Flows within rotating systems occur frequently in science and engineering applications…

Examples

• compressors and turbines

• fans and pumps

• rotating cavities, seals, and bearings

• mixing equipment

• fluid coupling devices and torque converters

• air motors

• marine and aircraft propellers

• and many more…

Computational Fluid Dynamics (CFD) now plays a central role in the design and analysis of these systems

Examine the four major classes of rotating machinery problems

– Single (rotating) reference frame (SRF)

– Multiple (rotating) reference frame (MRF)

– Mixing plane (MPM)

– Sliding mesh (SMM)

Present details on modeling rotating machinery problems with the Fluent CFD solver

– Problem definition

– Model setup

– Solution process (steady-state and unsteady)

– Post-processing

Moving Reference Frame Theory

• Introduction to Moving Reference Frames

• The Velocity Triangle

• Frame Acceleration

• Governing Equations for a Moving Reference Frame

– Relative and Absolution Velocity Formulations

– Scalar Transport Equations

• Boundary Conditions

Single Rotating Frame Modeling (SRF):

• Many problems which involve rotating components can be modeled using a Single (moving) Reference Frame (SRF)

• Why use a single moving reference frame?

– Flow field which is unsteady with respect to the stationary frame becomes steady with respect to the moving frame

• Easy to set up and solve!

• We will discuss issues related to SRF setup and modeling in this lecture, but most concepts will also apply to other modeling approaches

• Introduction to SRF

• SRF Modeling

– Solvers

– Physical Models

– Material Properties

– Cell Zone Conditions

– Boundary Conditions

– Solver Settings and Controls

– Initialization

Multiple-Rotating Reference Frame Modeling (MRF)

• Introduction to MRF Modeling

• What is the MRF Model?

• Interfaces

• MRF Problem Setup

• Troubleshooting MRF Problems

• Many rotating machinery problems involve stationary components which cannot be described by surfaces of revolution

– SRF is not valid!

– You must create interfaces between stationary and rotating regions

• Systems like these which involve stationary and rotating components separated by interfaces can be addressed with

FLUENT using three different approaches:

– Multiple reference frame model (MRF)

– Mixing plane model (MPM)

– Sliding mesh model (SMM)

• The MRF model, the simplest and most approximate of the three approaches, will be discussed in detail in this lecture

Mixing Plane Modeling (MPM)

Introduction to Mixing Planes

Mixing Plane Implementation

Mixing Plane Setup

Mixing plane Options

• Axial and centrifugal turbomachines are comprised of one of more stages, where a stage consists of vane or stator blade row (to turn the flow appropriately) and a rotor or impeller blade row (to add to or extract energy from the flow)

• For multistage problems, we often know the stage boundary conditions (e.g. inlet total pressure and temperature and stage outlet static pressure) but not the inter-stage conditions

• In addition, the blade counts will generally not be the same from one blade row to the next

• MRF in Fluent can only be used if we have equal periodic angles for each row. Thus stages with unequal blade counts may require computation of a large numbers of blade passages which can be computationally expensive!

• To overcome these problems, we can employ the Mixing Plane Model (MPM) to couple multiple single blade row models together to create models of one or more stages

Sliding Mesh Modeling (SMM)

Introduction to Sliding Mesh Modeling

The Navier-Stokes Equations: Moving Mesh Form

Sliding Mesh Setup

• Grid Interfaces

• Mesh Preview

• Choosing a Time Step

• The relative motion of stationary and rotating components in a rotating machine will give rise to unsteady interactions. These interactions are generally classified as

follows:

– Potential interactions – flow unsteadiness due to pressure waves which propagate both upstream and downstream

– Wake interactions – flow unsteadiness due to wakes from upstream blade rows advecting downstream

– Shock interactions – for transonic/supersonic flows,

unsteadiness due to shocks waves striking downstream blade row

• Both the MRF and Mixing Plane models neglect unsteady interaction entirely and thus are limited to flows where these effects are weak

• If unsteady interactions can not be neglected, we can employ the sliding mesh model to account for the relative motions of the stationary and rotating components

Post-processing for Rotating Machinery

Basic post-processing in FLUENT

• Surface and volume integrals

• Torques, forces, and power

• Contour and vector plotting

• Particle paths

Turbo-specific post-processing in FLUENT

• Turbo topology

• Turbo coordinates

• Turbo post-processing tools

Turbo post-processing with CFD Post

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