Yoshikuni Umeda

Observations of the oscillation modes of choked circular jets

Under‐expanded jets exhausting from convergent circular nozzles are known to emit a powerful acoustic tone called screech. The steady decrease of the screech frequency with increasing pressure ratio is interrupted by four frequency jumps as the mode of jet instability changes successively from an axially symmetric (varicose, torroidal) one labeled A1 (varicose, torroidal), to another but similar one labeled A2, a sinuous (lateral, flapping) one B, a helical one C, and finally one that is identified here as a sinuous one, D. Digital FFT analyses disclose the simultaneous presence of associated secondary tones about 25 dB weaker than the dominant ones, the frequency of these secondary tones forming smooth continuations of the dominant tones. As a secondary tone connects the dominant B and D modes, it is inferred that these are actually the same mode interrupted by the helical mode C. However, their detailed behavior is significantly different: the plane of oscillation of B appears to have a preferred orient...

Numerical analysis of gas-particle two-phase flows

This paper is concerned with a numerical analysis of axisymmetric gas-particle twophase flows. Underexpanded supersonic free-jet flows and supersonic flows around a truncated cylinder of gas-particle mixtures are solved numerically on the super computer Fujitsu VP-400. The gas phase is treated as a continuum medium, and the particle phase is treated partly as a discrete one. The particle cloud is divided into a large number of small clouds. In each cloud, the particles are approximated to have the same velocity and temperature. The particle flow field is obtained by following these individual clouds separately in the whole computational domain. In estimating the momentum and heat transfer rates from the particle phase to the gas phase, the contributions from these clouds are averaged over some volume whose characteristic length is small compared with the characteristic length of the flow field but large compared with that of the clouds. The results so obtained reveal that the flow characteristics of the gas-particle mixtures are widely different from those of the dust-free gas at many points.

On the sound sources of screech tones radiated from choked circular jets

The generation mechanism of the screech tone in the helical oscillation mode is mainly investigated using a series of instantaneous schlieren photographs. From the photographs, five evanescent sound sources are observed as prominent points along the jet axis. The sound source for the dominant helical oscillation mode is found to be the second prominent point which moves along a circular orbit in a plane perpendicular to the jet axis and just downstream of the rear edge of the third shock cell. It is shown that the speed of a moving sound source is supersonic and that the Mach cone generated by the moving sound source forms the helical-shaped wave front of the screech tone for the helical oscillation mode of the jet. This idea of the moving sound source is well supported by a measured directionality of the screech tone. Sound sources of the other oscillation modes appearing in the other pressure ratio ranges are also described.

Experimental and numerical analysis of circular pulse jets

Unsteady circular jets are treated experimentally and numerically. The time evolution of circular pulse jets is investigated systematically for a wide range of jet strength, with the focus on the jet evolution, in particular the formation processes of Mach disks in the middle stage and of shock-cell structures in the later stage. It is shown that unsteady second shocks are realized for all sonic underexpanded jets and they either breed conical shocks for lower pressure ratios or truncated cones (Mach disk and reflected shock) for higher pressure ratios. The vortex ring produced near the nozzle lip plays an important role in the formation of the shock-cell structure. In particular, interactions between the vortex ring and the Mach disk connected with a strong second shock affect remarkably the formation process of the first shock cell. Different formation processes of the first cell structure are found. It is also made clear that the Kelvin-Helmholtz instability along slip surfaces originating from the triple point at the outer edge of the Mach disk is responsible for the generation of large second vortices which entrain the first vortex. This results in strong mixing between the primary jet and surrounding gas for higher pressure ratios

Bubbly flows through a converging–diverging nozzle

Characteristics of bubbly flow through a vertical, two‐dimensional, converging–diverging nozzle are investigated theoretically and experimentally. First, a new model equation of motion governing a dispersed bubble phase is proposed. This is compared in detail with those proposed previously by several researchers. Next, hyperbolicity of the resultant systems of governing equations for the bubbly flow is investigated in detail. Numerical simulations of bubbly flows through a converging–diverging nozzle are carried out by using the proposed system of model equations. In order to check the validity of the numerical results and then the proposed system, they are compared with the experiments which were performed previously in the authors’ laboratory in a water–nitrogen blow‐down facility. It will be shown that the numerical results obtained by using the proposed system of model equations agree well with the experiments.

Discrete tones generated by the impingement of a high‐speed jet on a circular cylinder

Discrete tones generated from the interaction of high‐speed jets with solid boundaries are investigated experimentally. Measurement of far‐field pressure waves and Schlieren flow visualization were conducted. Emphasis is placed upon the generation mechanism of a discrete tone that is generated when a circular cylinder is placed in a jet at a high Reynolds number. This discrete tone was observed to be generated for jet exit Mach numbers greater than 0.6 and distances from the nozzle lip to the cylinder less than about eight times the nozzle diameter. By the Schlieren flow visualization, it is confirmed that this discrete tone is radiated from the jet by a strong feedback resonance. When an eddy (a ring vortex) interacts with the cylinder, an acoustic pulse is emitted with a strong directional peak in the upstream direction. The acoustic pulse propagates outside the jet and, upon reaching the nozzle lip, forces the generation of a new eddy. As the new eddy approaches the cylinder it regains the strength of the original eddy by extracting energy from the potential core of the jet and so maintains the cycle.

Nozzle flows of gas–particle mixtures

A numerical analysis of supersonic nozzle flows of gas–particle mixtures is described. A time‐dependent technique is applied to solve a two‐phase inviscid flow through an axially symmetric nozzle. The two‐step MacCormack algorithm is used for the gas‐phase flow and the method of characteristics is applied to the particle‐phase flow. Attention is mainly focused on the location of the limiting particle streamline. The numerical results are compared with the theoretical results obtained previously and also with experiments.

Supersonic gas-particle two-phase flow around a sphere

This paper describes supersonic flows of a gas-particle mixture around a sphere. The Euler equations for a gas-phase interacting with a particle one are solved by using a TVD (Total Variation Diminishing) scheme developed by Chakravarthy & Osher, and the particle phase is solved by applying a discrete particle-cloud model. First, steady two-phase flows with a finite loading ratio are simulated. By comparing in detail the dusty results with the dust-free ones, the effects of the presence of particles on the flow field in the shock layer are clarified. Also an attempt to correlate the particle behaviours is made with universal parameters such as the Stokes number and the particle loading ratio. Next, non-steady two-phase flows are treated. Impingement of a large particle-cloud on a shock layer of a dust-free gas in front of a sphere is numerically simulated. The effect of particles rebounded from the sphere is taken into account. It is shown that a temporal reverse flow region of the gas is induced near the body axis in the shock layer, which is responsible for the appearance of the gas flow region where the pressure gradient becomes negative along the body surface. These phenomena are consistent with the previous experimental observations. It will be shown that the present results support a flow model for the particle-induced flow field postulated in connection with ‘heating augmentation ’ found in the heat transfer measurement in hypersonic particle erosion environments. The particle behaviour in such flows is so complicated that it is almost impossible to treat the particle phase as an ordinary continuum medium.

Oscillation modes of underexpanded jets issuing from square and equilateral triangular nozzles

The oscillation modes of underexpanded jets exhausted from square and equilateral triangular nozzles were investigated experimentally. Two stages were observed for both jets. The oscillation modes corresponding to these stages were identified by using optical and acoustical methods. The experimental results show that oscillation modes for both jets are first axisymmetric and then sinuous with increasing jet pressure ratio. The helical oscillation mode, which appeared for the circular jet, was not observed in these jets. The hysteresis phenomenon found for the circular jet cannot be seen in these jets.

Existence of mach cones and helical vortical structures around the underexpanded circular jet in the helical oscillation mode.

Existence of Mach cone and helical vortical structure in the helical oscillation mode of an underexpanded circular jet was confirmed by using schlieren instantaneous photographs and drawing of the envelopes of the Mach cones by the superposition of spherical sound waves radiated from two moving sound sources about the jet axis at a supersonic speed. Existence of such structures was conjectured in our earlier paper [Umeda and Ishii, J. Acoust. Soc. Am. 110, 1845-1858 (2001)]. The envelopes of a Mach cone are observed as a V-shaped pattern composed of a pair of clear fine lines starting from a prominent point, which rotates about the jet axis. The helical vortical structure is observed as a bright pattern of the gathering of the tiny specks around the jet. It always appears to overlap on the envelopes of the moving Mach cones.