Daniele Violato

Three-dimensional evolution of flow structures in transitional circular and chevron jets

The three-dimensional behavior of flow transition in circular and 6-chevron jets at Re = 5000 is investigated with experiments conducted on a free water jet by time-resolved tomographic particle image velocimetry. The emphasis is on the unsteady organization of coherent flow structures, which play a role in the generation of acoustic noise. Shedding and pairing of vortices are the most pronounced phenomena observed in the near field of the circular jet. The first and second pairing amplify the axial pulsatile motion in the jet column and lead to the growth of azimuthal waves culminating in the breakup of the vortex ring. Streamwise vortices of axial and radial vorticity are observed in the outer region and move inward and outward under the effect of the vortex rings. In the jet with chevrons, the axisymmetric ring-like coherence of the circular jet is not encountered. Instead, streamwise flow structures of azimuthal vorticity emanate from the chevron apices, and counter-rotating streamwise vortices of axial and radial vorticity develop from the chevron notches. The decay of streamwise vortices is accompanied by the formation of C-shaped structures. The three-dimensional analysis allows quantifying the vortex stretching and tilting activity, which, for the circular jet exit, is related to the azimuthal instabilities and the streamwise vortices connecting the vortex rings. In the chevron jet, stretching and tilting peak during the formation of C-structures. Following Powell’s aeroacoustic analogy, the spatial distribution of the source term is mapped, evaluating the temporal derivative of the Lamb vector. The spatio-temporal evolution of such source term is visualized revealing that the events of highest activity are associated with the processes of vortex-ring pairing and vortex-ring disruption for the circular jet, and with the decay of streamwise instabilities and the formation of C-shaped structures for the chevron case.

Three-dimensional vortex analysis and aeroacoustic source characterization of jet core breakdown

The three-dimensional behavior of jet core breakdown is investigated with experiments conducted on a free water jet at Re = 5000 by time-resolved tomographic particle image velocimetry (TR-TOMO PIV). The investigated domain encompasses the range between 0 and 10 jet diameters. The characteristic pulsatile motion of vortex ring shedding and pairing culminates with the growth of four primary in-plane and out-of-plane azimuthal waves and leads to the formation of streamwise vortices. Vortex ring humps are tilted and ejected along the axial direction as they are subjected to higher axial velocities. By the end of the potential core, this process causes the breakdown of the vortex ring regime and the onset of streamwise filaments oriented at 30°-45° to the jet axis and “C” shaped peripheral structures. The latter re-organize further downstream in filaments oriented along the azimuthal direction at the jet periphery. Instead, in the vicinity of the jet axis the filaments do not exhibit any preferential direction resembling the isotropic turbulent regime. Following Powells aeroacoustic analogy, the instantaneous spatial distribution of the acoustic source term is mapped by the second time derivative of the Lamb vector, revealing the highest activity during vortex ring breakdown. A three-dimensional modal analysis of velocity, vorticity, Lamb vector, and Lamb vector second time derivative fields is conducted by proper orthogonal decomposition (POD) within the first 10 modes. The decomposed velocity fluctuations describe a helical organization in the region of the jet core-breakdown and, further downstream, jet axis flapping and precession motions. By the end of the potential core, vorticity modes show that vortex rings are dominated by travelling waves of radial and axial vorticity with a characteristic 40°-45° inclination to the jet axis. The Lamb vector and the Lamb vector second time derivative modes exhibit similar patterns for the azimuthal component, whereas the vortex ring coherence is described by the radial and the axial components. While velocity, vorticity, and Lamb vector modes are typically associated with Strouhal numbers (St) smaller than 0.9, the modes of the Lamb vector second time derivative are also related to higher frequencies (1.05 ? St ? 1.9) ascribed to the three-dimensional travelling waves. Far-field acoustic predictions are obtained on the basis of direct evaluation of Powells analogy with TR-TOMO PIV data. The spectral analysis returns peaks at pairing (St = 0.36) and shedding (St = 0.72) frequency. A broader distribution with a hump between St = 1 and 2.25 is observed, which corresponds to the breakdown of ring vortices.

Application of Powell's analogy for the prediction of vortex-pairing sound in a low-Mach number jet based on time-resolved planar and tomographic PIV

This paper describes an experimental investigation by time-resolved planar and tomographic PIV on the sound production mechanism of vortex pairing of a transitional water-jet flow at Re=5000. The shear layer is characterized by axisymmetric vortex rings which undergo pairing with a varicose mode. Three-dimensional measurements show the presence of longitudinal pairs of counter-rotating vortices inducing vortex azimuthal instabilities prior to the breakdown of the vortices. Based on Powells aeroacoustic analogy, flow structures responsible for noise generation are characterized by the second-time-derivative of the Lamb vector field, which is directly evaluated by planar and tomographic PIV. The analysis of the dynamics of such structures shows peak activity in correspondence of the vortex cores during the leapfrogging, vortex-azimuthal instabities and vortex breakdown mechanism. Under the hypothesis of axisymmetric flow, far field acoustic produced by vortex pairing is predicted by directly applying time-resolved planar PIV data to Powells acoustic analogy. Pronounced acoustic emission is found during the coalescence of the vortices.

On the suitability of direct application of acoustic theory to time-resolved tomographic PIV tested by DNS for low Mach number jet flows

In this paper, we investigate the minimum temporal requirements, spatial domain and spatial resolution requirements in order to make acoustic predictions by means of acoustic analogies, for low speed jet flows, by means of numerical simulations. This work is done to demonstrate the feasibility of indirect acoustic extrapolation from recent experiments undertaken by us and to develop robust techniques. For this paper we consider Direct Numerical Simulation of incompressible jets flows of Reynolds number 2,500 and Reynolds number 5,000 jets. Comparison of flow statistics and visualizations confirms that the simulations are representative of the jets obtained in our experiments. Lighthill’s and Powell’s aerocaoustic analogies are applied and demonstrated to yield indirect acoustic predictions of these jets, especially at 90o to the flow axis, where the coordinate origin dependency of Powell’s acoustic analogy is minimal. Finally we investigate the robustness of these indirect acoustic predictions when the DNS simulation data is reduced in temporal and spatial extent and resolution to those currently achievable by our Tomographic PIV setup. The results indicate that our current measurement capabilities are suitable for obtaining acoustic spectra up to a Strouhal number of 1.0. Moreover, in order to improve the spectral cut-off, it is preferable to increase the spatial resolution of the measurements at the expense of domain size and temporal resolution.

Investigation on Circular and Chevron Impinging Jets by IR Thermography and Time-Resolved Tomographic PIV

This paper describes an experimental investigation on impinging jets issued through a circular and a chevron nozzle by means of IR thermography and time-resolved tomographic particle image velocimetry. Measurements are performed at Reynolds number of 5000, based on the flow rate and on the nozzle diameter D. Time-resolved tomographic experiments are performed at kilo-hertz repetition rate in a tailored water jet facility where a plate is placed 4 D off the nozzle. The measurements are done at the nozzle exit in a cylindrical volume with a diameter of 4.5 D and an axial length of 4 D. Time-averaged heat transfer measurements of the convective heat transfer on the plate are performed by means of IR thermography with the heated-thin-foil heat transfer sensor for nozzle-to-plate distances ranging from 2 D to 10 D. Heat transfer measurements are shown in terms of Nusselt number (Nu) maps. The instantaneous three-dimensional organization of coherent structures is described for both the configurations. The heat transfer performances of the chevron jet are compared with those of the circular one, analyzing the influence of the nozzle-to-plate distance on the distribution of Nu. The most relevant issues for the chevron jet and the relation between the flow organization and the heat transfer are investigated.Copyright

Low-speed jet dynamics and sound radiation

Experimental velocity measurements of a low-speed jet, performed using time-resolved tomographic PIV, are used to study the dynamics of large-scale structures and their sound radiation. The experimental results show the roll-up of axisymmetric vortices that pair downstream, and subsequently lose their azimuthal coherence. Models of linear instability waves using both steady laminar and mean-field base flows flow are applied. While good agreement can be obtained for the vortex roll-up frequency in the near-nozzle region using the laminar base flow, non-linear effects must be included, via the mean field, in order to capture the downstream evolution of both the fundamental and subharmonic (vortex pairing). The velocity fluctuations for both frequencies have a wave-packet structure with some jitter in the form of modulations of the spatial extent and amplitude of the envelope. The sound radiation is modelled using a jittering wave-packet model, an shows agreement with the exponential directivity shape of Laufer and Yen (J. Fluid Mech. 134, 1983).

Characterization of noise sources in a rod-airfoil configuration by means of Time-Resolved Tomographic PIV

Time-resolved Tomographic PIV was used to characterize the flow around the leading edge of a NACA 0012 airfoil in rod-airfoil configuration at ReD = 3500. The volumetric approach at relatively high temporal resolution allows the measurement of the evolution of the 3D vortical structures constituting the Karman wake of the rod at interaction with the airfoil. The pressure gradient is reconstructed exploiting the definition of Lagrangian derivative and the pressure field is obtained by spatial integration of the Poisson equation. Time-correlation is performed between the aeroacoustic sources of Curles aeroacoustic analogy and Vortex Sound Theory represented by respectively, time derivative of the pressure fluctuations at the surface and time derivative of the Lamb vector in the surrounding flow. The region of highest correlation is located underneath the airfoil leading edge and appears not to coincide with the region of most intense vortex stretching.