Identifying regions of importance in wall-bounded turbulence through explainable deep learning.

Despite its great scientific and technological importance, wall-bounded turbulence is an unresolved problem in classical physics that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the energy-containing coherent structures in the flow. Such interactions are explored in this study using an explainable deep-learning method. The instantaneous velocity field obtained from a turbulent channel flow simulation is used to predict the velocity field in time through a U-net architecture. Based on the predicted flow, we assess the importance of each structure for this prediction using the game-theoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by revealing that the most important structures in the flow are not necessarily the ones with the highest contribution to the Reynolds shear stress. We also apply the method to an experimental database, where we can identify structures based on their importance score. This framework has the potential to shed light on numerous fundamental phenomena of wall-bounded turbulence, including novel strategies for flow control.

Aerosolisation in endonasal endoscopic pituitary surgery.

PURPOSE: To determine the particle size, concentration, airborne duration and spread during endoscopic endonasal pituitary surgery in actual patients in a theatre setting. METHODS: This observational study recruited a convenience sample of three patients. Procedures were performed in a positive pressure operating room. Particle image velocimetry and spectrometry with air sampling were used for aerosol detection. RESULTS: Intubation and extubation generated small particles (< 5 µm) in mean concentrations 12 times greater than background noise (p < 0.001). The mean particle concentrations during endonasal access were 4.5 times greater than background (p = 0.01). Particles were typically large (> 75 µm), remained airborne for up to 10 s and travelled up to 1.1 m. Use of a microdebrider generated mean aerosol concentrations 18 times above baseline (p = 0.005). High-speed drilling did not produce aerosols greater than baseline. Pituitary tumour resection generated mean aerosol concentrations less than background (p = 0.18). Surgical drape removal generated small and large particles in mean concentrations 6.4 times greater than background (p < 0.001). CONCLUSION: Intubation and extubation generate large amounts of small particles that remain suspended in air for long durations and disperse through theatre. Endonasal access and pituitary tumour resection generate smaller concentrations of larger particles which are airborne for shorter periods and travel shorter distances.

Monomerization of far-red fluorescent proteins.

Anthozoa-class red fluorescent proteins (RFPs) are frequently used as biological markers, with far-red (λem ∼ 600-700 nm) emitting variants sought for whole-animal imaging because biological tissues are more permeable to light in this range. A barrier to the use of naturally occurring RFP variants as molecular markers is that all are tetrameric, which is not ideal for cell biological applications. Efforts to engineer monomeric RFPs have typically produced dimmer and blue-shifted variants because the chromophore is sensitive to small structural perturbations. In fact, despite much effort, only four native RFPs have been successfully monomerized, leaving the majority of RFP biodiversity untapped in biomarker development. Here we report the generation of monomeric variants of HcRed and mCardinal, both far-red dimers, and describe a comprehensive methodology for the monomerization of red-shifted oligomeric RFPs. Among the resultant variants is mKelly1 (emission maximum, λem = 656 nm), which, along with the recently reported mGarnet2 [Matela G, et al. (2017) Chem Commun (Camb) 53:979-982], forms a class of bright, monomeric, far-red FPs.

Haemodynamic effects of incomplete stent apposition in curved coronary arteries.

Incomplete stent apposition (ISA, also known as malapposition) is a complication that affects day-to-day coronary stenting procedures. ISA is more prominent in complex arterial geometries, such as curvature, asa result of the limited conformability of coronary stents. These malapposed struts disturb the otherwise near-wall laminar blood flow and form a micro-recirculation environment. The micro-recirculation environment is often associated with low wall shear stress (WSS) and upsets the delicate balance of vascular biology, providing possible nidus for thrombosis and restenosis. In this study, a three-dimensional (3D) stent model was virtually deployed into an idealised curved coronary artery. Computational fluid dynamics (CFD) simulations were carried out to systematically analyse the haemodynamic effects of increasing maximum ISA distances, ranging from 180 (moderate), 400 (intermediate) to 910µm (severe) in an artery with decreasing radius of curvature (ROC). Micro-recirculations around both proximal and distal malapposed struts become more pronounced as compared to fully-apposed struts. The accompanying areas of low temporally-averaged WSS (AL-TAWSS) can increase twofold compared to the fully-apposed condition. Furthermore, substantial regions (∼5.2% and 9.0%) of AL-TAWSS are detached from the distal end of the malapposed struts in both moderate and intermediate cases respectively. Malapposed stents also induce more variation of TAWSS at the inner bend of the artery. At the stent surface, maximum WSS increases threefold from the fully-apposed case to intermediate ISA. High WSS on the strut surface is known to activate platelets which when exposed to the micro-recirculation environment may lead to their deposition and thrombosis.

Computational fluid dynamics study of common stent models inside idealised curved coronary arteries.

The haemodynamic behaviour of blood inside a coronary artery after stenting is greatly affected by individual stent features as well as complex geometrical properties of the artery including tortuosity and curvature. Regions at higher risk of restenosis, as measured by low wall shear stress (WSS < 0.5 Pa), have not yet been studied in detail in curved stented arteries. In this study, three-dimensional computational modelling and computational fluid dynamics methodologies were used to analyse the haemodynamic characteristics in curved stented arteries using several common stent models. Results in this study showed that stent strut thickness was one major factor influencing the distribution of WSS in curved arteries. Regions of low WSS were found behind struts, particularly those oriented at a large angle relative to the streamwise flow direction. These findings were similar to those obtained in studies of straight arteries. An uneven distribution of WSS at the inner and outer bends of curved arteries was observed where the WSS was lower at the inner bend. In this study, it was also shown that stents with a helical configuration generated an extra swirling component of the flow based on the helical direction; however, this extra swirl in the flow field did not cause significant changes on the distribution of WSS under the current setup.

Large-scale influences in near-wall turbulence.

Hot-wire data acquired in a high Reynolds number facility are used to illustrate the need for adequate scale separation when considering the coherent structure in wall-bounded turbulence. It is found that a large-scale motion in the log region becomes increasingly comparable in energy to the near-wall cycle as the Reynolds number increases. Through decomposition of fluctuating velocity signals, it is shown that this large-scale motion has a distinct modulating influence on the small-scale energy (akin to amplitude modulation). Reassessment of DNS data, in light of these results, shows similar trends, with the rate and intensity of production due to the near-wall cycle subject to a modulating influence from the largest-scale motions.