Inviscid flows and the turbulent (eddy) viscosity

In summary, the equations for an inviscid flow can be obtained by averaging the momentum equation, which includes a term for the partial derivative of the Reynolds stress tensor. By using the Eddy viscosity concept, the Reynolds stress tensor can be expressed in terms of the Eddy viscosity and turbulent kinetic energy. In the case of an inviscid flow, the Eddy viscosity and turbulent kinetic energy both go to zero. This leads to the answers to three questions: 1) The Reynolds stress comes from the equations of motion, 2) The term for viscosity is zero in the inviscid case, and 3) ##\mu_T## and ##k## refer to the Eddy viscosity and turbulent kinetic energy, which both go
  • #1
Joshua Pham
14
0
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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  • #2
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)?
 

1. What is an inviscid flow?

An inviscid flow is a fluid flow where there is no internal friction or viscosity present. This means that the fluid particles are able to move freely without any resistance or energy loss due to interactions between them.

2. How does the presence of turbulence affect inviscid flows?

Turbulence is the chaotic and random motion of fluid particles, and it can greatly affect inviscid flows by introducing eddies or vortices that disrupt the smooth flow of the fluid. These eddies can also cause mixing and transport of momentum, energy, and mass within the flow.

3. What is turbulent (eddy) viscosity?

Turbulent viscosity is a measure of the effective viscosity in a turbulent flow. It is caused by the interactions between the eddies and the fluid particles, and it can be thought of as an apparent or effective viscosity that accounts for the turbulent mixing and transport within the flow.

4. How is the turbulent viscosity modeled in fluid dynamics?

The turbulent viscosity is typically modeled using empirical equations or turbulent closure models, such as the Reynolds-Averaged Navier-Stokes (RANS) equations. These models use assumptions and mathematical formulations to approximate the effects of turbulent eddies on the fluid flow.

5. Why is understanding inviscid flows and turbulent viscosity important?

Inviscid flows and turbulent viscosity play a crucial role in many important applications, such as aerodynamics, oceanography, and weather prediction. Understanding how these concepts affect fluid flow can help improve the design and efficiency of various systems and processes, leading to advancements in technology and scientific research.

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