Buckling of elastic fibers in a shear flow

Three-dimensional dynamics of flexible fibers in shear flow are studied numerically, with a qualitative comparison to experiments. Initially, the fibers are straight, with different orientations with respect to the flow. By changing the rotation speed of a shear rheometer, we change the ratio A of bending to shear forces. We observe fibers in the flow-vorticity plane, which gives insight into the motion out of the shear plane. The numerical simulations of moderately flexible fibers show that they rotate along effective Jeffery orbits, and therefore the fiber orientation rapidly becomes very close to the flow-vorticity plane, on average close to the flow direction, and the fiber remains in an almost straight configuration for a long time. This ``ordering'' of fibers is temporary since they alternately bend and straighten out while tumbling. We observe numerically and experimentally that if the fibers are initially in the compressional region of the shear flow, they can undergo a compressional buckling, with a pronounced deformation of shape along their whole length during a short time, which is in contrast to the typical local bending that originates over a long time from the fiber ends. We identify differences between local and compressional bending and discuss their competition, which depends on the initial orientation of the fiber and the bending stiffness ratio A. There are two main finding. First, the compressional buckling is limited to a certain small range of the initial orientations, excluding those from the flow-vorticity plane. Second, since fibers straighten out in the flow-vorticity plane while tumbling, the compressional buckling is transient - it does not appear for times longer than 1/4 of the Jeffery period. For larger times, bending of fibers is always driven by their ends.

Navigation of micro-swimmers in steady flow: the importance of symmetries

Marine micro-organisms must cope with complex flow patterns and even turbulence as they navigate the ocean. To survive they must avoid predation and find efficient energy sources. A major difficulty in analysing possible survival strategies is that the time series of environmental cues in nonlinear flow is complex and that it depends on the decisions taken by the organism. One way of determining and evaluating optimal strategies is reinforcement learning. In a proof-of-principle study, Colabrese et al. (Phys. Rev. Lett., vol. 118, 2017, 158004) used this method to find out how a micro-swimmer in a vortex flow can navigate towards the surface as quickly as possible, given a fixed swimming speed. The swimmer measured its instantaneous swimming direction and the local flow vorticity in the laboratory frame, and reacted to these cues by swimming either left, right, up or down. However, usually a motile micro-organism measures the local flow rather than global information, and it can only react in relation to the local flow because, in general, it cannot access global information (such as up or down in the laboratory frame). Here we analyse optimal strategies with local signals and actions that do not refer to the laboratory frame. We demonstrate that symmetry breaking is required to find such strategies. Using reinforcement learning, we analyse the emerging strategies for different sets of environmental cues that micro-organisms are known to measure.

Synthesis of Vortex Rossby Waves. Part III: Rossby Waveguides, Vortex Motion, and the Analogy with Middle-Latitude Cyclones

Vortex Rossby Waves (VRWs) affect Tropical Cyclones’ (TCs’) motion, structure, and intensity. They propagate within annular waveguides defined by a passband between Ω1D, the Doppler-shifted frequency of a one-dimensional VRW, and zero. Wavenumber-1 VRWs cause TC motion directly and have wider waveguides than wavenumbers ≥ 2. VRWs forced with fixed rotation frequency propagate away from the forcing. Initially outward-propagating waves are Doppler shifted to zero at a critical radius, where they are absorbed. Initially inward-propagating waves are Doppler-shifted to Ω1D, reflect from a turning point, propagate outward, and are ultimately absorbed at the critical radius. Between the forcing and the turning radii, the VRWs have standing-wave structure; outward from the forcing they are trailing spirals. They carry angular momentum fluxes that act to accelerate the mean flow at the forcing radius and decelerate it at the critical radius.Mean flow vorticity monopoles are inconsistent with Stokes Theorem on a spherical Earth, because a contour enclosing the monopole’s antipode would have nonzero circulation but would enclose zero vorticity.The Rossby waveguide paradigm also fits synoptic-scale Rossby Waves in a meridionally sheared zonal flow. These waves propagate within a waveguide confined between a poleward turning latitude and an equatorward critical latitude. Forced waves are comma-shaped gyres that resemble observed frontal cyclones, with trailing filaments equatorward of the forcing latitude and standing waves poleward. Even neutral forced Rossby waves converge westerly momentum at the latitude of forcing.

Broadband Submesoscale Vorticity Generated by Flow around an Island

An array of moorings deployed off the coast of Palau is used to characterize submesoscale vorticity generated by broadband upper-ocean flows around the island. Palau is a steep-sided archipelago lying in the path of strong zonal geostrophic currents, but tides and inertial oscillations are energetic as well. Vorticity is correspondingly broadband, with both mean and variance O(f) in a surface and subsurface layer (where f is the local Coriolis frequency). However, while subinertial vorticity is linearly related to the incident subinertial current, the relationship between superinertial velocity and superinertial vorticity is weak. Instead, there is a strong nonlinear relationship between subinertial velocity and superinertial vorticity. A key observation of this study is that during periods of strong westward flow, vorticity in the tidal bands increases by an order of magnitude. Empirical orthogonal functions (EOFs) of velocity show this nonstationary, superinertial vorticity variance is due to eddy motion at the scale of the array. Comparison of kinetic energy and vorticity time series suggest that lateral shear against the island varies with the subinertial flow, while tidal currents lead to flow reversals inshore of the recirculating wake and possibly eddy shedding. This is a departure from the idealized analog typically drawn on in island wake studies: a cylinder in a steady flow. In that case, eddy formation occurs at a frequency dependent on the scale of the obstacle and strength of the flow alone. The observed tidal formation frequency likely modulates the strength of submesoscale wake eddies and thus their dynamic relationship to the mesoscale wake downstream of Palau.

Locomotion of juvenile silver carp (Hypophthalmichthys molitrix) near the separation zone at the channel confluence

<p>Knowledge of locomotion of fish with significant rheotaxis at river confluences is critical for prediction of fish distribution at a river network. Recently, less silver carps observed in the Poyang Lake should be related to the hydrodynamic change at the confluence of the lake outlet and the Yangtze River. The operation of the Three Gorges Dam has largely changed the hydrodynamics at this confluence. Silver carp is one of the four major Chinese carps, and has significant rheotaxis. In this study, a series of laboratory experiments were conducted to figure out the behavioral responses of juvenile silver carps to hydrodynamics near the separation zone at the channel confluence. The separation zone at a river confluence is one of the main zones for carp habitat and feeding. The locomotion and trajectory of juvenile silver carps were recorded through infrared thermal imaging at the confluence flume. Flow velocity field near the separation zone was measured by a Particle Image Velocimetry (PIV) system. A total of 40 juvenile silver carps were released from the separation zone and swam to the upstream, among which 24 carps swam to the tributary and the other to the main channel. Almost all 24 carps moved along the beginning of the boundary of the separation zone near the corner where the flow shear was strong. It seems that instead of avoiding places with great vorticity, they preferentially chose the trajectory where the flow vorticity was large continuously. They increased the tail-beat frequency and decreased the tail-beat amplitude to maintain body stability when they encountered the flow with large vorticity. These results are beneficial for the regulation of upstream dams to adjust the hydrodynamics at the confluence and improve local ecology.</p>

Computational fluid dynamics of the right atrium: Assessment of modelling criteria for the evaluation of dialysis catheters

Central venous catheters are widely used in haemodialysis therapy, having to respect design requirements for appropriate performance. These are placed within the right atrium (RA); however, there is no prior computational study assessing different catheter designs while mimicking their native environment. Here, a computational fluid dynamics model of the RA, based on realistic geometry and transient physiological boundary conditions, was developed and validated. Symmetric, split and step catheter designs were virtually placed in the RA and their performance was evaluated by: assessing their interaction with the RA haemodynamic environment through prediction of flow vorticity and wall shear stress (WSS) magnitudes (1); and quantifying recirculation and tip shear stress (2). Haemodynamic predictions from our RA model showed good agreement with the literature. Catheter placement in the RA increased average vorticity, which could indicate alterations of normal blood flow, and altered WSS magnitudes and distribution, which could indicate changes in tissue mechanical properties. All designs had recirculation and elevated shear stress values, which can induce platelet activation and subsequently thrombosis. The symmetric design, however, had the lowest associated values (best performance), while step design catheters working in reverse mode were associated with worsened performance. Different tip placements also impacted on catheter performance. Our findings suggest that using a realistically anatomical RA model to study catheter performance and interaction with the haemodynamic environment is crucial, and that care needs to be given to correct tip placement within the RA for improved recirculation percentages and diminished shear stress values.

Predictability of Orographic Rainfall Features Over Sylhet Using Nwp Model

Topography and orography are two physical factors which produce high impact rainfall over the North-eastern part of Bangladesh. To predict the orographic rainfall of 29 March 2017 over Sylhet, Bangladesh an attempt has been performed using Weather Research and Forecasting (WRF) model. The model has been run in a single domain of 10 km horizontal resolution for 48-h and 72-h using six hourly global final datasets from 0000 UTC of each initial day of the event as initial and lateral boundary conditions with NSSL 2-moment microphysics scheme, Kain–Fritsch cumulous scheme and Yonsei University Planetary Boundary Layer (PBL) scheme. The model outputs such as sea level pressure, wind flow, vorticity, wind shear, humidity, Convective Available Potential Energy (CAPE), Convective Inhibition, Lifted Index, K-index, Total Total Index and rainfall have been analyzed. The model predicted weather parameters were visualized by Grid Analysis and Display System (GrADS) and Geographic Information System (GIS) software and validated with observed data of Bangladesh Meteorological Department (BMD), Tropical Rainfall Measuring Mission (TRMM) and European Centre for Medium-Range Weather Forecasts (ECMRWF) data. The analysis determines that the CAPE of magnitude 800- 1000 JKg-1, positive vorticity of (6-10) ×10-5s-1 and relative humidity of 80-100% up to 500-400 hPa levels are accountable for the happening of the orographic extreme rainfall and other parameters are compatible with the observed or theoretical values. This study indicates that the model with an appropriate model set up is capable to predict the orographic precipitation realistically well and can be used for upcoming events. Journal of Engineering Science 11(2), 2020, 61-73

Abstract 14618: Bicuspid Aortic Valve Morphology and Risk Factors of Abnormal Hemodynamics: An Experimental Investigation of Velocity, Vorticity and Eccentricity of Systolic Jet Stream, Using a Magnetic Resonance Imaging Compatible Pulsatile Flow Circulation System

Introduction: Abnormal hemodynamics with bicuspid aortic valve (BAV) is influenced by the BAV morphology. However, due to the morphological variety, the relationship between BAV morphology and hemodynamics has not been well clarified. We experimentally investigated influences of BAV morphology on hemodynamics. Methods: An MRI compatible pulsatile flow circulation system incorporating an aortic valve model and morphologically relevant aortic arch model was developed. Two types of BAV models with cusp angles of 240-120 (asymmetric BAV) and 180-180 (symmetric BAV) were prepared using bovine aorta and pericardium. The vorticity and eccentricity of jet in the aortic arch model in systolic phase were assessed using 4D-flow MRI. Streamlines at peak systole were compared among 5 BAV morphologies defined based on the symmetry and the position of leaflets (Fig). Results: Eccentric supra-valvular jets directed to the aortic wall faced to the smaller leaflet were present in the 3 asymmetric BAVs. In the ABAV-1, a markedly eccentric jet impinging on the aortic outer curvature was present. In the ABAV-2, a left-posterior directed jet shifting to the outer curvature of proximal arch was present. The asymmetric BAVs induced larger flow vorticity in the ascending aorta than the symmetric BAVs (ABAV-1 vs -2 vs -3 vs SBAV-1 vs -2, 0.018 vs 0.019 vs 0.014 vs 0.010 vs 0.011 m 2 /s). The ABAV-2 had a larger vortex in the middle arch than the ABAV-1 (ABAV-2 vs -1, 0.010 vs 0.005 m 2 /s). Conclusion: Our study indicated that the angles and orientations of the BAV impacted on the locations of jets impinging on the aortic wall and magnitudes of vorticities in systole. In the asymmetric BAVs, the direction of jet was influenced by the position of smaller leaflet. Our data suggests that the ABAV-1 morphology may be a risk factor inducing asymmetric aneurysm bulged toward the aortic outer curvature, while the ABAV-2 morphology may be a risk factor of an aortic aneurysm involving the transvers arch.