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International Journal of Multiphase Flow 37 (2011) 596–608 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow Numerical simulation of shock propagation in a polydisperse bubbly liquid ⇑ Keita Ando , Tim Colonius, Christopher E. Brennen Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, United States article info abstract Article history: The effect of distributed bubble nuclei sizes on shock propagation in a bubbly liquid is numerically inves- Received 7 December 2010 tigated. An ensemble-averaged technique is employed to derive the statistically averaged conservation Received in revised form 12 February 2011 laws for polydisperse bubbly flows. A finite-volume method is developed to solve the continuum bubbly Accepted 7 March 2011 flow equations coupled to a single-bubble-dynamic equation that incorporates the effects of heat trans- Available online 25 March 2011 fer, liquid viscosity and compressibility. The one-dimensional shock computations reveal that the distri- bution of equilibrium bubble sizes leads to an apparent damping of the averaged shock dynamics due to Keywords: phase cancellations in oscillations of the different-sized bubbles. If the distribution is sufficiently broad, Continuum bubbly flow the phase cancellation effect can dominate over the single-bubble-dynamic dissipation and the averaged Bubble size distributions Shock dynamics shock profile is smoothed out. Bubble screen Ó 2011 Elsevier Ltd. All rights reserved. Finite volume method 1. Introduction 1988; Watanabe and Prosperetti, 1994; Kameda and Matsumoto, 1996; Kameda et al., 1998) reproduced averaged shock structures. Complex bubbly flows are ubiquitous in engineering flows, such The direct numerical simulations of shock propagation in bubbly liq- as underwater explosions, industrial piping systems and hydraulic uids (Delale et al., 2005; Delale and Tryggvason, 2008; Seo et al., machinery (Cole, 1948; Wylie and Streeter, 1993; Brennen, 1994). 2010) are an important step to quantify the effect of direct interac- Seawater near the sea surface contains fairly big bubbles due to tions between the neighboring bubbles. breaking waves. Even for liquid flows with small nuclei popula- A finite cloud of bubbles has been extensively studied as a tions, the interaction with structures often produces tension waves canonical example. One of the earliest attempts is the work of that can cause cavitation, and complex two-phase flow regimes are van Wijngaarden (1964) that considered the collapse of a bubbly encountered. The resulting gas or vapor bubbles lead to wave dis- layer near a wall. The study of the linearized dynamics of spherical persion; the dynamics of bubbly mixtures cannot be properly de- bubble clouds (d’Agostino and Brennen, 1989) identified the scribed by simple barotropic relations (Brennen, 1995, 2005). An natural frequencies of the cloud that can be much smaller than understanding of complex dynamics of such bubbly flows is still the natural frequency of individual bubble oscillations. The nonlin- inadequate for many engineering applications. ear computations of bubble clouds (Chahine, 1982; Omta, 1987; Theoretical and numerical models have been developed to inves- Wang and Brennen, 1999; Colonius et al., 2000) indicate that cloud tigate shock dynamics in bubbly flows. In the classic papers of van collapse can generate higher pressure than that predicted by Wijngaarden (1968, 1972), volume-averaged mixture quantities superposition of single-bubble calculations in the corresponding were defined in order to average out local scattering due to the one-way-coupling case (in which interactions among the bubbles dynamics of individual bubbles, and averaged conservation laws through the averaged field are ignored); this violent cloud collapse for bubbly flows were formulated based on heuristic reasoning. may account for cavitation noise and erosion. All of the above Zhang and Prosperetti (1994) developed an ensemble-averaging analyses were confined to the case of monodisperse mixtures. technique to derive the mixture-averaged equations. The computa- However, the computations of a spherical bubble cloud with a tions of the mixture-averaged equations coupled to a Rayleigh–Ples- distribution of equilibrium bubble sizes (Wang, 1999; Shimada set-type equation for single bubble dynamics (Nigmatulin et al., et al., 2000) suggest a profound impact of polydispersity on the averaged dynamics. The continuum models have been validated by experiments. In the pioneering work of Campbell and Pitcher (1958), shock propa- ⇑ Corresponding author. Present address: School of Electrical and Electronic gation in a bubbly liquid-filled, vertical tube was considered, and Engineering, Nanyang Technological University, Singapore 639798, Singapore. Tel.: +65 6790 6534. their results showed reasonable agreement with the steady shock E-mail addresses: [email protected] (K. Ando), [email protected] (T. Colo- relations derived from mixture-averaged equations. Subsequent nius), [email protected] (C.E. Brennen). experiments (Noordzij and van Wijngaarden, 1974; Beylich and 0301-9322/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2011.03.007 K. Ando et al. / International Journal of Multiphase Flow 37 (2011) 596–608 597 Gülhan, 1990; Kameda and Matsumoto, 1996; Kameda et al., 1998) where q is the mixture density, u is the mixture velocity, pl is the used a similar device with careful control of bubble sizes, and iden- averaged liquid pressure, I is the identity tensor, a is the void frac- tified oscillatory shock structures. Beylich and Gülhan (1990) re- tion, R is the bubble radius, and R_ is the bubble wall velocity. For di- ported on the polydisperse case in which the smoothed shock lute cases, the mixture density is well approximated by (1 À a)ql, structure was seen. Unfortunately, experimental observations on where ql is the liquid density. The averaged liquid pressure may the shock dynamics of polydisperse mixtures are rather limited. be described by the Tait equation of state (Thompson, 1972), The goals of this paper are to derive a continuum bubbly flow p þ B q m 1 q m model that incorporates a distribution of equilibrium bubble nuclei l ¼ l ¼ ; ð4Þ p B m 1 sizes and quantify the effect of polydispersity on averaged shock l0 þ ql0 ql0 À a dynamics. In Section 2, the continuum and single-bubble-dynamic where ql0 is the reference liquid density at the ambient pressure pl0, models with their limitations are presented. One-way-coupled and m and B denote stiffness and tensile strength of the liquid, flow computations are also considered in order to clarify some respectively. fundamental issues arising in polydisperse flows. Section 3 pro- The term p~ in the momentum flux in Eq. (2) represents pressure vides the numerical implementation and its verification with linear fluctuations due to the phase interactions, problems. In Section 4, one-dimensional shock propagation ! through a bubbly liquid and a bubble screen is computed, and R3p R3R_ 2 ~ bw the effect of polydispersity on the averaged shock dynamics is p ¼ a pl À À q ; ð5Þ R3 R3 investigated. Finally, we summarize the findings in Section 5. where pbw is the bubble wall pressure described by the dynamic 2. Physical model boundary condition (Brennen, 1995), 4l R_ 2Ç 2.1. Continuum bubbly flow model p ¼ p À l À : ð6Þ bw b R R 2.1.1. Ensemble-averaged equations Here, pb is the internal bubble pressure (sum of noncondensible gas With the ensemble-averaging technique of Zhang and Prosper- pressure pg and vapor pressure pv), ll is the liquid viscosity, and Ç is etti (1994), one may formally derive continuum models in order to the surface tension. evaluate the average mixture dynamics. The key concept is to sta- The overbar in Eqs. (3) and (5) denotes moments with respect to tistically treat the dynamical state of bubbly mixtures in which a the (normalized) distribution of equilibrium bubble sizes, f(R0), Z large number of realizations of spherical bubbles (or isotropic scat- 1 ters) exist. In other words, we discard any scattering effects in a u ðx; tÞ¼ uðx; t; R0Þf ðR0ÞdR0; ð7Þ specific realization, but explore the statistically averaged dynam- 0 ics. Even though direct interactions between the neighboring bub- where R0 is the equilibrium bubble radius corresponding to the bles are minimal, there can still appear indirect interactions among 2 _ 3 3 _ 2 3 ambient pressure and u represents any of R R; R pbw; R R or R . the bubbles through the averaged field; the problem is two-way- Note that the functional dependence of R0 on the distribution is as- coupled. In what follows, we present the ensemble-averaged con- sumed spatially uniform. This assumption is valid for all times due servation laws for polydisperse bubbly flows with the model to the no-slip assumption (d). For example, the void fraction is de- assumptions. The complete derivation can be found in Zhang and fined as Prosperetti (1994) and is also summarized in Ando (2010). 4p The continuum model assumes that (a) the bubbles are spheri- a ¼ nR3; ð8Þ cal; (b) mutual interactions among the bubbles are negligible ex- 3 cept through their effect on the mixture-averaged flow; (c) where n is the number of bubbles per unit volume of the mixture wavelengths in the mixture are large compared to the mean bubble and is conserved in time because of the assumption (a); namely spacing; (d) the bubbles advect with the ambient liquid velocity; @n and (e) density and velocity fluctuations in the liquid phase are þ r ðnuÞ¼0: ð9Þ uncorrelated. @t Assumption (a) implies that fission and coalescence of the bub- It should be noted that the phase interaction term p~ does not appear bles are not permitted, so that the bubble number is conserved in in the volume-averaged momentum equation of van Wijngaarden time. Assumption (c) is used for the model closure. Relative motion (1968, 1972).