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Flow Time (flow + time)

Distribution by Scientific Domains

Polymers and Materials Science	50%

Selected Abstracts

The spatial and temporal patterns of aggradation in a temperate, upland, gravel-bed river

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2009
Emma K. Raven
Abstract Intensive field monitoring of a reach of upland gravel-bed river illustrates the temporal and spatial variability of in-channel sedimentation. Over the six-year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land-use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Rapid Generation of Biologically Relevant Hydrogels Containing Long-Range Chemical Gradients

ADVANCED FUNCTIONAL MATERIALS, Issue 1 2010
Jiankang He
Abstract Many biological processes are regulated by gradients of bioactive chemicals. Thus, the generation of materials with embedded chemical gradients may be beneficial for understanding biological phenomena and generating tissue-mimetic constructs. Here a simple and versatile method to rapidly generate materials containing centimeter-long gradients of chemical properties in a microfluidic channel is described. The formation of a chemical gradient is initiated by a passive-pump-induced forward flow and further developed during an evaporation-induced backward flow. The gradient is spatially controlled by the backward flow time and the hydrogel material containing the gradient is synthesized via photopolymerization. Gradients of a cell-adhesion ligand, Arg-Gly-Asp-Ser (RGDS), are incorporated in poly(ethylene glycol)-diacrylate (PEG-DA) hydrogels to test the response of endothelial cells. The cells attach and spread along the hydrogel material in a manner consistent with the RGDS-gradient profile. A hydrogel containing a PEG-DA concentration gradient and constant RGDS concentration is also shown. The morphology of cells cultured on such hydrogel changes from round in the lower PEG-DA concentration regions to well-spread in the higher PEG-DA concentration regions. This approach is expected to be a valuable tool to investigate the cell,material interactions in a simple and high-throughput manner and to design graded biomimetic materials for tissue engineering applications. [source]

Optimizing injection gate location and cycle time for the in-mold coating (IMC) process

POLYMER COMPOSITES, Issue 5 2002
Mauricio Carbera-Rios
The standard practice when compression molding Sheet Molding Compound (SMC) exterior automotive body panels is to in-mold coat (IMC) the parts. Consequently, IMC needs to be considered an integral part when improving the process. Selecting the proper IMC injection gate location to obtain a defect-free coated part and properly setting the IMC processing conditions to reduce its cycle time are both key decisions for the IMC process. In the present work, an optimization method that involves metamodeling through either linear regression or artificial neural networks is explored with two purposes: first, to select the injection gate location(s) with the objective of minimizing the potential for surface defects in the coating; and second, to set the mold wall temperature and the initiator concentration to minimize the cure time for a given minimum required flow time. [source]

IMPROVING JOB SHOP PERFORMANCE THROUGH PROCESS QUEUE MANAGEMENT UNDER TRANSFER BATCHING

PRODUCTION AND OPERATIONS MANAGEMENT, Issue 4 2000
JOEL LITCHFIELD
This research investigated the value of protecting the continuity of release batchs in a transfer batching environment, by modifying the SPT rule. A simulation model of a job shop was used to test the modified SPT rule. The performance measures evaluated were mean flow time, flow time variance, and mean lateness. Conditions under which the SPT modification improved results were as follows: large number of transfer batches, small setup time to process time ratio, and large variation in process times from station to station. The results suggest that shop loading is not a significant factor affecting performance of the modified SPT rule. [source]

Normovolemia defined according to cardiac stroke volume in healthy supine humans

CLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 5 2010
Morten Bundgaard-Nielsen
Summary Background:, Both hypovolemia and a fluid overload are detrimental for outcome in surgical patients but the effort to establish normovolemia is hampered by the lack of an operational clinical definition. Manipulating the central blood volume on a tilt table demonstrates that the flat part of the Frank-Starling curve is reached when subjects are supine and that finding may be applicable for a clinical definition of normovolemia. However, it is unknown whether stroke volume (SV) responds to an increase in preload induced by fluid administration. Methods:, In 20 healthy subjects (23 ± 2 years, mean ± SD), SV was measured by esophageal Doppler before and after fluid administration to evaluate whether SV increases in healthy, non-fasting, supine subjects. Two hundred millilitres of a synthetic colloid (hydroxyethyl starch, HES 130/0·4) was provided and repeated if a ,10% increment in SV was obtained. Results:, None of the subjects increased SV ,10% following fluid administration but there was a minor increase in mean arterial pressure (92 ± 15 to 93 ± 12 mmHg, P = 0·01), while heart rate (HR) (66 ± 12 beats min,1; P = 0·32), cardiac output (4·8 ± 1·1 l min,1; P = 0·25) and the length of the systole corrected to a HR of 60 beats/min (corrected flow time; 344 ± 24 ms; P = 0·31) did not change. Conclusion:, Supporting the proposed definition of normovolemia, non-fasting, supine, healthy subjects are provided with a preload to the heart that does not limit SV suggesting that the upper flat part of the Frank-Starling relationship is reached. [source]

Flow front measurements and model validation in the vacuum assisted resin transfer molding process

POLYMER COMPOSITES, Issue 4 2001
R. Mathuw
Through-thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM). During the VART'M process, a highly permeable distribution medium is incorporated into the preform as a surface layer and resin is inftised Into the mold, under vacuum. During Infusion, the resin flaws preferentially across the surface and simultaneously through the thickness of the preform, giving rise to a three dimensional-flow front. The time to fill the mold and the shape of the flow front, which plays a key role in dry spot formation, are critical for the optimal manufacture of large composite parts. An analytical model predicts the flow times and flow front shapes as a function of the properties of the preform, distribution media and resin. It was found that the flow front profile reaches a parabolic steady state shape and the length of the region saturated by resin is proportional to the square root of the time elapsed. Experimental measurements of the flow front in the process were carried out using embedded sensors to detect the flow of resin through the thickness of the preform layer and the progression of flow along the length of the part. The time to fill the part, the length of flow front and its shapes show good agreement between experiments and the analytical model. The experimental study demonstrates the need for control and optimization of resin injection during the manufacture of large parts by VARTM. [source]