The effects of flow interference between three equal circular cylinders, which were arranged in an equidistant triangular cluster with their axes perpendicular to the uniform flow, were studied at various inclination angles to the uniform flow and at different spacing ratios. The static pressure around the circumference of each cylinder were measured individually and the drag and lift coefficients of each cylinder were determined at Reynolds number of 6.2×10⁴ based on the diameter of a single cylinder.
The effects of flow interference between the three cylinders are severe when the spacing ratio are small. When the two cylinders arranged side by side are upstream or behind the third one, the flow patterns are not symmetrical with respect to the uniform flow direction and the drag and lift coefficients of the two cylinders in the side-by-side arrangement are not always equal to each other at small spacing ratios.
When the inclination angles are changed, the drag and lift coefficients, the direction of force exerted on each cylinder and the flow patterns around the cylinders change sensitively. The effect of interference between three cylinders becomes weak as the spacing ratios are increased.

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Mansour and Lundgren investigated the three-dimensional linear instability of a swirl subject to a time-periodic axial strain. They reduced the problem to a matrix-Floquet problem under the low-Mach number approximation. The vortex is destabilized through resonant interaction between the vortex rotation and the oscillation of the strain field. In this paper, we generalize their analysis to cases where the direction of strain is not parallel to the vortex axis. When “the inclination angle between the direction of strain and the vortex axis : x” is non-zero, the wavenumber of disturbance bahaves time-periodically, quasi-time-peridically and exponentially, depending on “the ratio of the period of vortex rotation to the period of the strain field : s”. If the wavenumber is time-periodic, we can determine the instability growth rate based on the Floquet analysis. Otherwise, we calculate the approximate Liapunov exponents numerically and discuss the instability characteristics using them. The instability characteristics show sensitive dependence on the two parameters x and s.

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Premixed flame propagation of methane-air mixuture in a closed vessel is estimated through the direct numerical simulatinon of three dimensional unsteady Navier-Stokes equations. In order to deal with the strong density and temperature variation across the flame, the extended version of the MAC method is employed as a numerical method. The chemical reaction is assumed to be an irreversible single step reaction between methane and oxygen. The chemical species are CH₄, O₂, N₂, CO₂ and H₂O.
The simulation reproduces the transformation into a tulip flame during the flame propagation, and the results are agreed with experimental results. Especially, it is verified that the flame-flow interaction causes the flame transformation process. This work implies the possibility of the direct numerical simulation of the three dimensional flame propagation in the future.

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