Porous trailing edges attenuate hydrodynamic pressure fluctuations that scatter as trailing-edge noise, with their effectiveness governed by their material parameters. Conventional measurements of permeability rely on steady-flow rigs, which cannot capture the dynamic response of this parameter, which is relevant for predicting balancing pressure fluctuations under grazing-flow conditions. In this study, we introduce a method for directly determining the dynamic permeability of porous trailing edges from time-resolved particle image velocimetry (PIV) data. The approach employs a lumped-system circuit analogy that links unsteady pressure gradients to through-material velocities, enabling in situ characterisation without specialised porous rigs, thereby further extending its applicability to thin trailing-edge geometries. Two materials with similar porosity but distinct internal architectures are compared against a solid baseline: a structured porous trailing edge (SPTE) and a random foam trailing edge (RFTE). The extracted permeability curves show close agreement with the analytical model of Johnson et al. (J. Fluid Mech., 1987, vol. 176, pp. 379–402), validating the method for both structured and randomised porous materials. The procedure also allows for the estimation of the equivalent viscous characteristic length and tortuosity. A detailed comparison reveals that the SPTE exhibits a lower viscous length scale and tortuosity than the RFTE, with a relatively higher dynamic permeability response at high frequencies.

In tribonucleation, a liquid-to-gas phase transition induced by a local pressure drop (cavitation) is highly undesirable, as it causes surface erosion and noise. A paradigmatic flow characteristic of tribonucleation problems is the flow between two coaxial disks. The flow is produced by the rapid upward movement of the top disk, which is initially at rest and in contact with the bottom disk. An analytical model, the so-called negative squeeze film, is typically used to predict the flow in the gap between the disks in this class of problems. Such a model considers an azimuthally uniform inflow in the gap between the disks. In this study, we experimentally show that if a negligibly small misalignment between the axes of the two disks is introduced, the inflow is not azimuthally uniform as expected from the negative squeeze film, but an entry jet appears in the flow between the disks. This entry jet is associated with the formation of two counter-rotating vortices. From reconstructing the pressure field from PIV velocity data in the vortex regions, we find that the local pressure is lower than the vapor pressure. This indicates that the gaseous phase in the cores of the vortices, which is observed from shadowgraphy visualizations in our study, should be attributed to cavitation. The negative-squeeze-film model, however, largely fails to predict the minimum pressure. Therefore, the onset of cavitation is not correctly captured by the analytical model.

Interferometric particle imaging (IPI) is used to measure both the size distribution and concentration of microbubbles (with a diameter less than 100 micron) in water. Using a new method for calibration makes it possible to obtain quantitative results for the concentration of microbubbles. The results are validated using imaging with a long-range microscope shadowgraph (LMS). Estimates of the size distribution and concentration from both IPI and LMS agree within uncertainty limits. The relative uncertainty in the IPI concentration estimation is about 10% and is mostly due to the finite number of detected bubbles. It is shown that the performance of the bubble-image detection algorithm needs to be quantified to obtain a reliable estimate of the concentration obtained with IPI.

The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 63₃−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ^∗, where δ^∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.

The attachment of porous media to a blunt trailing edge (TE) can significantly suppress vortex shedding processes and the related tonal noise, yet the near-wall and internal flow fields of porous media are difficult to analyze experimentally and rely on numerical simulations to elucidate the internal flow features. A structured porous trailing edge (SPTE) has been recently designed that follows a methodology of a structured porous coated cylinder. The SPTE acoustic response was compared against randomized porous media with 10 and 30 pores/in. in an anechoic wind tunnel over a range of flow velocities. Acoustic beamforming revealed that the dominant acoustic sources were at the end of the solid plate, even when a porous TE was attached. A region of integration was used to extract acoustic spectra without additional noise sources, revealing that the SPTE possesses superior noise reduction capability. Dipolar directivity patterns were observed at the vortex shedding frequency for each TE, and the coherence between microphones revealed the complex acoustic propagation of the high-frequency content. A wavelet analysis revealed how the SPTE breaks periodic vortex shedding cycles into smaller cycles over a wider frequency range, leading to an overall noise reduction relative to the other TEs.

Tip-vortex cavitation is among the first forms of cavitation to appear around ship propellers. In the present study, the time-resolved three-dimensional flow field around non-cavitating and cavitating tip vortices in the wake of a marine propeller is investigated with tomographic PIV. The advance ratio of the propeller and the Reynolds number of the flow are kept constant, while the cavitation number is varied by changing the pressure inside the cavitation tunnel. The importance of masking the tip-vortex cavities before performing the tomographic reconstruction is firstly demonstrated, followed by a description of the applied masking algorithm. From the three-dimensional velocity vector fields, coherent structures of vorticity are identified using the Q-criterion. Three types of coherent structures are observed to populate the wake of the propeller, i.e. tip vortex, hub vortex, and secondary vortical structures. The secondary vortical structures surrounding the tip vortex appear to be progressively smaller in size and more chaotically-organized for decreasing cavitation number. This can be attributed to the pressure fluctuations induced by the cavity, which strengthen when the cavity size grows.

Abstract: This work combines the latest advancements in time marching of 3D vector fields from tomographic particle image velocimetry, with an adapted version of Lighthill’s formulation, for the prediction of far-field jet noise. Three-dimensional velocity vector fields of the jet flow are first reconstructed from a tomographic volume of 4× 3× 9.5 Dj3, with D_j = 5 cm being the jet-exit diameter. (The jet-exit Mach number M_j ranges from 0.10 to 0.20.) The obtained vector fields are then used as input to a recently developed procedure for the time marching of the vorticity field, which relies upon the vortex-in-cell methodology. This yields time series of each three-dimensional velocity field, from which the far-field pressure is computed via Lilley’s acoustic analogy (through evaluation of the Lighthill’s stress tensor). It is shown that the estimate of the far-field noise spectrum compares well with the spectrum measured directly from a far-field microphone in the anechoic A-tunnel facility of TU Delft, in the Strouhal number range from approximately 1 to 12. Graphical Abstract: [Figure not available: see fulltext.]

The characteristics of the internal layers of intense shear are examined in a mixing layer and in a jet, in the range of Reynolds numbers 134<Re λ <275. Conditionally averaged profiles of streamwise velocity conditioned on the identified internal layers present strong velocity jumps, which account for approximately 10% of the characteristic large-scale velocity of the flow. The thickness (δw) of the internal layers from the combined analysis of both the mixing layer and the jet scales with (δw)/λReλ-1/2, which suggests a scaling with the Kolmogorov length scale (η), analogous to recent observations on the turbulent/nonturbulent interface (TNTI). The thickness of the internal shear layers within the mixing layer is found to be between 9η and 11η. The concentration of a passive scalar across the internal layers is also examined, at the Schmidt number Sc=1.4. The scalar concentration does not show any jumps across the internal layers, which is an important difference between the internal layers and the TNTI. This can be explained from the analysis of the internal layers of intense scalar gradient, where the flow topology node/saddle/saddle dominates, associated with strain, whereas the internal layers of intense shear are characterized by a prevalence of focus/stretching. A topological content analogous to that obtained in layers of intense scalar gradient is found in proximity to the TNTI, at the boundary between the viscous superlayer and the turbulent sublayer. These observations evidence that the TNTI and the internal layers of intense scalar gradient are similar in several respects.

To investigate the influence of the orifice geometry on near-field coherent structures in a jet, Fourier proper orthogonal decomposition (Fourier-POD) is applied. Velocity and vorticity snapshots obtained from tomographic particle image velocimetry at the downstream distance of two equivalent orifice diameters are analyzed. Jets issuing from a circular orifice and from a fractal orifice are examined, where the fractal geometry is obtained from a repeating fractal pattern applied to a base square shape. While in the round jet energy is mostly contained at wave number m=0, associated to the characteristic Kelvin-Helmholtz vortex rings, in the fractal jet modal structures at the fundamental azimuthal wave number m=4 capture the largest amount of energy. In addition, energy is scattered across a wider range of wave numbers than in the round jet. The radial Fourier-POD profiles, however, are nearly insensitive to the orifice geometry, and collapse to a universal distribution when scaled with a characteristic radial length. A similar collapse was recently observed in POD analysis of turbulent structures in pipe flow. However, unlike in pipe flow, the azimuthal-to-radial aspect ratio of the Fourier-POD structures is not constant and varies greatly with the wave number. The second part of the paper focuses on the relationship between streamwise vorticity and streamwise velocity, to characterize the role of the orifice geometry on the lift-up mechanism recently found to be active in turbulent jets [P. Nogueira, A. Cavalieri, P. Jordan, and V. Jaunet, Large-scale streaky structures in turbulent jets, J. Fluid Mech. 873, 211 (2019)JFLSA70022-112010.1017/jfm.2019.365]. The averaging of the streamwise vorticity conditioned on intense positive fluctuations of streamwise velocity reveals a pair of vorticity structures of opposite sign flanking the conditioning point, inducing a radial flow towards the jet periphery. This pair of structures is observed in both jets, even if the azimuthal extent of this pattern is 30% larger in the jet issuing from the circular orifice. The coupling between streamwise vorticity and velocity motions is also examined using Fourier-POD. The analysis reveals that in the jet with a circular orifice lower wave-number modes, corresponding to structures at larger scales, capture a larger fraction of the vorticity-velocity coupling. This evidences that the orifice geometry directly influences the interaction between velocity and vorticity.