Abstracts - KTH Mechanics

40
Experimental and Theoretical Investigation into the Development
of Turbulent Spots under Varying Pressure Gradient
D. M. Hernon a, E. J. Walsh a, D. M. McEligot b
Recent DNS studies on zero pressure gradient flow have elucidated the precursors
to turbulent spot production; however, experimental and analytical techniques are not
capable of reproducing such detailed results. The capability of DNS to trace back in
time to the exact initiation of a turbulent spot can not be achieved in practice.
Therefore, in order to gain a similar level of insight into the transition process the
experimenter has to take a considered approach to the methodology of testing.
Furthermore, within the literature little information exists on the generation of
turbulent spots in both positive and adverse pressure gradients.
It is postulated that negative spikes observed in the outer portion of the boundary
layer, Fig. 1 at y/δ=1, are the receptivity sights where low speed streaks lift up and
couple to high frequency disturbances from the freestream, see recent DNS results on
the formation of negative jets 1,2. A correlation between the frequency of the negative
spikes upstream of transition onset in the outer layer and the development of the
turbulent spots in the near wall region, Fig. 1 at y/δ=0.3, is achieved for zero pressure
gradient flow giving further insight into the transition process.
Variable pressure gradient and turbulence intensity is imposed on a flat plate on
which a 32 element hotfilm array with 2.3mm element spacing is placed. The hotfilm
array is located within the transition region. The objective of this experimental set-up
is to gain further insight into the development of turbulent spots and spot production
rates in both positive and adverse pressure gradients under the influence of varying
freestream turbulence scales and intensity. Following this a theory relating the forcing
frequencies in the freestream and boundary layer to the initiation of transition will be
developed.
u (m/s)
t (s)
Figure 1. Perturbation velocity traces at transition onset,
Reθ=577 and %Tu=1.3. Black Line, y/δ=0.3; Red line, y/δ=1.
a
Stokes Research Centre, University of Limerick, Ireland.
b
Idaho National Laboratory (INL), Idaho Falls, Idaho 83415-3885, U.S.A.
1 Jacobs and Durbin, J. Fluid Mech. 428, 185 (2001).
2 Brandt et al, J. Fluid Mech. 517, 167 (2004).