Session G38: Vortex Dynamics and Vortex Flows: Boundary Interactions

Flow mechanism governing finite wall effects in the classical vortex ring-wall interaction

 Vortex ring-wall interaction applications appear in energy harvesting of vortices partially impinging on surfaces or interactions with close by structures. Canonically, when a vortex ring approaches an infinite surface, the induced flow along the surface forms a boundary layer that subsequently separates and rolls up into a secondary vortex structure. This phenomenon forces the primary vortex to rebound from the wall. When the finiteness of the wall is considered, the generation of secondary vorticity could be directly altered. To investigate this expectation, a programmable piston pump is used to drive a constant volume slug of fluid at various Reynolds numbers into a quiescent tank that contains interchangeable plates of various sizes. These plates are located more than 4 nozzle diameters downstream to allow full formation of the vortex ring without the influence of the plate. A control volume vorticity budget analysis is implemented that couples velocity field data from PIV and pressure gradient measurements from differential pressure transducers. Preliminary findings indicate that despite less space for vorticity growth, decreasing the plate size allows more vorticity to be ejected from the plate and roll up into the secondary vortex thereby increasing its strength. However, the effect on total secondary circulation generation through surface vorticity flux remains to be understood.   

Wall-bounded vortex destruction using unsteady actuators on a flat plate

Wall-bounded vortices in flows can dominate the flow dynamics and fluid forcing in many applications. In many cases, the impact of the vortex is negative. Our aim is to test a new method for destroying pre-existing coherent vortex motion in wall-bounded flow. We generate longitudinal streamwise vortex structures using rectangular tab-style vortex generators. The coherent vortex structures are then interrupted by a synthetic jet to decay the vortex structure more rapidly. While synthetic jets with rectangular orifices are commonly used as vortex generators, we used orifice orientations that specifically generate little coherent vortex motion downstream---minimizing the impact of the jet itself. Data were collected in a low-speed wind tunnel using stereoscopic particle image velocimetry. Two heights of vortex generators were explored along with different spanwise positions along the wind tunnel floor, shifting the center of the vortex core. The synthetic jets broke up the incoming vortex core into smaller and less coherent vortex structures. However, the velocity deficit region in the wake of the vortex generator persists downstream. The pressure field in the wake of the vortex generator and synthetic jet interaction region will be explored to gauge if the unsteady forcing is mitigated.   

Formation of Ground Vortices in the Cross Flow over an Axisymmetric Inlet Above a Plane

The formation and sustainment of a ground vortex at the inlet of a cylindrical nacelle operating in suction near a ground plane in the presence of crosswind is investigated in wind tunnel experiments. It is shown that the wall-normal columnar vortex originates at the ground plane and is drawn into the nacelle when its global dimensionless formation parameters (momentum flux and ground plane distance) exceed critical levels. The vortex is precipitated by the development of countercurrent shear flow that is induced over the ground plane on the leeward side of the inlet by interactions between the opposing cross flow and the suction flow into the nacelle. This countercurrent flow engenders rollup of wall-normal vortices by transport and turning of boundary layer vorticity concentrations that, depending on the direction of the instantaneous local velocity, are advected either downstream from or upstream towards the nacelle. When the formation parameters exceed the critical levels, an upstream advected vortex can become anchored to and drawn into the inlet. The anchored ground vortex is sustained by funneling, turning, and stretching vorticity concentrations off the ground plane, while its circulation above ground plane remains invariant as it is drawn into the inlet.   

How does ground effect impact the formation of three-dimensional instabilities in the wake of parallel oscillating foils?

Wake dynamics and flow instabilities around two pitching foils placed in side-by-side (parallel) configurations are numerically examined for Strouhal numbers of St = 0.3 and 0.5<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.5 at Reynolds number of 8000. Two phase offsets, φ = 0<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>ϕ=0 and π<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>π, representing in-phase and out-of-phase oscillations, are examined for a range of separation distances between 0.5c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.5c and 1.5c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>1.5c with increments of 0.25c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.25c, where c denotes the chord length. Ground effect, i.e., proximity to the adjacent foil, is revealed to be key in the formation of fine scale instabilities for all kinematic settings. While trailing edge vortices are shed from the closely spaced foils, braid region detaches from the rollers and subsequently convects downstream. They interact with the vortex dipoles in the mid-wake, leading to the formation of three-dimensional structures that enwrap the dipoles. These observations remain consistent for both Strouhal numbers, despite the wake at the higher Strouhal number demonstrating more pronounced instabilities. The detachment process differs significantly between in-phase and out-of-phase motions, possibly because of the contrasting growth of leading-edge vortices associated with these particular phase offsets. This study provides valuable insights into the underlying flow instability mechanisms of fish schools. We aim to expand on this study by developing a more comprehensive model of the wake across a broader parameter space.   

Vortex induced vibrations of a cylinder in a narrow channel.

The two-dimensional flow around cylinder tightly confined in a channel was numerically studied. Attached to a linear spring and subject to hydrodynamic forces and torques, the circular cylinder is free to move in both the cross-flow and in-line directions as well as to rotate about its axis. Low mass ratios and moderate Reynolds numbers were considered. The system behavior shares some features with the well studied unconfined case. Despite the similarities, the close presence of the channel walls has an impact in the cylinder's behavior, specially within the lock-in region. We managed to find a modified Strouhal parameter that collapses the range of excitation. Results offer a portrait of the motion, forces, and flow around the cylinder under such constrains.