Vortex shedding from flat plates with square leading and trailing edges having chord-to-thickness ratios 3–16 at Reynolds numbers (1–3) × 103 is investigated experimentally in low-speed wind tunnels. It is shown that vortex shedding from flat plates with square leading and trailing edges is characterized by the impinging-shear-layer instability where the separated shear layer becomes unstable in the presence of a sharp trailing edge corner. The Strouhal number which is based on the plate's chord is approximately constant and equal to 0.6 for chord-to-thickness ratios 3–5. With further increase in the ratio it increases stepwise to values that are approximately equal to integral multiples of 0.6.
This paper describes a numerical study of the flow around flat plates with square leading and trailing edges on the basis of a finite-difference analysis of the two-dimensional Navier—Stokes equations. The chord-to-thickness ratio of a plate, d/h, ranges from 3 to 9 and the value of the Reynolds number based on the plate's thickness is constant and equal to 103. The numerical computation confirms the finding obtained in our previous experiments that vortex shedding from flat plates with square leading and trailing edges is caused by the impinging-shear-layer instability. In particular, the Strouhal number based on the plate's chord increases stepwise with increasing d/h in agreement with the experiment. Numerical analyses also provide some crucial information on the complicated vortical flow occurring near the trailing edge in conjunction with the vortex shedding mechanism. Finally, the mechanism of the impinging-shear-layer instability is discussed in the light of the experimental and numerical findings.
Fast microtomography combined with local crack driving force analysis has been employed to analyze crack-tip stress/strain singularities in an aluminum alloy. The application of fast microtomography has made it possible to observe real crack initiation and propagation behaviors without intermediate unloading. The details of a crack and its local propagation behaviors are readily observed with this technique along with evidence of microstructure/crack interactions. After a preliminary investigation of the achieved spatial resolution, we show that conventional stationary and growing crack singularities can be quantitatively validated by deriving the local crack opening displacement. This is to our knowledge the first three-dimensional validation of conventional fracture mechanics during a real time continuous experiment that has been mainly developed via surface observations so far. We also reveal that there is a spatial transition from a stationary crack singularity to a growing crack singularity in addition to the well-known temporal transition that occurs with the onset of crack propagation. Local crack propagation behaviors are also discussed on the basis of this validation. To separate the effects of complex crack geometry from those of microstructure, we also perform an image-based numerical simulation.
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