Towing tank

The measurement of drag on a surface vessel in a towing tank is another problem that is difficult to model exactly. For surface vessels which operate at a water-air interface, the acceleration due to gravity (g) must be included in a dimensional analysis for drag, since water is lifted vertically from the level surface against gravitational attraction during the formation of a bow wave. The variables of importance in this case are  

The end result of a dimensional analysis for a surface vessel of fixed shape will be  

The dimensionless quantity (VVg ) is called the Froude Number (F). In this instance, three scale factors may be arbitrarily fixed. These will naturally be associated with size, density, and viscosity, since water is the only practical fluid to use in the towing tank. 

The second model is a thin rectangular plank having the same length and wetted area as the scale model. The drag in this case is the skin-friction drag D, since there is no bow wave. The wave-making drag for the model is obtained by difference  

The value is adjusted for Reynolds Number equivalency using 

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Information on ship resistance has been determined from large numbers of tests on scale models of ships and from full-size ships, and compilations of these experimental results have been published. For a new and innovative hull form the usual procedure is to construct a scale model of the ship and then to conduct resistance tests m a special test facility (towing tank). Alternatively, analytical methods can provide estimates of ship resistance for a range of different hull shapes. Computer programs have been written based on these theoretical analyses and have been used with success for many ship designs, including racing sailboats. 

A technique which can assist in the scale-up of commercial plants designs is the use of scale models. A scale model is an experimental model which is smaller than the hot commercial bed but which has identical hydrodynamic behavior. Usually the scale model is fluidized with air at ambient conditions and requires particles of a different size and density than those used in the commercial bed. The scale model relies on the theory of similitude, sometimes through use of Buckingham s pi theorem, to design a model which gives identical hydrodynamic behavior to the commercial bed. Such a method is used in the wind tunnel testing of small model aircraft or in the towing tank studies of naval vessels. 

A few other examples in which models may be used include ships in towing tanks airplanes in wind tunnels hydraulic turbines, centrifugal pumps, dam spillways, and river channels and the study of such phenomena as the action of waves and tides on beaches, soil erosion, and the transportation of sediment. It must be emphasized that the model may not necessarily be different in size from its prototype. In fact, it may be the same device, the variables in the case being the velocity and the physical properties of the fluid. 

A number of towing tank tests have confirmed that the drag of a ship model can be substantially reduced by adding polymers to the water. Dove [30], Canham [31]. The economics... 

Hoyt, J.W., Fabula, A.G. Frictional resistance in towing tanks. 10th Int. Towing Tank Conf., London 1963... 

Dove, H.L. The effect on resistance of polymer additives injected into a boundary layer of a frigate model. Proc. 11th Int. Towing Tank Conf., Tokyo, 1966... 

The second example is for the G Tanker, Condition C in a canal with vertical sides (similar to a restricted channel) from FHR and Ghent University. The 1 50 scale laboratory experiments were performed in a 7.0-m-wide (350-m prototype) towing tank. The measured 5b = l-18m for the ship sailing at 14 = 10 kt Vs = 5.14m/s). The ship and channel characteristics are listed in Table 26.4. 

Fig. 26.10. Effect of passing encounter on ship bow and stern squat as a function of ship speed in FHR tow tank for containership and bulk carrier. ...

Fig. 26.12. Effect of overtaking maneuver on bow and stern sqnat as a function of lateral distance between ship centerlines for a containership and bulk carrier in the FHR tow tank.

The FHR has conducted towing tank experiments with containerships to study ship... 

T = 14.5 m) sailing at constant speed in a channel with h = 19.6 m. Scale 1 80 towing tank tests, no propeller action. Open symbols stern closed symbols bow." ... 

Bretschneider spectrum or ITTC (International Towing Tank Conference) or ISSC (International Ship Structure Congress) spectrum. 

Experimental data on wake length, pressure distribution, and temperature profiles Drag measurements in towing tanks... 

The physical model tests are usually performed in towing tank to develop simulation model of ship motion (Eloot et al., 2010) or accessibility and sil-tation study (Willems et al., 2013). 

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