Numerical Investigation on a Rotor Tip-Vortex Instability in Very Low Advance Ratio Flight

  • Published : 2005.12.31


Helical tip vortex is known as stable vortex structure, however the specific frequency component of far wake perturbation induces the vortex pairing in hover and axial flight. It is expected that the tip vortex pairing phenomena may happen in transition flight and very low advance ratio flight so that inflow may be most nonuniform in the low advance ratio flight. The objectives of this paper are that a tip-vortex instability during the transition from hover into very low advance ratio forward flight is numerically predicted to understand a physics by using a time-marching free-wake method. To achieve the objectives, numerical method is firstly validated in typical axial and forward flights cases. Present scheme with trim routine can predict airloads and inflow distribution of forward flight with good accuracy. Then, the transition flight condition is calculated. The rotor used in this wake calculation is a small-scale AH-1G model. By using a tip-vortex trajectory tracking method, the tip-vortex pairing process are clearly observed in transient flight($\mu$=0.03) and disappears at a slightly higher advance ratio($\mu$=0.05). According to the steady flight simulation at $\mu$=0.03, it is confirmed the tip-vortex pairing process is continued in the rear part of rotor disk and not occurs in the front part. Time averaged inflow in this case is predicted as smooth distribution.


Rotor Tip Vortex;Vortex Instability;Time Marching Free Wake;Transition Flight;Low Advance Ratio Flight


  1. Chung, K.H. and Lee, D.J., ' Numerical Prediction of Rotor Tip Vortex Roll-up in Climb Flight by Using a Time-Marchig-Free-Wake Method', Computational Fluid Dynamics Journal, pp. 80-88, vol. 12, no. 1, April, 2003
  2. Landgrebe, A. J., 'The Wake Geometry of a Hovering Helicopter Rotor and Its Influence on Rotor Performance', J. American Helicopter Soc., 17, (4), Oct., 1972, pp. 3-15
  3. Tangier, J. L., Wohlfed, R. M., and Miley, S. J., 'An Experimental Investigation of Vortex Stability, Tip Shapes, Compressibility, and Noise for Hovering Model Rotor', NASA CR-2305, 1973
  4. Widnall, S. E., 'The Stability of a Helical Vortex Filament', Journal of Fluid Mechanics, Vol. 54, No. 4, 1972, pp. 641-663
  5. Gupta, B. P., and Loewy, R. G., 'Theoretical Analysis of the Aerodynamic Stability of Multiple, Interdigitated Helical Vortices', AlAA Journal, Vol. 12, No. 10, October 1974, pp. 1381-1387
  6. Martin, P. B., Bhagwat, M. J., and Leishman, J. G., 'Strobed Laser-Sheet Visualization of a Helicopter Rotor Wake', Paper PF118, Proceedings of PSFVIP-2, Honolulu, HI, May 1999
  7. Caradonna, F., Hendley, E., Silva, M., Huang, S., Komerath, N., Reddy, U., Mahalingam, R., Funk, R., Ames R., Darden, L., Villareal, L., Gregory, and Wong, O., 'An Experimental Study of a Rotor In Axial Flight', AHS Specialists' Meeting on Aerodynamics and Aeroacoustics, Williamsburg, VA, Oct. 1997
  8. Chung, K.H., Na, S.U., Jeon, W. and Lee, D.J., 'A Study on Rotor Tip Vortex Pairing Phenomenon by Using a Time-Marchig-Free-Wake Method', American Helicopter Society 56th Annual Forum, Virginia Beach, May 2-4, 2000
  9. Leishman, J. G., Principles of Helicopter Aerodynamics, 2000, Cambridge Univ. Press, pp.115-123
  10. Lee, D. J. and Na, S. U., 'Predictions of Helicopter Wake Geometry and Air Loadings by using a Time Marching Free Wake Method', Proc. 1st Forum Russian Helicopter Soc., Moscow, Russia, 1994, pp. 69-85
  11. Na, S. U. and Lee, D. J., 'Numerical Simulations of Wake Structure Generated by Rotating Blades Using a Time-Marching Free-Vortex-Blob Method', European Journal of Mechanics, 18, (1), 1999, pp. 147-159
  12. Moore, D. W., 'Finite Amplitude Waves on Aircraft Trailing Vortices,' Aeronautical Quarterly, 231972, pp. 307-314
  13. Rosenhead, L. 'The Spread of Vorticity in The Wake behind a Cylinder', Proc. Roy. Soc., A127, 1930, pp. 590-612
  14. Leishman, J. G., Baker, A. and Coyne, A., 'Measurements of Rotor Tip vortices Using Three-Component Laser Doppler Velocimetry', J. American Helicopter Society, 41, (4), Oct. 1996, pp. 342-353
  15. Cross, J. L., and Watts, M. E., 'Tip Aerodynamics and Acoustics TestA Report and Data Survey', NASA-RP-1179, NASA Ames Research Center, Dec. 1988
  16. Ahmad, J., and Duque, E. P. N., 'Helicopter Rotor Blade Computation in Unsteady Flows Using Moving Embedded Grids', AIAA Paper 94-19g22, June 1994
  17. Yang, Z., Sankar, L. N., Smith, M. J.and Bauchau O.,'Recent Improvements to a Hybrid Method for Rotors in Forward Flight', Journal of Aircraft, Vol. 39, No. 5, September-October, 2002
  18. Elliot, J. W., Althoff, S. L., and Sailey, R. H., 'Inflow Measurements Made with a Laser Velocimeter on a Helicopter Model in Forward Flight', Vol. I, NASA TM-100541, 1988