Two-dimensional Jet Flow Research Articles

Optimisation of initial velocity distribution of jets for entrainment and diffusion control using deep reinforcement learning

The control of the entrainment and diffusion of the heat and mass ejected along the jet flow requires the control of fluid motion. In the present study, optimal initial velocity distributions for suppressing and promoting jet entrainment and diffusion were identified using deep reinforcement learning under constant initial jet flow rate conditions. Two-dimensional jet flow simulations using OpenFOAM were combined with a deep Q-network as the learning algorithm. The spatial distribution of the initial jet (velocity and ejection angle) changed. The results show that to suppress the entrainment of the jet, an optimal initial velocity distribution shape in which the velocity in the centre is almost 0, with an inward velocity just outside of it, and a large outward velocity at the outer edge, is desirable. However, to promote entrainment, a shape with a large velocity in the centre portion and small velocity at the outer edge, with an overall inward angle, is desirable. To suppress the diffusion of the jet, an optimal initial velocity distribution shape with a large velocity in the centre portion, a small velocity at the outer edge, and an overall inward angle is desirable. Conversely, to promote diffusion, a shape with a large outward velocity at both the centre and outer edges is desirable. It is also indicated that, when the initial velocity distribution of the jet has a special shape, and the relationship between flow rate, centre velocity, and centre temperature that is normally observed in a jet flow may not hold. In other words, by changing the initial velocity distribution, the entrainment and diffusion of the jet can be separated and controlled, which is usually difficult to achieve using conventional engineering methods.

Exact solutions for permeable wall laminar jet with velocity slip: Momentum and thermal profiles

This study presents exact analytical solutions for the two-dimensional laminar jet flow and heat transport of Casson fluid under velocity slip condition. The laminar wall jet of classical Glauert type is extended to the Casson fluid model considering the flow to be along a flat permeable surface. The roles of velocity slip, wall temperature, and mass transfer on the fluid flow and rate of thermal exchange are investigated. The outcomes for momentum and thermal fields, skin friction and heat transfer gradient are obtained analytically with numerical solutions of the suction-far stream velocity expression. It is noted that mass transfer and velocity slip have significant effects on the flow of fluid and heat transfer. Thermal profile varies significantly for increasing Prandtl values with small suction.

Acoustic emission due to the interaction between shock and instability waves in two-dimensional supersonic jet flows

An analytical model is developed to study the sound produced by the interaction between shock and instability waves in two-dimensional supersonic jet flows. The jet is considered to be of vortex-sheet type and two-dimensional Euler equations are linearized to determine the governing equations for shock and instability waves and their interaction. Pack's model is used to describe shock waves, while instability waves are calculated using spatial stability analysis. The interaction between shock and instability waves can be solved analytically by performing Fourier transform and subsequently using the method of steepest descent. Sound produced by the interaction between the instability wave and a single shock cell is studied first, after which that due to a number of cells follows. We find that the model developed in this study can correctly predict the frequencies of the fundamental screech tone and its first and second harmonics. We show that the predicted sound directivity, even from a single shock cell, is in good agreement with experimental data. In particular, this model shows the strongest noise emission close to the upstream direction but the emitted noise starts to rapidly decay as the observer angle approaches $180^\circ$ , which is in accordance with experimental results; this suggests that the effective noise from a single shock cell is far from of the monopole type as assumed in the classical Powell's model. We find that the noise directivity is very sensitive to the local growth rate of the instability waves and the noise is generated primarily through the Mach wave mechanism.

Aerodynamic Modelling of the Airfoil Immersed in Two-Dimensional Jet Flow

In order to study the aerodynamic interaction of the airfoil and jet flow, the free streamline model and the panel method are combined to develop a fast calculation method for the airfoil in two-dimensional inviscid jet flow. The vortex strength and position of the jet boundary are determined by using the free streamline model and the constant total pressure difference assumption, the circulation of the airfoil is solved by the vortex panel method, and the whole process is coupled by relaxation iteration. Firstly, the convergence and effectiveness of the present method are verified. Next, the influence of the length ratio of jet height to airfoil chord, the velocity ratio of jet velocity to freestream velocity, and the ground effect on airfoil aerodynamics are studied. The results show that the aerodynamic characteristics of the airfoil in finite width jet flow and in freestream have a large difference, and it is important to consider the jet deflection for jet/airfoil interaction. In jet flow, the velocity ratio can be regarded as an aerodynamic similarity parameter for the airfoil. When the jet flow is deflected, the airfoil will not only generate lift but also drag, and the ground effect can be used to decrease drag. The developed method in this paper can not only capture the jet deflection but also has higher calculation efficiency than Computational Fluid Dynamic (CFD), which is beneficial for the preliminary design of a powered-lift device.

Calculation and analysis of aerodynamic characteristics for airfoils immersed in two-dimensional jet flow

In order to study the aerodynamic interaction of the airfoil and jet flow, the free streamline model and the panel method are combined to establish a calculation method for the aerodynamic characteristics of the airfoil in a two-dimensional inviscid jet flow. Using the assumption of constant total pressure difference and free streamline model to determine the vortex strength and position of the jet boundary, the aerodynamic force of the airfoil is solved by the vortex panel method, and the whole process is calculated by relaxation iteration. Firstly, the relevant theory in this paper is introduced, and the convergence and effectiveness of the present method are also tested. Next, the influence of the position and the ratio of jet height to airfoil chord on airfoil aerodynamics in the jet flow are studied. Finally, the momentum theory is used to analyze the aerodynamic characteristics of the airfoil in the jet flow. The results show that the aerodynamic characteristics of the airfoil in the jet flow is quite different from that in the freestream. When the jet flow is deflected, the airfoil will not only generate lift but also drag. The method in this paper can be used to study the aerodynamic characteristics of the airfoil immersed in the jet flow.

Ferrohydrodynamic and Magnetohydrodynamic Effects on Jet Flow and Heat Transfer of Fe3O4-H2O Nanofluid in a Microchannel Subjected to Permanent Magnets

In the present study, the two-dimensional jet flow of Fe3O4-H2O nanofluid was numerically investigated in a microchannel. The main objective of this article was to study the impact of permanent magnets on both ferromagnetic hydrodynamic and thermal behavior. A ferromagnetic hydrodynamic model, which includes the Brown effect and thermophoretic effect, was applied to simulate the problem through solving momentum, energy, and volume fraction equations. In this regard, different results, including the velocity vector, temperature distribution, and Nusselt number, were analyzed. Moreover, the influence of Kelvin force, inlet opening, permanent magnets position, and Reynolds number were studied on the jet flow and heat transfer. The obtained results demonstrate these factors significantly affect the jet flow and heat transfer of Fe3O4-H2O nanofluid in the microchannel. Moreover, it was found that the magnetic field originating from permanent magnets can effectively solve the problem of local high temperature on the wall at low inlet opening. The heat transfer gain was the most obvious when the position of the permanent magnet was close to the microchannel entrance. When inlet opening and permanent magnets position are 1/4 and 1, respectively, the heat transfer gain was largest, reaching 35.2%.

Large Eddy Simulation of Planar Free Jet Flow Using the Space-Time Conservation Element/Solution Element Scheme

In order to investigate the characteristics of planar free jet flows, the Monotonically Integrated Large Eddy Simulation (MILES) method with the high-resolution space-time conservation element/solution element (CE/SE) scheme was used to simulate the two-dimensional free jet flows. For free jet flows within the Reynolds number range 35300~2200, the flow structures, averaged and instant velocity, turbulence intensity, velocity profiles of different cross-sections were examined respectively. The simulation results agree well with the experiment data. A region of constant height exists right after the jet outlet, in which the average centerline velocity and turbulence intensity maintains same with the jet outlet, and the instant fluctuation amplitudes are also small. For turbulent flows, the length of the undisturbed region is equal to the length of region of constant height. While near the laminar region, the length of undisturbed region is slightly larger than the length of zone of constant height. The vortex structures start to generate after the undisturbed region, develop coherently in the region of flow establishment, and break down gradually in the region of fully established flow region. As the Reynolds number increases, the length of zone of constant height, the length of undisturbed region and the length of potential core region all decrease correspondingly.

Simulations of cavity flow noise and turbulent jet noise using a hybrid method

A hybrid method was explored to investigate the generation and near-field radiation of aerodynamic sound from an unsteady turbulent flow over a two-dimensional open cavity and three-dimensional jet flow. A two-dimensional cavity model was established to study the unsteady flow and radiated jet sound. It was revealed that the radiated sound that generated by the boundary layer separation and vortex impact cavity wall intervened in the front of the cavity, and an obvious interference phenomenon appeared. The far-field radiated sound generated by the cavity presented obvious directivity, and the sound pressure in the area located at 45°–135° interval was much higher. Then, the unsteady turbulence jet noises of the elliptical and rectangular nozzles were analyzed. It was revealed that the scale and intensity of the vortexes generated by the elliptical nozzle were larger than those by the rectangular nozzle. The jet noise of the elliptical nozzle is lower than that of the rectangular nozzle. Besides, the sound pressure distributions of the two nozzles presented obvious directivity. The sound pressure in the short-axis direction of the nozzle section was higher than that in the long-axis direction.

二次元オフセット噴流の流動特性と熱伝達

The flow field of an offset jet is complex and is frequently found in many engineering devices. In this paper, the fluid flow and heat transfer characteristics of two-dimensional offset jet are investigated experimentally. The offset jet is produced by the flow of air which issue from the end of a long parallel channel. The exit Reynolds number Re is changed in 1.5×103 to 7.0×103, and the offset ratio H/h (H : step height, h : channel height) is changed in 0.5, 1.0 and 1.5. The wall shear stress is measured using a micro flow sensor. Flow visualization is carried out using a CCD camera and laser light. The velocity profiles are measured by a PIV system. When the flow at the channel exit is laminar, fluid in the outer region has a counter-clockwise vorticity and fluid in the inner region has a clockwise vorticity, and a lifting-off of the wall jet flow into the ambient fluid is occurred. The Strouhal number of the vortex frequency is almost constant against the offset ratio. The formation of those vortices is not seen at the turbulent flow. Two peaks exist in the wall static pressure, Nusselt number and skin friction coefficient distributions of the laminar flow with H/h = 0.5 and 1.0. The 1st peak appears near the reattachment point, and the 2nd peak appears in the neighborhood of the lifting-off of the wall jet flow. The two peaks coalesce into one at H/h = 1.5. As the offset ratio increases, the reattachment point moves to downstream direction and Nu becomes large both the laminar and turbulent flows. The reattachment point of the turbulent flow moves to upstream direction compared with the laminar flow because Coanda-effect is strengthen.

Laminar Wall Jet Flow and Heat Transfer over a Shallow Cavity.

This paper presents the detailed simulation of two-dimensional incompressible laminar wall jet flow over a shallow cavity. The flow characteristics of wall jet with respect to aspect ratio (AR), step length (X u), and Reynolds number (Re) of the shallow cavity are expressed. For higher accuracy, third-order discretization is applied for momentum equation which is solved using QUICK scheme with SIMPLE algorithm for pressure-velocity coupling. Low Reynolds numbers 25, 50, 100, 200, 400, and 600 are assigned for simulation. Results are presented for streamline contour, velocity contour, and vorticity formation at wall and also velocity profiles are reported. The detailed study of vortex formation on shallow cavity region is presented for various AR, X u, and Re conditions which led to key findings as Re increases and vortex formation moves from leading edge to trailing edge of the wall. Distance between vortices increases when the step length (X u) increases. When Re increases, the maximum temperature contour distributions take place in shallow cavity region and highest convection heat transfer is obtained in heated walls. The finite volume code (FLUENT) is used for solving Navier-Stokes equations and GAMBIT for modeling and meshing.

Numerical Investigation on the Fluid Flow Characteristics of a Laminar Slot Jet on Solid Block Mounted on a Horizontal Surface

Numerical simulation is carried out to investigate the flow characteristics of two-dimensional laminar incompressible slot jet flows. Simulations are done for a vertical slot jet on a block at the bottom wall, and the top wall is confined by the parallel surface. The reattachment length, detachment length, vortex center and the coefficient of friction (Cf) are analyzed for different types of flow patterns. The correlation between the Reynolds number (Re) with vortex center and reattachment length is recommended. A detailed study is reported on the horizontal velocity profiles at various downstream locations by varying the aspect ratio (AR) (2 ≤ AR ≤ 5), block width (0.25 ≤ w/Dh ≤ 1.75), block height (0.25 ≤ h/Dh ≤ 1.0) and the jet exit Reynolds numbers (100 ≤ Re ≤ 300). The critical Reynolds number for the formation of the secondary vortex is obtained. The numerical results indicated that the Cf value increases while decreasing the distance between the jet and the impingement block. It is found that the effects of AR and Re are more influencing on the fluid flow field, compared to the block width and the block height.

Investigation of a Two-Dimensional Hot Jet’s Infrared Radiation Using Four Different Turbulence Models

A two-dimensional (2D) jet flow and temperature field are simulated by usingk-εT. C. model and compared with other three nontemperature corrected models, which are standardk-ε, RNGk-ε, and SSTk-ωmodel. Then based on the calculated results, the spectral infrared radiation characteristics within 4∼5 μm of the 2D jet flow were calculated. By comparing the computed results of the velocity, temperature field, and infrared radiation with the experimental measurements, it shows that thek-εT. C. model predicts mean flow mixing more rapidly and the turbulent kinetic energy dissipates earlier than with no temperature correction; thek-εT. C. model could give a good prediction for the velocity and temperature distributions on the centerline of the 2D hot gas jet, but not on the locations off the centerline. The maximum computation error of the 2D hot jet infrared radiation is decreased from 86% to 26%, and the accuracy of the computation is greatly improved.

Two-dimensional over-expanded jet flow parameters in supersonic nozzle lip vicinity

The mathematical model for two-dimensional (plane or axis-symmetric) over-expanded jet flow parameters analysis in the vicinity of supersonic nozzle lip is proposed. The variation of the key parameters of this problem (e.g., the geometrical curvature of oblique shock emanating from the nozzle edge) is studied parametrically depending of jet flow parameters, such as Mach number, jet incalculability, and the ratio of gas specific heats. It was proved that differential parameters of the flow field crucially depend not only of the key parameters, but on the symmetry type as well.

G0602 二次元オフセット噴流の流れの可視化と熱伝達(GS6 噴流・内部流)

G0602 二次元オフセット噴流の流れの可視化と熱伝達(GS6 噴流・内部流)

20906 低レイノルズ数領域における2次元噴流の蛇行現象(GS-18【流体(2)】)

A numerical simulation of a two-dimensional jet flow at Re=100 was carried out using the finite volume method The velocity profile at the exit of a slit was a top-hat defined by a hyperbolic tangent function. A flapping motion of the jet flow is observed at the 100 or more time downstream of the slit width, and a frequency of the flapping motion is approximately 10 Hz. Then, the streamwise velocities at two points symmetrical with a central line of the jet vary synchronously and asymmetrically.

Dynamic Mesh Method for Two-Dimensional Synthetic Jet

A numerical simulation method using dynamic mesh for two-dimensional synthetic jet flow was developed, in which dynamic mesh conservation equations and spring-based smoothing method were used. A computing model of an isolated jet to validate this method, the max velocity of oscillation is 0.47 m/s, Renault number is 180 and the Strouhal number is 0.056. The simulation results of different phases show that a shear layer is formed at the orifice edge and the shear layer rolls up to form a vortex ring, then fluid nearby the orifice enters into the cavity, and the vortex pair moves away from the orifice under its own momentum when the membrane moves to the bottom position. The suction process is affected by the outside vortex ring, and the second vortex pair impacts acutely the vortex ring outside. Comparing with other similar examples, the results show that the flow structure and action mechanism of synthetic jet flow are reasonable, and therefore this method could be used to simulate two-dimensional synthetic jet flow.

A spectral approach to thin jet flow

Steady and transient two-dimensional thin jet flow of a Newtonian fluid is examined numerically. The influence of inertia and gravity is emphasized. The fluid is assumed to emerge from a vertical channel, driven by a pressure gradient and/or gravity. The boundary layer equations are assumed for the thin film. In contrast to the commonly used depth-averaging solution method, the strong nonlinearities are preserved in the present formulation as the boundary layer equations are solved by expanding the flow field in terms of orthonormal shape functions in the transverse direction to the jet. It is found that the initial conditions strongly determine the stability of the film, which for all transient cases examined, were shown to be stable despite the presence of initial instabilities.

Numerical calibration of the Andersen cascade impactor using a single jet model

A single jet model is described and used to analyze the performance of an 8-stage Andersen cascade impactor (ACI). The two-dimensional axisymmetric jet flow field was calculated numerically by solving the Navier–Stokes equations and particle trajectories were then analyzed to obtain the collection efficiency curves at different flow rates. The resulting efficiency curves and corresponding cutoff diameters were compared and found to be in good agreement with the available experimental data. We also examined the effect of gravity on impactor performance and discuss the limitations of the single jet model.

Numerical study of mixed convection in a two-dimensional laminar incompressible offset jet flow

The mixed convection flow and heat transfer characteristics in a two-dimensional plane, laminar offset jet issuing parallel to an isothermal flat plate have been investigated numerically. The unsteady form of the vorticity transport equation has been solved to obtain the steady-state solution. The isothermal heated plate is 30 times the jet width and the offset ratio is 1. The behavior of the jet in the range of Reynolds number Re = 300 – 600 and Grashof number Gr = 10 3 – 10 7 is described in details. The medium considered is air ( Pr = 0.71). It is found that the reattachment length is strongly dependent on both Re and Gr for the range considered. Simulations are made to show the effect of entrainment on the recirculation eddy. The variation of the local Nusselt number is presented for various Re and Gr. An empirical correlation of average Nusselt number as a function of Richardson number ( Ri = Gr / Re 2 ) and Re has been given.

Heat transfer study of two-dimensional laminar incompressible offset jet flows

Heat transfer from an isothermal plate by a two-dimensional laminar incompressible offset jet is simulated numerically. The heat transfer characteristics of the jet with respect to offset ratio ( OR), Reynolds number ( Re) and Prandtl number ( Pr) are reported in details. At low Pr, conduction mode of heat transfer is dominant in the region. The isotherm is studied inside the recirculation region as well as in the reattachment location. The thermal boundary layer thickness is reduced when Pr is increased. Nusselt number is calculated for different OR and wide range of Re and Pr = 0.1 , 1 and 100. The variation of the average Nusselt number with different parameters is also reported in details.