bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooling in Constant Wall Temperature Microchannels With Thermal Creep Effects
Authors: Amiri-Jaghargh Ali , Niazmand Hamid , Renksizbulut Metin
Published: March 2011

Fluid flow and heat transfer in the entrance region of rectangular microchannels of various aspect ratios, 0.2 ≤ α* ≤ 1, are numerically investigated in the slip flow regime, 10−3 ≤ Kn ≤ 10−1, with particular attention to the thermal creep effects. Uniform inlet velocity and temperature profiles are prescribed in a microchannel with constant wall temperature. The gas inlet temperature is prescribed higher than the wall temperature in order to study the thermal creep effects in a fluid cooling process. To avoid unrealistically large axial temperature gradients due to the prescribed uniform inlet temperature and upstream conduction associated with low Reynolds number flows encountered in microchannels, an adiabatic section is added to the inlet of the channel, which resembles an adiabatic reservoir. A control volume technique is employed to solve the Navier-Stokes and energy equations which are accompanied with appropriate velocity-slip and temperature-jump boundary conditions at walls. Despite the constant wall temperature, axial and peripheral temperature gradients form in the gas layer adjacent to the wall due to temperature-jump. The simultaneous effects of velocity-slip, temperature-jump and thermal creep on the flow and thermal patterns along with the key flow parameters are examined in detail for a wide range of cross sectional aspect ratios, and Knudsen and Reynolds numbers (0.1 ≤ Re ≤ 5). Present results indicate that thermal creep effects influence the flow field and the temperature distribution significantly in the early section of the channel.
Gas Liquid Vane Separators in High Pressure Applications
Authors: Fadda Dani , Barker David
Published: March 2011

Vane separators are inertial devices used to remove entrained liquids from gas. They are utilized in pressure vessels operating at a wide range of temperatures and pressures. Computational Fluid Dynamics (CFD) modeling and sizing calculations are used to evaluate the loading to a vane separator and determine the maximum overall gas and liquid handling capacity of the pressure vessel. Test results, performed at operating pressures up to 133 bar (1931 psia) using live natural gas illustrate that, when sized correctly based on the vane’s capacity curves and CFD modeling, vane separators continue to have high separation efficiency at very high operating pressures.
Flow and Heat Transfer of Nanofluids With Temperature Dependent Properties
Authors: Ebrahimnia Bajestan Ehsan , Niazmand Hamid , Renksizbulut Metin
Published: March 2011

Numerical simulations of laminar convective heat transfer with nanofluids in two different geometries involving a straight pipe and a 90° curved pipe are presented. The Navier-Stokes and energy equations for an incompressible Newtonian fluid are solved in a body fitted coordinate system using a control-volume method. In the present work, the nanofluid is a mixture of water and alumina particles, and its thermophysical properties are considered as a function of temperature as well as particle concentration. The accuracy of the models employed for estimating the effective thermophysical properties of this nanofluid are first evaluated using available experimental data for heat transfer in a straight pipe. The same models are then employed for the simulation of flows in a curved pipe. Present results indicate that both the nanoparticle and curvature effects enhance the heat transfer performance but at the expense of increased pressure drop. However, in the present case, the nanoparticle contribution to the pressure drop is dominant, which increases by up to two orders of magnitude at higher nanoparticle concentrations. The ratio of the nanofluid Prandtl number to the based fluid Prandtl number is established as a criterion for the choice of a nanofluid. This ratio must be less than 1 to achieve higher heat transfer rates with relatively low pressure drops as the particle concentration is increased.
Liquid Deposition at Micro and Nanoscale
Authors: Lutfurakhmanov Artur , Sailer Robert , Loken Gregory , Kohut Ananiy , Wang Yechun , Voronov Andriy , Schulz Douglas L. , Akhatov Iskander S.
Published: March 2011

Micro/Nanolithography is a creation of micro/nano features on the substrate. This paper proposes a new capillary-based lithography method that is non-invasive to substrate. The application of the pressure from one side of capillary causes the liquid meniscus to form at the capillary outlet. Touching the substrate with the meniscus only causes the liquid deposition on the substrate. The size of deposited liquid droplet is about 10% of inner diameter of the capillary. Glycerol was chosen as a trial liquid of deposition, because the rate of evaporation for glycerol is low in comparison with other liquids. The deposition of glycerol was done in micro-scale. Comparison of theoretical results with experimental data is shown and discussed.
A Three Equation Curvature and Rotation Sensitive Variant of the SST K-Omega Turbulence Model
Authors: Dhakal Tej Prasad , Walters D. Keith
Published: March 2011

To date, eddy viscosity models are the most accepted and widely used RANS-based turbulence closures, attributable to their computational efficiency and relative robustness. One notable shortcoming of these models is their insensitivity to system rotation and streamline curvature. In this article, we present a variation of the SST k-ω model properly sensitized to system rotation and streamline curvature. The new model is based on a direct simplification of the Reynolds Stress Model under weak equilibrium conditions. To enhance stability and include history effects, an additional transport equation for a transverse turbulent velocity scale is added to the model. The new transport equation incorporates the physical effect of curvature and rotation on the turbulence structure. The eddy viscosity is then redefined based on the new turbulent velocity scale. The model is calibrated based on rotating homogeneous shear flow and implemented for a number of test cases including rotating channel, U-duct, and hump model flow. Compared to popular two equation models, the new model shows improved performance in system rotation and/or streamline curvature dominated flows.
Three Dimensional Volumetric Velocity Measurements in the Inner Part of a Turbulent Boundary Layer Over a Rough Wall Using Digital HPIV
Authors: Talapatra Siddharth , Katz Joseph
Published: March 2011

Microscopic digital Holographic PIV is used to measure the 3D velocity distributions in the roughness sublayer of a turbulent boundary layer over a rough wall. The sample volume extends from the surface, including the space between the tightly packed, 0.45 mm high, pyramidal roughness elements, up to about 5 roughness heights away from the wall. To facilitate observations though a rough surface, experiments are performed in a facility containing fluid that has the same optical refractive index as the acrylic rough walls. Magnified in line holograms are recorded on a 4864×3248 pixel camera at a resolution of 0.67μm/pixel. The flow field is seeded with 2μm silver coated glass particles, which are injected upstream of the same volume. A multiple-step particle tracking procedure is used for matching the particle pairs. In recently obtained data, we have typically matched ∼5000 particle images per hologram pair. The resulting unstructured 3D vectors are projected onto a uniform grid with spacing of 60 μm in all three directions in a 3.2×1.8×1.8 mm sample volume. The paper provides sample data showing that the flow in the roughness sublayer is dominated by slightly inclined, quasi-streamwise vortices whose coherence is particularly evident close to the top of the roughness elements.
Physically-Based Drop Size Distribution Evolution of Atomized Drops
Authors: Tembely Moussa , Soucemarianadin Arthur , Le´cot Christian
Published: March 2011

We report in this work the evolution of a physically-based drop size-distribution of atomized drops coupling the Maximum Entropy Formalism (MEF) and the Monte Carlo method. The atomization is performed using a Spray On Demand (SOD) print-head which exploits ultrasonic generation via a Faraday instability. The physically-based distribution is a result of the coupling of a MEF specific formulation and a general Gamma distribution. The prediction of the drop size distribution of the new device is performed. The dynamic model which prediction capability is fairly good is shown to be sensitive to operating conditions, design parameters and physico-chemical properties of the fluid. In order to achieve the drop size-distribution evolution, we solve the distribution equation, reformulated via the mass flow algorithm, using a convergent Monte Carlo Method able to predict coalescence of sprayed droplets.
A Valve-Less Rectification Minipump Based on Dynamic Rectifying Geometries
Authors: Fadl Ahmed , Demming Stefanie , Zhang Zongqin , Bu¨ttgenbach Stephanus , Krafczyk Manfred , Meyer Donna M. L.
Published: March 2011

The advantages of valve-less rectification micro pumps include having no moving parts, low cost, reliable, having the ability to pump particles-laden fluids and live cells, being compatible with a wide range of micro channel materials and working fluids. Most valve-less rectification micro pumps are based on passive rectifying geometries such as a nozzle/diffuser, Tesla (Valvular Conduit), and Bifurcation geometries. In this study, we present a new valve-less rectification minipump based on a dynamic rectifying geometry. The present work includes design, fabrication, and testing of the pump. The experimental results are presented in terms of flow rates and maximum back pressures.
Embedded Refractive Index Sensing in Microchannels Using Multiplexed Silicon-on-Insulator Microring Resonators
Authors: Grad Michael , Attinger Daniel , Wong Chee Wei
Published: March 2011

We describe the use of wavelength-multiplexed microring resonators as refractive index sensors embedded in microfluidic channels for parallel and differential sensing. The sensor geometry consists of two or more identical 45-micron diameter silicon ring resonators fabricated on silicon dioxide coupled to a single input/output bus waveguide. Two modes of sensing are described: parallel sensing with multiple sensors embedded in separate fluidic channels, and increased sensitivity refractive index sensing by the compensation of temperature dependence with a differential temperature referencing measurement. The first set of experiments demonstrates parallel sensing of water and aqueous salt solutions embedded in separate microchannels. The second set of experiments demonstrates the removal of temperature dependence, and thus improved refractive index detection limit, by temperature referencing using a differential measurement between two resonators.
Testing and Analysis of an Oscillating Hydrofoils Turbine Concept
Authors: Kinsey Thomas , Dumas Guy
Published: March 2011

A new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is presented, tested and analyzed in the present investigation. Due to its rectangular extraction plane, this technology is particularly well suited for river beds and shallow waters near the coasts. The present turbine is a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. The pitching motion of each hydrofoil is coupled to their cyclic heaving motion through four-link mechanisms which effectively yield a one-degree-of-freedom system driving a speed-controlled electric generator. The turbine has been mounted on a custom-made pontoon boat and dragged on a lake at different velocities. Instantaneous extracted power has been measured and cycle-averaged for several water flow velocities and hydrofoil oscillation frequencies. Results are demonstrated to be self-consistent and validate our extensive 2D flow simulation database. The present data show optimal performances of the oscillating hydrofoils concept at a reduced frequency of about 0.12, at which condition the measured power extraction efficiency reaches 40% once the overall losses in the mechanical system are taken into account. Further measurements of power extraction with a single oscillating hydrofoil have also been performed by taking out the downstream hydrofoil of the tandem pair. Those measurements favorably compare, quantitatively, with available 3D CFD predictions. The 40% hydrodynamic efficiency of this first prototype exceeds expectation and reaches levels comparable to the best performances achievable with modern rotor-blades turbines. It thus demonstrates the promising potential of the oscillating hydrofoils technology to efficiently extract power from an incoming water flow.
Pressure Drop of Accelerating Slug Flow in Microchannels: Modeling and Experiment
Authors: Molla Shahnawaz , Eskin Dmitry , Mostowfi Farshid
Published: March 2011

An investigation on the pressure drop of a gas-liquid slug flow through a long microchannel of rectangular cross-section is presented. A constant pressure gradient in the microchannel was observed in a flow where gas bubbles progressively expanded and the flow velocity increased due to significant pressure drop. In contrast to majority of the earlier studies of slug flow in microchannels, where void fraction was nearly constant throughout the channel, we investigated systems where the volume of the gas phase increased significantly due to large pressure drop (up to 2000 kPa) along the lengthy (∼1 m) channel. This expansion of the gas phase led to a significant increase in the void fraction, causing considerable increase in flow velocity. Local pressure was measured along the channel using a series of embedded membranes acting as pressure sensors. The axial pressure profile for a gas-liquid system, namely, Dodecane/Nitrogen was studied. Our investigation on pressure gradient showed linear trend over a wide range of void fractions (30–90%) and flow conditions in the two-phase flow. The lengths and the velocities of the liquid slugs and the gas bubbles were also studied along the microchannel by employing video imaging technique. Furthermore, a model describing the gas-liquid slug flow in long microchannels was developed. Excellent agreement between the developed model and the experimental data was obtained.
Prediction of Hydrodynamic Noise Scattered by the Non-Compact Circular Cylinder
Authors: Mao Yijun , Qi Datong , Gu Yuanyuan
Published: March 2011

A numerical investigation is carried out to predict the noise radiated from high Reynolds number incompressible flow over the circular cylinder. The study focuses on the scattering effect of cylinder surface to the propagation of noise at different frequencies. A boundary integral equation which extends the acoustic analogy to predict the hydrodynamic noise scattered by the non-compact surface is employed. Methods of unsteady flow simulation and noise prediction are validated by the experimental data and analytical result, respectively. Computational results of the noise at different frequencies show that the diffraction of cylinder surface to low-frequency noise is equivalent to a compact dipole source, but the scattering effect to the high-frequency noise gives a trefoil-like pattern which is much different from the directivity pattern of both quadrupole and dipole sources. Moreover, the advantage of the method is to avoid the time-consuming volume integral, which has to be implemented by using theory of vortex sound and method of the tailored Green’s function.
Analysis of Door Mirror Aeroacoustic Source Using Extended Proper Orthogonal Decomposition
Authors: Hekmati Abbas , Ricot Denis , Druault Philippe
Published: March 2011

Based on a 3D simulation of the air flow around a vehicle, the Extended Proper Orthogonal Decomposition (EPOD) is employed in order to identify the existing relation between aerodynamic events around the door mirror (Ω domain) and the near acoustic pressure field (S domain). The aerodynamic pressure field on a 2D plan of Ω is decomposed using a classical formulation of POD. These modes are then sorted according to their correlation to the acoustic pressure field. The modes for which this correlation value is higher than a given threshold are selected. The proposed threshold determination is detailed. The selected modes are then employed in an EPOD procedure in order to determine their contribution to the acoustic pressure field. The aerodynamic pressure field reconstructed by these selected modes has a good correlation with the acoustic pressure field. This correlation is higher than the correlation between the aerodynamic pressure field reconstructed by the most energetic modes and the acoustic pressure field.
DNS of Turbulent Spot Developing Into Turbulent Stripe in Plane Poiseuille Flow
Authors: Aida Hiroshi , Tsukahara Takahiro , Kawaguchi Yasuo
Published: March 2011

A structure consisting of quasi-laminar and turbulent regions in a stripe pattern, which can be found in a transitional plane channel flow, is called ‘turbulent stripe’. In this work, its formation from a turbulent spot has been investigated using a direct numerical simulation in a relatively large-scale computational domain of Lx × Ly × Lz = 731.4δ × 2δ × 365.7δ. We found that a spot developed into the turbulent stripes, as the spot split in the spanwise direction and took in the form of V-shape. Quasi-laminar and turbulent regions coexist inside the spot and each region became multiple V-shape. Several branching turbulent regions expanded from the edge of the spot and parallel to each other obliquely to the streamwise direction with an angle of about 30°. It was also found that the branching turbulent region gave rise to oblique waves at their edges.
Surface Acoustic Wave-Induced Microstreaming in Droplets for the Enhancement of Biosensing Performances
Authors: Ducloux Olivier , Galopin Elisabeth , Zoueshtiagh Farzam , Merlen Alain , Thomy Vincent
Published: March 2011

A droplet-based micro-total analysis system (microTAS) based on digital microfluidics, involving surface acoustic wave (SAW) microstreaming dedicated to the enhancement of the performance of biosensors was designed and studied. We first present the characterization of the flow induced in the droplet during SAW stirring. We then present the evaluation of the biosensor performance enhancement obtained using SAW, evaluated by finite element simulation. We finally stress on the importance of the Damko¨her and Peclet numbers in the design of stirring-based microTAS systems.
Computational Analysis of Fluid Flow in Pebble Bed Modular Reactor Using Different Turbulence Models
Authors: Gandhir Akshay , Hassan Yassin
Published: March 2011

A steady state computational study was done to obtain the pressure drop estimation in different packed bed geometries, and describe the fluid flow characteristics for such complex structures. Two out of the three Bravais lattices were analyzed, namely, simple cubic (symmetric) and body centered cubic (staggered). STARCCM+ commercial CFD software from CD-ADAPCO was used to simulate the flow. To account for turbulence effects standard k-epsilon and realizable k-epsilon models were used. Various cases were analyzed with Modified Reynolds number ranging from 10,000 to 50,000. Each model showed different results as far as the velocity and flow structure is concerned. However, for each case the flow structure showed similar features such as vortex formation downstream and between pebbles due to complex flow separation [1]. The pressure drop obtained from each model was found to be in reasonable agreement with the existing data.
Effect of Longitudinal Vortex on Boundary Layer State and Separation on NACA Symmetric Foil
Authors: Prothin Sebastien , Djeridi Henda , Billard Jean-Yves
Published: March 2011

Vortex generators have been widely used in aerodynamics to control the separation of boundary layers. In such application (Angele and Muhammad, 2005) vortex generators are embedded in the boundary layer and the vortex height, with regards to the wall, is of the boundary layer thickness. The objective of this configuration is obviously far from being the effects of a single longitudinal vortex (generated upstream by an elliptical plan form profile) on the turbulent boundary layer shape over a Naca0015 symmetric foil at different incidences at high Reynolds number 5 105. The vortex is situated outside the boundary layer (ten times the BL thickness over the wall) taking into account the small value of the thickness in our hydrodynamic application. Obviously, this situation is optimum as the vortex delays separation and increases the maximum lift but introduces drag penalty at small incidence. This is nevertheless frequently encountered in hydrodynamic applications (hub vortex upstream of a rudder) and of interest. To point out the mechanism of the boundary layer manipulation, both global efforts using gauge balance and velocity measurements using LDV and PIV have been performed and compared with and without vortex. The base flow is an APG boundary layer characterized by a predominant wake area. Effect of the vortex is analyzed via the shape factor both in inflow and outflow regions. The longitudinal vortex suppress the hysteretic loop classically described in this Reynolds number range (Djeridi et al., 2009) but an increase of the drag is observed in the range of incidence just before stall. Velocity measurements indicated that, for incidences near the stall appearance, the shape factor is decreased both in the inflow and in the outflow regions. Even for large incidences, in the inflow region the value of the shape factor is equivalent to the one found in the turbulent BL over a flat plate. In this region the vortex modifies the equilibrium state of the BL as attested by the Clauser parameter. Even for large distances between the vortex and the wall, the ability of the vortex to suppress the detachment of the BL is observed on the evolution of the backflow coefficient. This effect is greater pronounced in inflow area near the trailing edge region where the flow is locally reattached due to the high momentum fluid displacement.
The Effect of Mach Number and Aspect Ratio on the Interfacial Characteristics of a Submerged Rectangular Gas Jet
Authors: Weiland Chris , Vlachos Pavlos
Published: March 2011

Gas jets formed by rectangular nozzles submerged in water were studied using a non-invasive photographic technique which allowed simultaneous measurements of the entire interface. Three aspect ratios were considered corresponding to 2, 10, and 20 with all nozzles sharing a common width. As far as the authors know this study represents the first time the effects of aspect ratio and Mach number on a submerged gas jet have been studied. The results indicate aspect ratio and Mach number play a large role in dictating both the unsteadiness of the interface and the penetration of the gas jet into the surrounding liquid medium. The jet pinch-off is shown to have a logarithmic decay with increasing Mach number and when appropriately scaled by the total viewing length and a geometric length scale (LQ) is relatively constant across all aspect ratio nozzles. The location of pinch-off is also a function of aspect ratio, with the subsonic aspect ratio 2 nozzles showing maximum pinch-off at y/LQ ≈ 23–26 while sonic and supersonic Mach numbers have peaks over the range y/LQ ≈ 11–14. The AR 10 and 20 nozzles show no dependence on Mach number with the maximum number of pinch-off events observed over the interval y/LQ ≈ 3–5. Jet spreading which is indicative of liquid entrainment is also shown to increase with Mach number and aspect ratio. The jet penetration also increases with increasing Mach number and aspect ratio. The spatial instability growth rate was deduced from the downstream evolution of the interfacial unsteadiness and it is shown that the nozzle with aspect ratio of 2 follows a different trend than the aspect ratio 10 and 20 nozzles, suggesting a fundamentally different mechanism dominates the stability of large aspect ratio rectangular gas jets.
Application of an Immersed Boundary Method to Flow in Cerebral Aneurysms and Porous Media
Authors: Mikhal Julia , Lopez Penha David J. , Stolz Steffen , Geurts Bernard J.
Published: March 2011

We present the development and application of an immersed boundary (IB) method for the simulation of incompressible flow inside and around complex geometrical shapes and cavities. The IB method is based on a volume-penalization method that is applied throughout the domain, rendering the velocity in stationary solid parts negligibly small, while the flow in the open parts of the domain is governed by the Navier-Stokes equations. The flow solver is based on a skew-symmetric finite-volume discretization in combination with explicit time-stepping for the convective and viscous fluxes, and implicit time-stepping for the IB forcing term. The complex domain is characterized in terms of a so-called ‘masking function’ which equals unity in the solid parts and zero in the open parts of the domain. The focus is on the accuracy with which gradients of the solution close to solid walls can be approximated using the IB methodology. We investigate this for flow through a model of an aneurysm as may develop in the circle of Willis in a human brain, and to flow in a structured porous medium composed of a regular spatial arrangement of square rods. The shear stress acting on the vessel walls in case of flow through an aneurysm, and the permeability of the porous material, are analyzed. The computational method converges as a first order method for Poiseuille flow, with a considerable influence derived from the precise definition of the masking function near solid-fluid interfaces. We identify the best masking function strategy and show that for plane Poiseuille flow even second order convergence may be obtained. Qualitatively reliable results are obtained already at modest resolutions of 8–16 grid cells across a characteristic opening in the flow domain, e.g., the vessel diameter or the size of the gap between individual square rods.
Statistical Analysis of Correlation Between Simulation and Measurement of Interior Wind Noise
Authors: Powell Robert E. , Senthooran Sivapalan , Freed David M.
Published: March 2011

Aeroacoustic simulation with CFD (Computational Fluid Dynamics) generates large result datasets that need to be analyzed carefully to correlate with interior wind noise measurements. Recently, a new tool has been developed that simulates interior noise from exterior air flow by combining transient CFD for flow simulation and SEA (Statistical Energy Analysis) for vehicle structural acoustic response. Several interesting data reduction techniques have been employed to correlate dozens of separate passenger vehicle tests with the corresponding simulation results. This paper presents results of this correlation study, including statistical analysis of the resulting frequency domain comparison.
Viscous Oil-Water Flow in a Microchannel: Flow Patterns and Flow Development
Authors: Foroughi Hooman , Kawaji Masahiro
Published: March 2011

The flow characteristics of a highly viscous oil and water mixture in a circular microchannel have been investigated. Water and silicone oil with a viscosity of 863 mPa.s were injected into a fused silica microchannel with a diameter of 250 μm. Before each experiment, the microchannel was initially saturated with either oil or water. In the initially oil-saturated case, different liquid-liquid flow patterns were observed and classified over a wide range of oil and water flow rates. As a special case, the flow of water at zero oil flow rate in a microchannel initially filled with silicone oil was also studied. When the microchannel was initially saturated with water, the oil formed a jet in water at the injection point but developed an instability at the oil-water interface downstream and eventually broke up into droplets.
A Numerical Model on Secondary Flow and Mixing in Rotating Microfluidics
Authors: Leung Wallace Woon-Fong , Ren Yong
Published: March 2011

The fluid mechanics of mixing different species in a closed rotating microfluidics chamber was investigated. Complicated secondary flow, in form of vortices, in a rotating chamber was generated for mixing different species by continuous changing the rate of rotation over time until uniform mixing of the different species in the chamber has been attained. Three vortices are observed when reference to the rotating chamber — one main vortex in the R-θ planes, generated from the d’Alembert force, was responsible for momentum and mass transfer while two pairs of toroidal vortices, generated from the Coriolis force, is responsible for momentum and mass transfer in the direction parallel to the rotational axis. These secondary flow and vortices help to reduce the mixing length between species of a given concentration. With a much smaller mixing length, diffusion can further effect the remaining mixing in a more reasonable time for the mixture to attain a more uniform or homogenous condition of the species in the microfluidic chamber. Numerical simulation using ANSYS-CFX was used to solve the transient Navier-Stokes equation in the rotating frame simulating the Newtonian laminar flow pattern for linear acceleration and deceleration (in rotation) schemes. Mass transfer was also modeled using the convective-diffusion equation governing movement of the different species in the chamber after the NS and continuity equations have been solved for the velocity field. Using the numerical model, parametric study of the rotating truncated pie-shaped chamber (radius and angular span) have been carried out to investigate the effect of momentum and mass transfer. An index, the mixing quality based on concentration distribution for the whole domain, was used respectively to quantitatively evaluate the mixing performance.
Simulation of Dripping Using Dissipative Particle Dynamics
Authors: Khajepor Sorush , Joulaian Meysam , Pishevar Ahmadreza , Afshar Yaser
Published: March 2011

Dissipative Particle Dynamics (DPD) is a mesoscopic simulation approach used in wide range of applications and length scales. In this paper, a DPD simulation is carried out to study dripping flow from a nozzle. The results of this study are used to answer this question that whether DPD is capable of simulating the free surface fluid on all different scales. A novel wall boundary condition is developed for the nozzle surface that controls its penetrability, near wall fluid density oscillations and the fluid slip close to the wall. We also utilize a new method to capture the real-time instantaneous geometry of the drop. The obtained results are in good agreement with the macroscopic experiment except near the breakup time, when the fluid thread that connects the primitive drop to the nozzle, becomes tenuous. At this point, the DPD simulation can be justified by thermal length of DPD fluid and the finest accuracy of the simulation that is the radius of a particle. We finally conclude that in spite of the fact that DPD can be used potentially for simulating flow on different scales, it is restricted to the nanoscale problems, due to the surface thermal fluctuations.
An Immersed Boundary Method for General Flow Applications
Authors: Edwards Jack R. , Choi Jung-IL , Ghosh Santanu , Gieseking Daniel A. , Eischen Jeffrey D.
Published: March 2011

The development of a direct-forcing immersed-boundary method for general flow applications is outlined in this paper. A cell-classification procedure based on a signed distance to the nearest surface is used to separate the computational domain into cells outside the immersed object (‘field cells’), cells outside but adjacent to the immersed object (‘band cells’), and cells within the immersed object (‘interior cells’). Interpolation methods based on laminar / turbulent boundary layer theory are used to prescribe the flow properties within the ‘band cells’. The method utilizes a decomposition of the velocity field near embedded surfaces into normal and tangential components, with the latter handled using power-law interpolations to mimic the energizing effects of turbulent boundary layers. A procedure for directly embedding sequences of stereo-lithography files as immersed objects in the computational is described, as are extensions of the methodology to compressible, turbulent flows. Described applications include human motion, moving aerodynamic surfaces, and shock / boundary layer interaction flow control.
Numerical Analysis of the Conjugated Heat Transfer Problem for Mixed Electro-Osmotic and Pressure-Driven Flows of Phan-Thien Tanner Fluids in Microchannels
Authors: Escando´n Juan P. , Bautista Oscar E. , Me´ndez Federico
Published: March 2011

In this work we solve numerically the conjugated heat transfer problem of a non-Newtonian fluid and solid walls in a microchannel under the influence of pressure and electro-osmotic forces. The velocity field is determined taking into account a hydrodynamically fully-developed flow and a constitutive relation based in a viscoelastic rheological model with a simplified Phan-Thien Tanner fluid. The numerical process results in solid and fluid temperature distributions. Is shown the influence of nondimensional parameters involved in the analysis on the conjugated heat transfer problem: an indicator of viscoelastic behavior, the Peclet number, a normalized power generation term being the ratio of heat flow from the external wall to the Joule heating, a conjugation term which determines the basic heat transfer regimes between fluid and solid sections in the microchannel. For the flow field: the ratio of pressure forces to the electro-osmotic forces acts on flow as a drag reducer and drag increaser under favorable and adverse pressure gradients, respectively, moreover, for increasing values of the viscoelastic parameter, the velocity of the fluid increases with respect the Newtonian fluid flow case. These velocity perturbations resulting in cross-sectional variations of temperature.
Simulation on Flow Field Induced by Vibration Surface of Low Frequency
Authors: Shen Ling , Liu Shuhong , Wu Yulin
Published: March 2011

Ultrasonic cavitation generated by high-frequency ultrasonic transducer is widely studied because this phenomenon could be applied in a great variety of fields, including medical therapy, industrial cleaning as well as sewage treatment. Flow field influenced by vibration source of low frequency, however, is less studied. For the present study, a water tank of 1000×600×500mm is investigated when a vibration surface that represents a transducer of less frequency vibrates in the vicinity of one wall. Numerical computation based on the method of dynamic mesh is applied. Furthermore, two different vibration patterns are simulated, i.e., piston movement and drumhead vibration. Results show different pressure and velocity distribution within water tank when vibration surface is working at various frequencies and amplitudes. Differences of the flow fields are found between these circumstances, and similarity is found with that induced by ultrasonic transducer. Analysis on differences is discussed for further study.
DNS Study of Transitional Channel Flow Accompanied by Turbulent-Stripe Structures
Authors: Kaneko Shizuma , Tsukahara Takahiro , Kawaguchi Yasuo
Published: March 2011

A regular pattern of turbulent and quasi-laminar fluid motion is known to appear in plane Poiseuille flow near the lowest Reynolds number for which turbulence can be sustained. We focused on this transitional structure called the turbulent stripe and investigated its energy transport process, using a direct numerical simulation. We obtained the budget for Reynolds stresses including v′w′ and w′u′. The spatial outline of the energy transport with respect to the turbulent stripe is proposed. The turbulent energy is produced in both the turbulent region and the quasi-laminar region, and the energy transfer between these two regions is found to be small.
Design of a Mechanical Resonator to Be Coupled to a Thermoacoustic Stirling-Engine
Authors: Tijani M. E. H. , Vanapalli Srinivas , Spoelstra Simon
Published: March 2011

This paper describes the design of a mechanical resonator for a thermoacoustic Stirling-engine. The engine was previously run with a quarter-wavelength acoustic resonator. The advantage of the mechanical resonator is that it is compact and would dissipate less acoustic power. The mechanical resonator consists of a twin piston-spring assembly moving in opposite phase to cancel vibrations. The system uses flexure springs to suspend the piston in a cylinder leaving a narrow gap between them. The narrow gap acts as a dynamic seal between the fronts and back sides of the piston. Simulation calculations show that the mechanical resonator dissipates 40% less acoustic power than the acoustic one. This will lead to more useful acoustic power output from the thermoacoustic Stirling-engine. In addition, the size of the system is reduced considerably.
Hermite-DG Method for PDF-Equations Modelling Dispersions in Turbulent Boundary Layers Including Streamwise Velocity
Authors: van Dijk Peter J. , Swailes David C.
Published: March 2011

A novel methodology is presented for the numerical treatment of multi-dimensional pdf (probability density function) models used to study particle transport in turbulent boundary layers. A system of coupled Fokker-Planck type equations is constructed to describe the transport of phase-space conditioned moments of particle and fluid velocities, both streamwise and wall-normal. Unlike conventional moment-based transport equations this system allows for an exact treatment of particle deposition at the flow boundary. Moreover, the equations in the system are linear and can be solved in a sequential fashion; there is no closure problem to address. A Hermite-Discontinuous Galerkin scheme is employed to treat the system. The choice of Hermite basis functions in combination with an iterative rescaling approach, allows for efficient discretization of the, effectively, 5-dimensional phase-space domain. Results demonstrate the effectiveness of the methodology in resolving distributions near an absorbing boundary.
A Hybrid Model for Predicting Gas Entrainment in a Small Branch From a Co-Current Flowing Gas-Liquid Regime
Authors: Bowden Robert , Saleh Wael , Hassan Ibrahim
Published: March 2011

An analytical model was developed to predict the critical conditions at the onset of gas entrainment in a single downward oriented branch. The branch was installed on a horizontal square cross-sectional channel having a smooth stratified co-currently flowing gas-liquid regime in the upstream inlet region. The branch flow was simulated as a three-dimensional point-sink while the downstream run flow was treated with a uniform velocity at the critical dip location. A boundary condition was imposed in the model whereby the flow distribution between the branch and run was obtained experimentally and digital imaging was used to quantify the critical dip location through the dip angle. Three constant dip angles were evaluated in the model and results showed the dip height to have good agreement with experiments between angles of 50 and 60 degrees. The predicted upstream height, however, did not match well with the experimentally determined height due to the omission of shear and inertial effects between the upstream location and critical dip.
Numerical Modeling of a Mini/Microchannel Reactor for Methane-Steam Reforming
Authors: Peterson Daniel , Apte Sourabh V. , Narayanan Vinod , Schmitt John
Published: March 2011

Numerical modeling of methane-steam reforming is performed in a mini/microchannel with heat input through Nickel-deposited channel walls. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The surface reactions in the presence of Nickel catalyst are modeled as Neumann boundary conditions to the governing equations. Effects of the total heat input, heat flux profile, and inlet methane-steam molar concentration on production of hydrogen are investigated in detail.
Ex-Situ Characterization of Two-Phase Flow Regimes in Proton-Exchange Membrane Fuel Cells Under Non-Isothermal Conditions
Authors: Berson Arganthae¨l , Pharoah Jon G.
Published: March 2011

Efficient water management is crucial for the good performances of proton-exchange membrane fuel cells (PEMFCs). The geometric and physical characteristics of the components of a PEMFC as well as operating conditions have an impact on the transport of water through the porous transport layer (PTL) and the two-phase flow regimes in the microchannels. One parameter of importance is the local temperature, which affects properties such as surface tension and is coupled with phase change. Indeed, a temperature difference of about 5K is expected across the PTL, with spatial variations due to the geometry of the flow field plate. We present preliminary results obtained with a first experimental setup for the ex-situ characterization of two-phase flow regimes in the flow channels. Water is pushed through the PTL, which is sandwiched between a porous metal foam and the flow field plate. The air flow rate, temperature and humidity can be controlled. The cell can be heated up by applying an electrical current through the metal foam. A transparent window is located on top of the flow channel. The two-phase flow within the micro-channels is visualized using a high-speed camera and laser-induced fluorescence. Preliminary results obtained under isothermal conditions at room temperature show that different two-phase flow regimes occur in the channels depending on the operating conditions, in good qualitative agreement with data from the literature. Eventually, a new visualization cell is presented that is expected to correct the flaws of the previous design and will allow a better thermal control. It will be possible to adjust the temperature gradient and the mean temperature in order to observe their impact on two-phase flow regimes for different types of PTL and flow rates. The results will provide a better understanding of water transport in PEMFC and benchmark data for the validation of numerical models.
Effective Use of V&V20 Solution Verification Methodology
Authors: Freitas Christopher J.
Published: March 2011

Methods for the quantification of numerical uncertainty have been a subject of interest to the American Society of Mechanical Engineers (ASME) and the mechanical engineering community as a whole for over a decade. During this time period, ASME has promulgated three statements of standards for the reporting of numerical uncertainty in archival publications (Journal of Fluids Engineering). This paper summarizes the work that has gone into the specification of these standards and the continuing effort in formulation of methods and procedures for quantifying numerical uncertainty. Specifically, this paper discusses the efforts of the ASME V&V 20 Committee (Verification and Validation in Computational Fluid Dynamics and Heat Transfer) to lay a foundation and structure to verification and validation for fluid flow and heat transfer simulations. Issues and methods related to code verification and in particular solution verification are presented and discussed in the context of the recently released V&V20 Standard.
Experimental Study on Measurement of Tangential Momentum Accommodation Coefficient in Microtube
Authors: Yamaguchi Hiroki , Hanawa Tsuneo , Yamamoto Oto , Matsuda Yu , Egami Yasuhiro , Niimi Tomohide
Published: March 2011

Along with the progress in micro- and nano-technologies, such as Micro Electro Mechanical Systems (MEMS) and μ-TAS (Micro-Total Analysis Systems), the Knudsen number, which is a non dimensional parameter for rarefaction, of the flow around and inside the systems becomes large. In such high Knudsen number flows, gas-surface interaction has become important for flow field analyses. To illustrate overall gas-surface interaction without any detailed processes, an accommodation coefficient, α, is the most widely used as an empirical parameter for a practical purpose. One of accommodation coefficients, the tangential momentum accommodation coefficient (TMAC) αt, is in closely related to the loss of the pressure through a micro channel. Therefore, TMAC is an important coefficient for flow inside micro/nano fluidic devices. To obtain TMAC from experiments, the mass flow rate measurements in a microtube were carried out using the constant volume method. The results obtained from the experiments were analyzed in frame of the Navier-Stokes equation associated with the second order velocity slip boundary condition. The mean Knudsen number was less than 0.3, where the velocity slip boundary condition is applicable. From the mass flow rates, the slip coefficient of the boundary condition was obtained, and then, TMAC was determined. The experimental apparatus showed very low leakage rate, and TMAC was determined with a high degree of accuracy. The TMACs of the same surface material with different dimensional parameters were compared for validation of the system.
Analysis and Optimization of a Passive Micromixer With Curved-Shaped Baffles for Efficient Mixing With Low Pressure Loss in Continuous Flow
Authors: Cortes-Quiroz Cesar A. , Azarbadegan Alireza , Moeendarbary Emadaldin , Zangeneh Mehrdad
Published: March 2011

Numerical simulations and an optimization method are used to study the design of a planar T-micromixer with curved-shaped baffles in the mixing channel. The mixing efficiency and the pressure loss in the mixing channel have been evaluated for Reynolds number (Re) in the mixing channel in the range 1 to 250. A Mixing index (Mi) has been defined to quantify the mixing efficiency. Three geometric dimensions: radius of baffle, baffles pitch and height of the channel, are taken as design parameters, whereas the mixing index at the outlet section and the pressure loss in the mixing channel are the performance parameters used to optimize the micromixer geometry. To investigate the effect of design and operation parameters on the device performance, a systematic design and optimization methodology is applied, which combines Computational Fluid Dynamics (CFD) with an optimization strategy that integrates Design of Experiments (DOE), Surrogate modeling (SM) and Multi-Objective Genetic Algorithm (MOGA) techniques. The Pareto front of designs with the optimum trade-offs of mixing index and pressure loss is obtained for different values of Re. The micromixer can enhance mixing using the mechanisms of diffusion (lower Re) and convection (higher Re) to achieve values over 90%, in particular for Re in the order of 100 that has been found the cost-effective level for volume flow. This study applies a systematic procedure for evaluation and optimization of a planar T-mixer with baffles in the channel that promote transversal 3-D flow as well as recirculation secondary flows that enhance mixing.