Showing papers by "National Physical Laboratory published in 1975"

An experimental investigation of the formation and development of a wave packet in a laminar boundary layer

Abstract: The formation and development of a wave packet in the laminar boundary layer of a flat plate has been studied experimentally. The packet was arti­ficially generated by a short duration acoustic pulse, which was injected into the boundary layer flow through a small hole in the plate. The flow oscillations created by the passage of the packet were detected by a hot­wire anemometer which was positioned just outside the boundary layer. Repeated excitation of the disturbance enabled the noise content of the data to be significantly reduced by signal averaging which was carried out on­line by a small computer. From records generated at a number of probe locations over the plate it has been possible to show how the packet developed as it propagated downstream. Contours of the signal amplitude from each of the downstream stations showed that the patch of waves was initially roughly elliptic, but this evolved into a distinctly bowed shape as the patch spread out as it progressed downstream. The smoothly contoured wave packet, with peak amplitudes close to the centre, gradu­ally distorted and far downstream two amplitude maxima formed on either side of the centre line. The wave-like character of the fluctuations within the boundaries of the packet was revealed graphically through perspective projections of the signal records on the z, t plane ( z is across the span). These displays showed the gradual development from a small smooth packet close to the source to the final warped and distorted pattern. Spectral decomposition of the fluctuations at each streamwise measuring station provided information relating to the growth of narrow bands of waves. These showed that a particular band of oblique waves, which grew very rapidly at some stage, were linked to the observed distortions.

A Theoretical Model of a Wave Packet in the Boundary Layer on a Flat Plate

Abstract: A linear model of a wave packet in a laminar boundary layer is proposed. The model wave packet was chosen to conform to a particular experimental situation where a packet was artificially excited by a localized pulsed perturbation at the wall boundary. Appropriate quantities were com­puted from this model and these have been compared directly with experimental measurements. The model disturbance was built up from a linear combination of spatially growing modes summed numerically over all wavenumbers and frequencies. The input spectrum was assumed to be flat (all modes were equally excited), and the downstream development of the frequency—wavenumber spectrum was calculated on the basis of linear stability theory. The development of the model wave packet was compared with that of the experiment. It was found that the overall shapes of the disturbed region and the way it spread out as the packet travelled down­stream were well predicted by the model behaviour. Detailed comparisons of the wave motion within the wave packet were also made and although encouraging correlation was achieved at stations close to the source this was not maintained far downstream where the experimental packet showed various irregularities. The wave packet generated by the summation of modes developed smoothly downstream without forming any of the types of distortions which had been observed in the experiment.

Surface segregation and its relation to grain boundary segregation

Abstract: Equilibrium surface segregation concentrations have been measured for iron-tin alloys over the temperature range 500° to 850°C using Auger electron spectroscopy. The results are compared with the grain boundary segregation measurements reported earlier for these alloys, over the same temperature range, and also with the surface and grain boundary segregation data deduced from surface and interfacial energy measurements at 1420°C (Seah and Hondros 1973). The observations are all fully compatible with the theoretical predictions which incorporate the appropriate entropy terms. At low temperatures, in dilute solutions, it is found that the surface enrichment is approximately 130 times that at the grain boundary, whereas at 1420°C the ratio is only six. This difference, which will depend on the particular system considered, indicates that careful correlation experiments are required if surface segregation is to be used as a direct guide to grain boundary segregation. A second practical difference is ...

Itinerant-electron ferromagnetism in La 1−x Sr x CoO 3 : a Mössbauer study

Abstract: Mossbauer and other studies establish that in La 1-x Sr x CoO 3 (x>0.125), ferromagnetic Sr 2+ -rich clusters coexist with paramagnetic La 3+ -rich regions in the same crystallographic phase, with the ferromagnetic component increasing with increasing x and decreasing T. The 3d holes created by Sr 2+ substitution are itinerant both above and below T C . All the experimental observations on this system can be explained on the basis of itinerant-electron ferromagnetism.

A High Sensitivity Moire Grid Technique for Studying Deformation in Large Objects

Abstract: If a camera is to be used for the special purpose of recording periodic grid-like patterns, the range of spatial frequencies which it must handle is limited, and its imaging performance for a given range can be optimized by installing a suitably slotted mask inside the lens aperture. With this modification, a standard 35 mm single lens reflex camera has been found to resolve up to 600 lines/mm in both vertical and horizontal directions across the full format. The tuned response of this camera makes it well adapted for measuring lateral displacements of large incoherently lit structures by the moire grid technique. Some preliminary results are described, and the technique is compared to laser speckle photography.

Signal-to-noise ratio and other characteristics of heterodyne radiation receivers

Abstract: The theory of heterodyne radiation receivers for the infrared and far-infrared is reviewed. For simplicity, only systems based on photoemissive and photoconductive radiation mixers are considered. The signal-to-noise ratio is derived, and expressions for this quantity under various conditions are given. A brief comparison is made between heterodyne systems and high-resolution spectrometers using direct radiation detectors. It is concluded that for high spectral resolving powers and for relatively small astronomical sources, heterodyne systems are likely to offer a distinct advantage in signal-to-noise ratio, especially at long far-infrared wavelengths.

White-light speckle photography for measuring deformation, strain, and shape

Abstract: This describes a new optical technique for displaying both small surface displacements and the absolute shape of large objects. The method is based on recording fine detail appearing in random patterns applied to the surface. The detail is recorded at a high spatial frequency using a modified 35 mm camera and fine grain film. Information concerning relative and absolute displacements is contained in light diffracted by the processed negative.

Presentation and verification of a differential formulation for the diffraction by conducting gratings

Abstract: We present a rigorous formalism for the study of the diffraction by a grating of finite conductivity, possibly covered with a thin film. This diffraction problem is reduced to the numerical boundary-value problem of integrating a set of differential equations. A detailed experimental verification of the theory is presented, for the two fundamental cases of polarization. Specific applications of the method are given, such as the influence of conducting or dielectric layers on gratings. These should prove particularly interesting to opticians in the ultraviolet.

Storage and loss moduli in discontinuous composites

Abstract: The paper deals with viscoelastic, rubber-like material unidirectionally reinforced with discontinuous fibres. The longitudinal storage modulus is calculated not only from an equation based on an existing force balance treatment but also from the elastic strain energy stored in matrix and fibres, using two different models to derive the stress and strain distributions from which the stored energy is calculated. There is very good agreement between all the calculations. The energy calculations reveal that loss modulus is also greatly increased by discontinuous reinforcement and enable its value to be estimated. Experiments on storage and loss modulus are reported and show that the calculations underestimate storage modulus and overestimate loss modulus. In both cases the factor of error ∼ 2, and arises because the amplified matrix strain is underestimated and is partly hydrostatic; the hydrostatic strain is non-dissipative and therefore does not contribute to the loss modulus. Discontinuous reinforcement can increase loss modulus as well as storage modulus by more than 100 times, and this should help sound and vibration deadening. An estimate is made of the wide ratio of compliance ÷ breaking strength available with discontinuous but not with continuous reinforcement, which opens up new design latitude for components hitherto reinforced with continuous fibres.

Experience in computing wave loads on large bodies

Abstract: Designers of offshore installations for the North Sea and adjacent waters face a number of exceptional demands specially regarding the extreme and continuing severity of the weather and the large water depths. This paper describes experience in Ship Division of the National Physical Laboratory in the development, validation and application of a computer program written for this purpose that has been supplied to a number of computer bureaus and to Lloyds Register of Shipping and has been extensively used by the offshore industry.