8/29/2011

Introduction to CFX-Transient Brake Rotor

Transient Brake Rotor

This case models the transient heating of a steel rear disk brake rotor on a car as it brakes from 60 to 0 mph in 3.6 seconds.

To keep solution times to a minimum the case has been simplified by removing the wheel and brake assembly to leave only the brake rotor. The brake pad is modeled by applying a heat source to a small region of the brake rotor.

Assumptions

-The ambient air temperature is 81 F and the rotor is at ambient temperature before braking begins

-The vehicle tire size is 205/55/R16

-The total vehicle weight including passengers and cargo is 1609 kg

-The entire kinetic energy of the vehicle is dissipated through the brake rotors

-Energy dissipation during braking is split 70/30 between the front and rear brakes and split evenly between the left and right sides

-The vehicles speed reduces linearly from 60 to 0 mph in 3.6 seconds

Solution Approach

-The solution is transient, so you will need to begin by solving a steady-state case at a vehicle speed of 60 mph

-You will need two domains; a solid domain for the brake rotor and a fluid domain for the surrounding air

-The reference frame will be that of the vehicle. So the rotor will be spinning relative to this reference frame and air will be flowing past at the vehicle velocity

Start Steady-State Simulation

  1. Start CFX-Pre in a new working directory and create a new simulation named BrakeDisk
  2. Right-click on Mesh in the Outline tree and import the CFX-Mesh file named BrakeRotor.gtm

-The rotor mesh will be imported along with a bounding box surrounding the rotor

  1. In the Outline tree, expand Mesh > BrakeRotor.gtm > Principal 3D Regions

-There are two 3D regions in this mesh named B24 and B31

Examine Mesh Regions

  1. Click once in the tree on each of these 3D regions

-The mesh bounding each 3D region is displayed in the Viewer

-Notice that a mesh exists for the solid brake rotor and for the surrounding fluid region. These meshes are in separate 3D regions but still within the same Assembly

Create the Fluid Domain

By default the Simulation Type is set to Steady-State, so the next step is to create the fluid domain

  1. Select the Domain icon from the toolbarand enter the Name as AirDomain
  2. Pick the Location corresponding to the air region from the drop-down menu

-The regions are highlighted in the Viewer to assist you

  1. The fluid domain uses Air Ideal Gas as the working fluid at a Reference Pressure of 1 [atm]; the domain is Stationary relative to the chosen reference frame and Buoyancy (gravity) can be neglected. Use this information to set appropriate Basic Settings for this domain
  2. Switch to the Fluid Models tab for the domain
  3. Set the Heat Transfer Option to Thermal Energy and leave the Turbulence Option set to the default k-Epsilon model
  4. Switch to the Initialisation tab for the domain

Create the Fluid Domain

  1. Enable the Domain Initialisation, toggle

-All settings can then be left at their default values

  1. Click OK to create the domain

Create the Solid Domain

The next step is to create the solid domain for the brake rotor.

  1. Create a new domain named Rotor
  2. Pick the Location corresponding to the brake rotor
  3. Set the Domain Type to Solid Domain
  4. Set the Material to Steel
  5. Leave the Domain Motion Option as Stationary
  6. Switch to the Solid Models tab and enable the Solid Motion toggle

Create Expressions

  1. Set the Solid Motion Option to Rotating

The next quantity to enter is the Angular Velocity. This needs to be calculated based on the vehicle speed (60 mph) and the radius of the tire attached to the brake rotor. The tires were specified as 205/55/R16 (205 mm tire width, aspect ratio of 55, 16” rim diameter). Next you will create expressions to calculate the Angular Velocity.

  1. Switch to the Outline tab (do not close the Domain tab)
  2. Right-click on Expressions in the tree and select Insert > Expression

– You may need to expand the Expressions, Functions and Variables entry in the tree to be able to right-click on Expressions

  1. Enter the expression Name as Speed and click OK

– The Expressions tab will appear

  1. In the Definition window (bottom-left of the screen) enter
    60 [mile hr^-1] then click Apply

  1. Right-click in the top half of the Expressions window and select Insert > Expression; enter the Name as TireRadius
  2. Enter the Definition as (16 [in] / 2) + (205 [mm] * 0.55) and click Apply

14.Create another expression named Omega, type the Definition as Speed / TireRadius and then click Apply

15. Now switch back to the Domain: Rotor tab

Complete the Solid Domain

  1. Click the expression icon next to the Angular Velocity field and type in Omega (the name of the expression you just created)
  2. Pick the Rotation Axis as the Global X axis
  3. On the Initialisation tab set the Temperature Option to Automatic with Value and enter a Temperature of 81 [ F ]

-Make sure you have changed the units to F

  1. Now click OK to create the domain

Create Boundary Conditions

Boundary conditions are needed for the bounding box of the air domain. You will create an inlet boundary upstream of the rotor, an outlet boundary downstream of the rotor and an opening boundary for the remaining bounding surfaces. Start with the inlet boundary:

  1. In the Outline tree, right-click on AirDomain and select Insert > Boundary. Enter the Name as AirIn when prompted and click OK
  2. On the Basic Settings tab, set the Boundary Type to Inlet and the Location to Inlet
  3. On the Boundary Details tab, set the Mass And Momentum Option to Normal Speed
  4. In the Normal Speed field click the expression icon and enter Speed

-This is one of the expressions you created earlier

  1. Set the Heat Transfer Option to Static Temperature and enter the a value of 81 [ F ]
  2. Click OK to create the inlet boundary

Now create the outlet boundary condition:

  1. Right-click on AirDomain and insert a boundary named AirOut
  2. Use the following setting for this boundary:

-Boundary Type = Outlet

-Location = Outlet

-Mass And Momentum Option = Average Static Pressure

-Relative Pressure = 0 [ Pa ]

  1. Click OK to create the outlet boundary

Lastly, create the opening boundary condition:

  1. Insert a boundary named AirOpening into the AirDomain
  2. Use the following settings for this boundary:

-Boundary Type = Opening

-Location = OuterWalls

-Mass And Momentum Option = Entrainment

-Relative Pressure = 0 [ Pa ]

-Turbulence Option = Zero Gradient

-Heat Transfer Option = Opening Temperature

-Opening Temperature = 81 [ F ]

  1. Click OK to create the opening boundary

Create Domain Interface

Domain Interfaces are required when more than one domain exists in your simulation. Without domain interfaces one domain would not see or feel the effect of neighboring domains. A Default Fluid Solid Interface should already exist, but we will manually create the interface here as a practice exercise.

  1. Select the Domain Interface icon from the toolbarand enter the Name as RotorInterface
  2. Set the Interface Type to Fluid Solid
  3. For Interface Side 1, set the Domain (Filter) to AirDomain; pick both BrakePadsFluidSide and RotorFluidSide from the Region List

  1. For Interface Side 2, set the Domain (Filter) to Rotor. Pick BrakePadsSolidSide and RotorSolidSide from the Region List
  2. Under Interface Models, leave the Frame Change and Pitch Change Option set to None

http://www.cadfamily.com/html/Article/Introduction%20to%20CFX-Transient%20Brake%20Rotor_783_1.htm

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