Shear-induced aggregation and break up of fibril clusters close to the percolation concentration

To probe the behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the percolation concentration, c_p (7.5%), rheo-optical measurements and Fourier transform rheology were performed. Different results were found close to c_p (7.3%), compared to slightly further away from c_p (6.9 and 7.1%). For 6.9 and 7.1%, a decrease in complex viscosity, and a linear increase in birefringence, n, with increasing strain was observed, indicating deformation and orientation of the fibril clusters. For 7.3%, a decrease in complex viscosity was followed by an increase in complex viscosity with increasing strain, which coincided with a strong increase in n, dichroism, n, and the intensity of the normalized third harmonic (I₃/I₁). This regime was followed by a second decrease in complex viscosity, where n,n and I₃/I₁ decreased. In the first regime where the viscosity was decreasing with increasing strain, deformation and orientation of existing clusters takes place. At higher oscillatory shear, a larger deformation occurs and larger structures are formed, which is most likely aggregation of the clusters. Finally, at even higher strains, the clusters break up again. An increase in complex viscosity, n, n and I₃/I₁ was observed when a second strain sweep was performed 30 min after the first. This indicates that the shear-induced cluster formation and break up are not completely reversible, and the initial cluster size distribution is not recovered after cessation of flow.     

The measurement of ?U?, ?V? and ?uv? inside the turbulent spot using conditionally sampled hot-wire anemometer signals

By means of a tripping wire turbulent spots are generated in a conditionally stable laminar boundary layer along a flat plate in a low-turbulence windtunnel. At several vertical and lateral positions the streamwise (U) and vertical (V) velocity components of the spot are measured using anX-shaped hot wire anemometer. The spot's leading edge (LE) and trailing edge (TE) are determined by a high frequency detection criterion.The derived ensemble averaged shear stress uv shows a remarkable plateau over the last 3/4 of the spot length and over about 1/2 of the spot width, approximately independent of the height above the plate. The measurements suggest that the velocities of the LE and the TE can be explained by simple advection of the active turbulent region. Other mechanisms seem to be responsible for the lateral spread of the spot, as is suggested by the measurements outside the plane of symmetry.     

Wavefront sensing for single view three-component three-dimensional flow velocimetry

We present the application of wavefront sensing to particle image velocimetry for three-component (3C), three-dimensional (3D) flow measurement from a single view. The technique is based upon measuring the wavefront scattered by a tracer particle and from that wavefront the 3D tracer location can be determined. Hence, from a temporally resolved sequence of 3D particle locations the velocity vector field is obtained. Two approaches to capture the data required to measure the wavefronts are described: multi-planar imaging using a distorted diffraction grating and an anamorphic technique. Both techniques are optically efficient, robust and compatible with coherent and incoherent scattering from flow tracers. The depth (range) resolution and repeatability have been quantified experimentally using a single mode fiber source representing a tracer particle. The anamorphic approach is shown to have the greatest measurement range and hence was selected for the first proof of principle experiments using this technique for 3D particle imaging velocimetry (PIV) on a sparsely seeded gas phase flow.     

Numerical analysis of pressure oscillations over axisymmetric spiked blunt bodies at Mach 6.80

The pressure oscillations over a forward facing spike attached to an axisymmetric blunt body are simulated by solving time-dependent compressible Navier–Stokes equations. The governing fluid flow equations are discretized in spatial coordinates employing a finite volume approach which reduces the equations to semidiscretized ordinary differential equations. Temporal integration is performed using the two-stage Runge–Kutta time stepping scheme. A global time step is used to obtain a time-accurate numerical solution. The numerical computation is carried out for a freestream Mach number of 6.80 and for spike length to hemispherical diameter ratios of 0.5, 1.0 and 2.0. The flow features around the spiked blunt body are characterized by a conical shock wave emanating from the spike tip, a region of separated flow in front of the hemispherical cap, and the resulting reattachment shock wave. Comparisons of the numerical results are made with the available experimental results, such as schlieren pictures and the surface pressure distribution along the spiked blunt body. They are found to be in good agreement. Spectral analysis of the computed pressure oscillations are performed employing fast Fourier transforms. The surface pressure oscillations over the spike and phase plots exhibit a behaviour analogous to that of the Van der Pol equation for a self-sustained oscillatory flow. Received 28 February 2001 / Accepted 17 January 2002     

Heat transfer from non-isothermal surfaces in the stagnation-point flow of a micropolar fluid

Summary This paper contains a numerical study of the development of the thermal boundary layer on an impermeable flat surface, following a step change in surface temperature, for stagnation-point flow of a micropolar fluid. A study is made of the relative effects of the individual parameters in the equations, and a comparison of certain integral parameters, representative of the flow, is used to assess the accuracy of the results.

---

Zusammenfassung Diese Arbeit enthält eine numerische Untersuchung der Entwicklung der thermischen Grenzschicht auf einer undurchlässigen ebenen Oberfläche nach stufenförmigem Sprung in der Oberflächentemperatur für die Staupunktsströmung einer mikropolaren Flüssigkeit. Die relativen Auswirkungen der einzelnen Parameter in den Gleichungen werden untersucht und ein Vergleich von gewissen, für die Strömung repräsentativen integralen Parametern wird durchgeführt, um die Genauigkeit der Ergebnisse abzuschätzen.

---

---

With 4 figures and 1 table     

Balance equations and structural models for phase interfaces

The general balance equations are developed for an interface represented by a dividing surface and for a moving common line represented as an intersection of dividing surfaces. The surface excess variables associated with a dividing surface are expressed both in terms of those variables describing the three-dimensional interfacial region of finite thickness and in terms of those variables describing bulk phases that extend up to the dividing surface.A structural model for the interface is suggested in which a suspension of solid bodies representing surfactant molecules is distributed about a singular surface separating two adjacent bulk solvent phases. The suspension is required to have the same average behavior as the interfacial region. This is interpreted as meaning that the general jump balance for a continuum dividing surface represented by an interfacial suspension is a local area average. Specific results are derived for two structural models, each in the same simple shear field. One consists of a dilute suspension of neutrally buoyant spheres floating with their centers restricted to the dividing surface. The other is a dilute suspension of chains of neutrally buoyant spheres with the sphere at one end of the chain floating in the dividing surface.     

Deformation field of a misfitting inclusion

J.D. Eshelby (1957, 1959) has calculated the deformation field associated with an ellipsoidal inclusion in a state of homogeneous strain within an infinite matrix. Since most real precipitates occur with facets, the strain within such an inclusion is not uniform. Thus, plate precipitates of θ′ in Al-Cu and η in Al-Au have coherent broad faces with mismatches of 1.34 and 4.95 % respect- ively and semicoherent or disordered interfaces at the edges with residual mismatches of about ?4.3 and ?1.00% normal to the broad faces. The deformation field in the matrix around such precipitates has been calculated using Kelvin's (1848) result for the stress field due to a point force. The calculations show the existence of high stresses near the edges of the precipitates where they have an appreciable misfit. Unlike the case of an ellipsoidal inclusion, the stress fields of these precipitates have dilatational components which can affect the diffusion of solute atoms to them and, thus, the kinetics of interface migration. The behavior of alloys containing these precipitates indicates that the moduli of the precipitates are somewhat greater than those of the matrices. The present calculations, based on the assumption that the two moduli are the same, underestimate the actual deformation field in the matrix. In real systems, therefore, the effects of the deformation field on misfit dislocation nucleation and kinetics of interface migration are likely to be somewhat greater in general.     

Some experiments on the effect of isolated protuberances on flow through tubes

At various locations in the arterial system, plaques, or small relatively isolated protuberances, can develop on the inner wall of the vessel and project into the lumen. A number of investigators have suggested that the development and growth of these protuberances is related to the flow in the vicinity of the protuberance. In this study, the conditions under which the flow separates from an isolated protuberance located in a cylindrical tube were investigated. The critical Reynolds number at which separation first takes place for a given protuberance was determined. A series of tests was performed for steady flow of a Newtonian fluid through a rigid tube in which protuberances of various sizes were inserted. The results of the tests show the effect of the protuberance height and shape on the separation characteristics. In general, the results indicate that separation takes place at relatively small values of the Reynolds number; values that commonly occur in the arterial system, so that this phenomenon may be important in the study of the “coupling” between blood flow and arterial lesions.     

A multiple-pulse ruby-laser system for dynamic photomechanics: Applications to transmitted- and scattered-light photoelasticity

An ultrahigh-speed multiple-frame recording system for two- and three-dimensional dynamic photomechanics has been developed and is described here. The output from a ruby laser is modulated with a Pockels cell to produce a train of short, intense, monochromatic and polarized light pulses. Pulse widths of 50 nsec and repetition rates of up to 170,000 pulses/sec are obtained. These light pulses are synchronized with a “smear camera” and the event to produce a multiple-frame record of the phenomenon. The simplified camera requirements necessary for this purpose are indicated. The system is demonstrated by recording two-dimensional dynamic and scattered-light isochromatic fringe patterns. The capability of multiple recording of scattered-light fringe patterns, achieved here for the first time, has a tremendous potential for three-dimensional dynamic stress analysis. The developed system is also well suited for dynamic moiré, interferometry and holography.     

An acoustic/viscous splitting technique for computational aeroacoustics

A new technique for the numerical analysis of aerodynamic noise generation is developed. The approach involves first solving for the time-dependent incompressible flow for the given geometry. A hydrodynamic density correction to the constant incompressible density is then calculated from knowledge of the incompressible pressure fluctuations. The compressible flow solution is finally obtained by considering perturbations about the corrected incompressible flow. This fully nonlinear technique, which is tailored to extract the relevant acoustic fluctuations, appears to be an efficient approach to the numerical analysis of aerodynamic noise generation, particularly in viscous flows. Applications of this technique to some classical acoustic problems of interest, including some with moderately high subsonic Mach numbers, are presented to validate the approach. The technique is then applied to a fully viscous problem where sound is generated by the flow dynamics.