This animation was prepared by the Naval Surface Warfare Center - Carderock Division as part of the NOAA-Navy CFD ballast tank modeling study. It shows the flow and mixing in a computer model replicate of a full-size Great Lakes saltwater bulk-cargo vessel ballast tank during 3-tank-volume flow-through ballast water exchange. Each animation starts with the tank filled with fresh water (red). The influent seawater (salinity = 35) is blue and enters at a model flow rate scaled to equal 1,012 gpm (230 metric tonnes per hour).
The structural compartments of the tank are:
- Double-Bottom (DB) tank - the part of the tank closest to the lower left corner of the screen and shaped like a long, flat rectangular box. DB is adjacent and connected to the:
- Hopper Side Tank (ST) - the tall compartments of the lower tank structure that include the curviture of the ship's hull and taper to a triangular point at the top.
Both DB and ST are connected by two pipes to the:
- Overhead wing tank (WT) - the separate trapezoidal shaped tank closest to the top edge of the screen (only visible a Time greater than 200 seconds).
The complete ballast tank is composed of the interconnected double-bottom, hopper side- and overhead wing tanks. Tank structural elements are all shaded in gray. Color planes (initially shaded red) represent longitudinal data slices through DB-ST-WT sections 1 (bellmouth section), 2, 3, 5, 7, and 9 and are where the flow and seawater/freshwater composition can be followed during CFD animation.
The bellmouth is located in DB section 1, closest to the viewer. Initially one interconnecting pipe is seen at the top of ST section 2, but as the animation proceeds the viewpoint draws back and the second connecting pipe can be seen at the top of ST section 9. The overflow exits are not included in the animation, but occupy positions immediately above the two interconnecting pipes at the top of the WT.
The complete full-scale ballast tank model consists of ten adjacent DB-ST and WT sections with overall dimensions ~24 m long by ~5.2 m wide. The peak of the ST is ~5 m above the bottom of the tank; the top of the WT outer wall is ~4.7 m above the bottom edge of the WT, while the total height from the bottom of the DB to the top the WT is 14.7 m. The total volume (1 TV) of the entire tank is 185,740 gal not including the connecting pipes and interbay cutouts (manholes and drain holes),
Animation time volume-equivalents
3,723 s = 1 tank volume
7,446 s = 2 tank volume
11,169 s = 3 tank volumes
It is useful to pause the animation at appropriate times to view the features noted in the following narrative.
When the animation starts, note the intense vertical mixing and overturning waves early in the exchange (less than 60 sec) in the section containing the bellmouth. Also note the time delay in the appearance and vertical height of the saltwater interface down the length of the DB. For example, at ~300 seconds section 1 shows the saltwater interface at the top of the DB tank, but only a thin layer of partially mixed saltwater near the bottom in section 9.
After 400-500 seconds stratification is well formed across athe entire DB, although the height of the interface remains staggered. At around 700 seconds a diluted saltwater mixture has reached the first connecting pipe at the top of ST section 2, and by 1000 seconds, the mixture has reached the far connecting pipe at the top of ST section 9. Saltwater mixture is detectable in WT sections 2 and 9 by 1200 seconds, where it first appears to run up the outer tank wall then fold back into the body of the WT and start to stratify (~1600 sec).
It takes ~10,000 seconds for all vestiges of freshwater (red) to be completely eliminated from the DB-ST areas, although some areas of trappage of slightly diluted saltwater mixture are seen in the ST peaks of sections 1-7. At the end of 3 TV (~11,000 seconds) the WT still contains diluted saltwater with a relatively thin ~50% mixture (greenish layer) at the top to 100% saltwater (dark blue) near the bottom of the WT. Integration of volume elements over the entire tank volume indicates 93% replacement by saltwater at this point. Although less than the target of 95%, this is within the model error and statistically cannot be identified as significantly different from the target of 95%. In addition, the lowest salinity found in the tank after 3 TVE is ~50% saltwater (the thin layer at the WT top), with an estimated salinity between 15 and 20.
This animation is provided by the Great Lakes Environmental Research Laboratory - GLERL http://www.glerl.noaa.gov
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Model of Ballast Water Exchange in Great Lakes Bulk-cargo Vesselby noaaglerl1,196 views
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