dc.description.abstract |
Transition of the attachment fine boundary layer was investigated using a large
swept cylinder. Results for natural transition and transition tripping with two-dimensional
trip wires were simila to those obtained by Poll using a similar, but smaller, model. ]Lower
displacement thickness Reynolds numbers but larger trip sizes, than for the flat-plate
boundary layer, were required for transition. The investigation of transition tripping was
then extended to involve three-dimensional trips. The attachment line boundary layer was
less susceptible to three-dimensional trips than to two-dimensional trips but upper and lower
bounds of attachment line Reynolds number for transition were identical. It was also found
that the roughness Reynolds numbers for fully effective three-dimensional trips were similar
for the attachment line and flat-plate boundary layers. Another common feature was the
more abrupt upstream movement of the transition front with increasing Reynolds number
for three-dimensional trips than for two-dimensional trips. Turbulence spreading
downstream of a three-dimensional trip was also examined and, as in the flat-plate boundary
layer, was found to be heavily dependent on Reynolds number (varying from 3* at low
Reynolds number to a value approaching 10* as Reynolds number exceeded the value for
natural transition), but was also dependent on either the trip size or the initial conditions at
which the trip first introduced turbulent spots. The effects of higher levels of freestrearn
turbulence were then investigated for both two-dimensional and three-dimensional trips.
With a small increase in freestrea turbulence the conditions for transition with twodimensional trips were affected far more than those for three-dimensional trips, for which
only the transition completion conditions were affected signfficantly, resulting in a reduced
extent ofthe transition region. Larger levels of turbulence appeared to have similar effects
on the two trip types. Restrictions in model length and windspeed for the higher turbulence
tests prevented an accurate investigation of the effects of turbulence,on the upper and lower
bounds for transition tripping and on the influence of spanwise distance at higher levels of
turbulence. Finally, the interaction between two trips positioned on the attachment line was
examined. The effect of the second trip on the transition Reynolds number was found to a
function of the streamwise separation distance between the two trips. |
en_UK |