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Lstl Ter Tional FINITE ELEMENT MODELING OF FLUID SYSTEMS USING THE MOBILITY ANALOGY APPROACH Ronald J. Gaines* Rajendra Singh Research Assistant Assistant Professor Department of Mechanical Engineeri ng The Ohio State University 206 West 18th Avenue Columbus, Ohio 43210 Proceedings of the lstl ter tional •International Mo alAn...... ysis Modal Analysis Co...... nee 1ft·Exhibit November 8-10, 1982 Holiday Inn, International Driv~ Orlando, Florida FINITE ELEMENT MODELING OF FLUID SYSTEMS USING THE MOBILITY ANALOGY APPROACH Ronald J. Gaines* Rajendra Singh Research Assistant Assistant Professor Department of Mechanical Engineering The Ohio State University 206 West 18th Avenue Columbus, Ohio 43210 *Currently with Toledo Scale Div., Reliance Electric, Worthington, Ohio 43D85 ABSTRACT obtained good agreement between SUPERB and WAVENET (a fluid transients computer program based on the One-dimensional fluid systems can be analyzed for method of characteristics [9] ) . However, in order natural frequencies and modes using an available to establish the mobility analogy solution tech­ structural finite element program, wi th the aid of nique, a sufficient number of example cases must the mobility analogy. In this paper, the be analyzed, and an adequate error analysis should methodology, strength and limitation of the be performed. Thi s i s the focu s of this paper as solution technique are discussed . Th i s method is we will consider several one-dimensional fluid validated by considering several example cases and systems and compare the computed results, using comparing results with theory, experiment or other the mobility analogy-finite element analysis, with numerical techniques. the solutions obtained by theory, experiment, or other numerical techniques. INTRODUCTION SCOPE The physical variables of a dynamic system could be classified either according to the dynamic and We are interested in obtaining an eigenvalue kinematic variables (impedance approach) , or solution (i .e. fn and o/ n) of a fluid system over according to the across and through variables the plane wave regime. Accordingly we assume the (mobility approach) [1-3] . However, in order to following: (i) the fluid system is linear, develop an analogy between the mechanical and homogeneous, undamped, and perfectly elastic, fluid systems, the mobility approach i s often more (ii) the fluid i s at rest, (iii) the fluid is attractive because these physical systems are bounded by rigid walls and therefore solid-fluid generally excited by a through (flow/current) interactions are not included, and (iv) one­ variable T(t), and the response is often expressed dimensional plane wave propagation exists over the in terms of the across (effort/potential) frequency range of interest. variable A(t); see Table 1 for the classification of mechanical and fluid variables per mobility METHODOLOGY analogy. l. Convert a fluid system into a one-dimensional The impedance/mobility analogy is used extensively mechanical system using the mobility analogy: to solve dynamic problems for the fluid systems [1-6]. This is because the analogous mechanical a. geometry: 1 = 1f , where 1e or electrical systems are ea sier to analyze; also, .m e there is an abundance of solution techniques and [7,10]; sm =sf literature available for these physical systems . Conversely, the literature dealing with the fluid b. properties: pm = pf ; Em= Ef (bulk transients, especially the eigenvalue solution of modulus) ; poisson's ratio =< 0 fluid systems, is very limited [4,7]. Cory and Hatfield [6] have demonstrated that the force-flow c. boundary conditions : (i) open fluid end analogy can be used to determine natural frequen­ {p .= O) ... fixed mechanical termination cies fn and modes o/ n of fluid oscillations. They (s = 0), (ii) closed ·fluid end (q = 0) used a structural finite element program, SUPERB ... free mechanical termination (F = 0), [8], for a refinery piping example case, and or a very compliant spring, and 8 (iii) fluid branch (q = ql + qz) + in f1 and f2 values with different finite element mechanical branch (F = F1 + F2J· models. In Table 4 and Figure 4 the results of two typ.ical models are given; these are also compared 2. Develop a finite element model of the analo­ with an experiment and a two degrees of freedom gous mechanical system such that only lumped parameter analysis. We note that the translation in the longitudinal direction ( ~ x) finite element model requires a careful selection is allowed. For modeling a fluid branch, the of the master nodes. Similar results have been element-overlapping method should be used to found for other combinat ions of volumes and ensure this. orifices [12]. 3. Choose parameters related to the eigenvalue CONCLUDING REMARKS solution. The number (NM) and locations of master nodes [11] must be chosen judiciously The space limitation here prevents us from a in the x direction only, i.e. ~ Y = ~z = Gx = detailed discussion of the example cases presented Gy = Gz = 0. above. Based on these and other example cases we have studied, we can conclude that the mobility 4. Run the finite element model for eigenvalue analogy-finite element method predicts natural solution. frequencies and modes of one-dimensional fluid systems reasonably well. However, we have to pay 5. Finally, the results of the analogous adequate attention to the following modeling mechanical system can be interpreted for the aspects: (i) the number of elastic and inertia fluid system as follows: (f ) = (fn)f, and elements, (ii) the number and locations of master ('l'n)~ = ('l'n)p. n m nodes, and (iii) the employment of end corrections. For some physical systems, a selection of large RESULTS number of master nodes may not yield the "more correct" fn value as demonstrated here. This We will now apply our method to some basic fluid aspect of the finite element analysis is generally components which are often encountered in the not discussed well in the literature [6,8,11 ,13]; machines and piping networks. For the finite and therefore, it should be investigated further element analysis, we have used SUPERB with linear with reference to the fluid systems. Also, more beam type element [8]. fluid components and systems should be studied in order to establish some modeling guidelines. Example Case I: Closed-Open Tube The mobility analogy-finite element analysis Table 2 and Figure 1 show comparison between the method is an attractive solution technique for mobility analogy-finite element analysis and the practical one-dimensional fluid systems as any available structural finite element code could be closed-form solutions for fn and '~' n · We note excellent correl ation between theory and finite used readily. However, we should point out that element analysis, especially for the lower modes. sometimes it is difficult to construct an analogy The end corrections or additional kinetic energy between a mechanical and a fluid system. There­ effects must be applied at the open end [10]. fore, some user judgement and discretion is Since the finite element analysis ignored the end advised. corrections, its predictions are closer to the theoretical solution based on the geometric length LIST OF SYMBOLS (i.e. le lg). = A across variable Similar comparisons between theory and finite c speed of wave propagation element analysis have been found for the closed­ E modulus of elasticity closed and open-open tubes [12]. f frequency F force Example Case II: Helmholtz Resonator 1 length n number of modes We have examined a Helmholtz resonator whose geom­ N number of nodes etry is described in Reference [13]. Table 3 and p pressure Figure 2 show our results for the first mode; q volume flow rate these are compared with theory [7,10] and other r radius (hydraulic) three-dimensional finite element analyses [13,14]. S cross-sectional area T through variable We observe that our method predicts f1 well only when we employ only one master mode and apply an t time end correction; this model, however, does not x longitudinal coordinate predict the same mode shape as given by other p density three-dimensional finite element analyses. '¥ mode {pressure) Conversely, we predict mode shape well by employ­ e angular displacement ~ displacement (longitudinal) ing a large number of master modes (say~= 11). ~ end correction (length) Example Case III: Composite System Subscripts Figure 3 shows a composite system consisting of three volumes and two orifices [15]. For this e effective example case, we observe a substantial variation f fluid 9 g geometric Conference on Noise Control Engineering, pp. i inside 755-758. m mechanical M master n moda 1 index 0 outside T total/elastic REFERENCES 1. Olson, H. F., Dynamical Analogies, D. Van Nostrand, Princeton, 1958. 2. Karnopp, D. and Rosenberg, R., System Dynamics, Wiley-lnterscience, New York, 1975. 3. Lindsay, J. F. and Katz, S., Dynamics of Physical Circuits and Systems, Matrix Publishers, 1978. 4. Wylie, E. B. and Streeter, V. L., Fluid Transients, .McGraw-Hill, New York, 1978. 5. Schwirian, R. E. and Karabin, M. E., "Use of Spar Elements to Simulate Fluid-Solid Inter­ action in Finite Element Analysis of Piping System Dynamics," Proceedings of the Sympo­ sium on Fluid Transients and Structural Interactions in Piping Systems, American Society of Mechanical Engineers, 1981. 6. Cory, Jr., J. F. and Hatfield, F. J., "Force­ Flow Analogy for Pulsation in Piping," Finite Element Applications in Acoustics, American Society of Mechanical Engineers, pp. 121-126, 1981. 7. Blevins, R. D., Formulas for Natural Frequen­ cy and Mode Shape, Van Nostrand-Reinhold, 1979. See Chapter 13. 8. SUPERB User's Manual, Structural Dynamics Research Corp., Milford, Ohio, 1981. 9. WAVENET: Waves in Fluid Networks: User Guide, R. T. Bradshaw, Inc., Waltham, Massachusetts, 1976. 10. Rschevkin, S. N., A Course of Lectures on the Theory of Sound, MacMillan, New York, 1963. 11 , Bathe, K-J, Finite Element Procedures in Engineering Analysis, Prentice Hall, 1982 See Chapter 10.
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