bluuebunny/crossref_metadata_2025 huggingface dataset:
Improved Fluid Circulation and Heat Transfer in Geothermal Reservoirs due to Superior Fracture Network in Hot Dry Rocks (HDR)
Authors: Hessami Mir Akbar , White Justine
Published: July 2013

As the reduction of carbon emissions becomes an increasingly pressing issue, a larger emphasis is being placed on the need for the development of renewable energy. One such option is geothermal energy which utilizes the heat from the earth’s crust; it presents a vast potential for the production of commercial scale base-load power generation. However, the conventional techniques used in the stimulation of hot dry rocks (HDR) geothermal wells are not very effective in producing a permeable reservoir for heat exchange between the rock mass and the working fluid. To increase the permeability of geothermal reservoirs, a new stimulation technique (developed by CSIRO - Commonwealth Scientific and Industrial Research Organisation) which involves isolating sections of the well for controlled planar fracture growth can be used. However, if these notches/fractures are placed too closely together they will interact with one another, resulting in a deviated fracture path. A two dimensional numerical model has thus been developed to study conditions under which adjacent fractures will interact with one another. This study aims to verify the numerical model through stimulating a number of granite blocks, and drawing comparisons between the observed fracture pattern and that predicted by the model. To achieve this goal, the stimulated and fractured granite blocks were sectioned and their fracture patterns were extracted using a MATLAB code, before being reconstructed in their respective positions. Stimulation was carried out firstly using conventional techniques, and then by trialling the method proposed by CSIRO. Observation of the reconstructed images showed good agreement between the model predictions and the observed fracturing patterns in two-dimensions. However, the three-dimensional pattern in the notched, perpendicular well-bore was observed as a ‘half cylinder’. This was counter intuitive as it was expected that radial symmetry of the fractures would be observed resulting in a ‘bowl’ shape. It was therefore concluded that while the model was unable to accurately predict the three-dimensional geometry of an array of fractures, stimulation through a notched perpendicular wellbore was very effective in the production of a controlled system of fractures with an improved fluid flow and heat exchanging surface area of the reservoir in comparison to the conventional techniques.
Development of an Integrated Building Load-Ground Source Heat Pump Model as a Test Bed to Assess Short- and Long-Term Heat Pump and Ground Loop Performance
Authors: Gaspredes Jonathan L. , Masada Glenn Y. , Moon Tess J.
Published: July 2013

An integrated building load-ground source heat pump model is developed in this paper to serve as a test bed for assessing the short- and long-term performance of GSHP and HGSHP systems with vertical boreholes. The model uses the Simulink/Matlab environment to take advantage of their built-in functionality, allowing for full coupling of the component building load, heat pump, ground loop, and supplemental heat rejection models at every time step The building load model uses the HAMBASE thermal program which can model residential and commercial buildings. The heat pump model uses available data provided by GSHP manufacturers and sensible heat corrections to accurately model operation across a wide range of input conditions. The vertical borehole ground loop model is based on Eskillsons g-function model, but includes a one-dimensional numerical model by Xu to calculate the short-term thermal response of the borehole and ground. The supplemental heat rejection section allows various techniques to be tested. The integrated model captures system performance over a wide range of time scales from seconds to 10–20 years. Results of a 15-year simulation are presented to illustrate the different time scale information that reveal the slow degradation in system performance due to the effects of ground heating.
Cooling Towers as Supplemental Heat Rejection Systems in Ground Source Heat Pumps for Residential Houses in Texas and Other Semi-Arid Regions
Authors: Balasubramanian Siddharth , Gaspredes Jonathan , Moon Tess J. , Masada Glenn Y.
Published: July 2013

Simulation results from a hybrid ground source heat pump model are presented for a residential home that integrates a compact cooling tower into an existing ground source heat pump model. The tower is introduced to assess its impact on the operational and economic performance over that of a GSHP alone. Metrics include initial and lifetime operational costs, ground heating effects, heat pump efficiency, and ability to control the temperature of the conditioned space. A single story, 195 m2 house located in Austin, Texas is used as a cooling-dominated test case. Simulations spanning 10-years of operation show that adding the cooling tower is cost effective, but more importantly, it extends the lifetime of the borehole system and maintains the heat pump efficiencies at high levels.
A Direct-Steam Linear Fresnel Performance Model for NREL’S System Advisor Model
Authors: Wagner Michael J. , Zhu Guangdong
Published: July 2013

This paper presents the technical formulation and demonstrated model performance results of a new direct-steam-generation (DSG) model in NREL’s System Advisor Model (SAM). The model predicts the annual electricity production of a wide range of system configurations within the DSG Linear Fresnel technology by modeling hourly performance of the plant in detail. The quasi-steady-state formulation allows users to investigate energy and mass flows, operating temperatures, and pressure drops for geometries and solar field configurations of interest.

The model includes tools for heat loss calculation using either empirical polynomial heat loss curves as a function of steam temperature, ambient temperature, and wind velocity, or a detailed evacuated tube receiver heat loss model. Thermal losses are evaluated using a computationally efficient nodal approach, where the solar field and headers are discretized into multiple nodes where heat losses, thermal inertia, steam conditions (including pressure, temperature, enthalpy, etc.) are individually evaluated during each time step of the simulation.

This paper discusses the mathematical formulation for the solar field model and describes how the solar field is integrated with the other subsystem models, including the power cycle and optional auxiliary fossil system. Model results are also presented to demonstrate plant behavior in the various operating modes.
Modeling and Analysis of a Combined Photovoltaic-Thermoelectric Power Generation System
Authors: Najafi Hamidreza , Woodbury Keith
Published: July 2013

In the present paper, the possibility of using thermoelectric power generator modules (TEGs) to convert the heat generated by the photovoltaic/thermal (PVT) collector into electricity is investigated. A comprehensive heat transfer model for the combined PVT-TEG system is developed via MATLAB and simulated under different conditions. The hot side of the TEG module is considered connected to the top of the air channel which is attached to the backside of the solar panel and the cold side is cooled down by air flow through the air channel under the PV panel. The TEG modules convert the temperature gradient to electricity and generates extra power from the excess heat which results in higher total efficiency. The effect of using combined PVT-TEG on total generated power under different levels of irradiation is presented and discussed.
Characterization of Pyromark 2500 for High-Temperature Solar Receivers
Authors: Ho Clifford K. , Mahoney A. Roderick , Ambrosini Andrea , Bencomo Marlene , Hall Aaron , Lambert Timothy N.
Published: July 2013

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The cost, application, curing methods, radiative properties, and absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The directional solar absorptance was calculated from directional spectral absorptance data, and values for pristine samples of Pyromark 2500 were as high as 0.96–0.97 at near normal incidence angles. At higher irradiance angles (>40°–60°), the solar absorptance decreased. The total hemispherical emittance of Pyromark 2500 was calculated from spectral directional emittance data measured at room temperature and 600°C. The total hemispherical emittance values ranged from ∼0.80–0.89 at surface temperatures ranging from 100°C – 1,000°C. The aging and degradation of Pyromark 2500 with exposure at elevated temperatures were also examined. Previous tests showed that solar receiver panels had to be repainted after three years due to a decrease in solar absorptance to 0.88 at the Solar One central receiver pilot plant. Laboratory studies also showed that exposure of Pyromark 2500 at high temperatures (750°C and higher) resulted in significant decreases in solar absorptance within a few days. However, at 650°C and below, the solar absorptance did not decrease appreciably after several thousand hours of testing. Finally, the absorber efficiency of Pyromark 2500 was determined as a function of temperature and irradiance using the calculated solar absorptance and emittance values presented in this paper.
Distributed Generation for Village of Hope
Authors: Du Mez Kate , Krarti Moncef
Published: July 2013

The Village of Hope is an orphan community located in rural Zambia. The community is made up of several buildings of a variety of uses and schedules. They are currently tied to the grid, which is unreliable due to rolling blackouts for 2 to 4 hours per day. The community is looking for a financially beneficial solution to their electrical needs. A system optimization and sensitivity analysis was performed to determine system recommendations for the community. It was found that wind turbine systems supplementing a grid connection is the most realistic solution for the Village of Hope. However, there were many other factors identified that require further analysis to be able to truly optimize the system.
A Model of Transient Heat and Mass Transfer in a Heterogeneous Medium of Cerium Dioxide Undergoing Nonstoichiometric Reduction
Authors: Keene Daniel J. , Davidson Jane H. , Lipiński Wojciech
Published: July 2013

The redox chemistry of nonstoichiometric metal oxides can be used to produce chemical fuels by harnessing concentrated solar energy to split water and/or carbon dioxide. In such a process, it is desirable to use a porous reactive substrate for increased surface area and improved gas transport. The present study develops a macroscopic-scale model of porous ceria undergoing thermal reduction. The model captures the coupled interactions between the heat and mass transfer and the heterogeneous chemistry using a local thermal non-equilibrium (LTNE) formulation of the volume averaged conservation of mass and energy equations in an axisymmetric cylindrical domain. The results of a representative test case simulation demonstrate strong coupling between gas phase mass transfer and the chemical kinetics as well as the pronounced impact of optical thickness on the temperature distribution and thus global solar-to-chemical energy conversion.
Development and Validation of an Operational, Cloud-Assimilating Numerical Weather Prediction Model for Solar Irradiance Forecasting
Authors: Mathiesen Patrick J. , Collier Craig , Kleissl Jan P.
Published: July 2013

For solar irradiance forecasting, the operational numerical weather prediction (NWP) models (e.g. the North American Model (NAM)) have excellent coverage and are easily accessible. However, their accuracy in predicting cloud cover and irradiance is largely limited by coarse resolutions (> 10 km) and generalized cloud-physics parameterizations. Furthermore, with hourly or longer temporal output, the operational NWP models are incapable of forecasting intra-hour irradiance variability. As irradiance ramp rates often exceed 80% of clear sky irradiance in just a few minutes, this deficiency greatly limits the applicability of the operational NWP models for solar forecasting.

To address these shortcomings, a high-resolution, cloud-assimilating model was developed at the University of California, San Diego (UCSD) and Garrad-Hassan, America, Inc (GLGH). Based off of the Weather and Research Forecasting (WRF) model, an operational 1.3 km-gridded solar forecast is implemented for San Diego, CA that is optimized to simulate local meteorology (specifically, summertime marine layer fog and stratus conditions) and sufficiently resolved to predict intra-hour variability. To produce accurate cloud-field initializations, a direct cloud assimilation system (WRF-CLDDA) was also developed. Using satellite imagery and ground weather station reports, WRF-CLDDA statistically populates the initial conditions by directly modifying cloud hydrometeors (cloud water and water vapor content). When validated against the dense UCSD pyranometer network, WRF-CLDDA produced more accurate irradiance forecasts than the NAM and more frequently predicted marine layer fog and stratus cloud conditions.
Modeling Chemical and Thermal States of Reactive Metal Oxides in a CR5 Solar Thermochemical Heat Engine
Authors: Hogan R. E. , Miller J. E. , James D. L. , Chen K. S. , Diver R. B.
Published: July 2013

“Sunshine to Petrol” is a grand-challenge research project at Sandia National Laboratories with the objective of creating a technology for producing feedstocks for making liquid fuels by splitting carbon dioxide (and water) using concentrated solar energy [1]. A reactor-level performance model is described for computing the solar-driven thermochemical splitting of carbon dioxide via a two-step metal-oxide cycle. The model simulates the thermochemical performance of the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5). The numerical model for computing the reactor thermochemical performance is formulated as a system of coupled first-order ordinary differential equations describing the energy and mass transfer within each reactive ring and radiative energy transfer between adjacent rings. In this formulation, each of the counter-rotating rings is treated in a one-dimensional sense in the circumferential direction; supporting circumferential temperature and species gradients with assumed negligible gradients in both the radial and axial directions. The model includes radiative heat transfer between adjacent counter-rotating rings, variations in the incident solar flux distribution, heat losses to the reactor housing, and energy of reaction associated with the reduction and oxidation reactions. An overview of the physics included in this first-generation numerical model will be presented. Preliminary results include the circumferential distributions of temperature and species within each of the reactive rings. The computed overall chemical conversion efficiency will be presented for a range of design and operating parameters; including ring speed, carrier ring mass, reactive material loading, radiative emissivity, and differing incident flux distributions.
A Cost Effective Solar Tracker and Performance Monitoring System for Utility Scale Photovoltaic Installations
Authors: Sadineni Suresh B. , Madala Srikanth , Boehm Robert F.
Published: July 2013

A prototype of an integrated tracking and monitoring system is developed and experimentally tested in the environmental conditions of the desert south-west. This system hardware design incorporates both the tracker control and performance monitoring mechanism. A wireless communication protocol coupled with internet transmits the PV panel performance information collected by the electronic sensors to a remote user who can monitor the system performance. The initial experimental results of this newly developed system performance are presented. Finally, an economic analysis is conducted to evaluate the applicability of the current system to a utility scale installation.

The preliminary results suggest that there would be a 25% average increase in solar insolation received by a single-axis tracking surface over a fixed (tilt equal to local latitude) surface during the winter months. Although, limited amount of data is available for typical summer days, results from September days suggest an increase in solar insolation as high as 38%. Also, such an integrated tracking and monitoring system will yield in better monitoring resolution due to monitoring at the string level as opposed to central inverter level in the central inverter type PV power plant topology.

The monitoring sub-system design incorporates cutting edge electronic sensors. The measurements using these sensors were compared against the measurement through a programmable D.C load. The difference between both the power measurements is well within a median of 5–12% suggesting a good agreement between both the measurements.
Aerodynamic Forces and Vortex Shedding for Surface-Mounted Finite Square Prisms and the Effects of Aspect Ratio and Incidence Angle
Authors: McClean John F. , Sumner David
Published: July 2013

The flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of interest were the effects of aspect ratio and incidence angle on the mean aerodynamic forces and vortex shedding. Compared to the case of the “infinite” (or two-dimensional) square prism, the flow around the finite square prism has not been extensively studied. The experiments were conducted at a Reynolds number of Re = 7.2 × 104 for aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles of α = 0°, 15°, 30° and 45°. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe in the wake, and measurements of the mean drag and lift forces were obtained with a force balance.

For all aspect ratios and incidence angles, the Strouhal number and the mean drag coefficient were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The behaviours of the mean drag coefficient and Strouhal number with incidence angle were less sensitive compared to the case of the infinite square prism, although a minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number were found at α = 15°. The reduced sensitivity to incidence angle is attributed to the complex three-dimensional flow over the free end of the prism and the downwash flow that enters the near wake. The behaviour of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of drag coefficient and lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0° and 15°. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30° and 45°, vortex shedding peaks were absent in the power spectra in the upper part of the wake.
An Experimental Study of the Flow Above the Free Ends of Surface-Mounted Bluff Bodies
Authors: Rostamy Noorallah , Sumner David , Bergstrom Donald J. , Bugg James D.
Published: July 2013

The flow around surface-mounted finite-height bluff bodies is more complex than the flow around a two-dimensional or “infinite” cylinder. The flow over the free end and the boundary layer flow around the body-wall junction strongly influence the near-wake flow pattern. Streamwise tip vortex structures interact in a complex manner with Kármán vortex shedding from the sides of the body, and are responsible for a downward-directed local velocity field in the upper part of the wake known as “downwash.” A second pair of streamwise vortex structures, known as the base vortices, is found close to the ground plane. Upstream of the body the familiar horseshoe vortex is found. The interactions between the tip vortices, base vortices, and Kármán vortex shedding are strongly influenced by the aspect ratio, AR = H/D (for height, H, and width, D), the Reynolds number, Re, and the relative thickness of the boundary layer, δ/D.

The flow above the free ends of surface-mounted finite-height circular cylinders and square prisms was studied in a low-speed wind tunnel using particle image velocimetry (PIV). Cylinders and prisms of AR = 9, 7, 5, and 3 were tested at Re = 4.2 × 104. The bodies were mounted normal to a ground plane and were partially immersed in a turbulent flat-plate boundary layer with δ/D = 1.7. PIV measurements were made above the free ends in three vertical planes at different cross-stream locations (y/D = 0, 0.25, and 0.375). The ensemble-averaged streamlines, turbulence intensity and Reynolds shear stress fields were obtained in these planes.

The PIV results provide insight into the separated flow above the free ends, including the effects of AR and body shape. For the finite square prism, the large, separated, recirculating flow region extends into the near-wake. For the finite circular cylinder, this region is smaller and the separated flow reattaches onto the free-end surface. For the square prism of AR = 3, considerable difference is seen in the free-end flow pattern compared to the more slender prisms of AR = 9, 7 and 5. In particular, a cross-stream vortex is formed due to interaction between the separated flow from the leading edge of the prism and the reverse flow over the free end. This vortex is seen in all three planes for AR = 3 but only in the symmetry plane for AR = 9, while for the finite circular cylinder the flow pattern above the free end seems to be the same in all three planes for all aspect ratios, consisting of a cross-stream vortex at approximately x/D = 0.
Assessment of Mixing by Chemical Probe in Swirl Flow HEX Reactors
Authors: Ghanem Akram , Lemenand Thierry , Della Valle Dominique , Peerhossaini Hassan
Published: July 2013

Mixing is a fundamental issue in process engineering and many industrial fields. It is closely related to a large number of different applications, such as chemical reactions, thermal transfer, liquid-liquid extraction, crystallization, and the like. In fact, mixing whether at the reactor scale, sustained by the flow structures, or at molecular scales, influences the selectivity and hence the productivity of reactions. Understanding and quantification of the micromixing mechanism is critical in industrial chemical processes, especially for fast exothermal reactions. Micromixing can be characterized by chemical probe methods based on observation of a local chemical reaction that results from a competition between turbulent mixing at microscales and the reaction kinetics. A system of parallel competing reactions producing iodine was developed by Fournier et al. [1] to study partial segregation in stirred tanks. The coupling of the borate neutralization and the Dushman reaction in this system allows the measurement of micromixing efficiency in reactors by monitoring the amount of iodine produced. Called the iodide-iodate method, this technique has been extensively used in different types of reactors. A novel adaptive procedure recently developed by the authors to improve the reliability of the iodide-iodate method is used here. The heat exchanger-reactor presented here is an innovative geometry based on the addition in parallel of tubes equipped with helical inserts. It is expected to qualify as a low-cost compact heat-exchanger reactor and static mixer of high performance.
Calculation of Axial Hydraulic Thrust of Francis Turbine
Authors: Ji XingYing , Xu Lai , Liu Xiao
Published: July 2013

Put forward a method of calculating the axial thrust of Francis turbine. To use numerical simulation computing the axial thrust on hub, shroud and blade of inner runner, combine theoretical methods calculating the pressure on hub and shroud of outer runner, finally the axial thrust of Francis turbine is obtained. The results of calculation agree with the results of model test. It is an effective way of gathering the theoretical calculation and numerical simulation to calculate the axial thrust of Francis turbine. In addition the static suction of turbine has great effect on calculation results of the axial thrust. The static suction of turbine plays a significant role on the lifting of turbine.
Energy Production Characteristics of a Spring-Mounted Cantilevered-Free Flexible Plate in a Uniform Flow
Authors: Howell Richard M. , Lucey Anthony D.
Published: July 2013

We study a new fundamental system that comprises a cantilevered thin flexible plate exactly aligned with the direction of a uniform flow in which the upstream end of the flexible plate is not fixed. Instead, it is attached to a spring-damper system that allows the entire system to oscillate perpendicularly to the flow direction as a result of the mounting’s dynamic interaction with the flow-induced oscillations of the flexible plate. This models an energy-harvesting system whereby the rate of energy extraction by the damper represents power generation from the kinetic-energy flux of the mean flow transferred via fluttering motions of the flexible plate to the motion of the mounting system. The two-dimensional modelling presented is an extension of the methods in [1,2] that mixed numerical simulation with eigenvalue analysis to study a fixed cantilevered flexible plate. The present system also includes a rigid inlet surface upstream of and fixed to the spring-mounted cantilever. Ideal flow is assumed wherein the rotationality of the boundary-layers is modelled by vortex elements on the solid-fluid interface and the imposition of the Kutta condition at the plate’s trailing edge. The Euler-Bernoulli beam model is used for the structural dynamics. Results presented first show how the replacement of the fixed leading edge with an interactively oscillating mounting modify the well-known linear-stability characteristics of a fluttering plate. The overall effect is that the critical flow speed for flutter onset is reduced and this is desirable for the present energy-harvesting application. This entails some subtle but important changes to the destabilisation mechanisms. The power generating potential of the fluid-structure interaction system is then illustrated. The present model of the dynamics of the plate-support interaction has been simplified so as to demonstrate proof-of-concept; thus, a discussion of the way forward to a more complete model is presented to close the paper.
Perspective on the Tidal Bore: Background and Initiation
Authors: Graber S. David
Published: July 2013

Tidal bores have been the subject or cause of legend, poetry, recreation, tourism, shipping disasters, and scientific interest for centuries. They pose interesting theoretical questions for the engineer. Introductory details are presented regarding the remarkable tidal bore phenomenon, including discussion of the tidal bore in various locations and a recent photograph taken by the writer of the bore on China’s Qiantang River. A supplemental tabular summary is presented based on an extensive literature review. A simplified theory based on monoclinal rising waves is developed for the initiation of the tidal bore. The theory elucidates the mechanisms of the tidal bore more clearly and completely than more complex theories. Early studies of the tidal bore are cited and discussed in historical context. The intention is to provide greater understanding of the tidal bore for a broad set of readers. The theory is favorably compared to rectangular flume data and to all field data available to the writer. A companion paper builds on the present one by addressing additional features of the tidal bore.
Investigation of Turbulent Tip Leakage Vortex in an Axial Water Jet Pump With Large Eddy Simulation
Authors: Hah Chunill
Published: July 2013

Detailed steady and unsteady numerical studies were performed to investigate tip clearance flow in an axial water jet pump. The primary objective is to understand physics of unsteady tip clearance flow, unsteady tip leakage vortex, and cavitation inception in an axial water jet pump. Steady pressure field and resulting steady tip leakage vortex from a steady flow analysis do not seem to explain measured cavitation inception correctly. The measured flow field near the tip is unsteady and measured cavitation inception is highly transient. Flow visualization with cavitation bubbles shows that the leakage vortex is oscillating significantly and many intermittent vortex ropes are present between the suction side of the blade and the tip leakage core vortex. Although the flow field is highly transient, the overall flow structure is stable and a characteristic frequency seems to exist. To capture relevant flow physics as much as possible, a Reynolds-averaged Navier-Stokes (RANS) calculation and a Large Eddy Simulation (LES) were applied for the current investigation. The present study reveals that several vortices from the tip leakage vortex system cross the tip gap of the adjacent blade periodically. Sudden changes in local pressure field inside tip gap due to these vortices create vortex ropes. The instantaneous pressure filed inside the tip gap is drastically different from that of the steady flow simulation. Unsteady flow simulation which can calculate unsteady vortex motion is necessary to calculate cavitation inception accurately even at design flow condition in such a water jet pump.
Toward a GPU-Accelerated Immersed Boundary Method for Wind Forecasting Over Complex Terrain
Authors: DeLeon Rey , Felzien Kyle , Senocak Inanc
Published: July 2013

A short-term wind power forecasting capability can be a valuable tool in the renewable energy industry to address load-balancing issues that arise from intermittent wind fields. Although numerical weather prediction models have been used to forecast winds, their applicability to micro-scale atmospheric boundary layer flows and ability to predict wind speeds at turbine hub height with a desired accuracy is not clear. To address this issue, we develop a multi-GPU parallel flow solver to forecast winds over complex terrain at the micro-scale, where computational domain size can range from meters to several kilometers. In the solver, we adopt the immersed boundary method and the Lagrangian dynamic large-eddy simulation model and extend them to atmospheric flows. The computations are accelerated on GPU clusters with a dual-level parallel implementation that interleaves MPI with CUDA. We evaluate the flow solver components against test problems and obtain preliminary results of flow over Bolund Hill, a coastal hill in Denmark.
Numerical Simulation of Spacing Effects on the Flow Past Two 2:1 Rectangular Cylinders in Tandem at Re = 200
Authors: Huang Zhu , Xi Guang , Zhang Wei
Published: July 2013

The time-periodic flow over two rectangular cylinders in infinite medium is numerically investigated in this paper using the newly developed multidomain pseudospectral method. The two cylinders are arranged in tandem and both with length-to-width aspect ratio of 2; the incoming flow is assumed to be laminar and uniform, and the Reynolds number based on the cylinder width and the freestream velocity is fixed at 200.Computations are focusing on the effects of S, the ratio between the streamwise spacing of two cylinders (gap length) and the cylinder width, on the vortex shedding formations, evolving patterns and related aerodynamic performances of cylinders within the range 1 ≤ S ≤ 10.

Numerical results reveal that as S increases, there exists a critical value, Scr = 5, at which the mean drag coefficients of both cylinders behave notable jumps, and the root-mean-square values of lift and drag coefficients reach maxima. Meanwhile, the vortices behind the two cylinders are shedding in anti-phase mode at Scr, and their evolving patterns are further analyzed and interpreted.
The Development and Application of a Time-Domain Harmonic Balance Flow Solver
Authors: Du Pengcheng , Ning Fangfei
Published: July 2013

Time periodic unsteady flows are often encountered in turbomachinery. Simulating such flows using conventional time marching approach is very time-consuming and hence expensive. To handle this problem, several Fourier-based reduced order models have been developed recently. Among these, the time-domain harmonic balance method solves the governing equations purely in the time domain and there is also no need for the turbulence model to be linearized, making it easy to be implemented in an existing RANS code. Thus, the time-domain harmonic balance method was chosen and incorporated into an in-house Navier-Stokes flow solver. Several test cases were performed for the validations of the developed code. They cover standard unsteady test cases such as the low speed vortex shedding cylinder flow and the Sajben transonic diffuser under periodically oscillating back pressure. Further, two different practical turbomachinery unsteady flows were considered. One is a transonic fan under circumferential inlet distortion and the other is the rotor-stator interactions in a single stage compressor. The results illustrate the capability of the harmonic balance method in capturing the dominant nonlinear effects. The number of harmonics should be retained in the harmonic balance method is depend on the strength of the nonlinear unsteady effects and differs from case to case. With appropriate number of harmonics retained, it can resolve the unsteady flow field satisfactory, meanwhile, reducing the computational time significantly. In a word, the harmonic balance method promise to be an effective way to simulate time periodic unsteady flows.
3-D Euler-Lagrange CFD Simulation of Particle Impact on Carrier Fluid Through Gate Valve
Authors: Lin Zhe , Ruan Xiaodong , Cui Baoling , Zhu Zuchao
Published: July 2013

Gate valves are widely used in dilute Pneumatic conveying systems. The flow characteristic of carrier fluid through the valve changes under the effect of particles. In this study, in order to obtain the influence of particle parameters on carrier fluid while flowing through a gate valve, a three dimensional Euler-Lagrange model is used to simulate gas-solid flow at three opening degrees of valve. Since inlet velocity of air is very small and the Mach number is less than 10%, the carrier fluid is set as incompressible Newtonian fluid. The investigated particle parameters include mass flux ratio (κ) and diameter of particles (d). An important coefficient namely flow coefficient (Cv) is calculated to express the flow properties. Our results demonstrate that the particles do little, if any, effect on the flow properties when the valve is in full open position. However, with the closure of valve, the influence of particles on carrier fluid becomes more significant. Besides, the influence extent of particles on carrier fluid increases with mass flux ratio while decreases with the increasing of particle diameter. This study gives a suggestion that for dilute phase flow of Pneumatic conveying, the influence of particles on carrier fluid can be neglected if valve is of full open condition, otherwise the effect should not be neglected. Further study will focus on two phase flow field in valves under transient conditions.
Cross Jet Control for Manipulating the Separated Shear Flows Behind the Fence
Authors: Lee Seung Hwan , Jeong Tae-Sik , Tu Xin-Cheng , Kim Hyoung-Bum
Published: July 2013

The separated shear flows developed from the vertical fence was manipulated by using the vortex generator jets located in front of the fence. The experiments were carried out in a circulating water channel and the vertical fence was submerged in the turbulent boundary layer. The Reynolds number based on the height of fence and free stream velocity was 2000. DPIV method was used to measure the instantaneous velocity fields around a vertical fence. The mean properties were calculated using the ensemble average method. The obtained results were compared with uncontrolled fence flow. From the results, we found the vortex jets can alter the upstream flow and change the separation bubble behind the fence. As the jet speed increases, the suppression of separation bubble become stronger and the spanwise variation of separation bubble exists.
Eulerian-Eulerian Modeling of Multiple Jet Interactions for Different Distributor Plates
Authors: Kanholy Santhip Krishnan , Battaglia Francine
Published: July 2013

The hydrodynamics of fluidized beds involving gas and particle interactions are very complex, and must be carefully considered when modeling such a system using computational fluid dynamics (CFD). One of the issues involved is the interaction of multiple jets that develop above the distributor plate, which impacts the uniformity of fluidization. Using the common approach of a uniform gas velocity inlet boundary condition may not accurately represent distributor plates with nonuniform holes. The numerical approach will use a multi-fluid Eulerian-Eulerian CFD modeling to predict and examine the hydrodynamics of interacting jets. The present work will model a quasi-two-dimensional (2D) fluidized bed to compare with a corresponding experimental setup designed to examine multiple jet interactions for a distributor plate with 9 holes. Two-dimensional and three-dimensional simulations of the quasi-2D bed will be compared with experiments by investigating solid volume fraction distributions and solid flux distributions with agreeable results qualitatively. Use of experimental data in determining the amount of mass fluidizing will also be assessed using CFD. The efficacy of the new approach in capturing the hydrodynamics is demonstrated.
Experimental and Numerical Investigation of the Flow in a Micropump Model
Authors: Fernández Joaquín , Marcos Alberto , Montanero José María , Barrio Raúl , Parrondo Jorge L.
Published: July 2013

This work presents an experimental and numerical study of a nozzle/diffuser micropump working in the steady regime. Experiments were conducted with a model ten times bigger than the prototype. Preliminary results on the velocity distribution in the chamber were obtained with the PIV technique. The velocity distribution inside the chamber was calculated numerically and the predictions were compared with the experimental measurements showing a satisfactory agreement in the center and extremes, but not in the middle of them.
Error Estimation Using Hybrid Methods
Authors: Celik Ismail , Karaismail Ertan , Elizalde Blancas Francisco , Parsons Don , Sezer Hayri
Published: July 2013

New formalisms are developed for robust calculation of the discretization errors in CFD applications. The new methods are based on the premise that the true error (i.e. the difference between the exact solution and the numerical solution) on a given mesh is similar to the approximate error (i.e. the difference between the fine grid solution and the coarse grid solution). The proportionality constant can be calculated theoretically for a given scheme and it is, to a first order approximation, only a function of the grid refinement ratio. This method is called Approximate Error Scaling (AES) method.

Although AES is a viable method for error estimation it does not directly take into account the transport of error nor the error sources at the boundaries. To remedy this, a new method which is referred to, here and after, as the residual source in transport (REST-IC) method is formulated. This paper presents the theoretical details and the results from several test cases that are used for validation of the newly proposed methods.

Application to the 1D and 2D steady scalar transport equation and 2D Navier-Stokes (N-S) equations has revealed excellent performance with a pressure based segregated N-S finite volume solver. Implementation of these methods into CFD commercial codes such as ANSYS-FLUENT can be done with the help of expert developers of the code who can handle residuals for the selected equation and the variable through user defined subroutines.
Effect of Cell Size on Particle-Eddy Interaction and Erosion Predictions Using Commercial CFD Software (FLUENT)
Authors: Fard F. N. , McLaury B. , Shirazi S.
Published: July 2013

Some commercial CFD codes have the ability to predict erosion. For low concentration flows, a one-way coupling approach is used. The flow solution is determined then thousands of particle trajectories are determined using a Lagrangian approach, and the particle impact information is used in an erosion equation to predict the erosion. It is necessary practice to insure grid independence for the flow solution; however, discussion of the effect of grid size on particle tracking is not common. In this paper, the effect of cell size on particle tracking and erosion modeling is studied using a commercial computational fluid dynamics (CFD) code, FLUENT. Two surprising issues were found during detailed investigation of CFD results. First, eddy size is limited by cell size in FLUENT, which affects particle-eddy interaction in the discrete phase model and consequently erosion modeling. Limiting the eddy size based on the cell size can have a huge effect on the particle trajectories in geometries like sudden contraction/expansions since eddies play an important role in particle behavior in these geometries. This is particularly true for very small particles and when liquid is the carrier fluid. Second, the particle impact behavior seems unrealistic. Particles tend to stay near the wall and impact the wall over and over in a small area. During each impact series, the particle’s impact velocities do not reduce with successive impacts as expected. This promotes simulated erosion rates that are high. In addition to the CFD investigation, an experimental facility was designed and built to develop erosion equations for small particles (average size of 25 μm) to enable more appropriate comparisons of erosion data with predicted erosion.
An Efficient Immersed Boundary Method Based on Penalized Direct Forcing for Simulating Flows Through Real Porous Media
Authors: Breugem Wim-Paul , van Dijk Vincent , Delfos René
Published: July 2013

A computationally efficient Immersed Boundary Method (IBM) based on penalized direct forcing was employed to determine the permeability of a real porous medium. The porous medium was composed of about 9000 glass beads with an average particle diameter of 1.93 mm and a porosity of 0.367. The forcing of the IBM depends on the local solid volume fraction within a computational grid cell. The latter could be obtained from a high-resolution X-ray Computed Tomography (CT) scan of the packing. An experimental facility was built to determine the permeability of the packing experimentally. Numerical simulations were performed for the same packing based on the data from the CT scan. For a scan resolution of 0.1 mm the numerical value for the permeability was nearly 70% larger than the experimental value. An error analysis indicated that the scan resolution of 0.1 mm was too coarse for this packing.
Study of Vortex-Shedding-Induced Vibration of a Flexible Splitter Plate Behind a Cylinder
Authors: Lee Jinmo , You Donghyun
Published: July 2013

Integrated computational fluid dynamics (CFD) and computational structural dynamics (CSD) simulations of flow over a cylinder with a flexible splitter plate attached to the rear stagnation point, are performed. Flow over a cylinder produces vortex shedding, which causes unsteady pressure and shear stress distributions over a flexible splitter plate. As a result, the flexible splitter plate vibrates with distinct frequencies, which are different from the vortex-shedding frequency and natural frequencies of the plate. A systematic and detailed analysis of the effects of the flexible plate on fluid-structure dynamics and on the drag and lift of the cylinder, is presented.
Robust and Efficient Setup Procedure for Complex Triangulations in Immersed Boundary Simulations
Authors: Yang Jianming , Stern Frederick
Published: July 2013

Immersed boundary methods have been widely used for simulating flows with complex geometries, as quality boundary-conforming grids are usually difficult to generate for complex geometries, especially, when motion and/or deformation is involved. Complex geometries can be conveniently represented using triangulated surfaces in a Lagrangian manner. A major task in immersed boundary simulations is to inject the immersed boundary information into the background Cartesian grid, such as the inside/outside status of a grid point with regard to the immersed boundary and the accurate sub-cell position of the immersed boundary for a grid point next to it. For high resolution simulations with moving/deforming boundaries, this step can be very expensive. In this paper, a simple, fast, and robust procedure is developed for setting up complex triangulations. Several cases with complex geometries are performed to demonstrate the efficiency and robustness.
Comparison of Rotordynamic Fluid Forces Between Closed Impeller and Open Impeller
Authors: Uchiumi Masaharu , Nagao Naoki , Yoshida Yoshiki , Eguchi Masato
Published: July 2013

In the design of rocket turbopump with high energy density, rotordynamics considerations are very important to stabilize the motion of the shaft. Rotordynamic instabilities generated by rotordynamic fluid forces are often reported. It is recognized that the forces affect the whirling motion of the rotor, and induce the self-excited vibration. To understand the effect of the rotordynamic fluid forces of the impellers on rotor vibration, a tangential fluid force in a forward whirling direction and a normal fluid force in a radial direction were measured by using the experimental apparatus with an active magnetic bearing. As the experimental result for a closed impeller and an open impeller, it was clarified their fluid forces are quite different in the characteristics of destabilizing/damping effects and restoring/inertia effects.
Closed-Loop Feedback Control of Flow Over a Flapped Airfoil at High Angles of Attack Using Identified NARMAX Model
Authors: Obeid Sohaib , Jha Ratneshwar , Ahmadi Goodarz
Published: July 2013

This study investigates closed-loop feedback control system design aimed at reduction of turbulent flow separation over a NACA 0015 airfoil having 30% integral type trailing edge flap and equipped with leading-edge and trailing edge synthetic jet actuators (SJAs). The multiple-input single-output controller employs system identification techniques based on Nonlinear Auto Regressive Moving Average with eXogenous inputs (NARMAX) method to model nonlinear dynamics of the flow. RANS FLUENT simulations for 2-D airfoil are used besides an analytical modeling for the set of synthetic actuators. The resulting closed loop response using NARMAX tracks the desired pressure value and significant improvement in the transient response over the open-loop system at high angles of attack is realized. Improvements in aerodynamic efficiency and maximum lift values through active flow control would lead to better performance characteristics of airplanes.