Twenty-Second Symposium on Naval Hydrodynamics (1999) / Chapter Skim
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10 Weinblum Lecture
10 Weinblum Lecture
Pages 622-637
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From page 622... ...
whereas maximum lift is grossly in error due to scale effects; - Rans (Reynolds averaged Navier Stokes) calculations, the accuracy of which is not always beyond doubts, but which - after suitable verifications - appear to be the most reliable source of information; and - panel method calculations, which can give moderately accurate results with a minimum of effort if some empirical relationships and possibly corrections are applied.
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From page 623... ...
2. Radial distribution of velocity increment in propeller slipstream Fig.
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From page 624... ...
Another influence of substituting the vortex transport equation by rot u = 0 is an over-emphasis on the flow conditions just at the rudder surface while neglecting the conditions sideways from the rudder, which in reality influence rudder forces as well. To decrease this error, a weighted average of U(y)
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From page 625... ...
From the axial, circumferential and radial velocity the Cartesian components of the inflow velocity U are determined. 3.3 Desingularized direct pane} method For lift-generating flows, direct panel methods, which use the flow potential as primary unknown, proved superior in accuracy to indirect methods, which use source and vortex strength as primary unknowns.
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From page 626... ...
Table 1 shows various results for dCr/da corrected for infinite aspect ratio by using (18~. The values resulted from measurements, potential calculations and Rans calculations for various profiles in 2d and ad.
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From page 627... ...
0 experi- 0 04 meets by Whicker and Fehlner (1958J; · panel o method; curve Rans calculations by El Moctar 0 06 _ (1997J 627
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From page 628... ...
. The low lift gradient of the relatively blunt NACA0025 profile is not correctly predicted by the panel method.
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From page 629... ...
4.3 Maximum lift at stall angle Wind tunnel Rans 2d Wind tunnel Rans 2d Wind tunnel Rans 2d ,, 77 Rans ad Wind tunnel Rans ad Wind tunnel Rans ad Wind tunnel Rans ad Wind tunnel ~ s ad Wind tunnel Wind tunnel Rans ad ,, Wind tunnel Rans ad ,, Rans 2d a, ,, Rans ad ,, ,, Rans ad ,, Wind tunnel Rans ad ,, Table 3. CLAD and install Source ~ _ Chau 1997 Rans 2d NACA0012_ ~1.62 18 ,, ,, ,,,, _ 60 1.63 18 ,, _ 6 1.60 16 _ x 6 1.91 18 1.42 19 ,, _ 6 1.52 16 ,, _ 6 1.34 13 1 0.45 1.82 1.44 44 ,, ,, 1.82 1.26 39 2 ,, 1.82 1.29 30 ,, ,, 1.82 1.20 27 3 ,, 1.82 1.26 25 ,, ,, 1.82 1.15 23 3 ,, 18.2 1.37 27 ,, ,, 18.2 1.25 23 1 1.0 0.78 1.30 42 ,, ,, ,, 1.34 46 ,, ,, ,, 1.06 34 ,, ,, ,, 1.42 44 ,, ,, ', 1.19 37 ,, ,, ,, 1.47 46 3 0.45 1.82 1.26 25 ,, 1.0 1.82 1.26 25 oo _ 50 2.11 18 ,, _ ,, 2.06 21 ,, _ _ ,, 1.88 23 4 1.0 50 2.05 -31 ,, ,, ,, 1.96 35 ,, ,, ,, 1.74 33 1 0.45 2.7 1.29 45 3 ,, ,, 1.29 27 ,, ,, ,, 1.24 23 ,, ,, 50 1.48 27 NACA4412 ,, NACA652-415 ,, NACA0025 HSVA MP73-25 IfS 62 TR 25 NACA0015 ,, NACA0015 ,, NACA0015 ,, NACA0015 ,, NACA0025 NACA0015 HSVA MP73-25 IfS 62 TR 25 ,, NACA0015 ,, IfS 62 TR 25 lISVA MP73-25 NACA0025 IfS 62 TR 25 HSVA MP73-25 NACA0025 NACA0015 NACA0015 ,, a, 629
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From page 630... ...
All the lift data show very good coincidence; the same holds for the measured and Rans-computed drag and stock moment. The drag computed by the panel method is about 25~o too small, whereas the panel stock moment indicates a point of attack of the transverse force which, for 20° rudder angle, lies about 1.5% of c in front of the measured one.
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From page 631... ...
6. o: measurements of SVA Potsdam; curves: panel method results; broken line: extreme lift estimated from 2D RANS results 631
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From page 632... ...
The reason is, presumably, also the fundamental error of underestimating the interaction between the propeller slipstream and the flow induced by the rudder: The wakes at the top and bottom edges of the rudder, which are effective in shifting the attack point of the transverse force backward, were relatively weak in the computation because upper and lower end of the rudder were outside of the propeller slipstream; in reality, however, the propeller slipstream is elongated in vertical direction by the rudder at the aft end of the pressure side due to the vertical rudder-induced velocities. For the Reynolds number Rn = 0.26 106 of the experiments, 2D RANSE calculations in uniform flow gave Cat ma ~ = 1.24.
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From page 633... ...
However, in yaw checking large rudder angles opposite to the turning motion would lead to stall and the accompanied decrease of rudder transverse force. This is easily avoided by limiting the shaft moment which the rudder gear can generate to about the stall angle moment; then in yaw checking the rudder will just operate at the stall angle, producing maximum lift.
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From page 634... ...
Due to propeller slipstream contraction, the cylinder surface is not a streamline surface. Neglecting turbulent mixing, far behind the propeller the cylinder contains an inner region where the fluid ve locity is U`/1 + Cth according to the propeller momentum theory; in the outer cylinder region the fluid velocity is assumed to be U
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From page 635... ...
AUTHOR'S REPLY Regarding hull forces in maneuvering, I combine slender lifting body theory [7] win cross-flow resistance coefficients chosen to obtain correspondence of simulated with trial trip results.
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From page 636... ...
More important error sources in rudder forces influenced by hull and propeller are probably the turbulent mixing of the propeller slipstream with the surrounding fluid, the flow acceleration win increasing distance from the stern due to the hull displacement effect, and the flow curvature behind a drifting or turning ship hull. I thank all the discussers for their encouragement and valuable comments.
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From page 637... ...
y/rist ! cartesian velocity components u(3~= vt*
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