Time resolved measurements of the flow generated by suction feeding fish

The majority of aquatic vertebrates are suction feeders: by rapidly expanding the mouth cavity they generate a fluid flow outside of their head in order to draw prey into their mouth. In addition to the biological relevance, the generated flow field is interesting fluid mechanically as it incorporates high velocities, is localized in front of the mouth, and is unsteady, typically lasting between 10 and 50 ms. Using manometry and high-speed particle image velocimetry, this is the first study to quantify pressure within and outside the mouth of a feeding fish while simultaneously measuring the velocity field outside the mouth. Measurements with a high temporal (2 ms) and spatial (<1 mm) resolution were made for several feeding events of a single largemouth bass (Micropterus salmoides). General properties of the flow were evaluated, including the transient velocity field, its relationship to pressure within the mouth and pressure at the prey. We find that throughout the feeding event a relationship exists for the magnitude of fluid speed as a function of distance from the predator mouth that is based on scaling the velocity field according to the size of the mouth opening and the magnitude of fluid speed at the mouth. The velocity field is concentrated within an area extending approximately one mouth diameter from the fish and the generated pressure field is even more local to the mouth aperture. Although peak suction pressures measured inside the mouth were slightly larger than those that were predicted using the equations of motion, we find that these equations give a very accurate prediction of the timing of peak pressure, so long as the unsteady nature of the flow is included.     

Diffraction theory of optical interference moiré and a device for production of variable virtual reference gratings: A moiré microscope

Optical interference moiré methods are analyzed using Fraunhoffer diffraction theory to relate general large surface deformations to the fringes observed. This analysis determines the Almansl strain in the current configuration from the gradients of the fringe number function. The analysis shows the advantages of an experimental scheme that allows the virtual reference grating to be varied. The ability to vary the virtual reference grating results in a larger dynamic range and the ability to maintain a fringe spacing for maximum accuracy. A moiré microscope has been constructed which has this ability. Digital image processing coupled with optical filtering and phase control is used to enhance the accuracy of the fringe measurements. The variable virtual-reference-grating capability is demonstrated by using it to highlight several aspects of the deformation field near a crack tip in a single crystal of iron-silicon.     

Linear and non-linear pulse propagation in fluid-filled compliant tubes

Summary Here we study impact-initiated disturbances in fluid-filled elastic tubes. The undeformed tube diameter, wall thickness, and elastic modulus of the tube material are assumed to be functions of the distance along the centre line of the tube. The linearized version of the governing equations are solved by the Laplace transform, which is inverted by means of an approximate method. The original non-linear system of governing equations is solved numerically by the method of characteristics. Relationships between the axial fluid velocity and axial coordinate as well as between velocity and time are displayed for fixed values of time and axial coordinate respectively for the linear and nonlinear theory for ease of comparison.

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Sommario Si studiano le perturbazioni prodotte da impatto in tubi elastici riempiti di fluidi. Si assume che il diametro del tubo indeformato, lo spessore della parete e il modulo elastico del materiale del tubo siano funzioni della distanza misurata lungo la linea centrale del tubo. La versione linearizzata delle equazioni che governano il fenomeno è risolta con la trasformata di Laplace, invertita con un metodo approssimato. Il sistema originale non lineare delle equazioni è risolto numericamente con il metodo delle caratteristiche. Vengono rappresentate, per valori fissati del tempo e della coordinata assiale, le relazioni tra la velocità assiale del fluido e la coordinata assiale oltre alle relazioni fra velocità e tempo, sia per la teoria lineare sia per quella non lineare.

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Influence of long-chain branches in polyethylenes on linear viscoelastic flow properties in shear

 This contribution presents a survey on the influence of long-chain branching on the linear viscoelastic properties zero shear-rate viscosity and steady-state recoverable compliance of polyethylene melts. The materials chosen are linear and slightly long-chain branched metallocene-catalyzed polyethylenes of narrow molecular mass distribution as well as linear and highly long-chain branched polyethylenes of broad molecular mass distribution. The linear viscoelastic flow properties are determined in shear creep and recovery experiments by means of a magnetic bearing torsional creep apparatus. The analysis of the molecular structure of the polyethylenes is performed by a coupled size exclusion chromatography and multi-angle laser light scattering device. Polyethylenes with a slight degree of long-chain branching exhibit a surprisingly high zero shear-rate viscosity in comparison to linear polyethylenes whereas the highly branched polyethylenes have a much lower viscosity compared to linear samples. Slightly branched polyethylenes have got a higher steady-state compliance in comparison to linear products of similar polydispersity, whereas the highly branched polyethylenes of broad molecular mass distribution exhibit a surprisingly low elasticity in comparison to linear polyethylenes of broad molecular mass distribution. In addition sparse levels of long-chain branching cause a different time dependence in comparison to linear polyethylenes. The experimental findings are interpreted by comparison with rheological results from literature on model branched polymers of different molecular topography and chemical composition. Received: 12 July 2001 Accepted: 30 October 2001     

Energy Scaling of Compressed Elastic Films –Three-Dimensional Elasticity¶and Reduced Theories

We derive an optimal scaling law for the energy of thin elastic films under isotropic compression, starting from three-dimensional nonlinear elasticity. As a consequence we show that any deformation with optimal energy scaling must exhibit fine-scale oscillations along the boundary, which coarsen in the interior. This agrees with experimental observations of folds which refine as they approach the boundary. We show that both for three-dimensional elasticity and for the geometrically nonlinear Föppl-von Kármán plate theory the energy of a compressed film scales quadratically in the film thickness. This is intermediate between the linear scaling of membrane theories which describe film stretching, and the cubic scaling of bending theories which describe unstretched plates, and indicates that the regime we are probing is characterized by the interplay of stretching and bending energies. Blistering of compressed thin films has previously been analyzed using the Föppl-von Kármán theory of plates linearized in the in-plane displacements, or with the scalar eikonal functional where in-plane displacements are completely neglected. The predictions of the linearized plate theory agree with our result, but the scalar approximation yields a different scaling.     

The effect of spatial wandering on experimental laser velocimeter measurements of the end-wall vortices in an axial-flow pump

In a high Reynolds number axial-flow pump, laser velocimeter (LV) measurements were made to study the size and structure of the end-wall vortex. The time mean measurements show that the core size of the end-wall vortex increased with decreasing tip clearance, which is contrary to existing theory. Observations of cavitation in the vortex showed that the flow was unsteady. The vortices emanating from the smaller clearances were observed to wander or meander spatially and to develop kinks more than the vortices emanating from the larger tip clearances. This observed unsteadiness has a significant effect on the time mean size and velocity distribution of the vortex as measured with the LV employing the field point measurement technique. In order to obtain an estimate of the true size and velocity distribution, computational experiments were conducted which modelled a periodically wandering vortex and the LV measurement process. The computational and experimental results show good agreement, including a broadened and reduced tangential velocity distribution. In this paper, the end-wall vortex LV measurements are presented, and the method of analyzing the vortex wandering is described.     

The mechanism of polymer thread drag reduction

One very effective method of reducing the drag of a turbulent fluid flow is through the use of soluble, viscoelastic, long-chain, high-molecular-weight polymer additives. These additives have produced drag reduction of up to 80% in pipe flows. Polymers are typically added by injecting high concentration solutions into an established Newtonian flow.This study investigated the mechanism of drag reduction that occurs when a long-chain, high-molecular-weight polymer is injected along the centerline of a pipe with a concentration high enough to form a single, coherent, unbroken thread. In the present experiments, the unbroken threads existed for more than 200 pipe diameters downstream of the injector and produced drag reductions on the order of 40%. Previous authors have contended that this type of drag reduction is caused by the interaction of the thread with the outer flow. However, it has been proven in cases where the polymer is mixed throughout the flow that drag reduction requires the existence of polymer in the near-wall region. The objective of this study was to test the hypothesis that drag reduction from a polymer thread is caused by transport of polymer molecules from the thread into the near-wall region of the pipe. The objective was realized through the measurement of the drag reduction, the radial location of the thread, and the polymer concentration in the near-wall region. The concentration was measured by laser-induced fluorescence utilizing fluorescein dye as the tracer. This study provides strong evidence that the drag reduction from a polymer thread is caused by the transport of very low concentrations of polymer from the thread into the near-wall region.     

Testing theoretical bases of caustic methods in fracture mechanics and stress analysis

Determination of stress intensity factors using shadow optical method of caustics attracts increasing attention. However, the reliability and ranges of applicability of this method are seldom discussed. This paper presents such an analysis regarding the reliability and predictive power of caustic method in fracture mechanics. This analysis is performed according to the accepted methodology of testing any analytical, numerical, or experimental procedure, namely by testing its theoretical bases. The following four basic assumptions of the shadow optical method of caustics in fracture mechanics, and their consequences, are analyzed: assumption of a generalized plane stress state near crack tip; assumption that the radiant energy used to produce shadow images propagates rectilinearly inside a stressed body; assumption that the light velocity inside a stressed body depends only on stress components in planes normal to wave normals; assumption that there is no alteration in state of polarization of radiation impinging upon a stressed plate at oblique incidence.The first two basic assumptions are tested using results obtained by means of three analytical-experimental procedures, namely isodynes, gradient index method, and classical strain gages. It is known that the inaccuracies of the fourth assumption are within the noise level only when the angles of incidence are small.It is shown that the magnitudes of the effects caused by the geometric lens effect and the gradient index lens effect are comparable, and that the stress states near crack tips and bottoms of notches, which produce the geometric lens effect, are clearly three-dimensional.It is also indicated that the gradient index lens effect is caused jointly by the stress/strain-induced alteration of the optical path and by the bending of the light path caused by strain gradient.Obtained empirical evidence shows the existence of a particular relationship between the observed gradient index lens effect and the order of singularity in a particular singular solution of linear fracture mechanics, with the exception of the vicinity of the crack tip where singularities are inadmissible.     

Thermoacoustically driven flame motion and heat release variation in a swirl-stabilized gas turbine burner investigated by LIF and chemiluminescence

Laser-induced fluorescence and chemiluminescence, both phase-locked to the dominant acoustic oscillation, are used to investigate phenomena related to thermoacoustic instability in a swirl-stabilized industrial scale gas turbine burner. The observed sinusoidal phase-averaged flame motion in axial (main flow) direction is analyzed using different schemes for defining the flame position. Qualitative agreement between experimental data and theoretical analysis of the observed flame motion is obtained, interpreted as originating primarily from variation of the burning velocity. The heat release variation during an acoustic cycle is determined from the sinusoidally varying total OH* chemiluminescence intensity.

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Splitting,merging and spanwise wavenumber selection of Dean vortex pairs

Splitting, merging and spanwise wavenumber selection are studied during the initial development of Dean vortex pairs in a channel with mild curvature, an aspect ratio of 40, and an inner to outer radius ratio of 0.979. Two types of splitting events, and four types of merging events are evident from flow visualizations at Dean numbers from 75 to 220. These events are described in detail along with observations that occurrances of these different events are tied to spatial and temporal variations of spanwise wavenumbers of vortex pair spacing. Also discussed are frequency spectra of different events, recurrent phenomena, and the part played by splitting and merging in laminar to turbulent transition in curved channels.This study was sponsored by the Propulsion Directorate, U.S. Army Aviation Research and Technology Activity-AVSCOM, NASA-Defense Purchase Request 030030-P. Professor C. S. Subramanian provided assistance in setting up the traversing system and data acquisition systems used for this study. Mr. R. E. Hughes assisted in obtaining some of the results presented.