Showing papers on "Freestream published in 1971"

Breakup of liquid sheets and jets in a supersonic gas stream

Abstract: Experimental and analytical results are presented with the objective of denning the mechanism of liquid sheet and j et breakup when subj ected to a supersonic gas stream. Liquid sheets are studied with photomicrographs and high-speed movies of the activity of a liquid layer maintained upon a porous plate test model in a parallel Macli 2.2 freestream. Tests with several different liquids show wave motion, with droplet and ligament shedding across the liquid surface. Numerical results from a liquid surface stability analysis are used to explain these observations. Liquid jets are studied with spark shadowgraphs, high-speed movies and photomicrographs of the normal injection of various liquids into a Mach 2.1 freestream. The results show that the breakup mechanism is characterized by gross jet fracture, as opposed to surface disintegration. 'the degree of breakup at a given streamwise location and jet spread after injection are found to be related to injection diameter and dynamic pressure, and certain liquid properties.

A Turbulent Boundary Layer with Mass Addition, Combustion, and Pressure Gradients

Abstract: : A subsonic turbulent boundary layer with mass addition and combustion is studied to investigate the effects of combustion on the velocity field in constant pressure and accelerating flows. Particular attention is given to determining (1) the extent to which combustion alters the flow and (2) the mechanism whereby combustion interacts with the flow field. The experimental results demonstrate that combustion alters the velocity profiles in both constant pressure and accelerating flows. The velocity gradients at the surface in combusting flows differ markedly from those of corresponding isothermal flows and the velocity in the flame regions of accelerating flows actually exceed the freestream value. The results of analysis indicate that in a subsonic turbulent boundary layer with combustion the Reynolds stress is essentially kinematic and does not explicitly involve density fluctuations. This in turn indicates that the experimentally observed changes in the velocity profiles are attributable to the temperature dependence of the local mean density and molecular viscosity. Analytical results also indicate that the combustion-induced changes in velocity profile are strongly dependent on the axial pressure gradient. The consequence of combustion with regard to skin friction is also examined and it is found that effects on the skin friction coefficient are similar to those on the wall velocity gradient. A method of determining the velocity profiles in a combusting turbulent boundary layer is also presented. (Author)

Comparison of theory and experiment for ion collection by spherical and cylindrical probes in a collisional plasma

Abstract: Roshko for vortex shedding in steady flow. Our data points in general are lower than the Roshko curve, however, they do show a definite correlation between the instantaneous Strouhal and Reynolds numbers. These results suggest the following conclusions: 1) In an oscillatory freestream of 3 Hz and Reynolds number up to 4 X 10, the vortex shedding from a circular cylinder responds instantaneously to the freestream variations. 2) In the instantaneous Reynolds number range of 500 to 4 X 10, the instantaneous Strouhal number stays sensibly constant at 0.20, ±0.01. 3) With a limited set of data points in the instantaneous Reynolds number range of 3 to 8 X 10, the results show no systematic variation when the frequency is increased from 3 to 6 Hz.

An Experimental Investigation of the Stability of a Thin Liquid Layer Adjacent to a Supersonic Stream

Abstract: The experiments to study the interface response of a liquid film with a concurrent supersonic airflow were conducted at freestream Mach numbers of 4.95 and 7.3 with stagnation pressures of 3.4, 6.8, and 17.0 atm. A sphere-cone and a blunt wedge were used as test configurations. The liquid layer Reynolds number jR, based on interface velocity and liquid depth, ranged from approximately 0.3 to 300.0 as the thickness ranged independently from 0.01 to 0.05 cm. The nondimensional wave speed was found to decrease monotonically with increasing R from a value greater than the interface velocity ("fast" waves) to a value lower than the interface velocity ("slow" waves). The transition from fast waves to slow waves appeared to occur at a Reynolds number near 100. The dependence of wavelength and wave speed on Reynolds number, shear stress, Mach number, and depth are given. In the small Reynolds number range, the results are compared with a long wave analysis to show the destabilizing effects of Reynolds number and shear stress. In the high Reynolds number range, the results are compared with an analysis based on a Tollmien-Schlichting instability mechanism.

Effects of Engine Exhaust Flow on Boattail Drag

Abstract: Wind-tunnel tests were conducted to investigate the effects of jet plume shape and entrainment on hoattail pressure drag. The results were used to determine nozzle drag levels at various engine operating conditions as well as at conditions related to airplane force models. An isolated nozzle model with a pressure-tapped exterior and changeable internal parts was tested subsonically to examine changes in drag due to alterations in internal geometry and nozzle pressure ratio. In addition, tests were run with solid plume-shaped sleeves as a means to separate plume-shape effects from jet entrainment effects. Large differences in drag were measured with changes in plume shape, and, in certain regimes, jet entrainment also had a significant effect. The results of this study include boattail pressure distributions, integrated drag coefficients, and a comparison of test data with analytically predicted drag levels. Nom enclatur e A = cross-sectional area CD = boattail pressure drag coefficient (drag/g<^4.m) Cp = boattail pressure coefficient [(P — Po)/qQ] D,d = diameter h = boundary-layer height L = length of boattail MQ = freestream Mach number NPR = nozzle pressure ratio (PTJ/PO) P = local static pressure Po = freestream static pressure PTJ = exhaust jet total pressure q = dynamic pressure R = radius /3 = boattail trailing-edge angle Subscripts boattail terminal plane jet B,b J M,m T maximum freestream total

Surface mass injection at supersonic and hypersonic speeds as a problem in turbulent mixing. II - Axially-symmetric flow

Abstract: The effects of gas injection normal to the surface of two-dimensional bodies in supersonic or hypersonic flight were determined in the previously published Part I of the present study. In Part II the effects of axial-symmetry are considered, and for the two-dimensional case, it is found that significant streamline inclinations and surface pressures are generated by the action of turbulent viscous dissipation alone. For a "cold" surface at hypersonic speeds, similarity parameters are obtained which relate the induced streamline inclinations and surface pressures to the nondimensional blowing rate. The exact form of these relationships for a cone depends on the order of magnitude of the ratio of induced streamline deflection angle to cone angle. When the surface is not cold relative to the freestream total temperature, the magnitude of the induced streamline deflection depends on the velocity profile shape factor. For the two-dimensional case this shape factor becomes infinite as the nondimensional blowing rate approaches a fixed value corresponding to boundary-layer "blowoff." For the axially symmetric case, solutions can be obtained for blowing rates much larger than the rate corresponding to the two-dimensional blowoff value.

Hot wire measurements of freestream and shock layer disturbances.

Abstract: Preliminary results of hypersonic helium tunnel investigations of whether disturbance measurements in the freestream alone are adequate to describe the model boundary-layer input disturbances, or whether the model shock wave changes the freestream disturbances before they reach the model boundary layer. It was found that the spectra in the freestream are typical of the wide band turbulence as it exists for sound radiated from a turbulent boundary layer. In the shock layer some redistribution of the spectra seems to occur, especially at the highest stagnation pressure, but the more interesting feature is the gradual development of a discrete component, around 70 kHz in the spectra. This feature is believed to be associated with boundary-layer transition.

Interaction of a compressible fluid and an elastic membrane

Abstract: The paper presents results from a computer method for analyzing the unsteady interaction of a fluid and an initially flat circular elastic membrane. The method consists of an implicit coupling of computer programs for the fluid flowfield and for the membrane. The pressures and velocities in the flowfield are determined from a scheme based on the method of characteristics in three independent variables. The membrane is represented by a discrete number of points which are tracked as a function of time; lines connecting these points separate the fluid field into noninteracting regions. Deflected membrane shapes, pressure profiles, and the flowfield at various times are presented. The maximum deflection from the coupled analysis is shown to be only 60% of that obtained when the pressure on the membrane is maintained at the pressure associated with stagnating the freestream. Cp c E h hr M N P Ps R r s t U

Measurements in ablating air-teflon boundary layers

Abstract: Theoretical descriptions of the detailed structure of an ablating air-Teflon laminar boundary layer predict CF2 to be a major constituent in the boundary layer. This species has not been observed experimentally. This paper presents spectrally and spatially resolved uv thermal radiation measurements that demonstrate the presence of CF2 in the stagnation point boundary layer of ablating Teflon cylinder. The experiments were performed in an arc facility which provided subso nic air or nitrogen jets at 2500-4500°K and -^ atmospheric pressure. The state of the freestream was measured by the electron beam fluorescence technique.

Eddy Viscosity in Quiescent and Coflowing Axisymmetric Jets

Abstract: where, p — the mean density, U = the mean axial velocity, ( }e = freestream, r = the radial coordinate, a = the initial radius of the jet. The most important point in favor of applying eddy viscosity models to problems in turbulent mixing is that at this time it is the only model available for analysis of problems of engineering interest, e.g., a high-speed hydrogen jet exhausting into a hot air stream. Its most significant shortcoming is that it requires the shear stress be described in terms of local flow parameters, i.e., neglect upstream history effects. There have been attempts to incorporate history effects into the model by introducing an additional equation for either the Reynolds stress or eddy viscosity.' But these approaches are of little use until a means of specifying the initial shear stress distributions is obtained, e.g., a model such as eddy viscosity. Equation (1) was developed from an extension of Clauser's model for the wake region of a turbulent boundary layer to axisymmetric free shear layers. When the mass defect across a shear layer is small, but the viscosity gradient is not, the model erroneously predicts a vanishing eddy viscosity. However, this limitation must be accepted if the mass defect concept is to be retained. The subject of this Note is a

Results of a Shock Tube Experiment in Compressible Turbulent Mixing.

Abstract: : Two-dimensional supersonic turbulent mixing layers were produced with a small shock tunnel whose driven section is divided into two parallel chambers. When desired, combustion in the mixing layer was produced by use of oxygen in one stream and an argon-hydrogen mixture in the other. Variation in refractive index through the mixing layer was measured with a Mach-Zehnder interferometer and pressures were measured with piezo-electric transducers. From these measurements, freestream conditions could be computed. Approximate temperatures could be deduced directly from the interferograms throughout the mixing layer. A finite difference computer program which solves the turbulent conservation equations in terms of an empirical eddy viscosity model and a simple ignition delay-reaction time model for the chemistry was used to reconstruct the refractive index contours. Such an approach can describe the conditions in the mixing layer to an adequate degree of accuracy. It was found that for nonreacting mixing layers, the growth rates were comparable to those obtained from the literature for incompressible layers, and that the liberation of chemical energy increased mixing rates considerably, at least for the conditions of this experiment. (Author)

Measurements in ablating air Teflon boundary layers

Abstract: Theoretical descriptions of the detailed structure of an ablating air-Teflon laminar boundary layer predict CF2 to be a major constituent in the boundary layer. This species has not been observed experimentally. This paper presents spectrally and spatially resolved uv thermal radiation measurements that demonstrate the presence of CF2 in the stagnation point boundary layer of ablating Teflon cylinder. The experiments were performed in an arc facility which provided subso nic air or nitrogen jets at 2500-4500°K and -^ atmospheric pressure. The state of the freestream was measured by the electron beam fluorescence technique.