WAVE PROFILE 



LIFT FORCE 



-Fl i 



270* 



Figure 3. Lift force phenomenon. 



(a) Unrestricted flow through the bottom clearance at low velocities 

 results in downward lift force. 



(b) Unrestricted flow through the bottom clearance at higher velocities 

 increases the negative lift. ^ 



(c) Choking effect begins, so downward lift force decreases. 



(d) Velocities increase and pressures decrease at a faster rate over the 

 top of the pipe section than in the restricted flow through the bottom 

 clearance, so the lift force becomes positive. 



(e) Upward lift force increases with increasing velocities. 



(f) Positive lift reaches a maximum, 



(g) Positive lift force decreases and choking effect diminishes with 

 decreasing velocities of the passing wave crest. 



(h) Lift force again becomes negative as the flow through the bottom 

 clearance becomes less restricted. 



(1) Unrestricted flow through bottom clearance at low velocities results 

 in downward lift force. 



(j) Lift force cycle is repeated as the flow reverses with the approaching 

 wave trough. 



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