Jason Dahl

Dual resonance in vortex-induced vibrations at subcritical and supercritical Reynolds numbers

An experimental study is performed on the vortex induced vibrations of a rigid flexibly mounted circular cylinder placed in a crossflow. The cylinder is allowed to oscillate in combined crossflow and in-line motions, and the ratio of the nominal in-line and transverse natural frequencies is varied systematically. Experiments were conducted on a smooth cylinder at subcritical Reynolds numbers between 15 000 and 60 000 and on a roughened cylinder at supercritical Reynolds numbers between 320 000 and 710 000, with a surface roughness equal to 0.23 % of the cylinder diameter. Strong qualitative and quantitative similarities between the subcritical and supercritical experiments are found, especially when the in-line natural frequency is close to twice the value of the crossflow natural frequency. In both Reynolds number regimes, the test cylinder may exhibit a ‘dual-resonant’ response, resulting in resonant crossflow motion at a frequency fv, near the Strouhal frequency, and resonant in-line motion at 2 fv .T his dual resonance is shown to occur over a relatively wide frequency region around the Strouhal frequency, accompanied by stable, highly repeatable figure-eight cylinder orbits, as well as large third-harmonic components of the lift force. Under dualresonance conditions, both the subcritical and the supercritical response is shown to collapse into a narrow parametric region in which the effective natural-frequency ratio is near the value 2, regardless of the nominal natural-frequency ratio. Some differences are noted in the magnitudes of forces and the cylinder response between the two different Reynolds number regimes, but the dual-resonant response and the resulting force trends are preserved despite the large Reynolds number difference.

Lateral-line inspired sensor arrays for navigation and object identification

Found to affect numerous aspects of behavior, including maneuvering in complex fluid environments with poor visibility, the lateral line is a critical component of fish sensory systems. This sensory organ could fill the gap left by sonar and vision systems in turbid, cluttered environments; it has no analog in modern ocean vehicles, despite its utility and ubiquity in nature. A linear array of pressure sensors is used along with analytic models of the fluid in order to emulate the lateral line and characterize its object-tracking and shape recognition capabilities. Position, shape, and size of various objects in both passive and active sensing schemes are thus determined. The authors find that tracking a moving cylinder can be effectively achieved via a particle filter, based on pressure information. The authors are also able to reliably distinguish between cylinders of different cross section, using principal component analysis, as well as identify the critical flow signature information that leads to the shape identification. The authors employ pressure measurements on an artificial fish and an unscented Kalman filter in a second application in order to successfully identify the shape of an arbitrary static cylinder. The authors conclude that, based on the experiments, a linear pressure sensor array for identifying small objects should have a sensor-to-sensor spacing of less than 0.03 (relative to the length of the sensing body) and resolve pressure differences of at least 10 Pa. These criteria employ conductive polymer technologies to form a flexible array of small pressure sensors in order to be used in the development of an artificial lateral line adaptable to the curved hull of an underwater vehicle.

Mode Shape Variation for a Low-Mode Number Flexible Cylinder Subject to Vortex-Induced Vibrations

The excitation of two low-mode number, flexible cylinders in uniform-flow is investigated to determine effects of structural mode shape on vortex-induced vibrations. Experiments are performed in a re-circulating flow channel and in a small flow visualization tank using object tracking and digital particle image velocimetry (DPIV) to measure the excitation of the cylinder, to estimate forces acting on the structure, and to observe the wake of the structure under the observed body motions. Previous research has focused on understanding the effect of in-line to cross-flow natural frequency ratio on the excitation of the structure in an attempt to model the excitation of multiple structural modes on long, flexible bodies. The current research investigates the impact of structural mode shape on this relationship by holding the in-line to cross-flow natural frequency constant and attempting to excite a specific structural mode shape. It is found that the combination of an odd mode shape excited in the cross-flow direction with an even mode shape in the in-line direction results in an incompatible synchronization condition, where the dominant forcing frequency in-line may experience a frequency equal to the cross-flow forcing frequency, a condition only observed in rigid cylinder experiments when the natural frequency ratio is less than one. This is consistent with the first mode being excited in both in-line and cross-flow directions, however this leads to an asymmetric wake. The wake is observed using DPIV on a rigid cylinder with forced motions equivalent to the flexible body. A case of mode switching is also observed where the even in-line mode exhibits an excitation at twice the cross-flow frequency; however the spatial mode shape in-line appears similar to the first structural mode shape. It is hypothesized that this situation is possible due to variation in the effective added mass along the length of the cylinder.Copyright

Vortex-Induced Vibrations

Starting at a low Reynolds number of about 50, and reaching the highest Reynolds numbers recorded, bluff bodies placed within an external flow form an unstable wake that results in the formation of a regular pattern of vortices, the Karman street. If the structure is flexible or flexibly mounted, these vortices may cause vibrations, leading to stresses and fatigue damage. This motion of the body influences, in turn, the vortex formation process, establishing a feedback mechanism that may lead to stable or unstable dynamic equilibria. As a result, vortex-induced vibrations are controlled by complex physical mechanisms characterized by rich dynamic properties. When elongated, flexible structures are placed in a sheared cross-flow, the fluid–structure interaction process is distributed along their length, resulting in added complexity, as parts of the structure act to transfer energy from the flow to the structure, while other parts damp the response.

Active Control of Flexible Cylinders Undergoing Vortex-Induced Vibrations Using Piezo Stripe Actuators

In this study, piezo stripe actuators were used to control the vibrations of a flexible cylinder undergoing underwater flow-induced vibrations. The piezo actuators were attached at the anti-nodes of a rectangular plastic beam and urethane rubber was used to mold the test model to have a circular cylinder shape. Forced base oscillation experiments were first carried out in air to characterize the system and piezo responses. Experiments were then performed in a recirculating water channel with the flexible cylinder in a uniform free stream, where the cylinder undergoes self-excitation due to vortex shedding in the wake and forced excitation due to the piezo actuators. The actuators were oriented to apply an excitation only in the in-line direction of the flow. In the tests, two separate cases were investigated. In the first case, the piezo actuators were activated at a flow speed corresponding to a flow-induced response with a spatial mode change, causing the cylinder to be excited with a higher mode, leading to a significantly smaller amplitude response than without the piezo actuation (vibration suppression). In the second case, piezo actuators were activated at a flow speed corresponding to a significant flow-induced amplitude increase. The interaction of the piezo forcing and the forced response of the cylinder results in a jump to the higher amplitude response regime (vibration enhancement). This study presents two important observations: (1) it is possible that piezo actuators can trip the response frequency to force the cylinder to oscillate with a different mode thus reducing the total response amplitude significantly, (2) it is also possible to prematurely increase the response amplitude for a particular flow speed (i.e. jumping from a lower branch to upper branch response).

Rolling and pitching oscillating foil propulsion in ground effect

In this paper, we investigate the effect of operating near a solid boundary on the forces produced by harmonically oscillating thrust-generating foils. A rolling and pitching foil was towed in a freshwater tank in a series of experiments with varying kinematics. Hydrodynamic forces and torques were measured in the freestream and at varying distances from a solid boundary, and changes in mean lift and thrust were found when the foil approached the boundary. The magnitude of this ground effect exhibited a strong nonlinear dependence on the distance between the foil and the boundary. Significant effects were found within three chord lengths of the boundary, and ground effect can be induced at greater distances from the boundary by biasing the tip of the foil toward the boundary. Lift coefficients changed by as much as [Formula: see text] at the closest approach to the ground, with changes [Formula: see text] [Formula: see text] for all cases across Strouhal numbers ranging from [Formula: see text] to [Formula: see text], and nominal maximum angle of attack ranging from [Formula: see text] to [Formula: see text]. The ubiquity of the ground effect in high thrust kinematics suggests that the ground effect can provide a passive obstacle avoidance capability for foil propelled vehicles. By comparison with previous experimental work, we find that the ground effect experienced by a high-aspect ratio rolling and pitching foil is a fully three-dimensional phenomenon, as it is not accurately predicted when two-dimensional flow and/or two-dimensional kinematics are enforced. While two-dimensional foil kinematics are more easily modeled for numerical studies, three-dimensional foil kinematics may be more practical for real world implementation in underwater vehicles.

Principles of Wake Energy Recovery and Flow Structure in Bodies Undergoing Rapid Shape Change

For a body moving within a fluid, its shape and the manner in which it morphs greatly impact the energy transfer between it and the flow. In vanishing bodies, vorticity is globally shed, while added mass-related energy is released into the fluid. We investigate square-tipped, streamlined-tipped, and hollow foils towed at \(10^{\circ }\) angle of attack and quickly retracted in the span-wise direction, as generic models of bodies of different form undergoing rapid shape and volume change. Particle image velocimetry shows that large differences exist in their globally shed wakes. The retracting square-tipped foil forms a wake with energy in excess of the potential flow estimate before retraction starts; the extra energy results in the formation of an additional vortex ring that adds unsteadiness and complexity to the form of the wake. The streamlined-tipped foil avoids creating such ring vortices, but sheds a much less energetic wake: numerical simulation shows that energy is transferred back to the foil during the retraction phase through a thrust force. Circulation calculations show that energy transfer is enabled by the gradual shape change in this foil and is associated with simultaneous pressure gradient-induced and vorticity tilting-induced vorticity annihilation. Finally, the hollow foil combines the advantages of near-complete transfer of the original added mass-related energy to the wake and absence of a vortex ring formation, resulting in an energetic and also cleanly-evolving, stable wake. Hence, modest differences in morphing body shape are shown to result in significantly different flow patterns.

Coupled Inline-Cross Flow VIV Hydrodynamic Coefficients Database

Vortex Induced Vibrations (VIV) cause major fatigue damage to long slender bodies and have been extensively studied in the past decades. While most of the past research focused on the cross flow direction, it was recently shown that the inline motion in the direction of the flow has a major impact on the fatigue life damage due to its higher frequency (second harmonic) and more importantly, its coupling with the crossflow motion, which triggers a third harmonic stress component in the cross flow direction. In this paper, the coupled inline-crossflow VIV problem is addressed from semi-empirical modeling of fluid forces. Extensive fine grid forced inline-crossflow VIV experiments were designed and carried out in the MIT towing tank. An inline-crossflow VIV hydrodynamics coefficients database was newly constructed using the experimental results and it is expected to be useful for other semi empirical programs predicting coupled inline-crossflow VIV in the field. Several key hydrodynamic coefficients in the database, including lift force coefficients, drag force coefficients and added mass coefficients, were systematically analyzed. The coefficients in the crossflow and the inline directions were found to have strong dependency on the phase between the inline and crossflow motions.© 2014 ASME

Inline-Crossflow Coupled Vortex Induced Vibrations of Long Flexible Cylinders

The inline motion of long flexible cylinders caused by Vortex Induced Vibrations (VIV) has been long neglected due to its small amplitude compared to the cross-flow response amplitude. However, the inline motion has a major impact on fatigue life due to its higher frequency (second harmonic) and more importantly, because it triggers a third harmonic stress component in the crossflow direction along with a broad-band frequency stress component. We introduce an inline response prediction module to VIVA, a VIV response prediction program widely used in the offshore industry, to be able to consequently predict the higher harmonic and chaotic VIV response characteristics of flexible cylinders. Extensive forced inline and combined inline-crossflow experiments were employed to provide hydrodynamic coefficient databases for input to VIVA, in addition to existing crossflow hydrodynamic coefficients. The Norwegian Deepwater Programme (NDP) experimental data were used to validate this prediction methodology.© 2012 ASME