Flow past a normal flat plate undergoing inline oscillations

Abstract

The flow past an inline oscillating normal flat plate has been considered with the view to explore the variety of wake phenomena which arise even at the low Reynolds number (Re) equal to 100 based on the free stream velocity and the width of the plate. The three-dimensional Navier-Stokes equations were integrated in time over a wide range of excitation frequencies and amplitudes. A wake flow regime map was produced on the basis of the 24 computer simulations. For a certain excitation amplitude, the wake vortex shedding is first antisymmetric at low excitation frequencies fe. When fe is increased the wake first becomes chaotic and thereafter turns into a symmetric shedding mode, for instance the S-II mode with a binary vortex pair on each side of the wake. If fe is increased even further, more complex symmetric wake patterns may occur before the wake ultimately turns into chaos. Symmetric wakes are thus only observed in a band of intermediate excitation frequencies and then with the dominating flow frequency locked-on to fe. In one particular case, the S-II mode in the very near wake turned into what might be considered as a new S-IV mode which comprised four different vortex pairs per shedding cycle. In spite of the low Re considered, several cases exhibited distinct three-dimensionalities whereas some other cases remained strictly 2D. In some of the cases, at least, the transition from 2D to 3D wake flow was ascribed to a “mode competition.” Finally, for one of the two-dimensional cases the Reynolds number was first increased to 300 and then to 500 and a complex three-dimensional wake flow was observed. However, even at Re = 100, two-dimensional computer simulations are unable to reproduce the three-dimensional wake flow characteristics reported from the present study.

Linear drag law for high-Reynolds-number flow past an oscillating body

“…The back-and-forth motion generates the pair of counterrotating vortex rings that can be seen downstream. Shedding of such vortex pairs, or dipoles [17], has also been observed in other cases of oscillatory flows past an object [18][19][20][21]. The close-up photograph of 033202-5 Fig.…”

“…To quantitatively assess the dipole-jet model predictions of the flows generated, future studies will involve two-dimensional experiments using a long plate undergoing fast flapping. Computational fluid dynamics simulations will also likely prove advantageous in comparing both the flows and forces to the predictions of this model [18,34]. Finally, unsteady fluid forces might also be inferred from quantitative measurements of the flow velocity field [35].…”

Flow around an oscillating cylinder: computational issues

“…The forcing term i j in equation (3) stems from the acceleration of the cylinder in the inertial reference frame (see e.g. Rashid et al (2011) and Khaledi et al (2012)…”