Andreas Jocksch
Direct Numerical Simulation of
Turbulent Spots in High-Speed
Boundary Layers
Reprint of Diss. ETH No. 18104
1st edition Konstanz 2009, VI, 134 pages.
EURO 64,00.
ISBN
3-86628-275-3 and 978-3-86628-275-9
We investigate the development of isolated turbulent spots in boundary layers
at Mach numbers M = 1.1 and M = 5 for adiabatic and cooled walls by direct
numerical simulations. The turbulent spots show typical properties known from
the incompressible regime, e. g., the arrowhead-like shape of the spot with the
downstream overhang region and streaks in the tail region. At M = 5 spanwise
structures close to the wall appear in the front region. Strong wall cooling
leads to elongated turbulent spots. Besides the Mach number also the Reynolds number
affects the spot growth.
In the core region of the turbulent spots velocity profiles, skin friction and
heat transfer are evaluated. For one parameter set at M = 1.1 turbulence
statistics are analysed by considering an ensemble of turbulent spots with
slightly varying initial disturbances. For M = 5 the asymptotic spreading of
linear instabilities and its relation to the growth of turbulent spots are
investigated. The downstream spreading of the turbulent spots is faster than
for linear instabilities. In contrast to the adiabatic wall case, the slow tail
velocity of the cooled-wall spots seems to be related to the spreading of
linear instabilities. The near-field reveals waves propagating away from the
spots. In the freestream they are spherical and propagate according to
classical acoustics, while within the boundary layer they appear as distinct
semi-circular patterns.
Keywords: Turbulent spots, Transition to turbulence, Compressible
boundary, layer, Supersonic flow.
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