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3.3.3 Gravity effects

The gravity has a large effect only when gravity force is large relatively to other forces. A typical velocity ranges generated by the gravity is the same as for a object falling in air. The air effects can be neglected since the air density is very small compared with the liquid metal density. The momentum is the other dominate force in the filling the cavity. Thus, the ratio of the momentum forces to the gravity force, also known as Froude number, determines if the gravity effects are important. The Froude number is defined here as
    (3.9)

Where is the velocity, is the characteristic length is the gravity force. For example, the characteristic pouring length is in order of 0.1[m], in extreme cases the velocity can reach 1.6[m] with characteristic time of 0.1[sec]. The author is not aware of experiments to verify the flow pattern in such cases (low number due to solidification effect)11. Yet, it is reasonable to assume that the liquid metal in such a case is laminar even though the number is relatively large () because of the short time and the short distance. The number is defined by the flow rate and the thickness of the exiting typical dimension. Note, the velocity reach its maximum value just before impinging on the sleeve surface.

The gravity has dominate effect on the flow in the shot sleeve since the typical value of the Froude number in that case (especially during the slow plunger velocity period) are in the range of 1. Clearly, any analysis of the flow has to take the gravity into consideration (see Chapter [*]).


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