Numerical Simulation Study on the Asymmetric Structures of Oscillatory Flow Field in a Baffled Tube Reactor

The oscillatory flow in a baffled tube reactor provides a significant enhancement of radial transfer of momentum, heat and mass and a good control of axial back mixing at a wide range of net flow rate. But little has been known about reliable details of the three-dimensional structure of flow field in this kind of flow because most published studies in the area were based on the two-dimensional simulation techniques. This paper implemented a three-dimensional numerical simulation study on the asymmetry of flow pattern in the baffled tube reactor which was observed experimentally. A systematic study by numerical simulation was carried out which covered a range of oscillatory Reynolds number (Reo) from 100 to 5,000 and employed models respectively for laminar and turbulent flows. It was found in the simulation that under symmetric boundary conditions the transition from axially symmetric flow to asymmetric one depended on the numerical technique employed in simulation. With a structured grid frame the transition occurred at Reo much greater than that with an unstructured grid frame, for both laminar and turbulent flows. It is not rational that the onset of the transition changes with the accuracy of numerical technique. Based on the simulation results, it was postulated that the asymmetry appeared in simulations with symmetric boundary conditions might result from the accumulation of calculation errors but the asymmetry observed in experiments might result from the slight asymmetry of geometry which exists inevitably in any experiment apparatus. To explore the influence of the slight asymmetry of geometry, the effect of the eccentricity of baffles and the declination of oscillating boundary were studied by use of the finite volume method with a structured grid and adaptive time steps. The simulation result showed that both the eccentricity of baffles and the declination of oscillating boundary have obvious influence on the asymmetry of flow patterns for laminar and turbulent flow. More details were discussed in the paper.

Direct Numerical Simulation and RANS Modeling of Turbulent Natural Convection for Low Prandtl Number Fluids

Results of direct numerical simulation (DNS) of turbulent Rayleigh-Be´nard convection for a Prandtl number Pr = 0.025 and a Rayleigh number Ra = 105 are used to evaluate the turbulent heat flux and the temperature variance. The DNS evaluated turbulent heat flux is compared with the DNS based results of a standard gradient diffusion turbulent heat flux model and with the DNS based results of a standard algebraic turbulent heat flux model. The influence of the turbulence time scales on the predictions by the standard algebraic heat flux model at these Rayleigh- and Prandtl numbers is investigated. A four equation algebraic turbulent heat flux model based on the transport equations for the turbulent kinetic energy k, for the dissipation of the turbulent kinetic energy ε, for the temperature variance θ2, and for the temperature variance dissipation rate εθ is proposed. This model should be applicable to a wide range of low Prandtl number flows.

Study of Energy Spectra of Turbulence by the Vortex Method

The energy spectra produced by the vortex method are studied. The strengths of the vortices are determined so that the energy spectra correspond to the given target spectra for two different integral scales. Velocity fluctuations produced by the simulation of vortex shear layer are obtained and energy spectra of these fluctuations are examined. It is found that in the case of the larger integral scale, the spectra almost agree with the target in the range up to the cut-off wave length, and that in the case of the smaller scale, the deviation of the spectra from the target is quite large.

Improvements on a Newton-Krylov Based Solver for CFD Models Using Finite Rate NOx Chemistry

In this paper, we describe our progress on improving the performance of a newly developed Computational Fluid Dynamics (CFD) modeling tool, which uses reduced chemical kinetics mechanisms to model the finite rate chemistry effects and solves the resulting system of stiff partial differential equations with a matrix-free Newton-Krylov method. A multi-grid based preconditioner and a Newton iteration scheme have been implemented in the Newton-Krylov solver and the reduced mechanism module, respectively, to replace the original Picard based preconditioner and the point iteration scheme for steady state species evaluation. Preliminary tests of the improved modeling tool have been conducted using simple hotbox and a full-scale, coal fired electric utility boiler, and shown very promising results in terms of the accuracy, robustness, and efficiency of the new tool.

Immersive Visualization of Dynamic CFD Model Results

With immersive visualization the engineer has the means for vividly understanding problem causes and discovering opportunities to improve design. Software can generate an interactive world in which collaborators experience the results of complex mathematical simulations such as computational fluid dynamic (CFD) modeling. Such software, while providing unique benefits over traditional visualization techniques, presents special development challenges. The visualization of large quantities of data interactively requires both significant computational power and shrewd data management. On the computational front, commodity hardware is outperforming large workstations in graphical quality and frame rates. Also, 64-bit commodity computing shows promise in enabling interactive visualization of large datasets. Initial interactive transient visualization methods and examples are presented, as well as development trends in commodity hardware and clustering. Interactive, immersive visualization relies on relevant data being stored in active memory for fast response to user requests. For large or transient datasets, data management becomes a key issue. Techniques for dynamic data loading and data reduction are presented as means to increase visualization performance.

Research of Revised Thermal Design Parameter Optimization With Response Surface Method Application for Fitness Function

For the thermal management of the electric products, the compact modeling method is commonly applied to the numerical analysis with the simplification of the each component used in the products, on the view point of the best thermal design. In the generation of the compact modeling, the method that the model parameters are optimized toward to the fitness between the temperature value of numerical analysis result and the actual hardware testing data, is conventionally used, with some monitored points which are given in advance. In this parameter optimization, the fitness function is to be 1 on the no (0) temperature difference between them at all monitored locations, and on the other hand, that is to be 0 if the temperature difference between them is infinite. However, it was found that this conventional method brought the fitness function shape of one sharp mountain, as the analysis result, and the method, that represented the fitness function with the quadratic polynomial, caused the important problem on the view point of the analysis quality if the Response Surface Method is used of the parameter optimization. Therefore in this research to resolve this problem, we suggest the method that the response surfaces, which are based on the heat conduction law, are composed for each monitored location, and the fitness function is given with these surfaces. This method is applied into some compact modeling and the benefit is verified. The meanwhile of the temperature difference between the numerical analysis result and the solution field is decreased half, and the divergence makes one-tenth decrease.

Numerical Investigations of Arched Vortex Characteristics Generated at T-Junction Piping Systems

Thermohydraulic analyses for a fundamental water experiment simulating thermal striping phenomena at T-junction piping systems were carried out using a quasi-direct numerical simulation code DINUS-3, which is represented by instantaneous Navier-Stokes equations and deals with a modified third-order upwind scheme for convection terms. Calculated results were compared with experimental results on the flow patterns in the downstream region of the systems, the arched vortex structures under a deflecting jet condition, the generation frequency of the arched vortex, etc. in the various conditions; i.e., diameter ratio α, flow velocity ratio β and Reynolds number Re. From the comparisons, it was confirmed that (1) the DINUS-3 code is applicable to the flow pattern classifications in the downstream region of the T-junction piping systems, (2) the arched vortex characteristics with lower frequency components and their generation possibilities can be estimated numerically by the DINUS-3 code, and (3) special attentions should be paid to the arched vortex generations with lower frequency components of fluid temperature fluctuations in the design of T-junction systems from the viewpoints of the avoidances for the thermal striping.

Inviscid Store Separation During Transonic Flight With Deployable Fins Using (6+) DOF

The classical rigid body 6 DOF capability at Eglin AFB has been completely rewritten. The new (6+) DOF allows moving components relative to non-inertial reference frames during time accurate CFD simulations. An inviscid computational fluid dynamics model was prepared to investigate the separation of a 2,000 lb cluster munition (CBU104) with deployable fins from an F15-E at transonic speed using (6+) DOF. The Mach effects of the store with deployable fins were studied for a given fighter aircraft configuration prior to USAF flight clearance.

Computational Simulation of Flow to Improve Fluidic Stability in Cataract Surgery System

In this work, a CFD based design approach to improve fluidic stability of a cataract surgery system is presented. Cataract surgery is a procedure to remove hardened human lens (cataract) from the eye. Approximately two million cases of cataract surgery are done every year in the United States. The procedure starts with the incision of an aspiration port into the anterior chamber of the eye. The OD of the aspiration port is 0.9mm and is connected with vacuum pump and ultrasonic vibrator. After the incision, cataract is fragmented into small pieces using ultrasonic power. Finally, fragmented cataracts are extracted from the eye chamber using a vacuum pump. Current cataract surgery system has an issue of pressure surge followed by collapsing of the anterior chamber of the eye. In the extraction phase, often a big piece of cataract occludes the tip of the aspiration port to build up the pressure difference between the chamber and the pump. When the pressure difference reaches certain point the cataracts are abruptly pulled into the aspiration port. As a result of sudden displacement of cataract and the fluid from the chamber, pressure surges which causes eye chamber collapse. The collapsing of the chamber is not only dangerous to the organs in the anterior chamber such as cornea, but also it lifts the wall of posterior chamber and may damage the retina. Several different design concepts using mechanical and electrical feedback systems have been developed by Micro-Surgery Advanced Design Lab to improve fluidic stability of the system without significant influence upon the cycle time of the procedure. However, considering the size and precision required of the system and the complexity of the design parameters involved, feasibility test and design iterations using working prototypes may limit the possibility of finding an optimal solution to the design problem. In this work, a feasibility test method using computational flow analysis and bench-top simulation is proposed. In developing a design, it is suggested that the feasibility verification for the design concepts be divided into three different steps: CFD analysis, bench-top simulation and working prototype test. Each process filters the concepts before the concept is transferred to the next step and the results of each step are compared to improve the reliability. In CFD analysis, fluidic circuit is modeled to simulate the mechanisms of pressure surge and chamber collapse using CFDRC. Also, suggested design concepts are incorporated into the model to check the feasibility. In the interpretation of the results, the focus is on the estimation of time scale to see the validity of feedback system. Bench-top is an enlarged model of real eye and cataract surgery system. Dimensional analysis is used to design and interpret the result of the bench-top simulation. It has less flexibility in design changes than CFD analysis but easier to build and change than working prototypes because of the size. CFD analysis and bench-top simulation not only determine the initial feasibility of the design concept, but also it narrows down the design solution space to reduce the number of design iterations and save the time and cost for finding an optimal solution to the design problem.

A Pragmatic Coupling Strategy Between Commercial CFD and Structure Analysis Codes

The paper describes how a simple link has been established between the commercial Computational Fluid Dynamics (CFD) code CFX-4 and the two commercial structural analysis software packages ABAQUS and ANSYS. The interface has enabled separate, specialist groups to perform conjugate heat transfer and stress analysis computation within a common project, without the need to lease expensive interface software. The fluid/structure coupling operates primarily in steady-state mode, though transient coupling is possible at specified time intervals, provided there is no feed-back of the structural displacement on the thermal hydraulics. Illustrative examples are presented in the paper of how the coupled codes are being used to aid the design of a pilot spallation source target, and to analyze operational transients and accident sequences.