Publications & Talks

1. A New Definition of Vortex Formation Length

   Wenchao Yang, Emad Masroor, Mark A. Stremler

   In Fluid-Structure-Sound Interactions and Control, 2024

   Abs PDF

   We introduce a new definition of vortex formation length for uniform steady flow past a circular cylinder using Proper Orthogonal Decomposition (POD) of the pressure field. Previous definitions each identify a single characteristic length for a given value of Reynolds number. We use the leading modes of the pressure POD to define upper and lower bounds on the vortex formation length. Identifying a range for the vortex formation length is consistent with the hysteresis observed in the critical spacing of two tandem cylinders.

2. Reg. Chaot. Dyn.

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

   Emad Masroor, Mark A. Stremler

   Regular and Chaotic Dynamics, 2022

   Abs DOI PDF

   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 together 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: }\backslashvarepsilon = a/R}, the ratio of minor radius to major radius of the torus-shaped vortex rings, and }\backslashlambda = L/R}, the separation of the vortex rings normalized by their radii. Two bifurcations in the streamline topology are found that depend significantly on }\backslashvarepsilon} and }\backslashlambda}; 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.

3. DIB

   Flow visualization data from experiments with an oscillating circular cylinder in a gravity-driven soap film

   Emad Masroor, Wenchao Yang, Mark A. Stremler

   Data in Brief, 2022

   Abs DOI PDF

   This article contains flow visualization data from experiments conducted in an inclined gravity-driven soap film intersected by a circular cylinder undergoing controlled transverse oscillations at a Reynolds number of Re ≈235 . The dimensionless frequency and amplitude of cylinder oscillation were varied systematically over the ranges 0.2 < f^* < 1.8 and 0.1 < A^* < 1.3. A high-speed camera was used to capture the interference fringe patterns reflected from the soap film. These videos show the structure of the wake behind the cylinder, including the initial formation of vortices and the extended ‘vortex street’. Several wake patterns were identified, including the classic 2S, P+S, 2P, and 2T patterns, which are discussed in detail in the accompanying research article titled "The wake of a transversely oscillating circular cylinder in a flowing soap film at low Reynolds number" [1]. The videos presented in this article can be accessed through the Virginia Tech University Libraries’ Repository at https://doi.org/10.7294/14448027.v5 [2].

4. J. Fluids Struct.

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

   Wenchao Yang, Emad Masroor, Mark A. Stremler

   J. Fluids Struct., Aug 2021

   Abs DOI PDF

   An inclined gravity-driven soap film channel was used to study the wake patterns formed behind a transversely oscillating cylinder at Re=235±14. The natural frequency of vortex shedding from a stationary cylinder, fSt, was used to identify the oscillation frequencies of interest. The (dimensionless) frequency, f∗=f∕fSt, 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∗≈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 Kármán’ wakes were observed as the oscillation frequency became sufficiently different from fSt. The wake patterns and vortex formation processes found in this study, particularly the 2P mode wakes, bear a strong resemblance to previous three-dimensional experimental results, despite the physical constraint from the soap film that limits three-dimensional effects in the wake.

5. J. Fluid Mech.

   Erratum: Streamline patterns in 2P vortex street equilibria

   Mark A. Stremler, Saikat Basu, Emad Masroor

   Journal of Fluid Mechanics,, Aug 2020

   Abs DOI PDF

   Upon further examination, we have determined that the streamline patterns in figure 3 of Basu & Stremler (2017) are not shown in the co-moving frame as intended; the ide ntical error was also made in figure 4(C–F) of Basu & Stremler (2015) and figure 3.3(C–F) of Basu (2014). The vortex positions and strength ratios used in these figures do correctly correspond to equilibrium configurations that translate uniformly without relative motion, but the equations used to generate the corresponding flow patterns contained an error that resulted in streamlines being determined in a frame of reference for which the vortices move uniformly instead of remaining stationary. The equations governing passive particle motion, which were not included in these publications, are shown here for clarity.

2024

1. APS DFD

   Forces on a body with a vortex-dominated wake

   77th Annual Meeting of the American Physical Society’s Division of Fluid Dynamics

   Salt Lake City, UT, 24-26 Nov 2024

   Event Slides
2. SIAM NWCS

   Dynamics of vortex ring arrays

   SIAM Conference on Nonlinear Waves and Coherent Structures

   Baltimore, MD, 24-27 Jun 2024

   Event Slides
3. ICTAM

   Dynamics of coaxial thin vortex ring arrays

   ICTAM 2024: International Congress of Theoretical and Applied Mechanics

   Event Slides

2023

1. SIAM NNP

   Dynamics of coaxial arrays of vortex rings

   Emad Masroor, Mark A. Stremler

   SIAM Sectional Meeting, New York - New Jersey - Pennsylvania

   Event
2. SIAM DS

   Dynamics of coaxial arrays of vortex rings

   Emad Masroor, Mark A. Stremler

   SIAM Conference on Applications of Dynamical Systems

   Event

2022

1. APS DFD

   Fluid Dynamics of a Periodic Array of Axisymmetric Thin-cored Vortex Rings

   Emad Masroor, Mark A. Stremler

   75th Annual Meeting of the APS Division of Fluid Dynamics

   Event Abs

   It is well known that a spatially-periodic array of identical, axisymmetric, thin-cored vortex rings moves uniformly without change of shape or form in the direction of the central axis of symmetry, and is an equilibrium solution of Euler’s equations. We revisit this classical result, due to Vasilev (1914), using the modern formalism of Borisov et. al (2013). 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 nondimensional parameters that define this system: ε=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 ε 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 circle of fluid particles that is stationary in a frame of reference moving with the vortex rings.

2. A computationally efficient method for modeling the dynamics of swimming or flying flexible bodies in high-Re flows

   Emad Masroor, Anshul Nayak, Hodjat Pendar

   2nd Annual Engineering Mechanics Symposium

   Abs

   We propose a low-order computational method for simulating the fluid-structure interaction of 2D thin flexible fins immersed in a fluid at high Reynolds number. We use the panel method to discretize a bound vortex sheet representing the fin, and update its strength over time using the unsteady form of Bernoulli’s equation. A free vortex sheet is released from the trailing edge according to the unsteady Kutta condition. To satisfy the no-penetration boundary condition at each time step, we use a novel acceleration constraint to render the entire system of equations explicit in time, removing the need for iterative solution of a linear system in previous methods. This renders our method computationally efficient, and *more* effectively able to scale to large numbers of filaments compared to the state of the art. We demonstrate the application of our method to the dynamics of a single flexible fin with variable thickness along its length, subjected to prescribed, time-periodic, simultaneous heave and pitch motion of the leading edge, and calculate coefficients of efficiency and thrust.

2021

1. APS DFD

   Vortex patterns in the wake of a transversely oscillating circular cylinder at low Reynolds number

   Emad Masroor, Mark A. Stremler

   74th Annual Meeting of the APS Division of Fluid Dynamics

   Event Abs

   The flow patterns that form in the wake of an oscillating cylinder give us important information about the nature and extent of vortex-induced vibrations (VIV) of cylindrical structures such as oil risers, power lines and undersea cables. To investigate these patterns at low Reynolds number, we oscillated a circular cylinder transversely to a uniform free stream in a gravity-driven soap film over a large portion of the frequency-amplitude parameter space and visualized the resulting wakes using a high-speed camera. A rich variety of vortex patterns were found in the primary synchronization region of the parameter space, including the 2S, 2P, P+S, 2T and coalescing wake patterns. The presence of 2P wakes in a (nominally) two-dimensional flow suggests that spanwise instabilities in the cylinder wake are not necessary for the formation a 2P wake, since these instabilities are suppressed in our experiments. We will discuss these results, as well as the results of a follow-up study involving the oscillation of an extremely thin circular disk, instead of a cylinder, which was conducted to test alternative hypotheses for the formation of the 2P wake.

2. APS DFD

   A generalized Karman-like drag law for exotic vortex street equilibria

   Mark A. Stremler, Emad Masroor

   74th Annual Meeting of the APS Division of Fluid Dynamics

   Event Abs

   Th. von Karman showed that the time-averaged drag force on a submerged body producing a 2S vortex wake can be estimated by the features of a representative point vortex model. We generalize this approach, giving drag forces estimates for bluff bodies producing exotic wakes with N>2 vortices per period. The analysis consists primarily of a linear momentum balance applied to spatially periodic point vortex systems moving in relative equilibrium. Linear stability of some exotic vortex street equilibria help justify this approach. Drag force estimates are presented for ’P+S’ (N=3) and ’2P’ (N=4) vortex wakes, and comparisons are made with the classic ’2S’ drag estimate.

3. ICTAM

   Theoretical predictions for the drag force due to exotic wakes

   Emad Masroor, Mark A. Stremler

   25th International Congress of Theoretical and Applied Mechanics

   Event
4. Understanding the occurrence of the ‘2P mode’ in the wake of an oscillating cylinder at low Re

   Emad Masroor, Mark A. Stremler

   Inaugural Engineering Mechanics Symposium

2019

1. Drag forces on a bluff body shedding a 2P wake

   Emad Masroor, Mark A. Stremler

   Fall Fluid Mechanics Symposium

   Abs

   We develop analytical expressions for the drag experienced in a uniform flow by a bluff body shedding a 2P wake. Following von Karman’s well-known method for the drag experienced by a fixed cylinder with a typical ‘von Karman street’-type wake, we set up a potential flow past a general bluff body and model its wake as an infinite periodic collection of point vortices with 4 vortices per period. The relative positions of the 4 vortices are calculated by determining the relative equilibrium configurations of 2P wakes. By considering the momentum flux into and out of a rectangular region enclosing the body and cutting through the wake, we determine the drag forces exerted on the cylinder over one period of vortex shedding. The drag coefficients predicted by this method will be compared to those predicted for objects shedding a standard von Karman street, giving insight into the effect of wake type on the drag.

2. APS DFD

   Drag forces on a bluff body shedding a 2P wake

   Emad Masroor, Mark A. Stremler

   72nd Annual Meeting of the APS Division of Fluid Dynamics

   Event Abs

   We develop analytical expressions for the drag experienced in a uniform flow by a bluff body shedding a 2P wake. Following von Karman’s well-known method for the drag experienced by a fixed cylinder with a typical ‘von Karman street’-type wake, we set up a potential flow past a general bluff body and model its wake as an infinite periodic collection of point vortices with 4 vortices per period. The relative positions of the 4 vortices are calculated by determining the relative equilibrium configurations of 2P wakes. By considering the momentum flux into and out of a rectangular region enclosing the body and cutting through the wake, we determine the drag forces exerted on the cylinder over one period of vortex shedding. The drag coefficients predicted by this method will be compared to those predicted for objects shedding a standard von Karman street, giving insight into the effect of wake type on the drag.

3. IUTAM

   Vortex patterns in the two-dimensional wake of a transversely oscillating cylinder in uniform flow

   Emad Masroor, Wenchao Yang, Mark A. Stremler

   IUTAM Symposium on Vortex dynamics in science, nature and technology

   Event

2018

1. Vortex patterns in the two-dimensional wake behind an oscillating cylinder

   Wenchao Yang, Emad Masroor, Mark A. Stremler

   Fall Fluid Mechanics Symposium
2. APS DFD

   Wake Structure of an oscillating cylinder in a flowing soap film at low Reynolds number

   Emad Masroor, Wenchao Yang, Mark A. Stremler

   71st Annual Meeting of the APS Division of Fluid Dynamics

   Abs

   When a circular cylinder oscillates with respect to a uniform background flow, a variety of vortex wake patterns are observed as a function of the oscillation amplitude and frequency. Williamson and collaborators have determined pattern bifurcations in parameter space for 3D water flows with Re>300; Leontini and collaborators have studied the problem using a 2D computational method, which is physically relevant for Re<200. There are several differences between these prior computational and experimental results. We will present results of an ongoing experimental study of the wake structure produced by a circular cylinder undergoing forced oscillations transverse to the background flow in an inclined soap film system with Re<200. Wake patterns are observed directly and clearly via the interference fringes caused by thickness variations in the soap film. Comparisons will be made with the previous studies, and features of the two-dimensional experimental wake behavior will be explored.

3. BBVIV

   Classifying Relative Vortex Motions in 2P Mode Wakes

   Mark A. Stremler, Wenchao Yang, Emad Masroor, Saikat Basu

   7th Conference on Bluff Body Wakes and Vortex-Induced Vibrations