| 1. |
- Lacagnina, Giovanni, et al.
(författare)
-
Effects of spatial resolution on PIV investigation of a turbulent orifice jet
- 2012
-
Ingår i: 16th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon.
-
Konferensbidrag (refereegranskat)abstract
- Some of the most interesting turbulent phenomena are jets, the study of which is useful to reach a deep knowledge of mixing in fluids. Among all jets, there exists one, the orifice jet, which is still not fully understood because of its intrinsic complexity, e.g. the "vena contracta" phenomenon, though its wide range of applications ranging over underwater propulsion, pollution dispersion and studies on the cardiocirculatory system. In this work, the attention is focused to the large-scale statistics (mean field and higher statistical moments of the velocity) and to the small-scale statistics (velocity derivative statistical moments), with a special care regarding the effect of spatial resolution on Particle Image Velocimetry (PIV) measurements. To investigate the effect of spatial resolution on the measurements, framed areas of different size have been considered by changing the magnification factor (namely the object plane distance). Making reference to the orifice diameter D, squared regions 2D x 2D, 3D x 3D, 4D x 4D and 5D x 5D have been framed on the acquisition camera. Regarding to the large scales, first of all the occurrence of the vena contracta phenomenon for an orifice jet has been shown in several ways. In addition, it has been highlighted the small influence of spatial resolution on low order statistical moments, which slightly rises, due to local averaging, as the order of the statistics becomes higher. Then small scale features of the jet have been examined, particularly with interest to the evaluation of spatial derivatives of velocity components and then to the fulfillment of isotropy hypotheses of the jet. Based on those data, an incomplete local isotropy condition is attained, although restricted to the near zone, and an influence of spatial resolution high on the determination of local features of such a jet and moderate on the overall behavior of spatial derivatives and to the deductions about symmetry hypotheses.
|
|
| 2. |
- Lacagnina, Giovanni, 1982-, et al.
(författare)
-
PIV investigations on optical magnification and small scales in the near-field of an orifice jet
- 2015
-
Ingår i: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 56
-
Tidskriftsartikel (refereegranskat)abstract
- Among turbulent jets, the orifice jet is still far from being fully understood especially due to the phenomenon of “vena contracta”, which takes place in the near field. In this work, the attention is focused to large-scale statistics (mean field and higher statistical moments of the velocity) and to small-scale statistics (velocity derivative statistical moments) of such a jet, with a special focus onto the effect of optical magnification on particle image velocimetry measurements. In order to investigate this effect on the measurements, framed areas of different size have been considered by changing the magnification factor through the object plane distance. The obtained results show that a limit on the choice of this parameter exists for a proper description of both small and large scales.
|
|
| 3. |
- Lacagnina, Giovanni, 1982-, et al.
(författare)
-
Simultaneous size and velocity measurements of cavitating microbubbles using interferometric laser imaging
- 2011
-
Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 50, s. 1153-1167
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, interferometric laser imaging droplet sizing—ILIDS—is applied to incipient cavitation in the wake of a marine propeller model with the aim to evaluate simultaneously bubbles velocity and diameter. Until now, the feasibility of this technique has been demonstrated especially in sprays of water droplets in air where an optimal light scattering is obtained thanks to the spherical shape and to the given relative refractive index. In the present setup, to allow simultaneous size–velocity measurements, a single camera is used and the object distance over lens diameter ratio is kept as small as possible, thus increasing the size measurement resolution. These details, together with the algorithms used for image analysis at each single frame and in two consecutive frames, allow deriving cavitation bubble size and velocity distributions in the propeller wake.
|
|
| 4. |
- Sinibaldi, Giorgia, et al.
(författare)
-
Aeroacoustics and aerodynamics of impinging supersonic jets : Analysis of the screech tones
- 2013
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 25:8
-
Tidskriftsartikel (refereegranskat)abstract
- The interaction between acoustics and aerodynamics of a supersonic jet is an actual fundamental topic which has been a matter of discussion in the last decades. The present paper is devoted to the experimental analysis of free and impinging jets with particular attention on the effect of an impinging surface on screech tones. The acoustics is studied using free-field microphones, while Particle Image Velocimetry is used to investigate the velocity field. The analysis of acquired data allowed to verify and explain the coupling between acoustic discrete tones and mean and fluctuating flow velocities.
|
|
| 5. |
- Sinibaldi, Giorgia, et al.
(författare)
-
Analysis of the Characteristic Acoustic Tones of an Impinging Jet
- 2013
-
Ingår i: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134
-
Konferensbidrag (refereegranskat)abstract
- The acoustic emission of a supersonic jet is an interesting and fundamental issue due to the several technological applications and the diverse physical aspects to be still clarified. An experimental analysis of the characteristic acoustic tones of a supersonic impinging jet has been carried out with a particular attention on the effect of the nozzle pressure ratio and the distance of the impinging surface. The screech tones, typical of an imperfectly expanded supersonic free jet, have been first investigated to have an idea of the deflection from the free jet behavior due to the impinging surface. Then the analysis has been focused on smaller distances of the impinging plate from the jet exit to evaluate the behavior of the impinging tones. The difference between screech and impinging tones, due to the different interactions in the generating feedback mechanism, has been observed.
|
|
| 6. |
- Berk, Tim, et al.
(författare)
-
Simultaneous surface pressure and high-speed PIV measurements in stalled airfoil
- 2018
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The time-resolved velocity field (2D2C high-speed PIV) and surface pressure (pressure taps) of a stalled NACA0012 airfoil have been simultaneously measured. These measurements enable cross-evaluation of the velocity field and pressure time series. The present paper evaluates the flow field and surface pressure using POD, cross-correlation and conditional averaging. First, the flow and surface pressure are analysed independently. Both the time-average and the dynamic structures of the flow are presented. Distinct frequency bands are observed in the energy spectra of the surface-pressure signal. It is shown that a high-pressure event at the foremost pressure port (at x/c = 0.34) is followed by high-pressure events at the other pressure ports (x/c = 0.51-0.93), indicating a decaying pressure wave that travels over the surface with roughly half the free-stream velocity. Next, the link between the flow field and the surface pressure is explored. Using cross-correlation, it is shown that the pressure signal correlates with specific regions in the flow. Conditional averages of the flow fields, conditioned to high-pressure events at the surface, indicate that these pressure events are caused by coherent structures in the flow. These structures consist of a train of alternating vortices that induce velocity components toward and away from the surface, causing high-and low-pressure events at this surface. By filtering the pressure signal with different frequency bands, the flow structures responsible for different peaks in the energy spectra are recovered. A POD analysis indicates that the energy of the coherent flow structures leading to surface-pressure fluctuations only contain a fraction of the total energy of the flow. While POD and cross-correlation are applied to a full time-series, the conditional averages show that coherent structures in the flow can be identified in real-time using the surface pressure. Identification of coherent structures in the flow using only the surface pressure signal enables real-time control of such structures.
|
|
| 7. |
|
|
| 8. |
- Hyvärinen, Ann, et al.
(författare)
-
A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
- 2018
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 21:8, s. 605-617
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, the wake development behind small-scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind-tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind-turbine models was about 1000times smaller than full-size turbines, suggesting that the present results should only be qualitatively extrapolated to real-field scenarios. Wind-tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind-turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed-up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill-induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind-farm site to accurately capture the downstream wake development.
|
|
| 9. |
|
|
| 10. |
- Lacagnina, Giovanni, 1982-, et al.
(författare)
-
Experimental study on the forcing design for an intermittent injection
- 2018
-
Ingår i: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 59:8
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we consider the effects of acceleration and deceleration on the forcing of an intermittent jet. This experimental study specifically focuses on the effect of the acceleration and deceleration on the mixing of an intermittent jet with the ambient fluid and on the growth of disturbances that may lead to turbulence. The influence of different injection strategies has been evaluated. The results show that the deceleration phase may be able to contribute significantly to enhance the mixing of the jet with the ambient fluid. This effect is manifested primarily around the tail of the jet, towards the end of injection. The acceleration phase on the other hand has mainly impact at the leading part of the jet, where the leading part of the jet forms a mushroom shaped structure with minor mixing effect.
|
|
| 11. |
- Lacagnina, Giovanni, et al.
(författare)
-
Leading edge serrations for the reduction of aerofoil separation self-noise
- 2017
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents an experimental investigation into the use of LE serrations for the reduction of trailing edge self-noise, at least for the NACA-65 aerofoil family. It is shown that the leading edge serrations are able to reduce the self-noise in a low frequency range at small and negative angles of attack. The exact mechanism of this reduction is still not completely discovered, but the LE serrations are discovered able to modulate the mean velocity field and turbulent velocity spectrum in that range of frequencies, as well as to dampen the effect of the angle of attack on the pressure field and to reduce its coherence. We emphasise that this paper represents work in progress and further investigations are still necessary in order to completely understand the dynamics behind this reduction.
|
|
| 12. |
- Lacagnina, Giovanni, 1982-, et al.
(författare)
-
Mechanisms of airfoil noise near stall conditions
- 2019
-
Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 4:12
-
Tidskriftsartikel (refereegranskat)abstract
- The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈ 2× 10^5 – 4× 10^5. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation.
|
|
