Inviscid flows and the turbulent (eddy) viscosity

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SUMMARY

The discussion focuses on the analysis of inviscid flows and the implications of turbulent (eddy) viscosity in the context of the momentum equation. The derived equation demonstrates the relationship between Reynolds stress and eddy viscosity, specifically highlighting that in inviscid flows, the turbulent viscosity coefficient (μ_T) and turbulent kinetic energy (k) approach zero. Key questions raised include the origin of Reynolds stress in motion equations, the specific term in the Navier-Stokes equation that becomes zero for inviscid flows, and the behavior of μ_T and k in this context.

PREREQUISITES
  • Understanding of Favre averaging in fluid dynamics
  • Familiarity with the Navier-Stokes equations
  • Knowledge of Reynolds stress tensor concepts
  • Basic principles of turbulent flow and eddy viscosity
NEXT STEPS
  • Study the derivation and implications of the Navier-Stokes equations in inviscid flows
  • Explore the concept of Reynolds stress and its role in fluid dynamics
  • Investigate the mathematical formulation of eddy viscosity and its applications
  • Learn about the differences between laminar and turbulent flow characteristics
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Fluid dynamics researchers, mechanical engineers, and students studying turbulence and inviscid flow theories.

Joshua Pham
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Hello,
After Favre averaging the momentum equation for an inviscid flow, the following can be obtained:

$$\frac{\partial}{\partial t} \left(\overline{\rho}\tilde{u}_i \right) + \frac{\partial}{\partial x_j}\left( \overline{\rho}\tilde{u}_i \tilde{u}_j \right) + \frac{\partial \overline{p}}{\partial x_i} = \frac{\partial}{\partial x_j} \left(-\overline{\rho u_i'' u_j''}\right)$$

The term on the right hand side is the partial derivative of the Reynolds stress tensor with respect to ##x##.

By the Eddy viscosity concept,

$$\overline{\rho} \overline{u_i'' u_j''} = \mu_T \left( \frac{\partial \tilde{u}_i}{\partial x_j} + \frac{\partial \tilde{u}_j}{\partial x_i} \right) - \frac{2}{3} \left( \mu_T \frac{\partial \tilde{u}_i}{\partial x_j} + \overline{\rho} k \right) \delta_{ij}$$

For an inviscid flow do the ##\mu_T## and ##k##'s go to 0?
 
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Three relevant questions:
1) Where does the Reynolds stress come from in the equations of motion?
2) Which term in the Navier-Stokes equation is zero in the inviscid case?
3) What are ##\mu_T## and ##k##, and do they go to zero in case 2)?
 

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