Staff publications (SoE)

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Browsing Staff publications (SoE) by Publisher "American Institute of Physics"

Now showing 1 - 6 of 6
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    Collision dynamics of nanoscale Lennard-Jones clusters
    (American Institute of Physics, 2006-12-11) Kalweit, Marco; Drikakis, Dimitris
    An investigation of collision dynamics of nanoparticles for a broad range of impact factors and collision speeds is presented. The investigation is based on molecular dynamics simulations in conjunction with the Lennard-Jones interaction potential thus making the results applicable for a broad range of material properties. Identification criteria are used to classify the collision dynamics into different collision modes and submodes. Detailed analysis of the collision processes reveals the existence of coalescence and stretching separation modes, which are further classified according to their dynamics into sticking; slide-and-locking; droplet; normal stretching separation; stretching separation with satellite droplets; and shearing-off modes. Qualitative and quantitative comparisons with previous molecular dynamic studies and analytical prediction models derived for macroscopic droplet collisions are also discussed. The investigation reveals that the reflexive separation mode, which has been observed in macroscopic droplet collisions, does not occur for nanoparticles consisting of 10 000 (or less) atoms.
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    Design considerations for a three dimensional fiber optic laser Doppler velocimeter for turbomachinery applications
    (American Institute of Physics, 1997-12-31T00:00:00Z) James, Stephen W.; Tatam, Ralph P.; Elder, R. L.
    Single headed three dimensional (3D) laser Doppler velocimetry (LDV) geometries generally rely upon the use of three Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the nonorthogonal measurement coordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed fiber optic 3D LDV probe based on the use of two Doppler difference channels to directly measure the transverse velocity components, and a reference beam channel to measure the on axis velocity component. The f/4 probe head has a working distance of 200 mm, designed to operate within the constraints of the limited optical access available in turbomachinery applications.
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    Linear stability of ice growth under a gravity-driven water film
    (American Institute of Physics, 2006-07) Shapiro, Evgeniy; Timoshin, Sergei
    In this paper we consider linear stability of ice growth under a gravity-driven water film on a sloping wall. First, we derive an analytic solution of the stability problem in the long-wave limit, which shows that the presence of the ice layer generates an additional wave mode. Further, using a long-wave solution as an initial guess, we find the additional wave mode in the numerical solution of the complete Orr-Sommerfeld problem and investigate its behavior numerically for a wide range of problem parameters. We show that the ice mode can become unstable even at moderate Reynolds numbers, and that the ice layer alters the behavior of the mode corresponding to the waves on the liquid film surface. We also demonstrate that the presence of the ice layer stabilizes wave disturbances on the water surface and that, depending on the angle of the incline, the critical Reynolds number of the surface mode can be either increased or decreased.
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    Measurement of shock wave unsteadiness using a high-speed schlieren system and digital image processing
    (American Institute of Physics, 2008-12) Estruch-Samper, David; Lawson, Nicholas J.; MacManus, David G.; Garry, Kevin P.; Stollery, J. L.
    A new method to measure shock wave unsteadiness is presented. Time-resolved visualizations of the flow field under investigation are obtained using a high-speed schlieren optical system and the motion of the shock wave is determined by means of digital image processing. Information on the shock’s unsteadiness is subsequently derived with Fourier analysis. A sample study on shock unsteadiness in a shock-wave/turbulent boundary-layer interaction with separation is included. The method presented enables a measure of shock unsteadiness at locations in the imaged flow field not accessible by intrusive methods.
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    Modeling shock waves in orthotropic elastic materials
    (American Institute of Physics, 2008-08-01T00:00:00Z) Vignjevic, Rade; Campbell, James C.; Bourne, Neil K.; Djordjevic, Nenad
    A constitutive relationship for modeling of shock wave propagation in orthotropic materials is proposed for nonlinear explicit transient large deformation computer codes (hydrocodes). A procedure for separation of material volumetric compression (compressibility effects equation of state) from deviatoric strain effects is formulated, which allows for the consistent calculation of stresses in the elastic regime as well as in the presence of shock waves. According to this procedure the pressure is defined as the state of stress that results in only volumetric deformation, and consequently is a diagonal second order tensor. As reported by Anderson et al. [Comput. Mech.15, 201 (1994)], the shock response of an orthotropic material cannot be accurately predicted using the conventional decomposition of the stress tensor into isotropic and deviatoric parts. This paper presents two different stress decompositions based on the assumption that the stress tensor is split into two components: one component is due to volumetric strain and the other is due to deviatoric strain. Both decompositions are rigorously derived. In order to test their ability to describe shock propagation in orthotropic materials, both algorithms were implemented in a hydrocode and their predictions were compared to experimental plate impact data. The material considered was a carbon fiber reinforced epoxy material, which was tested in both the through-thickness and longitudinal directions. The psi decomposition showed good agreement with the physical behavior of the considered material, while the zdeta decomposition significantly overestimated the longitudinal stresses.
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    Quantitative analysis of a closed photoacoustic cell that uses a high compliance piezoelectric transducer
    (American Institute of Physics, 2005-01-01T00:00:00Z) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.
    A treatment of the operation of low-frequency closed photoacoustic cells is presented, which considers the compliance of various cell elements. The optimum detector, according to our predictions, has a sensor whose compliance is large compared to that of the fluid enclosed by the cell. A simple photoacoustic cell has been built, incorporating a thin-walled piezoelectric cylinder. It was used to test the theory, and demonstrate photoacoustic detection of an absorber in aqueous solution, excited by a low power 678nm laser diode modulated at 110Hz. We have detected absorbances as low as approximately 3ï ´10 -3 cm -1 (SNR=1), corresponding to the absorption of light with a modulated power of 50ï ­W