Two new optical techniques to measure strain

A grating engraved on the surface to be analyzed is utilized to measure strain. The grating is illuminated by a monochromatic, plane wavefront. A lens produces the diffraction spectrum of the grating, and an opaque screen is located in the plane of the spectrum. A window allows a single diffraction order to go through the screen. Two techniques are presented. In the point-by-point technique, the illuminating wavefront is reduced to a very small area. Behind the window a light sensor detects the changes of light intensity that are produced by loading the specimen. In the field technique, an image of the specimen is produced, and the light sensor is located in the image plane. In both cases, changes of light intensity are related to strains. Using crossed gratings, a rosette strain gage is obtained. Examples of application of both techniques are presented, showing a very good agreement between the strains measured by the proposed methods and by independent means.     

Permeability Effects of Turbulent Flow Through a Porous Insert in a Backward-Facing-Step Channel

In the present work, a k– model, based on the work of Lee and Howell (Proceedings of the ASME-JSME Thermal Engineering Hawaii, 1987), is rigorously derived based on time average of spatially averaged Navier–Stokes equations. The model is then employed to solve for a flow in a backward-facing step channel with a porous insert. The numerical solver is modified from the STREAM code (Lien and Leschziner, Comput. Meth. Appl. Mech. Eng. 114 (1994a) 123–148), and it has been validated against the experimental data of Seegmiller and Driver (AIAA Journal 23 (1985) 163–171). The code is then used to perform simulation for cases with a porous insert. The resistance of the porous insert can be altered by changing its permeability (), Forchheimers constant (F), or thickness (b). The goal is to examine the influence of each parameter on the resulting flow and turbulent kinetic energy (k) distributions. It is discovered that, by increasing the resistance of the insert, flow eventually enters a transitional regime towards relaminarization. This is due to the contribution of Darcys and Forchheimers terms in the governing equations, and modifying these two terms changes the levels of P_k and, hence, k and . Generally speaking, lowering or raising F results in a greater suppression of P_k than , causing the flow to relaminarize. Meanwhile, if the pore size is reasonably large to sustain turbulence within the porous media, increasing b reduces but does not eliminate the turbulent activity in the porous insert.     

The impact of high order refraction on optical microbubble sizing in multiphase flows

High order refractions have been found to have a great impact on optical particle sizing using phase-Doppler anemometry (PDA) especially when the relative refractive index of the media is less than one. The high order refractions cannot be neglected because of the differences in spatial frequencies and motion directions of each order. A model of phase-size correlation is proposed that considers these high order refractions. By using the conversion factors of the high order refractions, the particle diameter can be determined. The capability of the newly developed method was evaluated by using generalized Lorenz Mie theory (GLMT), which was validated by experiments. The results of the simulation and experiments are compared with those based on the conventional method. An optimization method for accurately sizing air bubbles in water has been suggested.     

Evolution of a forced counter rotating vortex pair for two selected forcing frequencies

In this paper, results from an experimental study of the natural and forced evolution of a pair of counter rotating wing-tip vortices are reported. The vortices were generated using a pair of opposed wing-tips in a wind tunnel and measurements made up to 77 tip-spacings downstream of the models at a chord Reynolds number of 1.3 × 10⁵. The wake was interrogated using 2D particle image velocimetry and the long-wave Crow instability observed. Velocity data were recorded throughout the lifetime of the instability from initial growth through linking, formation of vortex rings and their subsequent decay. Forcing was achieved using pulsed air jets blowing in the span-wise direction from the wing tip and imparting spatially periodic perturbations to the vortices. Forcing at a frequency within the range amplified by the Crow instability was found to enhance the instability growth rate whereas forcing at a frequency outside the amplified range was found to inhibit instability growth. In the latter case the imparted wavelength was observed to die out with a preferred wavelength growing in its place.     

Soil-tire/track interaction—current and future research needs

This paper deals with current and future research needs as related to the tractive and transport components of machinery used in agriculture. Research needs in both the basic and applied areas are addressed. An evaluation of the effects og inflation pressure on tire performance and soil compaction for all current tire designs appears to be the most pressing immediate need. The development of an adequate system to define the behavior of soil under load is the greatest long-range need.     

Critical needs of fringe-order accuracies in two-color holographic interferometry

Two-color holographic interferometry is a promising technique for separating simultaneous concentration and temperature variations in solution for crystal-growth experiments on earth and in microgravity conditions in space. The ultimate success depends on two linearly independent fringe patterns due to the different wavelengths. With available practical lasers and typical crystal-growth experimental conditions the two interference fringe patterns (other than scale effects) may not be too different. The slight error in the measured fringe order can then yield large uncertainty in temperature and concentration determination. This aspect is analytically considered for the first time in this article. A simple cell (rectangular parallelpiped) is considered containing the fluid. For simplicyt, we assume a constant field along the object-wave-propagation direction in the cell. The two reconstructed fringe patterns are then represented in terms of temperature and concentration variations. Solving two equations theoretically yields the desired temperature and concentration. However, once the fringe-postition measurement error is introduced, error in the temperature and concentration results. These errors are analytically determined. A particular case of TGS (triglycine sulfate) aqueous solution with HeNe (λ=632.8 nm) and HeCd (λ=441.6 nm) lasers for holography is critically discussed. It is found that a high degree of accuracy in the fringe order is required in this particular case. To improve fringe-position measurements, special techniques such as electronic phase measurement or heterodyne detection may be necessary. The study provides the analytical guidelines for designing the experiments and critical needs of desired physical parameters. Paper was presented at the 1990 SEM Fall Conference on Experimental Mechanics held in Baltimore, MD on November 5–8.     

Interaction between the effects of local and overall imperfections on the buckling of elastic columns

A. van derNeut has studied the buckling of elastic columns made from thin-walled members, with particular reference to the effects of imperfections. In this paper, a graphical method is used to extend his work and to make an exhaustive study of the combined effects of both ‘local’ and ‘overall’ imperfections. The resulting picture is remarkably simple, and the effects of imperfections are well described by the celebrated Perry formula in conjunction with a single imperfection parameter compounding simply the local and overall imperfections. Experiments on small-scale rubber model columns substantiate the main results of the theoretical analysis.     

Recoil in macromolecular solutions according to rigid dumbbell kinetic theory

A macromolecular solution is represented by the simple model of rigid dumbbells suspended in a Newtonian fluid with Brownian motion included. Hydrodynamic interaction is not taken into account. It is found that for this model there will be recoil after the cessation of steady shearing flow. The ultimate shear recovery S _∞ is developed as a power series in κ^?, the shear rate prior to the cessation of the steady shear flow: $$S_\infty = (\theta _0 /2\eta _0 ) \kappa ^\user1{ - } + O(\kappa ^\user1{ - } )^3$$ where η₀ and θ₀ values of the viscosity and primary normal stress functions respectively at zero-shear rate. The coefficient of the term in (κ^?)³ is calculated. In addition, the behavior of the normal stresses during the recoil process is found; during recoil τ₂₂?τ₃₃ has the opposite sign from τ₁₁?τ₂₂.     

Strain gages in cryogenic environment

The increasing use of cryogenics in all fields of endeavor necessitates the use of strain gages for stress analysis of structures under extreme temperatures. This has been a continuing program at the McDonnell Douglas Astronautics Co.'s Strain Gage Engineering Laboratory (Huntington Beach facility). Many characteristics of strain gages were investigated, and a majority of the commercial strain gage types were evaluated. Tests results are presented, as well as composite curves, showing the comparison between various types of strain gages.     

Evaluation of three-dimensional stresses in shrink-fit problems by scattered-light photoelasticity

Stresses caused by shrink fits are commonly evaluated by using Lame's solution or by other analytical methods in which it is assumed that the radial stresses on the surface of contact are distributed uniformly, or in some stepwise manner, and that friction forces between the bodies are zero. These assumptions were not necessary in the experimental problem solved. Heretofore, no experimental techniques have been available to check the existing theories on a three-dimensional basis. The stresses which result when a short, hollow, thick-walled cylinder is shrunk over a shaft which is a solid shaft or a thick-walled, hollow shaft were determined. Equipment and techniques using scattered-light three-dimensional photoelasticity with a laser light source were developed and applied.