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  • 1
    Electronic Resource
    Electronic Resource
    Lighthill transpiration velocity revisited: An exact formulation (1995)
    Springer
    Meccanica 30 (1995), S. 127-137 
    Details
    ISSN: 1572-9648
    Keywords: Potential-vorticity decomposition ; Transpiration velocity ; Boundary layer ; Fluid Mechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Sommario Nell'analisi di flussi viscosi di interesse aeronautico, la perturbazione al flusso esterno, dovuta agli effetti della viscosità che si manifestano al contorno, è comunemente approssimata mediante la velocità di traspirazione. Tale approssimazione consiste nell'assumere l'esistenza di un flusso uscente dalle pareti stesse, avente velocità (detta di traspirazione) determinata sulla base delle caratteristiche locali dello strato limite. In questo lavoro, tramite lo sviluppo di una nuova decomposizione potenziale-vorticità per il campo di velocità, viene derivata una formulazione esatta per la descrizione degli effetti dello strato limite sul flusso esterno. Questa viene confrontata con la rappresentazione approssimata del potenziale esterno basata sulla velocità di traspirazione: si mostra che, sotto tipiche ipotesi di strato limite, la nuova si riduce a quella di Lighthill, a meno di un termine correttivo di campo. Infine, per valutare quantitativamente il contributo del termine di campo presente nella nuova formulazione, vengono presentati i risultati numerici relativi al semplice caso di uno strato limite attaccato generato dal moto traslatorio uniforme di una lastra piana: essi indicano che il termine correttivo è trascurabile per numeri di Reynolds superiori a 104.
    Notes: Abstract The Lighthill transpiration-velocity correction is commonly used in the analysis of viscous flows of aeronautical interest, in order to take into account the perturbation to the potential flow caused by the presence of the vorticity in the boundary layer. This correction consists of considering the boundary as a permeable surface from which the fluid flows through the boundary surface, with a velocity (named the transpiration velocity) determined on the basis of the local boundary-layer characteristics. Here, we use a new potential-vorticity decomposition in order to derive an exact representation of the effects of the vorticity on the external flow. The relationship between approximate transpiration-velocity representation and the exact one presented here is analyzed: it is shown that, under typical boundary-layer assumptions, the new representation reduces to that by Lighthill, except for a corrective field term. Finally, in order to quantify, in a simple case, the contribution of the corrective terms which arise in the new formulation, we examine, as a numerical test case, the problem of an attached boundary-layer flow over a flat plate: the numerical results indicate that the corrective term is negligible for Reynolds numbers above 104.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00990451
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  • 2
    Electronic Resource
    Electronic Resource
    Helmholtz decomposition revisited: Vorticity generation and trailing edge condition (1986)
    Springer
    Computational mechanics 1 (1986), S. 65-90 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The use of the Helmholtz decomposition for exterior incompressible viscous flows is examined, with special emphasis on the issue of the boundary conditions for the vorticity. The problem is addressed by using the decomposition for the infinite space; that is, by using a representation for the velocity that is valid for both the fluid region and the region inside the boundary surface. The motion of the boundary is described as the limiting case of a sequence of impulsive accelerations. It is shown that at each instant of velocity discontinuity, vorticity is generated by the boundary condition on the normal component of the velocity, for both inviscid and viscous flows. In viscous flows, the vorticity is then diffused into the surroundings: this yields that the no-slip conditions are thus automatically satisfied (since the presence of a vortex layer on the surface is required to obtain a velocity slip at the boundary). This result is then used to show that in order for the solution to the Euler equations to be the limit of the solution to the Navier-Stokes equations, a trailing-edge condition (that the vortices be shed as soon as they are formed) must be satisfied. The use of the results for a computational scheme is also discussed. Finally, Lighthill's transpiration velocity is interpreted in terms of Helmholtz decomposition, and extended to unsteady compressible flows.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00298638
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  • 3
    Electronic Resource
    Electronic Resource
    Boundary integral equations and conservative dissipation schemes for full potential transonic flows (1993)
    Springer
    Computational mechanics 13 (1993), S. 90-99 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A boundary integral formulation for the nonlinear aerodynamic analysis of three-dimensional full-potential transonic flows is presented. The emphasis here is on the analysis of the effects on the solution of artificial dissipation schemes, which are necessary in order to capture properly the physics of the phenomenon. The main novelty is the use of conservative schemes, never previously used in boundary integral formulations where all the existing approaches are based on non-conservative ones. The conservative scheme presented here is an adaptation of concepts used in the CFD community. Specifically, a linear dissipation term is added directly to the continuity equation: hence the name artificial mass-generation scheme. Both conservative and non-conservative full-potential expressions for the nonlinear terms are discussed. The corresponding TSP (transonic small perturbation) formulation are also analyzed. Numerical results, for two-dimensional steady flows are presented in order to assess the different schemes. Good agreement is obtained with existing finite-difference and finite-volume results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00350705
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  • 4
    Electronic Resource
    Electronic Resource
    High-order BEM for potential transonic flows (1998)
    Springer
    Computational mechanics 21 (1998), S. 243-252 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A high-order boundary element method (BEM) for the analysis of the steady two-dimensional full-potential transonic equation is presented. The use of a high-order (piecewise cubic on the boundary and piecewise bi-cubic in the field) numerical formulation, the main novelty of the present work, is important in two respects: first, the convergence of the solution as h vanishes is faster than the zeroth-order (the piecewise constant) one, yielding more accurate results with coarser grid resolutions. In addition, in supercritical flows, the derivation of the velocity field from the high-order representation for potential gives, in the vicinity of the shock, a sharper discontinuity, and allows for an in-depth analysis of the shock properties. Both aspects are analyzed in the present paper through applications to steady, two-dimensional, full-potential flows. All the results obtained using the present method are validated through comparison to other computational fluid dynamics (CFD) solutions of the full-potential flows and, when applicable, Euler equations. A comparison to a zeroth-order BEM, based on the same integral formulation is also included.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004660050299
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  • 5
    Electronic Resource
    Electronic Resource
    Aerodynamics and aeroacoustics of wings and rotors via BEM - unsteady, transonic, and viscous effects (1998)
    Springer
    Computational mechanics 21 (1998), S. 265-275 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Recent developments on a general boundary integral formulation for the aerodynamic and aeroacoustic analyses of lifting bodies (e.g., wings and rotors) are reviewed. The emphasis is on recent numerical results, specifically on the effects of the unsteadiness, of the viscosity, and of the transonic nonlinearities. The boundary-element full-potential formulation for bodies in arbitrary motion is outlined along with the its extension to viscous/inviscid interaction. The effects of viscosity are taken into account through a coupled boundary-layer/full-potential technique similar to that used in the CFD community. Numerical results obtained with the present formulation are compared against numerical and experimental results available in literature. They include (i) aerodynamics and aeroacoustics of helicopter rotors in hover and forward flight in subsonic flows; (ii) transonic aerodynamics/acoustics results for steady potential flows around airfoils and hovering rotors, and (iii) viscous flows (subsonic and transonic) around airfoils.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004660050302
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  • 6
    Electronic Resource
    Electronic Resource
    A vorticity-only formulation and a low-order asymptotic expansion solution near Hopf bifurcation (1997)
    Springer
    Computational mechanics 20 (1997), S. 229-241 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A vorticity-only formulation is used in order to study the behavior of the solutions of the Navier-Stokes equations for two-dimensional incompressible flows as the Reynolds number is increased. This approach allows one to limit the numerical-solution domain to the vortical region of the flow, thereby reducing the number of the state variables of the system. The vorticity-only formulation is obtained from a vorticity-stream function formulation by inverting the Poisson equation relating the vorticity to the stream function and substituting the expression for the velocity in the vorticity-transport equation. The vorticity at the solid boundary is determined from the boundary conditions. The resulting dynamical system consists of a set of first-order ordinary differential equations having only quadratic nonlinearities. This system is then used to address the behavior of the solution beyond the stability boundary, within the context of the theory of dynamical systems. This part of the paper is general and is based on the use of a singular-perturbation technique, known as the method of multiple time scales; formulae for the nonlinear analysis near a Hopf bifurcation are given explicitly in terms of the coefficients of the dynamical system. Preliminary numerical results for the validation of the formulation are presented for the limited case of two-dimensional flows around a circular cylinder. These results include the steady-state solution with varying Reynolds number, the eigenvalues and the eigenvectors of the related stability matrix, and the characteristics of the corresponding limit cycle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004660050243
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  • 7
    Electronic Resource
    Electronic Resource
    Boundary integral equation for wave equation with moving boundary and applications to compressible potential aerodynamics of airplanes and helicopters (1989)
    Springer
    Computational mechanics 4 (1989), S. 231-243 
    Details
    ISSN: 1432-0924
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This work presents a general boundary-integral-equation methodology for the solution of the wave equation around objects moving in arbitrary motion, with applications to compressible potential aerodynamics of airplanes and helicopters. The paper includes the derivation of the boundary integral equation for the wave equation, in a frame of reference moving in arbitrary motion (in particular, in translation and in rotation). The formulation is then applied to study unsteady potential compressible aerodynamic flows around streamlined bodies, such as airplanes and helicopters. The formulation is given in terms of the velocity potential, for which an explicit treatment of the wake is required; a discussion of the formulation for the wake transport is included. The advantages of the velocity-potential formulation over the acceleration-potential formulation are discussed. The boundary-element algorithm used for the computational implementation is briefly outlined. Validation of the formulation is presented for airplane wings and helicopter rotors in hover. The test cases fall into two categories. prescribed-wake and free-wake analyses. The validation of the prescribed-wake analysis is presented for compressible flows, subsonic for helicopter rotors, transonic for airplanes. The numerical validation of the free-wake analysis of helicopter rotors is presented for incompressible flows.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00301382
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  • 8
    Electronic Resource
    Electronic Resource
    Thermal decomposition of bis(dimethylglyoximate) iron(II) complexes containing axial N-heterocyclic ligands (1990)
    Springer
    Journal of thermal analysis and calorimetry 36 (1990), S. 7-15 
    Details
    ISSN: 1572-8943
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Es wird eine systematische TG- und DTA-Untersuchung der thermischen Zersetzung einer Reihe von Bis(dimethylglyoximat)-Komplexe von Eisen(II) mit axialem N-heterocyclischen Liganden beschrieben. Die Zersetzungsreaktion ist sehr exotherm, was mit dem Verlust der axialen Liganden zusammenfällt. Die durchschnittliche Zersetzungstemperatur ist den Basizitäts eigenschaften der axialen Liganden proportional.
    Notes: Abstract A systematic TG and DTG study of the thermal decomposition of a series of bis(dimethylglyoximate)iron(II) complexes containing axial N-heterocyclic ligands is reported. The decomposition reaction is very exothermic, coinciding with the loss of the axial ligands. The average temperatures of decomposition correlate linearly with the basicity properties of the axial ligands.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01912062
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  • 9
    Electronic Resource
    Electronic Resource
    Lie transformation method for dynamical systems having chaotic behavior (1995)
    Springer
    Nonlinear dynamics 7 (1995), S. 403-428 
    Details
    ISSN: 1573-269X
    Keywords: Lie transformation (perturbation method) ; dynamical systems ; small divisors ; chaos
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The paper persents recent developments in a singular perturbation method, known as the “Lie transformation method” for the analysis of nonlinear dynamical systems having chaotic behavior. A general approximate solution for a system of first-order differential equations having algebraic nonlinearities is introduced. Past applications to simple dynamical nonlinear models have shown that this method yields highly accurate solutions of the systems. In the present paper the capability of this method is extended to the analysis of dynamical systems having chaotic behavior: indeed, the presence of “small divisors” in the general expression of the solution suggests a modification of the method that is necessary in order to analyze nonlinear systems having chaotic behavior (indeed, even non-simple-harmonic behavior). For the case of Hamiltonian systems this is consistent with the KAM (Kolmogorov-Arnold-Moser) theory, which gives the limits of integrability for such systems; in contrast to the KAM theory, the present formulation is not limited to conservative systems. Applications to a classic aeroelastic problem (panel flutter) are also included.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00121106
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  • 10
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    Boundary element analysis of unsteady aerodynamics of windmill rotors in the presence of yaw (1993)
    Elsevier
    Details
    Publication Date: 1993-05-01
    Print ISSN: 0167-6105
    Electronic ISSN: 1872-8189
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Published by Elsevier
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