Investigation on Plasma Jet Flow Phenomena During DC Air Arc Motion in Bridge-Type Contacts

Arc plasma jet flow in the air was investigated under a bridge-type contacts in a DC 270 V resistive circuit. We characterized the arc plasma jet flow appearance at different currents by using high-speed photography, and two polished contacts were used to search for the relationship between roughness and plasma jet flow. Then, to make the nature of arc plasma jet flow phenomena clear, a simplified model based on magnetohydrodynamic （MHD） theory was established and calculated. The simulated DC arc plasma was presented with the temperature distribution and the current density distribution. Furthermore, the calculated arc flow velocity field showed that the circular vortex was an embodiment of the arc plasma jet flow progress. The combined action of volume force and contact surface was the main reason of the arc jet flow.

Numerical study of hydrofoil surface jet flow on cavitation suppression

Cavitation caused vibration and noise of hydraulic machinery. To some extent,cavitation made fatigue damage in advance. Many scholars found that the re-entrant jets were the reasons of the shedding of cavities. To suppress cavitation,based on the idea of blocking the re-entrant jets,a special surface flow structure of 2D hydrofoil was proposed. The through-hole was made in the proper position of the hydrofoil. The incoming flow can outflow from this jet-hole automatically depending on the pressure difference between pressure side and suction side. Re-entrant jet growth can be weakened by optimizing the jet-hole geometry. Based on the standard k-ε turbulence model and Schnerr & Sauer cavitation model,under different cavitation numbers( σ) and jet-angles( β) for NACA0066( 2D) hydrofoil with 8° angles of attack,cavitation field numerical analysis was carried out. The results show that 2D hydrofoil cavitation flow had a strong unsteadiness. Making a jet-hole at the junction between the re-entrant jet and cavity can effectively minimize cloud cavitation. For a certain cavitation condition,optimal jet-angles( β) can be obtained to control cavitation growth. For the same β,the effects of cavitation suppression were changed with different cavitation numbers( σ). Consequently,suitable jet-angle and jet-position could extend the stable operating range of the hydrofoil.

Large Eddy Simulation of the Particle Coagulation in High Concentration Particle-Laden Planar Jet Flow

Particle coagulation by Brownian motion is an important but difficult research topic.When particle volume concentration is larger than 0.1%,the classic SMOLUCHOWSKI equation is not applicative anymore.The high concentration coagulation,with HEINE＇s correction,source terms for the Taylor-series expansion method of moments（TEMOM） are firstly driven in this paper.Ultra-fine particle（d0？100 mm） with initial volume fraction f？1% coagulation in a planar jet turbulence flow is simulated via the large eddy simulation（LES）.The instantaneous and time-averaged particle distributions and the high concentration enhancement are given out.The particle number concentration distribution results show that the coagulation is more intense comparing to dilute case in previous research,especially near the nozzle exit.After jet flow is fully developed,the effect is much more obvious at the region between vortexes.The time-averaged γ（the high concentration enhance factor） distributes sharply and symmetrically about the jet centerline at the upstream,but becomes broad and flat at downstream where the cross-stream averaged γ fluctuates drastically.As a new attempt,this paper shows Brownian coagulation with high concentration also can be calculated via TEMOM appropriately,and the coagulation at the region between vortexes is about 1.38 times intensive of the dilute result calculated by the classic Smoluchowski theory.

Transformation of single drop breakup from binary to ternary and multiple in turbulent jet flows

By releasing liquid drops in turbulent jet flows,we investigated the transformation of single drop breakup from binary to ternary and multiple.Silicone oil and deionized water were the dispersed phase and continuous phase,respectively.The probability of binary,ternary,and multiple breakup of oil drops in jet flows is a function of the jet Reynolds number.To address the underlying mechanisms of this transformation of drop breakup,we performed two-dimensional particle image velocimetry(PIV)experiments of single-phase jet flows.With the combination of drop breakup phenomenon and two-dimensional PIV results in a single-phase flow field,these transformation conditions can be estimated:the capillary number ranges from 0.17 to 0.27,and the Weber number ranges from 55 to 111.

Analysis on MHD Stability of Free Surface Jet flow in a Gradient Magnetic Fields

The simplified modeling for analysis on MHD stability of free surface jet flow in a gradient magnetic fields is based on the theoretical and experimental results on channel liquid metal MHD flow, especially, the results of MHD flow velocity distribution in cross-section of channels (rectangular duct and circular pipe), and the expected results from the modeling are well agreed with the recent experimental data obtained. It is the first modeling which can efficiently explain the experimental results of liquid-metal free surface jet flow.

Numerical Simulations of Jet Flow Inside Launching Box and Design of Pressure Opening the Covers of Missile Launching Box

Numerical simulations are presented for jet flow inside a launching box. The predictions are based on solutions of the unsteady three-dimensional Reynolds-averaged Navier-stokes equations. Since the pressure opening the forward cover is given, the pressure opening the backward cover is designed by analyzing the flow field inside the launching box. The κ -ε turbulent model is presented and the structured meshes are used through the whole computational field.

Numerical Experiments of Subsonic Jet Flow Simulations Using RANS with OpenFOAM

This study investigates the accuracy of different numerical schemes of OpenFOAM software to simulate compressible turbulent jets. Both pressure-based schemes utilizing the implicit PIMPLE algorithm and density-based schemes relying on AUSM scheme and explicit Runge-Kutta time integration are considered. The results of the numerical tests are compared and validated against data from NASA ARN nozzle geometry. The choice of parameter setting of the schemes is discussed in depth and possible optimization strategies are proposed to increase accuracy of RANS simulations of turbulent jets.

Numerical Simulation of MHD Jet Flows

Liquid metal free surface flows （films, jets and droplets） are considered as diverter/ limiter system and first wall in fusion reactor, but the knowledge Of liquid metal free surface under a non-uniform magnetic field is very limited. In this article, the stability of a jet flow under a gradient magnetic field is investigated, and its MHD effects are the top concern. Based on numerical simulation and experimental results, a simplified model is developed to analyze the MHD effects of the jet flow and to explain the reason why it can keep stable under a strong non-uniform magnetic field.

SOME RECENT PROGRESS IN STUDIES ON ACTIVE CONTROL OF WALL-JET FLOWS

Active control of wall his is important in both application and basic researches. In order to establish a solid background for the expected application,it is necessary to perform detailed studies on the base nows,on their stability characteristics and on the identification technique of coherent structures. This paper is a summarizing article rather than a detailed technical report, Its main purpose is just to introduce the recent progress in the related area.

Nanoparticle Transportation and Brownian Diffusion in Planar Jet Flow via Large Eddy Simulation

The nanoparticle transportation and Brownian diffusion in planar jet flow is simulated via large eddy simulation in this work. To thorough compare the Brownian diffusion with different particle size, we computed three particle diameter dp = 1 nm, 10 nm and 50 nm in one simulation process simultaneously. The numerical results showed that at the flow de- veloping stage, the particle mass concentration pattern develops as the flow vorticity develops. The distribution is nearly uniform at the lower reaches of the nozzle exit. When the jet flow is developing on, vortexes always carry the particle from upstream to downstream, from the central axis region to the outer mixing layer of jet. At the front of the jet flow, particles distribute more homogeneous for they have more residence time to diffuse from higher concentration region to the lower concentration region. The time averaged particle concentration distribution patterns are similar to Gaussian distribution form. The maximum concentration contributed by diffusion is present at the mixing layer near the nozzle exit. The farther away from the nozzle exit in the cross-stream direction, the smaller the concentration is. The maximum concentration contributed by diffusion is several orders smaller than that contributed by flow convection.

Stability of Subsonic Microjet Flows and Combustion

Results of experimental studies of round and plane propane microjet combustion in a transverse acoustic field at small Reynolds numbers are presented in this paper. Features of flame evolution under the given conditions are shown. Based on the new information obtained on free microjet evolution, new phenomena in flame evolution in a transverse acoustic field with round and plane propane microjet combustion are discovered and explained.

Discussion of Direct Numerical Simulation Method for Supercritical Carbon Dioxide Jet Flow

A kind of direct numerical simulation method suitable for supercritical carbon dioxide jet flow has been discussed in this paper. The form of dimensionless nonconservative compressible Navier-Stokes equations in a two-dimensional cartesian coordinate system is derived in detail. High accurate finite difference compact schemes based on non-uniform grid system are introduced to solve the equations. The simulation results of the three vortex pairing phenomenon of plane mixing layer and a compressible axisymmetric jet flow field show that the discussed numerical simulation method is feasible to calculate the supercritical carbon dioxide jet fluid. And it is found that the difficulties of splitting the convective terms in conservation Navier-Stokes equations, which are brought by the supercritical carbon dioxide fluid pressure state equation, can be avoided by solving the nonconservative compressible Navier-Stokes equations.

Stability of Subsonic Jet Flows

An overview of recent experimental results on instability and dynamics of jets at low Reynolds numbers is given. Round and plane, macro and micro jets are under the consideration. Basic features of their evolution affected by initial conditions at the nuzzle outlet and environmental perturbations are demonstrated.

Numerical Investigation of Flow Separation Control over an NACA0018 Airfoil Using Sweeping Jet Actuator

As an active flow control technology and with the advantages of no moving components, the Sweeping jet actuator has become a hotspot in the field of flow control. However, the linear relationship between oscillation frequency and momentum coefficient in a sweeping jet actuator makes it difficult to determine the dominant factors that affect control effectiveness. Decoupling the oscillation frequency and momentum coefficient, as well as determining the control mechanism, is the focus of studying the sweeping jet actuator. In this study, a novel sweeping jet actuator is designed using synthetic jets instead of feedback channels and applied to the flow separation control of NACA0018 airfoil. This article studies the control effect under three oscillation frequencies of F+ = f × c/U∞ = 1, 10, 100 and three momentum coefficients of Cμ = 0.45%, 0.625%, 0.9%. The numerical results indicate that all three oscillation frequencies have good control effects on flow separation, and the control effect is best when F+ = 1, with the maximum lift coefficient increasing by approximately 14% compared to the other two cases. And the sweeping jet actuator has a better ability to control flow separation as the momentum coefficient increases. By decoupling the characteristics of the sweeping jet actuator and conducting numerical analysis of the flow control effect, it will promote its better engineering application in the field of flow control. .

Numerical Study of Controlling Jet Flow and Noise using Pores on Nozzle Inner Wall

In this paper, the feasibility of controlling the subsonic jet flow and its noise using pores of blind holes added on the nozzle inner wall is explored numerically. These pores are intended to introduce disturbances to the shear layer so as to change the flow mixing. This passive strategy has not been attempted so far. A convergent nozzle with a cylindrical extension is selected as the baseline case. Three nozzles with pores on the inner wall are set up. Validations of the numerical settings are carried out, then the compressible turbulent jets at the exit Math number Mj = 0.6 in the four nozzles are calculated by large eddy simulations （LES）, while the ra-diated sounds are predicted by the FW-H acoustic analogy. The results show that the blind holes have produced some effects on weakening the turbulence intensity in the shear layer. Comparison reveals that both temporal and spatial correlations of the turbulent fluctuations in the modified cases are suppressed to some extent. Meanwhile, the porous nozzles are shown to suppress the pairing of vortices and enhance the flow mixing, and therefore, the development of shear layer and the fragmentation of large scale vortices are accelerated.

LOCAL CHAOS CHARACTERISTICS IN A SELF ASPIRATED REVERSED FLOW JET LOOP REACTOR

The local chaos characteristics of the time series pressure fluctuations of gas liquid two phase flow in a self aspirated reversed flow jet loop reactor are studied by the deterministic chaos analysis technique. It is found that the estimated local largest Lyapunov exponent is positive in all cases and the profile is similar to that of the local fractal dimension in this reactor. The positive largest Lyapunov exponent shows that the reactor is a nonlinear chaotic system. The obvious distribution indicates that the local nonlinear characteristic parameters such as the Lyapunov exponent and the fractal dimension could be applied to further study the flow characteristics such as the flow regine transitions and flow structures of the multi phase reactors.

LOCAL NONLINEAR CHARACTERISTICS OF GAS LIQUID SOLID THREE PHASE SELF ASPIRATED REVERSED FLOW JET LOOP REACTOR

Hursts rescaled range (R/S) analysis and Wolfs attractor reconstruction technique have been adopted to estimate the local fractal dimensions and the local largest Lyapunov exponents in terms of the time series pressure fluctuations obtained from a gas liquid solid three phase self aspirated reversed flow jet loop reactor,respectively.The results indicate that the local fractal dimensions and the local largest Lyapunov exponents in both the jet region and the tubular region inside the draft tube increase with the increase in the jet liquid flowrates and the solid loadings,the local fractal dimension profiles are similar to those of the largest Lyapunov exponent,the local largest lyapunov exponents are positive for all cases,and the flow behavior of such a reactor is chaotic.The local nonlinear characteristic parameters such as the local fractal dimension and the local largest Lyapunov exponent could be applied to further study the flow properties such as the flow regime transitions and flow structures of this three phase jet loop reactor.

Flow instability of nanofuilds in jet

The flow instability of nanofluids in a jet is studied numerically under various shape factors of the velocity profile, Reynolds numbers, nanoparticle mass loadings,Knudsen numbers, and Stokes numbers. The numerical results are compared with the available theoretical results for validation. The results show that the presence of nanoparticles enhances the flow stability, and there exists a critical particle mass loading beyond which the flow is stable. As the shape factor of the velocity profile and the Reynolds number increase, the flow becomes more unstable. However, the flow becomes more stable with the increase of the particle mass loading. The wavenumber corresponding to the maximum of wave amplification becomes large with the increase of the shape factor of the velocity profile, and with the decrease of the particle mass loading and the Reynolds number. The variations of wave amplification with the Stokes number and the Knudsen number are not monotonic increasing or decreasing, and there exists a critical Stokes number and a Knudsen number with which the flow is relatively stable and most unstable,respectively, when other parameters remain unchanged. The perturbation with the first azimuthal mode makes the flow unstable more easily than that with the axisymmetric azimuthal mode. The wavenumbers corresponding to the maximum of wave amplification are more concentrated for the perturbation with the axisymmetric azimuthal mode.

Laboratory model study of the effect of aeration on axial velocity attenuation of turbulent jet flows in plunge pool

In the laboratory model experiment, the velocities of the jet flow along the axis are measured, using the CQY-Z8a velocity-meter. The velocity attenuations of the jet flow along the axis under different conditions are studied. The effects of the aeration concentration, the initial jet velocity at the entry and the thickness of the jet flow on the velocity attenuation of the jet flow are analyzed. It is seen that the velocity attenuation of the jet flow along the axis sees a regular variation. It is demonstrated by the test results that under the experimental conditions, the velocity along the axis decreases linearly. The higher the air concentration is, the faster the velocity will be decayed. The absolute value of the slope K increases with the rise of the air concentration. The relationship can be defined as K = AC~ ＋ Kb. The coefficient A is 0.03 under the experimental conditions. With the low air concentration of the jet flow, the thinner the jet flow is, the faster the velocity will be decayed. With the increase of the air concentra- tion, the influence of the thickness of the jet flow on the velocity attenuation is reduced. When the air concentration is increased to a certain value, the thickness of the jet flow may not have any influence on the velocity attenuation. The initial jet velocity itself at the entry has no influence on the variation of the velocity attenuation as the curves of the velocity attenuation at different velocities at the entry are practically parallel, even coinciding one with another. Therefore, improving the air concentration of the jet flow and disper- sing the jet flow in the plunge pool could reduce the influence of the jet flow on the scour.

Large Eddy Simulation of Micro-Particles Transport with Different Mass Flow Rate in Turbulent Planar Jet Flow

The motion of micro-particles with different mass flow rate in the planer turbulent jet flow has been simulated, using LES method to obtain the flow vorticity evolution and Lagrangian method to track micro-particles. The re- suits showed that when the flow rate is small, the particles more likely to present in the vortex periphery, the dis- tribution pattern is similar to the flow pattern. When the flow rate is high, some particles will escape from the mo- tion region to the original static region, so that in the jet region, particles are relatively evenly distributed. When the flow field is full developed, the particles average concentration in the y direction affected by the mass flow rate relative slightly, the normalized mean particles concentrations at different flow rate were similar to Gaussian shape.