25,581 research outputs found
On the gas pressure inside cavitation bubbles
No full textTUBITAK (The Scientific and Technological Research Council of Turkiye) [117M072]; National Center for High Performance Computing of Turkiye (UHeM) [1006552019]This paper is in part supported by TUEBITAK (The Scientific and Technological Research Council of Tuerkiye) under Grant No. 117M072. Computing resources used in this work were provided by the National Center for High Performance Computing of Tuerkiye (UHeM) under Grant No. 1006552019.The validity of the reduced order [Delale and Pasinlioglu, Acoustic cavitation model based on a novel reduced order gas pressure law, AIP Adv. 11, 115309 (2021)] and of classical polytropic gas pressure laws during the response of a bubble to variations in the pressure of the surrounding liquid is investigated. In particular, from the exact expression of the gas pressure coupled to the thermal behavior of gas bubbles, we identify the conditions where the reduced order gas pressure law and the classical polytropic law hold. We then distinguish three regimes for the change of state of the bubble during its nonlinear oscillations as the nearly isothermal, transition, and nearly adiabatic regimes, depending on the value of the polytropic index, and we determine the mean value of the polytropic index in each regime by averaging over a parameter, which is a function of the Peclet number based on the characteristic thermal diffusion time. The results of the temporal evolution of the bubble radius, the bubble wall temperature, and the partial gas pressure inside the bubble are presented using an acoustic cavitation model based on the reduced order gas pressure law for both constant and variable interface properties.WOS:0009378540000032-s2.0-85149178094Science Citation Index ExpandedarticleUluslararası işbirliği ile yapılmayan - HAYIRMartYÖK - 2022-2
Acoustic cavitation model based on a novel reduced order gas pressure law
No full textThe thermal behavior of a spherical gas bubble in a liquid excited by an acoustic pressure signal is investigated by constructing an iterative solution of the energy balance equations between the gas bubble and the surrounding liquid in the uniform pressure approximation. This iterative solution leads to hierarchy equations for the radial partial derivatives of the temperature at the bubble wall, which control the temporal rate of change of the gas pressure and gas temperature within the bubble. In particular, a closure relation for the hierarchy equations is introduced based on the ansatz that approximates the rapid change of state during the collapse of the bubble from almost isothermal to almost adiabatic behavior by time averaging the complex dynamics of change of state over a relatively short characteristic time. This, in turn, leads to the desired reduced order gas pressure law exhibiting power law dependence on the bubble wall temperature and on the bubble radius, with the polytropic index depending on the isentropic exponent of the gas and on a parameter that is a function of the Péclet number and a characteristic time scale. Results of the linear theory for gas bubbles are recovered by identifying this parameter as a function of the Péclet number based on the Minnaert frequency. The novel gas pressure law is then validated against the near-isothermal solution and against the results of the numerical simulations of the original energy balance equations for large amplitude oscillations using spectral methods. Consequently, an acoustic cavitation model that accounts for phase change but that neglects mass diffusion is constructed by employing the reduced order gas pressure law together with the Plesset–Zwick solution for the bubble wall temperature and the Keller–Miksis equation for spherical bubble dynamics. Results obtained using variable interface properties for acoustically driven cavitation bubbles in water show that the time variations of the bubble radius and the bubble wall temperature lie between those obtained by the isothermal and adiabatic laws depending on the value of the Péclet number and the characteristic time scale.WOS:0007167554000132-s2.0-85118769383Science Citation Index ExpandedArticleUluslararası işbirliği ile yapılmayan - HAYIRNovemberYÖK - 2021-22Kası
Thermal Damping in Cavitating Nozzle Flows
No full textRecent investigations of bubbly cavitating
nozzle flows by Wang and Brennen (1998) and by Delale et al. (2001),
where various damping effects are lumped together in an
adhoc manner, have shown flow instabilities that lead to flashing
flow solutions. Here, we investigate the stabilizing effect of
thermal damping on these instabilities. For this reason we
consider the energy equation within the bubble, assumed to be
composed of vapor and gas, in the uniform pressure approximation
(similar to that given by Nigmatulin et al., 1981 and by
Prosperetti, 1991). The partial vapor pressure is fixed by the vapor
saturation pressure corresponding to the interface temperature,
which is evaluated by the Plesset-Zwick formula assuming the
thin boundary layer approximation within the liquid. Consequently,
the partial gas pressure is evaluated by its relation to the heat
flux through the interface in the uniform pressure approximation.
The model is then coupled to the steady-state cavitating nozzle
flow equations employed by Wang and Brennen and by Delale et al.,
replacing the previously assumed polytropic law for the partial
gas pressure. The instabilities arising from the use of the
polytropic law for the gas pressure in steady
cavitating nozzle flows are seen to be stabilized by thermal
damping with or without the occurrence of bubbly shock waves
Shock Propagation in Polydisperse Bubbly Liquids
No full textWe investigate the shock dynamics of liquid flows containing small gas bubbles with numerical simulations based on a continuum bubbly flow model. Particular attention is devoted to the effects of distributed bubble sizes and gas-phase nonlinearity on shock dynamics. Ensemble-averaged conservation laws for polydisperse bubbly flows are closed with a Rayleigh–Plesset-type model for single bubble dynamics. Numerical simulations of one-dimensional shock propagation reveal that phase cancellations in the oscillations of different-sized bubbles can lead to an apparent damping of the averaged shock dynamics. Experimentally, we study the propagation of waves in a deformable tube filled with a bubbly liquid. The model is extended to quasi-one-dimensional cases. This leads to steady shock relations that account for the compressibility associated with tube deformation, bubbles and host liquid. A comparison between the theory and the water-hammer experiments suggests that the gas-phase nonlinearity plays an essential role in the propagation of shocks
Numerical Simulation Of Unsteady Quasi-one-dimensional Bubbly Cavitating Nozzle Flows
No full textTez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2011Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2011Bu çalışmada, sanki-bir-boyutlu daimi olmayan kavitasyonlu kabarcıklı lüle akışları için oluşturulan model denklemlerin sayısal benzetim çözümleri incelenmiştir. Bu amaçla homojen kabarcıklı sıvı akışı modeli kullanılarak sanki-bir-boyutlu daimi olmayan kavitasyonlu lüle akış denklemleri kabarcık dinamiği yasasıyla birlikte (iyileştirilmiş Rayleigh-Plesset denklemi) gözönünde bulundurulmuştur. Çekirdekleşme, kabarcık bölünme ve birleşmeleri ihmal edilmiştir. Tüm sönüm mekanizmaları, viskoz yutulma biçiminde tek bir sönüm katsayısıyla ele alınmış, küresel kabarcıkların büyüme ve büzülmelerinde kabarcık içindeki gaz için politropik yasa kullanılmıştır. Başlangıç dağılımları, giriş koşulları ve lüle geometrisi, lülede kavitasyon oluşacak şekilde seçilmiştir. Bu varsayımlar altında model denklem sistemi, akış hızı ve kabarcık yarıçapı için iki evrim denklemine indirgenmiştir. Kavitasyonlu lüle akışlarının başlangıç/sınır değer problemi, evrim denklemlerinin başlangıç/sınır değer problemine indirgenerek, inşa edilen sayısal benzetim algoritması vasıtasıyla çözülmüştür. Önerilen modelin sayısal benzetim sonuçları, Preston ve diğ. tarafından (2002) elde edilen sayısal benzetim sonuçlarıyla ve İ.T.Ü.-TÜBİTAK lülesine ait deney sonuçlarıyla karşılaştırılmıştır. İnşa edilen model denklemlerin zamana bağlı çözümünden elde edilen kabarcık yarıçapı, basınç katsayısı ve akış hızı dağılımlarının, Preston ve diğ. (2002) tarafından elde edilen sayısal benzetim sonuçlarıyla birebir uyum sağladığı görülmüştür. İ.T.Ü.-TÜBİTAK lüle geometrisi için elde edilen daimi olmayan sayısal benzetim sonuçları da, kavitasyon deneylerinde ölçülen basınç kayıplarını yakalayabilmiştir.In this study, quasi-one-dimensional unsteady bubbly cavitating in converging-diverging nozzle flows are investigated by employing a homogeneous bubbly liquid flow model, where the nonlinear dynamics of cavitating bubbles is described by a modified Rayleigh-Plesset equation. Nucleation, coagulation of bubbles and bubble fission are neglected. The various damping mechanisms are lumped together by a single damping coefficient in the form of viscous dissipation. A polytropic law for the expansion and compression of the gas inside the bubble is assumed. The initial distributions, inlet conditions and nozzle geometry are choosen such that cavitation can occur in the nozzle. Under these assumptions the complete system of equations, by appropriate uncoupling, are reduced to two evolution equations, one for the flow speed and the other for the bubble radius. Consequently, the all hydrodynamic variables are calculated for the numerical simulation of quasi-one-dimensional unsteady bubbly cavitating nozzle flows. The numerical results are verified by a quantitative comparison of the results for two different type of nozzles. A comparison of the numerical results for the bubble radius, the flow speed and pressure coefficient distributions by the present algorithm show excellent agreement with those found by Preston et al (2002). Furthermore, the numerical results obtained for quasi-one-dimensional nozzle flows capture the measured pressure losses of the experimental study of the İ.T.Ü.-TÜBİTAK nozzle due to cavitation, but they turn out to be insufficient in describing the two-dimensional cavitation structures.DoktoraPh
Real Gas Effects in Thermally Chocked Nozzle Flows
No full textReal gas effects in condensing nozzle flows are discussed by the virial equation of state truncated after the second virial coefficient. The thermal choking conditions in nozzles previously derived for a perfect condensible vapor are generalized to include real gas effects. For these cases it is shown that the critical amount of heat necessary to thermally choke the flow can be defined explicitly only for the expansion of a pure vapor
A novel nonreflecting boundary condition for unsteady flow
No full textA novel nonreflecting boundary condition, which converges to the specified time-dependent boundary condition within any degree of accuracy, is introduced for the numerical simulation of hyperbolic systems and validated against the solution of two fundamental boundary value problems in fluids. First, transonic nozzle flow with backward acoustic disturbance is considered. Using high-order aeroacoustic numerical schemes, the proposed nonreflecting boundary condition yields results that are in excellent agreement with those obtained using conventional nonreflecting boundary conditions based on the method of characteristics as well as with the results of the exact solution. The novel nonreflecting boundary condition, implemented into a semi-analytical solution algorithm of unsteady bubbly cavitating nozzle flows, is also validated against results obtained using a Lagrangian finite volume scheme. Copyright (c) 2013 John Wiley & Sons, Ltd
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