Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session E18: Aerodynamics: Fluid-Structure InteractionAerodynamics FSI
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Chair: Banafsheh Seyed-Aghazadeh, Miami University Room: 607 |
Sunday, November 19, 2017 4:55PM - 5:08PM |
E18.00001: Instability-Driven Frequency Decoupling between Structure Dynamics and Wake Fluctuations Yaqing Jin, Jin-Tae Kim, Leonardo P. Chamorro Flow-induced dynamics of flexible structures appears to be strongly modulated by periodic vortex shedding. Experiments and numerical simulations suggest that the frequencies associated to the dominant motions of the structures are highly coupled with those of the wake under low turbulence and uniform flow. We present new experimental evidence that demonstrates a significant decoupling between the dynamics of simple structures and wake fluctuations for various geometries, Reynolds numbers and mass ratio. High-resolution 2D particle tracking velocimetry (PTV) was used to track instantaneous motions of fiducial points on the structure and a hotwire anemometry were used to characterize and gain the insight of wake fluctuations. For all samples oscillating in the streamwise direction, the natural frequency of the structure dominates the body dynamics, while the wake fluctuations are governed by the Strouhal-type vortex shedding. This phenomenon might be a consequence of flow instability, where the structural characteristics of the body dominate the oscillations. For structures allowing to oscillate in the spanwise direction, the footprint of both natural frequency and vortex shedding can be observed in the body dynamics. [Preview Abstract] |
Sunday, November 19, 2017 5:08PM - 5:21PM |
E18.00002: Spanwise structure of the flow past a fixed or freely vibrating cylinder in the early turbulent regime Remi Bourguet, Simon Gsell, Marianna Braza The flow patterns developing downstream of slender bodies with bluff cross-section have been the object of intense research in the past decades. Particular attention was paid to the vortex patterns emerging in the plane perpendicular to the body axis. In the present study, focus is placed on the spanwise structure of the flow, in the early turbulent regime. The existence of dominant spanwise wavelengths had already been reported. However, many aspects remained to be explored, among others, the streamwise evolution of the spanwise patterns and their possible alteration when the body oscillates. These aspects are examined here on the basis of direct numerical simulations of the flow past a circular cylinder at Reynolds number 3900. The body is either fixed or subjected to vortex-induced vibrations. A systematic analysis of the spanwise patterns reveals persistent trends of their amplitude and wavelength in the different compartments of the flow, i.e. the separating shear layer and wake regions. Physical mechanisms are proposed to explain these trends. It is also found that the spanwise structure of the flow is differently altered in these two regions once the cylinder vibrates, the alteration being concentrated in the separating shear layers. [Preview Abstract] |
Sunday, November 19, 2017 5:21PM - 5:34PM |
E18.00003: The dynamics of three-dimensional slung prisms under very low and high turbulence Sheng Ji, Yaqing Jin, Hongyi Zhou, Leonardo P. Chamorro The distinctive oscillation and pitching patterns of cubic and rectangular slung prisms were characterized at various Reynolds numbers under two free-stream turbulence levels. Laboratory experiments were performed using high-resolution telemetry and hotwire anemometry to quantify the dynamics of the prisms and wake fluctuation. Results show that the dynamics of the prisms depends on the prism shape, and can be categorized by two distinctive regions. Specifically, in the case of cubic prism, the regions are characterized by the growth rate of the pitching amplitude; whereas the dynamics of the rectangular prisms is more sensitive to the angle of attack. When the large side initially faces the flow, the regions are defined by the synchronization between the vortex shedding and pure oscillations under very low turbulence. However, this synchronization can be disturbed under high background turbulence. When the smaller side initially faces the flow, the regions are defined by the equilibrium pitching position. Regardless of the geometry of the prism and flow condition the dominant oscillation frequency resulted as being close to the natural frequency of the small-amplitude pendulum-like oscillation. [Preview Abstract] |
Sunday, November 19, 2017 5:34PM - 5:47PM |
E18.00004: Flow-induced vibration study by exploiting inherent nonlinearity of structure Banafsheh Seyed-Aghazadeh, Hamed Samandari Elastically mounted prismatic structures placed in flow can undergo Flow-Induced Vibration (FIV). Flow forces acting on these structures consist of a main frequency, close to the natural frequency of the system, as well as its higher harmonic components. Mostly in FIV studies, the structural stiffness is provided through linear springs. The linearity of the structure limits occurrence of potential large amplitude oscillations at higher harmonics of the main frequency. In this study, we propose implementing an inherently nonlinear structure in FIV study of a prismatic structure. Through this unique design, excitation of higher harmonics and coupling between those and natural frequencies of the system can be achieved. A square cross-section prism was mounted on the upstream tip of an elastic beam with inherent nonlinearity and was placed in the test-section of a subsonic wind tunnel. The tests were conducted in a Reynolds number range of 150\textless \textit{Re}\textless 20,000. Dynamic response of this configuration was examined for a range of prismatic tip mass aspect ratios, inertia and bending stiffness of the beam. The results show that large amplitude, low frequency galloping type oscillation is accompanied by large contributions from the higher harmonics in the frequency content of the oscillations. Numerical simulation using Differential Quadrature Method was conducted to identify the optimum structural configurations for coupling between the higher harmonics and natural frequencies of the system. [Preview Abstract] |
Sunday, November 19, 2017 5:47PM - 6:00PM |
E18.00005: Near wall effects on flexible splitter plate behind a cylinder Venkat Narayanan K, Dr Vengadesan S, Dr Murali K Vortex induced vibrations(VIV) of a rigid circular cylinder with a flexible plate attached to its rear end, close to the plane wall is numerically studied for Re = 200. Amplitude modulations were observed in the response of the flexible plate at the ground distance of $\frac{G}{D}$=0.5. Numerical simulations were conducted for a range of reduced velocities Ur(3,4,5 and 6), which appropriately captures the synchronization range of VIV of the structure. At U$_{r}$=3 there is no significant amplitude modulation. As Ur is increased further, the modulation appears. The modulation appears symmetric about the peak amplitude for successive cycles at U$_{r}$=4. The phase plots of lift coefficient C$_{L}$ and plate tip displacement revealed the change in sign of energy transfer between the plate and the wake. Amplitude modulation is reflected in the interaction of shed vortices and the plane wall. Shed vortices are convected parallel to the wall when the amplitude of the plate rises to its local maximum during modulation. During the growth and damping phase of the amplitudes in each modulation cycle, the vortex shedding is observed to be oblique towards the wall. [Preview Abstract] |
Sunday, November 19, 2017 6:00PM - 6:13PM |
E18.00006: Procedure to the collapse of the Tacoma Narrows Bridge Woojin Kim, Haecheon Choi The Tacoma Narrows Bridge (TNB) was collapsed in 1940 at the wind speed (U) of 18m/s corresponding to Re = 3000000 based on the height (H=2.4m) of the deck. The deck was lifted by two cables with 110 hangers and each cable was suspended by two towers. The deck experienced a vertical vibration with the spanwise wavelengths (l) of 1/4 and 1/4.5 of the deck length (L) and the non-dimensional frequency of St = fH/U = 0.08, followed by a torsional vibration with l = L and St = 0.027. We investigate the procedure before the collapse of the TNB using numerical simulation with fluid-structure interaction. We consider Re = 300 and 1000 because the non-dimensional frequency of vortex shedding is not much changed by the change in the Reynolds number due to a fixed separation point at the leading edge of the deck. A nonlinear model for a suspension bridge (Arioli & Gazzola, 2017) is used for the motion of the TNB. Initially, two-dimensional vortex shedding is generated with a weak vibration of the deck. Then, the deck oscillates vertically with l = L/4.5 and St = 0.09, together with three-dimensional vortical structures behind the deck. Finally, the deck oscillates torsionally with l = L and St = 0.033. These predicted vibrations of the TNB are in good agreements with those occurred in 1940. [Preview Abstract] |
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