Plane Jet Research Articles

Numerical Simulation of a Passive Control of the Flow Around an Aerofoil Using a Flexible, Self Adaptive Flaplet

Self-activated feathers are used by almost all birds to adapt their wing characteristics to delay stall or to moderate its adverse effects (e.g., during landing or sudden increase in angle of attack due to gusts). Some of the feathers are believed to pop up as a consequence of flow separation and to interact with the flow and produce beneficial modifications of the unsteady vorticity field. The use of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to the mentioned aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material. In this framework, this numerical study is divided in two parts. Firstly, in a simplified scenario, we determine the main characteristics that render a flap mounted on an aerofoil at high angle of attack able to deliver increased lift and improved aerodynamic efficiency, by varying its length, position and its natural frequency. Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the flaplet movement induced by the interaction with the flow field. The parametric study that has been carried out, reveals that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20o degrees. The results obtained from the direct numerical simulation of the flow field around the aerofoil equipped with the optimal flap at a chord Reynolds number of 2 × 104 shows that the flaplet movement is mainly induced by a cyclic passage of a large recirculation bubble on the aerofoil suction side. In turns, when the flap is pushed downward, the induced plane jet displaces the trailing edge vortices further downstream, away from the wing, moderating the downforce generated by those vortices and regularising the shedding cycle that appears to be much more organised when the optimal flaplet configuration is selected.

Effect of thickness of planar nozzles on erosion depth of levee soils subjected to plunging water

In this study, the effect of the thickness of a planar jet on the erosion depth when the jet impinges on a surface composed of cohesive soil was analytically and numerically evaluated. The results showed that the erosion depth was practically independent of the nozzle thickness for erosion depths shallower than the potential core length (i.e. the region of the jet in which the central flow velocity is the same as the nozzle velocity). The relation between nozzle thickness and erosion depth was non-linear with continuously variable slope for erosion depths deeper than the potential core length. Finally, the relation was approximately linear when the erosion depth converged to the equilibrium erosion depth. The findings of this study indicate that direct and fast prediction of the erosion depth in the field is possible using the data from a small scale soil erosion test with similar flow velocities.

Effect of Jet Oscillation on the Maximum Impingement Plate Skin Friction

The maximum impingement plate skin friction and flow field is measured for an acoustically forced planar impinging gas jet using oil film interferometry (OFI) and particle image velocimetry (PIV), respectively. The study is performed at a jet Reynolds number of Rejet = 11,000 and an impingement distance H, which is set to eight times the nozzle width W. The planar impinging gas jet is forced at the jet nozzle exit using Strouhal numbers StH = 0.39, 0.76, and 1.1, which are similar to those associated with the jet-plate tones measured in air-knife wiping experiments. The flow-field measurements indicate that the jet column oscillates at the applied forcing frequency, and depending on the forcing frequency, organized vortex structures can be identified in the shear layers that impinge on the plate surface. Both of these jet oscillation features result in a reduction in the time-averaged maximum impingement plate skin friction. This skin friction reduction is attributed to momentum loss at the jet centerline caused by increased levels of fluid entrainment and mixing of the surrounding quiescent fluid.

Understanding liquid-jet atomization cascades via vortex dynamics

Temporal instabilities of a planar liquid jet are studied using direct numerical simulation (DNS) of the incompressible Navier–Stokes equations with level-set (LS) and volume-of-fluid (VoF) surface tracking methods. $\unicode[STIX]{x1D706}_{2}$ contours are used to relate the vortex dynamics to the surface dynamics at different stages of the jet breakup – namely, lobe formation, lobe perforation, ligament formation, stretching and tearing. Three distinct breakup mechanisms are identified in the primary breakup, which are well categorized on the parameter space of gas Weber number ($We_{g}$) versus liquid Reynolds number ($Re_{l}$). These mechanisms are analysed here from a vortex dynamics perspective. Vortex dynamics explains the hairpin formation, and the interaction between the hairpins and the Kelvin–Helmholtz (KH) roller explains the perforation of the lobes, which is attributed to the streamwise overlapping of two oppositely oriented hairpin vortices on top and bottom of the lobe. The formation of corrugations on the lobe front edge at high $Re_{l}$ is also related to the location and structure of the hairpins with respect to the KH vortex. The lobe perforation and corrugation formation are inhibited at low $Re_{l}$ and low $We_{g}$ due to the high surface tension and viscous forces, which damp the small-scale corrugations and resist hole formation. Streamwise vorticity generation – resulting in three-dimensional instabilities – is mainly caused by vortex stretching and baroclinic torque at high and low density ratios, respectively. Generation of streamwise vortices and their interaction with spanwise vortices produce the liquid structures seen at various flow conditions. Understanding the liquid sheet breakup and the related vortex dynamics are crucial for controlling the droplet-size distribution in primary atomization.

Modeling of molecular diffusion and thermal conduction with multi-particle interaction in compressible turbulence

A mixing volume model (MVM) originally proposed for molecular diffusion in incompressible flows is extended as a model for molecular diffusion and thermal conduction in compressible turbulence. The model, established for implementation in Lagrangian simulations, is based on the interactions among spatially distributed notional particles within a finite volume. The MVM is tested with the direct numerical simulation of compressible planar jets with the jet Mach number ranging from 0.6 to 2.6. The MVM well predicts molecular diffusion and thermal conduction for a wide range of the size of mixing volume and the number of mixing particles. In the transitional region of the jet, where the scalar field exhibits a sharp jump at the edge of the shear layer, a smaller mixing volume is required for an accurate prediction of mean effects of molecular diffusion. The mixing time scale in the model is defined as the time scale of diffusive effects at a length scale of the mixing volume. The mixing time scale is well correlated for passive scalar and temperature. Probability density functions of the mixing time scale are similar for molecular diffusion and thermal conduction when the mixing volume is larger than a dissipative scale because the mixing time scale at small scales is easily affected by different distributions of intermittent small-scale structures between passive scalar and temperature. The MVM with an assumption of equal mixing time scales for molecular diffusion and thermal conduction is useful in the modeling of the thermal conduction when the modeling of the dissipation rate of temperature fluctuations is difficult.

Effects of combustion heat release on velocity and scalar statistics in turbulent premixed jet flames at low and high Karlovitz numbers

Theoretical scaling arguments for turbulent premixed combustion indicate that the pressure-dilatation source of turbulent kinetic energy becomes significant at low Karlovitz numbers, leading to potential invalidation of commonly-used turbulence models developed for non-reacting flow. Based on these arguments, a critical Karlovitz number is defined, below which dilatation effects are expected to become significant. Velocity and scalar statistics are obtained from Direct Numerical Simulation (DNS) calculations of low Mach number spatially-evolving turbulent premixed planar jet flames. At fixed bulk Reynolds number and stoichiometric equivalence ratio, two simulations are performed at Karlovitz numbers above and below the critical Karlovitz number. Hydrogen combustion with detailed transport is modeled using a detailed nine-species chemical kinetic mechanism, and coflows of combustion products are used to ensure flame stability at uniform equivalence ratio. The analysis of these statistics focuses on three key areas. First, the influence of the velocity-pressure gradient source of turbulent kinetic energy is confirmed at a low Karlovitz number, and the unimportance of these effects is confirmed at a high Karlovitz number. Similar effects are observed for the chemical source term in the scalar variance budgets. Second, the degree of alignment between the Reynolds stress tensor (scalar flux) and the strain-rate tensor (scalar gradient), the foundation of a majority of the turbulence models used in reacting flows, is assessed with the DNS databases. Additionally, consistency of anisotropic Reynolds stress and strain-rate tensor invariants is assessed using invariant maps. While good alignment and consistency are obtained for statistics and invariants at a high Karlovitz number, both alignment and consistency degrade at a low Karlovitz number. Third, turbulence models formulated for non-reacting flow are modified algebraically in the Bray–Moss–Libby (BML) formalism for turbulent premixed combustion. A variable efficiency function is defined to capture the regime dependence of heat release effects in these models. Model performance is evaluated at Karlovitz numbers above and below the critical Karlovitz number using the DNS databases, and satisfactory prediction of counter-gradient transport in the flame-normal direction is obtained. However, heat release effects are also observed in the flame-parallel directions in the low-Karlovitz number simulation, and the models developed in the formalism for statistically planar flames fail to capture these effects. Furthermore, in the low-Karlovitz number case, redistributive effects are active on the shear components of the Reynolds stress, which are not considered in the BML formalism. More advanced turbulence models are therefore necessary for turbulent premixed jet flames below the critical Karlovitz number.

Assessment of Numerical Simulation Capabilities of the Fire Dynamics Simulator (FDS 6) for Planar Air Curtain Flows

Computational Fluid Dynamics (CFD) results are discussed for momentum driven planar jet flows, resembling configurations in use for air curtain in the context of smoke control in building fire. The CFD package Fire Dynamics Simulator (FDS) is used. Special focus is given to the impact of grid resolution, synthetic turbulent inflow boundary condition and sub-grid scale eddy viscosity models. The computational results are compared with summarized literature data. Investigation of different set-ups of inlet boundary conditions, including the inlet duct length, velocity profile and method of generation of turbulence at the level of the inflow, reveals that the inlet boundary condition is the most influential factor governing the flow downstream. The FDS results successfully reproduce the planar jet flows, both in terms of mean variables and second-order statistics. ‘Reference’ results have been obtained with a fully developed turbulent flow emerging from a long inlet duct. By reducing the inlet duct length and applying the Synthetic Eddy Method (SEM) at the inflow boundary condition, the ‘reference’ results have been reproduced with a reduction in the computing times of approximately 20%. However, care must be taken when choosing the parameters of SEM, in particular the number of eddies and their length scale. The impact of turbulent viscosity model is noticeable, but not of primary importance for the flow at hand, provided that a sufficiently fine computational mesh is used.

Free-stream coherent structures in a planar jet

The free-stream coherent structure theory developed by Deguchi & Hall (J. Fluid Mech., vol. 752, 2014, pp. 602–625), valid in the large-Reynolds-number asymptotic limit, is extended and applied to jet flows. It is shown that a nonlinear exact coherent structure can be supported at the edge of the jet, and the structure induces a much bigger streaky flow in the centre of the jet. The lambda-shaped vortices that characterise the coherent structure are qualitatively consistent with those seen in experimental observations. Here a planar incompressible jet is investigated for the sake of simplicity, but the structure we describe could be used as a basis of more complex theories for incompressible and compressible jets of practical importance.

Experimental investigation of five parallel plane jets with variation of Reynolds number and outlet conditions

This article describes an experimental investigation in the near field of five parallel plane jets. The study applies 2D Particle Image Velocimetry (PIV) for ventilated and unventilated jets, where ventilated means exiting into a duct with expansion ratio 3.5 and unventilated means exiting to the free atmosphere. Results are presented for Reynolds numbers 1408, 5857 and 10510. The Reynolds number is calculated for the middle channel and is based on the height of the nozzle (channel) equivalent diameter 2h. All characteristic regions of the methodology to describe multiple interacting jets are observed by the PIV measurements - converging, merging and combined. Each of the five parallel channels has an aspect ratio of 25 defined as nozzle width (w) to height (h). The channels have a length of 185 times the channel height guaranteeing a fully developed velocity profile at the exit from the channel. Spacing between the single plane jets is 3 times the channel height. The near field of multiple mixing jets is depended on outlet nozzle geometry. Blunt geometry of the nozzle was chosen (sudden contraction).

Numerical simulation of planar liquid jets from collapsing bubbles in a U-shaped wall

Numerical simulation of planar liquid jets from collapsing bubbles in a U-shaped wall

Planar Helium Plasma Jet: Plasma "Bullets" Formation, 2D "Bullets" Concept and Imaging

Applications of dielectric barrier discharge (DBD)-based atmospheric pressure plasma jets are often limited by the relatively small area of treatment due to their one-dimensional (ID) configuration. This letter describes the first results demonstrating generation of two-dimensional (2D) plasma "bullets" and 2D plasma jets that permit rapid treatment of large targets. Imaging of pulsed DBD in helium (He) and ionization wave propagation along the dielectric surface shows that DBD evolution starts with formation of transient anode glow; only then does the development of cathode-directed streamers continue. The anode glow can propagate as an ionization wave along the dielectric surface outside of the discharge gap. We show that plasma "bullet" propagation is not limited to 1D geometry (i.e., tubes) and that it can be organized in a planar plasma jet form or other 2D (or even 3D) shapes.

A stochastic vortex structure method for interacting particles in turbulent shear flows

In a recent study, we have proposed a new synthetic turbulence method based on stochastic vortex structures (SVSs), and we have demonstrated that this method can accurately predict particle transport, collision, and agglomeration in homogeneous, isotropic turbulence in comparison to direct numerical simulation results. The current paper extends the SVS method to non-homogeneous, anisotropic turbulence. The key element of this extension is a new inversion procedure, by which the vortex initial orientation can be set so as to generate a prescribed Reynolds stress field. After validating this inversion procedure for simple problems, we apply the SVS method to the problem of interacting particle transport by a turbulent planar jet. Measures of the turbulent flow and of particle dispersion, clustering, and collision obtained by the new SVS simulations are shown to compare well with direct numerical simulation results. The influence of different numerical parameters, such as number of vortices and vortex lifetime, on the accuracy of the SVS predictions is also examined.

Cross wavelet analysis of Reynolds stress transport in turbulent planar jet

Cross wavelet analysis of Reynolds stress transport in turbulent planar jet

二次元乱流噴流における速度勾配と変動圧力の結合統計量

Joint statistics between velocity gradient and fluctuating static pressure in a turbulent planar jet are experimentally investigated. The experimental data was acquired by simultaneous measurements of velocity gradient ( ∂u’2/∂x1 and ∂u’1/∂x2, where u’1, u’2, x1 and x2 are fluctuating stereamwise velocity, fluctuating cross-streamwise velocity, streamwise coordinate, and cross-streamwise coordinate, respectively) and fluctuating pressure p′ using two combined probes. One of the combined probe consists of an X-type hot-wire probe and a pressure probe and the other combined probe consists of two I-type hot-wire probes arranged in vertical direction and a pressure probe. In the time-series, it is shown that the events in which the instantaneous Reynolds stress, squared cross-stramwise velocity, and product of velocity gradient and pressure shows significantly large value (so-called “significant events”) occur intermittently with negative pressure fluctuation, this implies that the Reynolds stress transport phenomena are locally active with the large-scale vortex structures observed in a turbulent planar jet. It was also found that the standard deviation of (p’/ρ)(∂u’2/∂x1) conditioned by the negative pressure fluctuation is 21% larger than the unconditioned standard deviation. The cospectra show that the correlation (p’/ρ)(∂u’2/∂x1) greatly contributes the local isotropy of the flow field. On the other hand, we confirmed that ∂u’1/∂x2 is almost unrelated to the pressure fluctuation in the present turbulent planar jet. The results show that there is little contribution on the Reynolds stress transport by the correlation (p’/ρ)(∂u’1/∂x2).

Flow Characteristics of a Plane Jet with Rectangular Tabs

Flow Characteristics of a Plane Jet with Rectangular Tabs

Effect of nozzle clustering on Mach wave emission in supersonic plane jets

Effect of nozzle clustering on Mach wave emission in supersonic plane jets

Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

A physical study of air–water flow in planar plunging water jet with large inflow distance

A plunging jet is commonly encountered in nature. It is also widely used in industry for its capacity to enhance fluid mixing and entrain gases into the liquid fluid when the impact velocity exceeds a critical value. This paper presents a physical study of vertical supported planar water plunging jets, in a relatively large-size facility. Air-water flow properties were measured in the falling jet and in the plunging pool using an intrusive phase-detection probe, with jet impact velocities between 2.5 m/s and 7.4 m/s and a fixed jet length. The falling jet was characterised by large disturbance and substantial pre-aeration. Intense air-water mixing was observed downstream of the impingement point. The development of air diffusion layer and turbulent shear layer was characterised by the streamwise evolution of void fraction, bubble count rate, bubble chord length and interfacial velocity profiles. The results compared favourably with the literature, albeit some difference was observed associated with different inflow jet turbulence levels as well as instrumentation development and signal processing refinement, including instrumental size, scanning rate and duration. The clustering properties were derived using the near-wake criterion. Results were comparable to those in horizontal hydraulic jumps. The air-entrainment rate was derived, highlighting the significant contribution of jet pre-aeration.

Volume reconstruction of a plane jet impinging on a slotted plate using the phase averaging technique

Impinging jets are widely used in ventilation systems in order to improve indoor air quality and energy consumption. Thus the flow dynamics involved in these configurations is very important and should be understood and controlled. In the present study, a plane jet issuing from a rectangular nozzle and impinging on a slotted plate was investigated experimentally by means of 3D-stereoscopic PIV. The velocity field was experimented for thirty parallel planes. The whole field was obtained by an interpolation between the reconstructed planes. The 3D reconstruction of the flow was obtained by using the phase averaging technique applied to each of the instantaneous fields. Self-sustained tones generated in this configuration were measured simultaneously with the PIV fields by the use of a microphone. The obtained acoustical signal was used as reference in the phase averaging technique.

Atmospheric Pressure Planar Radio Frequency Discharge With Isolated Electrodes: Glow Features and Application Prospects

Operation characteristics of RF planar barrier discharge in atmospheric pressure argon appropriate for obtaining the discharge extension outside the electrodes in a form of planar plasma jet are determined experimentally. The plasma jet images and spatial distributions of its light emission intensity are presented, and their features are discussed. The prospects of studied jet discharge for creation of liquid crystal alignment and etching polymer materials are demonstrated.