Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Near-and Far-Field CFD for a Naval Combatant Including Thermal-Stratification and Two-Fluid Modeling
Near-and Far-Field CFD for a Naval Combatant Including Thermal-Stratification and Two-Fluid Modeling
Pages 392-407
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From page 392... ...
It involves a deforming surface which undergoes breaking in certain regions, an extensive three dimensional boundary layer, air entrainment in the form of bubbles at the surface, and a strong momentum source at the propeller plane. There are additional influences including ambient density stratification due to thermal and salinity variations, sea state and other environmental effects, and Articulates and surfactants suspended in the fluid which have a poorly understood effect, if any, on the flow.
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From page 393... ...
Scalar fields such as temperature, micro-bubble populations, salinity and surface contaminants have been included because of their influence on the flow field and because they are of interest in their own right. The ability to predict the near and far fields of naval surface ships requires that these models be extended to include realistic hull geometries and operating environments and coupled together.
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From page 394... ...
where K iS the non-dimensional thermal diffusivity and T and t are the ensemble-averaged and fluctuating temperatures, respectively. The velocity and temperature fields interact through the previously mentioned buoyancy term in the Z-momentum equation, the energy equation (5)
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From page 395... ...
Possible sources include: wave breaking; bow, contact-line, and stern flows; propulsor aeration and cavitation; and ambient sea state. Properly stated boundary conditions must provide the gas flux through the generating surfaces including the gas-volume fraction, bubble-size distribution, and net bubble velocity.
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From page 396... ...
For stratified flows, the energy equation is solved implicitly using a tridiagonal algorithm and the method of lines and the buoyancy term in the Zmomentum equation is updated each time step. Convergence is determined using the L2 norms (residuals)
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From page 397... ...
Turbulent kinetic energy and dissipation rate, which are required for the far-wake simulation, are computed from the viscosity by assuming a length scale of t' = 0.01 and that the kinetic energy is given by: k- v' ~ _ Cptt, £ = kI/2e N1/2 5 Uncertainty Analysis Uncertainty analysis follows the ASME guidelines and implementation recommendations provided in (9~. Grid design was based on user experience and the Series 60 C - 0.6 zero and non-zero Fr results of (1~.
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From page 398... ...
Figure 6 shows details of the flow field through axial velocity (UJ contours, cross-plane (VW) vectors, piezometric pressure ~ p ~ contours, and 1 Note that for the Series 60 Cg-0.6, both EFD and CFD were conducted for zero sinkage and trim.
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From page 399... ...
The boundary layer over the hull and sonar dome is thin. The vectors show the displacement effects of the hull and the upward flow near the free surface due to the bow wave.
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From page 400... ...
show that the larger the bubble radius the larger the peak in volume fraction at the hull of the ship. This is due to two effects which attract the bubbles towards the hull, gravity and lift, the latter of which acts towards the hull over the entire boundary layer.
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From page 401... ...
The single-phase results show that both global and detailed features of Naval combatant flows can be accurately predicted, particularly the total resistance, wave profiles, and nominal wake. With regard to interaction with the ambient temperature field, the ship flow field transports cold water to the surface creating a thermal scar, which is an excellent tracer.
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From page 402... ...
9. Stern, F., Paterson, E., and Tahara, Y., "CFDSHIP-IOWA: Computational Fluid Dynamics Method for Surface-Ship Boundary Layers, Wakes, and Wave Fields," IIHR Report 666, Iowa City, Iowa, Februrary 1996.
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From page 403... ...
Resis~nce and grid convergence 2.41 2.67 - 1 ooJ/o 2.66 3.~0 -0.759 . Fr=0.39 ~v/ prop 20~7x60x40 0.116420 0.1 17609 0.118166 0.47% 5.25 97 _.
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From page 404... ...
so \ l.o 0.8 o.
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From page 405... ...
Axial velocity, cross-pl~e How, pressure, Ad temperature far unscathed 403 .
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From page 406... ...
Axial velocity, cross-plane flow, pressure, and temperature with body-force propeller.
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From page 407... ...
Void fraction as a Unction of bubble radius vs. distance perpendicular to hull at X/L=O.95.
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