Orr-sommerfeld Stability Analysis of Two-fluid Couette Flow

Orr-sommerfeld Stability Analysis of Two- gas Couette FlowORR-SOMMERFELD STABILITY ANALYSIS OF TWO-FLUID COUETTE FLOW WITH SURFACTANTV.P.T.N.C.Srikanth BOJJA1* , female horse FERNANDINO1, Roar SKARTLIEN2ABSTRACTIn the present work, the surface-active agent induced instability of a she atomic play 18d two fluid dodge is examined. The linear stability outline of two-fluid couette system with an amphiphilic surface-active agent is carried out by developing Orr-Sommerfeld type stability equations on with surfactant transport equation and the system of ordinary differential equations are solved by Chebyshev Collocation mode1,2. unidimensional stability analysis reveals that the surfactant either induces Marangoni instability or signifi tail endtly reduces the rate at which small perturbations decay.KeywordsLinear stability, Orr-Sommerfeld, Marangoni mode, Amphiphilic surfactant.NOMENCLATUREA complete list of symbols used, with dimensions, is addicted.Greek Symbols emersion rateS urfactant concent dimensionnMass density, kg/m3.Dynamic viscosity, kg/m.s.Height of perturbed intefaceSurface tension brandish number,Stream functionsLatin SymbolsCapillary numberMarangoni numberNumber of Collocation pintsReynolds numberPlate/Wall velocityComplex wave sppedWidth of fluid layer Amplitude of Pressure disturbanceAmplitude of surfactant submergence disturbanceAmplitude of port wine perturbationViscosity ratioDepth ratioShear of basic velocityVelocity, m/s.Sub/superscriptsIndex i.Index j.Perturbed quantitiesBase state quantitiesINTRODUCTIONTwo layer channel springs and immixs with and without surfactants have been tending(p) large importance because of its numerous industrial applications. Oil recovery3, lubricated pipelining4, liquid coating processes5 are typical industrial situations where Two layer channel hangs are a great deal seen. Surfactants also have wide range of industrial applications for example in enhanced oil recovery6.Using Perturbation analysis, the primary instability of the two-layer plane CouettePoiseuille fertilise was studied by Yih7 and his studies revealed that even at small Reynolds numbers, the interface is susceptible to long-wave instability associated with viscosity stratification. Yiantsios Higgins8 later extended this study for small to large comfort of wavenumber and confirmed the existence of the shear mode instability. Boomkamp Miesen9 came up with the method of an energy budget for studying instabilities in parallel two-layer flows, where energy is supplied from the primary flow to the perturbed flow and instability appears at sufficiently long wave numbers through the increase of kinetic energy of an infinitesimal disturbance with time. In the presence of surfactant at the sheared interface, Frenkel Halpern10,11 discovered that even in the stokes flow limit, the interface is unstable as the surfactant induces Marangoni instability, which was later confirmed by Blyth Pozrikidis12. In the movement o f Stokes flow, they identified two formula modes, the Yih mode due to viscosity stratification inducing a jump in the interfacial shear, and the Marangoni mode associated with the presence of the surfactant. In contrast, at finite Reynolds numbers, infinite number of normal modes are possible and by parameter continuation with respect to the Reynolds number the most dangerous Yih and Marangoni modes can be identified.In this article, the effect of an insoluble surfactant on the stability of two-layer couette channel flow is studied in detail for low to moderate determine of the Reynolds number. To isolate the Marangoni effect, gravity was suppressed in this fuss and this was done by considering equal density fluids. Linear stability analysis was carried out by formulating OrrSommerfeld leaping repute problem, which was solved numerically using Chebyshev collocation method12 for all wavenumbers. Both Marangoni mode and Shear mode are detected and utmost focus is given to Marango ni mode as Shear mode is always stable at moderate to long wavenumbers under the influence of inertia.The rest of the paper is organized as follows. In Model description, the governing equations for the system in question are laid out, Normal mode analysis of the physical system is carried out, OrrSommerfeld barrier value problem is formulated. General description of Chebyshev collocation method and detailed description of numerical simulation of OrrSommerfeld boundary value problem by Chebyshev collocation method and validation of numerical method with literature data is given in Numerical method. Detailed discussion of results done in Results. The concluding remarks and outlook for further-work in Conclusions. Finally acknowledgements in and Acknowledgemnts.Model descriptionConsider two super-imposed immiscible liquid layers between two infinite parallel plates located at, as in trope. 1. Let the basic flow be driven only by smasher motion of plates. It is well known that t he basic Couette velocity profiles are steady and vary only in the span-wise direction and in the basic state, the unperturbed interface between the liquids is flat and is located at. The gravity is suppressed in this problem by considering equal densities in order to investigate the effects of surfactant and inertia on the stability of the system under consideration. The subscripts 1 and 2 stir to the lower or upper fluid, one by one and channel rings move in the horizontal direction, x, with velocities and. The interface is occupied by an insoluble surfactant with surface concentration which is only convected and diffused over the interface, but not into the bulk of the fluids thus locally changing the surface tension .Governing equationsThe mass and momentum saving equations governing the two-layer system are, (1)Where subscript represents lower and upper liquid layers respectively.Here , Figure 1 Schematic sketch of Couette-Poiseuille flow with surfactant wet interface. The perturbed interface is shown as sinusoidal burn. is the concentration of insoluble surfactant.The associated boundary conditions for the system are no slip and no penetration boundary conditions at the walls.,at and,at The associated interface conditions are continuity of velocity, tangential examine and normal stress.Continuity of velocity at the interface, at The tangential and normal stress conditions at the interface are given by (2)Where are stress tensors, is unit normal, is unit tangent and Kinematic interfacial condition isThe surfactant transport equation13 at the interface is given by(3)Where is surface molecular diffusivity of surfactant. is usually negligible and neglected in this case.We introduce dimension less variables as follows , , , ,The dimensionless variables in base state for the couette flow with flat interface and uniform surfactant concentration are given by, ( ) and, ()Where is shear of basic velocity at interface and is given by We cons ider the perturbed state with small deviation from the base state,,,, Now we represent disturbance velocity in disturbance stream-functions and such that,,,Performing normal mode analysis by substitutingWhere is wave number of the disturbance, and are constants, and is the complex wave speed. Linearizing the kinematic boundary condition yields.Linerarizing the dimensionless x and y-components of Navier-Stokes equation (2) followed by price reduction from the corresponding base state equations and elimination of pressure terms, yields two 4th order Orr-Sommerfield ODEs in stream-functions, one for each fluid.(4a)(4b)Where is the Reynolds number and . (when,)Boundary conditions at wall in terms of stream-functions are(5a) (5b)Continuity of velocity at interface gives, (5c)Linearization of normal stress condition gives(5d)Linearization of surfactant transport equation givesLinearization of tangential stress balance condition givesWhere is the Marangoni number.By substituting th e value of from linearized surfactant transport equation in linearized tangential stress balance condition gives(5e)For each value of Eqs. (4),(5) forms a eigen value problem, which was numerically solved using chebyshev collocation method1,2 and QZ algorithmic program for determining the complex phase velocity .Numerical methodThe two Orr-Sommerfield equations eqs. (4) along with eight boundary conditions eqs. (5) are solved numerically using Pseudo-spectral Chebyshev collocation method1,2. To implement the Chebyshev method, we transformed each of the two fluid domains into standard Chebyshev domain that is liquid 1 domain is mapped to and Fluid 2 domain is mapped to by substituting and respectively.Next, we represent each stream function as truncated summation of nonmaterial Chebyshev polynomials by setting. and(6)Where and are unknown Chebyshev coefficients and N is the number of Cheyshev collation points in each domain. Upon substituting eq. (6) in eq. (4) and projec ting them on to arbitrary orthogonal functions and respectively by taking the Chebyshev inner product,.these two Chebyshev inner products forms N-3 equations each summing up to 2N-6 equations and N+1 coefficients in and N+1 coefficients . 2N-6 equations along with 8 boundary conditions obtained by substituting eq. (6) in eq. (5) and 2N+2 coefficients forms a linear systemWhere, and,are square matrices of size 2N+2.This generalized eigen value problem was solved by QZ algorithm to obtain and subsequently harvest-feast rate, .We used, above which the eigen values are independent of number of collocation points.The accuracy of the Numerical method is checked by canvas current results with published literature10 for the Two layer couette flow with an insoluble surfactant in stokes flow limit. To make this comparison, growth rates are calculated by muting the inertial terms by settingin the our code and with same parameters as in Halperns10 Fig 2a and Fig 2b, where growth rates are pr edicted by long-wave evolution equation. Fig xxx shows excellent agreement between two numerical procedures.Figure 2 Dispersion curves for the most (a)UnstableFigure 3 Dispersion curve for the ( secure line), (dashed line), at, ,, RESULTS and discussionsBlyth and Pozrikidis14 sight that in the Stokes flow limit, there exists two modes that govern the stability of a two-layer couette flow system with surfactant the Marangoni mode and the Yih mode associated with surfactant and the clean liquid-liquid interface respectively. But on the other hand, in flows with inertia, there exists more than two normal modes. From Fig. 3, the broken line corresponding to is above the solid line, which corresponds to , it is evident that the surfactant in the presence of inertia has significantly reduced the rate at which small perturbations decay. Earlier stability analysis for stoke flow in presence of surfactant opens up a range of unstable wave numbers extending from zero up to the critical wav enumber .The neutral stability curve Fig. 4 for values (,, and ) is in accordance with the earlier stokes flow stability analysis and in addition at , a wink small windowpane of stable wave numbers appears to form an island of stable modes, with the island tip located at . In Fig. 5 we plot the growth rate of the Marangoni mode against the Reynolds number, up to and beyond, for , corresponding to the stable island tip. At, linear stability for Stokes flow predicts the growth rate, for the Marangoni mode. The present results confirms that the Marangoni mode at marks the inauguration of the lower stable loop.In Fig. 6 for a fixed Reynolds number , we show the dependence of the growth rates of the Marangoni mode on the wave number. The close-up near , presented in Fig. 6(b), shows that the Marangoni mode has negative growth rate for small band of wave numbers ranging from and has positive growth rate thereafter up-to , beyond which the Marangoni mode is stable again. These results clearly demonstrate the critical role of the surfactant, which either provokes instability or significantly lowers the rate of decay of infinitesimal perturbations.Figure 4 Neutral stability curves for ,, and Figure 5 Growth rate vs. Reynolds number for the Marangoni mode for, , , , , Figure 6 Dispersion curve for the Marangoni mode (solid line) for,, , ,, (b) Zoom-in of (a) around Figure 7 Neutral stability curves for , , and (a) (b) (c) (d) (e) (f) Further, we investigated the effect of Marangoni number on the stability of the system under consideration via Fig 7(a) and this shows that in the devoid of surfactant that is at there is very small band of wavenumbers where the system is unstable for any Reynolds number. Moreover around the band of unstable wavenumbers is slightly larger than at any arbitrary Re. In presence of surfactant, Fig. 7(b)-7(e) a second small window of stable wave numbers appears to form