Coherent structures and modeling: Some background comments

Coherent structures are discussed as a sequence of events (identifiable motions) in the flow which convert significant amounts of mechanical energies of the mean flow stream, into turbulent fluctuations. The use of structure information in modeling is also discussed

Phase-lock loop frequency control and the dropout problem

Technique automatically sets the frequency of narrow band phase-lock loops within automatic lock-in-range. It presets a phase-lock loop to a desired center frequency with a closed loop electronic frequency discriminator and holds the phase-lock loop to that center frequency until lock is achieved

A review of quasi-coherent structures in a numerically simulated turbulent boundary layer

Preliminary results of a comprehensive study of the structural aspects of a numerically simulated number turbulent boundary layer are presented. A direct Navier-Stokes simulation of a flat-plate, zero pressure gradient boundary layer at Re0 = 670 was used. Most of the known nonrandom, coherent features of turbulent boundary layers are confirmed in the simulation, and several new aspects of their spatial character are reported. The spatial relationships between many of the various structures are described, forming the basis for a more complete kinematical picture of boundary layer physics than has been previously known. In particular, the importance of vortex structures of various forms to the generation of Reynolds shear stress is investigated

Experiments in free shear flows: Status and needs for the future

Experiments in free turbulent flows are recommended with the primary concern placed on classical flows in order to augment understanding and for model building. Five classes of experiments dealing with classical free turbulent flows are outlined and proposed as being of particular significance for the near future. These classes include the following: (1) Experiments clarifying the effect of density variation owing to use of different gases, with and without the additional effect of density variation due to high Mach number or other effects; (2) experiments clarifying the role and importance of various parameters which determine the behavior of the near field as well as the condictions under which any of these parameters can be neglected; (3) experiments determining the cumulative effect of initial conditions in terms of distance to fully established flow; (4) experiments for cases where two layers of distinctly different initial turbulence structure flow side by side at the same mean speed; and (5) experiment using contemporary experimental techniques to study structure in free turbulent shear flows in order to compliment and support contemporary work on boundary layers

Residual Stress Analysis using Multiparameter Tomographic Reconstruction

While tomographic reconstruction techniques are commonly utilized for the analysis of electromagnetic (typically x-ray) wave propagation data, this approach is infrequently used to examine acoustic data outside the geophysics community. However, acoustic tomography offers some distinct cost and performance advantages over conventional imaging techniques and some unique capabilities which are currently under investigation. One of the most intriguing of the enhanced capabilities is multiparameter imaging. In conventional ultrasonic testing, one usually concentrates on a single parameter of interest, whether it be amplitude, velocity, etc. and for most applications this is fully adequate. This is also true for most tomographic imaging situations such as x-ray tomography where attenuation is sought as the parameter to be obtained from the reconstruction process. However, in many cases, one parameter alone fails to yield full information about the material state even for isotropic media where two independent material stiffness parameters are required for complete characterization. For anisotropic media, the situation becomes increasingly complex with the degree of anisotropy with 21 independent material parameters required. In this work, we address the problem of multiparameter reconstruction and detail a way in which a standard reconstruction technique namely the algebraic reconstruction technique or ART can be modified to achieve this goal. Both isotropic and anisotropic situations are considered. Also, as a practical application of this approach, we address the problem of residual stress determination. Certainly, the use of tomography for residual stress analysis is not new. However, in all these studies, only a single residual stress parameter was reconstructed. This approach is quite satisfactory providing the stress state is uniaxial. Here, we develop a general approach for the tomographic reconstruction of a triaxial stress field

A Comparison and Analysis of Detection Methods for the Measurement of Production in a Boundary Layer

Two hot films and dye visualizations have been employed, simultaneously, in a turbulent boundary layer to explore the relations among visual observations and five kinds of detection methods using conditional sampling. The results show that all methods correlate positively with each other, but not with high enough values of correlation coefficients to indicate true correspondence between any two thus far studied. Moreover, none of the detection methods devised to date indicate a plateau in number of events as a function of trigger threshold. The results also provide additional information on several other matters: (i) the relationship of outward motions from the wall (bursts) to inward motions (sweeps); (ii) further details on the time and space location of periods of high uv-product with respect to the visual models and to fluctuation hodograph quadrant, and (iii) some data bearing on the transfer of energy in the frequency domain during turbulence production (cascade processes). The present paper emphasizes the relations among the various detection methods and visual observations during intervals of high uv-product; other results are reported in more detail elsewhere

Effect of well-width on the electro-optical properties of a quantum well

We record photoreflectance from Ge/GeSi modulation doped quantum wells possessing $10^4$ V/cm perpendicular electric fields. Qualitatively very different spectra are obtained from samples of well-width 100 \AA and 250 \AA. Comparing the wavefunctions calculated from an $8 \times 8$ \textbf{k.p} theory, we find that while they remain confined in the narrower 100 \AA QW, the electric field causes them to tunnel into the forbidden gap in the 250 \AA\ well. This implies that the samples should show a transition from the quantum confined Franz-Keldysh effect to the bulk-like Franz-Keldysh effect. Close to the band-edge where Franz-Keldysh effects are important, simulated photoreflectance spectra reproduce the essential features of the experiment, without any adjustable parameters.Comment: 8 pages, 8 figures. Submitted to Phys. Rev.

Two-dimensional cascade investigation of a turbine tandem blade design

Flow and wake characteristics on turbine tandem blades in two dimensional cascade tunne