CFD Case Studies

Brake Rotor Thermal Flow Analysis

Brake Rotor Thermal Flow Analysis

Background
High temperature in a vehicle brake system could cause brake pad/caliper distortion, accelerated corrosion of the cast iron rotor, and excessive brake pad wear. All these problems would result in a shorter life span of the brake assembly and consequently more customer complaints.

The new brake rotor design is thus focused on the effective heat removal from braking. Current brake rotor design usually relies on expensive and time consuming experimental study for each new design.

Optimal CAE provides an integrated CFD thermal analysis technique that can assist the brake rotor designer to achieve the goal in a quicker turnaround time and a lower cost.

Objectives

  • To evaluate and compare the temperature distribution and the airflow performance in different brake rotor design using CFD techniques.
  • To compare CAE predicted rotor temperature with the experimental data.
Approach
  • Construct modeling mesh for a slice of the brake rotor containing a pair of fins and the surrounding air.
  • Assign appropriate heat source generate from braking and inlet and outlet boundary conditions based on experimental measurements from the brake thermal capacity test at a constant rotation speed. Perform steady state turbulent flow analysis using FLUENT for two designs of fin configuration.
  • Transfer CFD result to ABAQUS to conduct transient heat transfer finite element analysis.
  • Compare transient temperature to testing temperature data.
Results - Temperature Distribution
The steady state CFD results show that the maximum temperature on the rotor surface with 37 straight fins is much higher than that with 72 curved fins.

Results - Heat Transfer Coefficients
The "steady state" heat transfer coefficient resulted from the CFD analysis is used as boundary conditions in the FEA thermal stress analysis.

Results - Velocity Contours
The airflow through the 72 curved fin rotor is much higher than that for the 37 straight fin rotor.

Conclusions
The CFD/FEA analysis correctly predicts the temperature difference trends corresponding to rotor design changes, however, the temperature after 25 stops for CAE prediction is higher than the testing data.

The discrepancy between CAE prediction and test data could be attributed to the simplification of the model.

For example:
  1. Thermal mass of hub and other components in the assembly is not included
  2. Heat loss due to radiation and conduction heat loss to the knuckle or axle are not modeled
  3. Mass transfer coefficient may change with time
  4. Mass transfer may be under-predicted by the wall-functions in the low velocity regions

Vibration Analysis

Vibration Analysis

Background
It was observed during field testing the mirror was vibrating excessively at certain speeds.

The vibration is due to the unsteady, flow-induced forces resulted from the vortex behind the mirror, which are close to the natural frequency of the mirror. The frequency of vortex shedding depends on the geometry of the mirror and the angle of impingement of the oncoming air. To avoid this resonance effect, an integrated CFD/FEA analysis could provide insight information in the mirror design process.

For example:

  1. Thermal mass of hub and other components in the assembly is not included
  2. Heat loss due to radiation and conduction heat loss to the knuckle or axle are not modeled
  3. Mass transfer coefficient may change with time
  4. Mass transfer may be under-predicted by the wall-functions in the low velocity regions
  5. Objectives
    To investigate vortex shedding process in the baseline mirror design that will vibrate at high speed.
    Based on the knowledge gained from the baseline design, modify the baseline design, conduct CFD simulation and compare to the baseline design.

    Approach
    • Construct 3D body fit, hexahedral mesh for the baseline mirror design, including the mirror and the side panel of a vehicle.
    • Apply boundary conditions and conduct a steady state flow analysis.
    • Conduct a FEA structural dynamics analysis of the mirror based on the pressure around the mirror obtained from the steady state CFD analysis.
    • Modify design geometry based on the steady state flow analysis and the structural dynamics analysis.
    • Conduct unsteady state flow analysis for the modified mirror design and compare the results to the baseline design.
    Results - Velocity Vectors
    Steady state velocity vector plots show that the modify mirror has a smaller re-circulation zone behind the side mirror.

    Results - 3D Streamlines
    The streamlines plots from the steady state flow analysis indicate that two recirculation zones can be found behind the mirror.

    Results - Relative Pressure
    The steady state pressure contour can be used as initial for structural dynamics analysis

    Conclusions
    The frequency of vortex shedding has been determined by using the cost-effective CFD/FEA model.

    The modified design, a result of CFD and FEA analysis from the baseline design, increases the natural frequencies of the side view mirror. The increase of the natural frequency of the mirror have improved the vibration significantly.

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