Trailing-edge far-field noise and noise source characterization in high inflow turbulence conditions.

Airframe noise currently is a bottle neck in various applications, e.g., wind energy, maritime applications, and aircraft. Airframe noise is significantly increased by the presence of inflow turbulence. High inflow turbulence influences the boundary layer and wall-pressure fluctuations close to the trailing edge of airfoils. In this research, measurements of boundary layer and wall-pressure fluctuations near the trailing edge of an airfoil are conducted to investigate how the inflow turbulence affects the trailing-edge noise generation mechanism. Far-field noise measurements of additional three airfoils are shown to understand the role of the airfoil geometry in the dominant noise source for the cases of inflow turbulence and to generalize the observed increase in trailing-edge noise. Inflow turbulence leads to an increase in both the wall-pressure spectrum and spanwise correlation length. Trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 dB up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry and inflow velocity, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil and for lower velocities.

Inflow turbulence distortion for airfoil leading-edge noise prediction for large turbulence length scales for zero-mean loading.

Turbulence distortion due to airfoil finite thickness is an important but not fully understood phenomenon that affects the airfoil radiated noise, resulting in inaccurate noise predictions. This study discusses the turbulence distortion in the leading edge (LE) region of an airfoil aiming to obtain more accurate LE noise predictions. Wind tunnel experiments were performed for National Advisory Committee for Aeronautics (NACA) 0008 and NACA 0012 airfoils at zero angle of attack subjected to large turbulence length scales (between 10 and 43 times the airfoil LE radius) generated by a grid and a rod. Hot-wire and surface pressure measurements were performed in the LE region. Results show that the root mean square of the velocity fluctuations urms and the turbulence integral length scale Λf at the stagnation line decrease considerably as the LE is approached. Rod-airfoil radiated noise was measured and compared with Amiet's model. The predicted noise overestimates the LE noise for high frequencies. However, the prediction agrees well with measurements when the turbulence spectrum based on the rapid distortion theory is used in Amiet's model, with as inputs the urms and Λf values measured close to the LE. This work's main contribution is to demonstrate that more accurate noise predictions are obtained when the inputs to the model consider the turbulence distortion effects.

High-Throughput Fabrication of Size-Controlled Pickering Emulsions, Colloidosomes, and Air-Coated Particles via Clog-Free Jetting of Suspensions.

Microcapsules with a liquid core and a solid shell composed of hydrophobic nanoparticles are broadly applied in food, pharmaceutics, and biotechnologies. For example, Pickering emulsions, colloidosomes, or antibubbles (droplets surrounded by air layers in water) enable controlled release of active agents, biocompatibility, and contact-less liquid transportation. However, producing controlled nanoparticle- or polymer-laden hydrophobic shells at scale is highly challenging, since bulk methods are polydisperse and microfluidic chips are prone to clogging and slow. Here, clog-free coating of an aqueous jet with silica nanoparticle suspensions with concentrations up to 10% (w/v), as well as high concentrations of polymers (30% (w/v) poly(lactic acid) (PLA)), is demonstrated, enabling continuous generation of microcapsules at flow rates up to 4 mL min-1 . Pickering emulsions are converted into capsules, providing hydrophobic shells consisting of nanoparticles for controlled release. As a highlight, the scalable fabrication of air-coated capsules (antibubbles) in the sub-millimeter range is demonstrated. The shell contains an air film that protects the liquid core for days yet enables ultrasound-induced release within 3 min. By enabling rapid fabrication of controlled Pickering emulsions, colloidosomes, antibubbles, and biodegradable capsules, jetting through a liquid layer (JetALL) provides a versatile platform for advanced applications in food, pharmacy, and life science.

Deposition Offset of Printed Foam Strands in Direct Bubble Writing.

Direct Bubble Writing is a recent technique to print shape-stable 3-dimensional foams from streams of liquid bubbles. These bubbles are ejected from a core-shell nozzle, deposited on the build platform placed at a distance of approximately 10 cm below the nozzle, and photo-polymerized in situ. The bubbles are ejected diagonally, with a vertical velocity component equal to the ejection velocity and a horizontal velocity component equal to the motion of the printhead. Owing to the horizontal velocity component, a discrepancy exists between the nozzle trajectory and the location of the printed strand. This discrepancy can be substantial, as for high printhead velocities (500 mm/s) an offset of 8 mm (in radius) was measured. Here, we model and measure the deviation in bubble deposition location as a function of printhead velocity. The model is experimentally validated by the printing of foam patterns including a straight line, a circle, and sharp corners. The deposition offset is compensated by tuning the print path, enabling the printing of a circular path to the design specifications and printing of sharp corners with improved accuracy. These results are an essential step towards the Direct Bubble Writing of 3-dimensional polymer foam parts with high dimensional accuracy.

Regimes of Soft Lubrication.

Elastohydrodynamic lubrication, or simply soft lubrication, refers to the motion of deformable objects near a boundary lubricated by a fluid, and is one of the key physical mechanisms to minimise friction and wear in natural and engineered systems. Hence it is of particular interest to relate the thickness of the lubricant layer to the entrainment (sliding/rolling) velocity, the mechanical loading exerted onto the contacting elements, and properties of the elastic boundary. In this work we provide an overview of the various regimes of soft lubrication for two-dimensional cylinders in lubricated contact with compliant walls. We discuss the limits of small and large entrainment velocity, which is equivalent to large and small elastic deformations, as the cylinder moves near thick or thin elastic layers. The analysis focusses on thin elastic coatings, both compressible and incompressible, for which analytical scaling laws are not yet available in the regime of large deformations. By analysing the elastohydrodynamic boundary layers that appear at the edge of the contact, we establish the missing scaling laws - including prefactors. As such, we offer a rather complete overview of physically relevant limits of soft lubrication.

Continuous High-Throughput Fabrication of Architected Micromaterials via In-Air Photopolymerization.

Recent advances in optical coding, drug delivery, diagnostics, tissue engineering, shear-induced gelation, and functionally engineered rheology crucially depend on microparticles and microfibers with tunable shape, size, and composition. However, scalable manufacturing of the required complex micromaterials remains a long-standing challenge. Here in-air polymerization of liquid jets is demonstrated as a novel platform to produce microparticles and microfibers with tunable size, shape, and composition at high throughput (>100 mL h-1 per nozzle). The polymerization kinetics is quantitatively investigated and modeled as a function of the ink composition, the UV light intensity, and the velocity of the liquid jet, enabling engineering of complex micromaterials in jetting regimes. The size, morphology, and local chemistry of micromaterials are independently controlled, as highlighted by producing micromaterials using 5 different photopolymers as well as multi-material composites. Simultaneous optimization of these control parameters yields rapid fabrication of stimuli-responsive Janus fibers that function as soft actuators. Finally, in-air photopolymerization enables control over the curvature of printed droplets, as highlighted by high-throughput printing of microlenses with tunable focal distance. The combination of rapid processing and tunability in composition and architecture opens a new route toward applications of tailored micromaterials in soft matter, medicine, pharmacy, and optics.

A multigrid method for N-component nucleation.

A multigrid algorithm has been developed enabling more efficient solution of the cluster size distribution for N-component nucleation from the Becker-Döring equations. The theoretical derivation is valid for an arbitrary number of condensing components, making the simulation of many-component nucleating systems feasible. A steady state ternary nucleation problem is defined to demonstrate its efficiency. The results are used as a validation for existing nucleation theories. The non-steady state ternary problem provides useful insight into the initial stages of the nucleation process. We observe that for the ideal mixture the main nucleation flux bypasses the saddle point.