FLOW IN PITS OF FLUID-DYNAMIC ORIGIN
A was made some years ago of the evidence of the nature of air flow over the surfaces of meteorites during their flight through air when they entered the earth's atmosphere. That evidence is left on the surface in the form of flow lines due to streaming of molten rock or iron oxide, just before the bodies in question were decelerated enough for ablation to cease. In that study the suggestion was made, as based on the observations, that the shallow pits that have often been observed on meteorites, particularly iron ones, might have been caused by peculiarities of the air flow over the meteorite surface rather than by inhomogeneities of the meteorite material. In particular, it seemed likely that a pit would form in a surface if the flow that causes ablation can exist in the form of a "bound vortex/ that is, a vortex with axis having a horseshoe shape which is held in one place on the ablating surface by the pressure gradients of the fluid flow. The pit would be formed at the apex of the horseshoe; the ends of the vortex would trail off to infinity in the wake. The horseshoe shape was concluded to be essential for the purpose of satisfying the continuity relation for the reversed flow implicit in any assumed vortex. As a rather direct method for demonstrating that vortices of the type proposed actually exist over the surface of an ablating body and that, when they do, pits will be formed, tests were made by use of steady water flow over the surface of large salt blocks such as are fed to cattle. The relation between bound vortices and surface pits was verified in that pits were formed where bound vortices were predicted to exist, and not where they were absent. Figure 1 shows one of the pitted blocks. The evidence of the horseshoe shape of the bound vortex, presumed to have carved the pit, is clear in the shape of the lower one. The question was posed how the ablation rate beneath a vortex could possibly be as great as that where no vortex exists; presumably, ablation rate depends on flow speed, which, in a vortex and especially inside a pit, certainly is less than it would be outside. An answer might be obtained if the flow about the pitted shape were studied in air flowing with the same Reynolds number as the water that shaped the block, under conditions of dynamic similarity, t For measurement of the air flow inside the pit, a special velocity probe was made. It consisted of two lengths of 0.046-in. capillary tubing, each length closed at one end and pierced with a small hole in a side as near to the closed end as possible. The two tubes were soldered together with the