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For the vertical velocities, the variations are more drastic as shown in Fig.20. The magnitude of the vertical component varies as much as of 10% of the freestream value in both upward and downward direction. For ship at even keel, however, only positive w components are seen for this flight path. The effect of ship motion becomes noticeable about one ship length away from the bulls eye, and intensifies as it approaches the stern. The upward w component reaches its maximum above the stern, and then decrease continuously over the missile deck where it changes direction again. Above the flight deck, the viscous/vortex interaction becomes so intensive that no consistent trend can be found for this fluctuating velocity component.

Fig. 20—Vertical velocities along flight path

Concluding Remarks

The airwake of a DD-963 ship configuration subject to atmospheric wind of 15.44 m/s (30 knots) at wind angle of 30 degrees is simulated by using a multi-zone, thin-layer Navier-Stokes method. The effect of ship motion is implemented by simulating the steady-state flowfield over the basic ship configuration that has pitched and/or rolled with respect to the water surface.

The resulting flow contains regions of massive flow separation along with free vortices. Major flow features including viscous-vortex interactions are captured. Some concluding remarks may be drawn:

  1. The ship pitched bow down yields an air burble (airwake) thinner than that of a ship at even keel traveling at the same speed. Similarly, the ship rolled towards starboard results in a thinner air burble in the transverse direction.

  2. The flow behind the superstructure or the hangar has characteristics of a backward-facing step with massive flow separation involving reversed flow accompanied by circulation. The pitched ship has larger local reversed flow regions than those of a ship at even keel.

  3. The transverse velocity distribution seems to be little influenced by the pitch angle of the ship, but much affected by ship roll angle. On the other hand, the vertical velocities are significantly influenced by both pitch and roll angles.

Acknowledgment

The present work was supported by the Naval Air Warfare Center, Aircraft Division. The NASA Ames Research Center and DoD High Performance Computing facility provided the Cray CPU time.

References

1. Garnett, T.S. Jr., “Investifation to Study the Aerodynamic Ship Wake Turbulence Generated by a DD963 Destroyer,” Report No. NADC-77214–30, Boeing Vertol Company, Philadelphia, PA, Oct 1979.

2. Healey, J.V., “The Prospects for Simulating the Helicopter/Ship Interface,” Naval Engineers Journal, Vol. 99, No. 2, 1987. pp.45–63.

3. Carico, D., Reddy, W., DiMarzic, C., “Ship Airwake Measurement and Modeling Options for Rotorcraft Applications, ” AGARD Symposium on Aircraft Ship Operations, Paper 100, Seville, Spain, May 1991.

4. Blanc, T.V. and Larson, R.E., “Superstructure Flow Distortion Corrections for Wind Speed and Direction Measurements Made from NIMITZ Class (CVN68-CVN73) Ships,” NRL Report 9215, Naval Research Laboratory, Washington, DC, Oct 1989.

5. Healey, J.V., “The Airwake of a DD-963 Class Destroyer,” Naval Engineers Journal, Vol. 101, No. 2, 1989. pp. 36–42.

6. Healey, J.V., “A Data Base for Flight in the Wake of a Ship,” AIAA Paper 92–0295, Jan 1992.

7. Healey, J.V., “Wind Tunnel Datatape on DD-963 Class Destroyer,” Naval Post-Graduate School, Monterey, CA, 1990.

8. Tai, T.C. and Carico, D., “Simulation of DD-963 Ship Airwake by Navier-Stokes Method,” AIAA Paper 93–3002, July 1993 . Also Journal of Aircraft, Vol. 32, No. 6, 1995. pp. 1399–1401.



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