Visualization of impinging supersonic free jet on a tilt plate by LIF and PSP

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

Free surface instability in a confined suspension jet

A suspension confined between two close parallel plates is studied in the Stokesian regime. The use of boundary integral equations and the lubrication approximation allows to compute the hydrodynamic forces acting on the particles. The forces are long ranged and depend on the orientation of the relative position and velocity of particles. This tensorial character predicts an “antidrag” that is observed in experiments. The effect of the computed hydrodynamic forces is studied in the dynamics of a jet of particles falling by a gravitational field, which shows a surface instability similar to the Kelvin–Helmholtz one. A theoretical model, based on hydrodynamic-like equations, is able to predict the instability that is produced by the interaction of the long-range forces and the free surface.     

Time-resolved PIV investigation on the unsteadiness of a low reynolds number confined impinging jet

The flow characteristics in a confined slot jet impinging on a flat plate were investigated in low Reynolds number regime by using time-resolved Particle Image Velocimetry technique. The jet Reynolds number was varied from 404 to 1026, where it is presumed that the transient regime exists. We found that the vortical structures in the shear layer are developed with increase of Reynolds number and that the jet becomes remains steady at the Reynolds number of 404. Vortical structures and their temporal evolution are verified and the results were compared with previous numerical studies.     

Visualization of concentration field in a vortex ring using acetone PLIF

To improve the understanding of flame propagation through a nonpremixed vortex ring, the characteristics of fuel concentration in a vortex ring have been investigated experimentally. The vortex ring was generated by the ejection of propane with a single stroke motion of a speaker. Planar laser-induced fluorescence (PLIF) technique was adopted by seeding acetone as a tracer to fuel stream, in which the PLIF signal intensity is directly proportional to the concentration of acetone. This technique provides non-intrusive and instantaneous measurement of concentration field. Results showed that fuel concentration and its gradient decreased with the evolution of a vortex ring. When a nonpremixed flame propagated through a vortex ring, the flame location coincides with the inner most spiral mixing layer of fuel and air in a vortex ring.     

Method for analyzing the gas jet impinging on a liquid surface

The impingement of a gas jet on a liquid surface in the stable-state regime is analyzed theoretically. We consider the case of the perpendicular jet action. It is found that for describing analytically the processes occurring in this case, it is necessary to employ the balance equation for forces at the interface and not the balance equation for pressures at the lowest point of cavity, which was used in most available publications. Recommendations for experimental studies of a gas jet impinging on a liquid surface are formulated. We report on the results of experiments confirming the correctness of our theoretical analysis and making it possible to determine the empirical value of the shape factor. The experiments were carried out with air and epoxy resin. The cavity formed on the liquid surface had radius R₀ = 1–8 mm and depth h = 0.2–12.5 mm.     

Visualization of a rotating flow under large-deformed free surface using anisotropic flakes

This study aims to clarify the relationship between the deformation of a free surface and flow transition in a “switching phenomenon” process. In a flow driven by a rotating disk in a cylindrical open vessel, the free surface irregularly changes its shape from axisymmetric to nonaxisymmetric and vice versa with repeating up-and-down motion (so-called “switching phenomenon”). The flow under the free surface was visualized by anisotropic flakes. When the free surface assumes a parabolic shape, the flow is distinguished by three regions; local circulation region, rigid vortex region and meridional circulation region. The flow transition in the switching phenomenon was shown by snapshots and movies of the visualized flow; the flow near the free surface is laminar even if the shape of the free surface changes to nonaxisymmetric during the time at which the free surface attaches to the bottom of the vessel. After the free surface detaches from the disk, the flow near the free surface becomes turbulent. When the free surface changes to axisymmetric while descending to the bottom, the flow changes from turbulent to laminar flow and the local circulation region reemerges at the center of the vessel.     

Interaction of a submerged horseshoe vortex with a free surface

In this paper a submerged horseshoe vortex under a free surface is discussed and the algebraic expression of the wave elevation is obtained. From this expression, some characteristics of the ship wave are described. There exists a smooth region near $\theta = 0^\circ$, but when the uniform stream passes the other singularities (source, sink, doublet, etc.) there is no smooth region. The mechanism of synthetic aperture radar (SAR) images of the narrow ship wakes is also explained.     

Visualization and force measurement of high-temperature,supersonic impulse jet impinging on baffle plate

The flow visualization and force measurement of a supersonic impinging jet on a center-holed vertical baffle plate were investigated. Center-holed baffle plates of 2d to 5d in diameter, with a 1d center hole were tested, where d is the bore of the launch tube. The standoff distance of the baffle plates from the open end of the launch tube were varied to be from 2d to 5d. The supersonic impulse jet, with an incident shock wave of Mach 2.89 was produced by a high-enthalpy blast-wave simulator. The direction-indicating color schlieren method produced a two-dimensional density gradient of the flow field around the baffle plate. In addition, the flow fields were numerically analyzed, using two-dimensional asymmetric Euler equations. The results of the numerically-analyzed and the experimentally-visualized flow field agreed well. The visualized flow field indicates that the baffle plate should be at least 3.5d in diameter to deflect the supersonic impinging jet at an angle greater than a right angle. We have concluded that the representative method of designing muzzle brakes for military purpose accurately predicts the force yielded by the supersonic impinging impulse jet on the vertical baffle plate only when there is a large ratio of the baffle-plate diameter to the bore of the launch tube.     

Visualization of dilute hydrogen jet flame in air flow

The structure of hydrogen jet flame diluted by CO₂ in air flow is studied by various visualization techniques, such as schlieren, direct photograph, tracer injection and reactive Mie scattering method, which allow understanding of the influence of CO₂ on the characteristics of the hydrogen jet flame. The experimental result indicates that the flame structure consists of laminar fuel jet and surrounding reaction zone near the nozzle exit. When the CO2 fraction is increased, the width of the fuel jet grows and the reaction zone is reduced in size. These observations are further confirmed by quantitative measurements of temperature and velocity fields in the flame, which are evaluated by thermocouple and particle image velocimetry (PIV), respectively. These results indicate that the flame temperature is decreased and the flow rate of the fuel jet is increased by the influence of diluents, which are due to the reduced calorific value and larger density of fuel, respectively.     

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.     

Microscale heat transfer in a free jet against a plane surface

A new two-layer model has been proposed to study microscale heat transfer associated with a developing flow boundary layer. As an example, a cold, microscale film of liquid impinging on an isothermal hot, horizontal surface has been investigated. The boundary layer is divided into two regions: a micro layer at microscale away from the surface and a macro layer at macroscale away from the surface. An approximate solution for the velocity and temperature distributions in the flow along the horizontal surface is developed, which exploits the hydrodynamic similarity solution for microscale film flow. The approximate solution may provide a valuable basis for assessing microscale flow and heat transfer in more complex settings.