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Roughness induced transition

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  1. Tez No: 540541
  2. Yazar: FAHRETTİN GÖKHAN ERGİN
  3. Danışmanlar: Dr. EDWARD B. WHITE
  4. Tez Türü: Doktora
  5. Konular: Astronomi ve Uzay Bilimleri, Makine Mühendisliği, Astronomy and Space Sciences, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2005
  8. Dil: İngilizce
  9. Üniversite: Case Western Reserve University
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 151

Özet

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Özet (Çeviri)

Surface roughness has numerous different effects on boundary layer stability. For roughness amplitudes lower than a critical level, the velocity disturbances roughness creates in laminar boundary layers experience a transient algebraic growth followed by exponential decay without leading to transition. When the critical level is exceeded unsteady disturbances are amplied and the ow rapidly transitions to turbulence. Whether the ow remains laminar or becomes turbulent is solely determined by the receptivity of the initial disturbance. Two experiments are performed that address two fundamental problems related to roughness induced disturbances. One examines the disturbances produced by subcritical elements. The other identies the instability mechanism responsible for breakdown in the wake of supercritical roughness elements. For subcritical conditions, multiple component velocity measurements are obtained in a at plate boundary layer in the wake of an array of subcritical cylindrical elements. These data reveal the topology of the steady horseshoe vortex that exists in the wake of roughness elements and leads to transient growth of streamwise and spanwise velocity disturbances. A theoretical technique that quanties the receptivity of velocity disturbances to surface roughness requires the complete velocity eld as input and the data obtained here are suitable for this purpose. For supercritical conditions, velocity measurements are obtained above and below the critical roughness height Reynolds number, Rek. The steady disturbance eld reveals local shear layers in the wall-normal and spanwise directions and the unsteady disturbance led reveals evidence of hairpin vortices observed in ow visualization studies. The locations of maximum uctuation intensity correspond to the locations of local inection points of the steady velocity eld suggesting that the uctuations result from a Kelvin–Helmholz-type instability. Rapid transition takes place at Rek = 334 but not below this value. The disturbances' energy growth rates indicate that the transition scenario can be understood as a competition between the unsteady disturbances' growth and the rapid relaxation of the steady ow that tends to stabilize these disturbances.

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