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Subject: Numerical solution ×

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    Acoustic scattering in a small centrifugal compressor based on the use of linearized equations in a rotating frame
    (London : Academic Press, 2022) Jacob, Stefan; Trigell, Emelie; Mihaescu, Mihai; Åbom, Mats
    Numerical solutions of acoustic wave scattering are often used to describe sound propagation through complex geometries. For cases with flow, various forms of the convected equation have been used. A better alternative that includes vortex-sound interaction is instead to use the linearized and harmonic forms of the unsteady fluid flow governing equations. In this paper, a formulation of the linearized equations that include rotational effects, in an acoustic computation using a rotating frame of reference in a stationary geometry, is presented. We demonstrate that rotational effects can be important, e.g., when computing the transmission loss through high-speed compressors. The implementation of the proposed addition to the existing schemes is both simple and numerically inexpensive. The results are expected to have an impact on the research and development related to noise control of high-performance turbo-machinery, e.g., used in automotive or aviation applications at operating conditions that can be represented by steady background flows.
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    Raman-Kerr Comb Generation Based on Parametric Wave Mixing in Strongly Driven Raman Molecular Gas Medium
    (2020) Benoît, Aurélien; Husakou, Anton; Beaudou, Benoît; Debord, Benoît; Gérôme, Frédéric; Benabid, Fetah
    We report on experimental and theoretical demonstrations of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerr-type nonlinearity. This combination enabled us to transform a conventional quasiperiodic Raman comb into a comb with a single and smaller frequency spacing. This phenomenon is achieved using a hollow-core photonic crystal fiber filled with 40 bars of deuterium and pumped with a high-power picosecond laser. The resultant comb shows more than 100 spectral lines spanning over 220 THz from 800 nm to 1710 nm, with a total output power of 7.1 W. In contrast to a pure Raman comb, a 120 THz wide portion of the spectrum exhibits denser and equally spaced spectral lines with a frequency spacing of around 1.75 THz, which is much smaller than the lowest frequency of the three excited deuterium Raman resonances. A numerical solution of the generalized nonlinear Schrödinger equation in the slowly varying envelope approximation provides very good agreement with the experimental data. The additional sidebands are explained by cascaded four-wave mixing between preexisting spectral lines, mediated by the large Raman-induced optical nonlinearity. The use of such a technique for coherent comb generation is discussed. The results show a route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing.
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    Interference structure of above-threshold ionization versus above-threshold detachment
    (Bristol : IOP, 2012) Korneev, Ph.A.; Popruzhenko, S.V.; Goreslavski, S.P.; Becker, W.; Paulus, G.G.; Fetić, B.; Milošević, D.B.
    Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schrödinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for nearperpendicular emission the qualitative appearance of the spectrum is unaffected.