A Dewpoint Meter Using Cooling by Expansion of CO2

For use in certain aerodynamical problems a dewpoint meter using the Joule-Thompson effect, with CO2 as cooling agent, has been developed. The instrument described here has some advantages over the common instrument which depends oupon the evaporation of ether. Two slightly different devices have been used successfully

On the Spectrum of Isotropic Turbulence

Measurements of the spectrum and correlation functions at large Reynolds number (RN ~ 10^5 based on the grid mesh) have been made, as well as a series of accurate spectrum measurements at lower Reynolds number (RN ~ 10^4). The results are compared with the theoretical laws proposed in recent years. It is found that the measurements at large Reynolds numbers exhibit a range of frequencies where the spectrum is nearly of the form n^- 5/3. The largest part of the spectrum in the initial stage of decay at the lower Reynolds number was found to follow closely the simple spectrum A/[B + n^2] , where A and B are constants and n is the frequency of fluctuation. At x/M = 1000 (where x is the distance behind the grid and M is the mesh size) the spectrum approaches a Gaussian distribution. The second, fourth, and sixth moments of the spectrum have been computed from the measurements and are discussed In relation to theoretical results. The significance of the number of zeros of the fluctuating velocity u(t) is discussed and examples of measurements for the determination of the microscale of turbulence [lambda] from zero counts are given

On the acoustic radiation from boundary layers and jets

In the following a general discussion of aerodynamically created sound is given. The study is essentially theoretical in nature, but arrives at a description of the physical phenomena in such a fashion as to yield an immediate access to experiments. First, the problem of aerodynamic noise is defined and two simple mechanical analogues discussed. Then, the general equations of motion of a viscous, compressible fluid are rearranged in a form suitable for a comparison with Lighthill's approach. However, this approach is not being followed through. Instead, the concept of induced velocities due to displacement effects is put forward and carried through for noise produced by boundary layer flow. The same concept is then extended to describe the sound field created by a jet

Fluid Dynamics of Liquid Helium

Liquid helium at low temperatures owes its existence to h through the zero point energy classically it should be solid. ^(4)He the common isotope, owes its peculiar behavior as a fluid to its spin and hence again to h; classically the difference between ^(3)He and ^(4)He should be trivial. In liquid helium flow we deal with a system which still shows all the usual behavior of a liquid plus some additional strange properties which reflect directly macroscopic quantum effects. The governing equations of motion due largely to Landau and London are, except in their linearized form, not as well founded and most certainly less well confirmed than one would like. Consequently, the experimental fluid dynamicist working with helium should have a field day exploring flow problems in an atmosphere more adventureous than with any ordinary fluid. This indeed is often the case. One does, however, ruefully discover that some of the more interesting and significant flow configurations which one likes to study in this strange field are by no means sufficiently well explored in the corresponding classical cases. One therefore likes to design simple fluid flow experiments which bring out the essentially new properties of He II and permit an experimental contribution to, or decision among, the theories of He II flow. In this spirit, experiments associated with the propagation of shock waves in liquid helium have been initiated at GALCIT. The design and construction of a cryogenic shock tube and its application to liquid helium are discussed in this paper

Magnetically driven cylindrical shock waves

Nearly every experiment on shock-wave propagation uses plane waves. Cylindrical and spherical waves are more difficult to produce, usually decay fast and do not offer any particular advantages. In magneto-fluid dynamics, however, it is possible to produce cylindrical waves easily and the axisymmetric geometry is a natural choice to study motion across magnetic field lines

Investigations of Free Turbulent Mixing

A discussion of the integral relations for the flow of the boundary-layer type is presented. It is shown that the characteristic laws of spread of jets, wakes, and so forth, can be obtained directly for the laminar case and, with the help of dimensional reasoning, for the turbulent case as well. Measurements of the mean velocity, the intensity and scale of the turbulent fluctuations, and of the turbulent shear in a two-dimensional mixing zone are presented. The results of these measurements are compared with the mixing-length theories. It is shown that both mixing length and exchange coefficient vary across the mixing zone. The theories based on the assumption of constant mixing length or exchange coefficient are thus in error. A discussion of the energy balance of the fluctuating motion is given and the triple point correlation is estimated

Investigations of Effects of Surface Temperature and Single Roughness Elements on Boundary-Layer Transition

The laminar boundary layer and the position of the transition point are investigated on a heated flat plate. It was found that the Reynolds number of transition decreases as the temperature of the plate is increased. It is shown from simple qualitative analytical considerations that the effect of variable viscosity in the boundary layer due to the temperature diference produces a velocity profile with an inflection point if the wall temperature is higher than the free-stream temperature. This profile is confirmed by measurements. Furthermore, it is confirmed that, even with large deviation from the Blasius condition, the velocity and temperature profiles are very nearly identical, as predictable theoretically for a Prandtl number [sigma] of the order of 1.0 (for air, [sigma]=0.76). The instability of injection-point profiles is discussed. Studies of the flow in the wake of large, two-dimensional roughness elements are presented. It is shown that a boundary laysr can separate and reattach itself to the wall without having transition take place

On the Contribution of Turbulent Boundary Layers to the Noise inside a Fuselage

The following report deals i preliminary fashion with the transmission through a fuselage of random noise generated on the fuselage skin by a turbulent boundary layer. The concept of attenuation is abandoned and instead the problem is formulated as a sequence of two linear couplings: the turbulent boundary layer fluctuations excite the fuselage skin in lateral vibrations and the skin vibrations induce sound inside the fuselage. The techniques used are those required to determine the response of linear systems to random forcing functions of several variables. A certain degree of idealization has been resorted to. Thus the boundary layer is assumed locally homogeneous, the fuselage skin is assumed flat, unlined and free from axial loads and the "cabin" air is bounded only by the vibrating plate so that only outgoing waves are considered. Some of the details of the statistical description have been simplified in order to reveal the basic features of the problem. The results, strictly applicable only to the limiting case of thin boundary layers, show that the sound pressure intensity is proportional to the square of the free stream density, the square of cabin air density and inversely proportional to the first power of the damping constant and to the second power of the plate density. The dependence on free stream velocity and boundary layer thickness cannot be given in general without a detailed knowledge of the characteristics of the pressure fluctuations in the boundary layer (in particular the frequency spectrum). For a flat spectrum the noise intensity depends on the fifth power of the velocity and the first power of the boundary layer thickness. This suggests that boundary layer removal is probably not an economical means of decreasing cabin noise. In general, the analysis presented here only reduces the determination of cabin noise intensity to the measurement of the effect of any one of four variables (free stream velocity, boundary layer thiclkness, plate thickness or the characteristic velocity of propagation in the plate). The plate generates noise by vibrating in resonance over a wide range of frequencies and increasing the damping constant is consequently an effective method of decreasing noise generation. One of the main features of the results is that the relevent quantities upon which noise intensity depends are non-dimensional numbers in which boundary layer and plate properties enter as ratios. This is taken as an indication that in testing models of structures for boundary layer noise it is not sufficient to duplicate in the model the structural characteristics of the fuselage. One must match properly the characteristics of the exciting pressure fluctuations to that of the structure

Shearing-Stress Measurements by use of a Heated Element

The rate of local heat transfer from a solid surface to a moving fluid is related to the local skin frinction. Measurements of the heat transmission from small elements embedded in the surface of a solid can thus be used to botain local skin-friction coefficients. This method was applied by Fage and Falkner for laminar boundary layers and by Ludwieg for turbulent boundary layers. The present report discussed the possible range of application of such an instrument in low- and high-speed flow and presents experimental data to show that a very simple instrument can be used to obtain laminar and turbulent skin-friction coefficients with a single calibration. The instrument consists of an ordinary hot-wire cemented into a groove in the surface. The heat loss from the wire is proportional to the cube root of the wall shearing stress, and the constant of proportionality may be found by one calibration, for example, in laminar flow

Active control of laminar-turbulent transition

Instability waves, commonly called T-S waves, can be introduced in a laminar boundary layer by periodic heating of flush-mounted heating elements. Experiments have demonstrated that nearly complete cancellation of a T-S wave excited in this way can be achieved by using a second downstream heating element with a suitable phase shift. As one application of the technique, a single element together with a feedback loop activated by measured wall shear stress has been used to reduce the amplitude of naturally occurring laminar instability waves. A significant increase in the transition Reynolds number has been achieved