116 results found
Cicolin M, Bearman P, Assi G, et al., 2023, Bistability in the wake of a circular cylinder with passive control using two leeward rods, Journal of Fluids and Structures, Vol: 120, Pages: 1-14, ISSN: 0889-9746
This paper presents an experimental investigation on the flow around a circular cylindercontrolled by a pair of rods placed close to the separated shear layers from the leewardface of the cylinder. Past research of Cicolin et al. (2021) has shown that one control rod,placed at specific positions, induces a late flow separation on the main cylinder’s surfacein a typical occurrence of the Coandă effect. As a result the control rod reduced the meandrag acting on the cylinder but also produced a net mean lift force. We now presentan extension of the aforementioned study to consideration of a pair of symmetricallyarranged control rods, aimed at eliminating the mean lift force whilst maintaining a dragreduction. The experiments were carried out at a Reynolds number Re = 20, 000 basedon the diameter of the main cylinder D, with the diameter of the control rods being tentimes smaller than that of the cylinder. The centre-to-centre distances between eachcontrol rod and the cylinder was 0.7D, and the angular position of the rods varied fromθ = 120◦to 125◦, where θ is measured from the front stagnation point. Time-resolvedPIV velocity fields and hydrodynamic forces were measured for the different setups.Results show that the mean flow is asymmetric in spite of the symmetry of the geometricmodel and position of the rods. The pair of control rods induces a bistable flow, inducedby the imbalance of two colliding jets in the near wake. The dynamics were found tobe random, with the average switching time between stable states depending on theposition of the rods. The mean drag force was reduced by up to 15%, and the mean liftforce was reduced by 80% compared to the cases with a single control rod. It was alsoobserved that the control rod can induce two different types of flow separation fromthe main cylinder, one in which the flow separates from the main cylinder only onceand one in which a small separation bubble forms in proximity to the control rod beforereatta
Cicolin M, Buxton O, Assi G, et al., 2021, The role of separation on the forces acting on a circular cylinder with a control rod, Journal of Fluid Mechanics, Vol: 915, Pages: 1-29, ISSN: 0022-1120
The development of the flow around a circular cylinder with a smaller diameter control rod in close proximity is the subject of this paper. It has long been known that this is an effective way to attenuate regular vortex shedding leading to reductions in its adverse effects on bluff body flow. The aim of this study is to improve understanding of the ways the control rod affects the near wake flow including how it influences the positions of boundary layer separation. Experiments were carried out in a water channel to measure lift and drag forces and PIV was employed to obtain detailed information on flow structure. The values of important properties were fixed as follows: Reynolds number20,000; ratio of cylinder and control rod diameters 10:1; centre to centre distance between main cylinder and control rod 0.7D, where Dis main cylinder diameter. The adjustable parameter was the angular position of the rod,θ, which was varied between 90◦and 180◦from the front stagnation line. Lift and drag forces were measured separately for the main cylinder and the control rod. A new method for identifying flow states is introduced using PIV to interrogate the instantaneous flow velocity in the gap between the main cylinder and the control rod. Similarly to previous studies, three stable flow states were observed together with a bi-stable state. The bi-stable state is very sensitive to the control rod angle with a small change of±1◦being sufficient to change the flow state.
Clapperton B, Bearman PW, 2018, Control of circular cylinder flow using distributed passive jets, Journal of Fluid Mechanics, Vol: 848, Pages: 1157-1178, ISSN: 0022-1120
A wind tunnel study has been carried out to investigate flow control around a hollow circular cylinder using passive jets driven by naturally occurring pressure differences. Flow enters the cylinder through spanwise holes along the stagnation line and exits through a spanwise distribution of holes at ±65∘ . The diameter of the entry and exit holes were 1 % and 0.5 % of the cylinder diameter, respectively. Reynolds numbers were at the upper end of the subcritical regime and ranged from 3×104 to 2.8×105 . Jet spacings of 10 % and 20 % of the cylinder diameter were investigated, and the ratio of the average jet exit velocity to the cross-flow velocity at the boundary layer edge was found to rise to approximately 0.35 and 0.4, respectively, above a Reynolds number of 1.5×105 . Findings based on using the surface oil flow technique revealed a repeating, organised cellular pattern downstream of adjacent jet exit holes consisting of a primary counter-rotating vortex pair structure, followed by a secondary weaker pair. Downstream of adjacent exit holes, and centred midway between them, there exists a separation bubble which delays final flow separation compared with the flow directly downstream of a jet. The variation in the angular position of boundary layer separation across the span had the effect of suppressing von Kármán vortex shedding. This resulted in a drag coefficient, at the upper end of the Reynolds-number range studied, 14.5 % lower than that found using trip wires to initiate boundary layer transition.
Assi G, Bearman PW, 2018, Vortex-induced vibration of a wavy elliptic cylinder, Journal of Fluids and Structures, Vol: 80, Pages: 1-21, ISSN: 0889-9746
This paper shows that three-dimensional separation lines on a wavy cylinder may be correlated by the lateral movement of the body responding to flow-induced excitations. Vortex-induced vibration (VIV) of a wavy elliptic cylinder is investigated by mean of experiments in a water channel in the range of Reynold number between 1,500 to 15,000. Results are compared with those for a plain circular cylinder of equivalent diameter with a combined mass–damping parameter of 0.018. Curves of displacement and frequency of vibration showed that the hydroelastic mechanism that drives the wavy cylinder into VIV is not different from that of a plain cylinder. Detailed decomposition of the fluid forces supports this conclusion. The reason for such similar behaviour is the correlation of the sinuous separation lines as the wavy cylinder starts to oscillate. Flow visualization reveals that the three-dimensional surface of the wavy cylinder affects the formation of vortices in the near wake, generating streamwise and cross-flow vorticity associated with the wavelength of the surface. However, once the cylinder is free to respond to VIV, moving in the cross-flow direction, coherent vortex filaments once more dominate the near wake.
Bearman PW, Gustavo A, 2018, Vortex-induced vibration of a wavy elliptic cylinder, Journal of Fluids and Structures, ISSN: 0889-9746
Assi GRS, Bearman PW, 2015, Transverse galloping of circular cylinders fitted with solid and slotted splitter plates, Journal of Fluids and Structures, Vol: 54, Pages: 263-280, ISSN: 0889-9746
The galloping response of a circular cylinder fitted with three different splitter plates and free to oscillate transverse to a free stream has been investigated considering variations in plate length and plate porosity. Models were mounted in a low mass and damping elastic system and experiments have been carried out in a recirculating water channel in the Reynolds number range of 1500 to 16 000. Solid splitter plates of 0.5 and 1.0 diameter in length are shown to produce severe galloping responses, reaching displacements of 1.8 diameters in amplitude at a reduced velocity of around 8. Fitting a slotted plate with a porosity ratio of 30% also caused considerable vibration, but with a reduced rate of increase with flow speed. All results are compared with the typical vortex-induced vibration response of a plain cylinder. Force decomposition in relation to the body velocity and acceleration indicates that a galloping mechanism is responsible for extracting energy from the flow and driving the oscillations. Visualisation of the flow field around the devices performed with PIV reveal that the reattachment of the free shear layers on the tip of the plates is the hydrodynamic mechanism driving the excitation.
Assi GRS, Bearman PW, Tognarelli MA, 2014, On the stability of a free-to-rotate short-tail fairing and a splitter plate as suppressors of vortex-induced vibration, Ocean Engineering, Vol: 92, Pages: 234-244, ISSN: 0029-8018
Experiments in the Reynolds number range of 1000 to 12,000 have been carried out on a free-to-rotate short-tail fairing fitted to a rigid length of circular cylinder to investigate the effect of rotational friction on the stability of this type of VIV suppressor. Measurements of the dynamic response are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown how vortex-induced vibration can be reduced if the rotational friction between the cylinder and the short-tail fairing exceeds a critical limit. In this configuration the fairing finds a stable position deflected from the flow direction and a steady lift force appears towards the side to which the fairing has deflected. The fluid-dynamic mechanism is very similar to that observed for a free-to-rotate splitter plate of equivalent length. A non-rotating fairing as well as splitter plates is shown to develop severe galloping instabilities in 1-dof experiments.
Assi GRS, Bearman PW, Carmo BS, et al., 2013, The role of wake stiffness on the wake-induced vibration of the downstream cylinder of a tandem pair, JOURNAL OF FLUID MECHANICS, Vol: 718, Pages: 210-245, ISSN: 0022-1120
When a pair of tandem cylinders is immersed in a flow the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. Assi, Bearman & Meneghini (J. Fluid Mech., vol. 661, 2010, pp. 365–401) concluded that the WIV excitation mechanism has its origin in the unsteady vortex–structure interaction encountered by the cylinder as it oscillates across the wake. In the present paper we investigate how the cylinder responds to that excitation, characterising the amplitude and frequency of response and its dependency on other parameters of the system. We introduce the concept of wake stiffness, a fluid dynamic effect that can be associated, to a first approximation, with a linear spring with stiffness proportional to Re and to the steady lift force occurring for staggered cylinders. By a series of experiments with a cylinder mounted on a base without springs we verify that such wake stiffness is not only strong enough to sustain oscillatory motion, but can also dominate over the structural stiffness of the system. We conclude that while unsteady vortex–structure interactions provide the energy input to sustain the vibrations, it is the wake stiffness phenomenon that defines the character of the WIV response.
Bearman PW, 2011, Circular cylinder wakes and vortex-induced vibrations, International Union of Theoretical and Applied Mechanics (IUTAM) Symposium on Bluff Body Wakes and Vortex-Induced Vibrations, Publisher: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, Pages: 648-658, ISSN: 0889-9746
Carmo BS, Sherwin SJ, Bearman PW, et al., 2011, Flow-induced vibration of a circular cylinder subjected to wake interference at low Reynolds number, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 27, Pages: 503-522, ISSN: 0889-9746
Huera-Huarte FJ, Bearman PW, 2011, Vortex and wake-induced vibrations of a tandem arrangement of two flexible circular cylinders with near wake interference, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 27, Pages: 193-211, ISSN: 0889-9746
Assi GRS, Bearman PW, Tognarelli MA, et al., 2011, THE EFFECT OF ROTATIONAL FRICTION ON THE STABILITY OF SHORT-TAILED FAIRINGS SUPPRESSING VORTEX-INDUCED VIBRATIONS, 30th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 389-+
Assi GRS, Bearman PW, Meneghini JR, 2010, On the wake-induced vibration of tandem circular cylinders: the vortex interaction excitation mechanism, JOURNAL OF FLUID MECHANICS, Vol: 661, Pages: 365-401, ISSN: 0022-1120
Assi GRS, Bearman PW, Kitney N, et al., 2010, Suppression of wake-induced vibration of tandem cylinders with free-to-rotate control plates, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 26, Pages: 1045-1057, ISSN: 0889-9746
Huera-Huarte FJ, Bearman PW, 2010, DPIV in the wake of a tandem arrangement of two flexible circular cylinders, JOURNAL OF VISUALIZATION, Vol: 13, Pages: 195-202, ISSN: 1343-8875
Assi GRS, Bearman PW, Meneghini JR, 2010, WAKE-INDUCED VIBRATION OF A PAIR OF CIRCULAR CYLINDERS AND ITS DEPENDENCY ON REYNOLDS NUMBER, 7th Int Symposium on Fluid-Structure Interactions, Flow-Sound Interactions, and Flow-Induced Vibration and Noise/Summer Meeting of the Fluids Engineering Division of ASME, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 233-240
Bearman PW, 2009, Understanding and predicting vortex-induced vibrations, JOURNAL OF FLUID MECHANICS, Vol: 634, Pages: 1-4, ISSN: 0022-1120
Huera-Huarte FJ, Bearman PW, 2009, Wake structures and vortex-induced vibrations of along flexible cylinder-Part 2: Drag coefficients and vortex modes, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 25, Pages: 991-1006, ISSN: 0889-9746
Huera-Huarte FJ, Bearman PW, 2009, Wake structures and vortex-induced vibrations of along flexible cylinder-Part 1: Dynamic response, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 25, Pages: 969-990, ISSN: 0889-9746
Assi GRS, Bearman PW, Kitney N, 2009, Low drag solutions for suppressing vortex-induced vibration of circular cylinders, 5th Conference on Bluff Body Wakes and Vortex-Induced Vibrations (BBVIV-5), Publisher: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, Pages: 666-675, ISSN: 0889-9746
Assi GRS, Bearman PW, 2009, VIV AND WIV SUPPRESSION WITH PARALLEL CONTROL PLATES ON A PAIR OF CIRCULAR CYLINDERS IN TANDEM, 28th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 263-268
Assi GRS, Bearman PW, Meneghini JR, 2008, UNSTEADY RESPONSE OF A CIRCULAR CYLINDER UNDER WAKE-INDUCED EXCITATION FROM A FIXED UPSTREAM CYLINDER, 9th International Conference on Flow-Induced Vibration, Publisher: ACAD SCI CZECH REPUBLIC, INST THERMOMECHANICS, Pages: 279-+
Assi GRS, Bearman PW, 2008, VIV SUPPRESSION AND DRAG REDUCTION WITH PIVOTED CONTROL PLATES ON A CIRCULAR CYLINDER, 27th International Conference on Offshore Mechanics and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 871-874
Huera Huarte FJ, Bearman PW, 2008, WAKE STRUCTURES AND DYNAMIC RESPONSE OF ONE AND TWO LONG FLEXIBLE CYLINDERS UNDERGOING VORTEX-INDUCED VIBRATIONS, 9th International Conference on Flow-Induced Vibration, Publisher: ACAD SCI CZECH REPUBLIC, INST THERMOMECHANICS, Pages: 419-+
Bruno S Carmo, Spencer J Sherwin, Peter W Bearman, et al., 2008, Wake Transition in the flow around two circular cylinders in staggered arrangements, Journal of Fluid Mechanics, Vol: 597, Pages: 1-29
The wake transition of the flow around two circular cylinders placed in staggered arrangements with fixed streamwise separation of 5D and cross stream separation varying from 0D to 3D has been studied. The wake transition is compared to that of a single isolated cylinder. Linear stability analysis utilising Floquet theory and direct numerical simulations using a spectral/hp element spatial discretisation were carried out. The unstable modes that first appear in the wake transition of the flow around a single cylinder, which are the long spanwise wavelength mode A and the short spanwise wavelength mode B, are also found in the flow around the staggered arrangements. However, a third mode, referred to as mode C, is also present in the wake transition of the flow around staggered arrangements, depending on the relative positioning of the cylinders. This mode has an intermediate spanwise wavelength and period-doubling character. The structure and onset characteristics of mode C are analysed and the non-linear character of the bifurcation for this mode is investigated.
Zhu G, Bearman PW, Graham JMR, 2007, Prediction of drag and lift of wings from velocity and vorticity fields, AERONAUTICAL JOURNAL, Vol: 111, Pages: 699-704, ISSN: 0001-9240
Assi GRS, Bearman PW, Meneghini JR, 2007, Unsteady force measurements on a responding circular cylinder in the wake of an upstream cylinder, 26th International Conference on Offshore Mechanics and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 633-639
Bearman PW, Chaplin JR, Fontaine E, et al., 2007, Comparison of CFD predictions of multi-mode vortex-induced vibrations of a tension riser with laboratory measurements, 6th Symposium on Fluid-Structure Interactions, Aeroelasticity and Flow-Induced Vibration and Noise, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 147-156
Bearman PW, Huera-Huarte FJ, Chaplin JR, 2007, The hydrodynamic forces acting on a long flexible circular cylinder responding to VIV, 6th Symposium on Fluid-Structure Interactions, Aeroelasticity and Flow-Induced Vibration and Noise, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 611-619
Brankovic M, Bearman PW, 2006, Measurements of transverse forces on circular cylinders undergoing vortex-induced vibration, 4th Conference on Bluff Body Wakes and Vortex-Induced Vibrations, Publisher: ACADEMIC PRESS LTD ELSEVIER SCIENCE LTD, Pages: 829-836, ISSN: 0889-9746
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