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An experimental study has been conducted on the near wake of a 6 : 1 spheroid, in both uniform and stratified backgrounds. The pitch angle, $\\theta$, was varied from $0^\\circ \\text { to }20^\\circ$. When $\\theta = 0^\\circ$, stratification decreases the characteristic wake element spacing so a characteristic Strouhal number ($St$) increases from 0.32 to 0.4. However, a similar measure scaled on wake momentum thickness shows the wake spacing to converge on those measured for other bluff and streamlined bodies. There is an apparent effect of Reynolds number, which changes the location of separation lines and hence the initial wake thickness. When $\\theta > 0^\\circ$, the wake is a combination of the usual drag wake together with a collection of streamwise vortices that have separated from the body, and this wake geometry can evolve in ways that are measurably different from the zero incidence case. These differences may be limited to the near wake, as the later evolution appears to converge with previous bluff- and streamlined bodies, with normalised wake height, $L_V = 0.5$ and centreline velocity, $\\bar {u}_0 = 0.3$ at $Nt = 10$, as the early wake enters the non-equilibrium regime with similar values to previously studied stratified wakes. In the presence of density stratification, the inclined wake itself generates large-scale internal wave undulations with time scale $2{\\rm \\pi} /N$, even when the background stratification is not strong and a body-based Froude number is $O(10)$. The geometry and strengths of the primary streamwise vortices are not symmetric, mirroring previous results from experiments and computations in the literature.

While many living organisms employ active feedback control during flapping locomotion, there is increasing evidence to suggest that passive fluid-structure interactions play an important role in mediating the dynamics and efficiency of insect locomotion. For example, it has been hypothesized that insect wing muscles store and release elastic energy periodically over a flapping cycle, and that the stiffness of these muscles could be tuned to enhance efficiency. To provide insight into these effects, we investigated experimentally the passive flapping and rotational dynamics of two-dimensional Λ-shaped flyers undergoing prescribed, periodic heaving motion in a rest fluid. The flyers were left free to rotate about the apex and, in the case of the flexible flyers, to flap. Three dimensionless parameters were varied independently for the rigid flyers, representing the normalized (i) heaving amplitude, (ii) acceleration, and (iii) opening angle. For the flexible flyers, the torsional spring stiffness at the apex was varied as well. Within the parameter ranges tested, we identify four types of behavior: periodic rotation, chaotic dynamics, stable behavior (apex-up position), and bistability (apex-up and apex-down position). The transition from stability to bistability is dependent on both the amplitude and acceleration, and occurs above a constant ratio of drag to gravity, indicating that the stabilizing effect in the inverted position is hydrodynamic. The introduction of flexibility minimizes unsteady rotation around the apex, suggesting that flexibility may passively enhance stability in flapping locomotion. Further, more effort is required to maintain the inverted apex-down position for the flexible flyer. The flapping amplitude increases as the normalized heaving acceleration increases, with little dependence on the heaving amplitude. A linear relationship between the flapping amplitude and the heaving acceleration suggests that the ratio of flapping amplitude to heaving acceleration is inversely proportional to the stiffness at the apex.

Bluff body wakes in stratified fluids are known to exhibit a rich range of dynamic behavior that can be categorized into different regimes based on Reynolds number (\\(Re\\)) and Froude number (\\(Fr\\)). Topological differences in wake structure across these different regimes have been clarified recently through the use of Dynamic Mode Decomposition (DMD) on Direct Numerical Simulation (DNS) and laboratory data for a sphere in a stratified fluid for \\(Re\\in [200,1000]\\) and \\(Fr\\in[0.5,16]\\). In this work, we attempt to identify the dynamic regime from limited measurement data in a stratified wake with (nominally) unknown \\(Re\\) and \\(Fr\\). A large database of candidate basis functions is compiled by pooling the DMD modes obtained in prior DNS. A sparse model is built using the Forward Regression with Orthogonal Least Squares (FROLS) algorithm, which sequentially identifies DMD modes that best represent the data and calibrates their amplitude and phase. After calibration, the velocity field can be reconstructed using a weighted combination of the dominant DMD modes. The dynamic regime for the measurements is estimated via a projection-weighted average of \\(Re\\) and \\(Fr\\) corresponding to the identified modes. Regime identification is carried out from a limited number of 2D velocity snapshots from numerical and experimental datasets, as well as 3 point measurements in the wake of the body. A metric to assess confidence is introduced based on the observed predictive capability. This approach holds promise for the implementation of data-driven fluid pattern classifiers.