共查询到20条相似文献,搜索用时 437 毫秒
1.
Alex Povitsky 《Computers & Fluids》2002,31(8)
The jet mixing reactor has been proposed for the industrial production of fullerence carbon nanotubes. The goal is to obtain a uniformly high temperature of a catalyst. The mixing flowfield is studied using the semi-implicit method for pressure-linked equations revised algorithm. Hot peripheral jets are used to enhance heating of the central jet by mixing with the ambience of the reactor. Numerous configurations of peripheral jets with various numbers of jets, distance between nozzles, angles between the central jet and a peripheral jet, and twisted configuration of nozzles are considered. Unlike the previous studies of jet mixing, the optimal configuration of peripheral jets produces strong stretching of a cross-section of the central jet that enhances the mixing of the central jet with the reactor ambience. 相似文献
2.
DNS for flow separation control around an airfoil by pulsed jets 总被引:1,自引:0,他引:1
Shutian Deng 《Computers & Fluids》2007,36(6):1040-1060
Direct numerical simulation (DNS) for flow separation and transition around a NACA-0012 airfoil with an attack angle of 4° and Reynolds number of 100,000 has been reported in our previous paper. The details of flow separation, formation of the detached shear layer, Kelvin-Helmholtz instability (inviscid shear layer instability) and vortex shedding, interaction of nonlinear waves, breakdown, and re-attachment are obtained and analyzed. The power spectral density of pressure shows the low frequency of vortex shedding caused by the Kelvin-Helmholtz instability still dominates from the leading edge to trailing edge. Based on our understanding on the flow separation mechanism, we try to reveal the mechanism of the flow separation control using blowing jets and then optimize the jets. DNS simulations for flow separation control by blowing jets (pulsed and pitched and skewed jets) are reported and analyzed. The effects of different unsteady blowing jets on the surface at the location just before the separation points are studied. The length of separation bubble is significantly reduced (almost removed) after unsteady blowing technology is applied. The mechanism of early transition caused by the blowing jets was found. A blowing jet with K-H frequency, sharp shape function (very small mass blowing), pitching and skewing obtained the best efficiency based on the increase of the ratio of lift over drag and decrease of blowing mass flow. In this work, a DNS code with high-order accuracy and high-resolution developed by the computational fluid dynamics group at University of Texas at Arlington is applied. 相似文献
3.
《Sensors and actuators. A, Physical》2006,125(2):159-169
In contrast to usual synthetic jets, the “hybrid-synthetic jets” of non-zero time-mean nozzle mass flow rate are increasingly often considered for control of flow separation and/or transition to turbulence as well as heat and mass transfer. The paper describes tests of a scaled-up laboratory model of a new actuator version, generating the hybrid-synthetic jets without any moving components. Self-excited flow oscillation is produced by aerodynamic instability in fixed-wall cavities. The return flow in the exit nozzles is generated by jet-pumping effect. Elimination of the delicate and easily damaged moving parts in the actuator simplifies its manufacture and assembly. Operating frequency is adjusted by the length of feedback loop path. Laboratory investigations concentrated on the propagation processes taking place in the loop. 相似文献
4.
具有侧向推力控制的自旋导弹建模与仿真 总被引:1,自引:0,他引:1
对于复合侧推力控制的自旋防空导弹,其末端控制时,由于侧喷发动机的工作,动力学特性属于具有离散事件的混杂系统。该文简单介绍了离散事件系统的有限状态机建模方法。利用有限状态机对姿控发动机的离散事件特性建模,同时实现导弹侧推力发动机的推力曲线仿真和复合控制系统仿真模型。由于有限状态机很容易通过仿真软件来实现,因此使用有限状态机能够很方便地建立导弹的一体化仿真环境。文中展示了建立在Simulink环境下的导弹仿真模型,并通过仿真结果展示了控制器的控制效果。 相似文献
5.
Computational fluid dynamics simulations using the WENO-LF method are applied to high Mach number nonrelativistic astrophysical jets, including the effects of radiative cooling. Our numerical methods have allowed us to simulate astrophysical jets at much higher Mach numbers than have been attained (Mach 20) in the literature. Our simulations of the HH 1-2 astrophysical jets are at Mach 80. Simulations at high Mach numbers and with radiative cooling are essential for achieving detailed agreement with the astrophysical images. 相似文献
6.
High Mach number astrophysical jets are simulated using a positive scheme, and are compared with WENO-LF simulations. A version
of the positive scheme has allowed us to simulate astrophysical jets with radiative cooling up to Mach number 270 with respect
to the heavy jet gas, a factor of two times higher than the maximum Mach number attained with the WENO schemes and ten times
higher than with CLAWPACK. Such high Mach numbers occur in many settings in astrophysical flows, so it is important to develop
a scheme that can simulate at these Mach numbers.
Research supported in part by the BK21 project at KAIST.
Research supported in part by the Space Telescope Science Institute under grant HST-GO-09863.06-A. 相似文献
7.
Blayne Field Sean O'Neill Victoria Interrante Thomas W. Jones Timothy Urness 《Computer Graphics and Applications, IEEE》2007,27(1):9-13
Astronomers have long been challenged to test theories of observable phenomena at great distances from Earth. One such area of active research is the study of fast, collimated jets of ionized matter, or plasma, formed near massive black holes at the centers of distant galaxies. Our group is involved in magnetohydrodynamic simulations that track the time and space evolution of the full 3D velocity and magnetic vector fields, plus fundamental scalar fields such as density and pressure. To accomplish the complex visualization of these jets, we developed FieldVis, a simulation tool that focuses primarily on representing 3D vector and scalar fields. Examining data from a sample 3D magnetohydrodynamic fluid simulation graphically illustrates the usefulness of our visualization package. Through our work, we found that streaklines with varying surface properties such as texture and color are the most effective way to extract information from our data. The techniques we used are not specific to astrophysical problems and can extend to other sets of vector and scalar fields. In the future, we plan to use FieldVis to visualize tangled magnetic fields in simulated galaxy clusters, as well as velocity and magnetic structures produced by intermittent jets 相似文献
8.
Ali Uzun Gregory A. Blaisdell Anastasios S. Lyrintzis 《Journal of scientific computing》2004,21(3):283-319
We present 3-D large eddy simulation (LES) results for a turbulent Mach 0.9 isothermal round jet at a Reynolds number of 100,000 (based on jet nozzle exit conditions and nozzle diameter). Our LES code is part of a Computational Aeroacoustics (CAA) methodology that couples surface integral acoustics techniques such as Kirchhoff's method and the Ffowcs Williams– Hawkings method with LES for the far field noise estimation of turbulent jets. The LES code employs high-order accurate compact differencing together with implicit spatial filtering and state-of-the-art non-reflecting boundary conditions. A localized dynamic Smagorinsky subgrid-scale (SGS) model is used for representing the effects of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in very good agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by the LES is coupled with the Ffowcs Williams–Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with experimental measurements of jets at similar flow conditions. The aeroacoustics results are encouraging and suggest further investigation of the effects of inflow conditions on the jet acoustic field. 相似文献
9.
In this paper, the results from 3D numerical simulations of circular synthetic jets issued into a zero-pressure-gradient laminar boundary layer developing along a flat plate are reported. The simulations are undertaken using FLUENT at a wide range of actuator operating conditions. The formation and development of the coherent structures produced as a result of the interaction between the synthetic jets and the boundary layer were examined using the Q-criterion. Non-dimensional parameter space maps were established to illustrate the variations in the appearance of these resultant structures and their shear stress footprints upon the changes in the operating conditions of synthetic jets. Finally, the parameter boundary separating the two distinct types of vortical structures and surface shear stress patterns is identified. It is found that the location of this boundary correlates closely with the jet-to-freestream velocity ratio of VR = 0.4 when the Strouhal number (Str) is less than 1, whereas for Str > 1 the boundary deviates from this trend, approaching the line of dimensionless stroke length of L = 1.6. In order to investigate the potential impact of the synthetic jets on the boundary layer, the increase in the space- and time-averaged skin friction coefficient relative to the baseline case without the synthetic jets is calculated. It appears that in order to maximise the impact on the near-wall flow while keeping the energy expenditure down, it is wise to maximise the accumulated effect of hairpin vortices by keeping the spacing between consecutive hairpin vortices similar to the local boundary layer thickness upstream of the separated flow instead of producing stronger individual structures. 相似文献
10.
J. Ma 《Computers & Fluids》2010,39(1):77-57
The accurate prediction of air entrainment is critical in simulating various important multiphase (air/water) flows. In this paper, we present a sub-grid air entrainment model that quantitatively predicts the rate of air entrainment and subsequent disperse bubbly flow for a plunging jet. The derivation of this model is based on the two-stage (i.e., low and high liquid jet velocity) air entrainment mechanisms suggested by Sene [Sene KJ. Air entrainment by plunging jets. Chem Eng Sci 1988;43(10):2615-23]. This model was validated against extensive experimental data for water jets in air over a wide range of liquid velocities (from around 1 to 10 m/s) for the total rate of air entrainment. It was then implemented into an Eulerian/Eulerian two-fluid computational multiphase fluid dynamics (CMFD) model, wherein the liquid and the bubbles are modeled as two distinct continua. This multiphase model, supplemented by the new sub-grid air entrainment model, was used to predict the void fraction distribution underneath plunging water jets at different depths and water jet velocities. It was found that this approach yields results that match the experimental observations very well. 相似文献
11.
A finite element method is used to study the effect of Reynolds number and surface tension on the expansion and contraction of jets of Newtonian liquids. For values of Reynolds numbers (based on tube diameter), below 14 the jets expand, and when Re > 14 the jets contract. For higher Reynolds numbers the jet diameter approaches a limiting value. It is also found that the surface tension has a considerable effect on low Reynolds number jet flows, becoming negligible at higher Reynolds numbers. As an example, if the surface tension parameter is equal to unity, the creeping flow jet expansion is reduced by 4% relative to the case with no surface tension but when Re is equal to 20 and 50 the final jet diameters increase by only 0.2%. The calculated jet shapes are compared with available experimental results. 相似文献
12.
13.
Fevzi Onen 《Neural computing & applications》2014,25(1):217-227
The flow characteristics of the plunging water jets can be defined as volumetric air entrainment rate, bubble penetration depth, and oxygen transfer efficiency. In this study, the bubble penetration depth is evaluated based on four major parameters that describe air entrainment at the plunge point: the nozzle diameter (D N), jet length (L j), jet velocity (V N), and jet impact angle (θ). This study presents artificial neural network (ANN) and genetic expression programming (GEP) model, which is an extension to genetic programming, as an alternative approach to modeling of the bubble penetration depth by plunging water jets. A new formulation for prediction of penetration depth in a plunging water jets is developed using GEP. The GEP-based formulation and ANN approach are compared with experimental results, multiple linear/nonlinear regressions, and other equations. The results have shown that the both ANN and GEP are found to be able to learn the relation between the bubble penetration depth and basic water jet properties. Additionally, sensitivity analysis is performed for ANN, and it is found that D N is the most effective parameter on the bubble penetration depth. 相似文献
14.
15.
Matthew S. Brown Nicholas T. Kattamis Craig B. Arnold 《Microfluidics and nanofluidics》2011,11(2):199-207
Laser-induced forward transfer (LIFT) is a high-resolution direct-write technique, which can print a wide range of liquid
materials without a nozzle. In this process, a pulsed laser initiates the expulsion of a high-velocity micro-jet of fluid
from a thin donor film. LIFT involves a novel regime for impulsively driven free-surface jetting in that viscous forces developed
in the thin film become relevant within the jet lifetime. In this work, time-resolved microscopy is used to study the dynamics
of the laser-induced ejection process. We consider the influence of thin metal and thick polymer laser-absorbing layers on
the flow actuation mechanism and resulting jet dynamics. Both films exhibit a mechanism in which flow is driven by the rapid
expansion of a gas bubble within the liquid film. We present high-resolution images of the transient gas cavities, the resulting
ejection of high aspect ratio external jets, as well as the first images of re-entrant jets formed during LIFT. These observations
are interpreted in the context of similar work on cavitation bubble formation near free surfaces and rigid interfaces. Additionally,
by increasing the laser beam size used on the polymer absorbing layer, we observe a transition to an alternate mechanism for
jet formation, which is driven by the rapid expansion of a blister on the polymer surface. We compare the dynamics of these
blister-actuated jets to those of the gas-actuated mechanism. Finally, we analyze these results in the context of printing
sensitive ink materials. 相似文献
16.
非约束环境下,光照、姿态、表情、遮挡等复杂背景因素给人脸识别带来严重影响。提出一种基于AAM(active appearance model)的图像对齐和局部匹配人脸识别算法,使之能够增强人脸识别算法对姿态、表情变化的鲁棒性。AAM能够快速准确地定位人脸的特征点,进而将图像扭转到一个标准正面人脸模型中。接着,提出一种新的基于信息熵的Gabor jet加权方法用于提高人脸识别率;并且对Borda count分类器组合方法进行了改进,认为在投票过程中为其设置阈值来排除“噪声”的干扰可以提高识别率。通过与多种人脸识别方法的实验结果比较表明,使用AAM矫正图像后,联合熵加权Gabor方法和加阈值Borda能够取得比单独使用更好的成绩。 相似文献
17.
We describe a C++ implementation of the Optimal Jet Definition for identification of jets in hadronic final states of particle collisions. We explain interface subroutines and provide a usage example. The source code is available from http://www.inr.ac.ru/~ftkachov/projects/jets/.
Program summary
Title of program: Optimal Jet Finder (v1.0 C++)Catalogue identifier: ADSB_v2_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADSB_v2_0Program obtainable from: CPC Program Library, Queen's University of Belfast, N. IrelandComputer: any computer with a standard C++ compilerTested with:- (1)
- GNU gcc 3.4.2, Linux Fedora Core 3, Intel i686;
- (2)
- Forte Developer 7 C++ 5.4, SunOS 5.9, UltraSPARC III+;
- (3)
- Microsoft Visual C++ Toolkit 2003 (compiler 13.10.3077, linker 7.10.30777, option /EHsc), Windows XP, Intel i686.
18.
Roman Kuc 《Robotics and Autonomous Systems》1993,11(3-4):213-219
A sonar-driven robot, ROBAT3D, has been implemented to track an object moving in three dimensions using qualitative interpretation of the sonar signals. ROBAT3D is equipped with a sonar system consisting of five identical transducers configured in the form of a cross. The center transducer emits an acoustic pulse and the echoes are detected by pairs of receivers that flank the transmitter horizontally and vertically. The vertically-oriented receivers are used to control the pitch while those oriented horizontally control the yaw. ROBAT3D is driven by four air jets that provide lateral forces that direct the heading toward the object. Two bits of information are extracted from each emission, one for pitch correction and the other for yaw correction. The correction is determined by which sensor in a pair first detects the echo. That sensor actuates an air valve supplying the contralateral jet to correct the heading. The range to the object can be determined from the echo time-of-flight. This simple nonlinear analog control system is very fast and can track an object that has an angular velocity upto π /2 rad /s. A trade-off exists between the size of a limit cycle oscillation and the bandwith of the system, determined by the thrust of the jets and the delay in the sonar sensing. 相似文献
19.
Differential and Integral Geometry of Linear Scale-Spaces 总被引:1,自引:0,他引:1
Alfons H. Salden Bart M. Ter Haar Romeny Max A. Viergever 《Journal of Mathematical Imaging and Vision》1998,9(1):5-27
Linear scale-space theory provides a useful framework to quantify the differential and integral geometry of spatio-temporal input images. In this paper that geometry comes about by constructing connections on the basis of the similarity jets of the linear scale-spaces and by deriving related systems of Cartan structure equations. A linear scale-space is generated by convolving an input image with Green's functions that are consistent with an appropriate Cauchy problem. The similarity jet consists of those geometric objects of the linear scale-space that are invariant under the similarity group. The constructed connection is assumed to be invariant under the group of Euclidean movements as well as under the similarity group. This connection subsequently determines a system of Cartan structure equations specifying a torsion two-form, a curvature two-form and Bianchi identities. The connection and the covariant derivatives of the curvature and torsion tensor then completely describe a particular local differential geometry of a similarity jet. The integral geometry obtained on the basis of the chosen connection is quantified by the affine translation vector and the affine rotation vectors, which are intimately related to the torsion two-form and the curvature two-form, respectively. Furthermore, conservation laws for these vectors form integral versions of the Bianchi identities. Close relations between these differential geometric identities and integral geometric conservation laws encountered in defect theory and gauge field theories are pointed out. Examples of differential and integral geometries of similarity jets of spatio-temporal input images are treated extensively. 相似文献
20.
《Computers & Fluids》2002,31(4-7):695-718
Large-eddy simulations of spatially developing planar turbulent jets are performed using a compact finite-difference scheme of sixth-order and an advective upstream splitting method-based method of second-order accuracy. The applicability of these solution schemes with different subgrid scale models and their performance for realistic turbulent flow problems are investigated. Solutions of the turbulent channel flow are used as an inflow condition for the turbulent jets. The results compare well with each other and with analytical and experimental data. For both solution schemes, however, the influence of the subgrid scale model on the time averaged turbulence statistics is small. This is known to be the case for upwind schemes with a dissipative truncation error, but here it is also observed for the high-order compact scheme. The reason is found to be the application of a compact high-frequency filter, which has to be used with strongly stretched computational grids to suppress high-frequency oscillations. The comparison of the results of the two schemes shows hardly any difference in the quality of the solutions. The second-order scheme, however, is computationally more efficient. 相似文献