Numerical studies of transition in wall-bounded flows
Ori Levin 2005
KTH Mechanics
SE-100 44 Stockholm, Sweden.
2005-12-16 10.15 room F3, Lindstedsvägen 26
Disturbances introduced in wall-bounded flows can grow and lead to
transition from laminar to turbulent flow. In order to reduce losses
or enhance mixing in energy systems, a fundamental understanding of
the flow stability and transition mechanism is important. In the
present thesis, the stability, transition mechanism and early
turbulent evolution of wall-bounded flows are studied. The stability
is investigated by means of linear stability equations and the
transition mechanism and turbulence are studied using direct numerical
simulations. Three base flows are considered, the Falkner-Skan
boundary layer, boundary layers subjected to wall suction and the
Blasius wall jet. The stability with respect to the exponential growth
of waves and the algebraic growth of optimal streaks is studied for
the Falkner-Skan boundary layer. For the algebraic growth, the optimal
initial location, where the optimal disturbance is introduced in the
boundary layer, is found to move downstream with decreased pressure
gradient. A unified transition prediction method incorporating the
influences of pressure gradient and free-stream turbulence is
suggested. The algebraic growth of streaks in boundary layers
subjected to wall suction is calculated. It is found that the spatial
analysis gives larger optimal growth than temporal
theory. Furthermore, it is found that the optimal growth is larger if
the suction begins a distance downstream of the leading edge.
Thresholds for transition of periodic and localized disturbances as
well as the spreading of turbulent spots in the asymptotic suction
boundary layer are investigated for Reynolds number Re=500, 800 and
1200 based on the displacement thickness and the free-stream
velocity. It is found that the threshold amplitude scales like
Re-1.05 for transition initiated by streamwise vortices and
random noise, like Re-1.3 for oblique transition and like
Re-1.5 for the localized disturbance. The turbulent spot is
found to take a bullet-shaped form that becomes more distinct and
increases its spreading rate for higher Reynolds number. The Blasius wall
jet is matched to the measured flow in an experimental wall-jet
facility. Both the linear and nonlinear regime of introduced waves and
streaks are investigated and compared to measurements. It is
demonstrated that the streaks play an important role in the breakdown
process where they suppress pairing and enhance breakdown to
turbulence. Furthermore, statistics from the early turbulent regime
are analyzed and reveal a reasonable self-similar behavior, which is
most pronounced with inner scaling in the near-wall region.