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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 14:14, 12 March 2017
, 12 March 2017→Modelling hydrodynamic coefficients
(→Modelling hydrodynamic coefficients: A bit more on planing drag.) |
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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 curve. | 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 curve. | ||
The actual physics of planing is very complex and the exact shape of the planing surfaces and the trim of the vessel play a large role in determining the drag (and other forces). The approach taken here to model this in JSBSim is very much simplified, using just a few functions and a handful of tuning parameters to estimate the hydrodynamic forces. | The actual physics of planing is very complex and the exact shape of the planing surfaces and the trim of the vessel play a large role in determining the drag (and other forces). The approach taken here to model this in JSBSim is very much simplified, using just a few functions and a handful of tuning parameters to estimate the hydrodynamic forces. For cases where appropriate and comprehensive tow test data exists coefficient tables could instead be extracted from that. | ||
[[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.]] | ||