Eli5: How do they effectively test scale models of giant ships in water? Doesn’t the water behave differently in the small vs large scale?
In: Physics
Certain behaviors of the water can be scaled, others not. As has already been pointed out, the Froude and Reynolds numbers are crucial. These numbers describe the similarity laws for different types of forces.
By adjusting the speed accordingly, a model ship can run at the same Froude number as the full scale ship, resulting in wave generation to scale and hence, the wavemaking resistance of the ship can be determined in the experiment. The same Reynolds number, however, cannot realistically be attained, which means that viscosity of the water, and therefore the viscous resistance, is not to scale. The viscous resistance, however, which is mostly dependent of the wetted surface area of the ship and the viscosity, can be calculated for the model ship, subtracted from the measured total resistance, and replaced by the viscous resistance calculated for the full scale ship.
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