Untitled - CNR

Marine research at CNRDoorHFa j q C D C L Eff HDS[deg.] [deg.] [deg.] [-] [-] [-] [m]ARCY1 22.2 2.3 9.8 0.83 1.87 2.25 76.52 28.6 3.0 10.2 0.91 2.07 2.29 93.93 34.2 4.7 11.2 1.12 2.14 1.91 99.34 39.1 7.0 12.6 1.41 2.10 1.49 96.65 43.1 9.5 14.3 1.74 2.02 1.16 89.31 * 24.5 7.0 12.7 0.36 1.06 2.99 56.22 33.8 9.1 13.7 0.38 1.12 2.91 59.73 40.1 11.9 15.5 0.51 1.25 2.45 68.94 43.5 13.8 16.9 0.61 1.35 2.20 76.15 * 44.0 14.1 17.2 0.63 1.37 2.17 77.3Note: for the Clarck-Y door, the values of the warp attachment position to the otterboard (HF) have been calculated on the basis ofthe results obtained with the modified otterboard used in the ST3.8[3] cruise.(*) The warp attachment positions to the otterboard (HF) nr. 1 and 5 have been extrapolated from the other tested HF.Table 3: Estimated values of full-scale traditional- (AR) and experimental (CY) doorsfor each warp attachment position to the otterboard (HF). Angle of attack of otterboard(α), heel angle of otterboard (φ), pitch angle of the otterboard (θ), drag force coefficient(CD), spreading force coefficient (CL), efficiency of otterboard (Eff), horizontal doorspread (HDS).ST3.2[2], ST3.8[3] or flume-tank) as factors.The use of further variables did notsubstantially improve the approximation ofdata. Considering that only the flume-tanktests at 0° heel have been analysed, thecurves have been produced by zeroing theheel term. It should be noted that flumetankcurves of CD, CL correspond to apure hydrodynamic effect while the coefficientsfrom sea trials have been affectedby the seabed effect also. For the AR otterboard,practically the same tendenciesbetween the model and full-scale experimentaltrials were found in all the curveswith angle of attack. For the drag- andlift-force coefficients, we obtained for thisdoor the expected results: sea trials resultin higher values than the flume-tankones (Figure 5), this is coherent with thefact that with real seabed there is an additionalfriction and ground shear effect.To be precise the maximum values obtainedin sea trials are for the drag and lift,around 50% and 100% respectively morethan flume-tank ones (see bold lines in Figure5). While for the Clarck-Y there is noevidence of coherent tendencies as abovementionedin section 3.2. For both doors,the graph for efficiency in Figure 5 showsthat, for a given attack angle, efficiency atsea is higher than in flume-tank, in particularthe maximum ratio is 1.9 and 3.9 for ARand Clarck-Y door respectively. To comparethe effect of seabed in vertical directionwe have estimated the values of reactionforce Rz. In Figure 5, the continuouslines are the scaled values of reactionforces measured in the flume-tank and dottedlines are the full-scale ones, discernedfor towing speeds 3.2 and 3.8 kn (ST3.2and ST3.8 respectively). Comparing thefull-scale doors, the reaction forces withangle of attack have inconsistent tendencies.We observe that reaction force, startingnow the discussion of it will be in absolutevalue, depends strongly on the tow-1883