PIV measurements of a turbulent jet issuing from round sharp-edged plate

An experimental investigation is presented of a turbulent jet issuing from a round sharp-edged orifice plate (OP) into effectively unbounded surroundings. Planar measurements of velocity were conducted using Particle Image Velocimetry (PIV) in the near and transition regions. The Reynolds number, based on the jet initial diameter and velocity, is approximately 72,000. The instantaneous and mean velocities, Reynolds normal and shear stresses were obtained. The centerline velocity decay and the half-velocity radius were derived from the mean velocity. It is revealed that primary coherent structures occur in the near field of the OP jet and that they are typically distributed asymmetrically with respect to the nozzle axis. Comparison of the present PIV and previous hot-wire measurements for the OP jet suggests that high initial turbulence intensity leads to reduced rates of decay and spread of the mean flow field and moreover a lower rate of variation of the turbulence intensity. Results also show that self-similarity of the mean flow is well established from the transition region while the turbulent statistics are far from self-similar within the measured range to 16 diameters.     

Experimental study and large eddy simulation for the turbulent flow around four cylinders in an in-line square configuration

The turbulent flows around four cylinders in an in-line square configuration with different spacing ratios of 1.5, 2.5, 3.5 and 5.0 have been investigated experimentally at subcritical Reynolds numbers from 11,000 to 20,000. The mean and fluctuating velocity distributions were obtained using the laser Doppler anemometry (LDA) measurement. The digital particle image velocimetry (DPIV) was employed to characterize the full field vorticity and velocity distributions as well as other turbulent quantities. The experimental study indicated that several distinct flow patterns exist depending on the spacing ratio and subcritical Reynolds number for turbulent flow. The three-dimensional numerical simulations were also carried out using the large eddy simulation (LES) at Reynolds number of 15,000 with the spacing ratio of 1.5 and 3.5. The results show that the LES numerical predictions are in good agreement with the experimental measurements. Therefore, the three-dimensional vortex structures and the full field instantaneous and mean quantities of the flow field such as velocity field, vorticity field, etc., which are very difficult to obtain experimentally, can be extracted from the simulation results for the deepening of our understanding on the complex flow phenomena around four cylinders in in-line configuration.     

Simultaneous DPTV/PLIF measurements of a turbulent jet

Simultaneous measurements of instantaneous velocity and concentration fields were performed using digital particle tracking velocimetry and planar laser- induced fluorescence for a turbulent jet at a Reynolds number of 3000. The measurements of mean velocity, turbulent stresses, mean concentration, concentration variance, and turbulent flux of tracer all collapse onto self-similar profiles in the far field of the jet. The measurements showed excellent agreement with previous point velocity and concentration measurements. It is concluded that the system is an effective means of measuring the velocity and concentration distributions and turbulent characteristics. Received: 7 July 1999/Accepted: 9 February 2000     

Visualization and two-color DPIV measurements of flows in circular and square coaxial nozzles

 High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle image velocimetry (DPIV) and thermal anemometry measurements in flows generated by equivalent coaxial circular and square jets are presented. Visualization results were obtained for three square, coaxial configurations, and a reference circular coaxial nozzle, at two Reynolds numbers of the outer jet (19,000 and 29,000) and for inner-to-outer jet velocity ratios of 0.15, 0.22, and 0.3. These indicated that the internal unmixed region diminished with decreasing velocity ratio. Strong evidence of unsteady recirculation and back-flow was observed at the end of the core of the inner jet, for the low velocity ratios. Comparisons between circular and square jet configurations indicated considerable mixing enhancement when square nozzles were used. Low-coherence, organized large-scale structure was evident from the visualizations and DPIV measurements near the origin of the inner mixing-region shear layers, and more so in the core region of the near field. These observations were confirmed by velocity spectra, which displayed peaks corresponding to a free shear-layer instability mode in the inner mixing-region shear layers, and a wake-type mode in the core region where the mean flow has a wake-like character. Although some large-scale structure was observed in the outer mixing layer during the visualizations, this was found to be incoherent on the basis of the DPIV measurements and the velocity spectra. It is noted that no axis-switching phenomena were observed in the square nozzle flows examined here. This is attributed to the absence of an organized structure in the outer shear layer, which was initially highly turbulent, and the weakly coherent nature of the organized structure observed in the inner mixing-region near field. Received: 2 November 1998/Accepted: 8 September 2000     

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k ^?1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.     

Quantifying the dynamics of flow within a permeable bed using time-resolved endoscopic particle imaging velocimetry (EPIV)

This paper presents results of an experimental study investigating the mean and temporal evolution of flow within the pore space of a packed bed overlain by a free-surface flow. Data were collected by an endoscopic PIV (EPIV) technique. EPIV allows the instantaneous velocity field within the pore space to be quantified at a high spatio-temporal resolution, thus permitting investigation of the structure of turbulent subsurface flow produced by a high Reynolds number freestream flow (Re _s in the range 9.8?×?10³?C9.7?×?10⁴). Evolution of coherent flow structures within the pore space is shown to be driven by jet flow, with the interaction of this jet with the pore flow generating distinct coherent flow structures. The effects of freestream water depth, Reynolds and Froude numbers are investigated.     

Experimental scanning for simplifying the model of a stirred-tank flow

 Velocity measurements by two-component laser Doppler velocimetry (LDV) and laser sheet visualizations were used to analyze the 3D turbulent flow field in a stirred tank, for the purpose of reducing the intricacy of the hydrodynamics to more simple models. We first deal with the mean flow field measured, and the instantaneous visualized patterns, to appreciate the deviation from steadiness and predict the dispersion of particles from the mean motion alone. The study then focuses on the discharge flow from the agitator (Rushton turbine). The jet at the exit from the turbine is modeled in mean values. The flow instability as shown experimentally indicates that the Reynolds number plays no part in this phenomenon. A study based on dynamical system theory, considering a frequency modulation of the flow, shows the possibility of describing rather simply the deterministic component in the velocity fluctuation due to the blades. Received: 29 July 1997/Accepted: 11 November 1997     

Use of simultaneous IR temperature measurements and DPIV to investigate thermal plumes in a thick layer cooled from above

 Simultaneous measurements of surface temperature and the underlying velocity field are presented for a thick horizontal layer evaporatively cooled from above. Previous studies have focused on either the temperature field at the cooled surface or, in a small number of cases, on point velocity measurements in the flow. The current investigation is, to the knowledge of the authors, the first to simultaneously and non-intrusively document both the surface temperature and underlying velocity field in this type of flow. An infrared (IR) sensing array was used to capture the instantaneous free surface temperature field while two-dimensional velocity measurements in planes either perpendicular or parallel to the free surface were acquired using digital particle image velocimetry (DPIV). Data from two cases are discussed. Received: 19 January 1998/Accepted: 22 October 1998     

Direct Numerical Simulation of Subsonic Round Turbulent Jet

Direct numerical simulation(DNS) of spatially developing round turbulent jet flow with Reynolds number 4,700 was carried out. Over 20 million grid points were used in this simulation. Fully compressible three-dimensional Navier–Stokes equations were solved. High order explicit spatial difference schemes and Runge–Kutta time integration scheme were used to calculate derivatives and time marching, respectively. Non-reflecting boundary conditions and exit zone techniques were adopted. Some refined computational grids were used in order to capture the smallest turbulent structures near the centerline of the jet. Low level disturbance were imposed on the jet inflow velocity to trigger the developing of turbulence. Turbulent statistics such as mean velocity, Reynolds stresses, third order velocity moments were obtained and compared with experimental data. One-dimensional velocity autospectra was also calculated. The inertial region where the spectra decays according to the k^− 5/3 was observed. The quantitative profiles of mean velocity and all of the third order velocity moments which were difficult to measure via experimental techniques were presented here in detail. The jet flow was proven to be close to fully self-similar around 19 jet diameters downstream of jet exit. The statistic data and revealed flow feature obtained in this paper can provide valuable reference for round turbulent jet research.     

Experimental investigation of the unsteady structure of a transitional plane wall jet

 A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional structure in the free shear layer initiates complete breakdown of the flow. Received: 8 November 1995/Accepted: 6 November 1996     

The near-field characteristics of circular jets at low Reynolds numbers

An experimental study has been undertaken to investigate the effect of Reynolds number on the near-field region of circular turbulent air jets. Measurements were made using a two-component Laser Doppler Anemometer, and included mean velocity, turbulence intensity, skewness factor, flatness factors and power spectrum. Measurements were taken up to 10 nozzle exit diameter in the downstream direction for different exit Reynolds numbers in the range of 1400 to 20000. The Reynolds number was found to have a strong effect on the jet flow behavior in the near-field region; the centerline velocity decays faster (decay constant = 6.11 for Re = 19400, = 1.35 for Re 1430) and the potential core gets shorter with decreasing Reynolds number. Profile measurements of the skewness and flatness factors indicate that the jet flow becomes more intermittent with decreasing Reynolds number. Power spectrum measurements of the streamwise fluctuating velocities reflects the high energy content of the high Reynolds number jet. It also reveals that there is greater energy at the higher frequencies with increasing Reynolds number.     

Measurements in the near flow field of an isosceles triangular turbulent free jet

The near field mean flow and turbulence characteristics of a turbulent jet of air issuing from a sharp-edged isosceles triangular orifice into still air surroundings have been examined experimentally using hot-wire anemometry and a pitot-static tube. For comparison, some measurements were made in an equilateral triangular free jet and in a round free air jet, both of which also issued from sharp-edged orifices. The Reynolds number, based on the orifice equivalent diameter, was 1.84×10⁵ in each jet. The three components of the mean velocity vector, the Reynolds normal and primary shear stresses, the one-dimensional energy spectra of the streamwise fluctuating velocity signals and the mean static pressure were measured. The mean streamwise vorticity, the half-velocity widths, the turbulence kinetic energy and the local shear in the mean streamwise velocity were obtained from the measured data. It was found that near field mixing in the equilateral triangular jet is faster than in the isosceles triangular and round jets. The mean streamwise vorticity field was found to be dominated by counter-rotating pairs of vortices, which influenced mixing and entrainment in the isosceles triangular jet. The one-dimensional energy spectra results indicated the presence of coherent structures in the near field of all three jets and that the equilateral triangular jet was more energetic than the isosceles triangular and round jets.     

Reynolds number effects on the behavior of a non-buoyant round jet

The behavior of a non-buoyant circular water jet discharged from a contraction nozzle was experimentally investigated. In this experiment, the Reynolds number of the jet, based on the mean velocity results obtained by particle image velocimetry (PIV), ranged from 177 to 5,142. From the experimental results, we found that the cross-sectional profile of the axial velocity for a laminar flow near the nozzle did not show a top-hat distribution, whereas the profiles with Reynolds number higher than 437 were almost top-hat. The length of the zone of flow establishment (ZFE) was found to decrease with increasing Reynolds number. The measured centerline velocity decayed more rapidly and, consequently, approached the theoretical equation earlier near the nozzle as the Reynolds number increased. The decay constant for the centerline velocity of the turbulent cases was relatively lower than that discovered in theory. It is assumed that this probably resulted from the use of the contraction nozzle. Verifying the similarity of the lateral velocity profiles demonstrated that the Gaussian curve was properly approximated only for the turbulent jets and not for the laminar or transitional flows. The jet half width seldom grew for the laminar or transitional flows, whereas it grew with increasing axial distance for the turbulent flows. The spreading rates for the turbulent flows gradually decreased with increasing Reynolds number. The normalized turbulence intensity along the jet centerline increased more rapidly with the axial distance as the Reynolds number increased, and tended to the constant values proposed by previous investigators. The Reynolds shear stress levels were also found to increase as the Reynolds number increased for the turbulent jets.     

Comparison of magnetic resonance concentration measurements in water to temperature measurements in compressible air flows

Magnetic resonance imaging (MRI) measurements in liquid flows provide highly detailed 3D mean velocity and concentration data in complex turbulent mixing flow applications. The scalar transport analogy is applied to infer the mean temperature distribution in high speed gas flows directly from the MRI concentration measurements in liquid. Compressibility effects on turbulent mixing are known to be weak for simple flows at high subsonic Mach number, and it was not known if this would hold in more complex flows characteristic of practical applications. Furthermore, the MRI measurements are often done at lower Reynolds number than the compressible application, although both are generally done in fully turbulent flows. The hypothesis is that the conclusions from MRI measurements performed in water are transferable to high subsonic Mach number applications. The present experiment is designed to compare stagnation temperature measurements in high speed airflow (M = 0.7) to concentration measurements in an identical water flow apparatus. The flow configuration was a low aspect ratio wall jet with a thick splitter plate producing a 3D complex downstream flow mixing the wall-jet fluid with the mainstream flow. The three-dimensional velocity field is documented using magnetic resonance velocimetry in the water experiment, and the mixing is quantified by measuring the mean concentration distribution of wall-jet fluid marked with dissolved copper sulfate. The airflow experiments are operated with a temperature difference between the main stream and the wall jet. Profiles of the stagnation temperature are measured with a shielded thermocouple probe. The results show excellent agreement between normalized temperature and concentration profiles after correction of the temperature measurements for the effects of energy separation. The agreement is within 1 % near the edges of the mixing layer, which suggests that the mixing characteristics of the large scale turbulence structures are the same in the two flows.     

Acoustic Lagrangian velocity measurement in a turbulent air jet

Measuring Lagrangian velocities in a turbulent flow is of a great interest for turbulence modeling. We report measurements made in an axisymmetric turbulent air jet at Reynolds number R _λ ≃ 320, using acoustical Doppler scattering. Helium-filled soap bubbles are used as Lagrangian tracers. We describe an experimental setup which allows the simultaneous measurement of the full three-component Lagrangian velocity and the longitudinal Eulerian one. Lagrangian velocity probability density functions (PDF) are found Gaussian, close to Eulerian ones. Velocity correlations are analysed as well as the statistical dependence between components.     

Review of Experimental Data on Incompressible Turbulent Round Jets

This article reviews measurements for the canonical flow: incompressible turbulent round jets issuing into a large, ideally infinite, quiescent domain. The available far-field data on mean velocity, momentum-flux conservation, Reynolds stresses and triple fluctuation correlations are presented. The budget equations for turbulent kinetic energy and for the individual Reynolds stresses are shown, including different formulations for dissipation and pressure-velocity correlations. Evidence of the persistence of source conditions in the self-preserving far field is observed in several cases. This article analyses and compares several data sets, obtained at different levels of detail, against analytical constraints, discusses limitations and provides insights aided by reference to recent numerical work. It is hoped that this will prove useful in guiding future experiments and numerical test cases, where complete details of the jet configuration are required for validation and comparison.     

Digital particle image velocimetry/thermometry and application to the wake of a heated circular cylinder

 Digital particle image velocimetry/thermometry (DPIV/T) is a technique whereby the velocity and temperature fields are obtained using thermochromic liquid crystal (TLC) seeding particles in water. In this paper, the uncertainty levels associated with temperature and velocity measurements using DPIV/T are studied. The study shows that large uncertainties are encountered when the temperature is measured from individual TLC particles. Therefore, an averaging procedure is presented which can reduce the temperature uncertainties. The uncertainty is reduced by computing the average temperature of the particles within the common specified sampling window used for standard DPIV. Using this procedure, the velocity and temperature distributions of an unsteady wake behind a heated circular cylinder are measured experimentally at Re=610. The instantaneous DPIV/T measurements are shown to be useful for computing statistical flow quantities, such as mean and velocity-temperature correlations. Received: 3 January 2000/Accepted: 26 June 2000     

Velocity field and surfactant concentration measurement techniques for free-surface flows

 Noninvasive measurement techniques were developed to obtain the surfactant concentration and the velocity field of a vortex pair interacting with a contaminated free surface. The optical method of second-harmonic generation (SHG) was utilized to determine the time-varying concentration of a surfactant at a point on the surface, and the established technique of digital particle image velocimetry (DPIV) was used for the measurement of the velocity field. The evolution of the initially uniform, insoluble monolayer along with the vorticity field are described. One of the present limitations is the temporal resolution of the concentration measurements which is of order 5 Hz. The extension of the SHG technique to high Reynolds number, turbulent flows is discussed. Received: 24 January 1996 / Accepted: 17 July 1996     

PIV–LES analysis of channel flow rotating about the streamwise axis

The flow field of a channel rotating about the streamwise axis is analyzed experimentally and numerically. The current investigations were carried out at a bulk velocity based Reynolds number of Re_m = 2850 and a friction velocity based Reynolds number of Re_τ = 180, respectively. Particle-image velocimetry (PIV) measurements are compared with large-eddy simulation data to show earlier direct numerical simulation findings to generate too large a reverse flow region in the center region of the spanwise flow. The development of the mean spanwise velocity distribution and the influence of the rotation on the turbulent properties, i.e., the Reynolds stresses and the two-point correlations of the flow, are confirmed in both investigations. The rotation primarily influences those components of the Reynolds shear stresses, which contain the spanwise velocity component. The size of the correlation areas and thus the length scales of the flow generally grow in all three coordinate directions leading to longer structures. Furthermore, experimental results of the same channel flow at a significantly lower bulk Reynolds number of Re_{m, l} = 665, i.e., a laminar flow in a non-rotating channel, are introduced. The experiments show the low Reynolds number flow to become turbulent under rotation and to develop the same characteristics as the high Reynolds number flow.     

Numerical investigation of turbulent free jet flows issuing from rectangular nozzles: the influence of small aspect ratio

In this research, the fluid and thermal characteristics of a rectangular turbulent jet flow is studied numerically. The results of three-dimensional jet issued from a rectangular nozzle are presented. A numerical method employing control volume approach with collocated grid arrangement was employed. Velocity and pressure fields are coupled with SIMPLEC algorithm. The turbulent stresses are approximated using k–e{\varepsilon} model with two different inlet conditions. The velocity and temperature fields are presented and the rates of their decay at the jet centerline are noted. The velocity vectors of the main flow and the secondary flow are illustrated. Also, effect of aspect ratio on mixing in rectangular cross-section jets is considered. The aspect ratios that were considered for this work were 1:1 to 1:4. The results showed that the jet entrains more with smaller AR. Special attention has been drawn to the influence of the Reynolds number (based on hydraulic diameter) as well as the inflow conditions on the evolution of the rectangular jet. An influence on the jet evolution is found for smaller Re, but the jet is close to a converged state for higher Reynolds numbers. The inflow conditions have considerable influence on the jet characteristics.