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Howto:Modelling hydrodynamics in JSBSim: Difference between revisions
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Howto:Modelling hydrodynamics in JSBSim (view source)
Revision as of 21:00, 5 December 2016
, 5 December 2016→Modelling hydrodynamic coefficients
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== Modelling hydrodynamic coefficients == | == Modelling hydrodynamic coefficients == | ||
The hydrostatic coefficients described above allow the vessel to float, so the next problem is to make it move through the water. Starting with drag there are different modes of movement | The hydrostatic coefficients described above allow the vessel to float, so the next problem is to make it move through the water. Starting with drag there are different modes of movement on the water, namely | ||
* displacement, the vessel pushing through the water; | * displacement, the vessel pushing through the water; | ||
* planing, the vessel skimming along the surface of the water pushing down on it rather than through it; and | * planing, the vessel skimming along the surface of the water pushing down on it rather than through it; and | ||
* semi-planing, a mix of the above. | * semi-planing, a mix of the above. | ||
For a non-planing vessel the drag is roughly proportional to the square of the speed through the water. | For a non-planing vessel the drag is roughly proportional to the square of the speed through the water while a planing vessel has much more complicated drag function as it goes through displacement, semi-planing and planing phases as speed increases. | ||
As an example consider the plot below of the towing power (drag times velocity) of a [[Swedish Navy T21 class MTB]] as speed increases from 0 to nearly 60 knots. The blue line is from a real tow tank test ("Resultat av släpförsök med svenska patent nr. 109626", A. F. Nordström, Statens Skeppsprovningsanstalt, Göteborg, 1941-10-01) and shows the complex shape of the drag function. One can guess that the curve up to about 10 knots is mostly due to quadratic displacement drag. Above this speed the vessel enters the planing phase and the displacement is reduced as more and more of the weight is supported by the planing forces. This reduces the increase in drag considerably. Finally, from about 38 knots and over another quadratic growth phase takes over. My hypothesis is that this phase is due to the rudders that remain in the water and ought to have (from standard fluid dynamics) a rather small but quadratic drag function. | |||
[[File:Swedish Navy T21 class MTB-drag.png|thumb|600px|center|Hydrodynamic drag of a MTB after ad hoc tuning. Total (green) and parts, displacement (red), planing (cyan) and rudder assembly (purple), of the hydrodynamic drag of the [[Swedish Navy T21 class MTB]] from a single acceleration run in JSBSim/standalone. The blue line is a real tow tank result traced from a diagram in "Resultat av släpförsök med svenska patent nr. 109626", A. F. Nordström, Statens Skeppsprovningsanstalt, Göteborg, 1941-10-01 available as an image (ID 2008:25:869:1) from Sjöhistoriska museet under CC BY-SA.]] | [[File:Swedish Navy T21 class MTB-drag.png|thumb|600px|center|Hydrodynamic drag of a MTB after ad hoc tuning. Total (green) and parts, displacement (red), planing (cyan) and rudder assembly (purple), of the hydrodynamic drag of the [[Swedish Navy T21 class MTB]] from a single acceleration run in JSBSim/standalone. The blue line is a real tow tank result traced from a diagram in "Resultat av släpförsök med svenska patent nr. 109626", A. F. Nordström, Statens Skeppsprovningsanstalt, Göteborg, 1941-10-01 available as an image (ID 2008:25:869:1) from Sjöhistoriska museet under CC BY-SA.]] | ||
''''To be continued... '''' | |||
== Examples == | == Examples == | ||