Transactions of the Korean Society of Mechanical Engineers B (대한기계학회논문집B)

The Korean Society of Mechanical Engineers (대한기계학회)
권호 표지
DOI QR코드

DOI QR Code

Double Frequency Forcing of the Laminar Separated Flow over a Backward-Facing Step

층류박리 후향계단 유동의 이중주파수 가진

  • 김성욱 (서울대학교 마이크로열시스템 연구센터) ;
  • 최해천 (서울대학교 기계항공공학부) ;
  • 유정열 (서울대학교 기계항공공학부)
  • Published : 2003.08.01
Download PDF
⟨ Previous Next ⟩

Abstract

The effect of local forcing on the separated flow over a backward-facing step is investigated through hot-wire measurements and flow visualization with multi-smoke wires. The boundary layer upstream of the separation point is laminar and the Reynolds number based on the free stream velocity and the step height is 13800. The local forcing is given from a slit located at the step edge and the forcing signal is always defined when the wind tunnel is in operation. In case of single frequency forcing, the streamwise velocity and the reattachment length are measured under forcing with various forcing frequencies. For the range of 0.010〈S $t_{\theta}$〈0.013, the forcing frequency component of the streamwise velocity fluctuation grows exponentially and is saturated at x/h = 0.75 , while its subharmonic component grows following the fundamental and is saturated at x/h = 2.0. However, the saturated value of the subharmonic is much lower than that of the fundamental. It is observed that the vortex formation is inhibited by the forcing at S $t_{\theta}$ = 0.019 . For double frequency forcing, natural instability frequency is adopted as a fundamental frequency and its subharmonic is superposed on it. The fundamental frequency component of the streamwise velocity grows exponentially and is saturated at 0.5 < x/ｈ < 0.75, while its subharmonic component grows following the fundamental and is saturated at x/ｈ= 1.5 . Furthermore, the saturated value of the subharmonic component is much higher than that for the single frequency forcing and is nearly the same or higher than that of the fundamental. It is observed that the subharmonic component does not grow for the narrow range of the initial phase difference. This means that there is a range of the initial phase difference where the vortex parring cannot be enhanced or amplified by double frequency forcing. In addition, this effect of the initial phase difference on the development of the shear layer and the distribution of the reattachment length shows a similar trend. From these observations, it can be inferred that the development of the shear layer and the reattachment length are closely related to the vortex paring.

Keywords

References

  1. Bhattacharjee, S., Scheelke, B. and Troutt, T. R., 1986, 'Modification of Vortex Interactions in a Reattaching Separated Flow,' AIAA J., Vol. 24, 623-629 https://doi.org/10.2514/3.9317
  2. Roos, F. W. and Kegelman, J. T., 1986, 'Control of Coherent Structures in Reattaching Laminar and Turbulent Shear Layers,' AIAA J., Vol. 24, 1956-1963 https://doi.org/10.2514/3.9553
  3. Hasan, M. A. Z., 1992, 'The Flow over a Backward-Facing Step Under Controlled Perturbation: Laminar Separation,' J. Fluid Mech., Vol. 238, 73-96 https://doi.org/10.1017/S0022112092001642
  4. Chun, K. B. and Sung, H. J., 1996, 'Control of Turbulent Separated Flow over a Backward-Facing Step by Local Forcing,' Exp. in Fluids, Vol. 21, 417-426 https://doi.org/10.1007/BF00189044
  5. Kiya, M., Shimizu, M. and Mochizuki, O., 1997, 'Sinusoidal Forcing of a Turbulent Separation Bubble,' J. Fluid Mech., Vol. 342, 119-139 https://doi.org/10.1017/S0022112097005521
  6. Songwan Jin, Sungwook Kim, Haecheon Choi, Jung Yul Yoo and Sa-ryang Kim, 2002, 'Effect of Two-Frequency Forcing on Flow Behind a Backward-Facing Step,' Trans. Of the KSME B, Vol. 26, No. 3, 423-431 https://doi.org/10.3795/KSME-B.2002.26.3.423
  7. Crow, S. C. and Champagne, F. H., 1971, 'Orderly Structure in Jet Turbulence,' J. Fluid Mech., Vol. 48, 547-591 https://doi.org/10.1017/S0022112071001745
  8. Zaman, K. B. M. Q. AND Hussain, A. K. M. F., 1980, 'Vortex Paring in a Circular jet Under Controlled Excitation. Part 1. General Jet Response,' J. Fluid Mech., Vol. 101, 449-491 https://doi.org/10.1017/S0022112080001760
  9. Ho, C. M. and Huang, L.-S., 1982, 'Subharmonic and Vortex Merging in Mixing Layers,' J. Fluid Mech., Vol. 119, 443-473 https://doi.org/10.1017/S0022112082001438
  10. Arbey, H. and Ffowcs Williams, J. E., 1984, 'Active Cancellation of Pure Tones in an Excited Jet,' J. Fluid Mech., Vol. 149, 445-454 https://doi.org/10.1017/S0022112084002743
  11. Husain, H. S. and Hussain, F., 1995, 'Experiments on Subharmonic Resonance in a Shear Layer,' J. Fluid Mech., Vol. 304, 343-372 https://doi.org/10.1017/S0022112095004459
  12. Sungkwon Cho, Jung Yul Yoo and Haecheon Choi, 1997, 'Vortex Pairing in an Axisymmetric Jet Using Fundamental and Subharmonic Forcing,' Trans. Of the KSME B, Vol. 21, No. 10, 1350-1362
  13. Adams, E. W., Johnston, J. P. and Eaton, J. K., 1984, 'Experiments on the Structure of Turbulent Reattaching Flow,' Rep. MD-43, Thermosciences Div. Mech. Engng. Dept., Stanford Univ
  14. Husain, M. A. Z. & Hussain, A. K. M. F., 1983, 'Natural Instability of Free Shear Layers,' AIAA J., Vol. 21, 1512-1517 https://doi.org/10.2514/3.8282
  15. Zaman, K. B. M. Q. and Hussain, A. K. M. F., 1981, 'Turbulence Suppression in Free Shear Flows by Controlled Excitation,' J. Fluid Mech., Vol. 103, 133-159 https://doi.org/10.1017/S0022112081001274