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IJCER (www.ijceronline.com) International Journal of computational Engineering research Document Transcript

  • 1. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 Numerical Analysis of Cavitating Flow over A2d Symmetrical Hydrofoil 1, Greshma P Rao, 2, Likith K, 3, Mohammed Naveed Akram, 4Adarsh Hiriyannaiah 1,2,3,4 Department of Mechanical Engineering R V College of Engineering, BangaloreAbstractThis report presents the numerical investigations of Adverse Effects of Cavitationcavitating flow over a 2D symmetrical hydrofoil. The main effects of cavitation are adverse effects: noise,Turbulent cavitating flow over a hydrofoil is simulated erosion, vibrations and disruption of the flow, whichusing a transport equation based model with results in loss of lift and increase of drag. Cavitation isconsideration of the influence of non condensable gases. known for its violent behaviour. That is caused by the factThe results presented in this report focuses on Cavitation that vaporization of water and condensation of vapour areinception, shape and behaviour of sheet cavity, lift and very fast processes, much faster than the dynamics of adrag forces with cavitation and the pressure distribution vapour cavity. As a result the growth and collapse of aaround the foil. Steady and unsteady simulations are cavity is not slowed down by these processes. Becausecarried out at different cavitation numbers ranging from cavitation is part of the flow, it can move rapidly fromnear inception conditions to developed conditions and regions of low pressure into regions of a higher pressure.almost super cavitating conditions. Standard k-ɛ model This leads to a very rapid collapse. The collapse is sowithout any modifications are used for simplicity. Effect rapid that the local speed of sound in the fluid is exceededof dissolved gas content is also considered. and shock waves occur. The consequence is that cavitation generates noise over a wide range ofIndex Terms— Cavitation, Hydrofoil, Standard k-ɛ frequencies, especially higher frequencies. Also the localmodel. pressure rises very strongly at collapse, leading to damage of adjacent surface. This effect is called erosion. WhenI. INTRODUCTION larger amounts of vapour are involved the implosion ofCavitation is defined as the process of formation of the cavitation can cause pressure variations in the fluid,vapour phase of a liquid when it is subjected to reduced which lead to vibrations of the cavitating structure.pressures at constant ambient temperature. Thus, it is the Cavitation can also alter the flow. This is the case onprocess of the boiling in a liquid as a result of pressure propellers when the cavitation becomes extensive. In thatreduction rather than heat addition. However, the basic case the flow over the blades and the lift of the blades isphysical and1, 2 :UndergraduateStudents(UG)4Assistant altered by cavitation and the thrust of the propeller isprofessor (RVCE)thermodynamic processes are the same strongly reduced. This is called ‘thrust breakdown’. Inin both cases. A liquid is said to cavitate hen vapour valves cavitation can also block or choke the flow. Thebubbles form and grow as a consequence of pressure volume of vapour in cavitation is much larger than thereduction.When the phase transition results from volume of water that has evaporated. In cases of extensivehydrodynamic pressure changes, the two-phase flow cavitation this leads to large volume increases andcomposed of a liquid and its vapour is called a ‘cavitating decreases when cavitation grows and collapses. Theflow’. Cavitation flow may be seen (and heard) as water volume variations cause pressure fluctuations in theflows through a glass venturi tube (fig 1), an experiment surrounding fluid, resulting in structural vibrations. Thefirst exhibited by Osborne Reynolds in 1894. According properties of cavitation and its implosion can also beto Bernoulli’s equation, when the velocity increases, the used, as mentioned below.pressure decreases. At sufficiently high flow rates theliquid in the throat begins to boil, because the velocity is 2. Cavitation numberhighest and the pressure is lowest at this section. The Generally tests are carried out in a cavitation tunnel. Insmall this case the easiest parameters to measure are:lowbubbles formed there are filled with cold steam and other velocity, pressure upstream of hydrofoilIt is customary togases diffused from the liquid. [1] define σ by the following expressionHydrofoilHydrofoils are foils operating in liquid medium. Theyneed not be as large as an aerofoil due to higher density ofLiquid than air. They help in increasing the performance Cp= (pv-pb )/ (0.5ρ 2 ) vof machinesIssn 2250-3005(online) September| 2012 Page 1462
  • 2. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 two-equation turbulence models to reproduce the cavity unsteadiness in the case of a venturi nozzle. They have Figure 2.1 stated that the use of the k- ɛ turbulence model leads toThe following figures (2.1, 2.2(a), 2.2(b), 2.2(c)) [3] steady-state cavity because of the high turbulent viscosityshow the relation between pressure coefficient cp and level induced by the turbulence model. Singhal et al [8]cavitation number σ for non cavitating, possible developed the full cavitation model in which the phasecavitation and developed cavitating conditions change expressions are derived from a reduced form of a Rayleigh-Plesset equation. The numerical model has been applied to the 2D configuration (fig 8.4). The flows corresponding to different cavitation numbers have been investigated, to obtain successively non cavitating flow, steady and unsteady sheet cavitation (cloud cavitation), finally supercavitation flow. Figure 2.2(a) 4. Mathematical Models Multi-Phase Model The multi-phase mixture model in FLUENT14.0 assumes that the working medium is a single fluid with a homogeneous mixture of two phases (liquid and vapour). Therefore, only one set of RANS equations is solved for the mixture fluid. Denoting the density of the mixture Figure 2.2(b) fluid by ρm, the continuity equation for the mixture flow becomes: The momentum equation for the mixture reads: Figure 2.2(c)3. Cavitation Simulation The density constitution of each phase in a mixture-flowCavitation is a two phase flow occurring in a liquid when cell is described by means of a scalar volume fraction.the local static pressure falls below the vapour pressure The relation between different volume fractions is linkedcorresponding to the liquid temperature. It affects the by:performance of hydraulic machines. Need forunderstanding cavitation in turbo machines is thusapparent. Because of experimental difficulties, Where αv and αl are the volume fraction of vapour andsimulation could be a useful tool to understand cavitation liquid respectively. To close the equations an additionalin such applications Simulating the occurrence of transport equation is solved for αv. To account for thecavitation in a hydraulic machine is a challenging task mass transfer between phases a cavitation model isbecause of the complex flow phenomenon as well as the needed, as discussed below.complicated geometry of hydro turbo machines; usuallynew cavitation models are tested against experimental Cavitation Modeldata obtained from much simpler geometry. One such Singhal et al cavitation model [8]This cavitation model isflow simulation widely used to validate numerical based on the "full cavitation model", developed byapproach is the flow over a hydrofoil. Hydrofoils form the Singhal et al. It accounts for all first-order effects (i.e.,basics of impeller blades of phase change, bubble dynamics, turbulent pressureaxial flow machines. Thus information of cavitation on a fluctuations, and non- condensable gases). It has thehydrofoil yields useful information about axial machine capability to account for multiphase (N-phase) flows orperformance under cavitation. flows with multiphase species transport, the effects of slipSimulation of cavitation requires a coupling between velocities between the liquid and gaseous phases, and theNavier-stokes equation, physical model for cavitation and thermal effects and compressibility of both liquid and gasclosure model for turbulence. Moreover such flows are phases. The cavitation model can be used with thesystematically unsteady at some scale thus an effective mixture multiphase model, with or without slipcavitation model is needed to correctly take into account velocities. However, it is always preferable to solve forthe different flow phenomenon Kubota et al[6] proposed a cavitationnew cavity model (bubble two-phase flow) that canexpress well the interactions between vortices andbubbles. Rebound et al [7] have investigated the ability ofIssn 2250-3005(online) September| 2012 Page 1463
  • 3. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 using the mixture model without slip velocity; slip Final surface is shown in fig (5.1.1) velocities can be turned on if the problem suggests that there is significant slip between phases. To derive an expression of the net phase change rate, Singhal et al uses the following two-phase continuity equations Liquid phase: Fig 5.1.1 The foil is 1m long in chord and maximum width 0.12m.The dimensions of the arc of the c mesh is 12.5 m Vapour phase: and length of the rectangular part is 12.5m with the trailing edge of the hydrofoil situated at the centre of the arc. Mixture: Once the c mesh outline is created a second surface (target body) is generated from this a hydrofoil surface is subtracted (tool body) shown in figure (5.1.2) The expression for the net phase change rate ( ) is finally obtained as Singhal et al. proposed a model where the vapour mass fraction is the dependent variable in the transport equation. This model accommodates also a single phase Figure 5.1.2 formulation where the governing equations are given by: Then the obtained surface is spilt into 4 quadrants to facilitate easy meshing. The mesh is generated using mapped face meshing and Turbulence modelling [4] suitable edge sizing In this simulation standard k ε model without any Thus obtained mesh is as shown below modifications is tested and it q has been found that this yields to inaccurate predictions especially in near wall conditions. For this model the transport equation for k is derived from the exact equation, but the transport for ε was obtained using physical reasoning and is therefore similar to the mathematically derived transport equation of k , but is not exact. The turbulent kinetic energy k and its rate of dissipation ε, for this model are obtained by the following equations. Figure 5.1.3   k    1/ 2   2 k u i      2  k   u j   ij    T   Computational domain x j x j x j    k  x j       x    j    k    2 k u i   T  k 1/ 2 u j   ij        x j  x j x j  k  x      2  x       j    j       2   u i  T  2 2T   2u  u j  C1  ij        x   C2 k    2  x  x j k x j x j     j   2 5. Procedure in ansys fluent Geometry and Meshing The shape of the hydrofoil is given by Y/c=a0(x/c)1/2+a1(x/c)+a2(x/c)2+a3(x/c)3+a4(x/c)4 Where a0=0.11858, a1=-0.02972, a2=0.00593, Fig 5.2 a3=-0.07272,a4=-0.02207 The geometry is created by using the equation above. And Solution set up- Grid and Boundary conditions the surface is created from the curve obtained by the coordinates satisfying the above equations. Issn 2250-3005(online) September| 2012 Page 1464
  • 4. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5The foil has a 7° angle of attack (AoA) that is the flow is cavitation. For σ=3 cp curve does crosses the –σ line theinclined at 7° and is operated at and is operated at various cavitation happens. Hence σ=3cavitation begins.cavitation numbers. Table 1 gives the flow and boundary (figure8.2) Cp v/s position for σ=2, that is developedconditions. The physical properties for the liquid and the cavitation (figure 6.3)vapor in Table 1 correspond to a water temperature at However cavitation itself is an unsteady phenomenon27°C (transient flow) and hence we can speak only of degree of unsteadiness in the flow.Table 1 – Boundary Conditions Development of cavity Sheet cavitation occurs when there is a strong low pressure peak at the leading edge of the foil and sheet cavitation therefore has its leading edge close to the leading edge of the foil. The closure of the cavity is shown in Fig6.7.At the beginning or leading edge of the cavity, constant pressure means that the streamlines separate tangentially from the surface of the foil at point A (assuming that the foil is a smooth surface). Tangential separation, however, means that there is a region just downstream of the separation location where the cavity is very thin, so thin that the surface tension becomes Table 2 Information on grids for hydrofoil recognizable and results in a curved surface, making the leading edge of the cavity at point B instead of A. Also point B experiences the lowest pressure marking the beginning of the cavity. (Indicated by red regions) The space around point A encounters flow separation leading Table 3 to reduced volume of vapour at that point, indicated by yellow region ( also near the trailing edge), The phase contours for σ=3 is plotted for different times. The fig (6.4, 6.5, 6.6), shows a few plots for 300,500,700ms are shown. The vapour reaches a maximum volume fraction of 1.91x10^-2 in 500 ms. Phase contours For σ=2 the phase contours are plotted fig (6.8, 6.9, 6.10,6.11)And reaches a maximum vapour6. Results and discussions volumefractionof2.03x10^-2 at 500ms which is greaterFor 7o angle of attack after the numerical simulations for than that ofσ=3.Also the length of the cavity is increased.a wide range of operating pressures the following five For σ=0.9 the phase contours are plotted –fig( 6.12, 6.13,main flow configurations have been observed in the 6.14, 6.15) And reaches a maximum vapour volumesystem fraction of 2.18x10^-2 at 450 ms. The vapour content is greater and reaches a maximum at a faster rate indicating Non cavitating flow(σ>4) that the cavitation gets more unsteady. Also the length of the cavity is increased.For the phase contour for σ=0.55 Near cavitation inception(σ=3) the maximum vapour volume fraction is 2.21x10^-2 which is reached at 500ms which is greater than the Sheet cavitation(α from 2 to 3) previous vapour factions. The fig (6.19) illustrates the Sheet cavitation(σ=0.9) vapour shedding at 168.2ms The vapour shedding takes place even before the full development of the cavity which Near super cavitation(σ=0.55) indicates that the shedding of the cavity takes place by parts and not as a whole which is an example for unsteadyThe cpmin value is -2.375 for σ=4(figure 6.1) cloud cavitation.Since the cp curve does not cross the –σ line the cavitationdoes not happen. Hence σ=4 is for non cavitating Effect of cavitation on lift and dragconditions. The drag coefficient is defined as:The cavitation begins at about σ=3.5 but is prominentlynoticed at σ=3 and this leads to developed cavitation atσ=2.5 followed by σ=2.at lower cavitation numbers i.e...σ= 0.55, 0.9 etc the cavitation gets unsteady or cloud Where:Issn 2250-3005(online) September| 2012 Page 1465
  • 5. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 Is the drag force, which is by definition the force phenomenon over different blade profiles is a cynosurecomponent in the direction of the flow velocity. of cavitation studies. is the mass density of the fluid, Hydrofoil is one of the simplest and most widely used profiles in impellers for axial flow machines. In the is the speed of the object relative to the fluid and analysis of a 2 D symetrical hydrofoil it is observed thatA is the reference area or projected area by decreasing cavitation number, different cavitationThe lift coefficient is equal to: regimes are formed.The near cavitation inception is obtained at 3 to 3.5 after which cavitation is developed and subsequently becomes unsteady. Cloud cavitation occurs at a cavitation number of 0.55. Further increase inWhere cavitation number leads to supercavitation conditions. It is also seen that the regions close to leading edge are sites is the lift force, is fluid density, is true airspeed, susceptible to this phenomenon. It is observed from is dynamic pressure and is plan form area. Or simulations that gas content also has a major effect on theprojected area cavitation formation. Lift decreases and drag increasesThe plots (Figure 6.20 (a) & (b)) show the lift and drag with the increase in cavitation.(which can thwart thecoefficient during the incipient flow for various cavitation performance of the machinery).numbers. Further the standard turbulence model k-ε used, withoutWe observe that the lift coefficient decreases and drag co any modification is not very accurate in predicting theefficient increases with decrease in cavitation number phase contours, vapour shedding and near wall.indicating increase in drag force and decrease in lift conditions.force with cavitation. In future the investigation can be extended to study theThe Graphs figs (6.21 (a), (b), (c)) show the variation of phenomenon for various angles of attack withabsolute pressure with respect to position on the hydro modifications of models used to improve the accuracy andfoil, for cavitation numbers 0.9 We observe that the reliability of the results.absolute pressure distribution over lower surface remainssame however the pressure distribution over the upper Bibliographysurface changes. It has observed that the area between the 1. Cavitation “section 12, handbook of fluidtwo curves decreases as the cavitation number decreases mechanics, McGraw-hill BookCo., Inc.,indicating decrease in lift force as 1961(section 12-I:”mechanics of cavitation,” by Phillip Eisenberg; section I2-II:”supercavitating flows”, by Marshall P Tulin)Effect of dissolved gas content For Cavitation number 2. Bertin, John and Michael Smith; Aerodynamics for0.9 and gas content 5, 10, 15 and 20ppm Engineers, Third Edition. Prentice Hall: New Jersey,Graphs 6.23(b),6.23(c),6.23(d) represents the variation 1998.of cp with different dissolved gas contents. we observe 3. Cavitation Bubble Trackers” BY Yves Lecoffrethat with the incresing gas content(from 10ppm to (2009)20ppm) the value of cpmin falls, indicating that the 4. Pope, S.B., Turbulent Flows, (2000)conditions get more favorable to cavitation. On thecontrary the value of cpmin for 5ppm(6.23 (a)) is lower Referencesthan that of 10ppm. This is probaly because very low gas 1. Introduction to Cavitation and Supercavitation Papercontent lead to lesser nuclei in water which may lead to presented at the RTO AVT Lecture Series onbubble cavitation which is a stronger form of cavitation “Supercavitating Flows”, held at the von Kármánthan sheet cavittion , justifying the fall in cpmin.Hence Institute (VKI) in Brussels, Belgium, 12-16 Februaryhigh nuclei density is favorable for sheet cavitation while 2001, and published in RTO EN-010.low nuclei density l leads to bubble cavitation 2. Kubota a Kato, and Yamaguchi, H 1992, A new modelling of cavitating flows; a numerical study ofConclusions unsteady cavitation on a hydrofoil section, J FluidCavitation is a phenomenon that has adverse affects on mech.240, pp 59-96the performance and life of the hydraullic machines, 3. Rebound J. L., Stutz B., and Coutier- Delgosha, o.,making it an inevitable subject of study.Analysisng 1998, two phase flow structure of cavitation:cavitation can provide us information,about the sites experiment and modelling of unsteady effects. Pre,vulnerable to cavitation damage and therefore help us to oct 3rd int.take suitable measures for its prevention. 4. Singhal A K Athavale M .M., Li ,H .Yi., and Jiang YThe phenomenon is known to have a devastating effect 2002, mathematical basis and validation of fullon the blades of the impellers used in various fluid cavitation model J. Fluid Engg, 124, pp 617-624machinery. Therefore the investigation of the 5. “Introduction to CFD Basics”, Rajesh Bhaskaran, Lance Collins (2010)Issn 2250-3005(online) September| 2012 Page 1466
  • 6. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 56. ranc J.P. and Michel J.M. (2004) "Fundamentals of cavitation" Kluwer.7. Kato H. (1984) "Thermodynamic effect on incipient and developed sheet cavitation" Proc. Int. Symp. On Cavitation inception, FED-Vol.16, New-Orleans (USA), Dec. 9-14, 1984, 127-136.8. A. Ducoin, J. A. Astolfi, F. Deniset, and J.-F. Sigrist, “Computational and experimental investigation of flow over a transient pitching hydrofoil,” European Figure 3 Journal of Mechanics B, vol. 28, no. 6, pp. 728–743, 20099. M. Morgut, E. Nobile, and I. Biluš, “Comparison of mass transfer models for the numerical prediction of sheet cavitation around a hydrofoil,” International Journal of Multiphase Flow, vol. 37, no. 6, pp. 620–626, 2011.10. G.Kuiper, Cavitation in Ship Propulsion, January 15, 2010 Figure 8.411. Stepanoff A.J. (1964) "Cavitation properties of liquids" J. of Eng. for Power, April 1964, 195-200.12. Numerical and experimental study on unsteady shedding of partial cavitation (2009) BIN JIn XIANWU LUO† and YULIN WU13. 5. J. Choi, C. T. Hsiao, G. Chahine and S. Ceccio, J. Fluid Mech. 624 (2009) 255.14. 8. B. Ji, F. W. Hong and X. X. Peng, Journal of Figure 5 Hydrodynamics, Ser. A 23(04) (2009) 412.15. De Lange D.F. & De Bruin G.J. (1998) "Sheet cavitation and cloud cavitation, re-entrant jet and threedimensionality" Appl. Sci. Res. 58, 91-114.16. MODELING OF CAVITATION FLOW ON NACA 0015 HYDROFOIL(2009) Jaroslav ˇStigler*, Jan Svozil* Appendix A Figure 6 Figure 7 Figure 1 Figure 8 Figure 2Issn 2250-3005(online) September| 2012 Page 1467
  • 7. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 Figure 14 Figure 9 Figure 15 Figure 10 Figure15 Figure 11 Figure 16 Figure 12 Figure 17 Figure 13 Figure 18Issn 2250-3005(online) September| 2012 Page 1468
  • 8. International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5 Gas content-5ppm Figure 19 Figure 23(a) Gas content-10ppm Figure 20(a) Figure 20(b)Absolute pressure v/s position for different Figure 23(b)cavitation numbers Gas content-15ppm Figure 21(a) Figure 23(c) Gas content-20ppm Figure 21(b) Figure 23(d) Figure 21(c)Issn 2250-3005(online) September| 2012 Page 1469