bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of the Stability of a High-Speed Gas Jet Under the Influence of Liquid Cross-Flow
Authors: Weiland Chris , Yagla Jon , Vlachos Pavlos
Published: March 2009

This paper reports on the interfacial character and deflection of a high-speed gas jet transverse to an aqueous cross-flow as a function of cross-flow speed and gas jet Mach number. Several gas exit velocities were tested including subsonic cases up to supersonic cases at cross-flow velocities from 0.3 m/s to 0.7 m/s. For the subsonic cases, it was found that the stability and resistance of the gas jet to deflect in the presence of cross-flow were increased with the jet Mach number. However, the Mach 1.6 jet was more stable than the Mach 1.9 jet, suggesting that there exists upper and lower bounds for jet stability which are Mach number dependent. Unstable gas jets were shown to pinch-off, meaning the interface of the gas jet in a plane parallel to the ejector exit collapsed to almost a point and an independent bubble rose to the free surface. The stagnation side gas/liquid interfaces were analyzed using the Proper Orthogonal Decomposition (POD) method to better understand the fundamental mode shapes contained in the interface waveforms. It was found that the subsonic jets shared many of the same characteristics in their first, second, and third mode shapes. The supersonic jets differed from the subsonic mode shapes. Interestingly, the mode shapes for the subsonic cases compared well to those of a beam in transverse vibration with sliding-free boundary conditions. The supersonic cases compared relatively well to pinned-free boundary conditions, owing to the more columnar nature of the gas jet as it exited the ejector.
Simulation of H2-Air Turbulent Diffusion Flame by the Partial Chemical Equilibrium Method
Authors: Fukumoto Kazui , Ogami Yoshifumi
Published: March 2009

This paper describes an application of the partial chemical equilibrium method considered chemical kinetics in computational fluid dynamics (CFD). In this method, fuels and oxidants are mixed at a turbulent rate so that a mixture gas of fuel and oxygen is generated. Next, the mixture gas of fuel and oxygen is burnt by molecular diffusion thereby resulting in combustion gases. The turbulent mixture rate is estimated by the eddy dissipation model and the burning velocity is evaluated by the Arrhenius equation. Finally, the combustion products are calculated by the chemical equilibrium method by using the combustion gases. One of the advantages of this method is its ability to calculate the combustion products without using chemical equations. The chemical equilibrium method requires only thermo-chemical functions (specific heat, standard enthalpy, etc). This method can be applied to incinerators or some complex combustion instruments and it can predict the intermediate chemical species of dioxins, etc.
Experimental Study of Axial Slit Wall Effect on Taylor-Couette Flow
Authors: Lee Sang-Hyuk , Kim Hyoung-Bum
Published: March 2009

Taylor-Couette flow has been studied extensively and lots of variables which affect the flow instability are being reported. The wall geometry effect of Taylor-Couette flow, however, has been less studied. In this study, we investigated the effect of axial slit of outer cylinder. This kind of configuration can be easily seen in rotating machinery. Particle image velocimetry method was used to measure the velocity fields in longitudinal and latitudinal planes. The index matching method was used to avoid light refraction. The velocity fields between the slit and plain model which has the smooth wall were compared. From the experiments, both models have the same flow mode below Re = 143. The transition from circular Couette flow to plain Taylor vortex flow began at Re = 103, and the next transition to wavy vortex flow occurred at 124. The effect of slit wall appeared when the Reynolds number is larger than Re = 143. Above this Reynolds number, there was no stable mode and plain and wavy Taylor vortex flow randomly appeared.
RANS and Large Eddy Simulation (LES) for Prediction of Separated Flow Characteristics for Flow Over a Wall-Mounted Hump
Authors: Gan Subhadeep , Poondru Shirdish , Ghia Urmila , Ghia Karman
Published: March 2009

One of the critical challenges in fluid flow research is to understand and predict separated flows. Separated flows are usually unsteady and turbulent. This work focuses on the calculations for the flow over a wall-mounted hump, an example of a turbulent separated flow. To validate the results of numerical simulation, the experimental data chosen was case 3 of the 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop (http://cfdval2004.larc.nasa.gov/case3.html) conducted by NASA. This particular hump is the upper surface of a Glauert-Goldschmied type airfoil and has a chord length of C = 0.42 m, a maximum height of 0.0537 m, and a span of 0.5842 m. For this configuration, as the flow approaches the hump, the boundary layer experiences an adverse pressure gradient. Over the front convex portion of the hump, the flow experiences a favorable pressure gradient and accelerates, and then it separates over a relatively short concave section in the presence of a strong adverse pressure gradient. The hump has a simple geometry, but, nevertheless, is rich in many complex flow phenomena such as shear layer, separation, reattachment, and vortex interactions. Advances made in Computational Fluid Dynamics (CFD) have made available wide variety of turbulence models. A variety of turbulence model and simulation approaches are being used for this work. The flow is simulated using steady and unsteady-state three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations based turbulence models and three-dimensional time-dependent Detached Eddy Simulation (DES) and Large-Eddy Simulation (LES) methods. Case 3 of the CFD Validation workshop was done for both the baseline case as well as with flow control. In this work, the baseline case with Reynolds number of 371,600 based on the hump chord length, C, and Mach number, M, of 0.04 is simulated and analyzed. The experimental data reported by Greenblatt et. al. is used to validate the results for this work. K-ω, the Spalart-Allmaras and the SST turbulence models are implemented for the RANS simulation. Mean-velocity profiles and turbulent kinetic energy profiles, is reported and analyzed at several streamwise (x/C) locations, where C is the chord length of the hump. Detailed comparisons have been made of mean and turbulence statistics such as the pressure coefficient, skin-friction coefficient, and Reynolds stress profiles, with experimental results. The location of the reattachment behind the hump has been compared with experimental results.
Behavior of Jet From an Orifice in a Pipe Under Pulsating Flow
Authors: Tamura Ema , Iwamoto Junjiro
Published: March 2009

An orifice flow meter is widely used in industry to measure the flow rate of gas or liquid in the pipe because of its simplicity. The accuracy of measurement is acceptable when the steady flow is measured. However, it is often the case in industry that the unsteady flow must be measured. It is not yet known, under unsteady flow conditions, how accurate the flow rate can be measured using the orifice flow meter and where the measurement errors come from. In the present paper to answer these questions a pulsating air flow is considered as the simplest of the various types of unsteady flow patterns and the measurement of the flow field in the pipe with orifice is made in detail. The sound pressure level is also measured by probe tube microphone. A comparison is made of power spectrum densities obtained by measurement of sound pressure level and by the velocity measurement from laser Doppler anemometer.
Biomedical Fluid Dynamics: Rich Flow Physics in Pulsatile Flow (Keynote)
Authors: Plesniak Michael W. , Peterson Sean D.
Published: March 2009

Many biological flows, e.g. speech production and the vasculature, are pulsatile and consequently exhibit a rich array of flow physics not observed in steady flows. A classical example in biomedical flows is flow in a rigid walled pipe subjected to non-zero mean harmonic forcing. This flow, solved analytically by Womersley [1], has retrograde flow in the Stokes layer during portions of the cycle. This paper will highlight some of interesting flow phenomena that result from pulsatile flow.
Reliability of Compressible Flow/Structure Coupling Method on Cartesian Grid With Signed Distance Field
Authors: Terashima Hiroshi , Ono Kenji
Published: March 2009

A compressible flow solver coupled with moving/deformed geometries on Cartesian grid with Signed Distance Field (SDF) is developed and its capability is investigated through computations of several basic flow fields for future applications with certain reliability. The flow solver is designed so that SDF includes sufficient geometrical information to compute flow fields. Since information of moving/deformed geometries is recognized as a change of the SDF between time steps, the flow solver can be coupled with moving/deformed geometries naturally. The implementation of this solver is simple and easy. No modification is needed in the main part of the flow solver. Furthermore, the interpolation and the corresponding stencils searching process are not required. Several basic flow fields around fixed/moving cylinders and a fixed sphere are computed in order to validate the proposed solver, in which the computed results are compared with available numerical and experimental results. The results demonstrated the method’s capability for moderate Reynolds number flows around both of fixed and moving geometries. Based on the results, some criteria and problems for obtaining reliable solution are suggested.
Numerical Simulation of a Polygon Motor
Authors: Akiyama Osamu , Kato Chisachi , Yoshimura Takuya , Miyazawa Masashi
Published: March 2009

A polygon motor consists of several mirror faces and it is one of the core components for a laser beam printer and a copier. Under high-speed rotation, aeroacoustic noise with a peak frequency of rotational speed times the number of mirror faces and its harmonics is prominently generated from a polygon motor covered with a casing. This paper describes a method to predict such noise that is generated from the flow in between the rotating mirror and stationary casing, propagates through the casing and is radiated into the ambient air. The unsteady flow is firstly computed by large-eddy simulation. The computed pressure fluctuations on the inner wall of the casing are then fed to structural analysis based on a dynamic explicit finite element method that computes the propagation of the elastic waves in the casing. Resulting external surface velocities are finally used for the computation of the acoustical field. A good correlation between the computed and measured results has been observed in terms of the pressure fluctuation spectrum on the inner surface of the casing, velocity spectra on the casing and spatial distribution of sound pressure level. The proposed method thus seems a promising engineering tool for predicting and identifying the generation mechanism of such noise that is generated from unsteady flow, propagates through solid walls and is radiated to ambient air, in general.
Numerical Analysis on Effects of the Contoured Endwall on the Three Dimensional Flow Characteristics in an Annular Turbine Nozzle Vane
Authors: Lee Wu Sang , Chung Jin Taek , Kim Dae Hyun , Choe Seung Joo
Published: March 2009

The three-dimensional flow in a turbine nozzle guide vane passage causes large secondary loss through the passage and increased heat transfer on the blade surface. In order to reduce or control these secondary flows, a linear turbine with contoured endwall configurations was used and changes in the three-dimensional flow field were analyzed and discussed. Contoured endwalls are installed at a location downstream of the saddle point near the leading edge of the pressure side blade and several positions along the centerline of the passage at constant distance. The objective of this study is to document the development of the three-dimensional flow in a turbine nozzle guide vane cascade with modified endwall. In addition, it proposes and appropriates endwall contouring which shows best overall loss reduction performance among the simulated contoured endwall. The results of this study show that the development of passage vortex and cross flow in the cascade composed of one flat and one contoured endwalls are affected by the acceleration which occurs in contoured endwall side. The overall loss is reduced near the flat endwall rather than contoured endwall, the best performance was shown for the case of 10–15% contoured for span-wise, 40–70% length of chord from trailing edge.
Numerical Simulation of Tip Clearance Flow in Semi-Open Impeller Pump
Authors: Shukla Shyam N. , Kshirsagar Jagadish
Published: March 2009

Semi-Open impellers are commonly used for handling fluids with abrasive particles. These impellers have back shroud and the blades do not have support from front shroud. The designer proposes standard clearance to allow free rotation of impellers in stationary casing. Design clearances are difficult to maintain during manufacturing as it calls for special manufacturing process. It is well established in experiments that both head and efficiency decrease with increase in tip clearance and are quite sensitive to rather small change in clearance. It was planned to carry out numerical experiments with such geometry and bring out effect of varying tip clearance on overall performance of pumps with semi-open impellers. In CFD analysis mostly pumps are analyzed with enclosed impeller as rotating component with stationery casing. The meshing of the tip clearance area was a matter of concern. This is overcome using commercial code CFX-Bladegen and CFX-Turbogrid software and good quality mesh of the impeller was prepared. The paper describes the numerical simulation of tip clearance in semi-open impeller pump. The flow pattern showing clearance vortices associated with tip clearance are brought out. Then effect of clearance on the overall performance of pump is established by varying tip clearance.
Transition Scenario of Periodic and Quasiperiodic Flow Bifurcations in Symmetric Communicating Channels
Authors: Guzman Amador M. , Beiza Maximiliano P. , Fischer Paul F.
Published: March 2009

The flow transition scenario in symmetric communicating channels has been investigated using direct numerical simulations of the mass and momentum conservation equations in the Reynolds numbers range of Re = [170–227]. The governing equations are solved for laminar and time-dependent transitional flow regimes by the spectral element method, using a periodic computational domain, for a periodic length of nL and an aspect ratio of r = aˆ / (2Lˆ) = 0.0405, where aˆ = 2a is the height of block within the channel, n an integer and Lˆ = L + 1 is the periodic length. Periodic computational domains with n = 1 and 2 are used in this investigation to determine the periodic length effect on the flow pattern characteristics. Numerical investigations with different domain meshes are carried out for determining the appropriate discretization for capturing transitional time-dependent flows. The numerical results show a transition scenario with two-flow Hopf bifurcations which develop as the pressure gradient is increased from a laminar to a time-dependent flow regime. The first Hopf bifurcation occurs to a critical Reynolds number of Rec1 and leads to a time-dependent periodic flow characterized by a fundamental frequency ω1. Further increases in the pressure gradient lead to successive quasi periodic flows after a second Hopf bifurcation B2 occurring to a critical Reynolds number Rec2 < Rec1, with two fundamental frequencies ω1 and ω2, and linear combinations of both frequencies—where the fundamental frequency ω1 increases continuously—and ω2 > ω1. This transition scenario is somewhat different from the Ruelle-Takens-Newhouse transition scenario obtained for symmetric wavy channels; in symmetric wavy channels, periodic and quasi periodic flow regimes develop as the Reynolds number increases. The friction factor for the symmetric communicating channel in the transitional regime is higher than the friction factor for the Poiseuille plane channel. The qualitative and quantitative behavior is compared to other channel geometries that also develop other transition scenarios.
Effects of Multiple Impacts and Size Distribution of Particles on Erosion Predictions Using CFD
Authors: Zhang Yongli , McLaury Brenton S. , Shirzai Siamack A.
Published: March 2009

Erosion equations are usually obtained from experiments by impacting solid particles entrained in a gas or liquid on a target material. The erosion equations are utilized in CFD (Computational Fluid Dynamics) models to predict erosion damage caused by solid particle impingements. Many erosion equations are provided in terms of an erosion ratio. By definition, the erosion ratio is the mass loss of target material divided by the mass of impacting particles. The mass of impacting particles is the summation of (particle mass × number of impacts) of each particle. In erosion experiments conducted to determine erosion equations, some particles may impact the target wall many times and some other particles may not impact the target at all. Therefore, the experimental data may not reflect the actual erosion ratio because the mass of the sand that is used to run the experiments is assumed to be the mass of the impacting particles. CFD and particle trajectory simulations are applied in the present work to study effects of multiple impacts on developing erosion ratio equations. The erosion equation as well as the CFD-based erosion modeling procedure is validated against a variety of experimental data. The results show that the effect of multiple impacts is negligible in air cases. In water cases, however, this effect needs to be accounted for especially for small particles. This makes it impractical to develop erosion ratio equations from experimental data obtained for tests with sand in water or dense gases. Many factors affecting erosion damage are accounted for in various erosion equations. In addition to some well-studied parameters such as particle impacting speed and impacting angle, particle size also plays a significant role in the erosion process. An average particle size is usually used in analyzing experimental data or estimating erosion damage cases of practical interest. In petroleum production applications, however, the size of sand particles that are entrained in produced fluids can vary over a fairly broad range. CFD simulations are also performed to study the effect of particle size distribution. In CFD simulations, particle sizes are normally distributed with the mean equaling the average size of interest and the standard deviation varying over a wide range. Based on CFD simulations, an equation is developed and can be applied to account for the effect of the particle size distribution on erosion prediction for gases and liquids.
Trial of Paste Backfill at Toyoha Mine (Keynote)
Authors: Asakura Kuniomi
Published: March 2009

Flow properties of paste made of dewatered mine tailings and a binder were investigated by means of a transportation test using pipelines of 4 inches and 8 inches in diameter at Toyoha Mine. In addition to full-scale tests at the mine, laboratory tests using small-scale pipes were carried out. In this paper, results of flow properties of the paste are presented and various problems during underground backfill are described.
Investigation of Flow Separation Inside a Conical Rocket Nozzle With the Aid of an Annular Cross Flow
Authors: Weiss Alexander , Abanteriba Sylvester , Esch Thomas
Published: March 2009

Flow separation is a phenomenon that occurs in all kinds of supersonic nozzles sometimes during run-up and shut-down operations. Especially in expansion nozzles of rocket engines with large area ratio, flow separation can trigger strong side loads that can damage the structure of the nozzle. The investigation presented in this paper seeks to establish measures that may be applied to alter the point of flow separation. In order to achieve this, a supersonic nozzle was placed at the exit plane of the conical nozzle. This resulted in the generation of cross flow surrounding the core jet flow from the conical nozzle. Due to the entrainment of the gas stream from the conical nozzle the pressure in its exit plane was found to be lower than that of the ambient. A Cold gas instead of hot combustion gases was used as the working fluid. A mathematical simulation of the concept was validated by experiment. Measurements confirmed the simulation results that due to the introduction of a second nozzle the pressure in the separated region of the conical nozzle was significantly reduced. It was also established that the boundary layer separation inside the conical nozzle was delayed thus allowing an increased degree of overexpansion. The condition established by the pressure measurements was also demonstrated qualitatively using transparent nozzle configurations.
Prediction of Transition Point on an Oscillating Airfoil Using Neural Network
Authors: Soltani M. R. , Seddighi M. , Masdari M.
Published: March 2009

Dynamic Neural network was used to minimize the amount of data required to predict the location of transition point on a 2-D oscillatory wing. For this purpose, various experimental tests were carried out on a section of a 660kw wind turbine blade. A multi layer non linear perceptrons network was trained using the output signals of four hot films attached on the upper surface of the model. Results show that using only 50% of the test data, the trained network was able to the transition point with an acceptable accuracy. Moreover, the method can predict the transition points at any position of the wing surface for different Reynolds numbers, amplitudes and initial angles of oscillation, and of course at various reduced frequencies.
The Study on the Flow Fields and Hydraulic Performance in the Pump Sump
Authors: Cheng Li , Liu Chao , Zhou Jiren , Tang Fangping , Yang Hua
Published: March 2009

The pump sump, which connects forebay and intake of pump station, supplies good flow condition for the intake of the pump. The flow inside pump sump is very complex and dominated by three dimensional viscous effects. In order to recognize the characteristic of pump sump, the control volume method is used to simulation the flow filed. The RNG k-ε turbulent model and SIMPLEC algorithm are applied to do analysis. Flow fields inside pump sump are analyzed in details. A lot of computational data, such as computational contour of sections, streamline, flow vectors and pressure contour, are given in the paper. On the based of the simulation results, best plane shapes and suitable dimensions of sump are presented. Relationship between flow patterns and dimensions of sump, such as back wall clearance, bellmouth height and shape of back wall etc, was discussed. The recommenced parameters are brought out. The results show that numerical flow fields agree well with experiment data measured by 3D-PIV model test.
Measurement of the Periodic Flow Field in a Radial Diffuser Pump by the PIV-Method
Authors: Benra Friedrich-Karl , Feng Jianjun , Dohmen Hans Josef
Published: March 2009

Detailed flow measurements are conducted by two-dimensional Particle Image Velocimetry (PIV) in a low specific speed radial diffuser pump at the design operating point, in order to investigate the impeller-diffuser interaction. The measurements are conducted at midspan for different relative positions between the impeller and diffuser. The measuring region covers a complete impeller channel and also a complete diffuser channel. The analysis of the experimental results shows that a jet-wake structure is observed near the impeller outlet. The presence of the diffuser vanes mainly affects the impeller flow at the rear part near the impeller outlet. The diffuser flow strongly depends on the relative impeller position which provides different inflow conditions for the diffuser.
Microbubble Generation Using High Turbulent Intensity Flow
Authors: Nouri N. M. , Maghsoudi E. , Sarreshtehdari A. , Yahyaei M.
Published: March 2009

In this paper, a new method for microbubble generation independent of porous media has been introduced. The approach used in the microbubble generator is based on high turbulent intensity rotational flow. At first, a simple cylindrical model is investigated experimentally. In the model, water enters the device via six diagonal nozzles and mixes with air coming from six holes at the bottom of the device. Passing a rotational trace around central motionless rod, air particles are broken up to tiny bubbles. Size of bubbles is measured and bubbles smaller than 1mm in diameter were found. Finally, improved microbubble generator with the similar mechanism has been introduced. Internal flow passes longer trajectory around the central rotating hub because of its design characteristic that let it rotate by inlet water excitation. Bubbles with diameters less than 300 microns are generated by this method. Size of bubbles is measured experimentally in different void fractions for different outlet gap size. Outlet flow gap changes from 20mm to 8mm and 3mm in three experiments. The best result is observed in the third experiment at which microbubbles smaller than 100 micron are observed. The desirable bubbles’ size is achieved at void fraction of 15% that could be increased up to 60%.
On the Analysis of Turbulent Flow Signals by Artificial Neural Networks and Adaptive Techniques
Authors: Lo´pez Pen˜a F. , Bellas F. , Duro R. J. , Farin˜as P.
Published: March 2009

Artificial Neural Networks (ANNs) and evolution are applied to the analysis of turbulent signals. In a first instance, a new trainable delay based artificial neural network is used to analyze Hot Wire Anemometer (HW) signals obtained at different positions within the wake of a circular cylinder with Reynolds number values ranging from 2000 to 8000. Results show that these networks are capable of performing accurate short term predictions of the turbulent signal. In addition, the ANNs can be set in a long term prediction mode resulting in a sort of non linear filter able to extract the features having to do with the larger eddies and coherent structures. In a second stage these networks are used to reconstruct a regularly sampled signal straight from the irregularly sampled one provided by a Laser Doppler Anemometer (LDA). The irregular sampling dynamics of the LDA signals is governed by the arrival of the seeding particles, superimposing the already complex turbulent signal characteristics. To cope with this complexity, an evolutionary based strategy is used to perform an adaptive and continuous online training of the ANNs. This approach permits obtaining a regularly sampled signal not by interpolating the original one, as it is often done, but by modeling it.
Numerical Prediction for the Bubbly Flow Around a Hydrofoil by Vortex Method
Authors: Uchiyama Tomomi , Degawa Tomohiro
Published: March 2009

This study is concerned with the numerical simulation for an air-water bubbly flow around a hydrofoil. The two-dimensional vortex method for bubbly flow, proposed by the authors in a prior paper, is applied for the simulation. A hydrofoil of NACA4412 with a chord length 100mm is mounted in an air-water bubbly flow. The Reynolds number is 2.5×105, the bubble diameter is 1 mm, and the volumetric flow ratio of bubble to whole fluid is 0.048. The simulation demonstrates that the two-phase flow features around the hydrofoil are successfully captured and that the vortex method is indeed applicable to the bubbly flow analysis around a hydrofoil.
Cavitation Noise Reduction Around a Butterfly Valve by Semicircular Fins
Authors: Ogawa K.
Published: March 2009

Butterfly valves are widely used in industry because of their compactness and simple construction. Many kinds of valves have been proposed to prevent cavitation. However, those valves are fairly expensive and the structures of the valve are complicated. In this paper, the attachment of fins to the valve body is proposed to reduce cavitation noise. The cavitation around the butterfly valve occurs because of the interference of the flow from the nozzle side and the flow from the orifice side. To avoid this interference, semicircular fins were attached to the valve body. From the experimental results, it was confirmed that the attachment of the fins was very effective in reducing the cavitation noise.
Study on the Hydraulic Transients of Pump-Turbines Load Successive Rejection in Pumped Storage Plant
Authors: Zhang Jian , Lu Weihua , Hu Jianyong , Fan Boqin
Published: March 2009

During the calculation of hydraulic transients in pumped storage plants, the speed change of reversible pump-turbine has great relation with the pressure of water conveyance system, and results in the minimum pressure at draft tube does not occur while all sets in the same hydraulic unit reject load simultaneously, but occurs in the combined conditions of load successive rejection. Based on the hydraulic characteristic of reversible pump-turbine, the reason why the minimum pressure at draft tube occur in the combined conditions and the dangerous interval time are analyzed bonding with the water conveyance system layout of pumped storage plants and the practical operation of pump-turbine. The research indicate that when reversible turbines in the same hydraulic unit reject load successively, the maximum pressure at spiral case maybe lower to some extent compared with load rejections simultaneously, but the change of minimum pressure drop at draft tube is great along with different interval time, which endangers the water conveyance system badly, especially in high head pumped storage plants, and should be pay great attention to it. Combining the practical operation of pump-turbine with calculation in theory, it would be the dangerous interval time of load successive rejection of pump-turbines after the first pump-turbine rejects load and its runaway speed achieve speed peak, the pressure of draft tube would drop the minimum once the other set in the same hydraulic unit also reject load at the moment.
Experimental Investigation of Pressure Fluctuation and Vibration in a Large Francis Turbine
Authors: Wang Fujun , Zhu Yuliang , Li Xiaoqin , Ma Jiamei , Wang Liping , Tang Xuelin
Published: March 2009

Vibration caused by unsteady flow usually leads to a problem of stability in operation for large hydraulic turbines. This paper reports the recent investigation and analysis of vibration and pressure fluctuation problem on a prototype 700MW Francis turbine unit at the Three-Gorges Hydropower Plant in China. The site investigation were carried out for a wide load range of 200–600MW at the condition of water head 57–76m. Significant vibrations were observed on head cover when the turbine was operated at load of 540–542MW with water head 68.3m. To identify the causes, investigations were done by measuring oscillations of the guide-bearing, pressure fluctuations under the head cover and in the draft tube. A special vibration zone at frequency of 4.56fn (fn is the runner rotation frequency) was found by further spectrum analysis for the vibrations and pressure fluctuations, while this special zone was not found in other turbines in the same site with same configurations. On the other hand, the low-frequency vibrations caused by pressure oscillations of vortex rope in draft tube were also measured and analyzed.
Numerical/Experimental Investigations on Inlet Back Flow in a Helical Inducer in Partial Flow Conditions
Authors: Watanabe Satoshi , Kim Jun-Ho , Ishizaka Koichi , Furukawa Akinori
Published: March 2009

The attachment of inducer upstream of main impeller is an effective method to improve the suction performance of turbopumps. However, the various types of cavitation instabilities are known to occur even at the designed flow rate as well as in the partial flow rate conditions. The cavitation surge is a viciously unstable phenomenon occurring at partial flow rates, in which all blade cavities are periodically and synchronously elongated and shortened, leading the strong vibration in pump systems. Because the cavitation surge is strongly associated with the inlet back flow of inducer, it is desirable to know the detailed structure of back flow for the development of its effective suppression methods/devices. Then, in the present study, we carried out the numerical and experimental investigations of non-cavitating flows in a two bladed helical inducer with/without an inlet ring plate, which has been found to be effective for the suppression of cavitation surge. The basic characteristics of the inlet back flow structure are obtained and discussed in terms of the occurrence/suppression mechanisms of cavitation surge.
Development of a Spectral Element DNS/LES Method for Turbulent Flow Simulations
Authors: Zhang Xu , Stanescu Dan , Naughton Jonathan W.
Published: March 2009

This paper describes a turbulent flow simulation method, which is based on combination of spectral element and large eddy simulation (LES) technique. The robust, high-order discontinuous Galerkin (DG) spectral element method for large-eddy simulation of compressible flows allows for arbitrary order of accuracy and has excellent stability properties. A local spectral discretization in terms of Legendre polynomials is used on each element of the (possibly unstructured) mesh, which allows for high-accurate simulations of turbulent flows. Discontinuities across the interfaces of the elements are resolved using a Riemann solver. An isoparametric representation of the geometry is implemented, with boundaries of the domain discretized to the same order of accuracy as the solution, and explicit low-storage Runge-Kutta methods are used for time integration. Large eddy simulation has proven to be a valuable technique for the calculation of turbulent flows. An element based filtering technique is used in conjunction with the standard Smagorinsky eddy viscosity model to estimate the effect of sub-grid scales stresses in this paper. The recently developed nonlinear model [1] will also be added in the future. The final aim of this project is to use the LES methodology in swirling jet flow simulation. As a first step towards these simulations, simulations of compressible turbulent mixing layer and back-facing step are also performed to evaluate the robust method. Initial results based on both DNS and large eddy simulations are presented in this paper. Future work will be to validate the code.
A Novel Design of Polymer Electrolyte Membrane Fuel Cell
Authors: Alhussan Khaled
Published: March 2009

A fuel cell is an energy conversion device that converts the chemical energy of fuel into electrical energy. Fuel cells operate continuously if they are provided with the reactant gases, not like batteries. Fuel cells can provide power in wide range. Fuel cells are environmentally friendly; the by-product of hydrogen/oxygen fuel cell is water and heat. This paper will show a numerical modeling for this spiral design of high pressurized Polymer Electrolyte Membrane fuel cell. Numerical modeling requires understanding the physical principles of fuel cells, fluid flow, heat transfer, mass transfer in porous media, electrochemical reactions, multiphase flow with phase change, transport of current and potential field in porous media and solid conducting regions, and water transport across the polymer membrane; and this will result in optimal design process. This paper will show fuel cell models that are used in this analysis. Such as; electrochemical model: predicts local current density, voltage distributions. Potential field model: predicts current and voltage in porous and solid conducting regions. Multiphase mixture model: predicts liquid water and gas flow in the porous diffusion layers. Thin film multiphase model: tracks liquid water flow in gas flow passages. The numerical results of the theoretical modeling are shown in this paper. This paper shows the contour plots of mole fraction of H2O, H2, and O2. Results in this research include the species concentration of H2O, H2, and O2. This research also shows the plot of mass concentration of H2O, H2 and O2.
Multi-Fins Decelerator for Blunt Body in Turbulent Flows
Authors: Alhussan Khaled
Published: March 2009

The aim of this research is to use the numerical analysis techniques to design deceleration devices for a blunt body. In this paper a qualitative analysis of the flow structure over a blunt body with and without multi-fins decelerator was shown. In some applications of aerodynamics, a deceleration of a moving body is required therefore the prediction and controlling of the drag is essential. The deceleration devices such as air bag or fins can be added to the body to increase the aerodynamics drag. For a supersonic speed, a flow around blunt body is complicated due to the detached shock wave, flow separation, boundary layer and their interactions. When a decelerator is integrated with the blunt body the flow is subject to sever change of aerodynamic forces and velocity. The results will show that adding a deceleration device will change the flow structure behind the body especially with regard to the pressure drag and wake. Results of contour plots of Mach number and static pressure for zero angles of attack will demonstrate that the aerodynamic forces and the velocity are changed when the deceleration device is integrated with the blunt body. A number of important conclusions follow from the current research. First, study of the actual flow configuration over a blunt body with a decelerator offers some insight into the complex flow phenomena. Second, adding the decelerator will increase the separation that will result in an increase of total drag.
The Effect of the Boundary Layer Separation in Y-Junction Shape
Authors: Alhussan Khaled
Published: March 2009

The work to be presented herein is a theoretical and numerical analysis of the complex fluid mechanism that occurs inside a Y-junction shape specifically with regard to the boundary layer separation, vortex shedding and generation of wake. The boundary layer separates from the surface forms a free shear layer and is highly unstable. This shear layer will eventually roll into a discrete vortex and detach from the surface. A periodic flow motion will develop in the wake as a result of boundary layer vortices being shed from the solid boundary. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure. This paper shows a numerical analysis of boundary layer separation that occurs in an internal flow; the Y-junction shape. This research shows a numerical simulation of mapping the flow inside Y-junction shape flow. The results show that for small divergent angle namely less that 30-degree the flow separation is almost negligible and that downstream, away from the junction, the boundary layer reattaches and normal flow occurs i.e. the effect of the boundary layer separation is only local.
Similarity Law of Air-Water Two-Phase Flow Performance of Centrifugal Pump
Authors: Matsushita Naoki , Watanabe Satoshi , Okuma Kusuo , Hasui Tomomichi , Furukawa Akinori
Published: March 2009

Air-water two-phase flow performance of conventional centrifugal pumps causes the impermissible head deterioration even at an inlet void fraction less than about 10%. A tandem arrangement of double rotating cascades and higher blade outlet angle more than 90° only in outer rotating cascade has been proposed as a centrifugal pump impeller with high performance in air-water two-phase flow condition. To obtain further improvement of pump performances, a diffuser cascade is installed downstream of impeller outlet. In design of air-water two-phase flow centrifugal pump in various size and operating conditions as well as in single-phase flow, similarity law of pump performances is very useful. The similarity law of impeller diameter, blade height and rotational speed is investigated for the proposed impeller in the present paper. As the results, the similarity law of impeller diameter and rotational speed is certified experimentally even in two-phase flow condition. In addition, influences of blade height on air-water two-phase flow performances indicate a little difference from the similarity law. This difference is, then, discussed by using the results of static head on the shroud wall and air behaviors in the impellers.
Transition of Scaling for Pinch-Off of an Axisymmetric Liquid Ligament in Another Immiscible Liquid
Authors: Homma Shunji , Koga Jiro
Published: March 2009

Scaling for pinch-off of an axisymmetric liquid ligament in another immiscible liquid is studied computationally for intermediate Reynolds number, where both inertial and viscous forces are dominant. Interface motion near pinch-off is traced by a front tracking/finite difference method with higher resolution. When the ligament breaks up, a thin thread forms between a bulb of the tip and the rest of the ligament. Computational results show that the minimum radius of the thread decreases with time and the exponent of time shifts from 2/3 to 1. This shift corresponds to the transition from potential flow scaling law to the scaling law for viscous thread in another viscous fluid. The rate of the thinning down of the thread is in good agreement with the one obtained from experiments for very small Reynolds number range, where viscous force is dominant. Since the length scale becomes very small close to the pinch-off, the viscous effect overcomes the inertial effect and the scaling alters from the potential flow scaling law to the viscous scaling law.
An Application of 3D-PIV to the Flow Measurements in Pump Sump
Authors: Liu Chao , Li Daliang , Zhou Jiren , Tang Fangping
Published: March 2009

Flow condition in the pump sump is of most importance to the pump performance. A test rig was designed for the measurements of velocity distribution. The 3D-PIV was applied to the measurements of the flow fields under the bell mouth of the pump in the opening sump. The calibrations for 3D-measurements at different sections in the pump sump were made before the test. The flow fields under the bell mouth of test pump were obtained and a 3D view of velocity vectors map was given. The measurement results of the flow fields are compared with the existing results from the experiments and numerical calculations. Analysis of flow condition in the pump sump is conducted.
Derivative Statistics of Axial Velocity and Passive Scalar in the Jet Diffusion Field of High-Schmidt-Number Matter
Authors: Sakai Y. , Uchida K. , Kubo T. , Nagata K.
Published: March 2009

In this study, a water solution of dye (whose Schmidt number is about 3,800) was issued into the quiescent water as an axisymmetric turbulent jet and the simultaneous measurements of axial velocity and concentration have been performed using the combined probe of I-type hot-film and fiber-optic concentration sensor based on the Lambert-Beer’s law. Then we calculated the PDF (Probability Density Function) for the streamwise velocity derivative ∂u/∂x and streamwise concentration derivative ∂c/∂x. It was confirmed that the PDFs for ∂u/∂x skew negatively, and the values of skewness (S∂u/∂x) and flatness factor (F∂u/∂x) are consistent with the other data (see Sreenivasan and Antonia, 1997). However, with regard to the PDFs for ∂c/∂x, the skewness (S∂c/∂x) show the values very close to zero, unlikely the past other data which show the magnitude of 0.5∼1.0. On the other hand, the flatness factor (F∂c/∂x) show the values of 7.0∼8.0 which are consistent with other data. This result suggests that the fine-scale structure of a high-Schmidt-number diffusion field is almost isotropic although it is intermittent.
Bimorph Type Soft Actuator Based on Conducting Polymer and Its Application to Micro Pump
Authors: Fuchiwaki Masaki , Naka Yoshitaka , Tanaka Kazuhiro
Published: March 2009

Various shapes of conducting polymer soft actuators attract the attention in recent years. We have proposed a soft actuator that performs open-close movement and micro pump using soft actuator as a driving source. We have developed a soft actuator that perform open-close movement by arranging a soft actuator in which bimorph structures with anion-driven layers and cation-driven layers are connected and cation-driven layers arranged in face-to-face relation. Moreover, the soft actuator performs optimum open-close movement when the ratio of single layer structures and bimorph structures is 1:2. A planner soft actuator performs open-close movement when slits are set on a wide planner soft actuator. The authors have built up a micro-pump with a planner conducting polymer soft actuator that performs open-close movement as a driving source. Its oscillation volume can be adjusted in the range of 2.2–31 [μ1/min].
A New Cavitation-Induced-Momentum-Defect (CIMD) Correction Approach to Track Cavitation Events
Authors: Srinivasan Vedanth , Salazar Abraham J. , Saito Kozo
Published: March 2009

A new unsteady cavitation event tracking model is developed for predicting vapor dynamics occurring in multi-dimensional incompressible flows. The procedure solves incompressible Navier-Stokes equations for the liquid phase with an additional vapor transport equation for the vapor phase. The model tracks regions of liquid vaporization and applies compressibility effects to compute the local variation in speed of sound using the Homogeneous Equilibrium Model (HEM) assumptions. The variation of local cell density as a function of local pressure is used to construct the source term in the vapor fraction transport equation. The novel Cavitation-Induced-Momentum-Defect (CIMD) correction methodology developed in this study serves to account for cavitation inception and collapse events as relevant momentum source terms in the liquid phase momentum equations. Effects of vapor phase accumulation and diffusion are incorporated by detailed relaxation models. A modified RNG K-ε model, including the effects of compressibility in the vapor regions, is employed for modeling turbulence effects. Turbulent kinetic energy and dissipation contributions from the vapor regions are integrated with the liquid phase turbulence using relevant source terms. Numerical simulations are carried out using a Finite Volume methodology available within the framework of commercial CFD software code Fluent v.6.2. Simulation results are in qualitative agreement with experiments for unsteady cloud cavitation behavior in planar nozzle flows. Multitude of mechanisms such as formation of vortex cavities, vapor cluster shedding and coalescence, cavity pinch off are sharply captured by the supplemented vapor transport equation. Our results concur with previously established theories concerning sheet and cloud cavitation such as the re-entrant jet motion, cavity closure and the impact of adverse pressure gradients on cavitation dynamics.
A Semi-Implicit Scheme for Modeling the Interaction Between Biological Cells and Electric Fields
Authors: Pierse John H. , Ferna´ndez Arturo
Published: March 2009

A numerical method for computing the simultaneous solution to the fluid flow equations and the electrostatic field equations is described. The methodology focuses on the modeling of biological cells suspended in fluid plasma. The fluid flow is described using the Navier-Stokes equations for incompressible flows. The electric field is computed trough the Maxwell equations neglecting magnetic effects. The effect of the electric field on the fluid flow is accounted for through the Maxwell stresses. The systems are described by a set of partial differential equations where the solution requires the simultaneous computation of the velocity, pressure and electric potential fields. A semi-implicit numerical scheme is proposed. In order to decrease the computational time required, it is proposed to use a semi-implicit splitting scheme where the Navier-Stokes and Maxwell equations are solved sequentially. The method is used to reproduce the response of human leukocytes immersed in a rotating electric field. An agreement between the numerical results and the data from experiments is observed.