Tham khảo tài liệu 'practical ship hydrodynamics episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Resistance and propulsion 71 limited to the propeller plane. The local velocities were traditionally measured by pitot tubes. Currently Laser-Doppler velocimetry also allows non-intrusive measurements of the flow field. The results are usually displayed as contour lines of the longitudinal component of the velocity Fig. . These data play an important role in the design of a propeller. For optimizing the propeller pitch as a function of the radial distance from the hub the wake fraction is computed as a function of this radial distance by integrating the wake in the circumferential direction _ 1 z w r r 0 d0 2n J 180 180 90 0 R V Vo 0 0 Figure Results of wake measurement The wake field is also used in evaluating propeller-induced vibrations. Method ITTC 1957 The resistance of the hull is decomposed as Rt Rf C Rr Rf is the frictional resistance RR the residual resistance. Usually the resistance forces are expressed as non-dimensional coefficients of the form R Ci 1 . r 2pv2s 72 Practical Ship Hydrodynamics S is the wetted surface in calm water Vs the ship speed. The resistance coefficient of the ship is then determined as cTs cFs cR cA cFs cTm cFm cA The index s again denotes values for the full-scale ship the index m values for the model. cR is assumed to be independent of model scale . cR is the same for model and full scale. The model test serves primarily to determine cR . The procedure is as follows 1. Determine the total resistance coefficient in the model test RTm cTm K t 2 c 2 Pm Vm Sm 2. Determine the residual resistance same for model and ship cR cTm cFm 3. Determine the total resistance coefficient for the ship cTs cR cFs cA 4. Determine the total resistance for the ship Rts cts 2PsV2Ss The frictional coefficients cF are determined by the ITTC 1957 formula cF ------------ 2 log 0 Rn - 2 2 This formula already contains a global form effect increasing the value of cF by 12 compared to the value for flat plates .
