Em. Saiki et S. Biringen, SPATIAL NUMERICAL-SIMULATION OF BOUNDARY-LAYER-TRANSITION - EFFECTS OF A SPHERICAL-PARTICLE, Journal of Fluid Mechanics, 345, 1997, pp. 133-164
In the present study, the effects of an isolated stationary spherical
particle on the transition process in a hat-plate boundary layer are e
xamined by a spatial direct numerical simulation. The full three-dimen
sional time-dependent incompressible Navier-Stokes equations are integ
rated by a time-splitting method and discretized spatially by a high-o
rder finite difference/spectral method. A virtual boundary technique d
efining the no-slip boundary of a sphere is implemented within the Car
tesian geometry of the computational grid. Two numerical simulations w
hich consider the effects of the sphere on the boundary layer are pres
ented. The subcritical Reynolds number case reveals the appearance of
hairpin vortices shed into the sphere wake which decay as they are con
vected downstream. The initial interaction of the sphere and the bound
ary layer produces a three-dimensional isolated disturbance comprising
a wave part and a transient part. The decaying transient part is conv
ected downstream at the local mean velocity, while the wave part induc
es a decaying Tollmien-Schlichting wave in the flow field. In the seco
nd case, an increase in the Reynolds number results in a wedge of inci
pient turbulent flow downstream of the sphere. The development of the
wake of the sphere is dominated by the appearance of an isolated distu
rbance which rapidly breaks down forming a structure resembling a turb
ulent spot. It is demonstrated that the transition induced by a sphere
in the boundary layer is due to a mechanism related to bypass transit
ion.