Near-wall turbulent characteristics along very long thin circular cylinders

Understanding the salient physics within the turbulent boundary layer of towed thin cylinders is paramount to the Navy sonar array communities. However, the required long array length to achieve wide acoustic aperture creates unique and consistent flow characteristics that suggest simplified tangential forcing expressions suitable for design purposes. One well-known fact is that the majority of the array surface experiences very thick turbulent boundary layers (TBL) and large Reynolds numbers. The resultant statistics are most commonly dependent on the inner and outer length scales. Herein, we resolve the near-wall TBL structure under those flow conditions by large-eddy simulation. The turbulent mean-flow statistics showed near-wall consistency using only inner scaling. But both inner and outer variables were found necessary to properly scale the turbulent fluctuations. An expression for the tangential wall-friction coefficient (C_t) indicates two distinct flow regimes as characterized by the near-wall turbulent flow structure. The respective parameters appear independent of the outer length scale. Thickening (or thinning) the cylinder near their common threshold (defined by a radius-based Reynolds number) transitioned the turbulent character between the two regimes.     

A thermodynamically-consistent microplane model for shape memory alloys

In microplane theory, it is assumed that a macroscopic stress tensor is projected to the microplane stresses. It is also assumed that 1D constitutive laws are defined for associated stress and strain components on all microplanes passing through a material point. The macroscopic strain tensor is obtained by strain integration on microplanes of all orientations at a point by using a homogenization process. Traditionally, microplane formulation has been based on the Volumetric–Deviatoric–Tangential split and macroscopic strain tensor was derived using the principle of complementary virtual work. It has been shown that this formulation could violate the second law of thermodynamics in some loading conditions. The present paper focuses on modeling of shape memory alloys using microplane formulation in a thermodynamically-consistent framework. To this end, a free energy potential is defined at the microplane level. Integrating this potential over all orientations provides the macroscopic free energy. Based on this free energy, a new formulation based on Volumetric–Deviatoric split is proposed. This formulation in a thermodynamic-consistent framework captures the behavior of shape memory alloys. Using experimental results for various loading conditions, the validity of the model has been verified.     

Characteristics of normal and tangential forces acting on a single lug during translational motion in sandy soil

Lugs (i.e., grousers) are routinely attached to the surfaces of wheels/tracks of mobile robots to enhance their ability to traverse loose sandy terrain. Much previous work has focused on how lug shape, e.g., height, affects performance; however, the goal of this study is to experimentally confirm the effects of lug motion on lug–soil forces. We measured normal and tangential forces acting on a single lug as functions of inclination angle, moving direction angle, sinkage length, horizontal displacement, and traveling speed. The experimental results were mathematically fitted by using least square method to facilitate quantitative analyses on effects of changes in these motion parameters. Moreover, we compared the measured tangential forces to values calculated from a conventional tangential force model to evaluate the effects of the lug-tip surface, which is generally ignored in existing terramechanics models. The conclusions from this study would be useful for estimating the traveling performance of locomotive mechanisms equipped with lugs, modeling interaction mechanics between lugged wheels and soil, etc.     

Particle dynamics in dense shear granular flow

The particle dynamics in an annular shear granular flow is studied using the discrete element method, and the influences of packing fraction, shear rate and friction coefficient are analyzed. We demonstrate the existence of a critical packing fraction exists in the shear granular flow. When the packing fraction is lower than this critical value, the mean tangential velocity profile exhibits a rate-independent feature. However, when the packing fraction exceeds this critical value, the tangential velocity profile becomes rate-dependent and varies gradually from linear to nonlinear with increasing shear rate. Furthermore, we find a continuous transition from the unjammed state to the jammed state in a shear granular flow as the packing fraction increases. In this transforming process, the force distribution varies distinctly and the contact force network also exhibits different features.     

Shape sensitivity for the Laplace-Beltrami operator with singularities

This paper considers shape sensitivity analysis for the Laplace-Beltrami operator formulated on a two-dimensional manifold with a fracture. We characterize the shape gradient of a functional as a bounded measure on the manifold and decompose it into a “distributed gradient” supported on the manifold, plus a singular part that we derive as the limit of a “jump” through the crack and Dirac measures at the crack extremities. The important point is that we introduce a technique that is not dimension dependent, and makes no use of classical arguments such as the maximum principle or continuation uniqueness. The technique makes use of a family of envelopes surrounding the fracture which enable us to relax certain terms and to overcome the lack of regularity resulting from the presence of the fracture. We use the min-max differentiation in order to avoid taking the derivative of the state equation and to manage the crack's singularities. Therefore, we write the functional in a min-max formulation on a space which takes into account the hidden boundary regularity established by the tangential extractor method.     

Helicity injection and diagnostics for orbitron millimeter wave source

Conventional orbitrons utilize the radially injected electrons for production of electromagnetic waves. In such a scheme, however, more than half of the electrons would not participate in the orbital motion around the anode due to the lack of acceleration. Only the electrons who did not suffer collisions till the radius 2/3 of the outer conductor (cathode) radius are possible to acquire the azimuthal velocity, via collisions, as large as the critical velocity with which the electrons can undergo circular equilibrium orbits. The axisymmetric injection is also a problem; 50% of electrons would be lost directly to the anode by the head-on collisions. This paper discusses various ways to enhance the efficiency and absolute power of an orbitron millimeterwave source. Experimental results are described on employment of a tapered metal-end, tangential injection of a thin electron beam, axial injection of rotating annular electron beams, and application of external magnetic fields. Further problem of conventional orbitrons is in its construction in which the potential-well is prematurely destroyed due to the shortening discharge current. Its diagnostics and consequence are discussed together with a new scheme leading towards the goal, an efficient injection of helicity (or helical electron-beam) into the potential-well conserved orbitron interation region.     

Determination of Lead(II) with the Rotating Sample System

As a result of industrialization lead is one of the most widely dispersed toxic heavy metals in the environment. There is a pressing need for a reliable, affordable and portable analytical technique for routine determination of lead at trace levels in biological and environmental samples. Despite their potential for portability and low cost, the currently available electrochemical stripping methods still have limited commercial availability. Among the reasons are the relatively large sample volumes and the large amount of reagents needed (1–3 mL), lower than required precision, and the inconvenience of a rotated electrode system. The Rotating Sample System is a unique approach to electrochemical stripping, devised for 20 μL sample droplets utilizing a large surface area electrode. This design combines the advantages of a microelectrode and a rotated electrode system. The semispherical sample drop itself is rotated by a fine gas jet directed at it tangentially, eliminating the need for a sample container. Neither fine moving mechanical parts nor sophisticated controls are required. The detection limit for lead(II) was found low enough and the reproducibility is sufficient for routine determinations in biomedical samples (5 ppb, 6%). The system can support a CDC recommended screening for blood lead levels and an on‐site analysis of environmental samples as well. Under suitable conditions calibration free direct determinations can also be performed.     

On the use of digital image correlation for slip measurement during coupon scale fretting fatigue experiments

Digital image correlation (DIC) is a full field three dimensional measurement technique that can quantify displacements and strains of a surface. In this paper, digital image correlation is used as a slip measurement technique during coupon scale fretting fatigue experiments. Slip measured with the novel DIC technique is compared to conventional slip measurement techniques as clip gauges and modified clip gauge measurements proposed by Wittkowsky et al. Slip measurements with the DIC system show lower slip values and higher tangential contact stiffness’s compared to (modified) clip gauge measurements. Slip measured with DIC is obtained closer to the contact compared to clip gauges, eliminating the influence of elastic deformations or fitting parameters. During the fretting fatigue experiments are two equal contacts simultaneously tested. However, the slip of both contacts is not identical with outliers of more than 10% difference in slip amplitude.