Effect of an inhibitor on high-speed turbulent flames and the transition to detonation

The influence of an inhibitor (CF₃Br or Halon 1301) on the propagation of high-speed turbulent flames, quasi-detonations and the transition to detonation has been investigated for methane-air, propane-air and acetylene-air mixtures. The experiments are carried out in a 13 m tube (15 cm diameter) filled with regularly spaced orifice plates (blockage ratio of 0.39) to ensure rapid flame acceleration. In all cases, the addition of the inhibitor reduces the turbulent flame velocity and extinguishes the flame with sufficient inhibitor concentration (2.7% and 7.5% for methane-air and propane-air, respectively). For acetylene-air mixtures, the quasi-detonation speed is progressively reduced with increasing inhibitor concentration and eventually causes the failure of the quasi-detonation and transition back to a fast turbulent flame. The inhibitor also narrows the propagation limits in all cases. To elucidate the inhibition mechanism, detailed modelling of both the turbulent flame structure as well as the chemical kinetics are required.     

The structure of compressible starting vortices

The structures of highly two-dimensional shock-induced compressible starting vortices were investigated. Density distributions across the vortex were measured by dual-pulsed holographic interferometry. Pressure profiles of the vortex were measured by fast-response miniature Kulite transducers. From these two independent measurements the velocity profile was calculated using the radial momentum equation. The detailed vortex structure is similar to that from the tip of a wing and consists of four well-defined regions: a core region, a logarithmic region, a transition region, and an inviscid region. The distribution of circulation of the vortices can be expressed by a set of empirical formulae. The proportionality constants of the logarithmic region were found to be 0.43±0.01 for three different two-dimensional vortices.Presently at U.S. Army Aeroflightdynamics Directorate, ATCOM, NASA-Ames Research Center, Moffett Field, CA 94035Professor     

Chaos and direct numerical simulation in turbulence

We show that direct numerical simulation will yield turbulent flowfields which are strongly dependent upon computer hardware and software. A computed flow trajectory is apparently uncorrelated to the true solution of a flowfield if it is allowed to evolve over a long time, and hence is called a pseudo-orbit. This is due to the trajectory instability of chaotic turbulent flows. All is not lost, however; a long-time average of flow quantities can now be computed using a pseudo-orbit by invoking the shadowing lemma. For the inviscid flow, this time average tends to approach asymptotically the phase average as predicted by the classical ergodic theorem. Although the inviscid two-dimensional flow has no real physical importance, the existence of canonical (equilibrium) distribution permits us to examine the accuracy of time averaging based on the pseudo-orbit and its inherent limitations.This work was supported by AFOSR task 2304N1.     

Nonlinear analysis of the forced response of a beam with three mode interaction

An analysis is presented for the primary resonance of a clamped-hinged beam, which occurs when the frequency of excitation is near one of the natural frequencies,_n . Three mode interaction (₂ 3₁ and _{3 1} + 2₂) is considered and its influence on the response is studied. The case of two mode interaction (₂ 3₁) is also considered to compare it with the case of three mode interaction. The straight beam experiencing mid-plane stretching is governed by a nonlinear partial differential equation. By using Galerkin's method the governing equation is reduced to a system of nonautonomous ordinary differential equations. The method of multiple scales is applied to solve the system. Steady-state responses and their stability are examined. Results of numerical investigations show that there exists no significant difference between both modal interactions' influences on the responses.     

A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

Fatigue crack growth behavior of Al7050-T7451 attachment lugs under flight spectrum variation

This paper discusses an analytical and experimental investigations of the fatigue crack growth behavior in attachment lugs subjected to a randomized flight-by-flight spectrum. In the analysis, the stress intensity factors for through-the-thickness cracks initiating from lug holes were compared by weight function method, boundary element method (BEM), the interpolation of Brussat’s solution. The stress intensity factors of a corner crack at a transition region were obtained using two parameter weight function method and correction factors. Fatigue life under a load spectrum was predicted using stress intensity factors and Willenborg retardation model considering the effects of a tensile overload. Experiments were performed under a load spectrum and compared with the fatigue life prediction using the stress intensity factors by different methods. Changes of fatigue life and aspect ratio according to the clipping level of the spectrum were discussed through experiment and prediction. Effect of the spectrum clipping level on the fatigue life was experimentally evaluated by using beach marks of fractured surface.     

Migration in an oscillatory flow of a laminar suspension measured by laser anemometry

Oscillatory flow occurs in a wide range of areas of engineering importance. Two-way lateral migration of particles was observed in the flow of a steady, laminar, dilute, neutrally-buoyant suspension of rigid particles in a tube. The equilibrium particle-position was dependent on the dimensionless Womersley parameter. Experiments were performed in which the Womersley parameter was varied between 1.12 and 8.0. For low values of the parameter, two-way radial migration was observed as in steady, laminar flow. For higher values of the parameter, two equilibrium positions were observed, together with three particle free layers across the radius of the tube.     

Hemostatic efficacy and tissue reaction of oxidized regenerated cellulose hemostats

Oxidized regenerated cellulose (ORC) has been used as an absorbable hemostat since World War II. In the present study, hemostasis time was determined in a spleen incision model in swine. The effect of mass on absorbable hemostat efficacy and hemostasis time was evaluated by standardizing the ORC materials on a mass basis. The median hemostasis time for a single layer of the new nonwoven ORC was as much as 51 % shorter than woven ORC (P < 0.001). The mean hemostasis time for nonwoven ORC was not affected by the mass of hemostat applied to the wound. The hemostatic efficacy of woven ORC increased with the mass (layers) of hemostat applied to the wound. Nonwoven ORC is significantly faster in achieving hemostasis than woven ORC, and its hemostatic efficacy is not influenced by the mass of material applied. Tissue reaction was minimal and the material was fully absorbed by 14 days.