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Deissler, R. G.

Publications and source records attributed to Deissler, R. G..

At least 37 records · Page 2
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NASA NTRS1976

Tornadoes and other atmospheric vortices

The growth of random vortices in an atmosphere with buoyant instability and vertical wind shear is studied along with the velocities in a single gravity-driven vortex; a frictionless adiabatic model which is supported by laboratory experiments is first considered. The effects of axial drag, heat transfer, and precipitation-induced downdrafts are then calculated. Heat transfer and axial drag tend to have stabilizing effects; they reduce the downdrafts of updrafts due to buoyancy. It is found that downdrafts or tornadic magnitude might occur in negatively-buoyant columns. The radial-inflow velocity required to maintain a given maximum tangential velocity in a tornado is determined by using a turbulent vortex model. Conditions under which radial-inflow velocities become sufficiently large to produce tangential velocities of tornadic magnitude are determined. The radial velocities in the outer regions, as well as the tangential velocities in the inner regions may be large enough to cause damage. The surface boundary layer, which is a region where large radial inflows can occur, is studied, and the thickness of the radial-inflow friction layer is estimated. A tornado model which involves a rotating parent cloud, as well as buoyancy and precipitation effects, is discussed.

Deissler, R. G.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the initial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given; the equations are used to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the intial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given, in order to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1975

Comparison of theory and experiment for homogeneous turbulence with shear

Solutions for uniformly-sheared turbulence, in which the interaction of the turbulence with the mean shear dominates the turbulent self-interaction, are compared with experiment. An anisotropic spectral tensor, which appears general enough to represent the initial experimental turbulence, is used for the initial condition in the calculations. The evolution of one-point turbulence components and microscales, as well as two-point velocity correlations, are considered. In most cases the agreement with experiment is good. The theory correctly predicts the presence of a negative region for two-point longitudinal-velocity correlations only for point separations in the direction normal to the flow and the mean gradient.

Deissler, R. G.↗
NASA NTRS1975

Turbulence processes and simple closure schemes

The closure problem in turbulence is briefly reviewed, and some simple closure schemes are introduced. Processes occurring in turbulent flow are discussed on the basis of solutions for some elementary flows such as homogeneous turbulence with and without uniform shear. Closure by specification of initial conditions, and finally practical closure schemes for more complicated flows (e.g., pipe flows) are considered briefly. The latter include Reynolds stress, eddy viscosity, and mixing-length closures.

Deissler, R. G.↗
NASA NTRS1975

Turbulence processes and simple closure schemes

The closure problem in turbulence is reviewed, and some simple closure schemes are introduced. Processes occurring in turbulent flow are discussed on the basis of solutions for some elementary flows such as homogeneous turbulence with and without uniform shear. Closure by specification of initial conditions, and finally practical closure schemes for more complicated flows are considered. The latter include Reynolds stress, eddy viscosity, and mixing-length closures.

Deissler, R. G.↗
NASA NTRS1974

Tornadolike gravity-driven vortex model

The buoyancy-induced vorticity concentration produced as the fluid in a vortex accelerates vertically was studied. The boiloff from liquid nitrogen, to which a small amount of initial vorticity was added, provided a source of cool, heavy gas in which a concentration of vorticity took place. Condensation streamers made the flow visible. It is shown that the presence of a surface boundary layer is not necessary for the effective concentration of vorticity. A simple theoretical analysis of the phenomenon was also made. A radial contraction of the flow with vertical position and a characteristic hook shape in the top view of the streamlines were observed in both theory and experiment. The vorticity concentration observed may be similar to that which occurs in tornadoes.

Deissler, R. G.↗
NASA NTRS1974

Evolution of a moderately short turbulent boundary layer in a severe pressure gradient

The early and intermediate development of a highly accelerated (or decelerated) turbulent boundary layer is analyzed. For sufficiently large accelerations (or pressure gradients) and for total normal strains which are not excessive, the equation for the Reynolds shear stress simplifies to give a stress that remains approximately constant as it is convected along streamlines. The theoretical results for the evolution of the mean velocity in favourable and adverse pressure gradients agree well with experiment for the cases considered. A calculation which includes mass injection at the wall is also given.

Deissler, R. G.↗
NASA NTRS1974

Remarks on the decay of homogeneous turbulence from a given state

A previous theory which did not require a usual closure assumption required three or more initial spectra. By allowing a simple physical assumption (a modification of Kovasznay's (1948) hypothesis), the required number of spectra is reduced to two. Agreement with experiment is good.

Deissler, R. G.↗
NASA NTRS1973

Nonlinear evolution of a disturbance in an unbounded viscous fluid with uniform shear

The evolution of a disturbance in the presence of a uniform mean velocity gradient is calculated by a power-series solution of the incompressible Navier-Stokes equations. Terms through those in time cubed are retained in the solution. For the initial condition a three-dimensional cosine distribution with two harmonic terms is assumed. The nonlinear interaction of these harmonic terms produces new harmonics which in turn interact. For large velocity gradients the energy of the disturbance can grow with time. The results shed some light on the maintenance or growth of turbulence in a shear flow.

Deissler, R. G.↗
NASA NTRS1972

Growth of turbulence in the presence of shear.

On the basis of calculated results for some simplified models, the mechanism of turbulence growth or maintenance in a shear flow is examined. The study is aimed mainly at determining whether, from a theoretical viewpoint, the effect of a mean shear can be great enough to offset the effects of viscosity and keep a turbulent field from decaying, regardless of whether the turbulence ultimately grows or reaches a steady state. The results obtained suggest that a mean shear can produce a nondecaying turbulence field.

Deissler, R. G.↗
NASA NTRS1972

Further comparison of theory and experiment for decay of homogeneous turbulence.

An analysis of the decay of homogeneous turbulence from a given initial state is compared with the experiment of Lin and Huang (1970). Comparisons were made for decay of turbulent energy, decay of energy-transfer spectra, decay of three-dimensional turbulent-energy spectra, and decay of higher-order spectral quantities V, R, and S, where these respectively represent functionals of three-, four-, and five-point spectral quantities. Good agreement between theory and experiment is noted.

Deissler, R. G.↗
NASA NTRS1972

Decay of homogeneous turbulence from a specified state

The homogeneous turbulence problem is formulated by first specifying the multipoint velocity correlations or their spectral equivalents at an initial time. Those quantities, together with the correlation or spectral equations, are then used to calculate initial time derivatives of correlations or spectra. The derivatives in turn are used in time series to calculate the evolution of turbulence quantities with time. When the problem is treated in this way, the correlation equations are closed by the initial specification of the turbulence and no closure assumption is necessary. An exponential series which is an iterative solution of the Navier stokes equations gave much better results than a Taylor power series when used with the limited available initial data. In general, the agreement between theory and experiment was good.

Deissler, R. G.↗