08-turb

# 08-turb - Lecture 8 Turbulence Applied Computational Fluid...

• Notes
• 40

This preview shows pages 1–12. Sign up to view the full content.

1 Lecture 8 - Turbulence Applied Computational Fluid Dynamics Instructor: André Bakker © André Bakker (2002-2006) © Fluent Inc. (2002)

This preview has intentionally blurred sections. Sign up to view the full version.

2 Turbulence What is turbulence? Effect of turbulence on Navier-Stokes equations. Reynolds averaging. Reynolds stresses.
3 Instability All flows become unstable above a certain Reynolds number. At low Reynolds numbers flows are laminar. For high Reynolds numbers flows are turbulent. The transition occurs anywhere between 2000 and 1E6, depending on the flow. For laminar flow problems, flows can be solved using the conservation equations developed previously. For turbulent flows, the computational effort involved in solving those for all time and length scales is prohibitive. An engineering approach to calculate time-averaged flow fields for turbulent flows will be developed.

This preview has intentionally blurred sections. Sign up to view the full version.

4 Time What is turbulence? Unsteady, aperiodic motion in which all three velocity components fluctuate, mixing matter, momentum, and energy. Decompose velocity into mean and fluctuating parts: U i (t) U i + u i (t). Similar fluctuations for pressure, temperature, and species concentration values.
5 Examples of simple turbulent flows jet mixing layer wake Some examples of simple turbulent flows are a jet entering a domain with stagnant fluid, a mixing layer, and the wake behind objects such as cylinders. Such flows are often used as test cases to validate the ability of computational fluid dynamics software to accurately predict fluid flows.

This preview has intentionally blurred sections. Sign up to view the full version.

6 Transition The photographs show the flow in a boundary layer. Below Re crit the flow is laminar and adjacent fluid layers slide past each other in an orderly fashion. The flow is stable. Viscous effects lead to small disturbances being dissipated. Above the transition point Re crit small disturbances in the flow start to grow. A complicated series of events takes place that eventually leads to the flow becoming fully turbulent.
7 Transition in boundary layer flow over flat plate

This preview has intentionally blurred sections. Sign up to view the full version.

8 Transition in boundary layer flow over flat plate Turbulent spots Fully turbulent flow T-S waves
9 Turbulent boundary layer Merging of turbulent spots and transition to turbulence in a natural flat plate boundary layer. Top view Side view

This preview has intentionally blurred sections. Sign up to view the full version.

10 Turbulent boundary layer Close-up view of the turbulent boundary layer.
11 Transition in a channel flow Instability and turbulence is also seen in internal flows such as channels and ducts. The Reynolds number is constant throughout the pipe and is a function of flow rate, fluid properties and diameter. Three flow regimes are shown: Re < 2200 with laminar flow. Re = 2200 with a flow that alternates between turbulent and laminar. This is called transitional flow.

This preview has intentionally blurred sections. Sign up to view the full version.

This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern