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Yayın Waves in an elastic tube filled with a heterogeneous fluid of variable viscosity(Pergamon-Elsevier Science Ltd, 2009-07) Demiray, HilmiBy treating the artery as a prestressed thin elastic tube and the blood as an incompressible heterogeneous fluid with variable viscosity. we studied the propagation of weakly non-linear waves in such a composite medium through the use of reductive perturbation method. By assuming a variable density and a variable viscosity for blood in the radial direction we obtained the perturbed Korteweg-deVries equation as the evolution equation when the viscosity is of order of epsilon(3/2). We observed that the perturbed character is the combined result of the viscosity and the heterogeneity of the blood. A progressive wave type of solution is presented for the evolution equation and the result is discussed. The numerical results indicate that for a certain value of the density parameter sigma, the wave equation loses its dispersive character and the evolution equation degenerates. It is further shown that, for the perturbed KdV equation both the amplitude and the wave speed decay in the time parameter tau.Yayın Modulation of non-linear axial and transverse waves in a fluid-filled thin elastic tube(Pergamon-Elsevier Science, 2000-07) Akgün, Güler; Demiray, HilmiIn the present work, utilizing the non-linear equations of a pre-stressed thin elastic tube filled with an incompressible inviscid fluid the propagation of weakly non-linear waves in such a medium is studied. Considering that the arteries are initially subjected to a large static transmural pressure P-0 and an axial stretch lambda(z) and, in the course of blood flow, a finite time-dependent displacement is added to this initial field, the non-linear equations governing the motion of the tube in the radial and axial directions are obtained. Utilizing the reductive perturbation technique the amplitude modulation of weakly non-linear but strongly dispersive waves is examined. The localized travelling wave solution to the evolution equation is given and the stability condition is discussed.Yayın On the derivation of some non-linear evolution equations and their progressive wave solutions(Pergamon-Elsevier Science, 2003-06) Demiray, HilmiIn the present work, utilizing the reductive perturbation method, the non-linear equations of a prestressed viscoelastic thick tube filled with a viscous fluid are examined in the longwave approximation and some evolution equations and their modified forms are derived. The analytical solution of some of these equations are obtained and it is shown that for perturbed cases, the wave amplitude and the phase velocity decay in the time parameter.Yayın Nonlinear waves in a viscous fluid contained in a viscoelastic tube(Birkhauser Verlag, 2001-11) Demiray, HilmiIn the present work the propagation of weakly nonlinear waves in a prestressed viscoelastic thin tube filled with a viscous fluid is studied. Using the reductive perturbation technique in analyzing the nonlinear equations of a viscoelastic tube and the approximate equations of a viscous fluid, the propagation of weakly nonlinear waves in the longwave approximation is studied. Depending on the order of viscous effects, various evolution equations like, Burgers', Korteweg-de Vries, Korteweg-de Vries-Burgers' equations and their perturbed forms are obtained. Ravelling wave type of solutions to some of these evolution equations are sought. Finally, utilizing the finite difference scheme, a numerical solution is presentede for the perturbed KdVB equation and the result is discussed.Yayın P-T phase diagram for NH4F(Editions Scientifiques Medicales Elsevier, 2002-04) Salihoğlu, Selami; Yurtseven, Hasan Hamit; Enginer, YücelIn this study we obtain a P-T phase diagram of NH4F using the mean-field theory. We fit our calculated phase line equations to the experimental P-T phase diagram. By choosing appropriately the coefficients in the free-energy expansions. our calculated phase diagram agrees well with the experimentally observed phase diagram of NH4F.Yayın On the contribution of higher order terms to solitary waves in fluid filled elastic tubes(Birkhauser Verlag, 2000-01) Demiray, HilmiIn the present work, employing the nonlinear equations of an incompressible, isotropic and elastic thin tube and the approximate equations of an incompressible inviscid fluid, the propagation of weakly nonlinear waves, in such a medium is studied through the use of the modified multiple scale expansion method. It is shown that the evolution of the lowest order (first order) term in the perturbation expansion may be described by the Korteweg-de Vries equation. The governing equation for the second order terms and the localized travelling wave solution for these equations are also obtained. The applicability of the present model to flow problems in arteries is discussed. Mathematics Subject Classification (1991).Yayın Non-linear waves in a fluid-filled inhomogeneous elastic tube with variable radius(Pergamon-Elsevier Scıence Ltd, 2008-05) Demiray, HilmiIn the present work, by employing the non-linear equations of motion of an incompressible, inhomogeneous, isotropic and prestressed thin elastic tube with variable radius and the approximate equations of an inviscid fluid, which is assumed to be a model for blood, we studied the propagation of non-linear waves in such a medium, in the longwave approximation. Utilizing the reductive perturbation method we obtained the variable coefficient Korteweg-de Vries (KdV) equation as the evolution equation. By seeking a progressive wave type of solution to this evolution equation, we observed that the wave speed decreases for increasing radius and shear modulus, while it increases for decreasing inner radius and the shear modulus.Yayın Non-linear waves in a viscous fluid contained in an elastic tube with variable cross-section(Elsevier Ltd, 2006-04) Demiray, HilmiIn the present work, treating the large arteries as a thin-walled, long and circularly cylindrical, prestressed elastic tube with variable cross-section and using the reductive perturbation method, we have studied the amplitude modulation of non-linear waves in such a fluid-filled elastic tube. By considering the blood as an incompressible viscous fluid, the evolution equation is obtained as the dissipative non-linear Schrodinger equation with variable coefficients. It is shown that this type of equations admit a solitary wave solution with a variable wave speed. It is observed that, the wave speed increases with distance for narrowing tubes while it decreases for expanding tubes.Yayın Weakly nonlinear waves in elastic tubes filled with a layered fluid(Freund Publishing House, 2002) Demiray, HilmiIn this work we studied the propagation of weakly nonlinear waves in a prestressed thin elastic tube filled with an incompressible layered fluid, where the outer layer is assumed to be inviscid whereas the cylindrical core is considered to be viscous. Using the reductive perturbation technique, the propagation of weakly nonlinear waves in the longwave approximation is studied. The governing equation is shown to be the Korteweg-de Vries-Burgers' equation. A travelling wave type of solution for this evolution equation is sought and it is shown that with increasing core radius parameter the formation of strong shock wave becomes inevitable.Yayın Modulation of non-linear waves in a viscous fluid contained in an elastic tube(Pergamon-Elsevier Science, 2001-06) Demiray, HilmiIn the present work, utilizing the non-linear equations of a prestressed thin elastic tube filled with an incompressible viscous fluid the propagation of weakly non-linear waves in such a medium is studied. Considering that the arteries are initially subjected to a large static transmural pressure P-0 and an axial stretch lambda (z) and, in the course of blood how, a finite-time-dependent displacement is added to this initial field, the non-linear equations governing the motion of the tube in the radial direction is obtained. Utilizing the reductive perturbation technique the amplitude modulation of weakly non-linear and dissipative but strongly dispersive waves is examined and the dissipative non-linear Schrodinger equation is obtained. Finally, the numerical solution of the evolution equation under the given initial condition is given and the stability condition of the solution is discussed.
