Frederick H. Abernathy

The formation of vortex streets

The formation of vortex streets in the wake of two-dimensional bluff bodies can be explained by considering the non-linear interaction of two infinite vortex sheets, initially a fixed distance, h , apart, in an inviscid incompressible fluid. The interaction of such sheets (represented in the calculation by rows of point-vortices) is examined in detail for various ratios of h to the wavelength, a , of the initial disturbance. The number and strength of the concentrated regions of vorticity formed in the interaction depend very strongly on h / a . The non-linear interaction of the two vortex sheets explains both the cancellation of vorticity and vortex-street broadening observed in the wakes of bluff bodies.

The Future of the Apparel and Textile Industries: Prospects and Choices for Public and Private Actors

The expectation that Chinese apparel and textile exports will swamp the US and EU retail markets now that international quotas on those products have been eliminated has fueled much of the discussion of the future of these industries. Although imports from China have surged since the elimination of quotas on 1 January 2005, this conventional wisdom masks important choices that remain for public and private policies over time. In particular, two factors will continue to have major effects on the location of apparel and textile production going forward. First, public-policy choices will continue to influence sourcing location; in particular, as they relate to tariffs and regional trade polices as well as to policies affecting the linkages between countries. Second, the lean-retailing model that now prevails requires apparel suppliers to replenish basic and fashion basic products on a weekly basis. As that retailing model became dominant in the 1990s, so too did the advantage of sourcing these apparel items closer to the US market so that products could be manufactured and delivered more rapidly from a smaller finished-goods inventory. Even though costs remain a driving factor, we show that proximity advantages for certain classes of products will continue in a postquota world as retailers raise the bar ever higher on the responsiveness and flexibility required of their suppliers.

Turbulent flow drag reduction and degradation with dilute polymer solutions

Experimental studies of drag reduction and polymer degradation in turbulent pipe flow with dilute water solutions of unfractionated polyethylene oxide are described. Drag reduction results indicate that the magnitude of the reduction cannot be correlated on the basis of weight average molecular weight, rather the phenomenon depends strongly on the concentration of the highest molecular weight species present in the molecular weight distribution. Polymer degradation in turbulent flow is found to be severe for high molecular weight polymers causing appreciable changes in drag reduction and molecular weight with the duration of flow. Data indicates that drag reduction exists in the limit of infinite dilution suggesting that the phenomenon is due to the interaction of individual polymer molecules with the surrounding solvent and that the extent of reduction is relatively independent of pipe diameter when a comparison is carried out at equal solvent wall shear stresses. Consideration of the high viscosity obtained with solutions in an irrotational laminar flow field suggests this is due to polymer molecule deformation and that this phenomenon is central to the mechanism of turbulent flow drag reduction.

Retailing and supply chains in the information age

Abstract This article describes how information technologies have reconfigured retailing and in turn the operation of a core US manufacturing industry, apparel. “Lean retailers” exchange point-of-sales information with their suppliers and require them to replenish orders quickly based on actual sales. This shifts part of the risk arising from changing consumer tastes from retailers and onto suppliers. In response to this shift in risk, we argue that manufacturers must reshape planning methods, cost models, inventory practices, production operations, and sourcing strategies. We then show that suppliers that adopt comprehensive changes to their manufacturing processes perform better along a number of dimensions compared to firms that have not.

The Information-Integrated Channel: A Study of the U.S. Apparel Industry in Transition

THE POPULAR PROGNOSIS for the U.S. apparel industry is bleak. Citing increased import penetration in many product segments and the concurrent erosion of domestic employment, many analysts regard apparel manufacturing in the United States as a dying industry. I The Department of Labor concurs, projecting a significant reduction in employment in the domestic apparel industry during the next decade. Under its most optimistic scenario, the department predicts employment will drop from a 1990 level of 839,000 to 649,000 in 2005; under its most

High-strain-rate free-surface boundary-layer flows

Two-dimensional boundary-layer flows of water down an inclined table were investigated in both the laminar and turbulent regimes. Mean, r.m.s. and skewness and velocity spectra were determined from streamwise velocity measurements. Two laser-Doppler anemometry methods were developed (for studying polymer-solution flows using this same water table) and compared with measurements obtained using hot-film anemometry. All three techniques obtained consistent results. An analysis based on a von Mises transformation is presented which accurately predicts the mean-velocity profile and flow development in the laminar regime. High strain rates are achieved which can be varied independently of Reynolds number, and turbulent flows are easily generated by inserting a disturbance. These turbulent flows are surprisingly similar to more commonly investigated turbulent boundarylayer flows of much greater y + extent. Turbulent water-table flows typically extend only to y + = 100, yet mean velocity essentially follows the law of the wall, and intensity and skewness measurements are similar to those obtained in flows much less limited in y + .

Transition to turbulence in pipe flow for water and dilute solutions of polyethylene oxide

The transition from laminar to turbulent flow was studied experimentally in a long 0.248 in. ID pipe for both water and for dilute water solutions of polyethylene oxide exhibiting a wide range of drag reduction. In all cases the transition pattern and the growth rates of turbulent patches are similar. The growth rate of turbulent patches indicates that drag reduction is associated with the small-scale structure of the turbulence near the pipe wall, while patch growth is associated with the larger-scale turbulence in the outer flow. For low-disturbance pipe inlet conditions, the strong drag-reducing solutions undergo transition at lower Reynolds numbers than the pure water.

Streamwise vortices and transition to turbulence

A series of experiments was conducted to determine the conditions under which streamwise vortices can cause transition to turbulence in shear flows. A specially designed obstacle was used to produce a single vortex in a water-table flow, and the design of this obstacle is discussed. Laser-Doppler velocimetry measurements of the streamwise and crossflow velocity fields were made in transitional and non-transitional flows, and flow visualization was also used. It was found that strong vortices (vortices with large circulation) lead to turbulence while weaker vortices do not. Determination of a critical value of vortex strength for transition, however, was complicated by ambiguities in calculating the vortex circulation. The profiles of streamwise velocity were found to be inflexional for both transitional and non-transitional flows. Transition in single-vortex and multi-vortex flows was compared, and no qualitative differences were observed, suggesting no significant vortex interactions affecting transition.