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Mark C. Thompson - One of the best experts on this subject based on the ideXlab platform.
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Wake states and frequency selection of a streamwise Oscillating Cylinder
Journal of Fluid Mechanics, 2013Co-Authors: Justin S. Leontini, David Lo Jacono, Mark C. ThompsonAbstract:This paper presents the results of an in-depth study of the flow past a streamwise Oscillating Cylinder, examining the impact of varying the amplitude and frequency of the oscillation, and the Reynolds number of the incoming flow. These findings are presented in a framework that shows that the relationship between the frequency of vortex shedding fs and the amplitude of oscillation A* is governed by two primary factors: the first is a reduction of fs proportional to a series in A*2 over a wide range of driving frequencies and Reynolds numbers; the second is nonlinear synchronization when this adjusted fs is in the vicinity of N = (1 - fs/fd)-1, where N is an integer. Typically, the influence of higher-order terms is small, and truncation to the first term of the series (A*2) well represents the overall trend of vortex shedding frequency as a function of amplitude. However, discontinuous steps are overlaid on this trend due to the nonlinear synchronization. When fs is normalized by the Strouhal frequency fSt (the frequency of vortex shedding from an unperturbed Cylinder), the rate at which fs/fSt decreases with amplitude, at least for fd/fSt = 1, shows a linear dependence on the Reynolds number. For a fixed Re = 175, the truncated series shows that the rate of decrease of fs/fSt with amplitude varies as (2 - fd/fSt)-1/2 for 1 < or egal fd/fSt < or egal 2, but is essentially independent of fd/fSt for fd/fSt < 1. These trends of the rate of decrease of fs with respect to amplitude are also used to predict the amplitudes of oscillation around which synchronization occurs. These predicted amplitudes are shown to fall in regions of the parameter space where synchronized modes occur. Further, for the case of varying fd/fSt, a very reasonable prediction of the amplitude of oscillation required for the onset of synchronization to the mode where fs = 0.5fd is given. In a similar manner, amplitudes at which fs = 0 are calculated, predicting where the natural vortex shedding is completely supplanted by the forcing. These amplitudes are found to coincide approximately with those at which the onset of a symmetric vortex shedding mode is observed. This result is interpreted as meaning that the symmetric shedding mode occurs when the dynamics crosses over from being dominated by the vortex shedding to being dominated by the forcing.
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A numerical study of an inline Oscillating Cylinder in a free stream
Journal of Fluid Mechanics, 2011Co-Authors: Justin S. Leontini, David Lo Jacono, Mark C. ThompsonAbstract:Simulations of a Cylinder undergoing externally controlled sinusoidal oscillations in the free stream direction have been performed. The frequency of oscillation was kept equal to the vortex shedding frequency from a fixed Cylinder, while the amplitude of oscillation was varied, and the response of the flow measured. With varying amplitude, a rich series of dynamic responses was recorded. With increasing amplitude, these states included wakes similar to the Kármán vortex street, quasiperiodic oscillations interleaved with regions of synchronized periodicity (periodic on multiple oscillation cycles), a period-doubled state and chaotic oscillations. It is hypothesized that, for low to moderate amplitudes, the wake dynamics are controlled by vortex shedding at a global frequency, modified by the oscillation. This vortex shedding is frequency modulated by the driven oscillation and amplitude modulated by vortex interaction. Data are presented to support this hypothesis.
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Modification of three-dimensional transition in the wake of a rotationally Oscillating Cylinder
Journal of Fluid Mechanics, 2010Co-Authors: David Lo Jacon, Justin S. Leontini, Mark C. Thompson, John SheridanAbstract:A study of the flow past an oscillatory rotating Cylinder has been conducted, where the frequency of oscillation has been matched to the natural frequency of the vortex street generated in the wake of a stationary Cylinder, at Reynolds number 300. The focus is on the wake transition to three-dimensional flow and, in particular, the changes induced in this transition by the addition of the oscillatory rotation. Using Floquet stability analysis, it is found that the fine-scale three-dimensional mode that typically dominates the wake at a Reynolds number beyond that at the second transition to three-dimensional flow (referred to as mode B) is suppressed for amplitudes of rotation beyond a critical amplitude, in agreement with past studies. However, the rotation does not suppress the development of three-dimensionality completely, as other modes are discovered that would lead to three-dimensional flow. In particular, the longer-wavelength mode that leads the three-dimensional transition in the wake of a stationary Cylinder (referred to as mode A) is left essentially unaffected at low amplitudes of rotation. At higher amplitudes of oscillation, mode A is also suppressed as the two-dimensional near wake changes in character from a single- to a double- row wake; however, another mode is predicted to render the flow three-dimensional, dubbed mode D (for double row). This mode has the same spatio-temporal symmetries as mode A.
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three dimensional transition in the wake of a transversely Oscillating Cylinder
Journal of Fluid Mechanics, 2007Co-Authors: Justin Scott Leontini, Mark C. Thompson, Kerry HouriganAbstract:A Floquet stability analysis of the transition to three-dimensionality in the wake of a Cylinder forced to oscillate transversely to the free stream has been undertaken. The effect of varying the oscillation amplitude is determined for a frequency of oscillation close to the natural shedding frequency. The three-dimensional modes that arise are identified, and the effect of the oscillation amplitude on their structure and growth rate quantified.It is shown that when the two-dimensional wake is in the 2S configuration (which is similar to the Karman vortex street), the three-dimensional modes that arise are similar in nature and symmetry structure to the modes in the wake of a fixed Cylinder. These modes are known as modes A, B and QP and occur in this order with increasing Re. However, increasing the amplitude of oscillation causes the critical Reynolds number for mode A to increase significantly, to the point where mode B becomes critical before mode A. The critical wavelength for mode A is also affected by the oscillation, becoming smaller with increasing amplitude. Elliptic instability theory is shown also to predict this trend, providing further support that mode A primarily arises as a result of an elliptic instability.At higher oscillation amplitudes, the spatio-temporal symmetry of the two-dimensional wake changes and it takes on the P + S configuration, with a pair of vortices on one side of the wake and a single vortex on the other side, for each oscillation cycle. With the onset of this configuration, modes A, B and QP cease to exist. It is shown that two new three-dimensional modes arise from this base flow, which we call modes SL and SS. Both of these modes are subharmonic, repeating over two base-flow periods. Also, either mode can be the first to become critical, depending on the amplitude of oscillation of the Cylinder.The emergence of these two new modes, as well as the reversal of the order of inception of the three-dimensional modes A and B, leads to the observation that for an Oscillating Cylinder wake there are four different modes that can lead the transition to three-dimensionality, depending on the amplitude of oscillation. Therefore this type of flow provides a good example for studying the effect of mode-order inception on the path taken to turbulence in bluff-body wakes.For the range of amplitudes studied, the maximum Re value for which the flow remains two-dimensional is 280.
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wake state and energy transitions of an Oscillating Cylinder at low reynolds number
Physics of Fluids, 2006Co-Authors: Justin Scott Leontini, Mark C. Thompson, Bronwyn Elaine Stewart, Kerry HouriganAbstract:This paper reports on an extensive parameter space study of two-dimensional simulations of a circular Cylinder forced to oscillate transverse to the free-stream. In particular, the extent of the primary synchronization region, and the wake modes and energy transfer between the body and the fluid are analyzed in some detail. The frequency range of the primary synchronization region is observed to be dependent on Reynolds number, as are the wake modes obtained. Energy transfer is primarily dependent on frequency at low amplitudes of oscillation, but primarily dependent on amplitude at high amplitudes of oscillation. However, the oscillation amplitude corresponding to zero energy transfer is found to be relatively insensitive to Reynolds number. It is also found that there is no discernible change to the wake structure when the energy transfer changes from positive to negative.
Joseph L Bull - One of the best experts on this subject based on the ideXlab platform.
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pulsatile flow past an Oscillating Cylinder
Physics of Fluids, 2011Co-Authors: Adnan Qamar, Robinson Seda, Joseph L BullAbstract:A fundamental study to characterize the flow around an Oscillating Cylinder in a pulsatile flow environment is investigated. This work is motivated by a new proposed design of the total artificial lung (TAL), which is envisioned to provide better gas exchange. The Navier–Stokes computations in a moving frame of reference were performed to compute the dynamic flow field surrounding the Cylinder. Cylinder oscillations and pulsatile free-stream velocity were represented by two sinusoidal waves with amplitudes A and B and frequencies ωc and ω, respectively. The Keulegan–Carpenter number (Kc=Uo/Dωc) was used to describe the frequency of the Oscillating Cylinder while the pulsatile free-stream velocity was fixed by imposing ω/Kc=1 for all cases investigated. The parameters of interest and their values were amplitude (0.5D
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pulsatile flow past an Oscillating Cylinder
Physics of Fluids, 2011Co-Authors: Adnan Qamar, Robinson Seda, Joseph L BullAbstract:A fundamental study to characterize the flow around an Oscillating Cylinder in a pulsatile flow environment is investigated. This work is motivated by a new proposed design of the total artificial lung (TAL), which is envisioned to provide better gas exchange. The Navier–Stokes computations in a moving frame of reference were performed to compute the dynamic flow field surrounding the Cylinder. Cylinder oscillations and pulsatile free-stream velocity were represented by two sinusoidal waves with amplitudes A and B and frequencies ωc and ω, respectively. The Keulegan–Carpenter number (Kc=Uo/Dωc) was used to describe the frequency of the Oscillating Cylinder while the pulsatile free-stream velocity was fixed by imposing ω/Kc=1 for all cases investigated. The parameters of interest and their values were amplitude (0.5D
Kerry Hourigan - One of the best experts on this subject based on the ideXlab platform.
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three dimensional transition in the wake of a transversely Oscillating Cylinder
Journal of Fluid Mechanics, 2007Co-Authors: Justin Scott Leontini, Mark C. Thompson, Kerry HouriganAbstract:A Floquet stability analysis of the transition to three-dimensionality in the wake of a Cylinder forced to oscillate transversely to the free stream has been undertaken. The effect of varying the oscillation amplitude is determined for a frequency of oscillation close to the natural shedding frequency. The three-dimensional modes that arise are identified, and the effect of the oscillation amplitude on their structure and growth rate quantified.It is shown that when the two-dimensional wake is in the 2S configuration (which is similar to the Karman vortex street), the three-dimensional modes that arise are similar in nature and symmetry structure to the modes in the wake of a fixed Cylinder. These modes are known as modes A, B and QP and occur in this order with increasing Re. However, increasing the amplitude of oscillation causes the critical Reynolds number for mode A to increase significantly, to the point where mode B becomes critical before mode A. The critical wavelength for mode A is also affected by the oscillation, becoming smaller with increasing amplitude. Elliptic instability theory is shown also to predict this trend, providing further support that mode A primarily arises as a result of an elliptic instability.At higher oscillation amplitudes, the spatio-temporal symmetry of the two-dimensional wake changes and it takes on the P + S configuration, with a pair of vortices on one side of the wake and a single vortex on the other side, for each oscillation cycle. With the onset of this configuration, modes A, B and QP cease to exist. It is shown that two new three-dimensional modes arise from this base flow, which we call modes SL and SS. Both of these modes are subharmonic, repeating over two base-flow periods. Also, either mode can be the first to become critical, depending on the amplitude of oscillation of the Cylinder.The emergence of these two new modes, as well as the reversal of the order of inception of the three-dimensional modes A and B, leads to the observation that for an Oscillating Cylinder wake there are four different modes that can lead the transition to three-dimensionality, depending on the amplitude of oscillation. Therefore this type of flow provides a good example for studying the effect of mode-order inception on the path taken to turbulence in bluff-body wakes.For the range of amplitudes studied, the maximum Re value for which the flow remains two-dimensional is 280.
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wake state and energy transitions of an Oscillating Cylinder at low reynolds number
Physics of Fluids, 2006Co-Authors: Justin Scott Leontini, Mark C. Thompson, Bronwyn Elaine Stewart, Kerry HouriganAbstract:This paper reports on an extensive parameter space study of two-dimensional simulations of a circular Cylinder forced to oscillate transverse to the free-stream. In particular, the extent of the primary synchronization region, and the wake modes and energy transfer between the body and the fluid are analyzed in some detail. The frequency range of the primary synchronization region is observed to be dependent on Reynolds number, as are the wake modes obtained. Energy transfer is primarily dependent on frequency at low amplitudes of oscillation, but primarily dependent on amplitude at high amplitudes of oscillation. However, the oscillation amplitude corresponding to zero energy transfer is found to be relatively insensitive to Reynolds number. It is also found that there is no discernible change to the wake structure when the energy transfer changes from positive to negative.
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variation in the critical mass ratio of a freely Oscillating Cylinder as a function of reynolds number
Physics of Fluids, 2005Co-Authors: Kris Ryan, Mark Christopher Thompson, Kerry HouriganAbstract:A two-dimensional numerical investigation of the flow-induced vibration of a circular Cylinder held free to oscillate transverse to the free-stream direction has been performed. The simulations were performed over a Reynolds number range Re=[30,200] and for an infinite reduced velocity. Two regions of high amplitude oscillations are observed and are referred to as the viscous and higher Reynolds number range, respectively. The viscous range was observed for Re=[40,95] and the higher Reynolds number range was observed above Re=180. A critical mass ratio, below which appreciable amplitude oscillations are observed, is determined as a function of Reynolds number. For Reynolds numbers between the two ranges, only very small oscillations were observed for all mass ratios investigated.
Z C Zheng - One of the best experts on this subject based on the ideXlab platform.
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nonlinear spacing and frequency effects of an Oscillating Cylinder in the wake of a stationary Cylinder
Physics of Fluids, 2010Co-Authors: Xiaofan Yang, Z C ZhengAbstract:Nonlinear responses to a transversely Oscillating Cylinder in the wake of a stationary upstream Cylinder are studied theoretically by using an immersed-boundary method at Re=100. Response states are investigated in the three flow regimes for a tandem-Cylinder system: the “vortex suppression” regime, the critical spacing regime, and the “vortex formation” regime. When the downstream Cylinder is forced to oscillate at a fixed frequency and amplitude, the response state of flow around the two Cylinders varies with different spacing between the two Cylinders, while in the same flow regime, the response state can change with the Oscillating frequency and amplitude of the downstream Cylinder. Based on velocity phase portraits, each of the nonlinear response states can be categorized into one of the three states in the order of increasing chaotic levels: lock-in, transitional, or quasiperiodic. These states can also be correlated with velocity spectral behaviors. The discussions are conducted using near-wake velocity phase portraits, spectral analyses, and related vorticity fields. A general trend in the bifurcation diagrams of frequency spacing shows the smaller the spacing, frequency, or amplitude, the less chaotic the response state of the system and more likely the downstream and upstream wakes are in the same response state. The system is not locked-in in any case when the spacing between the Cylinders is larger than the critical spacing. The near-wake velocity spectral behaviors correspond to the nonlinear response states, with narrow-banded peaks shown at the oscillation frequency and its harmonics in the lock-in cases. High frequency harmonic peaks, caused by interactions between the upstream wake and the downstream Oscillating Cylinder, are reduced in the near-wake velocity spectra of the upstream Cylinder when the spacing increases.
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study of heat transfer on the surface of a circular Cylinder in flow using an immersed boundary method
International Journal of Heat and Fluid Flow, 2008Co-Authors: Ning Zhang, Z C Zheng, Steven J EckelsAbstract:An immersed-boundary method, previously developed for flow-field simulation, is extended to study heat-transfer problems of flow over a circular Cylinder. The Dirichlet-type isothermal temperature boundary conditions and the Neumann-type iso-heat-flux boundary conditions are both implemented. To verify the accuracy of the simulation method, L2-norm are computed to test the order of accuracy of the scheme. Numerical solutions are then further validated by comparing simulated temperature distributions and local convective-heat-transfer coefficients in flow over a stationary circular Cylinder with data in the literature. Comparisons are made at different Reynolds-number flows and under different types of temperature boundary conditions. Finally, the effect of heat-transfer enhancement for flow over a transversely Oscillating Cylinder is investigated.
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frequency effects on lift and drag for flow past an Oscillating Cylinder
Journal of Fluids and Structures, 2008Co-Authors: Z C Zheng, N ZhangAbstract:A transversely Oscillating Cylinder in a uniform flow is modeled to investigate frequency effects of flow-induced wake on lift and drag of the Cylinder. Specifically, verified unsteady fluid dynamic simulations using an immersed-boundary method in a fixed Cartesian grid predict the flow structure around the Cylinder and reveal how the integration of surface pressure and shear distributions provides lift and drag on the Oscillating Cylinder. In this study, frequency ranges to be considered are both near and away from the natural frequency of wake vortex shedding. Subsequently, the effects of frequency lock-in, superposition and demultiplication on lift and drag are discussed based on the spectral analysis of time histories of lift and drag.
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an improved direct forcing immersed boundary method for finite difference applications
Journal of Computational Physics, 2007Co-Authors: Ning Zhang, Z C ZhengAbstract:A modified immersed-boundary method is developed using the direct-forcing concept. An improved bilinear interpolation/extrapolation algorithm is implemented for more accurate boundary forcing expressions and easier implementation. Detailed discussions of the method are presented on the stability, velocity interpolation on the immersed boundary, direct-forcing extrapolation to the grid points, resolution of the immersed boundary points, and internal treatment. The method can achieve second-order accurate solutions. The method is then applied to a finite-difference scheme to compute flow over a stationary Cylinder, an Oscillating Cylinder, and a stationary sphere. The accuracy of the computational results is verified using numerous computational and experimental results in the literature.
D Rockwell - One of the best experts on this subject based on the ideXlab platform.
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forces and wake modes of an Oscillating Cylinder
Journal of Fluids and Structures, 2001Co-Authors: Josie Carberry, John Sheridan, D RockwellAbstract:This investigation considers the wake states of a Cylinder subjected to forced oscillations at frequencies close to the Karman frequency. Two distinctly different wake states are observed. The emphasis is on the transition between these states, which is characterized in terms of the lift force on the Cylinder and the instantaneous patterns of vortex structures in the near-wake. As the frequency of oscillation increases, there is simultaneously an abrupt jump in the lift force and a change in the mode of vortex shedding. The jump in the lift force involves a sharp increase in the magnitude of the lift coefficient and a phase shift of the order 180°. The corresponding mode change involves an alteration in both the timing of the vortex initially shed from the Cylinder and the overall pattern of vortices in the near-wake. Whilst previously these changes have been observed individually in separate forced vibration investigations, we show conclusive evidence that these two events are intrinsically linked. Moreover, for a narrow band of frequencies, a self-excited transition is possible, where the wake state changes while the Cylinder oscillates at a constant frequency.
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vortices incident upon an Oscillating Cylinder flow structure and loading
Journal of Fluids and Structures, 1999Co-Authors: M Gaydon, D RockwellAbstract:Abstract Interaction of an incident vortex street with an Oscillating Cylinder is addressed using high-image-density particle image velocimetry and simultaneous force measurements. This approach reveals that the timing of the incident vortices relative to the Cylinder motion controls the large-scale vortex formation in the near-wake, and thereby the phase shift between the loading on the Cylinder and its motion. As a consequence, it is possible to change the sign of the fluid-dynamic work done by the fluid on the Cylinder. The incident vortices dramatically shorten the formation length of vortices in the near-wake and yield values of lift coefficient up to a factor of five larger than that for an isolated Cylinder subjected to controlled oscillations in the absence of incident vortices. These alterations of the wake structure and loading occur in conjunction with globally locked-on patterns of incident and shed vortices with respect to the Cylinder oscillation. Different states of global lock-on are attainable for different values of timing of the incident vortices.
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timing of vortex formation from an Oscillating Cylinder
Physics of Fluids, 1994Co-Authors: C K Chyu, D RockwellAbstract:The instantaneous structure of the near‐wake of a Cylinder subjected to forced oscillations is examined using particle imaging, which leads to representations of the streamline patterns and distributions of vorticity. As the frequency of excitation of the Cylinder is increased relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the Cylinder until a limiting position is reached; at this position, the vorticity concentration abruptly switches to the opposite side of the Cylinder. This process induces abrupt changes of the topology of the corresponding streamline patterns; such topological patterns alone, however, do not properly suggest the existence and rearrangement of the vorticity concentrations. Moreover, this vorticity‐switching concept persists to high values of Reynolds number, where the values of the mean base pressure coefficient and vortex formation length differ substantially from those at low Reynolds number. The switching mechanism is not significantly altered, either in an instantaneous or ensemble‐averaged sense, by the presence of small‐scale Kelvin–Helmholtz vortices that coexist with the large‐scale (Karman) vortices.