Our group puts special attention on the turbulent interaction (and feedback) between flow and structures to: i) understand the flow-induced mechanisms that modulates the rotations and translations of compact objects, and ii) the flow-induced oscillations in flexible structures. We are performing coupled characterization of the motion of objects with surrounding flow at high temporal resolution.
The distinctive pendulum-like oscillation and pitching patterns of cubic and rectangular slung prisms are inspected for two aspect ratios at various Reynolds numbers under two free-stream turbulence levels. Systematic experiments were performed using high-resolution telemetry and hotwire anemometry to quantitatively characterize the dynamics of the prisms and the wake fluctuation (Figure 1). Figure 2 shows the spectra of pitching angular velocity of the cubic prism under low turbulence.
Figure 1: Schematic of the experimental setup.
Figure 2: Spectra of pitching angular velocity of the cubic prism under low turbulence.
The pitching of hinged splitters in the trailing edge of elliptic cylinders was experimentally studied at various angle of attack of the cylinder, Reynolds numbers, splitter length, aspect ratio of the cylinder and freestream turbulence levels. High-resolution telemetry and hotwire anemometry were used to characterize and gain insight on the dynamics of splitters and wake flow (Figure 3). The spectra of splitter pitching and the joint-PDF of instantaneous aerodynamic forces are presented in figure 4.
Figure 3: Schematic of the experimental setup
Figure 4: Spectra of splitter pitching under low (a) and high (b) background turbulence; (c) joint–PDF of the instantaneous aerodynamic forces.
Flow-induced dynamics of flexible structures is, in general, significantly modulated by periodic vortex shedding. Here, we present new experimental evidence that demonstrates a significant decoupling between the dynamics of simple structures and wake fluctuations for various geometries, Reynolds numbers and mass ratio. High-resolution 2D particle tracking velocimetry (PTV) was used to track instantaneous motions of fiducial points on the structure and a hotwire anemometry were used to characterize and gain the insight of wake fluctuations (Figure 5). Figure 6 shows the dominating frequencies of structure oscillation and wake fluctuations.
Figure 5: Schematic of the experimental setup
Figure 6: (a) Spectra of the streamwise velocity motion of the plate; (b) 2D spectrum of the structure motion across various velocities; (c) 2D spectral counterpart of the wake flow. The white dashed line highlights the dominating oscillation frequency of the plate.
We are particularly interested on studying the effect of tip geometry on the unsteady oscillations of wall-mounted, thin plates. We first considered tips with flat, elliptic and aristate shapes and characterized the motion at various Reynolds numbers. High-resolution particle tracking velocimetry (PTV) and planar particle image velocimetry (PIV) were used to characterize the dynamics of the plates and surrounding flow (Figure 7). The flow characteristics of the flat plate under various Reynolds numbers are presented in figure 8.
Figure 7: Schematic of the experimental setup
Figure 8: Contours of the time-averaged streamwise velocity with superimposed streamlines for the rectangular plate.
Systematic wind tunnel experiments are underway to characterize and quantify the rotation and free pitching of flat plates as a function of thickness ratio, location of the axis of rotation and Reynolds number. High-resolution telemetry, laser tachometer, and hotwire were used to get time series of the plates motions and the signature of the wake flow at a specific location (Figure 9). Figure 10 illustrates the behavior of free pitching plates under various wind speed.
Figure 9: a) Schematic of the experimental setup; b) photograph of the plates and test section; c) characterization of scales.
Figure 10: Characterization of free pitching plates under various wind speed.
We are particularly interested on studying the influence of turbulence in the motion of plates. We use active grids and cylinders with different diameters to induce specified turbulence. PIV, hotwire, high-resolution telemetry and laser tachometer are used to track the motions of the structures and flow. Figure 11 shows a case with a cylinder and Figure 12 illustrates an example of the dominating frequency of plate oscillations and wake fluctuations, while Figure 13 shows the flow and vertical motions around a plate.
Figure 11: Schematic of the experimental setup.
Figure 12: a) Frequency of the dominating plate pitching and oscillations; b) velocity spectra downstream of the plate.
Figure 13: a, b) Mean velocity in streamwise direction around a plate; c) Instantaneous signed swirling strength Λci.
Laboratory experiments are under way to study the dynamics of falling objects and flow. Figure 14 shows the case of free-falling cone in a quiescent water tank. Telemetry is used to track the 3D translations and rotations. Flow field is captured via planar and 3D PIV (Fig. 15).
Figure 14: Basic setup of the falling cone, trajectory and Lagrangian features of a 30o cone durng the free fall.
Figure 15: Example of the flow field around a falling cone in stable (left) and unstable (middle) stages from planar PIV. Right: 3D field in the stable fall showing the rollup vortex formation.
Oscillating flexible plates with various amplitudes are investigated to study the interaction between flow and structures. High-speed PTV and PIV are used to track instantaneous deformation of moving plates, and measure the induced flow field, respectively. Figure 16 shows schematic of the experimental setup, and reconstructed plate at various thickness. Figure 17 illustrates the velocity of flexible plates. Figure 8 shows the instantaneous flow field induced by the flexible plates.
Figure 16: Schematic of the experimental setup and reconstructed plates at various thickness.
Figure 17: velocities of the flexible plates at various amplitudes; red and blue lines denote the base and tip velocities, respectively..
Our research group aims at providing fundamental insights on the role of turbulence in basic and applied problems of high interest, which can be divided in the following sub-areas:
i) structure of the boundary layer over complex topographies;
ii) wind & hydrokinetic energy technologies,
iii) scalar transport over urban and natural environments,
iv) flow-structure interaction; and
v) instrumentation for turbulence measurements.
We have developed a comprehensive research on these topics that are going to be sustained and expanded in the future. Our versatile experimental approach combines a set of state-of-the-art experimental techniques, including particle image velocimetry (PIV), computer vision, and our recently developed 3D particle tracking velocimetry (PTV). This framework allows us to study fluid dynamics from Eulerian and Lagrangian frame of references