Cylindrical bodies in uniform flows can be coated with a porous medium as a passive flow and noise control method in an effort to reduce the acoustic effects of vortex shedding. To date, the employed open-cell porous materials typically possess a randomized internal structure. This paper presents the design and validation of a novel 3-D printed structured porous coated cylinder that has significant flexibility, in that the porosity and pores per inch of the porous coating can be modified independently and relatively easily. The performance of the structured porous coating design is compared against porous polyurethane and metal foam with the same coating dimensions and similar pores per inch and porosity via an experimental acoustic investigation, revealing strong similarity in the passive noise control performance of each material type. A numerical comparison illustrates the similarities of the wake structure of the 3-D printed porous coated cylinder to an equivalent Darcy–Forchheimer model simulation that represents a randomized internal porous structure. The performance similarities of these different porous material types indicate that a structured porous geometry can be used to understand the internal flow behavior of the porous medium responsible for reducing the cylinder vortex shedding tone that is otherwise extremely difficult or impossible with typical randomized porous structures. Moreover, significant potential exists for the porous structure to be further optimized or smartly tailored by architectural design for different control purposes, coating geometries and dimensions, and working conditions.

The forward flight stage of multicopters accounts for a relatively large amount of flight time in comparison with hovering, ascending, and descending conditions; therefore, understanding its aerody...

Vortex shedding in the wake of a cylinder in uniform flow can be suppressed via the application of a porous coating; however, the suppression mechanism is not fully understood. The internal flow field of a porous coated cylinder (PCC) can provide a deeper understanding of how the flow within the porous medium affects the wake development. A structured PCC (SPCC) was three-dimensionally printed using a transparent material and tested in water tunnel facilities using flow visualisation and tomographic particle image velocimetry at outer-diameter Reynolds numbers of $Re = 7 \times 10^{3}$ and $7.3 \times 10^{4}$ , respectively. The internal and near-wall flow fields are analysed at the windward and mid-circumference regions. Flow stagnation is observed in the porous layer on the windward side and its boundary is shown to fluctuate with time in the outermost porous layer. This stagnation region generates a quasi-aerodynamic body that influences boundary layer development on the SPCC inner diameter, that separates into a shear layer within the porous medium. For the first time via experiment, spectral content within the separated shear layer reveals vortex shedding processes emanating through single pores at the outer diameter, providing strong evidence that SPCC vortex shedding originates from the inner diameter. Velocity fluctuations linked to this vortex shedding propagate through the porous layers into the external flow field at a velocity less than that of the free stream. The Strouhal number linked to this velocity accurately predicts the SPCC vortex shedding frequency.

To improve understanding of dual vortex shedding over a trailing edge with regard to airfoil tonal noise generation, synchronized velocity and noise measurements are conducted on a NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angle of attack near zero and a Reynolds number of 220 000. Instantaneous flow fields obtained by particle image velocimetry show the development of separated shear layer, vortex roll-up, and vortex breakup near the airfoil trailing edge. The time-averaged flow fields feature separation bubbles on both sides, and the root mean square values of streamwise velocity fluctuations show triple peak structures. The velocity spectra agree well with the noise spectra in terms of broadband humps and discrete tones. Through proper orthogonal decomposition (POD) analysis, the most energetic modes are identified, which represent global structures in the flow field. The trailing edge noise being an integral effect of the velocity fluctuation near the trailing edge, the POD analysis provides an alternative view for understanding the noise generation mechanism. The first and second modes are dominated by out-of-phase vortex shedding along the airfoil surface and in the near wake, which dominates the high amplitude noise emission process. The third and fourth modes represent in-phase vortex shedding, which dominates the low amplitude noise emission process. The noise source region is determined by the correlation between the velocity and sound pressure, which shows approximately the same periodic pattern as the first and second POD modes.