Abstract
The vibration and instability of functionally graded material (FGM) sandwich cylindrical shells conveying fluid are investigated. The Navier-Stokes relation is used to describe the fluid pressure acting on the FGM sandwich shells. Based on the third-order shear deformation shell theory, the governing equations of the system are derived by using the Hamilton’s principle. To check the validity of the present analysis, the results are compared with those in previous studies for the special cases. Results manifest that the natural frequency of the fluid-conveying FGM sandwich shells increases with the rise of the core-to-thickness ratio and power-law exponent, while decreases with the rise of fluid density, radius-to-thickness ratio and length-to-radius ratio. The fluid-conveying FGM sandwich shells lose stability when the non-dimensional flow velocity falls in 2.1—2.5, which should be avoided in engineering application.
Pipes conveying fluid are found in numerous industrial applications, in particular in water conservancy project and submarine oil transpor
Functionally gradient materials (FGMs) have some prominent advantages such as avoiding crack, avoiding delamination, reducing stress concentration, eliminating residual stress, etc
As one of the most prevalent composite structures applied in aerospace, naval, automotive and nuclear engineering, sandwich structures have attracted tremendous interests from academic and industrial communitie
Due to the good thermal insulation of sandwich shells, they can be used as pipelines for transporting petroleum to prevent the paraffin in the crude oil from depositing on the pipe wall after the oil temperature is lowered. The core layer usually uses a material with good heat insulation properties, such as ceramic
In the present study, we deal with the vibration and instability of FGM sandwich shells conveying fluid. The Navier-Stokes relation is used to describe the fluid pressure acting on the shells. The third-order shear deformation shell theory is used to model the present system. Then, the governing equations and boundary conditions are derived by using the Hamilton’s principle. Finally, the frequency and stability results are presented for FGM sandwich shells conveying fluid under various conditions.
As shown in

Fig.1 FGM sandwich cylindrical shell conveying fluid
For the FGM sandwich shell, the effective material properties of layer j (j = 1, 2, 3) can be expressed a
(1) |
where Pm and Pc denote the material properties of metal and ceramic, respectively; the volume fraction
(2) |
where is the power-law exponent.
Therefore, the Poisson’s ratio , Young’s modulus and mass density (j = 1, 2, 3) of the FGM sandwich shell are expressed as
(3) |
(4) |
(5) |
where , , are the Poisson ratio, mass density and Young’s modulus of metal, respectively; , , the Poisson ratio, mass density and Young’s modulus of ceramic, respectively.
The fluid inside the shell is assumed to be incompressible, isentropic and time independent. To simplify the problem, we ignore the influence of the deformation and vibration of the shell on the liquid flow, the shear force transferred from the flow, the flow separation and the Reynold number. The momentum-balance equation for the fluid motion can be described by the well-known Navier-Stokes equatio
(6) |
where is the flow velocity with components in the r, θ and x directions; P and μ are the pressure and the viscosity of the fluid, respectively; is the mass density of the internal fluid; the Laplacian operator and Fbody the body forces. In this paper, we neglect the action of body forces and consider Newtonian fluid, i.e., the viscosity is time-independent.
At the interface between the fluid and the shell, the velocity of the fluid is equal to the shell in the radial direction. These relationships can be written a
(7) |
where r is the distance from the center of the shell to an arbitrary point in the radial direction, and
(8) |
where U is the mean flow velocity.
Consider the fluid as inviscid. By substituting Eqs.(
(9) |
According to the third-order shear deformation shell theory, the displacement fields of the fluid-conveying FGM sandwich shell are expressed a
(10) |
where ; u, v and w are the displacements of an arbitrary point of the shell; u0, v0 and w0 the displacements of a generic point of the middle surface; ϕx and ϕθ the rotations of a normal to the mid-surface about and x axes, respectively.
The strain components at a distance z from the mid-plane ar
(11) |
(12) |
where
(13) |
(14) |
(15) |
(16) |
(17) |
where are the strains of an arbitrary point; the strains of a generic point of the middle surface; the curvatures of a generic point of the middle surface.
The relationship between stresses and strains of the fluid-conveying FGM sandwich shell is stated as
(18) |
where Qij is given by
(19) |
(20) |
(21) |
The strain energy of the FGM sandwich shell can be expressed as
(22) |
where the resultant forces Nx, Nθ and Nxθ, moments Mx, Mθ and Mxθ, shear forces Qxz and Qθz, and higher-order stress resultants Px, Pθ, Pxθ, Kxz and Kθz are defined by
(23‑1) |
(23‑2) |
(23‑3) |
(23‑4) |
(23‑5) |
The kinetic energy of the FGM sandwich shell is written as
(24) |
In addition, the potential energy associated to the fluid pressure is given b
(25) |
By using the Hamilton principle
(26) |
and then equating the coefficients of , , , and to zero, the motion equations of the fluid-conveying FGM sandwich shell can be obtained as
(27) |
(28) |
(29) |
(30) |
(31) |
where the inertias and (i = 1, 2, 3, 4, 5) are defined by
(32) |
(33) |
(34) |
(35) |
(36) |
(37) |
By substituting Eq.(23) into Eqs.(
(38) |
(39) |
(40) |
(41) |
(42) |
The simply supported boundary condition is considered in this study. It is given b
The solutions to Eqs.(
(43) |
(44) |
(45) |
(46) |
(47) |
(48) |
where ; ,,, and represent generalized coordinates.
Substituting Eqs.(
(49) |
(50) |
(51) |
(52) |
(53) |
where m = 1, 2, …,; i = 1, 2, …,; n = 1, 2, …, N; m ≠ i and m ± i are odd numbers. Mij and Kij are integral coefficients.
Eqs.(
(54) |
where M, C, K denote the mass, damping and stiffness matrices, respectively; X is a 5××N column vector consisting of ,,, and .
(55) |
where is the state vector. It should be noted that eigenvalue is a non-zero complex number. The imaginary part of is the frequency and the real part is the damping.
The natural frequency is obtained by finding the eigenvalues of the matrix .In order to demonstrate the accuracy of the present analysis, a comparison investigation related to a liquid-filled homogenous cylindrical shell is carried out. The used parameters are: h/R = 0.01, ρf = 1 000 kg/

Fig.2 Non-dimensional eigenvalue Ω versus non-dimensional flow velocity V
Next, we investigate the stability and free vibration of FGM sandwich shells conveying fluid. The fluid is considered as crude oil, with a mass density ρf = 0.81 g/c
Aluminum: Em = 70 GPa, νm = 0.3, ρm = 2 707 kg/
Zirconia: Ec = 151 GPa, νc = 0.3, ρc = 3 000 kg/

Fig.3 Non-dimensional natural frequency versus circumferential wave number n of FGM sandwich shell conveying fluid (m=1, L/R = 2.0, V = 0, k = 1, R/h = 80, h2/h = 0.2)

Fig.4 The first two non-dimensional natural frequencies versus non-dimensional flow velocity of FGM sandwich shell conveying fluid (n = 5, L/R = 2.0, = 2, k = 1, R/h = 80, h2/h = 0.2)

Fig.5 The first non-dimensional natural frequency versus length-to-radius ratio L/R of fluid-conveying FGM sandwich shell with different power-law exponents (n = 5, V = 1, = 2, R/h = 80, h2/h = 0.2)

Fig.6 The first non-dimensional natural frequency versus radius-to-thickness ratio R/h of fluid-conveying FGM sandwich shell with different power-law exponents (n = 5, L/R = 2.0, = 2, V = 1, h2/h = 0.2)

Fig.7 The first non-dimensional natural frequency versus the fluid density ρf of fluid-conveying FGM sandwich shell with different power-law exponents (n = 5, L/R = 2.0, = 2, V = 1, h2/h = 0.2, R/h = 80)

Fig.8 The first non-dimensional natural frequency versus core-to-thickness ratio h2/h of fluid-conveying FGM sandwich shell with different power-law exponents (n = 5, L/R = 2.0, = 2, V = 1, R/h = 80)
The vibration and instability of FGM sandwich shells conveying fluid are investigated based on the third-order shear deformation shell theory. By using the Hamilton’s principle, the governing equations of the present system are derived. Results show that as the flow velocity increases, the natural frequencies of the fluid-conveying FGM sandwich shells decrease at first. When reaching the critical velocity, the first frequency vanishes and the system loses its stability. Moreover, the fluid-conveying FGM sandwich shells lose stability when the non-dimensional flow velocity falls in 2.1—2.5, which should be avoided in submarine oil transport. Besides, the first non-dimensional natural frequency decreases with the rise of fluid density, radius-to-thickness ratio and length-to-radius ratio while increases with the raise of core-to-thickness ratio and power-law exponent. The fluid viscosity has insignificant effect on the first non-dimensional natural frequency of the fluid-conveying FGM sandwich shells.
Contributions Statement
Mr.LI Zhihang designed the study, complied the models, conducted the analysis, interpreted the results and wrote the manuscript. Prof. ZHANG Yufei and Prof. WANG Yanqing contributed to the discussion and background of the study. All authors commented on the manuscript draft and approved the submission.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos.11922205 and 12072201), and the Fundamental Research Fund for the Central Universities (No.N2005019).
Conflict of Interest
The authors declare no competing interests.
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