A ground vortex engendered by the interaction of uniform flow over a plane surface with suction into a cylindrical conduit whose axis is normal to the cross-flow and parallel to the ground plane is investigated in wind tunnel experiments. The formation and evolution of the columnar vortex and its ingestion into the conduit’s inlet are explored using planar/stereo particle image velocimetry over a broad range of formation parameters that include the speeds of the inlet and cross-flows and the cylinder’s elevation above the ground plane with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. The present investigations show that the appearance of a ground vortex within the inlet face occurs above a threshold boundary of two dimensionless formation parameters, namely the inlet’s momentum flux coefficient and its normalised elevation above the ground surface. Transitory initiations of wall-normal columnar vortices are spawned within a countercurrent shear layer that forms over the ground plane within a streamwise domain on the inlet’s leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream with the cross-flow but when their celerity is reversed with increased suction, they are advected towards the cylinder’s inlet, gain circulation and stretch along their centrelines and become ingested into the inlet at a threshold defined by the formation parameters. Finally, the present investigations demonstrated that reduction of the countercurrent shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical duct can significantly delay the ingestion of the ground vortex to higher level thresholds of the formation parameters.

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

The interfacial dynamics of a sessile water drop was investigated experimentally. The low-viscosity drop was forced by an underlying diaphragm driven vertically by a piezoelectric actuator. This high-frequency forcing produced very low diaphragm displacements, even at high acceleration amplitudes. As the driving amplitude was increased from zero, the drop exhibited several transitions to states of increasing spatio-temporal complexity. The first state of the forced drop consisted of harmonic axisymmetric standing waves that were present for even the smallest diaphragm motion. Wave modes up to 14 were observed an compared to theoretical results. As the forcing amplitude increased above a critical value, a parametrically driven instability occurred that resulted in the appearance of subharmonic azimuthal waves along the contact line. The critical accelerations and the resulting wavenumbers of the azimuthal waves were documented. For larger values of the forcing amplitude, the subharmonic azimuthal waves coupled with the harmonic axisymmetric waves to produce a striking new lattice-like wave pattern. With a further increase in the forcing amplitude, the lattice mode disappeared and the interface evolved into a highly disordered state, dominated by subharmonic wave motion. The characteristics of the lattice and pre-ejection modes were documented with phase-locked measurements and spectral analysis. Finally, as the forcing amplitude increased above another critical value, the interface broke up via droplet ejection from individual wave crests.