Natsuo Hatta

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.

Upper-bound approach to plane strain strip rolling with free deformation zones

SummaryA recently developed extension of the upper-bound theorem is applied to the problem of rigid/plastic deformation in case where the surface configuration of the concerned body is previously unknown. By assuming a simple velocity field the strip rolling process is analysed by considering the free deformation zones in front of, and behind the roll gap. The results so obtained show that the transitional plastic deformation zone exists only in front of the roll gap without any bulge. Furthermore, we find that this zone becomes larger with increase of the strain sensitivity as well as the strain rate sensitivity of material.ÜbersichtEine vor kurzem entwickelte Erweiterung des Satzes von der oberen Schranke für starr-plastische Umformvorgänge wird auf ein Problem angewendet, bei dem die Gestalt der Materialoberfläche im voraus unbekannt ist. Und zwar wird mittels eines elementaren Geschwindigkeitsansatzes der ebene Walzvorgang unter Berücksichtigung der freien Umformzonen vor und hinter dem Walzspalt ermittelt. Die berechneten Ergebnisse zeigen, daß nur vor dem Walzspalt eine Übergangszone der plastischen Umformung ohne Wulstbildung existiert und daß diese Zone größer wird mit anwachsender Verfestigungs- und Geschwindigkeitsabhängigkeit des Materials.

Numerical experiments for effects of elastic strain energy on decomposition process of binary alloy in a two‐dimensional system

This paper is concerned with phase decomposition of a supersaturated solid solution in a two‐dimensional system. We have investigated the behavior of numerical solutions to the Cahn equation including an elastic strain energy term in such a way as to follow the time evolution of composition modulation by computer. It is found that in our numerical experiment the introduction of the elastic strain energy gives no anisotropic effect on the crystallographic direction of the composition modulation, but leads to the decrease in the decomposition rate and quantity at the equilibrium state. Again, it is demonstrated that for a high solute alloy system inside the spinodal region a large circular precipitate is formed at the center of the flat matrix through a very complicated process accompanied with the aggregation and annihilation of particles. The results so obtained are discussed from a numerical point of view.

Tensile properties of fine-grained 7475 aluminium alloy at various temperatures and strain rates

For a superplastic 7475 alloy, tensile properties are tested under various conditions in order to investigate the effects of the tensile conditions on the mechanical properties as well as on the microstructure after working. The results obtained here suggest a possibility of high-speed forming of this alloy. Under conditions of 400°C and 8.3×10−2s−1 strain rate, a comparatively large elongation of about 200% can be attained. This may be due to the fact that the fine subgrains arise during the deformation. Also, mechanical properties at room temperature of the specimen prestrained under this condition are found to be fairly satisfactory.