The development of an axisymmetric curved turbulent wall jet

An experimental study has been carried out of the low speed Coanda wall jet with both streamwise and axisymmetric curvature. A single component laser Doppler technique was used, and by taking several orientations at a given point, values of the three mean velocities and five of the six Reynolds stresses were obtained. The lateral divergence and convex streamwise curvature both enhanced the turbulence in the outer part of the jet compared with a plane two-dimensional wall jet. The inner layer exhibited a large separation of the positions of maximum velocity and zero shear stress. It was found that the streamwise mean velocity profile became established very rapidly downstream of the slot exit. The profile appeared fairly similar at later downstream positions, but the mean radial velocity and turbulence parameters showed the expected nonself preservation of the flow. Removal of the streamwise curvature resulted in a general return of the jet conditions toward those expected of a plane wall jet. The range and accuracy of the data may be used for developing turbulence models and computational techniques for this type of flow.     

Moderate Reynolds number axisymmetric jet development downstream an extended conical diffuser: Influence of extension length

Low-velocity (bulk velocity of 4.4 m/s) and moderate Reynolds (7350) axisymmetrical jet development is studied by hot-film single sensor anemometry. The jet issues from a conical convergent-divergent diffuser with uniform extension (diameter 25 mm). Decreasing the length-to-diameter ratio of the extension tube from 20 down to 0.4 is shown to alter severely the mean velocity profile at the tube outlet from Blasius to top-hat whereas turbulence intensities increases from 6 up to 50%. Next, the influence of the initial velocity profile at the tube outlet on axisymmetrical jet development is assessed. The velocity development exhibits a self-similar far field characteristic for axisymmetrical jet development. Although, the jet centerline decay constant increases and the jet spreading rate decreases as length-to-diameter ratios are increased from 0.4 up to 7.2 for which the initial centerline velocity decreases. Therefore, scaling of the centerline decay constant and inverse scaling of the spreading rate with initial centerline velocity U₀ or initial velocity Reynolds number Re₀ reported for moderate Reynolds numbers and low initial turbulence levels [18,22] does not hold as the turbulence level exceeds a threshold value in the range 12–27%. In addition, the influence of initial conditions on near and far field turbulence properties is shown. A transition in near field behaviour is observed for length-to-diameters around 3.6. Flow and geometrical configurations under study are relevant to e.g. upper airway flow.     

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.     

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.     

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.     

Effect of particle size on a two-phase turbulent jet

The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 10⁴, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system.     

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.     

Numerical study of hysteresis in annular swirling jets with a stepped‐conical nozzle

This study investigates the experimentally observed hysteresis in the mean flow field of an annular swirling jet with a stepped‐conical nozzle. The flow is simulated using the Reynolds‐averaged Navier–Stokes (RANS) approach for incompressible flow with a k–ε and a Reynolds stress transport (RSTM) turbulence model. Four different flow structures are observed depending on the swirl number: ‘closed jet flow’, ‘open jet flow low swirl’, ‘open jet flow high swirl’ and ‘coanda jet flow’. These flow patterns change with varying swirl number and hysteresis at low and intermediate swirl numbers is revealed when increasing and subsequently decreasing the swirl. The influence of the inlet velocity profile on the transitional swirl numbers is investigated. When comparing computational fluid dynamics with experiments, the results show that both turbulence models predict the four different flow structures and the associated hysteresis and multiple solutions at low and intermediate swirl numbers. Therefore, a good agreement exists between experiments and numerics. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of drag-reducing polymers on turbulence: effects of Reynolds number, concentration and mixing

 Measurements of turbulence properties of solutions of polymers have been made over a large range of drag-reduction, in a fully-developed channel flow. At flows close to maximum drag-reduction the Reynolds stresses were approximately zero over the whole cross section of the channel. Added mean polymer stresses were observed in the viscous wall region for small drag-reduction and over the whole cross-section for large drag-reduction. Even though the Reynolds stresses are zero, the velocity profile is not parabolic because of the presence of these mean stresses. We interpret the results by arguing that the interaction of turbulence with the polymers introduces mean and fluctuating polymer stresses which can create turbulence. The observation that the turbulence modification depends on the manner by which the polymers are introduced into the flow supports the notion that the polymers need to form aggregates in order to be effective. Received: 29 September 1998 / Accepted: 10 February 1999     

Turbulent characteristics of shear-thinning fluids in recirculating flows

 A miniaturised fibre optic Laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1–0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present downstream turbulence field was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers. Received: 23 February 1999/Accepted: 28 April 1999     

On the effect of inflow conditions in simulation of a turbulent round jet

This paper investigates the impact of the inflow conditions on simulations of a round jet discharging from a wall into a large space. The fluid dynamic characteristics of a round jet are studied numerically. A numerical method based on the control volume approach with collocated grid arrangement is employed. The k-e{k-\varepsilon} model is utilized to approximate turbulent stresses by considering six different inlet conditions. The velocity field is presented, and the rate of decay at the jet centerline is determined. The results showed that inflow conditions had a strong influence on the jet characteristics. This paper also investigates both sharp-edged and contoured nozzles. The effects of velocity, turbulence intensity, turbulence kinetic energy, and turbulence dissipation rate on flow field characteristics are examined. Results showed that the present simulations in both types of nozzles are in good agreement with experiments when considering the appropriate inflow conditions.     

Investigation of the microstructure a turbulent jet in a wake

The study of the characteristics of the turbulence in the boundary layer and in free jets is one of the most important problems of the aerodynamics of viscous fluids. The accumulation of information on the pulsation characteristics of jet flows and the establishment of the corresponding governing laws may serve to verify the basic hypotheses of the semiempirical theories of turbulence, and also for the development of more advanced computational methods. In many cases the measurement of the pulsation characteristics of turbulent jets is of practical interest.The studies made up till now [1–5] of the microstructure of turbulent flow in the primary region of submerged axisymmetric jets have made it possible to obtain several interesting results. In particular, in addition to the average velocity profiles, hot-wire anemometric equipment has been used to measure the normal and tangential Reynolds stresses and also the intermittency factor in cross sections of the jet, the distribution of the intensity of the longitudinal and lateral velocity pulsations along the axis, the correlation coefficients and the corresponding integral turbulence scales, etc. These measurements have made it possible to draw several important conclusions on the mechanism of turbulent exchange, on the order of the terms omitted in the equation of motion, and on the semiempirical theories of turbulence [6–9].The common deficiency of the studies mentioned above is that near the boundary of a submerged jet, where the average velocity is practically equal to zero, the intensity of the pulsations is so great that it makes the reliability of the results obtained by means of the hotwire anemometer questionable. In this connection Townsend [6] indicated the advisability of studying the microstructure of a turbulent jet issuing into a low-velocity ambient flow.The present study had as its objective the investigation of the microstructure of the primary region of an axisymmetric jet in a wake flow over quite a broad range of the flow ratio parameter m=u/u₀;here u₀ is the average velocity at the nozzle exit, u is the velocity of the ambient stream. For various values of the parameter m in the primary region of the jet measurements were made of the profiles of the three components of the pulsation velocity and the Reynolds shear stresses, and also the values of the average velocity and two components of the pulsation velocity at a large number of points on the jet axis. The measured profiles of the Reynolds shear stresses were compared with the corresponding profiles calculated on the basis of the boundary layer equations from the experimentally determined average velocity profiles. For two values of the parameter m, in one of the sections of the jet measurements were made of the correlation coefficients of the longitudinal components of the pulsation velocity and the variation across the jet of the integral turbulence scale was determined.The results obtained give an idea of the influence of the parameter m on the characteristics of the turbulent jet in an ambient stream.     

A numerical study of a rectangular jet along a shear free boundary: Surface jet

A numerical simulation of a rectangular surface jet is performed at a Reynolds number of ${\mathit{Re}}_j=4400$. The global parameters of the jet e.g. maximum velocity decay, jet surface normal and lateral spread rates, entrainment, jet momentum flux and turbulent momentum flux are in agreement with several other studies reported in the literature. It is shown that the mean velocity and Reynolds stress profiles scale with the maximum local streamwise velocity and jet half width in the surface normal and lateral directions. The current simulation provides balance, explicitly calculated budgets for the turbulence kinetic energy, Reynolds normal and shear stresses. The surface jet develops a thin layer of fast moving fluid in the lateral direction near the surface. This layer is called the ‘surface current’. It has been suggested that the surface current arises due to the Reynolds stress anisotropy in the near surface region. The current study shows that this explanation is incomplete. The turbulence production for the Reynolds stress in the lateral direction is negative, which can drive the mean flow in the lateral direction. The higher level of negative production in the near surface region is responsible for the development of the surface current.     

Experimental investigation on a turbulence generation system with high-blockage plates

An experimental study was conducted to develop and characterize systematically a new turbulence generator system to yield large turbulent Reynolds numbers in a compact configuration. The effect of the geometric parameters of two families of high-blockage plates on the resulting turbulent flow field was systematically studied: one series of plates was characterized by the number and distribution of circular openings; a second series had non-circular opening(s) with different shapes, distribution and position of the opening(s). The plates were placed upstream of a contoured contraction and the near field at the centerline of the resulting turbulent free jet was characterized by hot-wire anemometry in terms of mean axial velocity, turbulence intensity, turbulence length scales and corresponding Reynolds numbers. The plate with a central, non-circular opening produced the best compromise of highest turbulence levels along with excellent uniformity in average velocity and turbulence intensity, as evidenced by scan in the transverse direction. It appears to be the most promising one. By comparison with more traditional approaches to turbulence generation, we increased the turbulent Reynolds numbers based on the integral length scale to values on the order of 1000, which was one of the design objectives. Other plate geometries also yielded intense turbulence, but, in some cases, exhibited spurious frequency peaks in their power spectrum. The turbulent generation approach is to be adapted to combustion studies to reproduce conditions typical of practical system in relatively small experimental set-ups that are well-suited for bench-top experiments.     

A laboratory study of irregular shoaling waves

The present research aims to investigate the dynamics of a single laboratory irregular wave, characterized by a narrow-banded spectrum and developing on a sloping sand bottom, in intermediate waters up to the surf zone. Experiments focused on the wave shoaling region, in order to examine how the wave is affected by breaking induced turbulence offshore the surf zone. A 3D acoustic Doppler velocimeter was used to measure the three wave velocity components, which were all processed to evaluate the time-averaged vertical distributions of orbital velocities, wave and turbulent Reynolds shear stresses and turbulent intensities. The vertical distributions of the phase-averaged velocity components, turbulent kinetic energy and transport of turbulence were also analysed. The adopted phase-averaging technique was applied to each investigated measurement point. Therefore, the crucial element of the study is that all the analysed values derive directly from real measurements and are not approximated by any kind of interpolation. The study confirmed some dynamic behaviour in the shoaling zone already known in the literature, such as the typical cell-type flow pattern of the mean flow and the necessity to evaluate the turbulent kinetic energy with all the three velocity components, when available, which would otherwise be underestimated. Referring to the time-averaged wave and Reynolds shear stresses, a contribution was added to the open debate on their order of magnitude. The measured wave Reynolds shear stresses were also compared with the results of the model by Zou et al. (J Geophys Res 111:C09032, 2006), confirming the behaviour typical of dissipative breaking waves. The analysis of turbulence transport in the shoaling zone revealed that it is seaward directed close to the surface and landward directed close to the bottom. The results presented in the paper can be extended only to other analogous flow conditions.     

Influence of the initial conditions on axisymmetric jets in a parallel flow

An experimental investigation was made of the initial-section flow of axisymmetric helium, air, and freon-12 jets in a parallel air flow for two different velocity profiles at the nozzle exit near the boundary of the jet. In one case, the velocity profile was determined by boundary layers on the nozzle walls; in the other case, it was produced artificially by means of a honeycomb of tubes of variable length. Measurements were made of the profiles of the mean and the pulsation velocity and the temperature. The flow was also photographed. The investigations showed that, depending on the initial conditions, the intensity of mixing of the jets in the initial section at Reynolds numbers Re 10⁴ (calculated using the jet diameter) can change from the level determined by molecular diffusion to the level characteristic of developed turbulent flow. The flow structure in the annular mixing layer also depends strongly on the initial conditions. The observed ordered structures in the mixing layer are related to a section of development of perturbations near the nozzle. The ordered structures are strongly influenced by the effect on the jet of acoustic vibrations from an external source. When the initial velocity profile is produced by the honeycomb, the transition to developed turbulence may be due to the development of long-wavelength perturbations or to the development of small-scale turbulence generated by the flow over the end of the honeycomb.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 18–24, July–August, 1980.We thank V. M. levlev and K. I. Artamonov for assistance and for discussing the work.     

Turbulent structure during transition to self-similarity in a round jet

 The developing turbulent region of a round jet was investigated using an improved implementation of digital particle image velocimetry (DPIV). The two-dimensional flow field in planes normal and parallel to the axial velocity was measured at locations between 15 and 30 diameters downstream, for two Reynolds numbers of 5500 and 16,000. The study consisted of instantaneous snapshots of the velocity and vorticity fields as well as measurements of velocity correlations up to third order. In this regime, the Reynolds number had a significant effect on both the instantaneous flow structure and the profiles of mean velocity across the jet. Coherent streamwise structures were present in the jet core for the lower Reynolds number. Additional structures whose evolution was governed by time scales two orders of magnitude larger than the convective scale inside the jet were observed in the entrainment field. The velocity correlations provided further support for the validity of DPIV turbulence measurements. The data was consistent with the equations of motion and momentum was conserved. DPIV measurements of turbulent kinetic energy components agreed with the hot-wire measurements of previous studies. Received: 27 November 1996/Accepted: 14 July 1997     

Numerical simulation of turbulent impinging jet on a rotating disk

The calculations of quasi‐three‐dimensional momentum equations were carried out to study the influence of wall rotation on the characteristics of an impinging jet. The pressure coefficient, the mean velocity distributions and the components of Reynolds stress are calculated. The flow is assumed to be steady, incompressible and turbulent. The finite volume scheme is used to solve the continuity equation, momentum equations and k–ε model equations. The flow characteristics were studied by varying rotation speed ω for 0?ω?167.6 rad/s, the distance from nozzle to disk (H/d) was (3, 5, 8 and 10) and the Reynolds number Re base on V_J and d was 1.45 × 10⁴. The results showed that, the radial velocity and turbulence intensity increase by increasing the rotation speed and decrease in the impingement zone as nozzle to disk spacing increases. When the centrifugal force increases, the radial normal stresses and shear stresses increase. The location of maximum radial velocity decreases as the local velocity ratio (α) increases. The pressure coefficient depends on the centrifugal force and it decreases as the distance from nozzle to plate increases. In impingement zone and radial wall jet, the spread of flow increases as the angular velocity decreases The numerical results give good agreement with the experiment data of Minagawa and Obi (Int. J. of Heat and Fluid Flow 2004; 25 :759–766). Copyright © 2006 John Wiley & Sons, Ltd.     

An experimental study of a two-dimensional plane turbulent wall jet

 Laser-Doppler measurements were conducted in a plane turbulent wall jet at a Reynolds number based on inlet velocity, Re ₀, of 9600. The initial development as well as the fully developed flow was studied. Special attention was given to the near-wall region, including the use of small measuring volumes and the application of specific near-wall data corrections, so that wall shear stresses were determined directly from the mean velocity gradient at the wall using only data below y ⁺=4. It was possible to resolve the inner peak in the streamwise turbulence intensity as well as the inner (negative) peak in the shear stress. Limiting values of (u′)⁺ and uv ⁺ were determined. Turbulence data from the outer region of the flow were compared to earlier hot wire measurements and large differences in the normal turbulence intensity and the shear stress were found. These differences can be attributed to high turbulence intensity effects on the hot-wires. Received: 17 October 1996 / Accepted: 8 December 1997     

Measurements in the vicinity of a stagnation point

This paper presents measurements of a plane jet impinging onto a normal flat plate placed up to five jet widths from the jet outlet. The small spacing ensured that the stagnation streamline remained in the potential core of the jet. The plate shear stress distribution compared well to that from an analytical solution for the laminar development of the plate boundary layer whose external velocity was determined from the measured pressure. By comparing the shear stress measured under the present low level of free stream turbulence (0.35%) at the jet exit with that of Tu and Wood [Exp. Thermal Fluid Sci. 13 (1996) 364–373] made at about 4%, it is concluded that the turbulence level at the nozzle exit has only a second-order influence on the surface shear stress around the stagnation point. Some spanwise non-uniformity was observed in the plate shear stress, but this was confined largely to the transition region. The mean velocity, Reynolds stresses, and fluctuating pressure were measured along the stagnation streamline using a fast-response pressure probe. A significant increase in the streamwise normal stress and the mean square of the pressure fluctuations occurred before they were eventually attenuated by the plate. This increase occurred in the region where the streamwise velocity was decreasing close to the plate causing extra energy production through the normal stresses. Spectra of the velocity and pressure fluctuations showed that the increase in level was mainly due to the low frequency motion, whereas the subsequent decrease occurred at higher frequencies.