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| 2. |
- Brunet, P., et al.
(författare)
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Control of thermocapillary instabilities far from threshold
- 2005
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 17:10
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Tidskriftsartikel (refereegranskat)abstract
- We report experiments on control of thermocapillary instabilities at high temperature differences, in an annular geometry. Previous studies [Phys. Fluids 14, 3039 (2002)] showed that a reasonable control of oscillatory instability could be achieved by optimizing a local heating feedback process. We conducted experiments with a basic flow converging from periphery to center. This constitutes a more unstable configuration than previously, and enables appearance of higher-order instabilities and chaos. Applying successfully local feedback control to the periodic state close to the threshold, we extend the process to higher temperature differences, where nonlinear as well as proportional/derivative control laws are necessary to obtain a significant decrease of the temperature fluctuations. Finally, proportional control allows us to synchronize a chaotic state, to a periodic one.
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| 3. |
- Castro, Ian P., et al.
(författare)
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Outer-layer turbulence intensities in smooth- and rough-wall boundary layers
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 727, s. 119-131
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Tidskriftsartikel (refereegranskat)abstract
- Clear differences in turbulence intensity profiles in smooth, transitional and fully rough zero-pressure-gradient boundary layers are demonstrated, using the diagnostic plot introduced by Alfredsson, Segalini & Orlu (Phys. Fluids, vol. 23, 2011, p. 041702) u'/U versus U/U-e, where u' and U are the local (root mean square) fluctuating and mean velocities and U-e is the free stream velocity. A wide range of published data are considered and all zero-pressure-gradient boundary layers yield outer flow u'/U values that are roughly linearly related to U/U-e, just as for smooth walls, but with a significantly higher slope which is completely independent of the roughness morphology. The difference in slope is due largely to the influence of the roughness parameter (Delta U+ in the usual notation) and all the data can be fitted empirically by using a modified form of the scaling, dependent only on Delta U/U-e. The turbulence intensity, at a location in the outer layer where U/U-e is fixed, rises monotonically with increasing Delta U/U-e which, however, remains of O(1) for all possible zero-pressure-gradient rough-wall boundary layers even at the highest Reynolds numbers. A measurement of intensity at a point in the outer region of the boundary layer can provide an indication of whether the surface is aerodynamically fully rough, without having to determine the surface stress or effective roughness height. Discussion of the implication for smooth/rough flow universality of differences in outer-layer mean velocity wake strength is included.
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| 4. |
- Elofsson, P. A., et al.
(författare)
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An experimental study of oblique transition in a Blasius boundary layer flow
- 2000
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Ingår i: European journal of mechanics. B, Fluids. - 0997-7546 .- 1873-7390. ; 19:5, s. 615-636
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Tidskriftsartikel (refereegranskat)abstract
- Transition initiated by a pair of oblique waves was investigated experimentally in a Blasius boundary layer how by using hot-wire measurements and flow visualisation. The oblique waves were generated by periodic blowing and suction through an array of pipes connecting to the flow through a transverse slit in the flat plate model. The structure of the flow held is described and the amplitude of individual frequency-spanwise wave number modes was determined from Fourier transforms of the disturbance velocity. In contrast to results from investigations of oblique transition at subcritical flow conditions, the transition process at the present conditions suggests the combined effect of non-modal growth of streaks and a second stage with exponential growth of oblique waves to initiate the final breakdown stage.
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| 5. |
- Facciole, L., et al.
(författare)
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Swirling jets issued from fully developed rotating pipe flow : Experiments and numerics
- 2005
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Konferensbidrag (refereegranskat)abstract
- Direct Numerical Simulation (DNS) and experimental data are used to study a rotating jet flow. A non-confined swirling jet is generated by a fully developed rotating turbulent pipe flow. Previous experiments have demonstrated the presence of a counter-rotating core appearing approximatively 6 diameters downstream the pipe outlet. The mean azimuthal velocity component changes its sign in the central part of the jet starting to move in the opposite direction with respect to the rotation imposed by the rotation of the pipe. The present paper introduces new investigations intended to analyse the jet flow in the proximity of this phenomenon.
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| 6. |
- Ford, C. L., et al.
(författare)
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Vortex-meter design : The influence of shedding-body geometry on shedding characteristics
- 2018
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Ingår i: Flow Measurement and Instrumentation. - : Elsevier Ltd. - 0955-5986 .- 1873-6998. ; 59, s. 88-102
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Tidskriftsartikel (refereegranskat)abstract
- The periodic vortex shedding from bluff bodies may be used in flow metering applications. However, because the bluff-body is highly confined (typically in a pipe) the shed vortices may interact with the pipe wall; causing an undesirable non-linear behaviour. An experimental investigation has been conducted; examining the vortex-shedding characteristics of highly confined bluff-bodies in pipe flow, at high Reynolds number (ReD=4.4×104 to 4.4×105). The bluff-bodies were comprised of a forebody and tail; both of which affected the primary-shedding characteristics. The shedders typically produced two unsteady modes: Mode-I was associated with the vortex shedding and mode-II resulted from a separation of the pipe-wall boundary layer. The mode-I behaviour allowed two classes of shedder to be defined: long-tails and short-tails. Modes I and II interacted, particularly for long-tailed geometries. When the length-scale of mode-II exceeded 0.8κ (where κ is the physical scale of the primary shedding vortex), mode-II disrupted mode-I, as the mode-frequency ratio (fII/fI) approached an integer value. The coupling of modes I and II caused mode-I to deviate from its preferred Strouhal number. When the deviation exceeded 25–30%, mode-I locked on to the mode-II frequency. This did not happen for short-tailed geometries, as the length-scale of mode-I was always dominant. Mode-coupling for short-tails occurred only when the mode frequencies were equal.
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| 7. |
- Haggmark, C. P., et al.
(författare)
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Experiments on a two-dimensional laminar separation bubble
- 2000
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Ingår i: Philosophical Transactions. Series A. - 1364-503X .- 1471-2962. ; 358:1777, s. 3193-3205
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Tidskriftsartikel (refereegranskat)abstract
- A two-dimensional separation bubble on a flat plate is studied experimentally by means of hot-wire anemometry and flow visualization. Separation of the laminar boundary layer on the plate is caused by an adverse pressure gradient imposed by a curved wall opposite to the plate. The instability of, and transition process in, the separation bubble are focused on. The bubble is found to be highly susceptible to high-frequency two-dimensional instability waves, which are studied under both natural and forced conditions. A similar development of these instability waves in the separation bubble is found in both cases. The exponential growth of the two-dimensional disturbances dominates the flow except for in the reattachment region, where large-scale three-dimensional structures appear. Some difficulties associated with experimental investigations of boundary-layer separation-bubble flows are discussed.
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| 8. |
- Haggmark, C. P., et al.
(författare)
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Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble
- 2000
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 28:3, s. 236-242
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Tidskriftsartikel (refereegranskat)abstract
- Measurements with a directional sensitive hot-wire probe have been carried out in a two-dimensional laminar separation bubble caused by an adverse pressure gradient. The probe has three parallel, in plane wires and can be traversed in the boundary layer in all spatial directions. The central wire, operated as a conventional hot-wire in CTA mode, and two surrounding resistance wires measure the instantaneous magnitude and direction of the flow, respectively. The probe is calibrated and operated in a similar way as a single hot-wire probe for boundary layer measurements. The frequency response is high enough for measurements of naturally occurring instability waves in the bubble. The flow direction intermittency was measured inside the bubble and regions with reversed flow were mapped out. Prior to reattachment periodical oscillations of the flow direction are found associated with shedding of vortical structures from the bubble.
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| 9. |
- Hyvärinen, Ann
(författare)
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Wind turbines over a hilly terrain: performance and wake evolution
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this licentiate thesis is to investigate wind-turbines placed in a complex-terrain environment. This is done by studying the flow around small-scale wind-turbine models placed over a landscape model with hills, and by comparing the results with corresponding data obtained over a flat terrain model. The studied flow features include the wind-turbine wake development and the turbine performance under different conditions, the effects from wake interactions, the influence of the ambient turbulence levels and the influence from a complex topography. Wind-tunnel measurements have been performed using particle image velocimetry and hot-wire anemometry to measure the velocity field. Additionally, numerical simulations, based on RANS modelling and actuator-disc techniques, have been made to support the experimental data and to gain further knowledge about the investigated flow cases.The results reveal that the hills promote a downward wake deflection behind the turbines and enhance the wind-turbine wake diffusion. As a consequence of this, and with the flow acceleration introduced by the hills, an improved power performance is seen for turbines exposed to wake-interference effects. A correlation is observed between the turbulence levels present in the flow, and the magnitude to which the hill-induced flow gradients influence the wake: Stronger wake deflections due to the hills are seen when the wind-turbine wake is more diffused. This is for instance the case when the wake of two tandem turbines is studied, or when higher ambient turbulence levels are present in the wind tunnel.A good qualitative agreement is seen when comparing the experimental and numerical results. The simulation results further indicate that the hills give rise to modulations of the wind-turbine wake. It is shown that these modulations can be reasonably captured by means of wake-superposition techniques, given that a wake model with sufficient accuracy is chosen.
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| 10. |
- Rueedi, J. -D, et al.
(författare)
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CICLoPE - A Large Pipe Facility for Detailed Turbulence Measurements at High Reynolds Number
- 2010
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Ingår i: PROGRESS IN TURBULENCE III. - Berlin, Heidelberg : SPRINGER-VERLAG BERLIN. - 9783642022241 ; , s. 73-
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Konferensbidrag (refereegranskat)abstract
- High Reynolds number turbulence is ubiquitous in a number of flow of practical interest and crucial to draw conclusions regarding the physics of turbulence. Although recent laboratory experiments, measurements in planetary boundary layer and direct numerical simulations provide a huge amount of information, none of these data sets provide high Reynolds number, high spatial resolution and well converged statistics at the same time. As a response to this problem, an international collaboration between a group of universities and research centers started some years ago to build large scale infrastructures for high Reynolds number experiments. The Center for International Cooperation in Long Pipe Experiments, CICLOPE (www.ciclope.unibo.it) at the University of Bologna, was created for this purpose and will be open to international scientists through different collaboration programs. The laboratory is currently under construction and the first facility, which will be installed there is a large pipe flow experiment that will allow fully resolved turbulence measurements even at high Reynolds number.
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