bluuebunny/crossref_metadata_2025 huggingface dataset:
Wall Modeled Large Eddy Simulation of Flow Over a Wall-Mounted Hump
Authors: Dilip Deepu , Tafti Danesh
Published: December 2014

Large Eddy Simulations (LES) of wall-bounded flows at high Reynolds numbers demand an extremely fine mesh resolution in the wall proximal inner layer. Accurate modeling of near wall turbulence is therefore crucial in reducing the computational cost of LES at practical Reynolds numbers. One approach is the implementation of a two-layer model that solves for a reduced one-dimensional boundary layer equation in the inner wall layer. A wall modeled LES thus allows for a coarser grid resolution than a wall resolved LES. This work evaluated the performance of a wall modeled LES against a wall resolved LES as well as experimental data for the flow over a wall mounted hump at Reynolds number 9.36×105. Results from the wall modeled LES were in good agreement with both wall resolved LES and experimental data in parameters such as surface pressure coefficient, skin friction, mean velocity profiles, Reynolds stresses and flow reattachment. It was observed that the wall modeled LES required only a fifth of the computational resources required for the wall resolved LES.
Numerical Simulation of Compressible Multi-Phase Flow in High Pressure Fuel Pump
Authors: Husmeier Frank , Greif David , Sampl Peter , Strucl Jure , Edelbauer Wilfried
Published: December 2014

Modern injection systems utilize high injection pressures to enhance the break-up of the injected fuel and the mixing of fuel with air. Elevated pressure level targets high performance, high efficiency and low tailpipe emissions. Such conditions lead to high internal loads of fuel injection equipment and aggressive conditions within fuel injectors and pumps. The high pressure pump is the most critical component assuring appropriate elevated pressure level. Under certain conditions cavitation can occur within the system, which will affect the performance of the pump and in long term also its durability. Namely, cavitation repeatedly appearing at the same location can lead to erosion damage, which is clearly not desired. Therefore, numerical analyses by means of Computational Fluid Dynamics (CFD), represent a powerful tool in the early stage of component definition or design of the pump itself. As the pressure appearing in such systems exceeds 300 MPa, the liquid fuel needs to be treated as compressible. Moving parts of the investigated fuel pump are displaced due to pressure forces, which means that pressure variations and pressure waves need to be accurately predicted in order to predict accurate part displacements and correct wetted volume shape. In order to achieve this, the liquid fuel is treated as compressible, otherwise exact inlet- and outlet check-valve displacements are not predictable. In present work the liquid compressible Euler-Eulerian multiphase model of the commercial CFD code AVL FIRE® has been applied. The domain has been geometrically discretized using the preprocessing part of the applied CFD tool, moving parts have been handled by a novel, so-called “mesh deformation by formula” methodology. The advantage of the approach is that it does not require the pre-definition of all moving parts but allows for an arbitrary, user-defined movement of all mesh nodes. The motion of internal floating parts is performed automatically during the calculation according to the local pressure forces. Due to high pressure levels local flow velocities are typically very high causing the fuel to undergo phase change from liquid to vapor called cavitation. To accurately account for the effect of cavitation, the applied CFD code offers advanced cavitation modeling options. The applied capability enables estimation of flow aggressiveness and the probability for the onset of cavitation erosion on the surface of the components with the objective to optimize or entirely eliminate cavitation. In the present study two simulations have been performed; (i) part load and (ii) full load condition.
On the Effect of Volute Design on Unsteady Flow and Impeller–Volute Interaction in a Centrifugal Pump
Authors: Litfin Oliver , Delgado Antonio
Published: December 2014

The main objective of the presented work is to investigate the effect of volute design on the impeller–volute interaction. Therefore a low specific speed centrifugal pump impeller has been employed and three different volutes with circular cross-sections have been designed using the constant-velocity approach. Numerical simulations of the simplified pump model have been performed by means of ANSYS® CFX®, solving the 3D unsteady Reynolds-averaged Navier–Stokes equations. The three designs are compared in terms of hydraulic performance and also the flow patterns in the different volutes are examined in detail, including velocity, swirl and pressure distributions on different cross-sections along the volute as well as the charging of the volute. To show the impact of the volute on the impeller flow, also the pump impeller has been simulated without the volute and a thorough comparison of the time averaged results has been conducted. This has been done at the design flow rate as well as for overload and part load conditions. Also the unsteady flow inside a blade channel of the impeller during a revolution is investigated, revealing significant variations of its operating point, especially at off-design conditions. As a result of the performed studies it can be seen how the different volute designs interact with the impeller depending on the operation point and how this affects the hydraulic performance of the pump.
CFD Investigation of the Effect of Viscosity on a Three-Stage Electric Submersible Pump
Authors: Stel Henrique , Sirino Thiago , Prohmann Pamella R. , Ponce Francisco , Chiva Sergio , Morales Rigoberto E. M.
Published: December 2014

Electric Submersible Pumps (ESP’s) are multistage pump arrangements used in offshore petroleum production. Most of their applications are subject to viscous oil pumping, which causes performance degradation with respect to the regular service with water and changes some characteristics related to the flow dynamics inside the pump. The purpose of this work is to use CFD to investigate numerically the flow in a semi-axial type ESP with three stages operating with fluids of different viscosities. Both design and off-design flow rates are simulated, as well as different impeller rotation speeds. Head curves of the ESP for these cases are compared with experimental data and show good agreement. The importance of considering more than a single stage when studying ESP’s is discussed. The flow fields inside the pump channels for different operating conditions are compared, showing for instance that the flow is not always blade-oriented at the best efficiency point for service with fluids more viscous than water. The effect of the fluid viscosity and the rotation speed on the performance degradation is also explored. In addition, dimensional analysis is used in favor of a better understanding on how the pump performance degrades when working out of the design figure.
Analysis and Control of Complex Flows in U-Bends Using Computational Fluid Dynamics
Authors: Güden Yiğitcan , Yavuz Mehmet Metin
Published: December 2014

Analysis and control of flow structure in U-bends are crucial since U-bends are used in many different engineering applications. As a flow parameter in U-bends, the ratio of inertial and centrifugal forces to viscous forces is called as Dean number. The increase of Dean number destabilizes the flow and leads to a three-dimensional flow consisting of stream wise parallel counter-rotating vortices (Dean vortices) stacked along the curved wall. Due to the curvature in U-bends, the flow development involves complex flow structures including Dean vortices and high levels of turbulence that are not seen in straight duct flows. These are quite critical in considering noise problems and structural failure of the ducts. In this work, computational fluid dynamic (CFD) models are developed using ANSYS FLUENT to simulate these complex flows patterns in square sectioned U-bend with a radius of curvature Rc/D=0.65. The predictions of mean velocity profiles on different angular positions of the U-bend are compared against the experimental results available in the literature and previous numerical studies. Performance of six different turbulence models are evaluated, namely: the standard k-ε, the k-ε Realizable, the k-ε RNG, the k-ω SST, the Reynolds Stress Model (RSM) and the Scale-Adaptive Simulation Model (SAS), to propose the best numerical approach with increasing the accuracy of the solutions while reducing the computation time. Numerical results show remarkable improvements with respect to previous numerical studies and good agreement with the available experimental data. The best turbulence model for this application is proposed considering both the computation time and the result accuracy. In addition, different flow control techniques are still under investigation to eliminate Dean vortices and to reduce turbulence levels in U-bends.
Interface Tracking Methods Applied to Phase Separation
Authors: Osmar L. , Vincent S. , Caltagirone J. P. , Estivalezes J. L. , Auguste F. , Magnaudet J. , Menard T. , Berlemont A. , Aniszewski Wojtek , Ling Y. , Zaleski S.
Published: December 2014

We are interested in the phase separation of a light fluid in a heavier fluid in a closed box. A simple initial configuration is considered. A cubic volume of light fluid with a characteristic length H/2 is contained in the bottom part of a cubic box of side H filled with a heavier fluid. This problem is typical of multiscale interfacial flows in which large interfacial scales coexist with small dispersed droplets interacting with larger drops according to coalescence or rupture of interfaces. A benchmark of phase separation is proposed here. Several CFD codes devoted to multiphase flow simulations are evaluated on DNS simulations for different meshes. The VOF method, the level set method and the AMR method are used for tracking interface, the final objective being to see if codes using different numerical methods converge to the same macroscopic quantities. Time evolution of potential energies, kinetic energies, and interfacial area are computed for each simulation and used for codes comparisons. These physical quantities are also used to ensure that grid convergence is achieved. The phase separation also induces turbulence effects. In order to quantify it, time evolution of enstrophy is evaluated for each simulation. It is shown that all the codes converge for all macroscopic quantities for two turbulent Reynolds numbers, except for enstrophy. This difference is due to an under resolution of the vorticity.
The Effects of Trees on Micrometeorology in a Medium-Size Mediterranean City: In Situ Experiments and Numerical Simulations
Authors: Pappaccogli Gianluca , Buccolieri Riccardo , Maggiotto Giuseppe , Leo Laura S. , Rispoli Gennaro , Micocci Francesco , Di Sabatino Silvana
Published: December 2014

This study analyses the aerodynamic effects of trees on local meteorological variables through in situ measurements and Computational Fluid Dynamics (CFD) simulations. Measurements are taken in the inner core of a medium-size Mediterranean city (Lecce, IT) where two adjacent street canyons of aspect ratio H/W∼1 (where H is the average building height and W is the average width of the street) with and without trees are investigated. Building façades and ground temperatures are estimated from infrared (IR) images, while flow and turbulence are measured through three ultrasonic anemometers placed at different heights close to a building façade at half length of the canyon. Tree crown porosity is evaluated through the Leaf Area Index (LAI) measured by a ceptometer. Numerical simulations are made using a CFD code equipped with the Reynolds Stress Model (RSM) for the treatment of turbulence. Overall, the analysis of measurements shows that trees considerably reduce the longitudinal wind speed up to 30%. Trees alter the typical diurnal cycle of surface and air temperature within the canyon, suggesting that in nocturnal hours the trapping of heat is more important than the power of passive cooling through evapo-transpiration. Comparative numerical simulations provide further evidence that flow velocity reduces in presence of trees and although the typical wind channeling observed without trees is still maintained, trees enhance the formation of a corner vortex leading to reverse flow at the openings of the street. The reduction of the exchange of momentum between the canyon and the atmosphere above, shown by the measurements in presence of trees is confirmed by numerical simulations.
Experimental Study of Non-Colloidal Mono and Polydisperse Suspension in Taylor-Couette Flow
Authors: Gheisari Reza , Mirbod Parisa
Published: December 2014

Monodisperse and polydisperse suspension flows form an extensive section of natural and technological flows. These flow structures can be categorized to sedimenting or neutrally buoyant suspensions considering the density ratio between particle phase to dispersion phase. Biological systems, food processing, ceramic injection, dynamic filtration and air conditioning are examples of areas that such flows arise. Various complicated interparticle interactions and their inevitable influence on and from the continuous phase result in some interesting phenomena which are challenging to justify. This research studies axial instabilities of suspension flow in a partially filled Taylor-Couette setup. Previous observations show that when a monodisperse suspension undergoes a rotational shear motion in a partially filled horizontal Couette cell, particles leave their initial uniform distribution and migrate to regions with lower shear rate. This migration helps formation of ring-shape axial concentrated bands. This study examines the noncolloidal neutrally buoyant suspensions of hard spherical particles with average diameters of 150, 360, 850 micron. Using UCON oil (poly ethylene glycol-ran-glycol) as suspending fluid, monodisperse and polydisperse suspensions in partially filled Stokesian Couette-Taylor flow were studied. The results show strong dependence of band number and profile on suspension concentration and filling level. Moreover interesting phenomena in polydisperse suspensions such as different band shape and weak dependence of band formation time on size of constituents were observed.
Modelling the Urban Boundary-Layer Over a Typical Mediterranean City Using WRF: Assessment of UHI and Thermal Comfort
Authors: Guarino M.-V. , Martilli A. , Di Sabatino S. , Leo L. S.
Published: December 2014

The aim of this work is to simulate the Urban Heat Island (UHI) in a medium size Mediterranean city (Lecce, IT) and to analyze its consequences for thermal comfort. We use the Weather Research and Forecasting (WRF) model (version 3.2), that accounts for the urban structure with a multilayer urban parameterization (BEP+BEM i.e. the Building Effect Parameterization (BEP) combined with the Building Energy Model (BEM)). Three hot and cloudless summer days have been simulated and results have been compared with field data collected during an experimental campaign performed over the whole summer in the city of Lecce, Italy. In the model, the structure and shape of the city are reproduced using detailed data related to different urban classes, urban fraction and building morphometry. For the residential urban classes, different thermal parameters that are representative of building materials in the oldest and the newer part of the city, are used. Results show that UHI reaches, on average, its maximum intensity (4–5 °C) just before sunrise, and its minimum (2 °C) occurs during the day. Model validation inferred through statistical analysis shows overall a better model performance for the historical city centre than for the suburban area. This suggests that further refinement of the building representation in the outskirts might still be required. Consequences of the increased urban temperature are evaluated in terms of thermal comfort. The maximum thermal stress occurs during the central hours of the day, while, the minimum thermal stress occurs during the twilight hours.
Large Eddy Simulation of the Flow Past Pitching NACA0012 Airfoil at 1E5 Reynolds Number
Authors: Kasibhotla Venkata Ravishankar , Tafti Danesh
Published: December 2014

The paper is concerned with the prediction and analysis of dynamic stall of flow past pitching NACA-0012 airfoil at 105 Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.188 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with dynamic Smagorinsky model at the sub grid scale. The lift hysteresis plots indicate closer match to experimental results, although discrepancies exist during the downstroke. The development of dynamic stall vortex, vortex shedding and reattachment as predicted by the present study are discussed in detail. This study has shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke.
PIV Measurements of the Flow in the Tip Region of a Compressor Rotor
Authors: Tan David , Li Yuanchao , Wilkes Ian , Miorini Rinaldo L. , Katz Joseph
Published: December 2014

An axial turbomachine, adapted from the NASA Glenn Low-Speed Axial Compressor (LSAC), has been assembled for detailed flow and turbulence measurements in the JHU optical refractive index matched facility. The test section consists of a row of twenty inlet guide vanes (IGV), followed by fifteen rotor blades, then twenty stator blades. The blades have the same geometry, but lower aspect ratio as the inlet guide vanes and the first stage of the LSAC facility at NASA Glenn. Although smaller in scale, the Reynolds number based on the tip speed and rotor blade chordlength are comparable to those of the LSAC. The casing, rotor blades, as well as half the IGVs and stator blades are made of transparent acrylic, matched with the refractive index of the working fluid, a concentrated solution of sodium iodide and water. The facility is designed to allow optical flow measurements in all three blade rows and from all directions. Results presented in this paper are based on 2D PIV measurements focusing on the flow structure in the tip region of the rotor blade for two flow rates, one of them just above the stall level. Included are phase-averaged distributions of velocity, circumferential vorticity, and turbulent kinetic energy in several meridional planes dissecting the tip at different chordwise locations. They follow the evolution of the Tip Leakage Vortex (TLV) as it rolls up near the blade suction side, migrates across the rotor passage, and subsequently bursts. Upon bursting, the distinct high vorticity core is replaced by a broad region of elevated vorticity, which occupy a substantial fraction of the passage. Sample instantaneous realizations and higher resolution stereo-PIV data are also provided. The turbulent kinetic energy is high near the vortex core, in the shear layer connecting the vortex to the blade tip, and around the point of flow separation on the endwall casing, where the leakage flow meets the main passage flow.
Experimental Study of Confined Turbulent Vortical Flow in a Narrow Annulus
Authors: Perelstein Yuri , Gancedo Matthieu , Farbos de Luzan Charles , Gutmark Ephraim , Frosell Thomas
Published: December 2014

An experimental study was carried out to investigate the turbulent flow field of confined vortical flow in a narrow annulus. The test section consists of two concentric annuli that are separated by a tube with four equally spaced rectangular apertures. Water is passing from the inner annulus to the outer annulus and the main region of interest is near aforementioned rectangular holes.

Flow velocity was measured using a stereoscopic particle image velocimetry (PIV) system. The small size of the outer annulus required development of multiple improvements to the PIV system. Stereoscopic calibration within the curved and narrow annulus required development of a calibration method that ensured accurate alignment of the laser sheet to provide an accurate polynomial mapping of the physical coordinates to the pixel location on the ICCD chips of the cameras. Macro lenses were used to provide the desired field of view. Test section dimensions required accurate vertical repositioning of the cameras and the laser within the measurement plane. This was implemented using a two-axis positioning system. Reflections within the outer annulus required development of a polyurethane based paint that reflected the laser light with a shifted wavelength. The paint survived high shear stresses and pressure gradients during testing.

Three dimensional flow field was assembled based on measurements taken, providing insight into the behavior of the flow as it exits the aperture and mixes in the outer annulus. A vortex core was identified.

The results of measurements are compared to CFD results and serve as a tool validating the CFD model. Both the CFD and the measured results predict similar trends in flow behavior, showing strong radial and tangential components of velocity at the plane coincident with the edge of the hole.
Numerical Investigation of Impact of Relative Humidity Condition on Droplet Accumulation and Film Cooling on Compressor Blades
Authors: Bugarin Luz , Badhan Antara , Mao Shaolin
Published: December 2014

In this study, the relationship between the inlet relative humidity (RH) condition, heat transfer, and droplet accumulation/motions on gas turbine’s compressor blades involved in enhanced film cooling was investigated. Wet compression has gained popularity as a highly effective way to increase power output for gas turbine systems due to its simple installation and low cost. This process involves injection of droplets into the continuous phase (air with high temperature) of the compressor to reduce the temperature of the flow leaving the compressor and in turn increase the power output of the whole gas turbine system. This study focused on a single stage rotor-stator compressor model; the simulations are carried out using the commercial CFD tool ANSYS (FLUENT). In particular, the study modeled the interaction between the two phases including mass and heat transfer, given different inlet relative humidity (RH) and temperature conditions. The Reynolds Averaged Navier-Stokes (RANS) equations with k–ϵ turbulence model were applied as well as the droplet coalescence and droplet breakup model considered in the simulation. The interaction between the blade and droplets was modeled to address all possible interactions, which include: stick, spread, splash, or rebound. The goal of this study is to quantify the relationship between the RH and the overall heat transfer coefficient, and the temperature on the blade surface.
Nonlinear Multi-Modal Panel Flutter Oscillations at Low Supersonic Speeds
Authors: Vedeneev Vasily , Shishaeva Anastasia , Kuznetsov Konstantin , Aksenov Andrey
Published: December 2014

In this paper aeroelastic instability of a plate in a gas flow is investigated by direct time-domain numerical simulation. Plate deformation and gas flow are simulated in solid and fluid codes, respectively, with direct coupling between these codes. A series of simulations under different parameters has been conducted.

Three types of the plate response have been observed: stability, static divergence and flutter. Depending on Mach number, two types of flutter were detected: single mode flutter and coupled mode flutter. At M = 1.8, a good correlation between the present study and the piston theory for coupled mode flutter has been obtained. At lower M, from 1 to 1.6, single mode flutter in 1st, 2nd and higher modes has been observed. Amplitudes and frequencies of flutter limit cycle oscillations have been studied. It is shown that limit cycle oscillations can occur in form of pure one-mode oscillations, or include 1:2 internal resonance, when fluttering mode excites another mode. In the region of Mach numbers from 1.3 to 1.5, where several plate modes are simultaneously unstable, transition from periodic to quasi-chaotic flutter oscillations occurs.
CFD Computations Using Preconditioned Krylov Solver on GPUs
Authors: Amritkar Amit , Tafti Danesh
Published: December 2014

GPU computation in recent years has seen extensive growth due to advancement in both hardware and software stack. This has led to increase in the use of GPUs as accelerators across a broad spectrum of applications. This work deals with the use of general purpose GPUs for performing CFD computations.

The paper discusses strategies and findings on porting a large multi-functional CFD code to the GPU architecture. Within this framework, the most compute intensive segment of the software, the BiCGSTAB linear solver using additive Schwarz block pre-conditioners with point Jacobi iterative smoothing is optimized for the GPU platform using various techniques in CUDA Fortran. Representative turbulent channel and pipe flow are investigated for validation and benchmarking purposes. Both single and double precision calculations are highlighted.

It is found that the precision has a negligible effect on the accuracy of predicted turbulent statistics. However, it was found that single precision calculations led to instabilities in the initial convergence of the pressure equation if the convergence criterion was set at too low a value. This was remedied by limiting the number of iterations during the initial stages of the calculation. For a modest single block grid of 64×64×64, the turbulent channel flow computations showed a speedup of about 8 fold in double precision whereas it was more than 13 fold for single precision on the NVIDIA Tesla GPU. For the pipe flow consisting of 1.78 million grid cells distributed over 36 blocks, the gains were more modest at 4.5 and 6.5 for double and single precision respectively.
Numerical Investigation of DC Dielectrophoretic Particle Transport
Authors: Hossan Mohammad Robiul , Dutta Prashanta , Dillon Robert
Published: December 2014

In this paper, we investigate the mechanism of two dimensional DC dielectrophoresis (DEP) using a hybrid immersed interface-immersed boundary method where both electric and hydrodynamic forces are obtained with interface-resolved approach instead of point-particle method. Immersed interface method is employed to predict DC electric field in a fluid media with suspended particles while immersed boundary method is used to study particle transport in a fluid media. The Maxwell stress tensor approach is adopted to obtain dielectrophoretic force. This hybrid numerical scheme demonstrates the underlying physics of positive and negative dielectrophoresis, and explains their contribution in particle assembly with consideration of size, initial configurations and electrical properties of particles as well as fluid media. The results show that the positive DEP provides accelerating motion while negative DEP provides decelerating motion depending on the electrode configurations and initial particle positions. The results also show that the local nonuniformity in electric field induced by the suspended particles guides the particles to form stable chain. Both positive and negative DEP can contribute in the process of particle assembly formation based on the properties of particles and fluid media. This hybrid immersed interface-immersed boundary scheme could be an efficient numerical tool for understanding fundamental mechanism of dielectrophoresis as well as designing and optimization of DEP based microfluidic devices.
Application of Multiphase Particle Methods in Atomization and Breakup Regimes of Liquid Jets
Authors: Farrokhpanah Amirsaman , Mostaghimi Javad
Published: December 2014

Multiphase Smoothed Particle Hydrodynamics (SPH) method has been used to study the jet breakup phenomena. It has been shown that this method is well capable of capturing different jet breakup characteristics. The value obtained for critical Weber number here in transition from dripping to jetting is a very good match to available values in literature. Jet breakup lengths are also agreeing well with several empirical correlations. Successful usage of SPH, as a comparably fast CFD solver, in jet breakup analysis helps in speeding up the numerical study of this phenomenon.
Shock-Induced Mixing With Chemical Reactions Using the FLASH Code
Authors: Varshochi H. , Attal N. , Ramaprabhu P.
Published: December 2014

FLASH is a massively parallel, multi-physics, open source code developed by the University of Chicago [1] for investigating astrophysical phenomena. FLASH was modified [2] to handle detailed chemical kinetics for hydrogen and methane combustion and heat release, in order to enable the code to be used for combustion applications. These capabilities have been tested and validated [2, 3] through an extensive suite of simulations. These modifications include the addition of detailed H2-air and CH4-air chemistry along with temperature dependent thermodynamic and transport properties.

The aim of this work is to apply the modified version of FLASH to three cases of highly compressible supersonic flows, involving chemical reactions. The first problem is a reacting shock-bubble interaction, in which the shock triggers combustion in a fuel bubble. In the second problem, a two-dimensional, Richtmyer-Meshkov Instability leading to combustion of the fuel at a non-premixed, single mode perturbed interface has been studied numerically. In these cases, the relationship between the integral heat release rate, the integral H2O production rate and total circulation is investigated. In the third example, a multimode perturbed interface has been implemented into a 3D simulation. Time evolution of the interface undergoing reacting RMI is studied. By defining a reflecting endwall for RMI simulations, the effect of a second reflected shock on the mixing behavior and combustion on an already shocked interface has been studied.

Numerical simulations of the interaction of a shock (Mach number 2) in air with H2 bubble was performed [2]. The misalignment between the pressure gradient across the shock front and the density gradient at the site of H2-Air interface generates baroclinc vorticity. This phenomenon generates counter-rotating vortices that breakdown the H2 bubble (fuel). The rapid breakdown of the H2 bubble transitions to turbulent mixing, intensifying the heat release rate. In two more general configurations, chemically reacting, single-mode and multimode Richtmyer-Meshkov Instability has been studied. RMI is the driving mechanism for growth of small interfacial perturbations. Initially single-mode perturbations of small amplitude grow linearly due to impulsive acceleration by shock. This is followed by non-linear growth at late times due to the formation of secondary Kelvin-Helmholtz instability. Heat release and product formation in the vicinity of interface will affect perturbation growth rates which will affect the mixing behavior and therefore the combustion efficiency [4].
DNS of Oscillatory Boundary Layer Over a Closely Packed Layer of Sediment Particles
Authors: Ghodke Chaitanya D. , Apte Sourabh V.
Published: December 2014

Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport. To study pickup and transport of sediment, it is necessary to have a detailed knowledge of the small amplitude oscillatory flow over the sediment layer near the sea bed. Fully resolved direct numerical simulations are performed using fictitious domain approach [1] to investigate the effect of a sinusoidally oscillating flow field over a rough wall made of regular hexagonal pack of spherical particles. The flow arrangement is similar to the experimental data of Keiller et al. [2]. Transitional and turbulent flows at Reδ = 95,150,200 and 400 (based on the Stokes layer thickness, δ=2ν/ω) are explored over a range of non-dimensional sphere sizes. Turbulent flow is characterized in terms of coherent vortex structures, Reynolds stress variation, turbulent kinetic energy budget and PDF distributions. The nature of unsteady hydrodynamic lift forces on sediment grains and their correlation to sweep and burst events is also reported. The dynamics of the oscillatory flow over the sediment bed is used to understand the mechanism of sediment pick-up.
Aerodynamic Optimization of Turboprop Turbine Blades Using a Response Surface Methodology Based Algorithm
Authors: Tosin Stefano , Friedrichs Jens , Sperling Johann , Kožulović Dragan
Published: December 2014

Turbomachinery blade design improvement and optimization by CFD is a time-consuming engineering challenge. Such an optimization process, which requires advanced numerical simulations, uses a large amount of computational resources to provide the required solutions. This paper presents a turbine blade optimization process which uses an algorithm based on response surface methodology (RSM) to increase the simulation speed. The main idea of RSM is to start with a lower number of sample points to generate an analytical model that describes the relationship between the pre-defined numbers of design parameters. In this study, the Kriging approximation is used to generate the surface model. The global minimum on the surface is searched by applying a gradient method. The increase in the convergence speed is achieved by using an adaptive scheme, which creates additional points around the previous minimum while reducing the solution space at each iteration step, until convergence is achieved. Each iteration step is composed of several CFD simulation runs where each point represents different designed geometries inside the n-dimensional parameter space. The process combines a Bezier-spline based airfoil-generator with a parametric meshing tool —G3DMESH— and a CFD solver —TRACE—, both developed and provided by DLR, into a MATLAB script function. A particular characteristic of this optimization method is its lower evaluation number requirement to reach convergence, as well as its capability to run multiple simultaneous RANS. The optimizer process was initially tested by using basic functions to analyze its solution behavior and its performance in comparison to a genetic algorithm (GA) type optimizer. It is observed from this comparison that RSM optimization reaches the convergence faster and more stable than the GA method applied on the test case. Preliminary optimization results show an improvement in function evaluation requirements by up to 50%, which depends on the complexity of the respective surface model of the test case. As an application, a 4-stage low pressure turbine for a turboprop engine is designed by multi-streamline analysis. 2D mid-span cross-sections from both rotor and stator are produced by the Bezier-spline based airfoil-generator. The basis tool requires input parameters as leading and trailing edge blade angles and maximum thickness position. The blade generator is further improved by the additional ability to work with high values of deviation angles between the leading and trailing edges, up to 90°. 6 control points are used to define the two curves, for pressure and suction sides, which encompass the cross-section geometry. Optimization process runs to improve these airfoil parameters. The 2D airfoils of the first stage are optimized by an objective function based on total pressure loss coefficient at the engine on-design point. The same geometry is also optimized using the GA method as a comparison case.
Numerical Investigation of the Effect of Solid Particles on Liquid Holdup in Stratified Wavy Gas-Liquid Flow in Natural Gas Pipelines
Authors: Jafari Mahdi , Mansoori Zohreh , Saffar Avval Majid , Ahmadi Goodarz
Published: December 2014

The effect of presence of solid particles on stratified wavy gas-liquid flows has been studied. The height of liquid phase in the natural gas pipeline is a key parameter in designing and can affect the corrosion/erosion rate. In present paper, the numerical four-way simulation of solid particles in gas-liquid wavy stratified flow has been used. The computational model is shown to be able to evaluate the effect of the particles on liquid holdup which is critical for gas pipeline design. The particles cause the liquid phase height in horizontal pipe decreases by increasing the solid phase concentration.
Some Canonical Examples of Streamwise Vortex Structure for Rotating Components
Authors: Gostelow J. P. , Garrett S. J. , Rona A. , Adebayo D. S.
Published: December 2014

Nose cones, turbine blades and bearings have rotating components and represent very practical geometries for which the modal behavior of vortex structures is not completely understood. These three rather different physical cases are being studied. A common theme of competition between modes and vortex types, whether counter-rotating or co-rotating, emerges. The objective of ongoing work is to obtain physical confirmation, enhanced understanding and predictive capability for the vortex structures encountered in rotating machines.
Self-Assembly and Manipulation of Particles on Drop Surfaces
Authors: Amah Edison C. , Shah Kinnari , Fischer Ian S. , Singh Pushpendra
Published: December 2014

It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or equator of the drop by subjecting it to a uniform ac electric field. The dielectrophoretic and hydrodynamic forces which cause the motion of particles depend on the parameters such as the particles’ and drop’s radii, the dielectric properties of the fluids and particles, and the frequency of the electric field. The hydrodynamic force, which arises because of the induced motion in the liquids, is the main focus of this paper. We show that it can be used to control the distribution of particle monolayers on the surface of a drop.
A Scaling Law for the Urban Heat Island Phenomenon: Deductions From Field Measurements and Comparisons With Existing Results From Laboratory Experiments
Authors: Neophytou Marina K.-A. , Fernando Harindra J. S. , Batchvarova Ekaterina , Sandberg Mats , Lelieveld Jos , Tryphonos Eleonora
Published: December 2014

We report results from a multi-scale field experiment conducted in Cyprus in July 2010 in order to investigate the Urban Heat Island (UHI) in Nicosia capital city and its interaction with multi-scale meteorological phenomena taking place in the broader region. Specifically, the results are analysed and interpreted in terms of a non-dimensional/scaling parameter dictating the urban heat island circulation reported from laboratory experiments (Fernando et al, 2010). We find that the field measurements obey the same scaling law during the day, in the absence of any other flow phenomena apart from the urban heating. During the night we find that the deduced non-dimensional value reduces to half (compared to that during the day); this is due to the presence of katabatic winds from Troodos mountains into the urban center of Nicosia and their cooling effect superimposed on diurnal urban heating. Based on this deduction, the impact of various proposed heat island mitigation measures in urban planning can be evaluated.
Numerical Simulation of Cavitating Flow Over 2D Hydrofoil Using OpenFOAM Adapted for Debian Operating System With LXDE Based in Kernel GNU/Linux
Authors: Diaz Victor Hugo Hidalgo , Luo XianWu , Huang RenFang , Cando Edgar
Published: December 2014

Though commercial CFD codes are widely used in flow analysis, but there are free/open source programs which have been applying for computational fluid dynamics. An open source software makes it possible to customize the solver according to the flow features. In the present paper, cavitating flows over 2D NACA66 hydrofoil were simulated based on open source software, where SALOME is used for mesh generation, OpenFOAM for flow solution under Debian GNU/Linux operating system. The results show the simulated cavitating flow characteristics such as cavity revolution, vortex shedding, cavitation induced pressure vibrations, etc. are validated by experiments and results obtained from proprietary software as Ansys Fluent. Thus, the proposed numerical methods based on open source platform are suitable for flow simulations, even for depicting the complicated physics of cavitation.
Application of Valve-Less Impedance Pumps to a Direct Sodium Borohydride–Hydrogen Peroxide Fuel Cell
Authors: Wen C. Y. , Yang A. S. , Tseng J. W.
Published: December 2014

In this study, two novel valve-less impedance pumps are applied, for the first time, in the liquid fuel supply of a Direct Sodium Borohydride–Hydrogen Peroxide Fuel Cell (DBHPFC). This valve-less pump prevents the pump corrosion and serves appropriately to reduce the volume and weight of fuel cell systems. It comprises an amber latex rubber tube, connected at both ends to rigid stainless steel tubes of different acoustic impedance, and a simple actuation mechanism which combines a small DC motor and a cam. The motor activated cam periodically compresses the elastic tube at a position asymmetric from the tube ends. Traveling waves, emitted from the compression, combine with reflected waves at the impedance-mismatched rubber tube/stainless tube interfaces. The resulting wave interference creates a pressure gradient and generates a net flow. A DBHPFC with the active area of 25 cm2 is constructed. It is shown that the maximum pumping rate can achieve 30 ml/min with the DBHPFC connected. The corresponding maximum power and current are 13.0 W and 25.5 A, respectively. Specific power, volumetric power density, and back work ratio of the DBHPFC using this pumping method are shown superior to those of the other pumping configuration with the peristaltic pumps.
PIV Measurement of the Transient Fluid Flow due to the Adsorption of Particles
Authors: Musunuri Naga A. , Singh Pushpendra , Fischer Ian S.
Published: December 2014

The particle image velocimetry (PIV) technique is used to study the physics of particle adsorption and the spontaneous dispersion of powders that occurs when particles come in contact with a fluid-liquid interface. The dispersion can occur so quickly that it appears explosive, especially for small particles on the surface of mobile liquids like water. The measurements show that the adsorption of a spherical particle causes an axisymmetric streaming flow about the vertical line passing through the center of the particle. The fluid directly below the particle rises upward, and near the surface, it moves away from the particle. The flow, which develops within a fraction of second after the adsorption of the particle, persists for several seconds. The flow strength, and the volume over which it extends, decrease with decreasing particle size. The streaming flow induced by the adsorption of two or more particles is a combination of the flows which they induce individually.
CFD Simulations of Low Liquid Loading Multiphase Flow in Horizontal Pipelines
Authors: Karami Hamidreza , Torres Carlos F. , Parsi Mazdak , Pereyra Eduardo , Sarica Cem
Published: December 2014

Low Liquid Loading is a very common occurrence in wet gas pipelines where very small amounts of liquid flow along with the gas, mainly due to condensation of hydrocarbon gases and water vapor. The effects of low liquid loading on different flow characteristics, and flow assurance issues such as pipe corrosion prove the necessity of analyzing the flow behavior in more depth. In this study, CFD simulations are conducted for a horizontal pipe where liquid and gas are supplied at separate constant rates at the inlet. The liquid is introduced at the bottom to help shorten the developing section. The simulations are conducted with Ansys Fluent v14.5 using Volume Of Fluid (VOF) as the multiphase model. The analysis targets, mainly, the shape of the interface, velocity fields in both liquid and gas phases, liquid holdup, and shear stress profile. On the other hand, experiments are conducted in a 6-inch ID low liquid loading facility with similar testing condition. Experiments are conducted with water or oil as the liquid phase for a liquid volume fraction range of 0.0005–0.0020 of the inlet stream. For all cases, several flow parameters are measured including liquid holdup and interface wave characteristics. A comparison is conducted between CFD simulation results, model predictions, and experimental results, and a discussion of the sources of discrepancy is presented. Overall, the results help understand the low liquid loading flow phenomenon.
Thermal and Dissolved Gas Effects on Cavitation in a 2-D Convergent-Divergent Nozzle Flow
Authors: Watanabe Satoshi , Enomoto Keisuke , Yamamoto Yuya , Hara Yoshinori
Published: December 2014

It is well known that the thermal effect of cavitation suppresses the development of cavity with the decreased local temperature due to the latent heat of evaporation. In the present study, to obtain the fundamental knowledge of thermal effect of cavitation, an experimental study of cavitation in a simple convergent-divergent nozzle was carried out using hydro-fluoro-ether (HFE) as a working fluid. HFE was expected to reveal the thermal effect at room temperature, which enabled us to simply neglect the heat flux between the test section and outside, and to roughly keep adiabatic conditions on nozzle wall surfaces.

It was found that the cavitation in HFE was a foam-like sheet cavity, which was different from a film like cavity in cold water. The temperature depression, near the leading edge of cavitation, was confirmed especially in more developed cavitation cases, revealing the existence of thermal effect of cavitation. However, in lower velocity cases, the measured cavity length was apparently longer in HFE despite of the expected thermal effect. The measured pressures inside the cavity were found to be apparently larger than the vapor pressure, indicating the possible separation of dissolved air into the cavity. Then, the cavitation number based on the measured cavity pressure instead of vapor pressure was proposed, and by using the proposed cavitation number, the thermal effect was clearly confirmed in terms of the cavity development. Frequency characteristics of measured pressure fluctuations were qualitatively similar regardless of the species of working fluid.
Thermal Effects on Micro-Sized Tesla Valves
Authors: Paudel Basil J. , Jamal Tausif , Thompson Scott M. , Walters D. Keith
Published: December 2014

The Tesla valve is a no-moving-parts check valve that may be used in a variety of mini- or micro-fluidic applications for passive flow promotion and/or rectification. Its effectiveness is measured via its diodicity which depends on the Reynolds number and its unique design features. The valve may be aligned in-series to further increase its fluidic diode effect — forming a multi-staged Tesla valve (MSTV). Using Computational Fluid Dynamics (CFD) and high performance computing, the current investigation assesses the effects of heat transfer on the overall MSTV diodicity and the extent to which the MSTV enhances heat transfer. For laminar, single-phase flow, thermal boundary conditions were imposed to include an isothermal MSTV wall at 20 °C and a flow inlet temperature of either: 20 °C, 50 °C or 80 °C. The flow outlet temperature and MSTV diodicity was then determined for the various inlet temperatures, Reynolds numbers, working fluids (i.e. Prandtl number, Pr) and number of Tesla valves in MSTV. Working fluids were varied between: air (Pr ∼ 0.7), water (Pr ∼ 5) and ethylene glycol (Pr ∼ 200). The MSTV channel cross-section was set to 1 mm2 and the valve-to-valve distance was held constant while varying the number of Tesla valves. Results indicate that there is a significant decrease in diodicity for water and ethylene glycol as the inlet temperature increases, suggesting higher performance of the MSTV when there is less heat transfer. For air, MSTV performance was found to actually increase with temperature. Due to mixing effects, the MSTV was demonstrated to function as an efficient heat exchanger relative to an un-valved mini-channel of similar size.