Effective control of propagation of a free turbulent jet

Experimental data are presented on the propagation of a free (submerged) turbulent air jet with artificially regulated-by exciting pulsations of a given frequency in the pipe-decay rate. The experimental results are easily generalized using the Strouhal number.     

Phase-averaged measurements in an isosceles triangular turbulent free jet

Digital time series hot-wire data, acquired in the near field of a turbulent free jet of air issuing from a sharp-edged isosceles triangular orifice, have been post-processed using a phase-averaging procedure to determine the coherent and random contributions to turbulence statistics. The Reynolds number, based on the equivalent diameter of the orifice, was 1.84×10⁵. It was found that momentum transfer by the Reynolds primary shear stress occurs mainly via the coherent primary shear stress.     

The axisymmetrical free laminar jet of an incompressible pseudoplastic fluid

Summary The equations of momentum and continuity of a free, axisymmetrical jet for an incompressible, inelastic, non-Newtonian fluid in isothermal flow are solved. Using the boundary layer approximations and a similarity transformation the resulting non-linear third order ordinary differential equation is solved numerically. Velocity profiles are obtained and compared to previously known solutions for the two-dimensional jet. The significance of the results is discussed in detail.At present on leave at the Department of Chemical Engineering, Stanford University, Stanford (Cal.), U.S.A.     

Effect of different initial conditions on a turbulent round free jet

Velocity measurements were made in two jet flows, the first exiting from a smooth contraction nozzle and the second from a long pipe with a fully developed pipe flow profile. The Reynolds number, based on nozzle diameter and exit bulk velocity, was the same (䏪,000) in each flow. The smooth contraction jet flow developed much more rapidly and approached self-preservation more rapidly than the pipe jet. These differences were associated with differences in the turbulence structure in both the near and far fields between the two jets. Throughout the shear layer for x<3d, the peak in the v spectrum occurred at a lower frequency in the pipe jet than in the contraction jet. For x́d, the peaks in the two jets appeared to be nearly at the same frequency. In the pipe jet, the near-field distributions of f(r) and g(r), the longitudinal and transverse velocity correlation functions, differed significantly from the contraction jet. The integral length scale L_u was greater in the pipe jet, whereas L_v was smaller. In the far field, the distributions of f(r) and g(r) were nearly similar in the two flows. The larger initial shear layer thickness of the pipe jet produced a dimensionally lower frequency instability, resulting in longer wavelength structures, which developed and paired at larger downstream distances. The regular vortex formation and pairing were disrupted in the shear layer of the pipe jet. The streamwise vortices, which enhance entrainment and turbulent mixing, were absent in the shear layer of the pipe jet. The formation of large-scale structures should occur much farther downstream in the pipe jet than in the contraction jet.     

Simplified method of numerical calculation of a three-dimensional turbulent free jet

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 170–173, January–February, 1989.     

Particle concentration and particle size measurements in a particle laden turbulent free jet

Particle concentration and particle size distribution curves have been measured for particle-laden jets of silica gel powder for different loading ratios and air velocities using a Laser Diffraction Method (LDM) and a tomography data transform technique. It was found that the mean particle size at the outer edge of the jet decreases with increasing gas velocity, and that the jet widens with decreasing particle concentration and increasing gas velocity.     

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.     

Inviscid,laminar and turbulent opposed flows

This paper attempts to reproduce numerically previous experimental findings with opposed flows and extends their range to quantify the effects of upstream pipes and nozzles with inviscid, laminar and turbulent flows. The choice of conservation equations, boundary conditions, algorithms for their solution, the degree of grid dependence, numerical diffusion and the validity of numerical approximations are justified with supporting calculations where necessary. The results of all calculations on the stagnation plane show maximum strain rates close to the annular exit from the nozzles and pipes for lower separations and it can be expected that corresponding reacting flows will tend to extinguish in this region with the extinction moving towards the axis. With laminar flows, the maximum strain rate increased with Reynolds number and the maximum values were generally greater than with inviscid flows and smaller than with turbulent flows. With large separations, the strain rates varied less and this explains some results with reacting flows where the extinction appeared to begin on the axis. The turbulent‐flow calculations allowed comparison of three common variants of a two‐equation first‐moment closure. They provided reasonable and useful indications of strain rates but none correctly represented the rms of velocity fluctuations on the axis and close to the stagnation plane. As expected, those designed to deal with this problem produced results in better agreement with experiment but were still imperfect. Copyright © 2004 John Wiley & Sons, Ltd.     

Phase-locked analysis of velocity fluctuations in a turbulent free swirling jet after vortex breakdown

Large-scale organized vortical structures were studied experimentally in a free swirling jet of air experiencing vortex precession (PVC) at ambient conditions. Detailed measurements were performed in the region near the nozzle exit using phase-locked LDV and PIV, at a Reynolds number of Re ?? 24,400 and a swirl parameter S ?? 1.0. The investigation allowed reconstruction of the time-averaged flowfield, with the associated distribution of turbulent fluctuations, the phase-locked structure of the jet and the associated precessing vortex structure. An original joint analysis of power spectra and probability density functions of velocity data led to quantification of the PVC effect on turbulent fluctuations. This analysis showed that the PVC contribution can be properly separated from the background random turbulence, reproducing the results of phase-locked measurements. It is found that the background turbulence in the near field is substantially weaker if compared to the coherent fluctuations induced by vortex precession.     

Optimization of turbulent jet mixing

We introduce an approach for controlling jet mixing that combines direct numerical simulation of an incompressible jet flow with stochastic optimization procedures. The jet is excited with helical and combined helical and axial actuations at the orifice. An objective function that measures the spreading of the jet evaluates the performance of the actuation parameters. The optimization procedure searches for the best actuation by automatically varying the parameters and calculating their objective function value. Solutions that lead to a pronounced spreading of the jet are found within reasonable time, although the evaluation of the objective function, the DNS of the jet, is expensive. For a jet flow at low Reynolds number the performance of different search algorithms (simulated annealing and evolution strategies) is evaluated. We compare various objective functions based on radial velocity and the concentration of a passive scalar, including functions that penalize actuation with high amplitudes. We find that a combined axial and helical actuation is much more efficient with respect to jet mixing than a helical actuation alone.     

Hydromagnetic capillary instability of a liquid jet with an axial flow

The hydromagnetic capillary instability of a jet of inviscid, impressible fluid of infinite electrical conductivity and subjected to a uniform axial magnetic field is studied, taking into account an axial flow in the jet. The results show that while the axial flow promotes instability due to capillary effects and the axial-flow effects can be completely suppressed by a magnetic field of sufficient strength.     

Effect of laden solid particles on the turbulent flow structure of a round free jet

The effect of solid particles on the flow structure of a round air jet in a stagnant surrounding was investigated experimentally. Information on the averaged two-component velocities, the kinetic energies, and the u′ v′-properties were obtained for both phases by means of a monochromatic three beam laser Doppler anemometer. The particle number density was also measured by this system. Glass beads of 64 μm and 132 μm diameter were used for a constant mass loading ratio of 0.3 in a jet with a Reynolds number of 20 000. The lateral mean velocity and number density profiles were expressed by best fitting functions and several invariable coefficients were found. The standard drag force coefficient C _D for a single particle was applicable for a dilute particle cloud even in a non-uniform air velocity field.     

Hydromagnetic free convection flow with induced magnetic field effects

An exact solution is presented for the hydromagnetic natural convection boundary layer flow past an infinite vertical flat plate under the influence of a transverse magnetic field with magnetic induction effects included. The transformed ordinary differential equations are solved exactly, under physically appropriate boundary conditions. Closed-form expressions are obtained for the non-dimensional velocity (u), non-dimensional induced magnetic field component (B _x ) and wall frictional shearing stress i.e. skin friction function (τ _x ) as functions of dimensionless transverse coordinate (η), Grashof free convection number (G _r ) and the Hartmann number (M). The bulk temperature in the boundary layer (Θ) is also evaluated and shown to be purely a function of M. The Rayleigh flow distribution (R) is derived and found to be a function of both Hartmann number (M) and the buoyant diffusivity parameter (ϑ ^*). The influence of Grashof number on velocity, induced magnetic field and wall shear stress profiles is computed. The response of Rayleigh flow distribution to Grashof numbers ranging from 2 to 200 is also discussed as is the influence of Hartmann number on the bulk temperature. Rayleigh flow is demonstrated to become stable with respect to the width of the boundary layer region and intensifies with greater magnetic field i.e. larger Hartman number M, for constant buoyant diffusivity parameter ϑ ^*. The induced magnetic field (B _x ), is elevated in the vicinity of the plate surface with a rise in free convection (buoyancy) parameter G _r , but is reduced over the central zone of the boundary layer regime. Applications of the study include laminar magneto-aerodynamics, materials processing and MHD propulsion thermo-fluid dynamics.     

Numerical simulation of confined laminar jet flows

Two‐dimensional incompressible jet development inside a duct has been studied in the laminar flow regime, for cases with and without entrainment of ambient fluid. Results have been obtained for the flow structure and critical Reynolds number values for steady asymmetric jet development and for the onset of temporal oscillations, at various values of the duct‐to‐jet width ratio (aspect ratio). It is found that at low aspect ratios and Reynolds numbers, jet development inside the duct is symmetric. For larger aspect ratios and Reynolds numbers, the jet flow at steady state becomes asymmetric with respect to the midplane, and for still higher values, it becomes oscillatory with respect to time. When entrainment is present, the instabilities of asymmetric development and temporal oscillations occur at a much higher critical Reynolds number for a given aspect ratio, indicating that the stability of the jet flow is higher with entrainment. Copyright © 2000 John Wiley & Sons, Ltd.     

Concentration fluctuations in a turbulent jet

Using Spalding's model of turbulence in a turbulent shear flow, we have calculated the root-mean-square value of the concentration fluctuations inside a turbulent jet. Although we used the same equations and the same solution technique as Spalding, we have not been able to find precisely his numerical results derived for a jet issuing into a fluid at rest with the same density as the jet. The differences between our numerical results, Spalding's numerical results and the experimental data of Becker, Hottel and Williams are fairly small only if the initial values of the turbulence energy and the mixing length inside the jet and the turbulence in the ambient fluid are taken into account in the model. For a turbulent jet issuing into a turbulently flowing surrounding stream of different density, we found that the relative concentration fluctuations can increase considerably. This brings out the importance of taking into account property variables in analysing turbulent mixing processes.     

Global mode-based control of laminar and turbulent high-speed jets

The emitted noise from round jets is reduced using linear feedback controllers designed using structural sensitivity analysis. Linear global modes inform the selection and placement of the controller, and Navier–Stokes simulations are used to demonstrate effectiveness in a Mach-1.5 cold axisymmetric jet and in a Mach-0.9 cold turbulent jet. In both jets, each fitted with a cylindrical nozzle, the control reduces the radiated noise and modifies the baseflow in a way that enhances the relative amplitudes of low-frequency $\mathit{St}\approx 0.05$ global modes that do not have significant support in the acoustic field.