Hongrae Park

Selective Roughness in the Boundary Layer to Suppress Flow-Induced Motions of Circular Cylinder at 30,000<Re<120,000

A passive control means to suppress flow-induced motions (FIM) of a rigid circular cylinder in the TrSL3, high-lift, flow regime is formulated and tested experimentally. The developed method uses passive turbulence control (PTC) consisting of selectively located roughness on the cylinder surface with thickness about equal to the boundary layer thickness. The map of “PTC-to-FIM,” developed in previous work, revealed robust zones of weak suppression, strong suppression, hard galloping, and soft galloping. PTC has been used successfully to enhance FIM for hydrokinetic energy harnessing using the VIVACE Converter. PTC also revealed the potential to suppress FIM to various levels. The map is flow-direction dependent. In this paper, the “PTC-to-FIM” map is used to guide development of FIM suppression devices that are flow-direction independent and hardly affect cylinder geometry. Experiments are conducted in the Low Turbulence Free Surface Water Channel of the University of Michigan on a rigid, horizontal, circular cylinder, suspended on springs. Amplitude and frequency measurements and broad field-of-view visualization reveal complex flow structures and their relation to suppression. Several PTC designs are tested to understand the effect of PTC roughness, location, coverage, and configuration. Gradual modification of PTC parameters, leads to improved suppression and evolution of a design reducing the VIV synchronization range. Over a wide range of high reduced velocities, VIV is fully suppressed. The maximum amplitude occurring near the system’s natural frequency is reduced by about 63% compared to the maximum amplitude of the smooth cylinder.

Selective Surface Roughness to Suppress Flow-Induced Motion of Two Circular Cylinders at 30,000<Re<120,000

In the Marine Renewable Energy Laboratory of the University of Michigan, selectively located surface roughness has been designed successfully to suppress vortex-induced vibrations of a single cylinder by 60% compared to a smooth cylinder. In this paper, suppression of flow-induced motions of two cylinders in tandem using surface roughness is studied experimentally by varying flow velocity and cylinder center-to-center spacing. The two identical cylinders are rigid, suspended by springs, and allowed to move transversely to the flow direction and their own axis. Surface roughness is applied in the form of four roughness strips helically placed around the cylinder. Results are compared to smooth cylinders also tested in this work. Amplitude ratio A/D, frequency ratio fosc/fn,water, and range of synchronization are measured. Regardless of the center-to-center cylinder distance, the amplitude response of the upstream smooth-cylinder is similar to that of the isolated smooth-cylinder. The wake from the upstream cylinder with roughness is narrower and longer and has significant influence on the amplitude of the downstream cylinder. The latter is reduced in the initial and upper branches while its range of VIV-synchronization is extended. In addition the amplitude of the upstream rough cylinder and its range of synchronization increase with respect to the isolated rough cylinder.Copyright

Map of passive turbulence control to flow-induced motions for a circular cylinder at 30,000<RE<120,000: Sensitivity to zone covering

Passive turbulence control (PTC) in the form of two straight roughness strips with variable width, and thickness about equal to the boundary layer thickness, is used to modify the flow-induced motions (FIM) of a rigid circular cylinder. The cylinder is supported by two end-springs and the flow is in the TrSL3, high-lift, regime. The PTC-to-FIM Map, developed in previous work, revealed zones of weak suppression, strong suppression, hard galloping, and soft galloping. In this paper the sensitivity of the PTC-to-FIM Map to: (a) the width of PTC covering, (b) PTC covering a single or multiple zones, (c) PTC being straight or staggered is studied experimentally. Experiments are conducted in the Low Turbulence Free Surface Water Channel of the University of Michigan. Fixed parameters are: cylinder diameter D = 8.89cm, m* = 1.725, spring stiffness K = 763N/m, aspect ratio l/D = 10.29, and damping ratio ζ = 0.019. Variable parameters are: circumferential PTC location αPTC ∈ [0°−180°], Reynolds number Re ∈ [30,000–120,000], flow velocity U ∈ [0.36m/s–1.45m/s]. Measured quantities are: amplitude ratio A/D, frequency ratio fosc/fn,w, and synchronization range. As long as the roughness distribution is limited to remain within a zone, the width of the strips does not affect the FIM response. When multiple zones are covered, the strong suppression zone dominates the FIM.Copyright

Effect of damping on galloping of circular cylinders

Galloping motion has long been differentiated from other flow-induced motions like VIV and flutter based on the asymmetry of the flow present causing instability. Galloping instabilities can occur for circular cylinders due to proximity to a boundary or asymmetric cross-sections from strakes or other flow interference. The response of a bluff body due to galloping instabilities is still an active area of research. For VIV, such parameters as variable added mass term [1] and a damping parameter c* [2] have been introduced. This paper demonstrates that these parameters have the potential of being applied to galloping to model it as a distinct resonance phenomenon from other flow-induced motions. Additionally, it is demonstrate that the amplitudes of motion, variable added-mass, and lift are very sensitive to the added damping coefficient, but the initiation of galloping remains largely dependent on the natural frequency of the cylinder with little sensitivity to the damping.

Effect of damping on variable added mass and lift of circular cylinders in vortex-induced vibrations

For many decades now, the idea of Vortex-Induced Vibrations (VIV) being modeled as a lock-in phenomenon of a mass-spring-dashpot system with an ideal added mass term has prevailed. In 2000, it was suggested by Vikestad et al. [1] that VIV may be modeled as a resonance phenomenon with variable natural frequency due to a variable added-mass term. In this paper, the variable added-mass approach is used for analysis of VIV at various added damping values. Additionally, Vandivers damping coefficient c* is used [2] to correlate damping to lift. The findings are that: 1. The oscillation frequency is in unity with the mean of the natural frequency with variable added mass for each period of oscillation during VIV lock-in no matter the damping value. 2. The time-averaged variable added mass coefficient is shown to vary with an increasing damping coefficient, where below a reduced velocity of approximately seven, increased damping indicates increased added mass. After a reduced velocity of approximately seven, however, increased damping results in decreased added mass. 3. Vandivers damping coefficient c* plotted against the nondimensional amplitude follows very closely to c*A/D = max lift coefficient = square root of 0.79 [3]. A handful of cases did exceed square root of 0.79 but only marginally.