Steady Translation Research Articles

Wall effects of an eccentric fluid sphere: Happel's and Kuwabara's models

AbstractIn the limit of very low Reynolds numbers, the steady translation of a fluid sphere situated at a non‐concentric position in a second immiscible fluid bounded by spherical cavity is investigated using boundary collocation technique. The flow inside and outside the fluid sphere is governed by Stokes equations. Boundary conditions at the fluid sphere are continuity of velocity and shear stress. Zero shear stress and zero vorticity are used on spherical cavity. By using superposition principle, a general solution is constructed from the basic solutions in the two spherical coordinate systems based on both the fluid sphere and spherical cavity. The normalized drag force exerted on the fluid sphere contained in a cavity is depended on the ratio of fluid sphere and cavity radii, viscosity ratio of fluid sphere and cavity, relative distance between the centers of fluid sphere and cavity. The results are in good agreement with previously published work in the limiting cases. The numerical results of the drag force are good agreement with the solutions of the translation of the fluid sphere in a concentric cavity and the translation of a solid sphere in a non‐concentric cavity. The drag force is increasing function of the viscosity ratio, volume fraction, and the relative distance. This work is extension of the previous work of Keh and Lee for the fluid sphere using Happel's and Kuwabara's models.

Flow past a sphere translating along the axis of a rotating fluid: revisiting numerically Maxworthy's experiments

We compute the flow induced by the steady translation of a rigid sphere along the axis of a large cylindrical container filled with a low-viscosity fluid set in rigid-body rotation, the sphere being constrained to spin at the same rate as the undisturbed fluid. The parameter range covered by the simulations is similar to that explored experimentally by Maxworthy (J. Fluid Mech., vol. 40, 1970, pp. 453–479). We describe the salient features of the flow, especially the internal characteristics of the Taylor columns that form ahead of and behind the body and the inertial wave pattern, and determine the drag and torque acting on the sphere. Torque variations are found to obey two markedly different laws under rapid- and slow-rotation conditions. The corresponding scaling laws are predicted by examining the dominant balances governing the axial vorticity distribution in the body vicinity. Results for the drag agree well with the semi-empirical law proposed for inertialess regimes by Tanzosh & Stone (J. Fluid Mech., vol. 275, 1994, pp. 225–256). This law is found to apply even in regimes where inertial effects are large, provided that rotation effects are also large enough. Influence of axial confinement is shown to increase dramatically the drag in rapidly rotating configurations, and the container length has to be approximately a thousand times larger than the sphere for this influence to become negligibly small. The reported simulations establish that this confinement effect is at the origin of the long-standing discrepancy existing between Maxworthy's results and theoretical predictions.

Steady sphere translation in weakly viscoelastic UCM/Oldroyd-B fluids with perfect slip on the sphere

We study analytically the influence of perfect slip boundary conditions to the steady translation of a spherical particle in a viscoelastic fluid which consists of a Newtonian solvent and a polymer solute. The viscoelastic fluid is modeled with the Oldroyd-B constitutive equation with the ratio of polymer zero shear rate viscosity to the total viscosity, η, considered as a fluid parameter and varying from zero (Newtonian limit) to 1 (Upper Convected Maxwell (UCM) limit). The solution for all the dependent flow variables is expanded as asymptotic power series with the small parameter being the Weissenberg number, Wi=λU/R, where λ is the single relaxation time of the fluid, R is the radius of the particle and U is its steady translational velocity. The resulting sequence of equations is solved analytically up to eight-order in Wi. Hence, the current work is a substantial expansion of the previous work by Gkormpatsis et al., (2020) which has examined the most general case of Navier slip, but the perturbation series has been found only up to fourth order in Wi. The current work confirms the results obtained in that study in the limit of perfect slip conditions. Moreover, the higher-order analytical solution allows for the better investigation of the convergence of the results with techniques that accelerate the convergence of series. Under perfect slip conditions, it is shown that the relative drag force on the sphere increases monotonically with viscoelasticity, as opposed to the cases of no-slip or partial slip where the drag can also decrease or attain a local minimum. This difference in behavior is attributed to the higher dominance of extensional deformations close to the stagnation points of the flow, while the shear deformations prevail away from these points. Finally, evidence is provided that the UCM/Oldroyd-B models become singular at a finite Weissenberg number, Wiu, which depends weakly on η. Although the exact value of Wiu could not be calculated, a sequence of approximations which results from the eight-order perturbation solution reveals that 0.5<Wiu<1. Wiu appears to decrease as η increases, with its minimum value being observed for the UCM model.

Swimming dynamics of a self-propelled droplet

Chemically active droplets often show intriguing self-propulsion behaviour in a surfactant solution. The drop motion is controlled by the nonlinear coupling among chemical transport in the bulk fluid, consumption of surfactant at the drop surface, and the fluid flow driven by the self-generated Marangoni stress. To quantify the underlying hydrodynamics, this work investigates the swimming motion of a two-dimensional drop that is determined by two dimensionless parameters, the Péclet number ($Pe$) and Damköhler number ($Da$). The weakly nonlinear analysis shows that near the instability threshold, the drop undergoes a supercritical bifurcation with velocity $U\sim \sqrt {Pe-Pe_c}$, where $Pe_c$ is the critical Péclet number for the onset of dipole mode. In the highly nonlinear regime, the drop transits from steady translation of pusher swimming to unsteady motion of mixed pusher–puller swimming along zigzaging trajectories of quadrangle and/or triangle waves. Mode decomposition shows that the zigzag motion is directly related to the interaction between the secondary wake of low surfactant concentration and the primary wake.

The singularity of the UCM/Oldroyd-B models at a finite Weissenberg number, for the steady sphere translation with Navier slip on the sphere

• The steady sphere translation with Navier slip on the sphere is studied. • A singular Weissenberg number is revealed for the UCM/Oldroyd-B models. • The singularity depends on the viscosity ratio and the slip coefficient. • Consistent and convergent approximations of the singularity are derived. For the steady sphere translation in an isothermal viscoelastic fluid which is modelled with the Upper Convected Maxwell and Oldroyd-B models, evidence of a singularity at a finite Weissenberg number is provided, when Navier type linear slip is applied at the surface of the sphere. The singular Weissenberg number depends on the dimensionless slip coefficient which appears in the slip-law, as well as on the polymer viscosity ratio. The analysis is performed following Housiadas et al. (2021) for similar flows of spherical micro-swimmers with prescribed slip velocity at the surface of the swimmer. Although here the exact singularity could not be found, the perturbation solution by Gkormpatsis et al. (2020) is utilized to evaluate consistent approximations of it is magnitude.

Quantitative Proteomics Links the LRRC59 Interactome to mRNA Translation on the ER Membrane

Protein synthesis on the endoplasmic reticulum (ER) requires the dynamic coordination of numerous cellular components. Together, resident ER membrane proteins, cytoplasmic translation factors, and both integral membrane and cytosolic RNA-binding proteins operate in concert with membrane-associated ribosomes to facilitate ER-localized translation. Little is known, however, regarding the spatial organization of ER-localized translation. This question is of growing significance as it is now known that ER-bound ribosomes contribute to secretory, integral membrane, and cytosolic protein synthesis alike. To explore this question, we utilized quantitative proximity proteomics to identify neighboring protein networks for the candidate ribosome interactors SEC61β (subunit of the protein translocase), RPN1 (oligosaccharyltransferase subunit), SEC62 (translocation integral membrane protein), and LRRC59 (ribosome binding integral membrane protein). Biotin labeling time course studies of the four BioID reporters revealed distinct labeling patterns that intensified but only modestly diversified as a function of labeling time, suggesting that the ER membrane is organized into discrete protein interaction domains. Whereas SEC61β and RPN1 reporters identified translocon-associated networks, SEC62 and LRRC59 reporters revealed divergent protein interactomes. Notably, the SEC62 interactome is enriched in redox-linked proteins and ER luminal chaperones, with the latter likely representing proximity to an ER luminal chaperone reflux pathway. In contrast, the LRRC59 interactome is highly enriched in SRP pathway components, translation factors, and ER-localized RNA-binding proteins, uncovering a functional link between LRRC59 and mRNA translation regulation. Importantly, analysis of the LRRC59 interactome by native immunoprecipitation identified similar protein and functional enrichments. Moreover, [35S]-methionine incorporation assays revealed that siRNA silencing of LRRC59 expression reduced steady state translation levels on the ER by ca. 50%, and also impacted steady state translation levels in the cytosol compartment. Collectively, these data reveal a functional domain organization for the ER and identify a key role for LRRC59 in the organization and regulation of local translation.

HIGD2A is Required for Assembly of the COX3 Module of Human Mitochondrial Complex IV

Assembly factors play a critical role in the biogenesis of mitochondrial respiratory chain complexes I-IV where they assist in the membrane insertion of subunits, attachment of co-factors, and stabilization of assembly intermediates. The major fraction of complexes I, III and IV are present together in large molecular structures known as respiratory chain supercomplexes. Several assembly factors have been proposed as required for supercomplex assembly, including the hypoxia inducible gene 1 domain family member HIGD2A. Using gene-edited human cell lines and extensive steady state, translation and affinity enrichment proteomics techniques we show that loss of HIGD2A leads to defects in the de novo biogenesis of mtDNA-encoded COX3, subsequent accumulation of complex IV intermediates and turnover of COX3 partner proteins. Deletion of HIGD2A also leads to defective complex IV activity. The impact of HIGD2A loss on complex IV was not altered by growth under hypoxic conditions, consistent with its role being in basal complex IV assembly. Although in the absence of HIGD2A we show that mitochondria do contain an altered supercomplex assembly, we demonstrate it to harbor a crippled complex IV lacking COX3. Our results redefine HIGD2A as a classical assembly factor required for building the COX3 module of complex IV.

Hydrodynamic impulse generated by slender bodies in viscous flow

The forces and moments on slender axisymetric bodies are studied using a hydrodynamic impulse formulation for viscous flow in non-inertial body frames of reference. The problems of steady translation and steady turning are specifically addressed. A computational fluid dynamics data base is used to evaluate the hydrodynamic impulse for 3 submarine shaped bodies with slenderness ratios of 7.35, 8.38, and 8.75. It was found that the forces and moments calculated using the hydrodynamic impulse formulation give equivalent forces and moment to the traditional method of integrating pressure and shear stress on the body. In addition, it is shown that the lateral force distribution can be calculated directly from the Lamb vector distribution over cross sectional planes. Linearized forms of the hydrodynamic impulse equations are derived for both translation and steady rotation.

Widespread Exon Junction Complex Footprints in the RNA Degradome Mark mRNA Degradation before Steady State Translation.

Exon junction complexes (EJCs) are deposited on mRNAs during splicing and displaced by ribosomes during the pioneer round of translation. Nonsense-mediated mRNA decay (NMD) degrades EJC-bound mRNA, but the lack of suitable methodology has prevented the identification of other degradation pathways. Here, we show that the RNA degradomes of Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), worm (Caenorhabditis elegans), and human (Homo sapiens) cells exhibit an enrichment of 5' monophosphate (5'P) ends of degradation intermediates that map to the canonical EJC region. Inhibition of 5' to 3' exoribonuclease activity and overexpression of an EJC disassembly factor in Arabidopsis reduced the accumulation of these 5'P ends, supporting the notion that they are in vivo EJC footprints. Hundreds of Arabidopsis NMD targets possess evident EJC footprints, validating their degradation during the pioneer round of translation. In addition to premature termination codons, plant microRNAs can also direct the degradation of EJC-bound mRNAs. However, the production of EJC footprints from NMD but not microRNA targets requires the NMD factor SUPPRESSOR WITH MORPHOLOGICAL EFFECT ON GENITALIA PROTEIN7. Together, our results demonstrating in vivo EJC footprinting in Arabidopsis unravel the composition of the RNA degradome and provide a new avenue for studying NMD and other mechanisms targeting EJC-bound mRNAs for degradation before steady state translation.

Steady sphere translation in a viscoelastic fluid with slip on the surface of the sphere

We study analytically the effect of Navier type slip on the surface of a spherical particle which translates with constant velocity U in a viscoelastic ambient fluid. We assume steady state, isothermal and creeping flow conditions. The fluid is modelled with the Upper Convected Maxwell (UCM), Oldroyd-B, exponential Phan-Thien and Tanner (ePTT), and Giesekus constitutive equations. The solution for all the dependent flow variables is expanded as asymptotic power series with the small parameter being the Weissenberg number, Wi=λU/R, where λ is the single relaxation time of the fluid and R the radius of the particle. The resulting sequence of equations is solved analytically up to fourth order in Wi. Techniques that accelerate the convergence of series solutions are also applied in order to derive more accurate expressions for the drag force on the particle applicable to an extended range up to order unity Weissenberg number. The effects of viscoelasticity, the rheological parameters, and the slip coefficient are presented and discussed. For fixed Weissenberg number, the relative drag with respect to the Newtonian value increases with increasing slip.

The RGG/RG motif of AUF1 isoform p45 is a key modulator of the protein’s RNA chaperone and RNA annealing activities

ABSTRACTThe RNA-binding protein AUF1 regulates post-transcriptional gene expression by affecting the steady state and translation levels of numerous target RNAs. Remodeling of RNA structures by the largest isoform AUF1 p45 was recently demonstrated in the context of replicating RNA viruses, and involves two RNA remodeling activities, i.e. an RNA chaperone and an RNA annealing activity. AUF1 contains two non-identical RNA recognition motifs (RRM) and one RGG/RG motif located in the C-terminus. In order to determine the functional significance of each motif to AUF1’s RNA-binding and remodeling activities we performed a comprehensive mutagenesis study and characterized the wildtype AUF1, and several variants thereof. We demonstrate that each motif contributes to efficient RNA binding and remodeling by AUF1 indicating a tight cooperation of the RRMs and the RGG/RG motif. Interestingly, the data identify two distinct roles for the arginine residues of the RGG/RG motif for each RNA remodeling activity. First, arginine-mediated stacking interactions promote AUF1’s helix-destabilizing RNA chaperone activity. Second, the electropositive character of the arginine residues is the major driving force for the RNA annealing activity. Thus, we provide the first evidence that arginine residues of an RGG/RG motif contribute to the mechanism of RNA annealing and RNA chaperoning.

Gyrotropic elastic response of skyrmion crystals to current-induced tensions

We theoretically study the dynamics of skyrmion crystals in electrically insulating chiral magnets subjected to current-induced spin torques by adjacent metallic layers. We develop an elasticity theory that accounts for the gyrotropic force engendered by the nontrivial topology of the spin texture, tensions at the boundaries due to the exchange of linear and spin angular momentum with the metallic reservoirs, and dissipation in the bulk of the film. A steady translation of the skyrmion crystal is triggered by the current-induced tensions and subsequently sustained by dissipative forces, generating an electromotive force on itinerant spins in the metals. This phenomenon should be revealed as a negative drag in an open two-terminal geometry, or equivalently, as a positive magnetoresistance when the terminals are connected in parallel. We propose nonlocal transport measurements with these salient features as a tool to characterize the phase diagram of insulating chiral magnets.

The annual cycle of the West African monsoon in a two‐dimensional model: mechanisms of the rain‐band migration

The processes that drive the annual cycle of the West African Monsoon (WAM) are analysed using an idealized meridional–vertical numerical model that includes moist physics. Using the work by Peyrillé and Lafore (2007) as a starting point, the framework has been adapted to studying the annual cycle. A suitable forcing methodology for temperature and humidity has been derived which allows the two‐dimensional model to reproduce the main features of the WAM.A budget analysis of the simulated temperature and humidity variables leads to a picture of the intertropical convergence zone (ITCZ) seasonal displacement, for which the moistening on the northern side of the ITCZ is a key. It is due to the near‐surface moisture advection by the monsoon flow to the north of the ITCZ, in addition to the turbulent fluxes and shallow convection, which transport moisture to the top of the planetary boundary layer. On a larger scale, the warming of the Saharan heat‐low by turbulence and radiation and the cooling/moistening within the ITCZ by convective downdraughts reinforces the monsoon flow. Inertial instability is also at play favouring the acceleration of the monsoon flow and the associated humidification. These mechanisms seem to be at play during the whole seasonal cycle, which is seen as a steady translation of these structures.Sensitivity experiments show the importance of the low‐level processes such as downdraughts, horizontal advection and water recycling. Although advection is the first‐order process, the water recycling appears as a key element by directly modulating the intensity of rainfall and by allowing the convective downdraughts to feed back onto the WAM.

Ground effect on the aerodynamics of a two-dimensional oscillating airfoil

This paper reports results of an experimental investigation into ground effect on the aerodynamics of a two-dimensional elliptic airfoil undergoing simple harmonic translation and rotational motion. Ground clearance (D) ranging from 1c to 5c (where c is the airfoil chord length) was investigated for three rotational amplitudes (α m) of 30°, 45° and 60° (which respectively translate to mid-stroke angle of attack of 60°, 45° and 30°). For the lowest rotational amplitude of 30°, results show that an airfoil approaching a ground plane experiences a gradual decrease in cycle-averaged lift and drag coefficients until it reaches D ≈ 2.0c, below which they increase rapidly. Corresponding DPIV measurement indicates that the initial force reduction is associated with the formation of a weaker leading edge vortex and the subsequent force increase below D ≈ 2.0c may be attributed to stronger wake capture effect. Furthermore, an airfoil oscillating at higher amplitude lessens the initial force reduction when approaching the ground and this subsequently leads to lift distribution that bears striking resemblance to the ground effect on a conventional fixed wing in steady translation.

Electromagnetic drag on a magnetic dipole caused by a translating and rotating conducting cylinder

We report an analytical and numerical investigation of the forces and torques on a magnetic point dipole due to the presence of a moving electrically conducting cylinder. The kinematic induction problem is formulated in the quasistatic approximation that is appropriate for low magnetic Reynolds numbers. The motion of the cylinder is either a steady translation along its axis or a steady rotation about it. We find different power laws for the dependence of the force on the distance between dipole and cylinder when it is either small or large compared with the cylinder radius. The power laws for large distances are derived systematically by a long-wave expansion in the axial coordinate. For rotation, the case of a finite cylinder is studied by means of a dipole approximation.

Global vorticity shedding for a shrinking cylinder

AbstractWe study numerically the viscous flow around a steadily moving two-dimensional cylinder undergoing a rapid reduction in its diameter as a model problem for force production through shape change which is encountered in the locomotion of certain animals. We consider first the case of a rapidly collapsing circular cylinder in steady translation, starting from an original diameter and reaching a final, smaller diameter under prescribed kinematics. We show that the difference in added mass energy is recovered by the body, and the boundary layer vorticity is reduced through annihilation with opposite-sign vorticity generated during the reduction phase. Next we consider a steadily moving circular cylinder which undergoes rapid but orderly melting, resulting in the same reduction of its diameter but which exhibits radically different flow patterns compared to the collapsing cylinder. The original vorticity in the boundary layer is shed instantaneously and globally in the fluid at the start of the melting phase, and then rapidly rolls up to form a pair of strong vortices, which contain the energy difference between the original and final cylinder states. The formation of the vortices in the melting cylinder takes less than a third of the time required by a rigid translating cylinder to form such vortices.

An Effective Heuristic DSR

In this paper, we come up with an optimal DSR based on ant colony algorithm to improve the operational efficiency of wireless sensor network. Firstly we introduce the concepts of virtual grid and effective information propagation radius, then accord to the sensor’s translational speed and the rest energy to estimate the sensors’ persistence of life; besides, depend on the radius of effective information propagation we set the size of virtual grid and use this virtual grid as the unit to divide clusters. Finally, we realize the goal of steady information translation and effective energy usage. Under the same simulation environment, we compared the running results of DSR, LEACH and HDSR, the results illustrate that HDSR is not only able to improve the efficiency of information transportation but also equipoise the energy cost, it effectively increases the robustness of WSN.

Induced‐charge electrophoresis near a wall

Induced-charge electrophoresis (ICEP) has mostly been analyzed for asymmetric particles in an infinite fluid, but channel walls in real systems further break symmetry andlead to dielectrophoresis (DEP) in local field gradients. Zhao and Bau (Langmuir 2007, 23,4053) predicted that a metal (ideally polarizable) cylinder is repelled from an insulating wall in a DC field. We revisit this problem with an AC field and show that attraction to the wall sets in at high frequency and leads to an equilibrium distance, where DEP balances ICEP, although, in three dimensions, a metal sphere is repelled from the wall at all frequencies. This conclusion, however, does not apply to asymmetric particles. Consistent with the experiments of Gangwal et al. (Phys. Rev. Lett. 2008, 100, 058302), we show that a metal/insulator Janus particle is always attracted to the wall in an AC field. The Janus particle tends to move toward its insulating end, perpendicular to the field, but ICEP torque rotates this end toward the wall. Under some conditions, the theory predicts steady translation along the wall, perpendicular to the field, at an equilibrium tilt angle around 451, consistent with the experiments, although improved models are needed for a complete understanding of this phenomenon.

Hybrid charge control for stick–slip piezoelectric actuators

The article describes an analog electronic circuit for driving stick–slip piezoelectric linear actuators. The task for the amplifier is to provide a high-voltage asymmetric sawtooth-like signal and feed it into a capacitive load. Generation of excessive heat must be avoided while maximizing the slew rate. In order to guarantee a steady translation, the hysteretic behaviour of the piezoelectric material must be compensated. Combination of a charge control scheme with switching is proposed as an efficient solution. Laboratory experiments confirm the superiority of this tailored solution over other existing techniques based on versatile linear voltage amplifiers.

Enhancement of transport from drops by steady and modulated electric fields

We consider the problem of transport of heat or mass from circulating droplets that are both settling and subject to an axial electric field. The electric field can be either steady or oscillatory in time and drives an electrohydrodynamic flow, called the Taylor circulation, which augments the Hadamard circulation caused by steady translation. The problem is governed by four dimensionless groups: the Peclet number Pe, the dimensionless amplitudes of both the steady and unsteady electric field, and the dimensionless frequency ω of the modulation. The convective diffusion equation is solved numerically by an efficient finite-difference scheme that allows a wide range of parameters—in particular, very large Peclet numbers—to be covered. The results are characterized by the asymptotic rate of extraction of heat or mass from the droplet, which is found to be exponential in time. The enhancement factor, defined as the ratio of this rate to that of a stagnant drop, is studied as a function of parameters. For steady drops, we find that transport remains diffusion controlled, but the enhancement factor is significantly higher with the Taylor flow than without. For modulated electric field the enhancement factor is not a simple function of parameters and exhibits spectral “resonant peaks” at particular values of ω for which the enhancement factor is extremely large. Movies of the simulations are used to study the underlying time-periodic spatial structures of the concentration field (so-called strange eigenmodes) and the complex time dependence that is responsible for these resonances.