A comparative study on instability of steady flows in helical pipes

Gelfgat, Alexander

doi:10.1088/1873-7005/ab6618

Cited by 4 publications

(3 citation statements)

References 59 publications

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“…In the cases of short-wave instability, the oscillation frequency is finite, which allows for a direct comparison of the experimental and numerical results. However, as was reported in our recent studies (Gelfgat (2020); Nezihovski et al, (2022)), such comparison is quite difficult because of experimental noise, as well as multiple frequencies that usually exhibit a large spectral power in experimentally obtained spectra. The appearance of the multiple frequencies seems to contradict the linear stability concept.…”

Section: Resultsmentioning

confidence: 75%

Experimental and numerical study of primary instability of a two-phase stratified flow in a circular pipe

Nezihovski,

Barmak,

Gelfgat

et al. 2024

Preprint

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The onset of the primary instability of two-phase air-water stratified flow in a circular pipe was studied experimentally by two independent non-intrusive techniques. The techniques analyze the steady or oscillatory response of either a laser beam passing through the air-water interface, or of an echo of an ultrasound wave reflected from the interface. The experiments are accompanied by linear stability analysis performed in both Cartesian and bipolar coordinates, thus yielding two independently obtained numerical results for comparison. The critical values of the phases’ superficial velocities corresponding to the flow primary instability calculated numerically by the two independent methods agree well with each other. The computed stability diagram shows that instability sets in owing to long-wave disturbances at large holdups, while at smaller holdups, the most unstable perturbations appear as finite-length (short) waves. A comparison of the experimental and numerical results shows a good agreement for the long-wave instability. For the short-wave instability, the measured critical superficial velocities are below the computed ones, while the frequencies found experimentally for supercritical oscillatory flow states agree with the numerical predictions. Several additional peaks in the frequency spectra are observed in the experiments. By comparing these frequencies with the numerical spectra, we argue that they correspond to stable eigenmodes with close to zero decay rates, which can be triggered either by non-linear mechanisms, or become unstable because of experimental imperfections.

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Section: Resultsmentioning

confidence: 75%

Experimental and numerical study of primary instability of a two-phase stratified flow in a circular pipe

Nezihovski,

Barmak,

Gelfgat

et al. 2024

Preprint

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show abstract

“…Linear stability of two-phase stratified flow in rectangular horizontal ducts is studied numerically by a comprehensive approach involving evaluation of the steady base flow state and calculation of the leading eigenvalues of the linearized stability problem. The numerical approach is based on propositions of Gelfgat (2007) with the extension to uniform spatial direction along which the disturbances are assumed to be periodic Gelfgat (2020). Additionally, the infinitesimal perturbations of the capillary boundary separating two phases are taken into account.…”

Section: Discussionmentioning

confidence: 99%

“…The numerical approach is the same as in Gelfgat ( 2007) with an addition that allows for minimization of a critical parameter over the wave number, as is realized in Gelfgat (2020). The problem was solved on staggered grids using the finite volume method.…”

Section: Methodsmentioning

confidence: 99%