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Suresh Behara

Bio: Suresh Behara is an academic researcher from University of Iowa. The author has contributed to research in topics: Vortex & Vortex shedding. The author has an hindex of 8, co-authored 12 publications receiving 278 citations. Previous affiliations of Suresh Behara include University of Minnesota & Indian Institute of Technology Madras.

Papers
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Journal ArticleDOI
TL;DR: In this article, the effect of blockage on the vortex-induced vibrations of a cylinder at low Re ( Re ⩽ 150 ) was investigated numerically via a stabilized space-time finite element formulation.

109 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the transition of the wake of a circular cylinder via a stabilized finite element method for 150≤Re≤350 and found that the vortex dislocations lead to time variation in the vortex shedding frequency.
Abstract: The transition of the wake of a circular cylinder is investigated numerically via a stabilized finite element method for 150≤Re≤350. Both the flow and aerodynamic coefficients are studied. The onset of the three-dimensionality of the flow takes place via the mode-A instability at Re=200. At this Re, the flow exhibits pure mode-A type flow structures for t<1800. At larger times, the vortex dislocations appear spontaneously and destroy the spanwise periodicity in the flow. This confirms the hypothesis that the fully developed mode-A flow structures cannot exist without vortex dislocations. The appearance of dislocations leads to time variation in the vortex shedding frequency. They also lead to a reduction in the global aerodynamic parameters such as drag coefficient, rms value of lift coefficient, and dominant vortex shedding frequency. The vortex dislocations repetitively appear and disappear from the flow. The aerodynamic coefficients achieve a relatively lower value at the time instant when vortex dislo...

48 citations

Journal ArticleDOI
TL;DR: In this article, fluid-structure interaction (FSI) simulations are carried out to investigate vortex-induced vibrations of a sphere, mounted on elastic supports in all three spatial directions.
Abstract: Fluid–structure interaction (FSI) simulations are carried out to investigate vortex-induced vibrations of a sphere, mounted on elastic supports in all three spatial directions. The reduced velocity () is systematically varied in the range , while the Reynolds number and reduced mass are held fixed at and , respectively. In the lock-in regime, two distinct branches are observed in the response curve, each corresponding to a distinct type of vortex shedding, namely, hairpin and spiral vortices. While shedding of hairpin vortices has been observed in several previous investigations of stationary and vibrating spheres, the shedding of intertwined, longitudinal spiral vortices in the wake of a vibrating sphere is reported herein for the first time. When the wake is in the hairpin shedding mode, the sphere moves along a linear path in the transverse plane, while when spiral vortices are shed, the sphere vibrates along a circular orbit. In the spiral mode branch, the simulations reveal hysteresis in the response amplitude at the beginning of the lock-in regime. Lower-amplitude vibrations are found as the sphere sheds hairpin vortices for increasing up until the beginning of the synchronization regime. On the other hand, higher-amplitude oscillations persist for the spiral mode as is decreased from the point of the start of the synchronization. The hairpin mode is found to be unstable for the value of reduced velocity where the spiral and hairpin solution branches merge together. When this point is approached along the hairpin solution branch, the sphere naturally transitions from shedding hairpin vortices and moving along a linear path to shedding spiral vortices and moving along a circular path in the transverse plane. The spiral mode was not observed in the work of Horowitz & Williamson (J. Fluid Mech., vol. 651, 2010, pp. 251–294), who studied experimentally the vibration modes of a freely rising or falling sphere and only reported zigzag vibrations. Our results suggest that this apparent discrepancy between experiments and simulations should be attributed to the fact that, for the range of governing parameters considered in the simulations, the elastic supports act to suppress streamwise vibrations, thus subjecting the sphere to a nearly axisymmetric elasticity constraint and enabling it to vibrate transversely along a circular path.

46 citations

Journal ArticleDOI
01 Apr 2009
TL;DR: A stabilized finite element formulation for three-dimensional unsteady incompressible flows is implemented on a distributed memory parallel computer and it is found that the formation of the RHS vector and the preconditioner achieves a very high level of superlinear speedup as the number of processors increase.
Abstract: A stabilized finite element formulation for three-dimensional unsteady incompressible flows is implemented on a distributed memory parallel computer. A matrix-free version of the GMRES algorithm is utilized to solve the equation systems in an implicit manner. The scalability of the computations on a 64-processor Linux cluster is evaluated for moderate to large size problems. A method for estimating the speedup for large-scale problems, where computations on a single processor is not possible, is proposed. Superlinear speedup is observed, perhaps for the first time, for a large-scale problem that is associated with more than 44 million nodes and 176 million equations. The performance of the various subactivities of the program is monitored to investigate the cause. It is found that the formation of the RHS vector and the preconditioner achieves a very high level of superlinear speedup as the number of processors increase. As a result, even though the network time for interprocessor communication increases with increase in processors, an overall superlinear speedup is realized for large-scale problems. The superlinear speedup is attributed to cache related effects. A comparison between the performance of matrix and matrix-free versions of the GMRES algorithm is carried out. It is found that for large-scale applications the matrix-free version outperforms its counterpart for reasonable dimensions of the Kyrylov subspace. The effect of mesh partitioning on the scalability is also studied. A significant reduction in communication time is observed with partitioning that leads to an overall improvement of speedup. The parallel implementation is utilized to study the wake instabilities in flow past a stationary circular cylinder at Re=150, 200 and 300. The Re=150 flow is found to be two-dimensional while mode-A and mode-B instabilities are observed at Re=200 and 300, respectively. The Re=300 flow is associated with a low frequency modulation in addition to the vortex shedding frequency.

43 citations

Journal ArticleDOI
TL;DR: Oblique shedding in the laminar regime for the flow past a nominally two-dimensional circular cylinder has been investigated numerically via a stabilized finite element method in this paper, where three cells are observed along the span for the Re=60 flow while only two cells are formed at Re=100 and 150.
Abstract: Oblique shedding in the laminar regime for the flow past a nominally two-dimensional circular cylinder has been investigated numerically via a stabilized finite element method. No-slip condition on one of the sidewalls leads to the formation of a boundary layer which promotes oblique vortex shedding. Computations are carried out for three values of Reynolds number (Re): 60, 100, and 150. Cellular shedding is observed in all cases. Three cells are observed along the span for the Re=60 flow while only two cells are formed at Re=100 and 150. Spotlike vortex dislocations form at the junction of the cells. The frequency of the appearance of the dislocations increases with Re. Cellular shedding leads to low frequency modulation in the time histories of aerodynamic coefficients. Lowest value of drag is achieved at a time instant corresponding to the appearance of a new dislocation in the near wake. The vortex shedding frequency as well as the oblique angle of the primary vortices is found to vary with time for t...

31 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the authors present applications of modal analysis techniques to study, model, and control canonical aerodynamic flows, including cylinder wakes, wall-bounded flows, airfoil wakes, and cavity flows.
Abstract: We present applications of modal analysis techniques to study, model, and control canonical aerodynamic flows. To illustrate how modal analysis techniques can provide physical insights in a complementary manner, we selected four fundamental examples of cylinder wakes, wall-bounded flows, airfoil wakes, and cavity flows. We also offer brief discussions on the outlook for modal analysis techniques, in light of rapid developments in data science.

343 citations

Journal ArticleDOI
TL;DR: Different IB approaches for imposing boundary conditions, efficient iterative algorithms for solving the incompressible Navier–Stokes equations in the presence of dynamic immersed boundaries, and strong and loose coupling FSI strategies are summarized and juxtapose.

336 citations

Journal ArticleDOI
TL;DR: In this paper, a numerical simulation of vortex-induced vibrations of a circular cylinder of low non-dimensional mass (m* = 10) in the laminar flow regime (60 < Re < 200) is presented.
Abstract: Results are presented for a numerical simulation of vortex-induced vibrations of a circular cylinder of low non-dimensional mass (m* = 10) in the laminar flow regime (60 < Re < 200). The natural structural frequency of the oscillator, fN, matches the vortex shedding frequency for a stationary cylinder at Re = 100. This corresponds to fN D2/ν = 16.6, where D is the diameter of the cylinder and ν the coefficient of viscosity of the fluid. A stabilized space–time finite element formulation is utilized to solve the incompressible flow equations in primitive variables form in two dimensions. Unlike at high Re, where the cylinder response is known to be associated with three branches, at low Re only two branches are identified: ‘initial’ and ‘lower’. For a blockage of 2.5% and less the onset of synchronization, in the lower Re range, is accompanied by an intermittent switching between two modes with vortex shedding occurring at different frequencies. With higher blockage the jump from the initial to lower branch is hysteretic. Results from free vibrations are compared to the data from experiments for forced vibrations reported earlier. Excellent agreement is observed for the critical amplitude required for the onset of synchronization. The comparison brings out the possibility of hysteresis in forced vibrations. The phase difference between the lift force and transverse displacement shows a jump of almost 180° at, approximately, the middle of the synchronization region. This jump is not hysteretic and it is not associated with any radical change in the vortex shedding pattern. Instead, it is caused by changes in the location and value of the maximum suction on the lower and upper surface of the cylinder. This is observed clearly by comparing the time-averaged flow for a vibrating cylinder for different Re. While the mean flow for Re beyond the phase jump is similar to that for a stationary cylinder, it is associated with a pair of counter-rotating vortices in the near wake for Re prior to the phase jump. The phase jump appears to be one of the mechanisms of the oscillator to self-limit its vibration amplitude.

240 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a review of the recent advances in the assessment of loads for ships and offshore structures with the aim to draw the overall technological landscape available for further understanding, validation and implementation by the academic and industrial communities.

226 citations