《船舶原理77Modeling》-公开课件（设计）.pptVIP

* 7.7 MODELING 35 So far we’ve discussed what resistance is and how it can quantified using: - RT by measuring the actual resistance force - CT dimensionless coefficients that can be used to compare resistance between dissimilar hull shapes and sizes We can now measure the resistance in a hull and use the data to designing a ship’s power plant - Using the resistance data, an effective power plant can be designed - Taking into account the relationship between - Effective Horsepower, EHP - Shaft Horsepower, SHP 32 = Rt Vs EHP 550 Resistance and power are related! Resistance can be measured in two ways: - Computer modeling - Can be very difficult to mathematically model viscous flow in 3 dimensions - Tow Tank testing - Producing a geometrically and dynamically similar model to test - Relate model performance to expected actual ship performance 25 Tow Tank testing is the obvious way to go! But to do so, your “model” ship must meet some criteria: …Note that a “minor” error in any length measurement will be cubed (n3)in volume scaling! Tow Tank Modeling l = LS (ft) LM (ft) l2 = SS (ft2) SM (ft2) l3 = VS (ft3) VM (ft3) Length Area Volume where: M = Model S = Ship 1. Geometric Similarity - The dimensions of the model and ship must be scaled exactly - The “Scale Factor” is called l (lambda) 54 Tow Tank Modeling 2. Dynamic Similarity - Motion of the vessel must also be scaled, including: - Ship’s velocity - Acceleration - Viscosity of the water - Dynamic similarity can only be approximated as water’s viscosity and the forces of gravity can not be manipulated (duh!) - The trade-off is a “partial similarity” - Froude’s Law of Comparison or “Law of Corresponding Speeds” CWM = CWS CVM = CVS 23 41 The Law of Corresponding Speeds says: VS = VM LS LM 7 Coefficient of Viscous Resistance - Viscous Flow around a ship Real ship: Turbulent flow exists near the bow. Model ship: St