The influence of nuclei content on cloud cavitation about a hydrofoil

International audienceThe dynamics of cloud cavitation about a 3D hydrofoil are investigated experimentally in a cavitation tunnel with both an abundance and dearth of freestream nuclei. The rectangular-planform, NACA hydrofoil was tested at a Reynolds number of $1.4 × 10^6$, a cavitation number of 0.55 and an incidence of 6°. High-speed photography of cavitation shedding phenomena was acquired simultaneously with unsteady force measurement to enable identification of cavity shedding modes corresponding with force spectral peaks. Two shedding modes are evident for both the nuclei deplete and abundant cases, although each are driven by different flow phenomena. The high-frequency mode for the nuclei deplete case is driven primarily by large-scale re-entrant jet formation during the growth phase, but shockwave propagation for the collapse phase of the cycle. The weaker low-frequency mode occurs because the strength of shedding at the hydrofoil tip varies at half the fundamental frequency. The dominant mode for the abundant case is the low-frequency mode which is some. times slower than the nuclei deplete case. The high-frequency mode for the nuclei abundant case is due to the propagation of two shockwaves; the passage of the first only partially condenses the cavity, while the second condenses a much larger region of the cavity

Cloud cavitation behaviour on a hydrofoil due to fluid-structure interaction

International audienceDespite recent extensive research into fluid-structure interaction (FSI) of cavitating hydrofoils there remains insufficient experimental data to explain many of these observed phenomena. The cloud cavitation behaviour around a hydrofoil due to the effect of FSI is investigated utilizing rigid and compliant 3D hydrofoils held in a cantilevered configuration in a cavitation tunnel. The hydrofoils have identical undeformed geometry of tapered planform with constant NACA section. The rigid model is made of stainless steel and the compliant model of carbon and glass fibre reinforced epoxy resin with the structural fibres aligned along the span-wise direction to avoid material bend-twist coupling. Tests were conducted at an incidence of 6°, a mean chord based Reynolds number of $0.7 × 10^6$, and cavitation number of 0.8. Force measurements were simultaneously acquired with high-speed imaging to enable correlation of forces with tip bending deformations and cavity physics. Hydrofoil compliance was seen to dampen the higher frequency force fluctuations while showing strong correlation between normal force and tip deflection. The 3D nature of the flow field was seen to cause complex cavitation behaviour with two shedding modes observed on both models

The contributions of molecular vibrations and higher triplet levels to the intersystem crossing mechanism in metal-free organic emitters.

Intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor-acceptor-donor (D–A–D) molecules. This dual-luminescence is stronger in the “angular” isomers, compared to their “linear” regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1-Tn, below the lowest singlet state, S1, in the “angular” regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin-vibronic mechanism. In the absence of this “triplet ladder”, due to the larger energy difference between T1 and Tn in the “linear” regioisomers, the ISC and rISC are not efficient. Remarkably the enhancement on the ISC rate in the “angular” regioisomers is accompanied by an increase on the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to harvest triplet states in organic compounds, and casts the TADF and RTP mechanisms into a common conceptual framework

Revisiting the Twentieth Century Through the Lens of Generation X and Digital Games: A Scoping Review

Video games have been around since the 1960s and have impacted upon society in a myriad of different ways. The purpose of this scoping review is to identify existing literature within the domain of video games which recruited participants from the Generation X (1965–1980) cohort. Six databases were searched (ACM, CINHAL Google Scholar, PubMed, Scopus, and Web of Science) focusing on published journal papers between 1970 and 2000. Search results identified 3186 articles guided by the PRISMA Extension for Scoping Reviews (PRISMA-ScR); 4 papers were irretrievable, 138 duplicated papers were removed, leaving 3048 were assessed for eligibility and 3026 were excluded. Articles (n = 22) were included into this review, with four papers primarily published in 1997 and in 1999. Thematic analysis identified five primary themes: purpose and objectives, respective authors’ reporting, technology, ethics and environment) and seven secondary themes: populations, type of participants (e.g. children, students), ethical approval, study design, reimbursement, language, type of assessments. This scoping review is distinctive because it primarily focuses on Generation X, who have experienced and grown-up with videogames, and contributes to several disciplines including: game studies, gerontology and health, and has wider implications from a societal, design and development perspective of video games

Experimental Investigation of Ventilated Cavity Flow Over a 3D Wall Mounted Fence

International audienceVentilated cavity flow over a 3-D wall mounted fence is experimentally investigated in a cavitation tunnel. The influence of air ventilation rate, fence height based Froude number and vapour pressure based cavitation number on the resulting cavity and associated drag coefficient is determined. Three different flow regimes are identified throughout the range of cavitation numbers for a constant set of free-stream conditions. The geometry of a fully formed ventilated cavity is described. The re-entrant jet cavity closure is observed as the primary mechanism for entrainment of ventilated air into the flow

The influence of nucleation on cloud cavitation about a sphere

International audienceThe influence of nucleation on cloud cavitation about a sphere is investigated using simultaneous high-speed photography and high-frequency surface pressure measurements. The 0.15 m diameter PVC sphere was located on the axial centreline of a variable-pressure water tunnel with a 0.6 m square test section and tested at a constant Reynolds number of 1.5×10 6 and a cavitation number of 0.7. The flush-mounted high-frequency pressure sensor was located within the shedding zone of the cloud cavitation and orientated such that it was in the field of view of the high-speed camera. Pressure measurements and high-speed photography were simultaneously obtained at 7 kHz. The time-series of spatially averaged image pixel-intensity was found to lead the pressure measurement. Wavelet analysis was used to compare both data sets obtained for cavitation about the sphere with and without upstream artificial nuclei seeding. PSDs generated from the wavelet analysis showed two peak frequencies for the cavity shedding, with the peaks being more defined for the nucleated case

Nucleation Effects on Tip Vortex Cavitation Inception Location

Nuclei, or microbubble, populations are inextricably linked to tip vortex cavitation (TVC) inception and dynamics. In order to gain a better quantitative understanding of this relationship, high-speed video measurements were taken in a cavitation tunnel of TVC inception locations about an elliptical hydrofoil in flows with mono- and polydisperse injected nuclei populations. Sample sizes of O(1000) were acquired. For both populations, inception occurred between 0.02 chord lengths upstream of the hydrofoil tip and about 2.1 chord lengths downstream along the cavity trajectory. However, inception location distribution varied significantly with nuclei population. This is explained by the higher concentrations of weaker nuclei in the monodisperse case, which increases the distance along the vortex within which nuclei are susceptible to cavitation. These results provide the foundation for studies on TVC dynamics and acoustics