arXiv subjects

Emad Masroor

Publications and source records attributed to Emad Masroor.

3 recordsLinked to original sources

arXiv2026

Performance comparison of tethered and self-propelled models of fish locomotion using unsteady thin airfoil theory

Numerous experimental and computational studies have been conducted in the past few decades to understand the swimming performance of fish, and to identify the optimal kinematic strategies for a swimming fish or fish-like robot. Many of these studies model the swimmer in a `tethered' condition, in which the swimmer is held fixed while it is subjected to a free stream. Its performance is then quantified using power expenditure, thrust generation, and efficiency. However, the dynamics of a tethered swimmer are different from those of a self-propelled swimmer, whose performance is best measured using its steady-state swimming speed in still fluid and its efficiency. It is an open question whether the conclusions drawn from studies of tethered swimmers can be directly applied to free swimmers. In this study, we use an unsteady panel method to systematically compare the swimming performance of a tethered fin and that of a self-propelled fin attached to a virtual drag-producing body to investigate how their performance varies over a set of prescribed kinematics. After validating the numerical model against previous experimental results, we show how the pitch amplitude, heave amplitude and the phase offset between them affect the efficiency and thrust generation in the tethered case, and how they affect the speed and efficiency in the self-propelled case. We find that the kinematic strategies that optimize the performance of a tethered swimmer do not necessarily optimize the performance of a self-propelled swimmer.

cond-mat.soft↗

arXiv2022

On the topology of the atmosphere advected by a periodic array of axisymmetric thin-cored vortex rings

The fluid motion produced by a periodic array of identical, axisymmetric, thin-cored vortex rings is investigated. It is well known that such an array moves uniformly without change of shape or form in the direction of the central axis of symmetry, and is therefore an equilibrium solution of Euler's equations. In a frame of reference moving with the system of vortex rings, the motion of passive fluid particles is investigated as a function of the two non-dimensional parameters that define this system: $\varepsilon = a/R$, the ratio of minor radius to major radius of the torus-shaped vortex rings, and $λ= L/R$, the separation of the vortex rings normalized by their radii. Two bifurcations in the streamline topology are found that depend significantly on $\varepsilon$ and $λ$; these bifurcations delineate three distinct shapes of the 'atmosphere' of fluid particles that move together with the vortex ring for all time. Analogous to the case of an isolated vortex ring, the atmospheres can be 'thin-bodied' or 'thick-bodied'. Additionally, we find the occurrence of a 'connected' system, in which the atmospheres of neighboring rings touch at an invariant ring of fluid particles that is stationary in a frame of reference moving with the rings.

physics.flu-dyn↗

arXiv2021

The wake of a transversely oscillating circular cylinder in a flowing soap film at low Reynolds number

An inclined gravity-driven soap film channel was used to study the wake patterns formed behind a transversely oscillating cylinder at $Re =235 \pm 14$. The natural frequency of vortex shedding from a stationary cylinder, $f_{\text{St}}$, was used to identify the oscillation frequencies of interest. The (dimensionless) frequency, $f^*=f/{f_{\text{St}}}$, and amplitude, $A^*=A/D$, of the cylinder's motion was varied over a large portion of the fundamental synchronization region (i.e., for $f^* \approx 1$), and a `map' of wake patterns was constructed in ${(f^*, A^*)}$ space. Lock-on between the frequency of the cylinder's motion and the dominant frequency of the resulting vortex wake was observed for a large range of this parameter space, predominantly manifested as synchronized `2S' and `2P' wake modes. Synchronized `P+S', `2T', and `transitional' wakes were also found in smaller regions of parameter space. Unsynchronized `coalescing' and `perturbed von~\karman' wakes were observed as the oscillation frequency became sufficiently different from $f_{\text_{St}}$. The wake patterns and vortex formation processes found in this study, particularly for the 2P mode wakes, are more similar to those observed by \cite{Williamson1988} in three-dimensional experiments than those found by \cite{Leontini2006} in two-dimensional simulations, despite the physical constraint from the soap film that prevents three-dimensional effects in the wake.

physics.flu-dyn↗