bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Investigations of Active Flow Control Using Synthetic Jets on a Highly Loaded Compressor Stator Cascade
Authors: Gmelin Christoph , Steger Mathias , Zander Vincent , Nitsche Wolfgang , Thiele Frank , Huppertz Andre´ , Swoboda Marius
Published: March 2011

Time-resolved Reynolds-Averaged Navier-Stokes simulations of a 3D stator compressor cascade are performed. At the design point of the airfoil under investigation, pronounced secondary flow effects are observed. Strong corner vortices emerge from the casing walls and the flow separates from the blade suction side towards the trailing edge. Transition from laminar to turbulent flow occurs within a laminar separation bubble. Using a commercial CFD software, the influence of the spatial resolution is investigated by means of a spanwise coarsening and refinement of the created mesh. Zero net mass flux synthetic jet actuation is used to control the separated regions. The work presents a variation of the temporal discretization and an analysis of the driving parameters of the actuation.
Convective Heat Transfer in Wall-Bounded Flows Affected by Severe Fluid Properties Variation: A Second-Moment Closure Study
Authors: Jakirlic´ S. , Jester-Zu¨rker R.
Published: March 2011

Different flow configurations subjected to increasingly enhanced wall heating were selected to be computationally investigated by means of a differential, near-wall second-moment closure model based on the solution of transport equations for second moments of the fluctuating velocities and temperature, ui″uj″͠ and ui″θ͠ respectively. Both Reynolds stress model and heat flux model represent wall-topography free formulations with quadratic pressure-strain term and pressure-temperature-gradient correlation. The transport equations for the turbulent stress tensor and the turbulent heat flux are solved in conjunction with the equation governing a new scale supplying variable, so-called “homogeneous” dissipation rate, Jakirlic and Hanjalic (2002). Such an approach offers a number of important advantages: proper near-wall shape of the dissipation rate profile was obtained without introducing any additional term and the correct asymptotic behaviour of the stress dissipation components by approaching the solid wall is fulfilled automatically without necessity for any wall geometry-related parameter. The configurations considered include fully-developed and developing flows in channel (without and with a sudden expansion) and pipe in conjunction with the scalar transport under conditions of variable fluid properties for which an extensive experimental and numerical (DNS and LES) reference database exists.
Numerical Investigation of the Flow Field Around a Low Rise Building Using LES Technique
Authors: Motallebi Hasankola S. S. , Abouali O.
Published: March 2011

In this research the flow field obtained by LES technique is compared with laboratory data measured around a low rise building model with 1:1:0.3 (length: width: height) aspect ratio. The results also are compared with some other turbulence models. The turbulence models include standard k-ε, Durbin’s revised k-ε (DBN), and LRR Reynolds stress models (RSM). Firstly the limitations of the mentioned turbulence models are described for capturing the recirculation zone above and behind of the building model. Among the RANS models, Durbin’s revised model with low Reynolds type of the boundary condition predicts the flow field around the building better compared with other RANS models. The Smagorinski and WALE models among SGS models of LES were employed for the simulation of flowfield around the building. The LES results show generally a better agreement with experimental data compared with RANS models for the streamwise velocity at the roof and behind of building. This improvement is mainly due to the fact that the periodic velocity fluctuation behind the building is well reproduced in LES.
Experimental Study of Structure and Dynamics of Turbulent Stratified Jet
Authors: Xu Duo , Chen Jun
Published: March 2011

Stratified flows are frequently observed in environmental and oceanic applications, which often involve interaction of momentum and scalar flux. In this study, Particle Image Velocimetry and Planar Laser Induced Fluorescence are applied to simultaneously measure the velocity and density fields of a turbulent jet discharged horizontally into an environment with density difference, for studying the mixing and entrainment process in stratified flows. The data are analyzed to gain understanding of the physical mechanism of vertical mixing (mixing along gravity direction) and horizontal mixing (mixing along horizontal direction) introduced by the turbulent jet flows. The dataset also provides a test platform for mixing models used in stratified flow simulations.
Submerged Jet Impingement Boiling of Saturated Water Under Sub-Atmospheric Conditions
Authors: Cardenas Ruander , Mani Preeti , Narayanan Vinod
Published: March 2011

An experimental study of mini-jet impingement boiling is presented for saturated conditions. Unique to this study is documentation of boiling characteristics of a submerged water jet under sub-atmospheric conditions. Data are reported at a fixed nozzle-to-surface distance that corresponds to a monotonic decrease in heat transfer coefficient for single-phase jet impingement. A mini nozzle is used in the present study with an internal diameter of 1.16 mm. Experiments are performed at three sub-atmospheric pool pressures of 0.2 bar, 0.3 bar and 0.5 bar. At each pressure, jet impingement boiling at four Reynolds numbers are characterized and compared with the pool boiling heat transfer. Enhancements in critical heat flux with increasing Re are observed for all pressures.
A Non-Local Convective Flux Limiter for Upwind-Biased Finite Volume Simulations
Authors: Poe Nicole M. W. , Walters D. Keith
Published: March 2011

Finite volume methods on structured and unstructured meshes often utilize second-order, upwind-biased linear reconstruction schemes to approximate the convective terms, in an attempt to improve accuracy over first-order methods. Limiters are employed to reduce the inherent variable over- and under-shoot of these schemes; however, they also can significantly increase the numerical dissipation of a solution. This paper presents a novel non-local, non-monotonic (NLNM) limiter developed by enforcing cell minima and maxima on dependent variable values projected to cell faces. The minimum and maximum values for a cell are determined primarily through the recursive reference to the minimum and maximum values of its upwind neighbors. The new limiter is implemented using the User Defined Function capability available in the commercial CFD solver Ansys FLUENT. Various simple test cases are presented which exhibit the NLNM limiter’s ability to eliminate non-physical oscillations while maintaining relatively low dissipation of the solution. Results from the new limiter are compared with those from other limited and unlimited second-order upwind (SOU) and first-order upwind (FOU) schemes. For the cases examined in the study, the NLNM limiter was found to improve accuracy without significantly increasing solution convergence rate.
Numerical Multiphysics Modeling of Microdroplet Motion Dynamics in Digital Microfluidic Systems
Authors: Ahmadi Ali , Holzman Jonathan F. , Najjaran Homayoun , Hoorfar Mina
Published: March 2011

In this paper a novel numerical algorithm is proposed for modeling the transient motion of microdroplets in digital microfluidic systems. The new methodology combines the effects of the electrostatic and hydrodynamic pressures to calculate the actuating and opposing forces and the moving boundary of the microdroplet. The proposed model successfully predicts transient motion of the microdroplet in digital microfluidic systems, which is crucial in the design, control and fabrication of such devices. The results of such an analysis are in agreement with the expected trend.
The Effects of Blocking Ratio and Atmospheric Altitude on Starting Annular Jets
Authors: Abdel-Raouf Emad , Baker John , Sharif Muhammad
Published: March 2011

The effects of the blocking ratio and atmospheric altitude on starting annular air jets at a low Reynolds number are investigated by examining the velocity profiles, jet entrainment and vortex formation. A 2D axisymmetric numerical model is developed to perform the analysis. The numerical model is validated with theoretical and experimental results from other studies. In order to achieve a comprehensive analysis, the annular jet is tested for blocking ratios with the values of 0.00 (i.e. a round jet), 0.50 and 0.75. Air properties at altitudes of 0.00 km, 18.90 km and 33.75 km are tested to simulate atmospheric environments at sea level, edge of near space and near space, respectively. The results showed that the starting jet velocity profile, entrainment and vortex formation is a strong function of blocking ratio. On the other hand, the same parameters are shown to be nearly independent, strong and weak functions of atmospheric altitude, respectively.
Computer Simulations of Airflow Field Around a Cube: Comparisons of LES and RANS Models
Authors: Tavakoli Behtash , Ahmadi Goodarz
Published: March 2011

Simulations of flow field around wall mounted square cylinders have been used extensively for validation of computational models in the literature. In this paper the airflow fields around a square cylinder were simulated using the Reynolds Averaged Navier-Stokes (RANS) models as well as the Large Eddy Simulations (LES). Particular attention was given to the case with Reynolds number of 80,000 for which the experimental data of Hussein and Matinuzzi [1] are available. The nature of the 3D wakes behind the cube as well as the vortices in front and at the back of the cube were investigated. The simulation results were compared with the experimental data and the accuracy of different models were studied. While the LES better captured the features of this separated flow, it is computationally intensive. The Reynolds Stress Transport Model (RSTM) did not properly predict some features of this separated flow, but is comparatively more economical. The accuracy of RSTM for predicting the turbulence features of separated flows was discussed, and its application for the flow around a realistic model of a building was pointed out.
Various Approaches to Compute Fluid Residence Time in Mixing Systems
Authors: Li Genong , Mukhopadhyay Aniruddha , Cheng Chi-Yang , Dai Yi
Published: March 2011

Residence time including mean residence time and residence time distribution (RTD) is a very important parameter to characterize a mixing system. In practice, tracer study has been widely used in experiments to obtain residence time distribution. There are several numerical approaches available to compute the average residence time and the residence time distribution of a system. This paper attempts to summarize those available approaches through an example.
Flowfield Analysis in T-Junction Microchannel With Bubble Formations
Authors: Pidugu Srikanth , Abdel-Salam Tarek , Bayraktar Tuba
Published: March 2011

Rapid fluid mixing phenomenon in microchannels offers significant advantages in lab-on-chip testing, drug preparations, micro-assay, and micro-combustor applications. Development of new materials for microchannels and advances in micro fabrication techniques continue to aid fluid mixing research that is vital to microfludic applications. Due to the need for low cost and biocompatibility, polymers began to play important role in design of micromixers. Recently, a number of methods and devices are designed to enhance mixing at the microscale [1–3]. A novel idea of introducing bubbles into two mixing streams with three mixing chambers downstream is tested by means of experiments [3]. The interaction among the bubbles in the mixing chamber results in the stretching and folding of the laminar flow interface leading to a rapid chaotic mixing in short period of time. However, the physics of recirculation zones, bubble formation, and bubble fragmentation must be fully understood in order to design efficient micromixers using this technique. The objective of the present work is to numerically study the formation of recirculation zones, bubble formation, and bubble break-up in microchannels. Numerical calculations were performed with finite volume CFD code ANSYS Fluent. Results obtained with structured grids with about 47,000 grid points. Different gas velocities, liquids velocities and inlet angles were used to investigate the flowfield. Results show that the liquid velocity has a major effect on the circulations inside the channel which impact the formation of the gas bubble. Also, at low liquid velocities, the length of the gas slug is affected by the gas velocity.
A Vortex Immersed Boundary Method for Bluff Body Flows
Authors: Cottet Georges-Henri , Gallizio Federico , Magni Adrien , Mortazavi Iraj
Published: March 2011

The aim of this work is to couple vortex methods with the penalization methods in order to take advantage from both of them. This immersed boundary approach maintains the efficiency of vortex methods for high Reynolds numbers focusing the computational task on the rotational zones and avoids their lack on the no-slip boundary conditions replacing the vortex sheet method by the penalization of obstacles. This method that is very appropriate for bluff-body flows is validated for the flow around a circular cylinder on a wide range of Reynolds numbers.
A New Procedure to Capture Temperature Fluctuations Using Cold Wires: Application to Thermoacoustic Systems
Authors: Berson Arganthae¨l , Poignand Gae¨lle , Blanc-Benon Philippe , Comte-Bellot Genevie`ve
Published: March 2011

The improvement of the thermal coupling between the stack of a thermoacoustic refrigerator and the heat exchangers is necessary to achieve high-efficiency and stable operation. Heat transport by the thermoacoustic effect depends on both the velocity and temperature fields. Inside the stack, it can be described by the linear theory of thermoacoustics. However, departures from linear behaviours are expected near the edges of the stack and in the heat-exchangers due to the generation of vorticity and temperature harmonics. The present work focuses on the experimental characterization of temperature harmonics near the edges of a thermoacoustic stack. Experiments are conducted in an 18cm-long resonator operated with air at atmospheric pressure at the resonance frequency of approximately 464Hz. Drive ratios up to 3% are achieved, which corresponds to temperature oscillation amplitudes up to 2.5K. Temperature measurements are performed using a novel procedure recently proposed by Berson et al., Rev. Sci. Instrum. 81, 015102 (2010). The instantaneous temperature is measured with a cold wire operated by a Constant-Current Anemometer (CCA). In addition, we record the output signal of the same wire, under the same flow conditions — which is made possible by the periodicity of the acoustic wave — and operated in the heated mode by a Constant-Voltage Anemometer (CVA). During post-processing, the thermal inertia of the cold wire operated with the CCA is corrected using the CVA signal. This procedure does not require any physical properties of the wire such as the diameter. In addition, it does not require the knowledge of a heat-transfer/velocity relationship for the wire. This is all the most important for thermoacoustic systems since no such relationship is available in oscillating flows. Results validate the generation of temperature harmonics near the stack edges. The spatial distributions of the first and second harmonic amplitudes are compared with a one-dimensional model. The model is an extension of an analytical model from the literature [Gusev et al., J. of Sound and Vibration 235, (2000)] that takes into account axial conduction. Experimental results show an excellent qualitative agreement with the model and demonstrate the importance of axial conduction on the nonlinear thermal field behind the stack.
Modeling Steam Dryers
Authors: Fadda Dani , Taylor David , Burr Jason , Sredzienski Michael , Gardner Jeff
Published: March 2011

Nuclear steam dryers are used to reduce the moisture carryover (MCO) to levels often well below 0.1%, by weight, water in the steam. The dryers are designed to provide very high quality steam at the full capacity of the steam generator. The purpose of this paper is to present computational fluid dynamics (CFD) models of the steam flow in a generator and the decisions that are required to evaluate different designs. These computational models are successful and proven in field operations.
Air Flow Simulation in a Diesel Engine
Authors: Ferna´ndez Joaqui´n , Marcos Alfonso , Barrio Rau´l , Blanco Eduardo , Castilla Alejandro
Published: March 2011

The intake manifold of a Diesel engine is of significant importance. Several modifications are usually carried out in this system to increase the power generated without modifying the general design of the engine. This work presents a numerical and experimental investigation on the air flow in a commercial Diesel engine. The air average mass flow through the intake manifold was obtained experimentally for the D909 Deutz-Diter Diesel engine. Additionally, a numerical model was created for the test engine. The model reproduces the operating and boundary conditions of the flow and integrates the equations that describe the motion of the fluid. The average mass flow through the engine was obtained from the model and compared with the experimental measurements. The model was used subsequently to investigate the air flow within the engine.
Investigation of Streaming Flow Patterns in a Thermoacoustic Device Using PIV
Authors: Babaei Hadi , Siddiqui Kamran
Published: March 2011

We report on an experimental study conducted to study the streaming velocity fields in the vicinity of the stack in a thermoacoustic device. Synchronized Particle Image Velocimetry (PIV) technique was used to measure the two-dimensional streaming velocity fields. The streaming velocity fields were measured at both sides of the porous stack over a range of pressure amplitudes (drive ratios). The results show that the streaming flow structure is significantly different on hot and cold sides of the stack. The hot side of the stack experienced higher magnitudes and higher spatial variability of the streaming velocities compared to the cold side. The difference in the velocity magnitude between the hot and cold sides of the stack showed a significant increase with an increase in the drive ratio.
Visualizing Principal Mesh Modification Actions According to an Interpolation Error Based Estimator on the Diffuser
Authors: Labbe´ Paul , Guibault Franc¸ois
Published: March 2011

Anisotropic mesh adaptation offers a methodology that iteratively marches towards a mesh that equidistributes the interpolation error of scalars. In order to quantify potential improvements to a mesh that can be brought by mesh adaptation, a novel mesh descriptor and associated visualization tool are presented. The combination of the mesh descriptor, which is based on interpolation error estimation, and the associated visualization approach, aims to provide CFD analysts with a quantitative tool to guide manual or automatic mesh adaptation. The experiment of Clausen on the swirling boundary layer in a conical diffuser is used as a real world test case to assess the potential of the approach.
Numerical Simulation of a Synthetic Jet Actuator Using a Simplified Periodic Boundary Condition
Authors: Pascau Antonio , Garci´a Nelson
Published: March 2011

When using a collocated grid for the discretization of the unsteady Navier-Stokes equations special care has to be taken in the evaluation of the cell face velocity as if it is not properly calculated the resulting flow field may be dependent on the time step. This dependency increases as the time step is reduced so this problem can be of paramount importance in rapidly varying flows. As an illustration of the problem in a flow of industrial interest a synthetic jet has been chosen. Although the primary goal of the paper is not to compare computational and experimental results, the assessment with experimental data will highlight the discrepancies in the computational results with different time steps. For comparison purposes a well documented case was chosen: the first test case of the synthetic jet workshop organized by NASA in 2004, but with the new 2006 data. This flow is produced by a moving diaphragm at one of the sides of a cavity connected to an otherwise stagnant air through a slot. Near the slot exit the flow is almost bidimensional so in order to reduce computational time it has been modelled in a 2D domain with a transpiration velocity at the bottom boundary of a simplified cavity. This velocity tries to reproduce the waveform of the measurements at the slot exit with an appropriate combination of Fourier modes.
Flow Structure in the Wake of a Wall-Mounted Bluff Body
Authors: Buchholz James H. J. , Hajimirzaie Seyed Mohammad , Wojcik Craig J.
Published: March 2011

This paper discusses an experimental study of the flow field around three different wall-mounted obstacles in a boundary layer within a relatively shallow flow (d/H ≈ 4, where d is the flow depth and H is the obstacle height). The first two obstacles consist of a semi-ellipsoidal body with the major axis of the base ellipse oriented in the streamwise and the transverse direction. The third obstacle is a low-aspect-ratio cylinder, bearing greater similarity to geometries investigated in other studies. Reynolds numbers of 5500 and 17800, based on obstacle height, were investigated. Digital particle image velocimetry was used to characterize the flow field in each case. At ReH = 17800, isocontours of streamwise vorticity reveal the presence of a tip vortex pair and a base vortex pair for each geometry; however, the ratio of the relative strengths of the tip and base vortices varies considerably, with the streamwise-oriented ellipsoid having the largest ratio of tip/base vortex circulation and the transverse ellipsoid having the smallest ratio. The presence of a base vortex for such low-aspect-ratio obstacles is unexpected, and therefore we hypothesize that its existence is related to the small d/H. It is anticipated that variation of geometry and d/H will help to elucidate the role of the base vortex in the three-dimensional vortex structure of the wake.
A Multifunctional Microfluidic Device Based on Bifurcation Geometry
Authors: Fadl Ahmed , Demming Stefanie , Zhang Zongqin , Hoxhold Bjo¨rn , Bu¨ttgenbach Stephanus , Krafczyk Manfred , Meyer Donna M. L.
Published: March 2011

Developing multifunctional devices are essential to realize more efficient Microsystems. With miniaturization processes taking place in many different applications, the rooms for single function microfluidic devices are limited. In this study, we introduce a multifunctional micro fluidic device based on bifurcation geometry which is capable of performing pumping and mixing at the same time. Optical lithography is used to fabricate the designed microfluidic device. The microfluidic device is tested at low actuator frequencies, and ethanol is employed as a working fluid. The operational principles are based on rectifying the oscillatory flows by using bifurcation structures for flow rectification. The results prove the feasibility of the novel design, and results are presented in terms of flow rates and maximum back pressures.
An Experimental Study of the Flow Phenomena of R134a Flowing Through a Capillary Tube
Authors: Kaew-On Jatuporn , Nilpueng Kitti , Chingulpitak Sakkarin , Wongwises Somchai
Published: March 2011

In the present work, the flow phenomena of the R-134a flowing through the adiabatic capillary coiled tube are experimentally studied. The main parameters relating flow conditions such as inlet pressure, degree of subcooling mass flow rate, are examined. The test section is made from copper tubing with inner diameters of 1.07mm and the coil diameters of 50 mm. The local pressure and temperature distribution along the length of the capillary coiled tube are measured at several inlet pressures and various degrees of subcooling. The experiments are performed at inlet pressure ranging from 10 to 12 bar, mass flow rate from 9 to 11 kg/hr, and the degrees of subcooling from 6 to 10.5 °C. It was found that there are three regions of the refrigerant flow inside capillary tube including subcooled liquid region, metastable liquid region, and metastable two-phase region and these are discussed in details.
Experimental Investigation of Flow Patterns, Pressure Drop, and Flow Instability During Flow Boiling in a Cross-Linked Microchannel Heat Sink
Authors: Megahed Ayman
Published: March 2011

This paper investigates experimentally flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow maldistribution. Flow visualization, flow instability, and two-phase pressure drop measurements are conducted using the dielectric coolant FC-72 for the range of heat flux from 20.1 to 104.2 kW/m2, mass flux from 109 to 290 kg/m2.s, and exit quality from 0.02 to 0.65. Flow visualization studies indicate that the observed flow regime is primarily slug. Instability results show that the periods and amplitudes of inlet pressure and outlet saturation temperature oscillations decrease with increasing mass flux. The two-phase pressure drop strongly increases with the exit quality and the two-phase frictional pressure drop increases by a factor of 1.6–2, at xe, o < 0.3, as compared with that in the straight microchannel heat sink.
Numerical Analysis of Mixing in a Multifunction Electromagnetic Micropump
Authors: Al-Halhouli Ala’aldeen , Bu¨ttgenbach Stephanus
Published: March 2011

This work presents numerical study on the feasibility of implementing a newly introduced electromagnetic pump for mixing fluids in microscales. It also introduces an interesting energization scheme that can be used to adjust the mixing length until obtaining complete mixing. The electromagnetic gentle pump was proposed for biomedical applications. It consists of an annular fluidic channel, set of solenoids, two permanent magnets and a cover. The pumping concept depends on controlling the rotation of the two hard magnets placed in an annular channel in opposing polarity under the influence of an electronically controlled moving electromagnetic field. This concept is currently under development for microfluidic applications using polymer micromachining. Results have been visualized through plotting mass fraction contours along the channel length. Results are motivating and showed higher mass fraction of H2O at low channel widths.
Saturated-Subcooled Stratified Flow in Horizontal Pipes
Authors: Schultz Richard R. , Kadakia Hiral J. , Liou Jim C. P. , Williams Brian G.
Published: March 2011

Advanced light water reactor systems are designed to use passive emergency core cooling systems with horizontal pipes that provide highly subcooled water from water storage tanks or passive heat exchangers to the reactor vessel core under accident conditions. Because passive systems are driven by density gradients, the horizontal pipes often do not flow full and thus have a free surface that is exposed to saturated steam and stratified flow is present. The subcooled water flows into the reactor vessel via the downcomer—which is an annular region separated from the core by a cylindrical, steel, inner liner. Under accident conditions, where the horizontal pipes are only partially full, both saturated water and saturated steam from the downcomer are likely to be present in the pipe. The saturated water is quasi-static and the fraction of the free surface that is saturated water is dictated by the physics of the flow. The saturated steam is usually flowing in the countercurrent direction to the subcooled water. Consequently the flow may be a three-layered system with saturated steam over a static saturated liquid layer over a flowing subcooled layer. The conditions leading to a saturated liquid layer that separates the saturated steam from flowing subcooled water are explored. The variables that influence the formation of the saturated liquid layer and enable the saturated layer to be maintained for a spectrum of conditions, including steam flow in the countercurrent direction to the subcooled water, are derived, and compared to experimental data. Conclusions regarding this type of flow are given. Finally, typical steam flow velocities that may induce wave-bridging, leading to condensation-induced-water hammer, are identified.
Flow Boiling Heat Transfer Characteristics of R-134a in Horizontal and Vertical Mini-Channels
Authors: Saisorn Sira , Kaew-On Jatuporn , Wongwises Somchai
Published: March 2011

Heat transfer experiments are carried out to obtain the data of R-134a during flow boiling in a circular mini-channel having a diameter of 1.75 mm and a length of 600 mm. The DC power supply generating 80 A at 12 V is used to apply heat to the test section. T-type thermocouples are installed at the inlet and outlet of the test section to measure the system temperature. The 10 thermocouples are installed on the top and bottom sides at equal distances along the tube to measure the wall temperature. Variable area type flow meters calibrated specially in the range 0.02–0.2 LPM for R-134a by the manufacturer are used to measure the refrigerant flow rate. A transparent tube is installed to match up with the test section outlet and to serve as a viewing window for visualisation. Two sets of the horizontal flow and vertical upward flow data are analysed. Flow regime map and heat transfer coefficient tend to be dependent with flow direction under the certain experimental conditions.
Adaptive Anisotropic Stochastic Discretization Schemes for Uncertain Conservation Laws
Authors: Tryoen Julie , Le Maiˆtre Olivier , Ern Alexandre
Published: March 2011

This paper deals with the design of adaptive anisotropic discretization schemes for conservation laws with stochastic parameters. A Finite Volume scheme is used for the deterministic discretization, while a piecewise polynomial representation is used at the stochastic level. The methodology is designed in the context of intrusive Galerkin projection methods with Roe-type solver. The adaptation aims at selecting the stochastic resolution level with regard to the local smoothness of the solution in the stochastic domain. In addition, the stochastic features of the solution greatly vary in the space and time so that the constructed stochastic approximation space depends on space and time. The overall method is assessed on the stochastic Burgers equation with shocks, showing significant computational savings.
Experimental Analysis of Non-Uniform Surface Roughness Affecting to Flow Resistance
Authors: Matsumoto A. , Ito T. , Motozawa M. , Kawashima H. , Ando H. , Senda T. , Kawaguchi Y.
Published: March 2011

Experimental investigation on non-uniform surface roughness affecting to flow resistance has been carried out. Experiment was performed with the concentric cylinder device. In this experiment, we evaluated non-uniform surface roughness of various surface by the optical method and measured friction coefficient in turbulent flow over each surface roughness. As an evaluation of non-uniform surface roughness, we measured surface roughness profiles of each surface by a laser displacement sensor. Based on this surface roughness profiles, we calculated some roughness parameters such as the root mean square roughness. One important result indicates the relationship between friction coefficient and roughness Reynolds number. The friction coefficient increases logarithmically with increasing roughness Reynolds number. Moreover, to discuss the effect of non-uniform roughness in detail, the Probability Density Function (PDF) of the roughness height and the spectrum of the surface roughness profiles were analyzed. As a result, the frictional drag over the rough surface can be mostly evaluated by the roughness Reynolds number which is defined by the root mean square roughness when the probability density distribution of the surface roughness profile has Gaussian distribution. However, if the probability density distribution does not have Gaussian distribution, kurtosis and skewness of surface roughness profile are also important parameter for the evaluation of the surface roughness.
Experimental Investigation on Flow and Mixing in Rotating Microfluidics
Authors: Leung Wallace Woon-Fong , Ren Yong
Published: March 2011

Mixing in rotational micro-chamber has been carried out both experimentally and numerically with intent of improving mixing in viscous dominated microfluidics. In experiment, an enclosed chamber made of machined PMMA cut-out with a PDMS cover was set-up on the rotational platform. Different geometries of the chamber (on the order of several mm) with constant height yet different angular span 5, 10, 15, 20 deg. and radial extents 1.5 and 3 mm were used in the tests. While the two different dyes each occupying half of the chamber took a long time (e.g. nearly 40 minutes for a 20-degree span chamber with inner and outer radii, respectively, 35 and 36.5 mm and height 0.5 mm) for mixing by molecular diffusion in the chamber, faster mixing (about 2.5 minute) can be achieved under continuous acceleration and deceleration rotation with a linear rate of 25 rad/s2. The time for mixing per unit volume (Specific Mixing Time - SMT) was determined experimentally as a function of geometry. The SMT increases with increasing vorticity strength as a result of increasing size of the rotating chamber and/or increasing the magnitude of the linear acceleration and deceleration rates. Mixing is also studied by numerical simulation of the tested geometry. Comparing with the experiments, we found good agreement for the SMT between test results and numerical model for small-angled chamber while the numerical model seems to under-estimate the SMT for large-angled chamber. Further, we have also verified qualitatively in the experiment the flow pattern of the primary vortex in the radial-azimuthal plane responsible, to a large extent, for mixing in the chamber. Two approaches have been taken to observe and confirm the primary vortex being set up by continuous acceleration-and-deceleration of the chamber. The first approach adopted color tracing with two miscible dyed liquids, and the second approach used neutrally buoyant particles to trace fluid particle flow in the rotating chamber.
Symbolic Calculation of Laminar Convection in Uniformly Heated Horizontal Pipe at High Prandtl Number
Authors: Mansour Kamyar
Published: March 2011

We consider fully developed steady laminar flow through a uniformly heated horizontal pipe is simplified by assuming infinite Prandtl number. The solution is expanded in powers of a single combined similarity parameter which is the product of the Prandtl, Rayleigh, and Reynolds numbers and the series extended by means of symbolic calculation up to 16 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of exactness is almost the same order of the radius of convergence but Pade approximation lead our result to be good even for much higher value of the similarity parameter.
2D Measurement and Simulation of Microparticle Cross-Sectional Distribution by Micro Process Tomography
Authors: Choi Je-Eun , Zhao Tong , Takei Masahiro
Published: March 2011

The application of micro process tomography to microchannels is considered in this paper. Using the micro process tomography system in microchannel, a particle distribution image can be obtained in a two phase flow. Based on the reconstructed image, it is easy to estimate the particle movement in a fluid flow. A novel microchannel system is designed for electrical process tomography that includes 60 electrodes distributed cross-sectional at five positions. The particle and water were injected into the microchannel, and then the capacitance of the fluid flow was measured at cross-section of microchannel. The result showed the 2D image of particle distribution of cross-section in the microchannel. Also, cross-sectional images of the particle distribution of a two phase flow are simulated.
Magnus-Effect Rotors for Solar Chimney Power Plants
Authors: Serag-Eldin Mohamed A. , Latif Mohammed A. Abdul
Published: March 2011

The paper proposes the use of spinning and rotating cylinders to replace the axial turbines of Solar Chimney power plants. A large number of circular cylinders are placed equidistant, on a circular track concentric with the solar chimney axis. The cylinders spin around their own axis while simultaneously rotating about the chimney axis. By virtue of the Magnus effect, Lift forces arise which create force components tangential to the track in the direction of motion of the cylinders; thus mechanical work is produced. Using CFD modeling, the paper analyzes the resulting flow pattern and presents the expected performance of the hypothetical design for different geometric parameters and operating conditions. It is demonstrated that the design is indeed promising, and worthy of further investigation and development. It is also revealed that good performance of the proposed rotor is highly dependent on the proper choice of operating parameters.
A Molecular Dynamics Study of Fluid Flows Through Slit-Like Nanochannels Using Two Different Driving Systems
Authors: Darbandi Masoud , Khaledi-Alidusti Rasool , Sabouri Moslem , Abbasi Hossein Reza
Published: March 2011

The Poiseuille flow through slit-like nanochannels is investigated using the nonequilibrium molecular dynamics simulations. To drive a dense flow through the channel, we use two self-adjusting vertical plates strategy. These plates force the liquid to flow through the nanochannel under adjustable inlet and outlet boundary conditions. Comparing with the dual-control-volume grand-canonical molecular dynamics method, the current strategy provides many advantages. The current strategy does not need particle insertion and deletion, therefore, the system dynamics would not be affected at all. Moreover, the number of particles in the simulation system is fixed due to inserting the two self-adjusting vertical plates at the two ends of the nanochannel. The motion of these plates are controlled using a combination of an externally applied force and an internal force produced by the molecules in the system. Using this strategy, we study the transport of liquid argon and oxygen through a few slit-like nanochannels having different sizes. We benefit from the nonequilibrium molecular dynamics (NEMD) strategy in our simulations. To expand our study, we consider different back pressure implementations in the flow through the nanochannel. The current results are eventually compared with those derived by applying a uniform driving force method and their advantages are described.
Recognize the Cold Flow Perturbation Sources in a Dump Combustor With Taper Exit
Authors: Yadav N. P. , Kushari Abhijit
Published: March 2011

This paper reports on experimental investigation of the flow field inside a low aspect ratio dump combustor with tapered exit. The length of the combustor studied was less than the reattachment length for the separated flow. The flow field behavior in the combustor is evaluated from pressure and velocity measurement at varying flow Reynolds number. The pressure measurement inside the combustor at different locations was performed by the piezo-resistive pressure sensors. The velocity measurement inside the combustor was carried out by a hot wire probe. The rms (root mean square) velocity and turbulence intensity distributions are found to be axisymmetric due to the geometry of the combustor. The variation in pressure and velocity with locations and Reynolds number was studied. The velocity variation near the surface of the combustor (r/R = 0.66) was showing the opposite behaviour upto x/h = 4.837 than the radial locations (r/R = 0.0, 0.33) due to presence of recirculation and high separation of shear layer by deceased in the strength of the potential core. The pressure and velocity measurement studies support that the presence of the strong recirculation and high turbulence inside the combustor. The power spectral studies of the pressure and velocity fluctuations also suggested the presence of strong recirculation, acoustic modes and turbulence behaviour inside the combustor. The velocity distributions were corroborated by comparing the frequency spectrum with the wall pressure distributions and the results were found to be in very good qualitative agreement with each other.
Simulation of Polymer Chain Driven by DPD Solvent Particles in Nanoscale Flows
Authors: Darbandi Masoud , Zakeri Ramin , Schneider Gerry E.
Published: March 2011

In this study, we simulate the motion and reformation of polymer chain in the nanoscale fluid flow motion of the DPD (Dissipative Particle Dynamics) solvent. The behavior of polymer chain through DPD solvent is studied for 2D and 3D considerations. We implement two body forces of Poiseuille flow and electroosmotic flow to the DPD fluid particles. In case of the electroosmotic flow force, we show that the movement of polymer chain via the electroosmotic phenomenon provides less dispersion than that of the Poiseuille flow for the same polymer chain movement.
Dissipative Particle Dynamics Simulation of Nano Taylor Cone
Authors: Joulaian Meysam , Khajepor Sorush , Pishevar Ahmadreza , Afshar Yaser
Published: March 2011

Dissipative particle dynamics (DPD) is an emerging method for simulating problems at mesoscopic time and length scales. In this paper, we present a new algorithm to describe the hydrodynamics of a perfect conductive fluid in the presence of an electric field. The model is based on solving the electrostatic equations in each DPD time step for determining the charge distribution at the fluid interface and, therefore, corresponding electrical forces exerted by the electric field to the particles near the interface. The method is applied to a perfect conductive pendant drop which is immersed in a perfect dielectric and hydrodynamically inactive ambient. We have shown that when the applied voltage is sufficiently high, the drop shape is changed to a cone with an apex angle which is near to the Taylor analytical estimation of 98.6°. Our results reveal that the presented algorithm gives new capabilities to the conventional DPD method for simulating nanoscale problems in the presence of an electric field.
Effect of Shear Rate on Thrombus Growth
Authors: Tolouei Elham , Fouras Andreas , Carberry Josie
Published: March 2011

In this study micro and nano scale measurement techniques are applied to platelet studies and determination of factors in platelet aggregation and thrombus formation. Conventionally it has been assumed that platelets are stimulated by blood clotting factors and platelet activators to aggregate and form a thrombus at sites of vascular injury. We have recently shown that a primary factor in initiating platelet aggregation is hemodynamic shear. This paper presents the effect of shear rate on the time evolution of thrombus formation and the final geometry of a mature thrombus. A relationship between maximum mature thrombus height and local shear rate is formulated. We have shown that the shear rate is not only an important factor in initiating platelet aggregation but is also one of the main inhibitors of platelet aggregation and thrombus formation. We propose that when the platelets reach a critical height, they encounter a specific local hemodynamic range, which prevents further thrombus growth.
An Analytical Model of the Instantaneous Transvalvular Pressure Gradient Through an Aortic Stenosis
Authors: Keshavarz-Motamed Zahra , Maftoon Nima , Kadem Lyes
Published: March 2011

Diagnosis and treatment of aortic stenosis largely depends on accurate determination of the pressure difference before and after the valve, known as transvalvular pressure gradient (TPG). Clinically, TPG is obtained using Doppler echocardiography though sometimes invasive cardiac catheterization has to be used to confirm Doppler echocardiography findings. By solving analytically coupled fluid and solid domain equations, we suggest a formulation that with a good degree of accuracy can be used to calculate TPG. Analytical result is validated using experimental data from literature. The suggested methodology is an alternative to cardiac catheterization and helps to prevent its risks.
An Experimental Study of a Synthetic Jet in Cross Flow in a Microchannel
Authors: Sinclair Alexander , Reizes John , Timchenko Victoria , Rosengarten Gary
Published: March 2011

Laminar flow limits the mixing performance and heat transfer rates that occurs within microdevices. Synthetic jets in the microscale could disrupt laminar flow and improve the performance of such devices. In this paper a synthetic microjet integrated in a microchannel was designed and fabricated using micromachining techniques. The channel flow was driven by a syringe pump at a rate of 1.39μL/s and the device was actuated using a piezoceramic disc at a frequency of 600Hz. Flow fields were measured phase locked to the actuation cycle using the MicroPIV technique in the mid plane of the jet. The resultant fields revealed a jet with a largest velocity of 2.3m/s. The average velocity during expulsion was estimated to be 0.73m/s using a comparison to the oscillatory solution to flow in a square duct. Measurements at different phases in the cycle revealed a jet strong enough to impinge on the opposing wall and the growth and decay of a pair of vortices formed at the edge of the orifice. It was also shown that the synthetic jet significantly altered the flow patterns showing promising signs for enhancing mixing and heat transfer in microchannels.
Flow Simulation and Diagnosis Through Hydraulic Turbines Using a RNG k-ω Turbulence Model
Authors: Liu Shuhong , Wu Xiaojing , Wu Yulin
Published: March 2011

Francis turbine is widely employed in large scale hydro-power stations in the world with main characteristics of efficiency, stability and cavitation. In practical establishment, each large power station must develop a new Francis turbine for its special natural condition and requirement, such as higher efficiency for utilization of natural resources. CFD has been developed greatly and helped a lot in hydraulic design stage of the turbine. In this paper, firstly, a new RNG k–ω turbulence model is proposed based on the RNG k–ε model, which brings the nonlinear term of the mean fluid flow transition to the ω equation in the original k–ω model. And, this RNG k–ω model has been used to predict the energy performances for Francis turbine. Then, the flow diagnosis method in the turbine runner based on vorticity parameters is presented, following the detailed flow behavior revealed. Finally, the simulation results for different model Francis turbines have been compared and analyzed for optimizing the energy performances of the turbine. The model test results indicate that the efficiency of hydraulic turbine has been improved from 93.6% to 94.5%.