Mitsutomo Hirota

Correction Method for Particle Velocimetry Data Based on the Stokes Drag Law

A correction method based on the Stokes drag law was developed for flow velocimetry using tracer particles. The present method can be used to calculate the corrected velocity from the measured velocity and its spatial gradient without the need to solve the whole flowfield. Velocity measurement was conducted for sonic transverse injection into Mach 1.8 flow using particle image velocimetry. The correction method was applied to the velocity field obtained from the particle image velocimetry data. The corrected data showed the positions and shapes of the shock waves in the measurement region more clearly. The corrected peak velocity downstream of the shock waves became closer to the theoretical values. The jet streamlines calculated from the corrected and original particle image velocimetry data were compared. The streamlines from the corrected data went lower than those from the original data.

Effects of Injection and Main Flow Conditions on Supersonic Turbulent Mixing Structure

The effects of injection and main flow conditions on the turbulent structure produced by a gaseous sonic transverse injection into a supersonic airstream were investigated. The turbulent structure inherent in the mixing flowfield was characterized by the single-time two-point spatial correlations based on the concentration fluctuation and measured with acetone planar laser-induced fluorescence. Results revealed that the most intensively fluctuating region appeared on the 50%-averaged-concentration track, irrespective of the injection conditions. The highly correlated region, which indicates an organized large-scale structure, appeared as a tilting ellipse in the lower-compressibility cases and an ellipse almost parallel to the streamline in the higher-compressibility cases. These differences in structure result in the difference of the averaged jet width. The injection with the higher-compressibility condition showed more stable structures and slower development in the size due to suppression of the instability growth. The structure was likely to be stretched toward the averaged-velocity direction as the main flow Mach number increased. Increasing the velocity difference between the jet and main flow caused the large-scale eddies to appear more frequently.

Extended Quantitative Fluorescence Imaging for Multicomponent and Staged Injection into Supersonic Crossflows

The fluorescence ratio technique for processing planar laser-induced fluorescence data was extended for quantitative imaging of the injectant mole-fraction and density in multicomponent and multiinjection configurations in a nonreacting supersonic mixing flowfield. The method was experimentally validated in a mixing flowfield formed by single or staged sonic transverse injection into a Mach 2.0 supersonic air stream. Gaseous helium and air were used as injectants. Injectant mole-fraction distributions obtained by the present method were confirmed by comparison with gas-sampling data. Density distributions were validated with the oblique shock relation. The new method worked well for the multistaged injection configuration. Phase-locked measurement for the pulsed secondary injection was also tested. Results demonstrated the applicability of the new method to the pulse injection as well.

Strategy to diagnose ultra-lean (ϕ < 0.6) premixed flames by acetone-OH simultaneous PLIF with one-laser and one-detector combination

A strategy to diagnose ultra-lean flames utilizing the “acetone-OH simultaneous PLIF” concept via a one-laser and one-detector combination system is presented. The main aim of the present work is to overcome difficulties encountered in our previous studies; namely, that the seeding amount of acetone used for visualization purposes must be sufficiently small in order to avoid its effect on flame structure (at least <5% of fuel), while clear imaging also must be accomplished under such conditions. For this purpose, several important revisions have been made: (1) the 266-nm excitation line has been added to improve the fluorescence from acetone, (2) a dual-peak band-pass filter has been introduced instead of conventional blue filter and (3) controllability of fine acetone seeding has been improved. The effects of these changes on flame imaging are also investigated. Clear visualization of the flame zone of a very lean premixed flame, of which the limiting equivalence ratio is below 0.6, has been successfully achieved for the first time with one-laser and one-detector system. The time-dependent, near-extinction flame behavior is also clearly imaged, suggesting that this method could utilize to investigate the flame extinction study.Graphical Abstract

Supersonic Turbulent Mixing Structure in Staged Injection Flowfields

Scalar spatial correlations in the supersonic mixing flowfield produced by staged sonic transverse injection into a Mach 2.0 supersonic air stream were investigated using acetone planar laser-induced fluorescence (acetone PLIF) data. Gaseous helium was used for the primary injection and air was used for the secondary injection. Turbulent behavior was examined from side-view images. The single-time two-point spatial correlations based on the concentration fluctuations of injectant were obtained from the acetone PLIF data. Contour maps of the correlations indicate that significantly organized large-scale flow structures were recognized as inclined ellipses toward flow direction. They were seen in the upper jet boundary region and result in the width of the time-averaged jet boundary. By comparing with helium and air single injection cases, the staged injection exhibited enhancement of growing the large scale structure in size, and strong merging between the primary and the secondary jet which indicates the main flow air does not mix from the interface region of them but the periphery of the combined jet. PDF maps based on the instantaneous PLIF intensity revealed highly intermittent feature of the large-scale structure observed in the single-shot image and correlation maps.

Important Factors in the Starting Process of a Two-Dimensional Nozzle

This paper describes an experimental and numerical simulation study of the unsteady flow of the starting process of a two-dimensional nozzle. The analysis focuses on changes in the flow-separation point, in addition to the overall flow structure. The position of the separation point in the nozzle-starting process is simulated using a turbulent model and an appropriate arrangement of the computational grid. It was found that the flow structure near the nozzle throat wall is important in the nozzle-starting process. The boundary layer including the vortical structure in this area characterizes the flow structure of the downstream side. Moreover, the aspect ratio of the computational grid should be 1:1 near the nozzle throat wall in order to reproduce the results of the nozzle-starting experiment. Furthermore, a simplified RANS turbulence model cannot reproduce the separation point in the nozzle-starting process.

Measurement of Flow Field Produced by Ramp Vortices and Twin Swirl Jets

** †† Influence of the interaction between the streamwise vortices produced by a ramp in a supersonic flow and twin swirling jets for the mixing enhancement was experimentally investigated. Nominal Mach number of the main air stream and the twin swirl jets were 2.35 and 2.0, respectively. The directions of swirls were opposite and same with the streamwise vortices produced by the ramp. Three dimensional velocity distributions were measured using a Particle Image Velocimetry (PIV). Mie scattering images of the particles seeded in the injectant on cross section of the duct were recorded. In the opposite swirl case, strong outward flow was observed at the interaction point of the swirl in the jet and the ramp vortex. Mie-scattering images showed the difference of the injectant trajectories with the swirling condition. In the opposite swirl case, the injectant spread more widely and the injectant attached on the wall most slowly in the three cases.

Numerical and Experimental Investigations of Fluidic Thrust Vectoring with Oblique Shock Waves

Mechanical thrust vectoring (MTV) has already been put to practical use. It controls the thrust direction of jet propulsion system by mechanically moving structural components such as exhaust nozzles and paddles. MTV has several advantages such as high mobility during supersonic flight in the high-altitude where effective turning is difficult to perform due to low atmospheric density.