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AE/ME 339 Computational Fluid Dynamics (CFD) K. M. Isaac 12/3/2018

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1 AE/ME 339 Computational Fluid Dynamics (CFD) K. M. Isaac 12/3/2018
Topic7_NS_ F02

2 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
12/3/2018 Topic7_NS_ F02

3 Momentum equation Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR Momentum equation 12/3/2018 Topic7_NS_ F02

4 Consider the moving fluid element model shown in Figure 2.2b
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Consider the moving fluid element model shown in Figure 2.2b Basis is Newton’s 2nd Law which says F = m a Note that this is a vector equation. It can be written in terms of the three cartesian scalar components, the first of which becomes Fx = m ax Since we are considering a fluid element moving with the fluid, its mass, m, is fixed. The momentum equation will be obtained by writing expressions for the externally applied force, Fx, on the fluid element and the acceleration, ax, of the fluid element. The externally applied forces can be divided into two types: 12/3/2018 Topic7_NS_ F02

5 Body forces: Distributed throughout the control volume. Therefore,
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR Body forces: Distributed throughout the control volume. Therefore, this is proportional to the volume. Examples: gravitational forces, magnetic forces, electrostatic forces. Surface forces: Distributed at the control volume surface. Proportional to the surface area. Examples: forces due to surface and normal stresses. These can be calculated from stress-strain rate relations. Body force on the fluid element = fx r dx dy dz where fx is the body force per unit mass in the x-direction The shear and normal stresses arise from the deformation of the fluid element as it flows along. The shape as well as the volume of the fluid element could change and the associated normal and tangential stresses give rise to the surface stresses. 12/3/2018 Topic7_NS_ F02

6 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
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7 type of fluid we are dealing with.
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR The relation between stress and rate of strain in a fluid is known from the type of fluid we are dealing with. Most of our discussion will relate to Newtonian fluids for which Stress is proportional to the rate of strain For non-Newtonian fluids more complex relationships should be used. Notation: stress tij indicates stress acting on a plane perpendicular to the i-direction (x-axis) and the stress acts in the direction, j, (y-axis). The stresses on the various faces of the fluid element can written as shown in Figure 2.8. Note the use of Taylor series to write the stress components. 12/3/2018 Topic7_NS_ F02

8 The normal stresses also has the pressure term.
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR The normal stresses also has the pressure term. Net surface force acting in x direction = 12/3/2018 Topic7_NS_ F02

9 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
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10 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
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11 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
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12 The term in the brackets is zero (continuity equation)
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR The term in the brackets is zero (continuity equation) The above equation simplifies to Substitute Eq. (2.55) into Eq. (2.50a) shows how the following equations can be obtained. 12/3/2018 Topic7_NS_ F02

13 The above are the Navier-Stokes equations in “conservation form.”
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR The above are the Navier-Stokes equations in “conservation form.” 12/3/2018 Topic7_NS_ F02

14 For Newtonian fluids the stresses can be expressed as follows
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR For Newtonian fluids the stresses can be expressed as follows 12/3/2018 Topic7_NS_ F02

15 viscosity coefficient.
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR In the above m is the coefficient of dynamic viscosity and l is the second viscosity coefficient. Stokes hypothesis given below can be used to relate the above two coefficients l = - 2/3 m The above can be used to get the Navier-Stokes equations in the following familiar form 12/3/2018 Topic7_NS_ F02

16 Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR
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17 The energy equation can also be derived in a similar manner.
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR The energy equation can also be derived in a similar manner. Read Section 2.7 12/3/2018 Topic7_NS_ F02

18 For a summary of the equations in conservation and non-conservation
Computational Fluid Dynamics (AE/ME 339) K. M. Isaac MAEEM Dept., UMR For a summary of the equations in conservation and non-conservation forms see Anderson, pages 76 and 77. The above equations can be simplified for inviscid flows by dropping the terms involving viscosity. (read Section 2.8.2) 12/3/2018 Topic7_NS_ F02

19 Program Completed University of Missouri-Rolla
Copyright 2002 Curators of University of Missouri 12/3/2018 Topic7_NS_ F02

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