Local Density Variations Research Articles

Local Density Variations in Planetary Nebulae

Local Density Variations in Planetary Nebulae

Turbulent gravitational convection from maintained and instantaneous sources

Theories of convection from maintained and instantaneous sources of buoyancy are developed, using methods which are applicable to stratified body fluids with any variation of density with height; detailed solutions have been presented for the case of a stably stratified fluid with a linear density gradient. The three main assumptions involved are (i) that the profiles of vertical velocity and buoyancy are similar at all heights, (ii) that the rate of entrainment of fluid at any height is proportional to a characteristic velocity at that height, and (iii) that the fluids are incompressible and do not change volume on mixing, and that local variations in density throughout the motion are small compared to some reference density. The governing equations are derived in non-dimensional form from the conditions of conservation of volume, momentum and buoyancy, and a numerical solution is obtained for the case of the maintained source, This leads to a prediction of the final height to which a plume of light fluid will rise in a stably stratified fluid. Estimates of the constant governing the rate of entrainment are made by comparing the theory with some previous results in uniform fluids, and with the results of new experiments carried out in a stratified salt solution. For the case of an instantaneous source of buoyancy there is an exact solution; the entrainment constant is again estimated from laboratory results for a stratified fluid Finally, the analysis is applied to the (compressible) atmosphere, by making the customary substitution of potential temperature for temperature. Predictions are made of the height to which smoke plumes from typical sources of heat should rise in a still, stably stratified atmosphere under various conditions.

Diastrophism During Historic Time in Gulf Coastal Plain

The Gulf Coastal Plain is an area of active diastrophism. Five classes of evidence are discussed--earthquakes, fissures and faulting, regional warping, local subsidence and uplift, and pseudo-diastrophic features. Although the Gulf Coast is described as an area of low seismicity, many earthquakes have occurred in the past and more should be expected in the future. Fissuring and faulting have been taking place in the Gulf Coast during historic time. The same statement is true for regional warping. Many cases of local subsidence have been recorded and in some localities subsidence and uplift are taking place to-day. Hoskins Mound salt dome is believed to have risen ½ foot in 23 years. Much of the diastrophism occurring in the Gulf Coast is believed to be the result of the force of gravity. Two important factors influencing such diastrophism are, local inequalities in support and large local variations in density. Where such conditions and forces are present, compaction of the sediments by grain rearrangement often influences the form which results and may in some cases be a cause of diastrophism. Compaction may be aided by microseisms. The writer asks if the warping noted might be the result of waves in the earth's crust traveling at geologically slow speeds. The basin of the Gulf of Mexico is compared with a shallow rubber pan partly filled with stiff mud which is constantly being stretched, squeezed, and warped. Many diastrophic features affect physiography and can be traced by use of aerial photographs even where they can not be followed on the ground. More publicity should be given to the subject of historic diastrophism to encourage individuals to make known their personal observations.