Transitions in the wake of a flapping foil

Abstract

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using 2D Particle Image Velocimetry (PIV). An analysis in terms of a flapping frequency-amplitude phase space allows to identify: 1) the transition from the well-known B\'enard-von K\'arm\'an (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and 2) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.Comment: Revised version includes minor changes in the discussion of the drag coefficient calculation. The 4 figures are unchange

PROTEUS: A coupled iterative force-correction immersed-boundary cascaded lattice Boltzmann solver for moving and deformable boundary applications

“…The governing parameters of the flow are the Reynolds number Re = U ∞ D/ν and the Strouhal number 11a shows the normalised average velocity profile at X = 12D downstream from the trailing edge. Overall, the computed structures of the vortices, as well as the mean velocity fields are in very good agreement with the experimental results of Diana et al [61]. It has been found that at A D = 1.77 the wake is not symmetric leading to net lift force generation, as shown in Fig.…”

“…In Fig. 11b, we compare the computed mean drag coefficients with the values obtained by Diana et al [61] using the momentum balance approach in a control volume around the foil. The values obtained with the LPA scheme and with the momentum balance approach are highlighted in red and black respectively.…”

Numerical study on hydrodynamic effect of flexibility in a self-propelled plunging foil

“…Further comments regarding the influence of flexibility on the stability property of the wake are presented here. As shown in Godoy-Diana et al [49], the up-down symmetry of the reversed Karman vortex street can be persevered if the Strouhal number is less than a critical value (in the range of 0.33-0.44) for a rigid pitching foil at the Re number of 255. At the first glance, the 'symmetry-preserving' effect of flexibility can be attributed to reducing St number (to a value below the critical one).…”

Mode Transition and Symmetry-Breaking in the Wake of a Flapping Foil

“…Karman and Burgers [3] offered the first theoretical explanation of drag or thrust production based on the wake vortices, where the wake of the flow past bluff bodies is modeled by an infinite row of alternating vortices commonly known as von Karman Vortex Streets. In following years, many researchers investigated the RKVS in different situations, such as the wakes of fish [4] in experiment [5] and numerical simulations [6,7]. More recently, Godoy-Diana et al [7] experimentally investigated the transition from the KVS to the RKVS in the wake of pitching foil.…”

“…In following years, many researchers investigated the RKVS in different situations, such as the wakes of fish [4] in experiment [5] and numerical simulations [6,7]. More recently, Godoy-Diana et al [7] experimentally investigated the transition from the KVS to the RKVS in the wake of pitching foil. We will simulate the transition process using the immersed boundary method and compare the results obtained with Godoy-Diana et al' s experimental results.…”