On the skin friction coefficient in viscoelastic wall-bounded flows

Analysis of the skin friction coefficient for wall bounded viscoelastic flows is performed by utilizing available direct numerical simulation (DNS) results for viscoelastic turbulent channel flow. The Oldroyd-B, FENE-P and Giesekus constitutive models are used. First, we analyze the friction coefficient in viscous, viscoelastic and inertial stress contributions, as these arise from suitable momentum balances, for the flow in channels and pipes. Following Fukagata et al. (Phys. Fluids, 14, p. L73, 2002) and Yu et al. (Int. J. Heat. Fluid Flow, 25, p. 961, 2004) these three contributions are evaluated averaging available numerical results, and presented for selected values of flow and rheological parameters. Second, based on DNS results, we develop a universal function for the relative drag reduction as a function of the friction Weissenberg number. This leads to a closed-form approximate expression for the inverse of the square root of the skin friction coefficient for viscoelastic turbulent pipe flow as a function of the friction Reynolds number involving two primary material parameters, and a secondary one which also depends on the flow. The primary parameters are the zero shear-rate elasticity number, El₀, and the limiting value for the drag reduction at high Weissenberg number, LDR, while the secondary one is the relative wall viscosity, μ_w. The predictions reproduce both types A and B of drag reduction, as first introduced by Virk (Nature, 253, p. 109, 1975), corresponding to partially and fully extended polymer molecules, respectively. Comparison of the results for the skin friction coefficient against experimental data shows good agreement for low and moderate drag reduction which is the region covered by the simulations.     

Mean first-passage time of a bistable kinetic model driven by two different kinds of coloured noises

The transient properties of a bistable system are investigated when both the multiplicative noise and the coupling between additive and multiplicative noises are coloured with different correlation time τ₁ and τ₂. The mean first-passage time (MFPT) is obtained. The numerical computations show that the intensities of additive and multiplicative noises and the coupling strength λ affected the MFPT in the same way, while τ₁ and τ₂ play different roles in the MFPT. The increase of τ₁ can reduce the escape rate, while increase of τ₂ can enhance the escape rate.     

Catalytic Asymmetric Inverse‐Electron‐Demand Oxa‐Diels–Alder Reaction of In Situ Generated ortho‐Quinone Methides with 3‐Methyl‐2‐Vinylindoles

The first catalytic asymmetric inverse‐electron‐demand (IED) oxa‐Diels–Alder reaction of ortho‐quinone methides, generated in situ from ortho‐hydroxybenzyl alcohols, has been established. By selecting 3‐methyl‐2‐vinylindoles as a class of competent dienophiles, this approach provides an efficient strategy to construct an enantioenriched chroman framework with three adjacent stereogenic centers in high yields and excellent stereoselectivities (up to 99 % yield, >95:5 d.r., 99.5:0.5 e.r.). The utilization of ortho‐hydroxybenzyl alcohols as precursors of dienes and 3‐methyl‐2‐vinylindoles as dienophiles, as well as the hydrogen‐bonding activation mode of the substrates met the challenges of a catalytic asymmetric IED oxa‐Diels–Alder reaction.     

A hybrid feedback control strategy for autonomous waypoint transitioning and loitering of unmanned aerial vehicles

We consider the problem of autonomously controlling a fixed-wing aerial vehicle to visit a neighborhood of a pre-defined waypoint, and when nearby it, loiter around it. To solve this problem, we propose a hybrid feedback control strategy that unites two state-feedback controllers: a transit controller capable of steering or transitioning the vehicle to nearby the waypoint and a loiter controller capable of steering the vehicle about a loitering radius. The aerial vehicle is modeled on a level flight plane with system performance characterized in terms of the aerodynamic, propulsion, and mass properties. Thrust and bank angle are the control inputs. Asymptotic stability properties of the individual control algorithms, which are designed using backstepping, as well as of the closed-loop system, which includes a hybrid algorithm uniting the two controllers, are established. In particular, for this application of hybrid feedback control, Lyapunov functions and hybrid systems theory are employed to establish stability properties of the set of points defining loitering. The analytical results are confirmed numerically by simulations.     

Efficient method for the synthesis of 2-(3-arylbenzo[f]quinolin-2-yl)ethanol derivatives through an unusual ring-opening of THF-involved reaction

An efficient synthesis of 2-(3-arylbenzo[f]quinolin-2-yl)ethanol derivatives by an unusual THF-involved reaction of aromatic aldehyde and naphthalen-2-amine promoted by iodine has been demonstrated with good yields. The ring-opening of THF reaction is proposed in the possible mechanism.     

Simultaneous speciation of inorganic arsenic and antimony in water samples by hydride generation-double channel atomic fluorescence spectrometry with on-line solid-phase extraction using single-walled carbon nanotubes micro-column

A new method was developed for the simultaneous speciation of inorganic arsenic and antimony in water by on-line solid-phase extraction coupled with hydride generation-double channel atomic fluorescence spectrometry (HG-DC-AFS). The speciation scheme involved the on-line formation and retention of the ammonium pyrrolidine dithiocarbamate complexes of As(III) and Sb(III) on a single-walled carbon nanotubes packed micro-column, followed by on-line elution and simultaneous detection of As(III) and Sb(III) by HG-DC-AFS; the total As and total Sb were determined by the same protocol after As(V) and Sb(V) were reduced by thiourea, with As(V) and Sb(V) concentrations obtained by subtraction. Various experimental parameters affecting the on-line solid-phase extraction and determination of the analytes species have been investigated in detail. With 180 s preconcentration time, the enrichment factors were found to be 25.4 for As(III) and 24.6 for Sb(III), with the limits of detection (LODs) of 3.8 ng L^− 1 for As(III) and 2.1 ng L^− 1 for Sb(III). The precisions (RSD) for five replicate measurements of 0.5 μg L⁻¹ of As(III) and 0.2 μg L⁻¹ of Sb(III) were 4.2 and 4.8%, respectively. The developed method was validated by the analysis of standard reference materials (NIST SRM 1640a), and was applied to the speciation of inorganic As and Sb in natural water samples.     

Bilinear approach for a symmetry constraint of the modified KdV equation

The paper investigates the mKdV equation with the potential under symmetry constraint through bilinear approach, i.e., Hirota method and Wronskian technique. We show that the potential can be a summation of squares of wave functions and these wave functions can precisely be described as Wronskians.     

Improved synthesis of dihydrothiophenes derivatives under ultrasound irradiation

A series of dihydrothiphenes derivatives were synthesized via the four‐component reaction of aldehyde, malononitrile, 1,3‐thiazolidinedione, and piperidine at room temperature under ultrasound irradiation. Compared with the conventional methods, the remarkable advantages of this method are operational simplicity, higher yield, and shorter reaction time. J. Heterocyclic Chem., (2011).