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Finally, it is useful to compare the amplitude coefficients of the harmonics of the wake survey data. These amplitude coefficients provide useful information when designing the propulsion system.

Comparison between the coefficients derived from the measured data and the predictions are given below in table 4.

Table 4 Amplitude coefficient of the wake harmonics (×1000)

Harmonic Number

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Experiment

4.28

12.58

6.51

102.36

10.74

5.66

8.63

50.00

9.48

4.48

2.61

10.72

2.10

0.58

0.80

1.65

H-H grid

2.11

8.85

10.70

63.09

12.73

3.40

1.23

26.86

2.45

0.21

0.17

6.21

0.67

0.01

0.21

2.03

C-H grid

7.17

16.00

14.27

66.18

12.32

4.18

1.73

26.63

1.56

1.10

0.10

5.61

0.16

0.12

0.32

2.64

C-O grid

6.32

30.45

16.00

71.50

12.12

2.73

1.59

26.12

1.08

1.15

0.05

5.58

0.28

0.57

0.37

1.03

O-O grid

0.03

34.90

13.06

74.88

13.57

2.37

2.75

21.87

1.96

0.52

0.44

4.52

0.24

0.61

0.07

1.09

k-ε

0.40

2.44

7.22

39.41

7.42

5.93

2.80

14.25

1.18

1.89

0.38

4.18

0.27

0.19

0.17

0.25

RNG k-ε

1.31

1.32

9.13

51.65

8.38

5.51

2.79

15.41

1.52

1.49

0.67

4.87

0.49

0.32

0.25

0.12

DSM

2.50

1.38

7.61

42.60

7.01

4.84

3.60

24.87

1.52

1.66

0.50

12.06

0.08

0.07

0.04

2.28

Conclusions

The capability of the CFD approach to the prediction of nominal wake has been quantified by comparison with high quality experiment data. In the region of the propulsor, the predictions are qualitatively correct in that the flow features are captured. The predictions have an overall error of around 5% of the reference velocity in the fluid velocity distribution. This compares with an error of 2.5% associated with the measured data. The predictions also fail to capture adequately the harmonics of the measured velocity distribution.

However, the predictions obtained appear to be independent of grid resolution and topology for a given flow solution algorithm. Changing from the isotropic two-equation turbulence models to the anisotropic Reynold Stress model requires further evaluation.

The author wishes to acknowledge the invaluable help and assistance in the preparation of these predictions and this paper by the Computational Hydrodynamics Team at DRA Haslar.

References

1. Report of the 20th ITTC Resistance and Flow Committee. Proceedings of the 20th ITTC. Volume 1. San Francisco, California, September 1993.

2. Huang TT, Liu H-L, Groves NC, Forlini TJ, Blanton JN and Gowing S. “Measurements of Flows Over an Axisymmetric Body with Various Appendages (DARPA SUBOFF Experiments).” Proceedings of the 19th Symposium on Naval Hydrodynamics. Seoul, Korea. August 1992.

3. Groves NC, Huang TT, Chang MS. “Geometrical Characteristics of DARPA SUBOFF Hulls (DTRC Models Nos. 5470 and 5471)”. DTRC/SHD-1298–01 March 1989. David Taylor Research Center, Bethesda, Maryland 20084–5000

4. Lin CW, Smith GD, Fisher SC. “Numerical Flow Simultations on the DARPA SUBOFF Configurations”. DTRC/SHD-1298–09 July 1990. David Taylor Research Center, Bethesda, Maryland 20084–5000

5. Shaw JA, Georgala JM, May NE, Pocock MF. “Application of Three-Dimensional Hybrid Structured /Unstructured Grids to Land, Sea and Air Vehicles”. Proceedings of Numerical Grid Generation in Computational Fluid Dynamics and Related Fields. Swansea, UK. 1994, pp 687–695

6. Patis CCP, Bull PW. “The Generation of Viscous Grids for Hydrodynamic Vehicles”. Mississiippi State University, Starkville, April 1996.

7. “CFDS-FLOW3D Users' Guide”. CFDS. Building 8.19, Harwell Laboratory, Oxfordshire, OX11 0RA. United Kingdom

8. “FLUENT/UNS Users' Guide”. Fluent Inc., Centerra Resource Park, 10 Cavendish Court, Lebanon. USA. NH 03766

9. “FIDAP 7.5 Users' Guide” FDI Inc., 500 Davis Street, Suite 600, Evanston, USA. Illinois 60201 .



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