Abstract:
The present numerical study has been conducted to investigate the unsteady
boundary layer characteristics over a flat plate under the influence of wake vortices
induced from an elliptic cylinder positioned in the free stream by numerical
computations. This flow situation is an idealization of that occurring on
turbomachinery blades where unsteady wakes are generated by the preceding row of
blades. The major-minor axis ratio of the elliptic cylinder is taken 0.6 and the angle
of attack 00 positioning the cylinder at different locations from the plate. The
investigation covers a Reynolds number range up to 1000 based on the focal distance
of the elliptic cylinder and undisturbed free stream velocity. The Reynolds numbers
are relatively low so that in the limiting case of a boundary layer undisturbed by
wakes this remained laminar over the full length of the test plate. The time
dependent, two dimensional flow is simulated numerically using finite element
formulation. Quadratic triangular elements and discontinuous linear triangular
elements are used for velocity and pressure interpolation respectively. A finite
element package COMSOL Multiphysics is used for the present computation. The
development of the flow field up to certain time period is considered. Instantaneous
streamlines of the disturbed flow field, instantaneous velocity field, instantaneous
boundary layer integral parameters, and skin friction on different plate streamwise
locations are presented for all the cases. The wake vortices strongly affect the
boundary layer compared to undisturbed flow over flat plate. From the above results,
two types of interaction can be obtained, one is the strong wake-boundary layer
interaction and another is the weak wake-boundary layer interaction depending on
the cylinder to plate relative position. Comparisons are also carried out for circular
cylinder having the same height of the elliptic cylinder cross-section, for a particular
cylinder to plate relative position and Reynolds number.
