scholarly journals Mass Transport and Turbulent Statistics within Two Branching Coral Colonies

Fluids ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 153
Author(s):  
Md Monir Hossain ◽  
Anne E. Staples

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched structure (P. meandrina). Detailed velocity vector and streamline fields were obtained inside both corals for the same unidirectional oncoming flow, and significant differences were found between their flow profiles and mass transport mechanisms. For the densely branched P. meandrina colony, a significant number of vortices were shed from individual branches, which passively stirred the water column and enhanced the mass transport rate inside the colony. In contrast, vortices were mostly absent within the more loosely branched P. eydouxi colony. To further understand the impact of the branch density on internal mass transport processes, the non-dimensional Stanton number for mass transfer, St, was calculated based on the local flow time scale and compared between the colonies. The results showed up to a 219% increase in St when the mean vortex diameter was used to calculate St, compared to calculations based on the mean branch diameter. Turbulent flow statistics, including the fluctuating velocity components, the mean Reynolds stress, and the variance of the velocity components were calculated and compared along the height of the flow domain. The comparison of turbulent flow statistics showed similar Reynolds stress profiles for both corals, but higher velocity variations, in the interior of the densely branched coral, P. meandrina.

2013 ◽  
Vol 2013 ◽  
pp. 1-7 ◽  
Author(s):  
H. F. Lopez ◽  
H. Mendoza

The effect of temperature on nano-CeO2 particle coarsening is investigated. The nanoceria powders were synthesized using the microemulsion method and then exposed to temperatures in the range of 373–1273 K. It was found that the nanoparticles exhibited a strong tendency to form agglomerates and through the application of ultrasound these agglomerates could be broken into smaller sizes. In addition average nanoparticle sizes were determined by powder X-ray diffraction (XRD). The outcome of this work indicates that the initial nano-CeO2 powders are amorphous in nature. Annealing promotes CeO2 crystallization and a slight shift in the (111) XRD intensity peaks corresponding to CeO2. Moreover, at temperatures below 773 K, grain growth in nano-CeO2 particles is rather slow. Apparently, mass transport through diffusional processes is not likely to occur as indicated by an estimated activation energy of 20 kJ/mol. At temperatures above 873 K, the measured activation energy shifted to 105 kJ/mol suggesting a possible transition to Ostwald-Ripening type mass transport mechanisms.


Author(s):  
Bohua Sun

This paper proposed an explicit and simple representation of velocity fluctuation and the Reynolds stress tensor in terms of the mean velocity field. The proposed turbulence equations are closed. The proposed formulations reveal that the mean vorticity is the key source of producing turbulence. It is found that there are no velocity fluctuation and turbulence if there were no vorticity. As a natural consequence, the laminar- turbulence transition condition was obtained in a rational way.


2006 ◽  
Vol 63 (3) ◽  
pp. 262-269 ◽  
Author(s):  
Florence Siepmann ◽  
Juergen Siepmann ◽  
Mathias Walther ◽  
Ross MacRae ◽  
Roland Bodmeier

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