8 research outputs found
Controlling a re-entrant manufacturing line via the push-pull point
A reduced model of a re-entrant semiconductor factory exhibiting all the important features is simulated, applying a push dispatch policy at the beginning of the line and a pull dispatch policy at the end of the line. A commonly used dispatching policy that deals with short-term fluctuations in demand involves moving the transition point between both policies, the push-pull point (PPP), around. It is shown that, with a mean demand starts policy, moving the PPP by itself does not improve the performance of the production line significantly over policies that use a pure push or a pure pull dispatch policy, or a CONWIP starts policy with pure pull dispatch policy. However, when the PPP control is coupled with a CONWIP starts policy, then for high demand with high variance, the improvement becomes approximately a factor of 4. The unexpected success of a PPP policy with CONWIP is explained using concepts from fluid dynamics that predict that this policy will not work for perishable demand. The prediction is verified through additional simulations
History and sensitivity comparison of the
The history of toxicity tests with duckweeds shows that these assays with free-floating
aquatic angiosperms are gaining increasing attention in ecotoxicological research and
applications. Standard tests have been published by national and international
organizations, mainly with the test species Lemna minor and Lemna
gibba. Besides the former two test species the great duckweed Spirodela
polyrhiza is to date also regularly used in duckweed testing. Under unfavorable
environmental conditions, the latter species produces dormant stages (turions) and this
has triggered the attention of two research groups from Belgium and Greece to jointly
develop a âstock culture independentâ microbiotest with S. polyrhiza. A
72 h new test has been worked out which besides its independence of stock culturing and
maintenance of live stocks is very simple and practical to perform, and much less
demanding in space and time than the conventional duckweed tests. Extensive International
Interlaboratory Comparisons on the S. polyrhiza microbiotest showed its
robustness and reliability and triggered the decision to propose this new assay to the ISO
for endorsement and publication as a standard toxicity test for duckweeds. Sensitivity
comparison of the 72 h S. polyrhiza microbiotest with the 7d L.
minor assay for 22 compounds belonging to different groups of chemicals
revealed that based on growth as the effect criterion both duckweed assays have a similar
sensitivity. Taking into account its multiple advantages and assets, the S.
polyrhiza microbiotest is a reliable and attractive alternative to the
conventional duckweed tests
History and sensitivity comparison of the Spirodela polyrhiza
The history of toxicity tests with duckweeds shows that these assays with free-floating
aquatic angiosperms are gaining increasing attention in ecotoxicological research and
applications. Standard tests have been published by national and international
organizations, mainly with the test species Lemna minor and Lemna
gibba. Besides the former two test species the great duckweed Spirodela
polyrhiza is to date also regularly used in duckweed testing. Under unfavorable
environmental conditions, the latter species produces dormant stages (turions) and this
has triggered the attention of two research groups from Belgium and Greece to jointly
develop a âstock culture independentâ microbiotest with S. polyrhiza. A
72 h new test has been worked out which besides its independence of stock culturing and
maintenance of live stocks is very simple and practical to perform, and much less
demanding in space and time than the conventional duckweed tests. Extensive International
Interlaboratory Comparisons on the S. polyrhiza microbiotest showed its
robustness and reliability and triggered the decision to propose this new assay to the ISO
for endorsement and publication as a standard toxicity test for duckweeds. Sensitivity
comparison of the 72 h S. polyrhiza microbiotest with the 7d L.
minor assay for 22 compounds belonging to different groups of chemicals
revealed that based on growth as the effect criterion both duckweed assays have a similar
sensitivity. Taking into account its multiple advantages and assets, the S.
polyrhiza microbiotest is a reliable and attractive alternative to the
conventional duckweed tests
History and sensitivity comparison of the Spirodela polyrhiza microbiotest and Lemna toxicity tests
The history of toxicity tests with duckweeds shows that these assays with free-floating
aquatic angiosperms are gaining increasing attention in ecotoxicological research and
applications. Standard tests have been published by national and international
organizations, mainly with the test species Lemna minor and Lemna
gibba. Besides the former two test species the great duckweed Spirodela
polyrhiza is to date also regularly used in duckweed testing. Under unfavorable
environmental conditions, the latter species produces dormant stages (turions) and this
has triggered the attention of two research groups from Belgium and Greece to jointly
develop a âstock culture independentâ microbiotest with S. polyrhiza. A
72 h new test has been worked out which besides its independence of stock culturing and
maintenance of live stocks is very simple and practical to perform, and much less
demanding in space and time than the conventional duckweed tests. Extensive International
Interlaboratory Comparisons on the S. polyrhiza microbiotest showed its
robustness and reliability and triggered the decision to propose this new assay to the ISO
for endorsement and publication as a standard toxicity test for duckweeds. Sensitivity
comparison of the 72 h S. polyrhiza microbiotest with the 7d L.
minor assay for 22 compounds belonging to different groups of chemicals
revealed that based on growth as the effect criterion both duckweed assays have a similar
sensitivity. Taking into account its multiple advantages and assets, the S.
polyrhiza microbiotest is a reliable and attractive alternative to the
conventional duckweed tests
The influence of environmental changes on local and regional vegetation patterns at Rieme (NW Belgium): implications for Final Palaeolithic habitation
Late-glacial vegetation changes were studied at Rieme, NW Belgium. Human occupation of this cover sand area occurred from the Final Palaeolithic onwards. The research area is situated on the northern side of a large cover sand ridge in an undulating landscape with small ridges and depressions. The past landscape was reconstructed using a multi-disciplinary approach, including geomorphological, sedimentological, loss-on-ignition, botanical (micro- and macrofossil) and zoological analyses. AMS 14C dating provided an accurate chronology for the sediments. Analyses were performed on three sequences located ~200â300 m apart. Our study shows that during the BĂžlling (GI-1e) wet meadows developed on the sandy soils and groundwater levels increased probably as result of permafrost melting. Shallow pools formed in depressions. During the Older Dryas (GI-1d) shrubs with juniper, seabuckthorn and willow developed. Many shallow depressions were overblown with sand and deposition of organic material almost ceased. In the early AllerĂžd (GI-1c) open birch woodlands developed. Due to the final melting of permafrost, groundwater levels rose further and ponds with floating-leaved open water vegetation developed. Large water level fluctuations occurred in one of the ponds. Accumulation of organic deposits ceased during the mid- AllerĂžd. Indirect evidence for human occupation during the AllerĂžd (GI-1c) was found in indications of burning of the reed-swamps in combination with the presence of large herbivores. Final Palaeolithic people probably used the northern side of the cover sand ridge as hunting area, while they settled their temporary (base) camps on the steep southern side along the extensive and deeper Moervaart lake