Turbulent Plane Jet Research Articles

Lagrangian properties of the entrainment across turbulent/non-turbulent interface layers

Lagrangian statistics obtained from direct numerical simulations of turbulent planar jets and mixing layers are reported for the separation distance between the tracer particles at the outer edge of the turbulent/non-turbulent interface layer, and the entrained fluid particles. In the viscous superlayer (VSL) the mean square particle distance exhibits a ballistic evolution, while the Richardson-like scaling for relative dispersion prevails inside the turbulent sublayer (TSL). The results further support the existence of two different regimes within the interface layer, where small-scale outward enstrophy diffusion governs the entrained particles in the VSL, while inviscid small-scale motions govern the TSL.

A numerical study of a plane turbulent wall jet in a coflow stream

This work is a numerical study of an isothermal and a non-isothermal turbulent plane wall jet emerging in a coflow stream with different velocity ratios ranging from 0 to 0.2. Turbulence modeling is performed by using a modified low-Reynolds number k–ε model. The numerical resolution of the governing equations was carried out via finite difference method. The present predictions are compared with those suggested in the literature. It was found that the studied model reasonably predicts the mean flow proprieties of the flow field. The main purpose of this work is to determine the influence of the velocity ratio on the dynamic, thermal and turbulent characteristics of the flow. A comparison with a simple wall jet is made. Besides, the influence of Reynolds and Richardson numbers on the wall jet emerging in a coflowing stream is examined. As far as the isothermal flow is concerned, results show that the potential core length decreases in accordance with the velocity ratio. It was also found that, downstream from the jet exit (in the established region), the longitudinal distributions of the normalized forms of the excess maximum x-velocity and the turbulent maximum kinetic energy converge to a single curve at different velocity ratios. The present investigation suggests that the effect of coflowing jet on the dynamic, thermal and turbulent parameters is negligible in the potential core area (ZFE) and the jet is similar to that of a simple jet. Further downstream of this region, the velocity ratio affects the flow as far as the high coflow streams are concerned.

Simultaneous measurement of velocity and pressure near the turbulent/non-turbulent interface of a planar turbulent jet

Simultaneous measurement of velocity and pressure near the interface of turbulent/non-turbulent region of a planar turbulent jet is performed. Simultaneous measurement is achieved by means of a combined probe comprising an X-type hot-wire and a static pressure probe. The measurement data are analyzed by the conditional sampling technique and the ensemble-averaging technique on the basis of the intermittency function for the turbulent/non-turbulent decision. Obtained results show that there is a thin interfacial layer between the turbulent/non-turbulent region accompanied with the rapidly changing of velocity, Reynolds stress, and pressure. The thickness of the interfacial layer is 0.08 times the half-width of the cross-streamwise profile of the mean streamwise velocity and almost the same as Taylor transverse micro scale at the measurement position. Further, the velocity and pressure field near the interfacial layer indicates the existence of vortices whose core is at the middle of the interfacial layer and its diameter is the same as the thickness of the interfacial layer. The turbulent energy production and diffusion near the interface are also investigated by estimating the production term and the diffusion term in the transport equation for the turbulent energy. Estimated results show that the turbulent energy is transported from the turbulent region to the outer non-turbulent region by the pressure diffusion. Therefore, the existence of the vortices near the interface has an important role to transport the turbulent energy to the outer non-turbulent region.

A dynamic regularized gradient model of the subgrid-scale stress tensor for large-eddy simulation

Large-eddy simulation (LES) solves only the large scales part of turbulent flows by using a scales separation based on a filtering operation. The solution of the filtered Navier-Stokes equations requires then to model the subgrid-scale (SGS) stress tensor to take into account the effect of scales smaller than the filter size. In this work, a new model is proposed for the SGS stress model. The model formulation is based on a regularization procedure of the gradient model to correct its unstable behavior. The model is developed based on a priori tests to improve the accuracy of the modeling for both structural and functional performances, i.e., the model ability to locally approximate the SGS unknown term and to reproduce enough global SGS dissipation, respectively. LES is then performed for a posteriori validation. This work is an extension to the SGS stress tensor of the regularization procedure proposed by Balarac et al. [“A dynamic regularized gradient model of the subgrid-scale scalar flux for large eddy simulations,” Phys. Fluids 25(7), 075107 (2013)] to model the SGS scalar flux. A set of dynamic regularized gradient (DRG) models is thus made available for both the momentum and the scalar equations. The second objective of this work is to compare this new set of DRG models with direct numerical simulations (DNS), filtered DNS in the case of classic flows simulated with a pseudo-spectral solver and with the standard set of models based on the dynamic Smagorinsky model. Various flow configurations are considered: decaying homogeneous isotropic turbulence, turbulent plane jet, and turbulent channel flows. These tests demonstrate the stable behavior provided by the regularization procedure, along with substantial improvement for velocity and scalar statistics predictions.

The effect of coflows on a turbulent jet impacting on a plate

The study examines the dynamic and the turbulent behavior of a plane jet impinging normally on a flat target plate in a flow at rest or in a coflow stream. First, numerical and experimental investigations of the characteristics of a turbulent plane jet impinging on a flat surface in still air are depicted. The measurements are conducted using Particle Image Velocimetry (PIV). A comparison has been carried out between numerical results of three turbulence closure models (k–ε standard model, RNG k–ε model and a second-order model RSM) and experimental data. It has been found that the second order turbulence model (RSM) reasonably predicts the mean flow properties of the flow field. Next, a numerical study of the flow field of an impinging jet in a coflow stream is conducted. The study of the jet in a no-directed coflow stream shows the presence of a phenomenon of recirculation near the flat plate. In addition, the study focuses on the influence of the coflow velocity ratio on the behavior of an impinging jet and the influence of the directed co-flow on various physical parameters such as the mean and the turbulent quantities. The results clearly show that the coflow stream imposed noticeable restrictions on the spreading of the impinging jet and decreases considerably the entrainment of air jet.

Hydrodynamic instability and shear layer effects in turbulent premixed combustion

A turbulent premixed plane jet flame is analyzed by large-eddy simulations. The analysis shows that the flame front wrinkling is strongly influenced by the shear layer effect when the gas expansion effects are small leading to larger flame front amplitudes at the flame base than at high gas expansion ratios. However, the hydrodynamic instability effect induces a continuously increasing flame front amplitude which yields an enhanced flame pocket generation at the flame tip. Both phenomena influence the magnitude of the turbulent burning area and burning area rate response through the flame front deflections which are determined by the contribution coefficient. This coefficient represents the mutual interaction between the flame and the flow. At low gas expansion ratios, the total heat release rate spectra of the turbulent flame are wider in terms of dominant modes at Strouhal numbers which are linked to the mean flame height oscillations. Thus, at low gas expansion ratios, the vortex-flame interaction is less damped by the flame in the sense that vortices can perturb the flame front stronger. The total heat release rate trend of St−2.2 previously found for a round jet flame is also determined for the current slot jet at realistic gas expansion ratios indicating a general tendency to transfer energy from large to small flame structures. At high gas expansion ratios, an increasing Markstein length leads to an energy transfer between neighboring dominant modes in the low frequency range 1 < St < 10 and the burning area rate response becomes more important for the total heat release rate spectra of the turbulent slot flames which agrees with recent findings for a laminar premixed plane flame.

M1005 2色LIFとPIVによる平面乱流浮力噴流の温度場と速度場の同時計測(GS6 熱工学(2),修士研究発表セッション)

M1005 2色LIFとPIVによる平面乱流浮力噴流の温度場と速度場の同時計測(GS6 熱工学(2),修士研究発表セッション)

Steady interaction of a turbulent plane jet with a rectangular heated cavity

Turbulent heat transfer between a confined jet flowing in a hot rectangular cavity is studied numerically by finite volume method using the k-w SST one point closure turbulence model. The location of the jet inside the cavity is chosen so that the flow is in the non-oscillation regime. The flow structure is described for different jet-to-bottom-wall distances. A parametrical study was conducted to identify the influence of the jet exit location and the Reynolds number on the heat transfer coefficient. The parameters of this study are: the jet exit Reynolds number (Re, 1560< Re <33333), the temperature difference between the cavity heated wall and the jet exit (DT=60?C) and the jet location inside the cavity (Lf, 2? Lf? 10 and Lh 2.5<Lh?10). The Nusselt number increased and attained its maximum value at the stagnation points and then decreased. The flow structure is found in good agreement with the available experimental data. The maximum local heat transfer between the cavity walls and the flow occurs at the potential core end. The ratio between the stagnation point Nusselt numbers of the cavity bottom (NuB0) to the maximum Nusselt number on the lateral cavity wall (NuLmax) decreased with the Reynolds number for all considered impinging distances. For a given lateral confinement, the stagnation Nusselt number of the asymmetrical interaction Lh?10 is almost equal to that of the symmetrical interaction Lh=10.

Steady regime of a turbulent plane jet flowing into a rectangular hot cavity

Turbulent heat transfer between a confined jet and the walls of a hot rectangular cavity is studied numerically by finite volume method using the k-ω SST one point closure turbulence model. The parameters considered in this study were the jet exit Reynolds number, the temperature difference between the cavity heated wall and the jet exit and the jet location inside the cavity. The flow structure predictions were in good agreement with the available experimental data. The maximum local heat transfer between the cavity walls and the flow occurred at the potential core end. The ratio between the stagnation point Nusselt numbers of the cavity bottom to the maximum Nusselt number on the lateral cavity wall decreased with the Reynolds number for all considered impinging distances. For a given lateral confinement, the stagnation Nusselt number of the asymmetrical interaction is almost equal to that of the symmetrical interaction.

Three-dimensional study of turbulent flow characteristics of an offset plane jet with variable density

An experimental and numerical investigation of the flow field of variable density turbulent offset jet is presented. The velocity measurements are performed using a Velocimetry Laser Doppler technique for an offset height h. Three cases of variable-density turbulent plane jets discharging from a rectangular nozzle into a quiescent medium are studied. The variation density jets considered were revealed at different Reynolds numbers. In the second step of this work, a numerical three-dimensional model of the problem is simulated through the resolution of the Navier–Stokes equations by means of the finite volume method and the Reynolds stress model second-order turbulent closure model. A non-uniform mesh system tightened close to the emitting nozzle and both the vertical and horizontal walls is also adopted. A good level of agreement was achieved, between the experiments and the calculations. Once the model validated, our model allowed the evaluation of the influence of the variation density on the characterizing features of the resulting flow filed. It is found that the centerline velocity and concentration of the heavier jet decays much faster than in the two other jets, and a similar behavior for the vertical profiles in the three variable-density jets is well reproduced in the simulation.

Bubble breakup in a turbulent shear layer

The breakup of a millimetre size buoyantly rising bubble encountering a horizontal plane turbulent jet is experimentally investigated using high-speed shadowgraphy and acoustic techniques. The bubble diameter to jet height ratio is 0.75 and the jet height based Reynolds number is 4000. The high-speed imaging was recorded at 7 kHz simultaneous with hydrophone output at 100 kHz. Bubble breakup events were seen to produce simple binary divisions into products of similar size as well as three products where at least one was much smaller than the other products. Coalescence of products was also observed. In almost all cases time-frequency analysis of the acoustic emissions enabled the products to be identified and sized.

Numerical Investigations on Heat Transfer of Self-Sustained Oscillation of a Turbulent Jet Flow Inside a Cavity

Computations of heat transfer and fluid flow of a plane isothermal fully developed turbulent plane jet flowing into a rectangular hot cavity are reported in this paper. Both velocity and temperature distributions are computed by solving the two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations. This approach is based on one-point statistical modeling using the energy-specific dissipation (k-ω) turbulence model. The numerical predictions are achieved by finite volume method. This problem is relevant to a wide range of practical applications including forced convection and the ventilation of mines, enclosure, or corridors. The structural properties of the flow and heat transfer are described for several conditions. An oscillatory regime is evidenced for particular jet location, inducing for each variable a periodic behavior versus time. The jet flapping phenomena are detailed numerically by the instantaneous streamlines contours and the vorticity magnitude contours within one period of oscillation. The heat transfer along the cavity walls is also periodic. Time average of mean Nusselt number is correlated according with some problem parameters.

Numerical study of amplitude and frequency effects upon a pulsating jet

Sinusoidal pulsation effects on a turbulent plane jet are investigated numerically. In the nozzle exit, the flow is characterized by a uniform temperature, and submitted to a longitudinal and periodic velocity disturbance: u¯(x,y,t)=u¯0+asin(ωt). Upon mixing the spreading and heat exchange with the surrounding medium, properties are influenced obviously by pulsation in the potential core region but they are similar to the steady jet after the end of the potential core.

The asymptotic downstream flow of plane turbulent wall jets without external stream

The plane turbulent wall-jet flow without externally imposed stream is considered. It is assumed that the wall jet does not emerge from a second wall perpendicular to the velocity vector of the initial wall jet. The (kinematic) momentum flux$K(x)$of the wall jet decreases downstream owing to the shear stress at the wall. This investigation is based on the hypothesis that the total friction force on the wall is smaller than the total inflow momentum flux. In other words, the turbulent wall jet tends to a turbulent ‘half-free jet’ with a non-zero momentum flux$K_{\infty }\;(\text{m}^{3}~\text{s}^{-2})$far downstream. The fact that the turbulent half-free jet is the asymptotic form of a turbulent wall jet is the basis for a singular perturbation method by which the wall-jet flow is determined. It turns out that the ratio between the wall distance$y_{m}$of the maximum velocity and the wall distance$y_{0.5}$of half the maximum velocity decreases downstream to zero. Dimensional analysis leads immediately to a universal function of the dimensionless momentum flux$K(\mathit{Re}_{x})/K_{\infty }$that depends asymptotically only on the local Reynolds number$\mathit{Re}_{x}=\sqrt{(x-x_{0})K_{\infty }}/{\it\nu}$, where$x_{0}$denotes the coordinate of the virtual origin. When the values$K$and${\it\nu}$at the position$x-x_{0}$are known, the asymptotic momentum flux$K_{\infty }$can be determined. Experimental data on all turbulent plane wall jets (except those emerging from a second plane wall) collapse to a single universal curve. Comparisons between available experimental data and the analysis make the hypothesis$K_{\infty }\neq 0$plausible. A convincing verification, however, will be possible in the future, preferably by direct numerical simulations.

On the turbulent energy transport related to the coherent structures in a planar jet

In this study, the turbulent energy transport related to the coherent vortex structures in the self-similarity region of a turbulent planar jet is investigated experimentally. First, two velocity components and the pressure are measured at multiple points simultaneously through the use of a “combined probe” comprising of a pressure probe and an X-type hot-wire probe. Then, proper orthogonal decomposition (POD) is applied to obtained velocity and pressure field data to extract the coherent vortex structures in the jet. A complementary technique combining POD and linear stochastic estimation is then used to reconstruct the spatiotemporal velocity and pressure field for the dominant POD modes. In the reconstructed velocity and pressure field, a coherent vortex structures which have counter-rotating vortices staggered with respect to the jet centerline are observed. This structure contained approximately 42% of the total energy. Finally, the turbulent energy transport related to the coherent vortex structure is evaluated from the reconstructed velocity and pressure field. This estimation shows that the production and pressure diffusion of the turbulent energy in the jet strongly depend on the discussed coherent vortex structures.

Numerical investigation of a gas–solid turbulent jet flow with Reynolds number of 4500 using lattice Boltzmann method

The gas–solid two-phase turbulent plane jet flow with high Reynolds number of 4500 was numerical investigated by means of lattice Boltzmann method (LBM). The multiple relaxation time (MRT) was employed to deal with the high Reynolds number fluid flows, and the particles were traced by the Lagrangian method. The results show that the flow changes from initial symmetric mode to asymmetric mode with the development of the flow. And asymmetric pattern appears first at the position of x/d=4, where the vortex structures begin to form. The dispersion of particles at different Stokes number shows various distributions. The MRT-LBM shows its good ability in simulating turbulent flow with the high Reynolds number.

Incomplete similarity of a plane turbulent wall jet on smooth and transitionally rough surfaces

This study assesses the hypothesis of incomplete similarity for a plane turbulent wall jet on smooth and transitionally rough surfaces. Typically, a wall jet is considered to consist of two regions: an inner layer and an outer layer. The degree to which these two regions reach equilibrium with each other and interact to produce the property of self-similarity remains an open question. In this study, the analysis of the outer and inner regions indicates that each region is characterised by a half-width which exhibits its own distinct dependence on the streamwise distance x from the slot, and a single self-similar structure for both regions does not exist. More specifically, the inner and outer layers of the wall jet exhibit different scaling laws, which results in two self-similar mean velocity profiles, both of which retain a dependence on the slot height H. As such, incomplete similarity of the wall jet on smooth and transitionally rough surfaces is confirmed by this study. In addition, comparison of the experimental results for the transitionally rough surface with the smooth wall case indicates that the surface roughness modifies the development of the mean velocity profile in both the inner and outer regions, although the effect on the outer region is relatively small and close to the experimental uncertainty.

Flow Characteristics of Flapping Motion of a Plane Water Jet Impinging onto Free Surface

A submerged turbulent plane jet in shallow water impinging vertically onto the free surface will produce a large-scale flapping motion when the jet exit velocity is larger than a critical one. The flapping phenomenon is verified in this paper through a large eddy simulation where the free surface is modeled by volume of fluid approach. The quantitative results for flapping jet are found to be in good agreement with available experimental data in terms of mean velocity, flapping-induced velocity and turbulence intensity. Results show that the flapping motion is a new flow pattern with characteristic flapping frequency for submerged turbulent plane jets, the mean centerline velocity decay is considerably faster than that of the stable impinging jet without flapping motion, and the flapping-induced velocities are as important as the turbulent fluctuations.

On the velocity and pressure statistics during the flapping motion in a planar turbulent jet

The role of a large-scale coherent vortex structure in a planar jet, which is intermittently observed and called “flapping motion”, on the turbulent energy transport process is investigated experimentally. The experiment is performed by simultaneously measuring the two components of velocity and pressure in the self-preserving region of the jet. The probe for the simultaneous measurement of the velocity and pressure consists of an X-type hot-wire sensor and a static pressure probe. The measurement data are analyzed using a conditional sampling technique and the phase-averaging technique, on the basis of an intermittency function which shows whether the jet is now flapping or not. This intermittency function is obtained by the measurement results of the streamwise velocity fluctuation by means of the two I-type hot-wire sensors set in the self-preserving region of the jet with applying a continuous wavelet transform analysis to the data. The experimental results show that the phase-averaged velocity field during the flapping motion shows a good agreement with those obtained through the 23 points simultaneous measurement of the streamwise velocity in the previous studies. Further, the phase-averaged pressure field during the flapping motion indicates the existence of a large-scale coherent vortex structure, interpreted as a combination of the flapping and puffing motions in the self-preserving region of the jet. In addition, it is found that the production of the turbulent energy and its diffusion from the inner region to the outer region of the jet are enhanced by the flapping motion. In particular, this enhancement of the turbulent energy diffusion is caused by the combination of an increase of the turbulent diffusion to the outer region of the jet and a decrease of the pressure diffusion to the inner region of the jet.

The effect of subgrid-scale models on the entrainment of a passive scalar in a turbulent planar jet

Classical large-eddy simulation (LES) modelling assumes that the passive subgrid-scale (SGS) models do not influence large-scale quantities, even though there is now ample evidence of this in many flows. In this work, direct numerical simulation (DNS) and large-eddy simulations of turbulent planar jets at Reynolds number ReH = 6000 including a passive scalar with Schmidt number Sc = 0.7 are used to study the effect of several SGS models on the flow integral quantities e.g. velocity and scalar jet spreading rates. The models analysed are theSmagorinsky, dynamic Smagorinsky, shear-improved Smagorinsky and the Vreman. Detailed analysis of the thin layer bounding the turbulent and non-turbulent regions – the so-called turbulent/non-turbulent interface (TNTI) – shows that this region raises new challenges for classical SGS models. The small scales are far from equilibrium and contain a high fraction of the total kinetic energy and scalar variance, but the situation is worse for the scalar than for the velocity field. Both a-priori and a-posteriori (LES) tests show that the dynamic Smagorinsky and shear-improved models give the best results because they are able to accurately capture the correct statistics of the velocity and passive scalar fluctuations near the TNTI. The results also suggest the existence of a critical resolution Δx, of the order of the Taylor scale λ, which is needed for the scalar field. Coarser passive scalar LES i.e. Δx ≥ λ results in dramatic changes in the integral quantities. This fact is explained by the dynamics of the small scales near the jet interface.