Effect of noncircular orifice plates on the near flow field of turbulent free jets

In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters（De） and their aspect ratios（AR） range from 1 to 6.5. Planar particle image velocimetry（PIV） is used to measure the velocity field at the same Reynolds number of Re = 5 × 10^4,where Re = Ue De/ν with Ue being the exit bulk velocity and ν the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core,expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet（AR = 6.5） produces the greatest decay rate of the near-field mean velocity, postpones the position at which the 鈥榓xis-switching鈥檖henomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.     

Investigation into higher-order mode propagation through orifice plates in circular ducts

Most established techniques for analyzing sound transmission in ducts containing orifices plates are only applicable for plane wave propagation. Once the wavelength of the sound approaches the cross section of the duct, higher order mode propagation in the system must be considered in the analysis. This is a numerically intensive activity if fully coupled calculations of the higher order modes are undertaken. This investigation estimates the acoustic fields in a duct with a simple orifice plate installed using an uncoupled model to estimate the higher order mode contribution. The uncoupled model is then used as the basis for a hybrid decomposition approach to estimate the sound field in the regions before and after the orifice plate installed in a circular duct. This approach is applied to a duct, excited by a point source over a wide frequency range, containing a single orifice plate installed a distance inside the duct. Different orifice plates with one, two and multiple openings are investigated. Of particular interest is the location of the point source relative to the duct axis. If the source is located concentric to the duct axis then, without any orifice plate present, only axially symmetric higher order modes may be excited in the duct. Thus, the investigation considers the point source located in the concentric position and in eccentric positions to vary the contribution from the different types of higher order mode. Estimates of the acoustic fields in the duct obtained using the hybrid decomposition approach are compared with measured data and the applicability of using an uncoupled estimate for the acoustic fields is commented on.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

Characterisation of the Eulerian and Lagrangian accelerations in the intermediate field of turbulent circular jets

A rich data-set of Lagrangian trajectories from 3D particle tracking velocimetry is used to study the structure of various acceleration components, vorticity, and strain in the intermediate field of a circular jet at Reynolds number Re = 6000. The total acceleration is decomposed into three distinctive sets: (1) streamwise–radial; (2) tangential–normal; and (3) local–convective components. Probability density function (PDF) and joint distributions of each set are characterised at various radial locations from the jet core within a streamwise band 16 ≤ x/d_h ≤ 17, where d_h is the diameter of the pipe. The PDF of the relative angle between the acceleration components and the velocity vector is also included to aid the characterisation. Results show that the acceleration components are described by two distinctive distributions: one of them exhibits symmetry and heavy tails, while the other is best fitted by a power-law type. The tails of acceleration PDFs are heavier with larger radial distance from the core. The increased departure from the Gaussian distribution with the distance from the core is a result of the increasing turbulence levels promoted by the mean shear. The variation of the third and fourth moments between the streamwise–tangential and the radial–normal accelerations indicate the anisotropy of the jet. Joint PDF of each acceleration decomposition exhibits distinctive distribution that appears to depend from the distance from the jet core. However, the vorticity and strain show similar PDF across radial distances. Finally, complementary analysis of a jet from a semicircular pipe shows the footprint of the nozzle geometry in the acceleration structure of jets.     

Spatial resolution analysis of planar PIV measurements to characterise vortices in turbulent flows

The effects of spatial resolution of planar particle image velocimetry (PIV) on vortex size, swirling strength, circulation and population density characterisation are analysed using a series of experimental and numerical databases. The databases comprise a PIV database of an adverse-pressure-gradient turbulent boundary layer (APG TBL), a PIV database of a zero-pressure-gradient (ZPG) TBL in streamwise-wall-normal planes and streamwise-wall-normal slices of a direct numerical simulation (DNS) of a ZPG TBL. The effects of interrogation window and mesh sizes on the vortex parameters are analysed in the outer region of these flows using different qualitative and quantitative approaches. The quantitative analysis mainly capitalises on the possibility of mimicking the PIV data-sets with the DNS one. These approaches allow us to not only isolate the effects of mesh size and the interrogation window size but also to deduce the combined effects of other measurement errors in PIV. Typical values of mesh size and interrogation window size (0.01–0.03 of the boundary layer thickness) and typical levels of measurement uncertainties have significant effects on the vortex parameters. Moreover, each PIV error source affects the vortex parameters in different and frequently opposite manners. Hence, an optimal selection of measurement parameters such as the interrogation window size is indispensable in order to minimise the effects of spatial resolution and other measurement errors on the vortex parameters. Guidelines are presented in the Conclusions section of this paper. Finally, it is found that all the vortex parameters, when averaged across the outer region, are reasonably comparable in the ZPG and APG TBLs despite the fact that these are very different flows.     

Dynamic PIV measurement of a high-speed flow issuing from vent-holes of a curtain-type airbag

A curtain-type airbag is a safety device designed to protect passengers from the side collisions of a car. The curtain-type airbag system consists of an inflator, a fill-hose, and a curtain-airbag. The fill-hose is a passageway and distributor of the exploded gases from the inflator to the airbag through vent-holes. Although the design of vent-holes is important for proper deployment of the airbag, it is very difficult to measure the exceedingly high speed flow issuing from the vent-holes by using conventional measurement methods. In this study, we employed a dynamic PIV technique to measure the temporal evolution of instantaneous velocity fields of the flow ejecting from the vent-holes. From the velocity field data measured at a frame rate of 2000 fps, the temporal variation of the volume flux from vent-holes was also evaluated for the diagnosis of airbag performance. The flows ejecting from the vent-holes showed high velocity fluctuations, and the maximum velocity was about 480 m/s. The instantaneous velocity fields in the initial stage showed a swaying motion of a high-speed jet. The accumulated volume flux from the vent-holes was also compared at each vent-hole region.     

空气湍射流速度时间序列的最大Lyapunov指数以及湍流脉动

用热线风速仪采集了圆喷嘴空气射流的速度时间序列,并采用一种基于最大Lyapunov指数不变性的混沌时间序列分析方法,计算了出口雷诺数在939≤Re≤3758范围内的速度信号的最大Lyapunov指数以及湍流的非拟序脉动.结果表明,最大Lyapunov指数随着雷诺数的增加而增大,随着离开喷嘴出口距离的增加而减小,而且最大Lyapunov指数的倒数与关联时间是正相关的.湍流的非拟序脉动随着雷诺数的增加以及随着离开喷嘴出口距离的增加均是逐渐增大的,而且湍流的非拟序脉动与Kolmogorov尺度是负相关的.     

Numerical model of round turbulent jets

Using a second-order semi-empirical turbulence model that includes the differential equations for the normal Reynolds stresses transport, numerical simulation of the flow in round turbulent jets is performed. The calculation results are in good satisfactory accordance with the experimental data. The results of numerical analysis of the self-similar degeneration are presented.     

Recording of PIV images on photothermoplastic plates

In this paper we explore the possibility of utilizing commercially available photothermoplastic plates as recording support in Particle Image Velocimetry (PIV), to allow real time analysis of recorded data. An experimental apparatus has been developed which allows holographic recording and reconstruction of simulated PIV images. We show that double exposed photothermoplastic plates can successfully be analysed provided that a suitable recording/reconstruction scheme is used.     

Vortex dynamics and entrainment mechanisms in low Reynolds orifice jets

Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.     

3C PIV and PLIF measurement in turbulent mixing

The present work focuses on the measurements of instantaneous concentration fields of a passive scalar due to an impinging round jet injection into a liquid filled rectangular tank. Simultaneous measurements of velocity and passive scalar concentration fields have been conducted by using Particle Image Velocimetry (planar 2C and 3C PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. The mixing injection behavior is analyzed for several injection values of depth and flow rate. Results showed the classical developing and self-similar regions of the jet, the mixing layer and the coupled concentration and velocity fields due to impingement. Finally, 3C PIV reveals a 3D flow jet structure which seems to be a swirl that does not disturb 2D analysis.     

Helical modes in low- and high-swirl jets measured by tomographic PIV

We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.     

Flow and mixing fields of turbulent bluff-body jets and flames

The mean structure of turbulent bluff-body jets and flames is presented. Measurements of the flow and mixing fields are compared with predictions made using standard turbulence models. It is found that two vortices exist in the recirculation zone; an outer vortex close to the air coflow and an inner vortex between the outer vortex and the jet. The inner vortex is found to shift downstream with increasing jet momentum flux relative to the coflow momentum flux and gradually loses its circulation pattern. The momentum flux ratio of the jet to the coflow in isothermal flows is found to be the only scaling parameter for the flow field structure. Three mixing layers are identified in the recirculation zone. Numerical simulations using the standard k-? and Reynolds stress turbulence models underpredict the length of the recirculation zone. A simple modification to the C₁ constant in the dissipation transport equation fixes this deficiency and gives better predictions of the flow and mixing fields. The mixed-is-burnt combustion model is found to be adequate for simulating the temperature and mixing field in the recirculation zone of the bluff-body flames.     

Large eddy simulation of turbulent horizontal buoyant jets

Large eddy simulations (LESs) of turbulent horizontal buoyant jets are carried out using a high-order numerical method and Sigma subgrid-scale (SGS) eddy-viscosity model, for a number of different Reynolds (Re) and Richardson (Ri) numbers. Simulations at previous experimental flow conditions (Re = 3200, 24, 000 and Ri = 0, 0.01) are carried out first, and the results are found to be qualitatively and quantitatively similar to the experimental results, thus validating the numerical methodology. The effect of varying Ri (values 2×10^?4, 0.001, 0.005, and 0.01) and Re (3200 and 24, 000) is studied next. The presence of stable stratification on one side and unstable stratification on the other side of the jet centreline leads to an asymmetric development of horizontal buoyant jets. It is found that this asymmetry, the total radial spread and the vertical deflection are significantly affected by Ri, while Re affects only the radial asymmetry. The need for developing improved integral models, accounting for this asymmetry, is pointed out. Turbulent production and dissipation rates are investigated, and are found to be symmetric in the horizontal plane, but asymmetric in the mid-vertical plane. A previously proposed model, for correlation between the vertical component of the fluctuating scalar flux vector and the vertical cross-correlation component of the Reynolds tensor, is modified based on the current LES results. Instantaneous scalar and velocity fields are analysed to reveal the structure of horizontal buoyant jets. Similar to the developed turbulent jet, the flow close to the nozzle too is found to be markedly different in the stable and unstable stratification regions. Persistent coherent vortex rings are found in the stable stratification region, while intermittent breakdown of vortex rings into small-scale structures is observed in the unstable stratification region. Similarities and differences between the flow structures in the horizontal buoyant jet configuration and those in the jet in crossflow configuration are discussed. Finally, a dynamic mode decomposition analysis is carried out, which indicates that the flow in the unstable stratification region is more energetic and prone to instabilities, as compared to the flow in the stable stratification region.