Abstract: The present paper reports the results of the Particle
Image Velocimetry (PIV) visualization in a fully developed turbulent plane
channel flow subjected to a localized wall blowing applied through a spanwise
porous strip. The PIV visualization is used to measure the total two dimensional
velocity fields in a streamwise-wall-normal
-plane downstream the porous strip. Three blowing rates are used with a
magnitude less than 6% of the mean centreline velocity
m/s corresponding to the Reynolds number The results obtained from ensemble
averaging of the instantaneous velocity vector fields of PIV images reveal that
blowing significantly decreases the magnitude of the streamwise velocity in the
inner layer immediately behind the porous strip, whereas the outer layer
characterized by is not affected. The downward shift
of streamwise mean velocity significantly increases with increasing of wall
blowing velocity Note that the increase in removes the appearance of the
log-layer far the wall up to about 100 viscous wall units for delaying the extent of the inner
region. The data of streamwise mean velocity extracted from the PIV images
exhibit a similar behavior to that measured by hot-wire anemometry at same
station. The magnitude of the streamwise velocity fluctuations significantly
increases in the inner region downstream of the porous strip. Immediately behind
this strip, the turbulent plane channel flow quickly responds to the wall
condition with the streamwise mean velocity and fluctuation distributions which
highly depend of the blowing rate in the inner layer. Using the basic Reynolds
decomposition of the turbulent flow fields, the issues of visualization in the
wall region show structures located above the
wall and lifted up by wall blowing. The PIV instantaneous field of spanwise
vorticity shows that these vortices are more
titled and aligned in form of the packets in the streamwise direction downstream
the blowing strip. Using the Willmarth and Lu [28] decomposition, these vortices
packets are located above a region of strong second-quadrant fluctuations generating a shear layer inclined
up to to the wall for the highest wall
blowing velocity For the no-blowing case, the
individual vortices packets grow upwards in the streamwise direction at a mean
angle from to the wall. This
result agrees well with those of literature.
Keywords and phrases: channel flow, blowing, coherent structures, HWA, PIV.
