When a fluid in a tube is occluded, one finds a static configuration in which the occluding free surface of the fluid is an equilibrium capillary surface spanning the tube. We extend known criteria for existence and non-existence of such a surface, leading to an explicit mathematically rigorous occlusion criterion for cylindrical tubes in a transverse body force field, depending on the force magnitude and contact angle. For any contact angle γ = π/2, we provide further an explicit design of a tube section, which will not occlude in a downward gravity field, regardless of the field strength. In addition, we derive a precise analytic occlusion criterion for liquid partially filling a circular vessel spinning about its axis.
Simple, well-characterized experimental perturbations are needed to validate CFD simulations of supersonic and hypersonic boundary-layer receptivity. A laser-generated hot-spot perturbation has been characterized and modeled, based on experiments in the Purdue University Mach-4 quiet-flow Ludwieg tube. The model provides a detailed density profile and time evolution for the perturbation, which consists of a hot spot and the surrounding weak shock. The model is validated by comparing simulated optical path differences to measurements from a highsensitivity, high-bandwidth laser differential interferometer. CFD simulations are compared with receptivity experiments on a hemisphere to illustrate the usefulness of the perturbation model.
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