Passive control of a falling sphere by elliptic-shaped appendages

The majority of investigations characterizing the motion of single or multiple particles in fluid flows consider canonical body shapes, such as spheres, cylinders, discs, etc. However, protrusions on bodies -- being either as surface imperfections or appendages that serve a function -- are ubiquitous in both nature and applications. In this work, we characterize how the dynamics of a sphere with an axis-symmetric wake is modified in the presence of thin three-dimensional elliptic-shaped protrusions. By investigating a wide range of three-dimensional appendages with different aspect ratios and lengths, we clearly show that the sphere with an appendage may robustly undergo an inverted-pendulum-like (IPL) instability. This means that the position of the appendage placed behind the sphere and aligned with the free-stream direction is unstable, in a similar way that an inverted pendulum is unstable under gravity. Due to this instability, non-trivial forces are generated on the body, leading to turn and drift, if the body is free to fall under gravity. Moreover, we identify the aspect ratio and length of the appendage that induces the largest side force on the sphere, and therefore also the largest drift for a freely falling body. Finally, we explain the physical mechanisms behind these observations in the context of the IPL instability, i.e., the balance between surface area of the appendage exposed to reversed flow in the wake and the surface area of the appendage exposed to fast free-stream flow.Comment: 16 pages, 13 figures, 2 tables, under consideration for publication in Phys. Rev. Fluids; revisio

Secondary threshold amplitudes for sinuous streak breakdown

The nonlinear stability of laminar sinuously bent streaks is studied for the plane Couette flow at Re=500 in a nearly minimal box and for the Blasius boundary layer at Re_d*= 700. The initial perturbations are nonlinearly saturated streamwise streaks of amplitude AU perturbed with sinuous perturbations of amplitude AW. The local boundary of the basin of attraction of the linearly stable laminar flow is computed by bisection and projected in the AU – AW plane providing a well defined critical curve. Different streak transition scenarios are seen to correspond to different regions of the critical curve. The modal instability of the streaks is responsible for transition for AU ~ 25%–27% for the considered flows, where sinuous perturbations of amplitude below AW ~ 1%–2% are sufficient to counteract the streak viscous dissipation and induce breakdown. The critical amplitude of the sinuous perturbations increases when the streamwise streak amplitude is decreased. With secondary perturbations amplitude AW ~ 4%, breakdown is induced on stable streamwise streaks with AU ~ 13%, following the secondary transient growth scenario first examined by Schoppa and Hussain [J. Fluid Mech. 453, 57 (2002)]. A cross-over, where the critical amplitude of the sinuous perturbation becomes larger than the amplitude of streamwise streaks, is observed for streaks of small amplitude AU < 5%–6%. In this case, the transition is induced by an initial transient amplification of streamwise vortices, forced by the decaying sinuous mode. This is followed by the growth of the streaks and final breakdown. The shape of the critical AU – AW curve is very similar for Couette and boundary layer flows and seems to be relatively insensitive to the nature of the edge states on the basin boundary. The shape of this critical curve indicates that the stability of streamwise streaks should always be assessed in terms of both the streak amplitude and the amplitude of spanwise velocity perturbations

A Limited Self-Claimed Web-Based Survey COVID-19 Contamination among Iranian Healthcare Workers

Introduction: It is likely that high rate of healthcare workers (HCWs) infection has occurred in Iran, but there is not any proof yet.&nbsp;Objective: This study was conducted to highlight the rate of Iranian HCWs infected by COVID-19 and some of its surrounding points.&nbsp;Methods: This cross-sectional study was conducted in Tehran, Iran. Using web-based applications including WhatsApp, Telegram, Instagram and Facebook, the link to the questionnaire was sent and exposed to the eligible ones. The target population of the study was HCWs who were diagnosed as approved cases of COVID-19. They were asked about their baseline characteristics and also possible source of infection, symptoms onset, hospitalization and etc. All findings presented by frequency and percent.&nbsp;Results: From March 29, 2020 to April 5, 2020, a total of 452 HCWs had completed the online questionnaire of whom 50.9% were women; mostly were in the age range of 25-29 years old. Among the participants, physicians had the largest population with 312 people (69.0%). The most frequent clinical symptoms were fatigue, fever and myalgia, respectively. The highest frequency with 85 cases (18.8%) was reported their symptoms onset within 20-24th February, 2020. The most commonly used piece of equipment was gloves, which was used in 57.3% of the cases, followed by simple surgical mask, which was used by 47.1% of the participants. In 21.9% cases no personal protective equipment was used. Totally, 348 cases (91.6%) were treated in an outpatient setting and only 36 cases (9.5%) needed to be hospitalized. In 160 cases (35.4%), at least one other person was infected with COVID-19 in their household.&nbsp;Conclusions: Considerable number of participants that declared their infection in this study, emphasizes on the considerable rate of Iranian HCWs infected by COVID-19

Stability of a jet in crossflow

We have produced a fluid dynamics video with data from Direct Numerical Simulation (DNS) of a jet in crossflow at several low values of the velocity inflow ratio R. We show that, as the velocity ratio R increases, the flow evolves from simple periodic vortex shedding (a limit cycle) to more complicated quasi-periodic behavior, before finally exhibiting asymmetric chaotic motion. We also perform a stability analysis just above the first bifurcation, where R is the bifurcation parameter. Using the overlap of the direct and the adjoint eigenmodes, we confirm that the first instability arises in the shear layer downstream of the jet orifice on the boundary of the backflow region just behind the jet.Comment: Two fluid dynamics videos, high-resolution 1024x768 (~80MB), and low resolution 320x240 (~10MB), included in the ancillary file