterogeneous medium cases, using regular or irregular computational meshes, but has a smaller number of unknowns for given mesh geometry.

Keywords: Groundwater, finite elements method, mixed hybrid finite elements method, finite volumes method, positive-definite matrix, M-property, Raviart-Thomas basis function


Title of the Paper: Air Curtains Integrity when Misusing the Refrigerated Display Cabinets

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Authors: Samir R. Traboulsi, Ali Hammoud, M. Farid Khalil

Abstract: Tilted air curtains are used as barriers between two environments of different temperature, humidity and quality and are the core elements in Refrigerated display cabinets. Energy consumption and performance became the concern of end users of cabinets as the Entrainment of one environment Fluid (air) into the curtain by shear layer mixing contributes to both the sensible and the latent heat load on the other environment and the impingement of the air curtain formed. Obstructions of any type on the path of the air curtain endanger its integrity. Protrusion present in the direction of the flow impacts the performance of the air curtain and defeats its purpose of existence. Computational Fluid Dynamics (CFD) software method is used to evaluate the impact of such intentional and non intentional obstructions, on the performance of the air curtain formed by the tilted jet plane and is also validated by comparing the CFD calculations results with experimental results. Qualitative design combination of various geometrical parameters and various levels of obstruction in the direction of the flow(s) are proposed in order to guarantee the existence of the air curtain.

Keywords: Display Cabinet, Simulation, Velocity, Turbulence, Infiltration Rate, Air Curtain


Title of the Paper: Pre-mixed and Diffusion Flames Assessment using CFD Tool for Natural Gas and Biogas Fuels in Gas Turbine Combustion Chambers

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Authors: Lucilene De Oliveira Rodrigues, Marco Antonio Rosa Do Nascimento

Abstract: In the past few years, with the development of advanced numerical computational codes, numerical simulation became a promising option to develop and improve technology in different fields. The results obtained through simulations are used to gather important information during the design phase or optimization of industrial equipment. Its employment generates reliable results at low cost due to the reduced number of experiments as well as the opportunity to develop new products and perform many simulations before construction of a new product. With the recent energy shortage and the reduction of the fossils fuel reserves, the industrial sector starts to develop more compact equipment that can be fed with different fuels, attending a wide range of heat and power demand. The aim of the work is to assess thermal-aerodynamics and emission using numerical simulation (Computational Fluid Dynamics CFD) of a 600kW simple cycle gas turbine combustion chamber. The thermal-aerodynamics assessment of the combustion chamber will be presented, showing the profiles of temperature and emissions, when used the premixed and diffusion flame for the natural gas and biogas fuels, respectively.

Keywords: Combustion Chamber, Flame, Premixed, Diffuse, Different Fuels, Gas Turbine


Issue 3, Volume 4, July 2009


Title of the Paper: Hydrodynamics of Vertical Semi-Immersed Slotted Barrier

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Authors: E. V. Koutandos

Abstract: In the present study, wave interaction with fixed, vertical, semi-immersed, slotted barriers is investigated numerically. Numerical results are obtained with the use of the COBRAS (Cornell breaking Wave and Structures) wave model for regular waves, revealing the effects of relative depth d/L (d: water depth, L: wave length) and the porosity of the barrier on the hydrodynamic characteristics (wave transmission, reflection, dissipation, velocity field, turbulence kinetic energy field). Numerical results concerning wave transmission, reflection, dissipation against the porosity of the structure, are well compared with experimental results by Isaacson et al. [13], revealing the credibility of the wave model. Detailed computed velocities and turbulence kinetic energy in the vicinity of the structure indicate the effects of the special breakwater on the flow pattern and the turbulence structure.

Keywords: Slotted barrier, Permeable barrier, Special breakwater


Title of the Paper: Universal Solutions of Unsteady Two-Dimensional MHD Boundary Layer on the Body with Temperature Gradient along Surface

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Authors: Zoran Boricic, Dragisa Nikodijevic, Bratislav Blagojevic, Zivojin Stamenkovic

Abstract: In this paper, we consider multi-parametric method for solution of unsteady temperature twodimensional MHD laminar boundary layer. Outer magnetic field induction is assumed as function of longitudinal coordinate and time with force lines perpendicular to the body on which boundary layer forms. Temperature varies along body surface with longitudinal coordinate, but not with time. Further, electric field is neglected and value of magnetic Reynolds number is significantly less then one i.e. problem is considered in induction-less approximation. According to temperature differences under 50oC physical properties of fluid are constant. Introducing new variables and then similarity parameters, starting equations are transformed into universal form. Obtained universal equations and corresponding boundary conditions do not contain explicit characteristics of particular problems. Appropriate approximations of obtained equations are solved numerically in this paper, and a part of obtained results is given in the form of figures and corresponding conclusions.

Keywords: MHD, multi-parametric method, boundary layer, similarity parameters, temperature, universal solutions


Issue 4, Volume 4, October 2009


Title of the Paper: Investigation into LBM Analysis of Incompressible Laminar Flows at High Reynolds Numbers

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Authors: A. C. Benim, E. Aslan, I. Taymaz

Abstract: The Lattice Boltzmann Method is applied to incompressible, steady, laminar flows at high Reynolds numbers varying in a range form 50 to 2000. The developing channel flow and the lid driven cavity flow are analyzed. The effect of the model Mach number on accuracy is investigated by performing computations at different Mach numbers in the range 0.1 - 0.4 and comparing the results with finite-volume predictions of the incompressible Navier-Stokes equations. It is observed that the Mach number does not effect the results within this range, and the results agree perfectly well with the finite-volume solution of the incompressible Navier-Stokes equations. An important purpose of the study has been to explore the stability limits of the method. It is observed that the maximum allowed collision frequency decreases with increasing Reynolds and Mach numbers, and this dependency is more predominant, and the limiting collision frequencies are lower for the channel flow compared to the lid driven cavity flow.

Keywords: Lattice Boltzmann Method, Computational Fluid Dynamics, Incompressible Flow, Laminar Flow, High Reynolds Number


Title of the Paper: Computational Analysis of Gas Turbine Preswirl System Operation Characteristics

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Authors: M. Cagan, A. C. Benim, D. Gunes

Abstract: Gas turbine engine efficiency can be increased by the improvements of the cooling air system. In this study, gas turbine preswirl cooling system is computationally analyzed using a validated three-dimensional quasi-steady model. In the first phase of the study, the system is analyzed with respect to different operating conditions, for understanding its behavior. In this part, the dependence of the discharge coefficient and the temperature drop on the inverse of the swirl ratio parameter are employed to characterize the system. In the second phase of the analysis, the possibilities of improving the system performance is investigated by implementing different modifications on the system geometry.

Keywords: Gas turbine cooling, Preswirl system, Discharge coefficient, Performance, CFD


Title of the Paper: The Analysis of the Influence of the Initial Impeller on the Discharge and the Delivery Head of High Speed Pump with Radial Blades

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Authors: Andrzej Wilk

Abstract: The article discusses a theoretical analysis of fluid flow through the initial impeller which is mounted at the inlet to the centrifugal impeller in the impeller pump. The article presents the results of laboratory tests for the influence of various initial impellers in the impeller pump with an open-flow impeller and rectilinear radial blades on the pump operating parameters and the course of performance characteristics. The research has been performed at the rotational speed from 3000 to 7000 rpm.

Keywords: High-speed pump, open-flow impeller, radial blades, initial impeller, performance characteristics


Title of the Paper: Estuarine Hydrodynamic as a Key-Parameter to Control Eutrophication Processes

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Authors: Antonio A. L. S. Duarte, Jose M. P. Vieira

Abstract: Estuaries and coastal lagoons are commonly subjected to intensive anthropogenic stress due to massive pollutant loading from urbanised riparian areas. Nutrient enrichment is a key-factor for habitat degradation, leading to sensible structural changes in estuarine ecosystems with the consequent occurrence of eutrophication processes. The influence of hydrodynamics must not be neglected on estuarine eutrophication vulnerability assessment. In fact, flushing time affects the transport and the permanence of water and its constituents inside an estuary, so the increase of estuarine flushing capacity can be seen as a management measure to mitigate or to invert eutrophication processes, like the one observed in the Mondego River estuary (Portugal), during the last two decades. In this work, an integrated 2-DH water quality model was applied to calculate water residence time values, at different simulated management scenarios of the Mondego estuary. The results of the performed simulations illustrate the strong asymmetry of flood and ebb duration time at the inner sections of this estuary, a keyparameter for a correct tidal flow estimation, as the major driving force of the southern arm flushing capacity, as well as a spatial and temporal variability of calculated RT values. The conclusions of this work confirm the crucial influence of hydrodynamics on estuarine water quality status and the usefulness of this hydroinformatic tool as contribution to support the design of a sustainable management plan, based on nutrient loads reduction and hydrodynamic circulation improvement. So, the proposed methodology, integrating hydrodynamics and water quality, constitutes a powerful hydroinformatic tool for enhancing estuarine eutrophication vulnerability assessment, in order to contribute for better water quality management practices and to achieve a true sustainable development.

Keywords: Water sustainable management, estuarial modelling, eutrophication, hydroinformatics, residence time, Mondego estuary


Title of the Paper: Is a Flat Inlet Profile Sufficient for WSS Estimation in the Aortic Arch?

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Authors: J. Renner, D. Loyd, T. Lanne, M. Karlsson

Abstract: Atherosclerosis is one of the main reasons for cardivascular disease which cause many deaths every year especially in the Western world. The development of atherosclerosis is strongly believed to be influenced by hemodynamic forces in the arteries e.g. wall shear stress (WSS). Estimations of WSS are therefore very important. By combining magnetic resonance imaging (MRI), image processing and computational fluid dynamic (CFD) simulations, it is possible to estimate subject specific WSS in the human arteries. The framework for performing such work includes i.e. using inlet boundary conditions which, however, will influence the final result i.e. the WSS distribution. This paper aims to investigate the influence of the inflow boundary condition in the human aorta with comparing two settings for the inflow: 1) subject specific inlet profile measured with MRI and 2) uniform profile with the subject specific mass flow rate. The analysis of WSS will be performed both on spatial location along the artery as well as on the temporal location in the cardiac cycle. Subject specific data have been used for geometry, inflow velocity profile and blood viscosity. The recommendation due to our findings from nine healthy subjects, is that a measured subject specific inlet boundary condition must be used in order to get a subject specific WSS distribution; the difference in WSS is 8-34% compared to using a mass-flow correct uniform profile. Temporal variations were clearly seen in the WSS differences due to the different inflow velocity profiles used. The lowest influence of the inlet boundary condition was found at peak velocity in the cardiac cycle. The aortic geometry does not form the flow in such extent (compared to the influence by inlet boundary condition) to obtain a more correct WSS distribution further away from the inlet at the systolic parts of the cardiac cycle. The shape of the vessel has only a significant influence at low velocities i.e. the diastolic phase of the cardiac cycle.

Keywords: Wall Shear Stress Aorta - CFD - Subject Specific - Inlet Boundary Condition - Uniform Velocity Profile


 

   
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