Drag reduction in a flat-plate turbulent boundary layer flow by polymer additives

This paper presents a theoretical study on the velocity distribution and the friction factor of boundary layer flows with polymer additives starting from the concept of “stress deficit.” A novel method of order of magnitude analysis is developed, which converts the governing equations of boundary layer flow into a solvable ordinary differential equation, thus the total shear stress distribution is obtained, then the formulas for the mean velocity profiles and the friction factor for a boundary layer flow are derived after introducing appropriate expressions for the “effective viscosity” and the thickness of viscous sublayer. The derived velocity equation is able to depict the velocity from a solid wall to the outer edge of boundary layer with or without polymer additives using only one fitted parameter D* that is a function of polymer species, its concentration, and Reynolds number. By integrating the velocity profiles, the friction factor and the thickness of boundary layer development are obtained. Experimental data agree well with the theoretical results

Review of Cross-border Electronic Retail Logistics Research in the Last Five Years

Nearly five years, retail in cross-border e-commerce (CBER) has gradually attracted more attention with the expansion of domestic e-commerce in several retail major economies. Scholars both here and abroad have become increasingly interested in CBER. As the important supporting services, logistics services of CBER have also be deeply concerned. However, compared with the rich practice, the theoretical outputs of logistics services in CBER are still less. Previous studies have focused on the relationship between cross-border e-commerce and logistics services , and international distribution network. So far, there is little research on content-based logistics services concerning CBER. Based on the literature review, this paper has determined a series of possible research directions, including strategic significance of cooperation in forming CBER distribution structure and how to implement customer-driven logistics service improvement. Finally, some future research directions are proposed

High-contrast coronagraph for ground-based imaging of Jupiter-like planets

We propose a high-contrast coronagraph for direct imaging of young Jupiter-like planets orbiting nearby bright stars. The coronagraph employs a step-transmission filter in which the intensity is apodized with a finite number of steps of identical transmission in each step. It should be installed on a large ground-based telescope equipped with state-of-the-art adaptive optics systems. In that case, contrast ratios around 10^-6 should be accessible within 0.1 arc seconds of the central star. In recent progress, a coronagraph with circular apodizing filter has been developing, which can be used for a ground-based telescope with central obstruction and spider structure. It is shown that ground-based direct imaging of Jupiter-like planets is promising with current technology

An iterative wave-front sensing algorithm for high-contrast imaging systems

Wave-front sensing from focal plane multiple images is a promising technique for high-contrast imaging systems. However, the wave-front error of an optics system can be properly reconstructed only when it is very small. This paper presents an iterative optimization algorithm for the measurement of large static wave-front errors directly from only one focal plane image. We firstly measure the intensity of the pupil image to get the pupil function of the system and acquire the aberrated image on the focal plane with a phase error that is to be measured. Then we induce a dynamic phase to the tested pupil function and calculate the associated intensity of the reconstructed image on the focal plane. The algorithm is to minimize the intensity difference between the reconstructed image and the tested aberrated image on the focal plane, where the induced phase is as the variable of the optimization algorithm. The simulation shows that the wave-front of an optics system can be theoretically reconstructed with a high precision, which indicates that such an iterative algorithm may be an effective way for the wave-front sensing for high-contrast imaging systems.Comment: 7 pages, 7 figure

An assembly gap control method based on posture alignment of wing panels in aircraft assembly

The gaps between two mating surfaces should be strictly controlled in precision manufacturing. Oversizing of gaps will decrease the dimensional accuracy and may reduce the fatigue life of a mechanical product. In order to reduce the gaps and keep them within tolerance, the relative posture (orientation and position) of two components should be optimized in the assembly process. This paper presents an optimal posture evaluation model to control the assembly gaps in aircraft wing assembly.Based on the step alignment strategy, i.e. preliminary alignment and refined alignment, the concept of a small posture transformation (SPT) is introduced. In the preliminary alignment, an initial posture is estimated by a set of auxiliary locating points (ALPs), with which the components can be quickly aligned near each other. In the refined alignment, the assembly gaps are calculated and the formulation of the gaps with component posture is derived by the SPT. A comprehensive weighted minimization model with gap tolerance constraints is established for redistributing the gaps in multi-regions. Powell-Hestenes-Rockafellar (PHR) optimization, Singular Value Decomposition (SVD) and KD-tree searching are introduced for the solution of the optimal posture for localization. Using the SPT, the trigonometric posture transformation is linearized, which benefits the iterative solution process. Through the constrained model, overall gaps are minimized and excess gaps are controlled within tolerance. Practical implications – This method has been tested with simulated model data and real product data, the results of which have shown efficient coordination of mating components.This paper proposed an optimal posture evaluation method for minimizing the gaps between mating surfaces through component adjustments. This will promote the assembly automation and variation control in aircraft wing assembly