Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
would be sufficient to support a small rotorcraft such as the Bell 206-B3. The geometry of the yacht is shown in Figure 3.
Where, Helideck
Ventilation Exhaust
z U C
0 = *2 g (3)
Here the Charnock number, C, relates the equivalent roughness of the sea surface to the mean wind speed and typically takes a value of 60. Equations (2) and (3) can be iteratively solved for U*. Figure 4 shows the resulting distribution of wind velocity with height above the water surface which is utilised within the CFD model.
Figure 3: Yacht geometry with helideck identified.
The computational domain around the yacht measures 218m long by 45m wide by 50m tall and extends approximately 45m forward and 70m aft of the vessel.
It
is essential to use a large domain to ensure that the blockage effects caused by the presence of the yacht are minimised in order to provide the best representation of unbounded flow.
4.2 COMPUTATIONAL MESH
Forward and aft of the yacht a hexahedral mesh scheme is used with refinement close to the water surface to resolve and preserve the incoming atmospheric boundary layer (see Section 4.3). The region immediately adjacent the yacht is filled with tetrahedral cells which allow the complex geometry to be represented. The resolution of the tetrahedral mesh is increased immediately adjacent the yacht and in the wake region of the superstructure. The mesh grows in size with distance from the yacht where the flow gradients become negligible.
The separation of flow from the superstructure is likely to be dominated
by pressure geometric features, discontinuities and sharp corners. gradients will be less significant. namely These features will
define and stabilise the location of the separation and therefore flow separation off smooth surfaces under adverse
Therefore, no boundary layer mesh is typically applied over the superstructure.
4.3 OPERATING CONDITIONS
The scenario of interest considers a stationary yacht in a 25kt head wind. The ambient conditions are taken to be 30°C and 1atm. The selection of a 25kt head wind scenario refers back to the previous velocity criteria listed in the now superseded 5th edition of CAP437. The influence of the sea surface on the incoming wind is represented using the following log law approximation to the atmospheric boundary layer [13]:
Figure 4: Definition of the atmospheric velocity boundary layer profile at the domain inlet.
The propagation of thermal plumes formed by warmed air issuing from the outlets to the yacht’s engine room ventilation system is examined. Figure 3 shows the position of the ventilation air outlets. The temperature of air leaving the yacht is taken to be 45°C which is 15°C above ambient conditions.
The symmetry of the geometry and flow conditions on the yacht centreline computational
was
superstructure, particularly around the helideck region, a full model would be required.
4.4 CFD SOLUTION
The flow around the yacht was calculated using the RANS solver of the commercial CFD code Fluent 6.3. The Shear Stress Transport (SST) k-ω turbulence model is
used, which from previous experience has been observed to preserve the atmospheric boundary layer.
structure of the incoming
The calculation took approximately one day to complete using one quad core processor of a 3.16GHz 64bit machine. Therefore, if the capabilities of modern multi- processor computer clusters were fully utilised, it would be feasible to complete an extensive airwake survey (i.e.
exploited to reduce the
solution. For non-head wind cases or for yacht geometries with significant
requirement and provide an efficient asymmetries in the
U U=
*
0.41ln z0 z
(2)
©2010: The Royal Institution of Naval Architects
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