Direct Numerical Simulation of a Recirculating, Swirling Flow

A direct numerical simulation (DNS) of a recirculating, swirling flow is performed at a Reynolds number of 5000. Detailed one and two point statistics are presented in this paper. Flow visualization and frequency analysis are used to identify a precessing vortex core and to characterize its position, extent and influence on the flow field. The results are compared with laser Doppler velocimetry (LDV) measurements as well as large eddy simulation (LES) data reported in the literature. The present work constitutes a first step in setting up a DNS data base for complex flows.     

Fluid Dynamical Analysis of Atmospheric Reacting and Isothermal Swirling Flows

A series of unconfined swirling premixed natural gas/air flames was investigated. Reynolds-numbers spanned from 10,000 to 42,300. Respective isothermal flows were studied additionally to gain insight into changes of fluid dynamical features caused by combustion. Statistical moments, Reynolds-stresses, temporal time scales, spatial length scales, and power spectral densities were deduced from one- and two-point laser Doppler velocimetry (LDV) data. Properties of the turbulent flows and dependencies on Reynolds-number, swirl number, and chemical reactions are discussed. Most distinct differences between combusting and isothermal flows were precessing vortex cores (PVC) occurring only for the latter cases. The study is aimed to serve as a database of a generic flame geometry featuring important characteristics of industrial applications for validation of numerical simulations. Therefore, nozzle exit profiles as important inlet conditions to numerical simulations are thoroughly documented.     

Computation of a drastic flow pattern change in an annular swirling jet caused by a small decrease in inlet swirl

This paper investigates the flow pattern change in an annular jet caused by a sudden change in the level of inlet swirl. The jet geometry consists of an annular channel followed by a specially designed stepped‐conical nozzle, which allows the existence of four different flow patterns as a function of the inlet swirl number. This paper reports on the transition between two of them, called the ‘open jet flow high swirl’ and the ‘Coanda jet flow.’ It is shown that a small sudden decrease of 4% in inlet swirl results in a drastic and irreversible change in flow pattern. The objective of this paper is to reveal the underlying physical mechanisms in this transition by means of numerical simulations. The flow is simulated using the unsteady Reynolds‐averaged Navier–Stokes (URANS) approach for incompressible flow with a Reynolds stress turbulence model. The analysis of the numerical results is based on a study of different forces on a control volume, which consists of the jet boundaries. The analysis of these forces shows that the flow pattern change consists of three different regimes: an immediate response regime, a quasi‐static regime and a Coanda regime. The simulation reveals that the pressure–tangential velocity coupling during the quasi‐static regime and the Coanda effect at the nozzle outlet during the Coanda regime are the driving forces behind the flow pattern change. These physical mechanisms are validated with time‐resolved stereo‐PIV measurements, which confirm the numerical simulations. Copyright © 2008 John Wiley & Sons, Ltd.     

Algebraically explicit analytical solutions for the unsteady non-Newtonian swirling flow in an annular pipe

This paper presents two algebraically explicit analytical solutions for the incompressible unsteady rotational flow of Oldroyd-B type in an annular pipe. The first solution is derived with the common method of separation of variables. The second one is deduced with the method of separation of variables with addition developed in recent years. The first analytical solution is of clear physical meaning and both of them are fairly simple and valuable for the newly developing computational fluid dynamics. They can be used as the benchmark solutions to verify the applicability of the existing numerical computational methods and to inspire new differencing schemes, grid generation ways, etc. Moreover, a steady solution for the generalized second grade rheologic fluid flow is also presented. The correctness of these solutions can be easily proven by substituting them into the original governing equation.     

Study of turbulent swirling flow in a rotating separation element of variable cross section

In the present study, the distribution of circumferential velocity components of carrier flow in separation elements of air-centrifugal classifiers of powder materials has been experimentally examined. The study was carried out for variously contoured separation channels, and also for various conditions of a carrying agent (air) input in a separation zone. Optimal conditions for particle separation throughout the whole volume of the separation element can be organized by making the channel contour diverging towards the center of rotation. The experimental data gained in this study supplement the mathematical model for the aerodynamics of turbulent carrier flow in the shaped rotating separation elements of air-centrifugal classifiers and make an analysis of the separation-zone flow possible.     

环形扩压通道内有旋绕流动的研究

本文计算了环形截面的扩压通道内带进气旋绕的流动.在小横向流假定下.用三维边界层积分方程法求解内外壁面附近的流动.通过对子午面上与流线子午投影准正交方向的速度梯度方程和流量不变方程的迭代求解得出边界层外的势流场.计算与实验结果基本符合.本研究可用于分析环形扩压器内带进气予旋的流动.     

SIMULATION OF NOx FORMATION IN TURBULENT SWIRLINGCOMBUSTION USING A USM TURBULENCE—CHEMISTRY MODEL

A unified second-order moment (USM) turbulence-chemistry model for simulating Nox formation in turbulent combustion is proposed. All of correlations, including the correlation of the reaction-rate coefficient fluctuation with the concentration fluctuation, are closed by the transport equations in the same form. This model discards the approximation of series expansion of the exponential function or the approximation of using the product of several 1-D PDF‘s instead of a joint PDF. It is much simpler than other refined models, such as the PDF transport equation model and the condi-tional moment closure model. The proposed model is used to simulate methane-air swirling turbulent combustion and Nox formation. The prediction results are in good agreement with the experimental results.     

旋流和无旋突扩流动的LES和RANS模拟

本文用smagorinsky-Lilly亚网格尺度湍流模型对旋流突扩流动(s=0.53)和无旋突扩流动(s=0)进行了大涡模拟(LES模拟),同时分别用压力应变项为IPCM和IPCM+Wall模型的雷诺应力方程模型进行了RANS模拟,和LES的统计结果对比。LES的统计结果与雷诺应力模型的模拟结果及实验对照表明,LES结果与实验结果的吻合比雷诺应力模型的好,说明所用的亚网格尺度湍流模型对旋流流动是适用的,LES结果是可信的。LES的瞬态结果揭示出在旋流作用下,流场中存在复杂的旋涡脱落现象。大涡结构极易破碎成小涡,而在无旋突扩流动的情况下,由于剪切的作用更强,大涡结构的尺寸和范围比旋流流动的要大。