Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
ten different wind speed and headings) in as little as two weeks, including the initial model build and mesh. This represents a significant saving compared to the extent of an experimental programme which is the sum total of time take to build the scale model, set up the wind tunnel instrumentation and acquire the results.
4.5 RESULTS
The results of the CFD model are interrogated to identify regions of the airwake which could pose problems with regard to rotorcraft operations. The specific elements of the flow field examined are:
• Overall velocity field. • Gust velocity. • Local temperature perturbations.
Gust velocity and local temperature perturbations are examined in the critical region above the helideck as defined by LY2 standard (i.e. a cylindrical volume of air of a basal area equal to that of the helideck and with a height of 22.52m based on the requirements for
the
popular Bell 206-B3 helicopter. This is equal to the sum of the wheel-to-rotor distance (approximately 3.2m) plus one rotor diameter (10.18m) plus 30ft
(9.14m). volume is shown in relation to the yacht in Figure 5. This
Figure 6: Contours of velocity magnitude (ms-1) plotted on a vertical plane through the centre of the yacht (top) and on a horizontal plane 10m above the water surface (bottom).
disturbance caused by the superstructure can clearly be seen, particularly in the region immediately aft of the bridge.
Forward and far aft of the bridge the pathlines are parallel to the free stream flow.
Figure 5: Airwake assessment volume. 4.5 (a) Overall Velocity Distribution
Figure 6 shows plots of velocity magnitude on vertical and horizontal planes through the yacht located on the centreline and 10m above the water surface.
From
Figure 6 the reduction in velocity magnitude from the free stream value in the wake region, and just forward of the yacht where the flow stagnates, can clearly be seen. The contours indicate that the largest reductions in velocity magnitude are located immediately aft of the yacht, close to the helideck.
Examining the velocity
distribution away from the yacht the variation in free stream velocity, as defined in Figure 4, can also be seen.
Figure 7 shows pathlines coloured by velocity magnitude (ms-1) released from a vertical plane just ahead of the upper deck and bridge.
From this B-28 figure the flow
Figure 7:
(ms-1) released forward of the bridge showing the free stream flow disturbances induced by the superstructure in the region of the helideck.
Pathlines coloured by velocity magnitude 4.5 (b) Gust Velocity
The gust velocity represents local fluctuations in the velocity field due to turbulent structures. The turbulence modelling approach adopted explicitly assumes that the mean velocity fluctuations in each direction (u’, v’ and w’), are equal (isotropy), therefore the turbulent kinetic energy (k) is defined by,
k u
= ′ + ′ + ′ = ′2 v w
2 2 2 2
3w 2
The gust velocity (σ) is equivalent (4) to the standard deviation of vertical velocity in the flow and therefore is ©2010: The Royal Institution of Naval Architects
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60