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Computational Fluid Dynamics: Governing Equations and Boundary Conditions

This topic discusses the governing equations (continuity, momentum, and energy equations) in Computational Fluid Dynamics (CFD) and their conservation/non-conservation forms. It also covers the physical boundary conditions and their specifications for different types of flow problems.

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Computational Fluid Dynamics: Governing Equations and Boundary Conditions

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  1. AE/ME 339 Computational Fluid Dynamics (CFD) K. M. Isaac Professor of Aerospace Engineering topic7_NS_equations

  2. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Governing equation summary Continuity equation Non-conservation form ………….(2.29) Conservation form topic7_NS_equations

  3. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Governing equation summary Momentum equation Non-conservation form topic7_NS_equations

  4. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Governing equation summary Momentum equation conservation form topic7_NS_equations

  5. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Governing equation summary Energy equation non-conservation form topic7_NS_equations

  6. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Governing equation summary Energy equation conservation form topic7_NS_equations

  7. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR • Exercise: Write the corresponding equations for inviscid flow. • Observations: • Equations are coupled and non-linear • Conservation form contains divergence of some quantity • on the LHS. This form is sometimes known as divergence • form. • Normal and shear stress terms are functions of velocity gradient. • We have six unknowns and five equations • (1 continuity + 3 momentum + 1 energy). • For incompressible flow r can be treated as a constant. • For compressible flow, the equation of state can be used as an • additional equation for the solution. • The set of equations with viscosity included, is known as the • Navier-Stokes equations. • The set of inviscid flow equations is also known as the Euler • equations. These naming conventions are not strictly followed • by everyone. topic7_NS_equations

  8. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR • Physical Boundary Conditions • The above equations are very general. For example, • they represent flow over an aircraft or flow in a hydraulic pump. • To solve a specific problem much more information would be • necessary. Some of them are listed below: • Boundary conditions (far field, solid boundary, etc) • Initial conditions (for unsteady problems) • Fluid medium (gas, liquid, non-Newtonian fluid, etc.) • BC specification depends on the type of flow we are interested in. • e. g., velocity boundary condition at the surface • “No slip condition” for viscous flow. All velocity • components at the surface are zero. • Zero normal velocity of inviscid flow. topic7_NS_equations

  9. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Temperature BC at the wall. Temperature, Tw, heat flux, qw, etc. can be specified. Note If is a known quantity, an expression for normal to the surface can be written in terms of known quantities. topic7_NS_equations

  10. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Conservation form of the equations All equations can be expressed in the same generic form fluxes can be written as topic7_NS_equations

  11. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Conservation form contains divergence of these fluxes. topic7_NS_equations

  12. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR topic7_NS_equations

  13. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR topic7_NS_equations

  14. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR topic7_NS_equations

  15. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR In the above U is called the solution vector F, G, H are called flux vectors J is called the source term vector The problem is thus formulated as an unsteady problem. Steady state solutions can be obtained asymptotically. Once the flux variables are known from the solution, the “primitive” variables, u, v, w, p, e etc. can be obtained from the flux variables. Exercise write the vector form of the equations for inviscid flow (Euler equations). topic7_NS_equations

  16. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Note that the following equations can be used to determine T topic7_NS_equations

  17. Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR topic7_NS_equations

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