tailieunhanh - Finite Element Analysis - Thermomechanics of Solids Part 13

Tham khảo tài liệu 'finite element analysis - thermomechanics of solids part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 13 Thermal Thermoelastic and Incompressible Media TRANSIENT CONDUCTIVE-HEAT TRANSFER Finite-Element Equation The governing equation for conductive-heat transfer without heat sources assuming an isotropic medium is kVT pce T. With the interpolation model T t - T0 ạ Ị x T9 t and VT PT T0 t the finite-element equation assumes the form __ _ _ _ Ktớ Mrớ q t kt I TPAPT dv MT I bTyTpcyT T-dV This equation is parabolic first-order in the time rates and implies that the temperature changes occur immediately at all points in the domain but at smaller initial rates away from where the heat is added. This contrasts with the hyperbolic second-order time rates solid-mechanics equations in which information propagates into the medium as finite velocity waves and in which oscillatory response occurs in response to a perturbation. Direct Integration by the Trapezoidal Rule Equation is already in state form since it is first-order and the trapezoidal rule can be applied directly M 9n 1 9n K 9n 1 9n q n 1 q n from which K Dt9 i r i h __ h__ h. K. M Kr . M_ 9 K K 9-- q_ q DT T 2 T n 1 Tn 2 Tn 2 1 n 1 n 173 2003 by CRC CRC Press LLC 174 Finite Element Analysis Thermomechanics of Solids For the assumed conditions the dynamic thermal stiffness matrix is positive-definite and for the current time step the equation can be solved in the same manner as in the static counterpart namely forward substitution followed by backward substitution. Modal Analysis Modes are not of much interest in thermal problems since the modes are not oscillatory or useful to visualize. However the equation can still be decomposed into independent single degree of freedom systems. First we note that the thermal system is asymptotically stable. In particular suppose the inhomogeneous term vanishes and that 0 at t 0 does not vanish. Multiplying the equation by 0T and elementary manipulation furnishes that d 0TMT0 -0TKT 0 0. Clearly the product 0TMT0 .

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