tracing fidelity is controlled by the flow rates of helium, liquid soap and air in HFSB production. A characteristic time response, defined as the ratio of slip velocity and the fluid acceleration, is obtained. The feasibility of performing time-resolved tomographic PIV measurements over large volumes in aerodynamic wind tunnels is also studied. The flow past a 5-cm-diameter cylinder is measured over a volume of 20 × 20 × 12 cm 3 at a rate of 2 kHz. The achieved seeding density of <0.01 ppp enables resolving the Kármán vortices, whereas turbulent substructures cannot be captured.

A novel technique is proposed and investigated for the estimation of the body force field resulting from the operation of a dielectric barrier discharge plasma actuator. The technique relies on the measurement of the spatio-temporal evolution of the induced velocity field using high-speed particle image velocimetry (PIV). The technique has the advantage of providing spatial distribution of the body force vector field. A full Navier-Stokes term decomposition is applied on the evolving field along with additional closure norms in order to decouple the pressure gradient and body force terms. Results are compared with load-cell measurements of the direct reaction force and also momentum balance calculations based on the PIV field. Agreement between the different methods is observed. The data can easily be incorporated in computational flow solvers and also be used for validation and calibration of numerical plasma models.

No abstract

The positive and negative high-amplitude pressure peaks (HAPP) are investigated in a turbulent boundary layer at Re θ = 1900 in order to identify their turbulent structure. The three-dimensional velocity field is measured within the inner layer of the turbulent boundary layer using tomographic particle image velocimetry (tomo-PIV). The measurements are performed at an acquisition frequency of 10 000 Hz and over a volume of 418 × 149 × 621 wall units in the streamwise, wall-normal and spanwise directions, respectively. The time-resolved velocity fields are applied to obtain the material derivative using the Lagrangian method followed by integration of the Poisson pressure equation to obtain the three-dimensional unsteady pressure field. The simultaneous volumetric velocity, acceleration, and pressure data are conditionally sampled based on local maxima and minima of wall pressure to analyse the threedimensional turbulent structure of the HAPPs. Analysis has associated the positive HAPPs to the shear layer structures formed by an upstream sweep of high-speed flow opposing a downstream ejection event. The sweep event is initiated in the outer layer while the ejection of near-wall fluid is formed by the hairpin category of vortices. The shear layers were observed to be asymmetric in the instantaneous visualizations of the velocity and acceleration fields. The asymmetric pattern originates from the spanwise component of temporal acceleration of the ejection event downstream of the shear layer. The analysis also demonstrated a significant contribution of the pressure transport term to the budget of the turbulent kinetic energy in the shear layers. Investigation of the conditional averages and the orientation of the vortices showed that the negative HAPPs are linked to both the spanwise and quasi-streamwise vortices of the turbulent boundary layer. The quasi-streamwise vortices can be associated with the hairpin category of vortices or the isolated quasi-streamwise vortices of the inner layer. A bi-directional analysis of the link between the HAPPs and the hairpin paradigm is also conducted by conditionally averaging the pressure field based on the detection of hairpin vortices using strong ejection events. The results demonstrated positive pressure in the shear layer region of the hairpin model and negative pressure overlapping with the vortex core.

Mussel-inspired dopamine chemistry has increasingly been used for surface modification due to its simplicity, versatility, and strong reactivity for secondary functionalization with amine or thiol containing molecules. In this work, we demonstrate a facile surface modification technique using dopamine chemistry to prepare a zwitterionic polymer coating with both antifouling and antimicrobial property. Catechol containing adhesive monomer dopamine methacrylamide (DMA) was copolymerized with bioinspired zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) monomer, and the synthesized copolymers were covalently grafted onto the amino (−NH 2 ) rich polyethylenimine (PEI)/polydopamine (PDA) codeposited surface to obtain a stable antifouling surface. The resulting surface was later used for in situ deposition of antimicrobial silver nanoparticles (AgNPs), facilitated by the presence of catechol groups of the coating. The modified surface was characterized using X-ray photoelectron spectroscopy (XPS), water contact angle measurements, and atomic force microscopy (AFM). This dual functional coating significantly reduced the adhesion of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria and showed excellent resistance to bovine serum albumin (BSA) adsorption. This bioinspired and efficient surface modification strategy with dual functional coating promises its potential application in implantable biomedical devices.