Development and characterization of a variable turbulence generation system

Experimental turbulent combustion studies require systems that can simulate the turbulence intensities [u′/U ₀ ~ 20–30% (Koutmos and McGuirk in Exp Fluids 7(5):344–354, 1989)] and operating conditions of real systems. Furthermore, it is important to have systems where turbulence intensity can be varied independently of mean flow velocity, as quantities such as turbulent flame speed and turbulent flame brush thickness exhibit complex and not yet fully understood dependencies upon both U ₀ and u′. Finally, high pressure operation in a highly pre-heated environment requires systems that can be sealed, withstand high gas temperatures, and have remotely variable turbulence intensity that does not require system shut down and disassembly. This paper describes the development and characterization of a variable turbulence generation system for turbulent combustion studies. The system is capable of a wide range of turbulence intensities (10–30%) and turbulent Reynolds numbers (140–2,200) over a range of flow velocities. An important aspect of this system is the ability to vary the turbulence intensity remotely, without changing the mean flow velocity. This system is similar to the turbulence generators described by Videto and Santavicca (Combust Sci Technol 76(1):159–164, 1991) and Coppola and Gomez (Exp Therm Fluid Sci 33(7):1037–1048, 2009), where variable blockage ratio slots are located upstream of a contoured nozzle. Vortical structures from the slots impinge on the walls of the contoured nozzle to produce fine-scale turbulence. The flow field was characterized for two nozzle diameters using three-component Laser Doppler velocimetry (LDV) and hotwire anemometry for mean flow velocities from 4 to 50 m/s. This paper describes the key design features of the system, as well as the variation of mean and RMS velocity, integral length scales, and spectra with nozzle diameter, flow velocity, and turbulence generator blockage ratio.     

Fractional conservation laws in optimal control theory

Using the recent formulation of Noether’s theorem for the problems of the calculus of variations with fractional derivatives, the Lagrange multiplier technique, and the fractional Euler–Lagrange equations, we prove a Noether-like theorem to the more general context of the fractional optimal control. As a corollary, it follows that in the fractional case the autonomous Hamiltonian does not define anymore a conservation law. Instead, it is proved that the fractional conservation law adds to the Hamiltonian a new term which depends on the fractional-order of differentiation, the generalized momentum and the fractional derivative of the state variable. Partially presented at FDA ’06—2nd IFAC Workshop on Fractional Differentiation and its Applications, 19–21 July 2006, Porto, Portugal.     

Droplets splashing upon films of the same fluid of various depths

We explore the effects of fluid films of variable depths on droplets impacting into them. Corresponding to a range of fluid “film” depths, a non-dimensional parameter—H*, defined as the ratio of the film thickness to the droplet diameter—is varied in the range 0.1≤H*≤10. In general, the effect of the fluid film imposes a dramatic difference on the dynamics of the droplet–surface interaction when compared to a similar impact on a dry surface. This is illustrated by the size distribution and number of the splash products. While thin fluid films (H*≈0.1) promote splashing, thicker films (1≤H*≤10) act to inhibit it. The relative roles of surface tension and viscosity are investigated by comparison of a matrix of fluids with low and high values of these properties. Impingement conditions, as characterized by Reynolds and Weber numbers, are varied by velocity over a range from 1.34 to 4.22 m/s, maintaining a constant droplet diameter of 2.0 mm. The dependence of splashing dynamics, characterized by splash product size and number, on the fluid surface tension and viscosity and film thickness are discussed.     

Modeling of the catalytic properties of high-temperature surface insulation in a dissociated carbon dioxide — Nitrogen mixture

A model of the heterogeneous catalysis of a dissociated carbon dioxide — nitrogen mixture on high-temperature heat-shield coatings is developed; the model takes into account nonequilibrium adsorption-desorption reactions of nitrogen and oxygen atoms and carbon dioxide molecules and their recombination in the Eley-Rideal reactions. On the basis of a comparison of the calculated heat fluxes in dissociated carbon dioxide with those measured on the VGU-4 plasma generator of the Institute for Problems in Mechanics of the RAS, the parameters of the catalysis model in question are chosen for three modern oxidant-resistant coating materials. The performances of these coatings are compared for the conditions of Mars Miniprobe entry into the Martian atmosphere. Their usability for the entry path considered is shown. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 106–116, January–February, 2000. The study was carried out with the support of the Russian Foundation for Basic Research (project No. 99-01-00259) and the “Integratisya“ Federal Program (project No. 2.1–414).     

Rotationally symmetric spontaneous swirling in MHD flows

The stability of steady axisymmetricMHD flows of an inviscid, incompressible, perfectly conducting fluid with respect to swirling—perturbations of the azimuthal components of the velocity field—is studied in a linear approximation. It is shown that for flows similar to a magnetohydrodynamic Hill-Shafranov vortex, the problem reduces to a one-dimensional problem on a closed streamline of the unperturbed flow (the arc length of the streamline is the spatial coordinate). A spectral boundary-value eigenvalue problem is formulated for a system of two ordinary differential equations with periodic coefficients and periodic boundary conditions. Sufficient conditions under which swirling is impossible are obtained. Numerical solution of the characteristic equation shows that, under certain conditions, for each streamline there is a real eigenvalue that yields monotonic exponential growth of the initial perturbations. Lavrent’ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 5, pp. 120–129, September–October, 2000.     

Hypertractions and hyperstresses convey the same mechanical information

A strengthened and generalized version of the standard Virtual Work Principle is shown to imply, in addition to bulk and boundary balances, a one-to-one correspondence between surface and edge hypertractions and hyperstress fields in second-gradient continua. When edge hypertractions are constitutively taken null, the hyperstress is shown to take the form it has for a relevant example of second-gradient fluid-like material, referred to as a Navier–Stokes—α fluid.     

Deformation and fracture of a spheroplast under low-cycle loading at various temperatures

This paper gives the results of an experimental study of the deformation and fracture of a spheroplast under uniaxial low-cycle loading (compression and unloading) at a temperature T = 25 and 100°C. Various mechanisms of damage accumulation at various temperatures and degrees of damage to the material are studied. The experimental results are compared with the well-known dependences taking into account damage accumulation for metals. It is established that the basic propositions of these theories are suitable for the low-cycle fracture of spheroplast — a ductile material of complex structure. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 1, pp. 197–204, January–February, 2009.     

An investigation of the losipescu and asymmetrical four-point bend tests

The Iosipescu shear test, utilizing a notched specimen in bending and a modification—the asymmetrical four-point bend (AFPB) test—were evaluated as shear tests for composites. This paper summarizes the results of an extensive numerical and experimental investigation of the Iosipescu and AFPB test methods. Finite-element analyses were conducted to assess the influence of notch parameters and load locations on the stress state in the specimen. The shear moduli and the shear strengths were experimentally measured for a quasiisotropic graphite/epoxy laminate using both the Iosipescu and the AFPB test methods. The tests were conducted for various combinations of notch parameters and load locations. The test results indicate that changes in the notch geometry and load locations aimed at improving the stress distribution in the test section resulted in unexpected changes in the failure mode. Paper was presented at the 1985 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 9–14.     

Use of magnetocumulative generators in experiments on current disruption of shaped-charge jets

Results of experiments on disruption of shaped-charge jets by a pulsed current are reported. An industrially produced helical-coaxial magnetocumulative K-80 generator with transformer energy output was used as a source of energy. The operation of the generator in the experiments performed and the effect of the current-pulse parameters on jet disruption and depth of penetration of a shaped-charge jet into a target are discussed. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 3, pp. 30–35, May–June, 1998.     

Numerical design of multimodal axisymmetric hypersonic nozzles for wind tunnels

A method for design of hypersonic nozzles for wind tunnels is developed and implemented on the basis of solving direct problems with various models of the medium and numerical methods of integration of gas-flow equations. Multimodal nozzles for operation in Mach number ranges M _out =8–14 and M _out =14–20 satisfying specified requirements are designed.     

Specific features of laser cutting of steel sheets and monitoring of sample quality after laser influence

An automated laser system with an 8-kW quantum generator with the beam quality not worse than that of a single-mode laser is described. The possibility of using such a system in procurement production for cutting carbon and stainless steel sheets is demonstrated. The quality control of the blank material shows that the properties of the latter satisfy appropriate standards. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 176–184, July–August, 2006.     

Aerodynamic modification of flow over bluff objects by plasma actuation

Particle image velocimetry and smoke visualization are used to study the alteration of the flow field in the wake of a bluff body by use of an alternating current (AC) surface dielectric barrier discharge. Staggered, surface, and buried electrodes were positioned on the downstream side of circular cylinders at conditions of Re _D = 1 × 10⁴−4 × 10⁴ configured to impose a force due to the ion drift that is either along or counter to the free-stream flow direction. Smoke visualization and Particle Image Velocimetry (PIV) in the wake of the flow confirms that the configuration of the surface electrodes and operation of the discharge significantly alters the location of the flow separation point and the time-averaged velocity profiles in the near and distant wake. Measurements of the vibrational and the rotational temperature using optical emission spectroscopy on the N₂ second positive system (C³Π_u–B³Π_g) indicates that the resulting plasma is highly non-equilibrium and discounts the possibility of a thermal effect on the flow separation process. The mechanism responsible for reduction or enhancement of flow separation is attributed to the streamwise force generated by the asymmetric ion wind—the direction of which is established by the electrode geometry and the local surface charge accumulated on AC cycles.     

Nonequilibrium thermal dissociation of diatomic molecules with VV,VT, and VT′ exchanges

Nonequilibrium dissociation in a mixture of anharmonic oscillators — molecules of a diatomic gas and its atoms — is investigated with allowance for the temperature gradient in the boundary layer. The effect of VT, VV, and VT′ exchanges on the dissociation rate is considered, and the nonequilibrium correction to the macroscopic reaction rate, which depends, in particular, on the temperature gradient and degree of dissociation, is determined. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 171–179, January–February, 1994.     

Effect of an electric field on the stability of convective flow in a vertical layer

The effect of such factors as the electrization of the fluid and the action of an electric field on the stability of the flow in a vertical layer is investigated numerically. Volume electrization of a nonuniformly heated weakly conducting fluid with a linear dependence of conductivity on temperature is used as a model for describing the electrical effects. The influence of electrical factors is taken into account by means of two parameters — the electrical Prandtl and Grashof numbers (Pe and Ge, respectively). For small Pe low values of Ge correspond to stabilization of the flow expressed as an increase in the critical Grashof numberG. However, as Ge increases, stabilization is replaced by destabilization and the criticalG decreases to zero as Ge increases without bound. As Pe increases, the initial stabilization may disappear, but the nature of the destabilization at large values of Ge remains unchanged. The actual form of the dependence of the criticalG on Ge is determined not only by the value of Pe but also by the Prandtl numberP. Perm. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 35–41, July–August, 1994.     

The Semiclassical Regime for Ablation Front Models

In this paper, we study two problems appearing in two-dimensional fluid mechanics in a constant gravity field . These two problems—the Rayleigh convection problem and the ablation front problem—generalize the Rayleigh–Taylor model in compressible flows. The analysis of their stability relies on semiclassical techniques for the linearized system around a reference solution. We consider normal modes which are approximate solutions corresponding to large wave numbers associated with , and we discuss the existence or the non-existence of such normal modes. The results depend on the value of the dimensionless growth rate Γ compared with two relevant mathematical parameters, namely σ _p (p standing for the model) and some effective semiclassical parameter h.     

Thermovibrational convective instability of mechanical equilibrium of an inclined fluid layer

The convective instability of mechanical equilibrium of an inclined plane layer of fluid developing under the action of a static gravity field and high-frequency vibration is studied. Configurations corresponding to four directions of the equilibrium temperature gradient — vertical, longitudinal, horizontal, and transverse — are considered for an arbitrary orientation of the vibration axis. The stability limits and the characteristics of the critical perturbations are determined. Perm’. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 8–15, January–February, 1998. This investigation was carried out with partial support form RSA-NASA (contract No. 920/18 — 5208/96).     

Shock wave standoff distance for a sphere slightly above Mach one

This paper describes a numerical simulation of bow shock formation ahead of a sphere at steady supersonic flow in the Mach number range of 1.025–1.20. Turbulent viscous flow results are presented using the Spalart–Allmaras turbulence model. The purpose of this study is to determine the shock standoff distance for a spherical projectile at slightly supersonic free flight speeds. Results are compared to experimental data, including double exposure holographic interferograms obtained from a 40 mm polycarbonate sphere launched by a light gas gun. The shock standoff distance was determined from the interferograms. The present numerical simulations were found to agree with previously published data, and reached down to M = 1.025—a range where almost no previously published data exists. The computed flow structure and shock wave locations agree well with recently obtained free-flight interferograms.     

Design of Modified Tabs for Testing of Symmetric Composite Thin Walled Tubes Under Pure Torsional and Tensile Loadings

This paper is mainly concerned with designing modified tabbing systems for testing of composite thin-walled tubes with symmetric layup. The modified tabs are presented based on the concept of iso-displacement points—the points of the specimen that move together and behave similarly during a test. Numerical solutions are carried out for tubular specimens in order to find the displacement field and hence the iso-displacement points. The analyses are carried out for two loading cases: pure torsion and pure tension. Finite element analyses are conducted for both cases, to estimate the displacement fields of thin-walled tubular specimens and then predictive equation were determined using multiple nonlinear regression models. The accuracy of these models is evaluated using a multiple coefficient of determination, R ², where 0 ≤ R ² ≤ 1, coupled with predicted squared error (PSE) and mean squared error (MSE). R ² is found to be greater than or equal to 0.98 for all cases considered in this study while PSE and MSE values are minimized, demonstrating the reliability of the model. Comparison between the modified tabs, based on the iso-displacement points determined by predictive equations, with conventional tabs indicates an improvement in the uniformity of stress and strain distribution (which is an important factor in order to have a successful test) within the specimen for both load cases. It is also demonstrated that due to the elimination of the stress and strain non-uniformities, the measured permissible maximum load of the specimens is significantly increased during the test.     

On study of nonclassical problems of fracture and failure mechanics and related mechanisms

Nonclassical problems of fracture and failure mechanics that have been analyzed by the author and his collaborators at the S. P. Timoshenko Institute of Mechanics (Kiev, National Academy of Sciences of Ukraine) during the past forty years are considered in brief. The results of the analysis are presented in a form that would be quite informative for the majority of experts interested in various fundamental and applied aspects of fracture and failure problems including the identification of related mechanisms. This paper was prepared on invitation of the Editorial Board of the journal “Annals. The European Academy of Sciences” and may be considered as an Extended Pascal Medal Lecture (The 2007 Blaise Pascal Medal in Materials Sciences of the EAS) This is an updated edition of the author’s lecture prepared at the invitation of the Annals—The European Academy of Sciences Magazine on the occasion of awarding him the 2007 Blaise Pascal Medal in Materials Sciences by the EAS. The author’s speech at the award ceremony at the General Assembly of the Academy has already been published in International Applied Mechanics [75]. The electronic version of the paper in Annals has been prepared; this issue of Annals is to be published as a book. The paper includes an additional section and extended list of references [41–99]. Published in Prikladnaya Mekhanika, Vol. 45, No. 1, pp. 3–40, January 2009.     

Correlation of the separation region length in shock wave/channel boundary layer interaction

 Results of an experimental investigation of the characteristics of a separation region induced by the interaction of an externally generated oblique shock with the turbulent boundary layer formed in a rectangular half channel are discussed. The experiments were carried out in the supersonic wind tunnel of the Institute of Theoretical and Applied Mechanics SB RAS at a free-stream Mach number M _∞=3.01 over a range of Reynolds numbers Re ₁=(9.7–47.5)×10⁶ m^-1 and at zero incidence and zero yaw of the model. Particular attention is paid to the size of the zone of the upstream propagation of disturbances (upstream influence region) under different experimental conditions: with varied values of the shock wave strength, half channel width, and Reynolds number. It is shown, in particular, that the normalized upstream influence region length as a function of inclination angle of the shock generator in a rectangular half channel is readily approximated by a simple exponential function. In support of the known reference data obtained for supersonic numbers M _∞ and moderate Re in other configurations, it is also shown that the upstream influence region length decreases with increasing Reynolds number. Generalization of experimental data on the length of the upstream influence region formed in similar geometric configurations is possible using an additional reference linear scale which is the distance from the leading edge of the shock generator to the exposed surface. A substantial dependence of the reference dimensions of separation region on the half channel width is also established. Received: 20 January 1997/Accepted: 30 June 1997