Peter Freymuth

On transition in a separated laminar boundary layer

This paper deals with the growth of small disturbances in a separated laminar boundary layer for high Reynolds numbers as a function of the dimensionless flow parameters. Using a hot-wire technique, the experiments show that spatially growing disturbances are only affected by the Strouhal number. Thus the basic equations of the process become relatively simple. The experiments show good agreement with theoretical results obtained by means of hydrodynamic stability theory for spatially growing disturbances.

Frequency response and electronic testing for constant-temperature hot-wire anemometers

The author aims to identify theoretically and to measure electronically the basic dynamic performance parameters of the constant-temperature hot-wire anemometer. It is found that a well designed anemometer behaves as a third-order control system which allows for a two-parameter optimization of its dynamic response. The cut-off frequency of the anemometer and related dynamic parameters can be estimated by means of electronic square- and sine-wave tests.

Turbulent Energy Balance and Spectra of the Axisymmetric Wake

Axisymmetric turbulent wake behind a sphere in an incompressible fluid has been experimentally investigated from 50 to 300 diam downstream from the sphere at Reynolds numbers from 4000 to 150 000. Mean and turbulent velocity measurements show that the region of self‐preservation starts 50 sphere diam downstream, and the virtual origin of the wake is 12 sphere diam downstream. Detailed measurements were made in the self‐preserving region of the wake. The three components of the turbulent velocity, turbulent shear, and the mean velocity defect were measured across the wake using the hot‐wire anemometer. From the measurements the turbulent energy production, dissipation, convection, and diffusion across the wake were determined. The one‐dimensional energy spectra of the three components of the turbulent velocity, their dependence on distance along and across the wake and on Reynolds number have been measured. The large scale motion (low wavenumbers) is dynamically similar and the small scale motion (large wavenumbers) exhibits Kolmogoroffs universal equilibrium. The present spectral measurements are compared with other published measurements of the universal equilibrium spectra in various turbulent flows.

Structure of Temperature Fluctuations in the Turbulent Wake behind a Heated Cylinder

Convection, production, diffusion, and dissipation of temperature fluctuations have been measured in the dynamically similar turbulent wake behind a headed circular cylinder 1140 diameters downstream from the cylinder at a Reynolds number of 960. The measurements show strong production and dissipation and moderate convection and diffusion. In addition, the dependence of one‐dimensional spectra of temperature fluctuations on wake parameters and the skewness of the temperature derivative have been measured for the Reynolds number range 440‐69 000. All measurements are consistent with the assumption of local isotropy except for the skewness of the temperature derivative.

Spectra of Turbulence in Wakes behind Circular Cylinders

One‐dimensional spectra of turbulent wakes behind circular cylinders are measured in the range of 50‐800 diam downstream of the cylinders and for a Reynolds number range of 320‐95 000. The spectra of large‐scale (low‐wavenumber) turbulence are anisotropic and dynamically similar. The small‐scale turbulence is locally isotropic and an inertial subrange was found for large Reynolds numbers. A proposed formula describes the spectra over the entire wavenumber range and for all Reynolds numbers.

Flow visualization in fluid mechanics

The large field of flow visualization is briefly reviewed. Definition, purpose, and classification into subfields are outlined, and the reader is directed toward more detailed and more specialized reviews. The basic methods of flow visualization are explained and the long history of flow visualization is sketched by tracing the introduction of major methods and of discoveries made by flow visualization. Limitations and problem areas in the visual evaluation of velocity and vorticity fields are discussed, and a glimpse into the future role of flow visualization is ventured.

Temperature fluctuations in the turbulent wake behind an optically heated sphere

Convection, production, diffusion, and dissipation of temperature fluctuations have been measured in the dynamically similar turbulent wake behind an optically heated sphere 136 sphere diameters downstream and at a Reynolds number 4300. The measurements show strong convection and dissipation and moderate diffusion and production. In addition, the dependence of one‐dimensional spectra of temperature fluctuations on wake parameters, intermittency, and the skewness of temperature and velocity derivatives have been measured in a Reynolds number range 1000‐38 000. The skewness of temperature derivative exhibits locally nonisotropic behavior especially at low Reynolds numbers.

The vortex patterns of dynamic separation: A parametric and comparative study

Abstract The vortical pattern development during dynamic separation in accelerating flow around airfoils is visualized and described. The descriptive language developed for this task is subsequently presented in a ‘dictionary’ for generic use. This dictionary is applied to a comparative study of vortex patterns of several dynamically separating flow configurations.

Further Visualization of Combined Wing Tip and Starting Vortex Systems

Visualisation des systemes de tourbillons dans un ecoulement demarrant a acceleration constante

Noise in Hot‐Wire Anemometers

An analysis is given of the electronic noise and the signal‐to‐noise ratio for constant‐temperature and constant‐current hot‐wire anemometers. The analysis shows that both anemometers are equivalent in their signal‐to‐noise ratio if the noise sources are equivalent and the hot‐wires are operated at the same conditions. A discussion of the theoretical results and a comparison of both anemometer types based on these results are given. In addition, some noise measurements on a constant‐temperature anemometer and the methods to obtain them are reported and compared with theoretical results. The agreement between measured and theoretical results is rather good.