Theoretical studies of internal flow-induced instabilities of cantilever pipes

A theoretical analysis is made of the stability of a circular cross-section cantilever pipe containing an incompressible, inviscid fluid flowing uniformly and steadily in the axial direction. Beam and shell mode instabilities are examined. The study is based on the idea of obtaining a simple approximation for the fluid pressure and determining the importance of the downstream fluid behaviour. Results indicate that a pressure approximation (of the travelling wave type) can be made and that a reduction in the necessary numerical work can be achieved, with some small loss of accuracy, by making approximations to the in-plane inertia terms in the pipe equations.     

Peculiarities of deformation of compliant coatings with increased strength

Influence of elasticity module of coating material on the parameters of hard compliant coatings deformation has been analysed. Calculation using two-dimensional model has shown that maximum coating deformation is achieved at the ratio of flow rate U to the parameter C _t ⁰ = (E/3ρ)^0.5 approximately equal to 2.5, however, velocity of wall surface motion has first local maximum at U/C _t ⁰≈1. The range of coating parameters’ values at which compromise between its hardness and intensity of interaction with turbulent flow is provided has been determined. For rubbery materials with Poisson coefficient of about 0.5, correlations of the flow velocity and parameter C _t ⁰ shall be in the range 1÷1.5. It is shown that at such parameters, the mean square value of the coating surface deflection/inflection is less than the viscous sublayer thickness, its correlation with the wavelength is very small and equals (1÷5)·10⁻⁴. Such form of deformed surface fundamentally differs from the parameters of the wave wall in Kendall’s experiments which results are used for calculation of inverse influence of wall deformation on the flow. It was assumed that solid compliant coatings do not cause instability of interaction with the ambient flow.     

光子加速器的理论研究

研究了光子加速器(即利用等离子体中的朗谬尔波实现光脉冲频率上转换)所涉及的基本物理问题.文中导出了在存在朗谬尔波的条件下,光脉冲频率对于等离子体本底密度及电子密度扰动的依赖关系,并就最大频率转换、光脉冲俘获、波的加速位相(accelerating phase)以及位相滑动(phase slippage)效应进行讨论.     

Experimental studies of compliant coatings for reduction of turbulent friction

Measurement results on turbulent friction are presented for nine single-layer compliant coatings in comparison with a polished solid surface streamlined by the water flow. Three experimental series separated by time passed after coating fabrication were carried out. Results on monitoring of the viscoelastic properties of coating materials are shown; they allow analysing coating aging effect as the instability factor of coating hydrodynamic efficiency. The work was financially supported by the US Navy Research Department (No. 000140210743).     

Experimental investigation of electrohydrodynamic instabilities in micro channels

The European Physical Journal Special Topics - An electric field is applied to destabilize the interface between two Newtonian and immiscible liquids flowing in a rectangular micro channel. The...     

Drag reduction by compliant coatings made of a homogeneous material

The possibility of drag reduction due to compliant coatings of viscoelastic silicone rubbers has been tested experimentally. For this purpose, a series of single-layer coatings of various thicknesses was made of a homogeneous material. The experiments were carried out in a high-speed cavitation tunnel of Pusan National University. Dynamic viscoelastic properties of coating materials were carefully measured. The range of flow rates and coating thicknesses was calculated assuming that the coatings can interact intensively with the dynamic structures of the turbulent boundary layer only in the region of frequencies of their maximum compliance. The predicted range of coating parameters and flow velocities, in which the coatings reduce drag, is compared with experimental data.     

An investigation of oscillating instabilities in deflagrations with edges

We examine the Lewis-number-greater-than-1 stability of a deflagration sitting on a porous-plug burner with an inert coflow. The flame edges generated by the coflow influence the stability, and this influence is examined. Very wide flames display the same stability characteristics as unbounded flames (flames sans edges), but for moderately wide flames the instability is suppressed. A new two-dimensional instability can occur for narrow flames. There is a range of mass fluxes for which a monotonic decrease in burner (flame) width generates a transition from unstable flames to stable flames, to unstable flames, to quenching. The insertion of a cold probe into the combustion field can stabilize an unstable flame or destabilize a stable flame, depending on the point of insertion.     

A theoretical investigation of shell effects in deep inelastic collisions

The relative importance of three different influences of independent-particle shell structure on the nucleon transport process during heavy-ion collisions are discussed. Results of calculations for the 620-MeV⁸⁶Kr +¹⁹⁷Au system are presented.     

Strategy of a choice of single-layer compliant coatings for turbulent drag reduction

The main principles and methods of developing the single-layer monolithic compliant coatings for turbulent drag reduction are presented.     

A theoretical investigation of HeH+

Results are reported for the ground state (X¹Σ⁺) of the HeH⁺ ion. They include an improved Born-Oppenheimer potential curve, adiabatic corrections, and rovibronic energies for the three lowest vibrational states. The dissociation energy in the adiabatic approximation is found to be 14 873.6 cm^?1, and adiabatic effects decrease the two lowest vibrational spacings $\text{Δ}\text{G(}\text{1}\text{2}\text{)}$ and $\text{Δ}\text{G(}\text{3}\text{2}\text{)}$ by 0.9 and 0.8 cm^?1, respectively. The adiabatic rovibronic energies reported are thought to be accurate to within 1 cm^?1.     

A theoretical investigation of chirp insonification of ultrasound contrast agents

A theoretical investigation of second harmonic imaging of an Ultrasound Contrast Agent (UCA) under chirp insonification is considered. By solving the UCA’s dynamical equation analytically, the effect that the chirp signal parameters and the UCA shell parameters have on the amplitude of the second harmonic frequency are examined. This allows optimal parameter values to be identified which maximise the UCA’s second harmonic response. A relationship is found for the chirp parameters which ensures that a signal can be designed to resonate a UCA for a given set of shell parameters. It is also shown that the shell thickness, shell viscosity and shell elasticity parameter should be as small as realistically possible in order to maximise the second harmonic amplitude. Keller-Herring, Second Harmonic, Chirp, Ultrasound Contrast Agent.     

A theoretical investigation of electronic ground state of parent sulfur diimide

In this paper various properties of parent sulfur diimides (PSD) as the simplest member of sulfur diimides family have been considered using both DFT and ab initio methods employing correlation consistent basis set hierarchies. According to these calculations, EZ conformer is predicted to be the most stable conformer in accord with experimental observation, while its energy difference with ZZ conformer is completely minute. Comparison of microwave gas phase spectra and dense phase IR experimental data with theoretical calculations show a great coincidence. The properties of EE conformer have also been anticipated and its possible presence in dense phase have been discussed although no firm experimental data exists for this conformer. The possible pathways for interconversion between various conformers have been investigated and only two unimolecular type processes namely EZ ↔ EE and EZ ↔ ZZ interconversions have been established according to a pseudo-rotation mechanism as constituting the essential dynamic of system.     

A theoretical investigation of hydrolysis of borides and borohydrides of alkali metals

A theoretical investigation of phase transitions of components of chemical reactions in the course of hydrolysis of borohydrides of alkaline metals МВН ₄ (M = Li, Nа, K, Rb, etc.) and dehydrogenation of borohydrides is performed; these reactions giving different hydrogen yield are compared. The percentage of the extracted hydrogen is estimated and the temperature dependence of its concentration is revealed. The phase transformation diagrams are constructed using the experimental data.     

A theoretical and matrix-isolation FT-IR investigation of the conformational landscape of N-acetylcysteine

The conformational landscape of N-acetylcysteine (NAC) has been investigated by a combined experimental matrix-isolation FT-IR and theoretical methodology. This combination is a powerful tool to study the conformational behavior of relatively small molecules. Geometry optimizations at the HF/3-21 level resulted in 438 different geometries with an energy difference smaller than 22 kJ mol⁻¹. Among these, six conformations were detected with a relative energy difference smaller than 10 kJ mol⁻¹ at the DFT(B3LYP)/6-31++G∗∗ level of theory. These were finally subjected to MP2/6-31++G∗∗ optimizations which resulted in five minima. The vibrational and thermodynamical properties of these conformations were calculated at both the DFT and MP2 methodologies. Experimentally NAC was isolated in an argon matrix at 16 K after being sublimated at 323 K. The most stable MP2 form appeared to be dominant in the experimental spectra but the presence of three other conformations with ΔE_MP2 < 10 kJ mol⁻¹ was also demonstrated. The experimentally observed abundance of the H-bond containing conformations appeared to be in good accordance with the predicted MP2 value.     

Vibrational investigation of 1-cyclopentylpiperazine: A combined experimental and theoretical study

FT-IR and Raman spectra of 1-cyclopentylpiperazine(1cppp)have been experimentally examined in the region of 4000–200cm-1.The optimized geometric parameters,conformational equilibria,normal mode frequencies and corresponding vibrational assignments of 1cppp(C9H18N2)are theoretically examined by means of B3LYP hybrid density functional theory(DFT)method together with 6-31++G(d,p)basis set.On the basis of potential energy distribution(PED)reliable vibrational assignments have been made and the thermodynamics functions,highest occupied and lowest unoccupied molecular orbitals(HOMO and LUMO)of 1cppp have been predicted.Calculations are employed for four different conformations in C1 and Cs point groups of 1cppp in gas phase.Comparison between the experimental and theoretical results indicates that B3LYP method is able to provide satisfactory results for predicting vibrational frequencies and the structural parameters,vibrational frequencies and assignments.Furthermore,C1(equatorial-axial)point group has been found as the most stable conformer of 1cppp.     

A theoretical and experimental investigation of long-pulse,electron-beam-produced rare gas plasmas

Visible emission spectroscopy (380-650 nm) has been performed on intense, electron beam (1 kA, 300 ns, 300 kV) produced Ar, Kr, and Xe plasmas at pressures ranging from 10 to 750 torr. Singly ionized and neutral lines dominated the spectra in all cases except argon, where only singly ionized lines occurred. The temperature of the plasma as determined by comparing line intensities was between 1 and 2 eV. A computer model for chemical kinetics of the electron-beam-produced plasmas in Ar, Kr, and Xe was formulated. The model predicts time, pressure, and temperature trends of the ions, neutrals, and electrons, which agree with trends found experimentally     

Experimental investigation of combustion instabilities of a mesoscale multinozzle array in a lean-premixed combustor

Self-excited combustion instabilities in a mesoscale multinozzle array, also referred to as a micromixer-type injector, have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. The injector assembly consists of sixty identical swirl injectors of 6.5 mm inner diameter, which are evenly distributed across the combustor dump plane. Their flow paths are divided into two groups – inner and outer stages – to form radially stratified reactant stoichiometry for the control of self-excited instabilities. OH PLIF measurements of stable flames reveal that the presence of radial staging has a remarkable influence on stabilization mechanisms, reactant jet penetration/merging, and interactions between adjacent flame fronts. In an inner enrichment case, two outer (leaner) streams merge into a single jet structure, whereas the inner (richer) reactant jets penetrate far downstream without noticeable interactions between neighboring flames. The constructed stability map in the 〈?_i, ?_o〉 domain indicates that strong self-excited instabilities occur under even split and outer enrichment conditions at relatively high global equivalence ratios. This is attributed to large-scale flame surface deformation in the streamwise direction, as manifested by vigorous detachment/attachment movements. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations, because the center flames are securely anchored during the whole period of oscillation, giving rise to a moderate lateral motion. We demonstrate that the collective motion of sixty flames – rather than the individual local flame dynamics – play a central role in the development of limit cycle oscillations. This suggests that the distribution pattern of the injector array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.