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Figure 1: SWATH prospective and lateral orthographic views

Figure 1 SWATH prospective and lateral orthographic views

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Table 2: Operative conditions Exploiting the flexibility of the chimera overlapping grids a high-quality body-fitted mesh has been realized. Figure 2 shows an overview of the main topologies realized. The grids is composed by 92 partially overlapping blocks, for a total of about 5.6 millions volumes. Grid resolution is clustered to the wall with normal spacing ensuring yt < 1 for the Reynolds numbers considered. The boundary conditions are set considering the symmetry on the y = 2.5m plane (this allows to simulate the presence of the twin hull), the uniform inflow condition on the x = 0 plane and the outflow (non reflecting) boundary condition for the other boundaries.
Table 2: Operative conditions Exploiting the flexibility of the chimera overlapping grids a high-quality body-fitted mesh has been realized. Figure 2 shows an overview of the main topologies realized. The grids is composed by 92 partially overlapping blocks, for a total of about 5.6 millions volumes. Grid resolution is clustered to the wall with normal spacing ensuring yt < 1 for the Reynolds numbers considered. The boundary conditions are set considering the symmetry on the y = 2.5m plane (this allows to simulate the presence of the twin hull), the uniform inflow condition on the x = 0 plane and the outflow (non reflecting) boundary condition for the other boundaries.
Figure 2: Topology overview: black far background, light green near background, light blue free surface, blue near free surface, orange hull, red struts
Figure 2: Topology overview: black far background, light green near background, light blue free surface, blue near free surface, orange hull, red struts
Table 3: Non-dimensional resistance results and verification analysis Grid convergence results are rather satisfactory in the whole speed range, though a high uncertainty value (i.e. 7.35%) is observed at low Froude numbers. This is due to lack in the grid resolution around the free surface, yelding to inaccuracies in the computation of the short and small waves at the lower Froude numbers.
Table 3: Non-dimensional resistance results and verification analysis Grid convergence results are rather satisfactory in the whole speed range, though a high uncertainty value (i.e. 7.35%) is observed at low Froude numbers. This is due to lack in the grid resolution around the free surface, yelding to inaccuracies in the computation of the short and small waves at the lower Froude numbers.
fn Dee ae ee a a ee ae ee ee ee a ee ee a ee In order to evaluate the interaction effects of the twin hulls, the pressure, the viscous and the total resistance are evaluated for both the complete and the demi SWATH. Figure 3 shows the resistance trends for both configurations. The total resistance curves show the presence of two humps, which are typical of multihull vessels as it has been already shown in previous studies (e.g. [15, 3]). The local maximum and minimum are related to the position of the wave trough. Both complete and demi SWATH configurations show the local minimum close to U = 4.5kn, Fr=0.35. Figure 3: Resistance analysis: complete and demi SWATH
fn Dee ae ee a a ee ae ee ee ee a ee ee a ee In order to evaluate the interaction effects of the twin hulls, the pressure, the viscous and the total resistance are evaluated for both the complete and the demi SWATH. Figure 3 shows the resistance trends for both configurations. The total resistance curves show the presence of two humps, which are typical of multihull vessels as it has been already shown in previous studies (e.g. [15, 3]). The local maximum and minimum are related to the position of the wave trough. Both complete and demi SWATH configurations show the local minimum close to U = 4.5kn, Fr=0.35. Figure 3: Resistance analysis: complete and demi SWATH
Figure 4: Interference Factor analysis The estimation of the augmented resistance due to the interference between the twin hulls is evaluated using R°—2R4 the interference factor, defined as IF = “S77 and the demi SWATH, respectively). (where C and D indicates quantities corresponding to the complete
Figure 4: Interference Factor analysis The estimation of the augmented resistance due to the interference between the twin hulls is evaluated using R°—2R4 the interference factor, defined as IF = “S77 and the demi SWATH, respectively). (where C and D indicates quantities corresponding to the complete
Figure 5: Wave elevation patterns respectively at U = lkn, U = 2kn, U = 3kn, U = 4kn, U = 5kn and U = 6kn
Figure 5: Wave elevation patterns respectively at U = lkn, U = 2kn, U = 3kn, U = 4kn, U = 5kn and U = 6kn
=e ee es l—F es ee ee ae ee Figure 6 shows the comparison between potential flow results and grid convergent RANS solutions. The agreement is reasonable in the whole speed range. Specifically, the wave resistance coefficient trend is well cap- tured, although a difference between the peak values (Fr = 0.294, U = 4kn) is significant. The friction resistance coefficients evaluated with the two solvers, are considerably close and the total resistance coefficient behaves ac- cordingly. The dimensional total resistance shows a remarkable agreement in the entire speed range, especially for low and high speed values. Figure 6: Potential flow vs RANS computations (wave, frictional, total resistance coefficient and total resistance) A preliminary parametric analysis is performed considering a variable struts clearance, namely C (equal to 0.5 0.75 and | the original struts clearance Cg) and a variable interaxis distance (J) between torpedoes (equal to 0.75 1, 1.5 and 2 the original interaxis distance Jy). This is performed by linear superposition of strut and torpedo effect [11]. Figure 7 shows the contour plots of the total resistance coefficients Cr versus C and J, obtained using a thi plate spline (TPS) interpolation of the simulation results (represented by circles). It may be noted that the strut clearance has significant effects on the resistance at low speeds. Furthermore, an increase of the interaxis distanc induces a reduction of the total resistance coefficient.
=e ee es l—F es ee ee ae ee Figure 6 shows the comparison between potential flow results and grid convergent RANS solutions. The agreement is reasonable in the whole speed range. Specifically, the wave resistance coefficient trend is well cap- tured, although a difference between the peak values (Fr = 0.294, U = 4kn) is significant. The friction resistance coefficients evaluated with the two solvers, are considerably close and the total resistance coefficient behaves ac- cordingly. The dimensional total resistance shows a remarkable agreement in the entire speed range, especially for low and high speed values. Figure 6: Potential flow vs RANS computations (wave, frictional, total resistance coefficient and total resistance) A preliminary parametric analysis is performed considering a variable struts clearance, namely C (equal to 0.5 0.75 and | the original struts clearance Cg) and a variable interaxis distance (J) between torpedoes (equal to 0.75 1, 1.5 and 2 the original interaxis distance Jy). This is performed by linear superposition of strut and torpedo effect [11]. Figure 7 shows the contour plots of the total resistance coefficients Cr versus C and J, obtained using a thi plate spline (TPS) interpolation of the simulation results (represented by circles). It may be noted that the strut clearance has significant effects on the resistance at low speeds. Furthermore, an increase of the interaxis distanc induces a reduction of the total resistance coefficient.
Figure 7: Effects of variable interaxis distance and variable struts clearance on total resistance coefficient (U = 3,4,5 and 6kn) using TPS interpolation
Figure 7: Effects of variable interaxis distance and variable struts clearance on total resistance coefficient (U = 3,4,5 and 6kn) using TPS interpolation
Figure 8: Optimal configuration and resistance parametric analysis Figure 8 shows a comparison between the optimized and the original total resistance versus the speed. The optimized configuration performs slightly better than the original for U = 3 + 4kn and for U = 5 + 6kn.
Figure 8: Optimal configuration and resistance parametric analysis Figure 8 shows a comparison between the optimized and the original total resistance versus the speed. The optimized configuration performs slightly better than the original for U = 3 + 4kn and for U = 5 + 6kn.
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Naval ArchitectureFree surface flowsSimulation Based OptimizationShip DesignSimulation Based Design
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