Numerical simulation on the behavior of a liquid jet into an air flow

A numerical simulation has been performed to clarify the effects of turbulence in a liquid on the deformation of the liquid jet surface into an air flow. The turbulences in the liquid jet were simulated by the Rankin vortices, and the liquid jet surface was tracked numerically by the volume of fluid method. By numerical simulations, the onset of the protrusions on the liquid jet surface is caused by the vortices in the liquid, and the surrounding air flow plays an important role in the amplification of the protrusions. The amplification rate of the trough displacement is proportional to the air‐to‐liquid velocity ratio. At large imposed vortex intensities, the trough displacement increases with the vortex intensity. On the other hand, at small imposed vortex intensities, the amplification of the trough displacement is also affected by factors other than vortex intensity. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 473–484, 2001     

Numerical simulation of vortex structures and heat transfer behind a hill in a laminar boundary layer

The present study examines a three‐dimensional numerical simulation of vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. A vortex pair is formed symmetrically in the separation bubble behind the hill, and a hairpin vortex is periodically shed in the wake. The hairpin vortex moves downstream with time, and the gradient of the head of the hairpin vortex increases. Further downstream, the hairpin vortex is deformed to an Ω‐shaped structure. In the growth process of the hairpin vortex, horn‐shaped secondary vortices grow near the wall. The dissipation rate of the temperature fluctuation around the hairpin vortex increases because the heated fluid near the wall is removed to the free stream by Q2 ejection. Heat transfer increases due to the legs of the hairpin vortex and secondary vortices. These vortices generate high turbulence in the flow field and also contribute to an increase in Reynolds shear stress and turbulent heat flux. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 398–411, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20217     

Numerical simulation of asymmetrical flow and heat transfer behind a hill in shear flows

Three‐dimensional numerical simulations of asymmetrical flows and heat transfer around a hill in shear flows were performed. When shear velocity distributions are introduced at the inlet, a vortex pair is formed asymmetrically to the spanwise direction behind the hill. Further, an asymmetrical hairpin vortex is periodically generated downstream. The leg of the asymmetrical hairpin vortex on the high‐speed side collapses first. Further downstream, the asymmetrical hairpin vortex breaks down earlier than the symmetrical hairpin vortex, and streamwise vortices appear on the high‐speed side. These streamwise vortices increase the heat transfer downstream. In contrast, no hairpin vortex appears in the case of a strong shear velocity distribution, but instead a streamwise vortex appears. The heat transfer decreases downstream since the turbulence generated by streamwise vortices is weak. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20223     

Numerical simulation of separated flow transition and heat transfer around a two‐dimensional rib

Numerical simulations of separated flow transition and heat transfer around a two‐dimensional rib mounted in a laminar boundary layer were performed. The separated shear layer becomes unstable due to the Kelvin–Helmholtz instability and generates a two‐dimensional vortex. This vortex becomes three‐dimensional and collapses in the downstream part of the separation bubble. As a result, transition from laminar to turbulent flow occurs in the separated shear layer. Streamwise vortices exist downstream of the reattachment flow region. The low‐frequency flapping motion and transition of the separated shear layer are influenced by three‐dimensional dynamics upstream of the separation bubble. Large‐scale vortices around the reattachment flow region have substantial effects on heat transfer. Downstream of the reattachment point, the surface friction coefficient and Nusselt number are different from their profiles in the laminar boundary layer and approach the distributions seen in the turbulent boundary layer. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 513–528, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20177     

LES analysis of flow and turbulent mixing in an unsteady jet

The flow and mixing process of unsteady jets are fundamentally analyzed by large eddy simulations. The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between the nozzle fluid and ambient fluid were investigated. The results show that a toroidal‐shaped vortex, which emerges around the jet tip, primarily accelerates the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages mixing in the jet without changing the jet‐contour. Furthermore, when the rise‐up time of the initial nozzle velocity is elongated, turbulent mixing is suppressed. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 303–313, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20158     

Streamwise Vortex Interaction with a Horseshoe Vortex

How control in turbomachinery is very difficult because of the complexity of its fully 3-D flow structure. The authors propose to introduce streamwise vortices into the control of internal flows. A simple configuration of vortices was investigated in order to better understand the flow control methods by means of streamwise vortices. The research presented here concerns streamwise vortex interaction with a horseshoe vortex. The effects of such an interaction are significantly dependent on the relative location of the streamwise vortex in respect to the leading edge of the profile. The streamwise vortex is induced by an air jet. The horseshoe vortex is generated by the leading edge of a symmetric profile. Such a configuration gives possibility to investigate the interaction of these two vortices alone. The presented analysis is based on numerical simulations by means of N-S compressible solver with a two-equation turbulence model.     

Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

This study presents a three‐dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn‐shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn‐shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high‐turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20261     

Stability analysis of the tip vortices of a wind turbine

The aim of the present paper is to obtain a better understanding of the stability properties of wakes generated by wind turbine rotors. To accomplish this, a numerical study on the stability of the tip vortices of the Tjaereborg wind turbine has been carried out. The numerical model is based on large eddy simulations of the Navier–Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies, the flow is disturbed by inserting harmonic perturbations, giving rise to spatially developing instabilities. The results show that the instability is dispersive and that growth arises only for some specific frequencies and type of modes, in agreement with previous instability studies. The result indicates two types of modes; one where oscillations of neighboring vortex spirals are out of phase and one where oscillations in every vortex spiral in phase. The mode with spirals out of phase results in the largest growth with the main extension of the disturbance waves in radial and downstream directions. The out‐of‐phase disturbance leads to vortex pairing once the development leaves the linear stage. The study also provides evidence of a relationship between the turbulence intensity and the length of the near wake. The relationship, however, needs to be calibrated against measurements. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental and computational analysis of stall cells on rectangular wings

The present paper is the second part of a combined (experimental and computational) study on stall cells (SCs) on a rectangular wing. In the first part, tuft data were used in order to geometrically characterize a stabilized SC resulting from a localized spanwise disturbance introduced by a zigzag tape. Here, pressure measurements on the model and in the wake and aerodynamic polars at midspan are reported. The wing model had an aspect ratio value of 2, the Reynolds number was 10⁶ and the range of angles of attack (α) was from ?6^° to 16^°. Experimental results confirm previous findings. Furthermore, two‐dimensional and three‐dimensional Reynolds Averaged Navier‐Stokes RANS simulations are used in order to better understand the structure of SCs. 3D simulations reproduce the experimental data with a 3° delay in α and permit a qualitative analysis. It is found that the SC vortices start normal to the wing surface and extend downstream in the wake; the evolution of the SC vortices in the wake is in strong interaction with the separation line vortex and the trailing edge line vortex; as the SC vortex develops downstream in the wake, its centreline is contracted towards the SC centre; the wing wake is pushed upstream at the centre of the SC and downstream at the sides by the SC vortices; spanwise lift and drag distributions always attain their minimum at the SC centre. Copyright © 2013 John Wiley & Sons, Ltd.     

TRANSIENT NATURAL CONVECTION OF WATER NEAR ITS DENSITY EXTREMUM IN A RECTANGULAR CAVITY FILLED WITH POROUS MEDIUM

A numerical study is performed to analyze the transient flow field and heat transfer behavior of cold water in a rectangular enclosure filed with a porous medium. The numerical method was employed by the SIMPLE-C algorithm. The accurate nonlinear relation between density and temperature developed by Gebhart and Mollendorf was used. The numerical results show that, for R < 0 · 5, the size of the clockwise vortex beside the high-temperature surface increases as time increases. The clockwise vortex occupies almost the whole space when the steady state is reached. For R = 0 · 5, the size of the clockwise vortex adjacent to the high-temperature surface increases as time increases. However, two counterrotation vortices having the same strength and size exist in the space when steady state is reached. For R > l, only one counterclockwise vortex exists in the space the entire time. For τ > 10^?3, the heat transfer rate increases with increase in Darcy number for the entire time. However, as time increases, the heat transfer rate on the high-temperature surface decreases and that on the low-temperature surface increases.     

Turbulence and Heat Transfer of a Hairpin Vortex Formed Behind a Cube in a Laminar Boundary Layer

We report numerical simulations of three-dimensional unsteady separated flow and heat transfer around a cube mounted in a laminar boundary layer. The separated shear layer rolls up and a hairpin vortex is generated periodically behind the cube. A horseshoe vortex is also formed ahead of the cube. Heat transfer around the cube is high due to the horseshoe vortex. Since the hairpin vortex interacts with horseshoe vortices downstream of the cube, the heat transfer increases around the center of the spanwise direction. The hairpin and horseshoe vortices generate local areas of high turbulence.     

Large eddy simulation of coherent structure of impinging jet

The flow field of a rectangular exit, semi-confined and submerged turbulent jet impinging orthogonally on a flat plate with Reynolds number 8500 was studied by large eddy simulation (LES). A dynamic sub-grid stress model has been used for the small scales of turbulence. The evolvements such as the forming, developing, moving, pairing and merging of the coherent structures of vortex in the whole regions were obtained. The results revealed that the primary vortex structures were generated periodically, which was the key factor to make the secondary vortices generate in the wall jet region. In addition, the eddy intensity of the primary vortices and the secondary vortices induced by the primary vortices along with the time were also analyzed.     

Fluid flow and heat transfer of natural convection adjacent to upward‐facing inclined heated plates

Turbulent transition mechanism and local heat transfer characteristics of the natural convective flows over upward‐facing inclined plates were investigated experimentally. The experiments were performed in the range of modified local Rayleigh numbers from 10⁴ to 8 × 10¹⁴ and of inclination angles θ from 0 to 90°. The flow fields over the plate and the surface temperatures of the plate were visualized with dye and liquid crystal thermometry. The results showed that longitudinal vortices play a main role in the turbulent transition over the plate of θ < 72°. These vortices appear first in the laminar boundary layer, then detach from the plate and, finally become distorted. It is found that the heat transfer is enhanced markedly by the detachment and the distortion of these vortices. The local heat transfer coefficients were measured in the laminar, transitional, and turbulent regions. The results show that the coefficients in the turbulent region become identical and independent of inclination angles. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(3): 278–291, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10091     

Influence of inclination angle,attack angle,and arrangement of rectangular vortex generators on heat transfer performance

We have studied the enhancement of heat transfer by vortex generators. Experiments were performed on rectangular‐type vortex generators mounted on a parallel‐plate heater, and the heat transfer coefficient of the heater surface and pressure drop in the duct were measured. These measurements indicated that a rectangular vortex generator (called a double‐inclined winglet), with inclination angle of the vortex generator surface to the heater surface (β) at 60°, and the attack angle to the flow direction (γ) at 45°, maximizes the local Nusselt number of the heater surface. It was also found that a group of double‐inclined winglets has an optimal arrangement in a winglet array, longitudinal pitch and transverse pitch, that maximizes the ratio [Colburn's dimensionless heat transfer coefficient J_H]/[friction factor f]. The results of numerical calculations showed that the double‐inclined winglet was superior to the conventional rectangular vortex generator in heat transfer. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(3): 253–267, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10089     

HAWT dynamic stall response asymmetries under yawed flow conditions

Horizontal axis wind turbines can experience significant time‐varying aerodynamic loads, potentially causing adverse effects on structures, mechanical components and power production. As designers attempt lighter and more flexible wind energy machines, greater accuracy and robustness will become even more critical in future aerodynamics models. Aerodynamics modelling advances, in turn, will rely on more thorough comprehension of the three‐dimensional, unsteady, vortical flows that dominate wind turbine blade aerodynamics under high‐load conditions. To experimentally characterize these flows, turbine blade surface pressures were acquired at multiple span locations via the NREL Phase IV Unsteady Aerodynamics Experiment. Surface pressures and associated normal force histories were used to characterize dynamic stall vortex kinematics and normal force amplification. Dynamic stall vortices and normal force amplification were confirmed to occur in response to angle‐of‐attack excursions above the static stall threshold. Stall vortices occupied approximately one‐half of the blade span and persisted for nearly one‐fourth of the blade rotation cycle. Stall vortex convection varied along the blade, resulting in dramatic deformation of the vortex. Presence and deformation of the dynamic stall vortex produced corresponding amplification of normal forces. Analyses revealed consistent alterations to vortex kinematics in response to changes in reduced frequency, span location and yaw error. Finally, vortex structures and kinematics not previously documented for wind turbine blades were isolated. Published in 2000 by John Wiley & Sons, Ltd.     

Tip‐vortex breakdown of wind turbines subject to shear

Sheared velocity profiles pervade all wind‐turbine applications, thus making it important to understand their effect on the wake. In this study, a single wind turbine is modeled using the actuator‐line method in the incompressible Navier–Stokes equations. The tip vortices are perturbed harmonically, and the growth rate of the response is evaluated under uniform inflow and a linear velocity profile. Whereas previous investigations of this kind were conducted in the rotating frame of reference, this study evaluates the excitation response in the fixed frame of reference, thus necessitating a frequency transformation. It is shown that increasing the shear decreases the spatial growth rate in the upper half of the wake while increasing it in the lower half. When scaled with the local tip vortex parameters, the growth rate along the entire azimuth collapses to a single value for the investigated wavenumbers. We conclude that even though the tip‐vortex breakdown is asymmetric in sheared flow, the scaled growth rates follow the behavior of axisymmetric helical vortices. An excitation amplitude reduction by an order of magnitude extends the linear growth region of the wake by one radius for uniform inflow. In the sheared setup, the linear growth region is extended further in the top half than in the bottom half because of the progressive distortion of the helical tip vortices. An existing model to determine the stable wake length was shown to be in close agreement with the observed numerical results when adjusted for shear.     

Coherent structures in a fully developed stage of a non‐isothermal round jet

Direct numerical simulation (DNS) was performed for a non‐isothermal air jet with a Reynolds number of 1200 in order to reveal coherent structures of the developed jet. A fourth‐order central finite difference was applied to the simulation. An effort was also made to enable experimental visualization (dye mixing and PTV) to support the validity of the instantaneous structures by DNS. Computational results for two types of inlet profiles suggested that nozzle conditions scarcely affect the turbulence statistics and the coherent structures in a jet‐established stage. Two‐point correlations of velocity and temperature show that similar distributions denoting the temperature can be used as an indicator of a vortex. A conceptual model of a hairpin‐shaped vortex was proposed and validated by two‐point correlations and PDF analysis for vortex alignment. The hairpin‐shaped vortex stands with legs inclined downstream. The inclination angle and the tilting angle between the two legs are ?45° and 40°, respectively. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 342–356, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20014     

Experimental study on flow characteristics of a sleeved jet into a main crossflow

Experiments were carried out on the hydraulic mechanism of the thermal shock caused by cold jet injection at a T‐junction with thermal sleeve in the reactor cooling system using digital particle imaging velocimetry (DPIV) technique to measure the flow in the main duct and in the annular space of the sleeve tube. The flow and vorticity characteristics were investigated at jet‐to‐crossflow velocity ratios of 0.5 to 4. There was a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which resulted in decreasing the influence of the jet on the downstream wall. The intensive vorticity in the near wake mainly originated from the shear layer vorticity of the jet and the annular discharge stream. The intensive vorticity soon broke down and dissipated, and further developed into the counterrotating vortex pair in the far wake. The flow in the annulus was closely dependent on R, and thermal protection of the sleeve would become evident at higher R. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 24–31, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10131     

Flow-field analysis of anti-kidney vortex film cooling

Film cooling is an important measure to enable an increase of the inlet temperature of a gas turbine and, thereby, to improve its overall efficiency. The coolant is ejected through spanwise rows of holes in the blades or endwalls to build up a film shielding the material. The holes often are inclined in the downstream direction and give rise to a kidney vortex. This is a counter-rotating vortex pair, with an upward flow direction between the two vortices, which tends to lift off the surface and to locally feed hot air towards the blade outside the pair. Reversing the rotational sense of the vortices reverses these two drawbacks into advantages. In the considered case, an anti-kidney vortex is generated using two subsequent rows of holes both inclined downstream and yawed spanwise with alternating angles. In a previous study, we performed large-eddy simulations (which focused on the fully turbulent boundary layer) of this anti-kidney vortex film-cooling and compared them to a corresponding physical experiment. The present work analyzes the simulated flow field in detail, beginning in the plenum (inside the blade or endwall) through the holes up to the mixture with the hot boundary layer. To identify the vortical structures found in the mean flow and in the instantaneous flow, we mostly use the λ 2 criterion and the line integral convolution (LIC) technique indicating sectional streamlines. The flow regions (coolant plenum, holes, and boundary layer) are studied subsequently and linked to each other. To track the anti-kidney vortex throughout the boundary layer, we propose two criteria which are based on vorticity and on LIC results. This enables us to associate the jet vortices with the cooling effectiveness at the wall, which is the key feature of film cooling.     

液体射流在旋转气流中的雾化机理研究

对旋转气流中的空心柱形液体射流的雾化过程进行了数值模拟。结果表明：Reynolds数Re、Weber数We、空心柱内部及外部气液密度比Q和Qh及喷嘴内半径与液膜厚度比Ah，在射流的雾化过程中均起不稳定性的作用；空心柱射流内部的气流旋转强度Ea在雾化过程中加强了自由面上的轴对称扰动和非轴对称扰动；空心柱射流外部的气流旋转强度Eb削弱了自由表面上的轴对称扰动强度，却加强了自由表面的非轴对称扰动。此外，Eb的增加，能大大增加扰动种类。