Structure and dynamics of the turbulent flow through a central baffle

Central baffle flume (CBF) can be utilized as a control structure to measure flow discharge in irrigation channels under free and submerged flow conditions. Stage-discharge relationship has been extensively studied for various geometrical parameters and flow conditions, whereas internal structure of the flow around a baffle has not been investigated in the literature. To address this need, the present work investigates the turbulent flow around a central baffle through high-resolution numerical simulations using an open source computational model. Velocity measurements were conducted in a laboratory flume to setup and validate the numerical model. Comparison of the numerical results with the experimental measurements proves that the present numerical model can predict water depth and velocity field. Longitudinal distance from the apex to the intersection point of water and critical depths can be estimated as L_xc = 2L_e, where L_e is the longitudinal length of the guide walls. A horseshoe vortex system identified in front of the baffle produces a significant bump on the free-surface and rib vortices generated from the baffle extend up to the sidewalls of the channel. The vertical separation layer observed downstream of the baffle results in a reverse flow and a vortex pair is formed by the impingement of the resulting reverse flow on the back of the baffle. Reverse flow, plunging flow structure, splash and rebounding wave events observed at the downstream produce substantial hydrodynamic effects on the baffle. Geometry of the central baffle was modified to suppress recirculation effects based on the insights into the complete flow structure around the baffle. Eventually, vortex structures were suppressed and the length of the recirculation zone was reduced by 76%.     

A method for measuring the dynamics of velocity vector fields in a turbulent flow using smoke image-visualization videos

A new optical method for determining flow-velocity vector fields that allows investigations of unsteady and fast processes is described. The method is based on measuring the displacements of turbulent structures, which are visualized in a light sheet, within a fixed time interval between consecutive video frames. The method was tested using measurements of pulsating-flow velocity. It was shown that the difference between the mean flow velocity in the investigated flow measured using this method and a hot-wire anemometer does not exceed 1%, while the deviation of the rms velocity pulsations is within 4.5%.     

Laminar flow past a sphere rotating in the transverse direction

Laminar flow past a sphere rotating in the transverse direction is numerically investigated in order to understand the effect of the rotation on the characteristics of flow over the sphere. Numerical simulations are performed at Re = 100, 250 and 300, where the Reynolds number is based on the free-stream velocity and the sphere diameter. The rotational speeds considered are in the range of 0 ≤ ω* ≤ 1.2, where ω* is the maximum velocity on the sphere surface normalized by the free-stream velocity. Without rotation, the flow past a sphere experiences steady axisymmetry, steady planar-symmetry, and unsteady planar-symmetry, respectively, at Re = 100, 250 and 300. With rotation, however, the flow becomes planar-symmetric for all the cases investigated, and the symmetry plane of flow is orthogonal to the rotational direction. Also, the rotation affects the flow unsteadiness, and its effect depends on the rotational speed and the Reynolds number. The flow is steady irrespective of the rotational speed at Re = 100, whereas at Re = 250 and 300 it undergoes a sequence of transitions between steady and unsteady flows with increasing ω*. As a result, the characteristics of vortex shedding and vortical structures in the wake are significantly modified by the rotation at Re = 250 and 300. For example, at Re = 300, vortex shedding occurs at low values of ω*, but it is completely suppressed at ω* = 0.04 and 0.6. Interestingly, at ω* = 1 and 1.2, unsteady vortices are newly generated in the wake due to the shear layer instability. The critical rotational speed, at which the shear layer instability begins to occur, is shown to be higher at Re = 250 than at Re = 300. This paper was recommended for publication in revised form by Associate Editor Dongshin Shin Dongjoo Kim is an associate professor in the School of Mechanical Engineering at Kumoh National Institute of Technology. His research interests include computational fluid dynamics, bluff-body wakes, and control of turbulent flows. He has a PhD in mechanical engineering from Seoul National University. He is a member of the American Physical Society and the American Institute of Aeronautics and Astronautics.     

On the mixing flow structure of a turbulent cross jet

An experimental study on the structure of a turbulent cross jet mixing flow is presented. Diffusion rates, two and three dimensional flow structures, mean velocities turbulence intensities and turbulent shear stresses of the mixing flow were measured as varying the velocity ratio. Self-similar forms for the dimensionless mean velocity and turbulent shear stresses was obtained by correlating the measurement data resulting a remarkable agreement. It was found that the deviation gradient(a) is linearly correlated with the velocity ratio(R), and the cross section of the mixing flow is an elliptic form.     

Large eddy simulation of turbulent premixed flame in turbulent channel flow

Large eddy simulation of turbulent premixed flame in turbulent channel flow is studied by usingG-equation. A flamelet model for the premixed flame is combined with a dynamic subgrid combustion model for the filtered propagation flame speed. The objective of this work is to investigate the validity of the dynamic subgridG-equation model to a complex turbulent premixed flame. The effect of model parameters of the dynamic subgridG-equation on the turbulent flame speed is investigated. In order to consider quenching of laminar flames on the wall, wall-quenching damping function is employed in this calculation. In the present study, a constant density turbulent channel flow is used. The calculation results are evaluated by comparing with the DNS results of Bruneaux et al.     

Measurement of developing turbulent flow in a U-bend of circular cross-section

Hot-wire measurements of the full mapping of the velocity and Reynolds stress components are reported for developing turbulent flow in a strongly curved 180 deg pipe and its tangents. A slanted wire is rotated into 6 orientations and the voltage outputs from wires are combined to obtain the mean velocity and Reynolds stress components. The strength of secondary flow reaches up to the 28% of bulk mean velocity. The strong counter-rotating vortex pair induced by the transverse pressure gradient and centrifugal force imbalance grows up to Θ = 67.5° into the bend. But the vortex pair breaks down into two cell pattern after Θ=90° Core vortex formation and reversal of secondary flow direction along the bend symmetry plane is cleanly found in the secondary vector plot. At Θ=67.5° and Θ = 90° into bend a large “trough” develops in the longitudinal velocity toward the inside of the bend due to the breakdown of secondary flow. In the bend, the mean longitudinal velocity component changes little after Θ=90°, but secondary flow never achieves fully-developed state. Similar behaviors are observed in the radial and circumferential stresses.     

Reynolds number effect on turbulent secondary flow in a duct

Numerical simulations of fully-developed turbulent flow through a straight square duct at three Reynolds numbers of 190, 300, and 550 based on the friction velocity averaged over the duct perimeter and duct half width are reported. The effect of Reynolds number on the mean and turbulence statistics and secondary flow is investigated. The mean streamwise-velocity profiles along the wall bisector are found to obey a logarithmic scaling when they are normalized by the friction velocity at the mid-wall. Magnitudes and spatial distributions of the peak production and diffusion terms in the mean streamwise-vorticity equation normalized by the wall units are found to be unaffected by the Reynolds-number variation when they are considered in wall-unit coordinates.     

Large-eddy simulation and acoustic analysis of a turbulent flow field in a swirl-stabilized combustor

To conduct a comprehensive study on the flow characteristics and acoustic oscillation in a gas turbine combustor, a 3D large-eddy simulation (LES) was implemented. The formulation consists of the Favre-filtered conservation equations of mass, momentum, and energy. The subgrid-scale dynamics are modeled using a compressible flow version of the Smagorinsky model. To investigate the dominant coherent structure, the proper orthogonal decomposition (POD) method was used for post-processing. The combustor of concern is the LM6000, lean-premixed dry low-NOx annular combustor, developed by General Electric Aircraft Engines (GEAE). Four important characteristics of swirl flow are visualized: vortex breakdown, procession and dissipation of vortical structures, recirculation zones, and helical waves immediately downstream of the swirl injector. It is shown that the turbulent motion of swirl flow directly affects acoustic oscillation through the cycle and spectral analysis. The four most dominant acoustic modes are extracted from the flow field by the POD analysis. The transverse modes in the y and z directions are dominant in all four modes, since the pressure fields are significantly affected by swirl flow.     

Isoviscous flow past a rigid sphere partially immersed in a thin oil film

Numerical solutions to steady state circular contact elastohydrodynamic lubrication problems normally employ Reynolds equation, forcing the assumption that the flow through the conjunction is two‐dimensional and single‐phase. In this paper, the full Navier–Stokes equations are used on a simpler rigid isoviscous sliding point contact. The aim of the study was to compute the lubricant flow patterns through and around the conjunction of a point contact when the oil inlet is a free surface that has a similar wavy shape to the wake. This model is like some seen experimentally in a ball and plate machine or industrially in a radial ball bearing with insufficient lubricant supply. There is first a discussion on the alteration with time of the oil wake geometry after it has left the conjunction. Then, the computed results mapped the areas of varying concentration between oil and air. They showed an accumulation of oil and vapour forming a reservoir at the inlet to the conjunction. Within it was a vortex attached to the stationary surface, just downstream of which, the main pressure distribution commences. The computed flow patterns were compared with experimental pictures wherever possible. Copyright © 2007 John Wiley & Sons, Ltd.     

Numerical investigation of turbulent cavitating flow in an axial flow pump using a new transport-based model

Journal of Mechanical Science and Technology - With the aim to enhance the capability of predicting cavitating flows for conventional cavitation models, a developed alternative numerical model was...     

Characteristics of turbulent flow in the annuli with smooth and rough surfaces

The structure of turbulence of fully developed flow through three concentric annuli with rough walls shown in Fig. 1 was investigated experimentally for Reynolds number range of Re =15000−66000. Time mean velocity distribution, friction factor, turbulence intensity and turbulent kinetic energy in annuli of three radius ratios of α=0.26, 0.39 and 0.56 for four cases of roughness type were measured. The results show that the structure of turbulence in these asymmetric flows is significantly different depending on the roughness wall position. An experimental study for the case of a smooth annulus was also carried out for a benchmarking purpose. The velocity distributions and friction factors are also presented and discussed.     

Inertia and thermal effects in turbulent flow journal bearings

A semi-analytical study of the influence of inertia terms and the effects of convection and dissipation on an infinitely long bearing is presented. The distribution of the effective viscosity is assumed to be dependent on local shear stress and temperature. The momentum and energy equations are evaluated by assuming that the shape of the velocity profiles is not strongly affected by the presence of the inertia forces. Approximate integral momentum equations are thus obtained. The results show that inclusion of inertia terms has a pronounced effect on the dimensionless load capacity of the bearing. The inclusion of thermal effects results in a smaller dimensionless capacity.     

Effect of engine variables on the turbulent flow of a spark ignition engine

The turbulent flow in a spark ignition engine plays an important role in determining its combustion characteristics and thermal efficiency. In order to analyse the combustion process, the turbulent flow and its turbulence intensity must be studied. To study the turbulent flow as varying various factors in a combustion chamber of a spark ignition engine, the L-head with or without squish area are selected. The turbulent as varying flow on the piston speed, inlet flow velocity, and squish velocity are measured by using hot wire anemometer. To examine the characteristics of turbulent flow, the ensemble averaged mean velocity, turbulence intensity, turbulence intensity decrease ratio, production rate of turbulence intensity, production coefficient of turbulence intensity are analysied.     

A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals.Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.     

LDV measurements of turbulent flow behavior of droplets in a two-phase coaxial jet

Coaxial nozzles are frequently utilized for the atomization of liquids in sprays. The performance of a nozzle is generally evaluated by its atomizing characteristics, which are actually governed by the turbulence interactions of two fluids. With this point of view, this experimental study was carried out to investigate the turbulent behavior of the droplets atomized in a two-phase coaxial jet. Air and water have been used as the working fluids, and the measurements have been made by an on-line data acquisition system connected to a two-channel LDV set(DISA, 5W, Argon laser, blue: 488 nm, green: 514.5 nm). In order to generate a two-phase mixing jet, two types of coaxial nozzles (liquid column type, liquid sheet type) were used. For the investigations of the turbulent flow structure of this two-phase mixing jet, the spreading rates, mean and fluctuating components, intermittency factors and the iso-contours of joint probability densities were measured and analyzed. The results from the both types of nozzles did not show remarkable differences in mean and fluctuating velocity distributions, intermittency factors or the iso-joint probability density contours. Since the measurements were made in the fully developed turbulent mixing regions, the mean velocity distribution profiles showed good similarities and agreed well with the semi-empirical curves. The RMS values were represented as high order levels and so were the intermittency factors. The typical development trends of turbulent components ofu′ andv′ for both types were illustrated in the iso-joint probability density contours.     

SK型静态混合器内的流动特性数值研究

基于微尺度流动特征的角度,研究SK型静态混合器内部湍流时的流动规律.利用计算流体力学专业软件并采用雷诺时均方程和标准的k-ε湍流模型对SK静态混合器内的湍流状态下的三维不可压缩流场进行数值模拟.数值计算和研究表明在含有多个SK型螺旋元件的静态混合器内,受元件衔接处的切割作用的流体将在其后的混合元件的3L/8处完成重新汇合;研究了SK型静态混合器的主要参数对流动阻力的影响,从而为静态混合器的优化设计提供了依据.     

TR-PIV measurement of separated and reattaching turbulent flow over a surface-mounted square cylinder

The separated and reattaching turbulent flow over a surface-mounted two-dimensional square cylinder was experimentally studied by using time-resolved particle image velocimetry (TR-PIV). A total of 61,440 instantaneous image frames were acquired at a framing rate of 125 Hz, yielding a reliable result of the statistical quantities. The time-averaged features of the separated and reattaching flow were analyzed in terms of distributions of the velocity vectors, vorticity, the streamwise velocity fluctuation intensity and shear stress. The association between the large-scale vortical structures and spatial variation of these time-averaged quantities were thoroughly discussed. The unsteady features of the flow were revealed from distributions of the reverse-flow intermittency, space-time contour plot of the fluctuating streamwise velocity, and cross-correlation of the streamwise velocity. Subsequently, a comprehensive understanding of the contribution of the flow structures into the fluctuating flow field was gained by using a snapshot proper orthogonal decomposition (POD) analysis. The results showed that the linear combination of the first five POD modes, which capture 57% of the fluctuation energy, was capable of representing the large-scale behaviors of the separated and reattaching turbulent flow in the senses of spectrum, instantaneous feature and spatial variation of the velocity fluctuation intensity.     

Non-linear analysis of two-lobe bearings in turbulent flow regimes

By adopting the non-linear turbulent theory proposed by Elrod and Ng, the classical Navier-Stokes equations were modified. These modified momentum equations and the continuity equation were solved by the finite element method using Galerkin's technique and a suitable iterative scheme.The performance characteristics of a finite two-lobe hydrodynamic bearing operating in a fully developed turbulent regime were studied in terms of the load-carrying capacity, the stiffness and damping coefficients and the non-dimensional critical journal mass at various eccentricities for Reynolds' numbers up to 12000. Computed results were compared with the results obtained using linearized theory.