Numerical investigation of flow over elastically-mounted circular cylinders

Wu, Xingeng
Major Professor
Anupam Sharma
Committee Member
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Aerospace Engineering
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Aerospace Engineering

Fluid flow over smooth circular cylinders is numerically investigated using a k-ω delayed detached eddy simulation model. The methodology is verified with experiments for rigid cylinders that are rigidly-mounted (static), as well as cylinders that are elastically-mounted and allowed to oscillate in the transverse direction (dynamic). The self-excited oscillation phenomenon of vortex-induced vibration (VIV) is investigated via the elastically-mounted cylinder simulations. The method can accurately predict the aerodynamic loads (mean, variance, and spectra are compared) for the static cases and displacement amplitude and the phenomenon of frequency lock-in for the dynamic cases. These analyses are carried out for flow that is normally-incident on the cylinders as well as for yawed flow.

After verifying the computational methodology with measured data, it is used to investigate the following: (i) the validity of the independence principle for VIV, (ii) the effects of finite span in VIV simulations, (iii) VIV in turbulent separation regime (Re=8×10^6), and (iv) modal decomposition of flow behind oscillating cylinders.

According to the independence principle, nondimensional parameters, e.g., pressure and force coefficients, are independent of the incoming flow angle (yaw) if the component of the flow velocity normal to the cylinder axis is used as the reference velocity scale to nondimensionalize the variables. Through careful numerical simulations, the independence principle is demonstrated to hold not only for static cylinders but also in dynamic cases (transversely oscillating cylinders in free vibration). VIV simulations for an elastically-mounted rigid cylinder are performed at two inflow angles, β=0^∘ and 45^∘ and the independence principle is found to be valid over the entire range of reduced velocities tested with a slightly greater discrepancy when the vortex shedding frequency is close to the natural frequency of the system.

Span-periodic boundaries are often employed in fluid flow simulations to (theoretically) simulate infinitely long structures. In eddy-resolving simulations, however, the finite simulated span introduces artificial effects, particularly when the coherence length scale becomes comparable to the span of the numerical model. The artefacts of finite-span computational domain with periodic boundaries for VIV simulations are investigated. The results show that simulations with span less than five-cylinder diameters give erroneous results including, under/overprediction of displacement amplitude, premature lock-in, etc. Spectra of transverse loading and displacement, instantaneous oscillation frequency, and phase lag between transverse force and displacement are analyzed. High spanwise coherence is identified to be the primary cause of these artefacts observed in small-span simulations.

The same computational methodology is also used to investigate VIV in the turbulent separation (TS) regime where the diameter-based Reynolds number is very high (Re=8×10^6 here). Such high Re is possible in large underwater structures, e.g., monopile foundations of offshore wind turbines. The numerical predictions are compared with VIV in the laminar separation (LS) regime (Re=2×10^4). Two regions, Region I and Region II, are identified over a range of reduced velocities V_R for TS cases. In Region I (V_R<6), the displacement amplitude and oscillating frequency are similar between LS and TS cases. However, TS cases show a distinct large peak in the displacement amplitude in Region II (V_R>6). This large peak is related to the phase transition from 0^∘ to 180^∘ occurring over a large range of reduced velocities.

The DES methodology is also used to simulate the wake behind a cylinder that is oscillating transversely in a prescribed harmonic motion. Three dimensional-reduction techniques, namely, Fourier analysis, Proper Orthogonal Decomposition, and Dynamic Mode Decomposition are used to investigate the vorticity in the cylinder wake. Previously-observed wake patterns, such as 2S, 2P, and P+S, as well as some intermittent switching between these patterns, are observed in the flow as the oscillation amplitude is varied. Dimensional reduction is found to be possible when the force spectra are tone dominated; this generally occurs when the peak frequencies are also harmonics of the vortex shedding frequency. In such cases, the choice of modal decomposition approach does not matter -- all three approaches give verysimilar shapes for the dominant modes. For the case where mode switching is observed (between 2P and P+S), none of the techniques identify the 2P and P+S as distinct modes.