Multiple hydrodynamic states were observed during cocurrent downflow of air and water under trickling flow conditions in a packed bed of rectangular cross section. Although the multiplicity was exhibited by both pressure gradient and liquid holdup, the pressure gradient showed the largest variations at identical conditions. The multiplicity is interpreted as being due to the liquid flowing in two different modes, namely, film flow and rivulet flow.The characteristics of pulsing flow in a packed bed of rectangular cross section were found to be appreciably different from those reported in the literature for flow in small-diameter cylindrical columns. The most significant observation in the packed bed of rectangular cross section was that the pulses did not always span the column cross section, unlike the case in small-diameter columns. The lower pressure drop and pulse velocity in the packed bed of rectangular cross section are believed to result from the bypassing of gas around the edge of the pulses. The location of the pulses was found to depend on the quality of gas and liquid distribution at the top of the column, and the shape of the top surface of the packing. A distributor configuration in which the gas was injected directly into the bed was found to be the most desirable, and is recommended in industrial practice.

Model calculations and experiments have been performed to explore the modes of photoacoustic signal generation from highly porous powders. These indicate that two modes of signal generation are important: (i) a thermal mode resulting from heating of the sample and conduction to the transducer gas above the sample, and (ii) a pressure mode resulting from the volumetric expansion of the interstitial gas in the porous sample. For highly porous materials, such as silica, the pressure mode can dominate the photoacoustic signal. Under conditions where the pressure mode is dominant, the frequency dependence of the acoustic magnitude and phase is similar to that of a homogeneous sample in photoacoustic saturation, however, the photoacoustic signal will still scale with absorption coefficient in this regime. The pressure mode of signal generation can be attenuated by compaction of the powdered sample to reduce the porosity. Unambiguous determination of photoacoustic saturation is shown to require measurements as functions of both frequency and porosity. The experimental results show that for highly porous silica even the strongest absorption bands are not in photoacoustic saturation. The results also suggest that infrared light scattering by powders of submicron-sized particles is not of major significance. This suggests that photoacoustic spectroscopy is extremely well suited to the study of highly porous materials.

This article seeks to illuminate the role of elite ideologies in understanding how cities have sought to transform their waterfronts during the postindustrial era. A case study of Philadelphia's long-term efforts to develop Penn's Landing, a valuable thirty-five-acre site along the Delaware River at the eastern edge of Center City, reveals how shifting perspectives among mayors, their top advisors, and leading activists with respect to the proper scope of government and the appropriate source of political power have resulted in significant changes in the city's approach to waterfront planning and policy making.

This article examines the Neighborhood Transformation Initiative (NTI), Mayor John F. Street's plan to revitalize Philadelphia's distressed neighborhoods by issuing $295 million in bonds to finance the acquisition of property, the demolition of derelict buildings, and the assembling of large tracts of land for housing redevelopment. Despite its resemblance to the discredited urban renewal programs of the past, this plan offered real potential for reducing blight by leveraging substantial private investment at a time when public subsidies for affordable housing and community development have been steadily diminishing.However, NTI did not promote equitable development that might have fostered broader support for an inherently controversial plan. Moreover, Street's initial leadership in proposing this bold initiative was followed by a reluctance to promote NTI aggressively after it was adopted in 2002. The result was a watered-down effort that achieved some goals but has fallen short of what might have been accomplished.

The mayor of Philadelphia, Edward G. Rendell, has garnered attention throughout the United States for guiding his city out of a fiscal crisis and then aggressively pursuing an ambitious plan for economic growth. This article examines Rendell’s economic development policies during his first term in office while highlighting the mayor’s leadership role in reviving the city. Although the mayor has been effective so far in stimulating investment in Philadelphia’s downtown business district, his leadership skills have not yet benefited the city’s struggling neighbourhoods. The case study concludes by considering implications for other nations contemplating implementation of a strong-mayor form of local government.