Vortex-Induced Vibration of a Neutrally Buoyant Tethered Sphere

Recent preliminary experiments have indicated that a neutrally buoyant tethered sphere develops a large diameter quasi-circular trajectory, unlike the oscillations observed for non-neutrally buoyant tethered spheres. This shows similarities to the path of buoyant bubbles, which may follow zig-zag and/or helical paths depending on the Reynolds number. The current study explores the behaviour using well resolved numerical simulations. The forces like tension, buoyancy and fluid force are considered. It is found that there exist six different flow regimes within the range of the Reynolds number = [50, 800] according to the sphere response. Regime I (Re = [50, 205]) and Regime II (Re = [210, 260]) are characterised by steady axisymmetric flow structure without body movement except the loss of axisymmetry in Regime II. The sphere starts to vibrate from Regime III (Re = [270, 280]). Regime IV (Re = [300, 330]) shows suppressed body oscillation and steep decrease of off-centered distance in the plane normal to streamwise direction (yz plane). In Regime V (Re = [335, 550]), the sphere oscillates around (0, 0) in yz plane. The sphere of Regime VI (Re = [600, 800]) oscillates rather irregularily. The transitions are compared with those for a fixed sphere. In addition, the effect of moving away from neutral buoyancy is examined

Experimental investigation of in-line flow-induced vibration of a rotating circular cylinder

This study experimentally investigates the in-line flow-induced vibration (FIV) of an elastically mounted circular cylinder under forced axial rotation in a free stream. The present experiments characterise the structural vibration, fluid forces and wake structure of the fluid–structure system at a low mass ratio (the ratio of the total mass to the displaced fluid mass) over a wide parameter space spanning the reduced velocity range

Simulation of the Control of Vortex Breakdown in a Closed Cylinder Using a Small Rotating Disk

The enhancement or suppression of vortex breakdown in a closed cylinder caused by a small rotating disk embedded in the nonrotating endwall is simulated in this study. This paper shows that corotation or counter-rotation of the control disk with respect to the driving lid is able to promote or suppress the “bubble-type” vortex breakdown. This is achieved using only a small fraction of the power required to drive the main lid. The simulations show that the vortex breakdown induced or suppressed by flow control displays similar characteristics near the breakdown region as produced by varying the flow Reynolds number. These include near-axis swirl, centerline axial velocity, and centerline pressure. The influence of the size of the control disk is also quantified

Vortex-induced vibration of a transversely rotating sphere

Vortex-induced vibration (VIV) of a sphere is one of the most basic fluid-structure interaction problems. Since such vibrations can lead to fatal structural failures, numerous studies have focused on suppressing such flow-induced vibrations. In this study, for the first time, the effect of an imposed transverse rotation on the dynamics of the VIV of an elastically mounted sphere has been investigated. It was observed that the non-dimensional vibration amplitude for a rotating sphere (A∗ = √2yrms/D, where yrms is the root mean square of the displacement in the transverse direction and D = sphere diameter) exhibits a bell-shaped evolution as a function of reduced velocity, similar to the classic VIV response of a non-rotating sphere. The sphere is found to oscillate freely up to a rotation ratio α (ratio of the equatorial velocity of the sphere to the free-stream velocity) close to 0.5. For lower rotation ratios (α ≤ 0.3), the response looks similar to the non-rotating case but with slightly smaller vibration amplitude. For higher α values, the amplitude was found to decrease significantly with the rotation up to α = 0.5. The amplitude dropped drastically after it reached the peak amplitude. This is unlike the VIV response of a rotating circular cylinder where the vibration amplitude increases up to three times the maximum vibration amplitude in the non- rotating case due to a novel asymmetric wake pattern (see [1]

Effect of Central Body Size on the Leading Edge Vortex of a Rotating Insect Wing

The stable attachment of a leading-edge vortex (LEV) is responsible for the high lift observed from insect wings. In experiments, we study the flow structure over a model wing mounted on a central body. The diameter of the central body and the change in Rossby number (Ro) due to placement of the wing root away from the centre can affect the flow structure. Normally, the LEV splits to form dual LEVs in a rotating wing, with the spanwise split location changing with Reynolds number. The results presented here show that the LEV structure is minimally affected by changes in the central body size for a wide range of body sizes

REPORTAJE FOTOGRÁFICO DEL TABOR DE AGÜIMES [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201

Vortex-induced vibration of a rotating sphere

Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–structure interaction problems. Since vortex-induced vibration can lead to structural failure, numerous studies have focused on understanding the underlying principles of VIV and its suppression. This paper reports on an experimental investigation of the effect of imposed axial rotation on the dynamics of vortex-induced vibration of a sphere that is free to oscillate in the cross-flow direction, by employing simultaneous displacement and force measurements. The VIV response was investigated over a wide range of reduced velocities (i.e. velocity normalised by the natural frequency of the system): 3 U∗ 18, corresponding to a Reynolds number range of 5000 < Re < 30 000, while the rotation ratio, defined as the ratio between the sphere surface and inflow speeds, α = |ω|D/(2U), was varied in increments over the range of 0 α 7.5. It is found that the vibration amplitude exhibits a typical inverted bell-shaped variation with reduced velocity, similar to the classic VIV response for a non-rotating sphere but without the higher reduced velocity response tail. The vibration amplitude decreases monotonically and gradually as the imposed transverse rotation rate is increased up to α = 6, beyond which the body vibration is significantly reduced. The synchronisation regime, defined as the reduced velocity range where large vibrations close to the natural frequency are observed, also becomes narrower as α is increased, with the peak saturation amplitude observed at progressively lower reduced velocities. In addition, for small rotation rates, the peak amplitude decreases almost linearly with α. The imposed rotation not only reduces vibration amplitudes, but also makes the body vibrations less periodic. The frequency spectra revealed the occurrence of a broadband spectrum with an increase in the imposed rotation rate. Recurrence analysis of the structural vibration response demonstrated a transition from periodic to chaotic in a modified recurrence map complementing the appearance of broadband spectra at the onset of bifurcation. Despite considerable changes in flow structure, the vortex phase (φvortex), defined as the phase between the vortex force and the body displacement, follows the same pattern as for the non-rotating case, with the φvortex increasing gradually from low values in Mode I of the sphere vibration to almost 180◦ as the system undergoes a continuous transition to Mode II of the sphere vibration at higher reduced velocity. The total phase (φtotal), defined as the phase between the transverse lift force and the body displacement, only increases from low values after the peak amplitude response in Mode II has been reached. It reaches its maximum value (∼165◦) close to the transition from the Mode II upper plateau to the lower plateau, reminiscent of the behaviour seen for the upper to lower branch transition for cylinder VIV. Hydrogen-bubble visualisations and particle image velocimetry (PIV) performed in the equatorial plane provided further insights into the flow dynamics near the sphere surface. The mean wake is found to be deflected towards the advancing side of the sphere, associated with an increase in the Magnus force. For higher rotation ratios, the near-wake rear recirculation zone is absent and the flow is highly vectored from the retreating side to the advancing side, giving rise to large-scale shedding. For a very high rotation ratio of α = 6, for which vibrations are found to be suppressed, a one-sided large-scale shedding pattern is observed, similar to the shear-layer instability one-sided shedding observed previously for a rigidly mounted rotating sphere

Canadian physiotherapists' views on certification, specialisation, extended role practice, and entry-level training in rheumatology

<p>Abstract</p> <p>Background</p> <p>Since the last decade there has been a gradual change of boundaries of health professions in providing arthritis care. In Canada, some facilities have begun to adopt new arthritis care models, some of which involve physiotherapists (PT) working in extended roles. However, little is known about PTs' interests in these new roles. The primary objective of this survey was to determine the interests among orthopaedic physiotherapists (PTs) in being a certified arthritis therapist, a PT specialized in arthritis, or an extended scope practitioner in rheumatology, and to explore the associated factors, including the coverage of arthritis content in the entry-level physiotherapy training.</p> <p>Methods</p> <p>Six hundred PTs practicing in orthopaedics in Canada were randomly selected to receive a postal survey. The questionnaire covered areas related to clinical practice, perceptions of rheumatology training received, and attitudes toward PT roles in arthritis care. Logistic regression models were developed to explore the associations between PTs' interests in pursuing each of the three extended scope practice designations and the personal/professional/attitudinal variables.</p> <p>Results</p> <p>We received 286 questionnaires (response rate = 47.7%); 258 contained usable data. The average length of time in practice was 15.4 years (SD = 10.4). About 1 in 4 PTs agreed that they were interested in assuming advanced practice roles (being a certified arthritis therapist = 28.9%, being a PT specialized in rheumatology = 23.3%, being a PT practitioner = 20.9%). Having a caseload of ≥ 40% in arthritis, having a positive attitude toward advanced practice roles in arthritis care and toward the formal credentialing process, and recognizing the difference between certification and specialisation were associated with an interest in pursing advanced practice roles.</p> <p>Conclusion</p> <p>Orthopaedic PTs in Canada indicated a fair level of interest in pursuing certification, specialisation and extended scope practice roles in arthritis care. Future research should focus on the effectiveness and cost-effectiveness of the emerging health service delivery models involving certified, specialized or extended scope practice PTs in the management of arthritis.</p