Download - Brake Analysis-Temperature Approach
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Thermal Analysis of a Vehicle DiscBrake in a Multi-Stop Scenario
Josh Pryor | [email protected]
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Introduction
Disc brake simulations can becomplex to mesh and setup
Previous UGM talks havediscussed: Meshing techniques (shell vs. solid)
Heat application techniques CFD convection
This effort starts from a solid-meshed vented disc brake
Main question: how to effectivelyuse CFD for a multi-stopscenario?
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Convection options
RadTherm convection (automatic library or assigned HTC) Fast and easy to setup
Difficult to capture effects of vent designs or upstream flow effects
(vents, shields, etc.) Typical transient CFD coupling
(local fluid temperature-based HTC): Run multiple steady-state cases corresponding to different points in
time of a RadTherm transient model
Different CFD case is needed anytime flow conditions or surfacetemperatures change significantly Depending on the stop profile, this most likely requires 6-20 CFD cases
per stop (60-500 total cases!)
One-way CFD coupling(reference fluid temperature-based HTC): Assume that convection effects are primarily based on flow conditions
(which are cyclical) and not on local surface temperatures
Run enough CFD cases to cover velocity profile of one stop (6-20total cases)
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Thermal model
Brake geometry only Rotor, pads, caliper
Heat applied to pad surface
Pads linked to rotor with generic thermal
link (intermediate node approach) 276,000 total elements
Rotor & pads primarily hexa & prismelements
Caliper uses tet elements
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CFD model
Front quarter-car model Includes heat
exchangers, grille, frontfascia, basic underhood,
wheels/tires,suspension, brake parts
5.7 million volumeelements (polyhedral)
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CFD selected cases
6 points were used tocharacterize the velocityprofile for each stop
6 cases are set up andrun for eachcorresponding velocity Surface temperatures
estimated from middle stopof standalone thermal model
Reference temperatureapproach: Convection from these 6
cases are importedrepeatedly to capture fullprofile
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Selecting the convection coefficient
reference temperature Most straightforward approach is to use the ambient
temperature as the reference This will result in valid convection coefficients if all surface elements
in question are experiencing local cooling
This works well for hotter parts (rotor/pads), but breaks down forcooler parts (caliper)
In this model, the caliper was experiencing local heating
Reference temperature therefore needs to be hotter than the typicalcaliper temperature and cooler than typical rotor temperature
57o C was used
This temperature is consistent with typical fluid temperatures near the
brake (due to heating from the brake and underhood) and results inrealistic convection coefficients
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Importing CFD Results
After 6 neutral files arederived from CFD cases
with convection data, eachmust be imported 20 times Once during speed-up and
slow-down of each stop
Batch script is used toautomate this process radtherm importCFD
settings.txt save model.tdf Before each import, the time
value in settings.txt isreplaced with the next value
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CFD Flow Results (50 kph)
Wheel-well vent has
little impact on brake Most incoming
flow from
under wheel-
well
Some from
underhood
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CFD Convection Results
96 kph 67 kph 50 kph 38 kph 19 kph 4 kph
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Thermal Results
Part average temperatures
0
50
100
150
200
250
300
350
75 175 275 375 475 575 675 775 875 975
Time (s)
Temperature(degC)
Rotor contact face (inboard) Rotor contact face (inboard) (standalone)
Rotor Rotor (standalone)
Pads Pads (standalone)
Caliper Caliper (standalone)
Rotor and padtemperaturessignificantlycooler with CFDconvection
Peak
temperaturessimilar
Calipertemperaturessomewhatwarmer
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Thermal Results
Temperature around circumference of rotor contact face
295
300
305
310
315
320
Elem 45600 Elem 46938 Elem 47322 Elem 45822 Elem 46991 Elem 46213 Elem 45341
Tem
perature(degC)
Standalone (RadTherm convection) Reference temp (CFD convection)
Small variation around rotor in bothcases
Convection approach has significantimpact on average
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Thermal Results (CFD convection)
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Comparison of Convection Methods
Part aver age temperatures
220
240
260
280
300
320
340
775 825 875 925 975
Time (s)
Temperature(degC)
Rotor contact face (inboard) (standalone)
Rotor contact face (inboard) (local CFD convection)
Rotor contact face (inboard) (part-averaged CFD convection)
Part-averaged CFD convection resultssimilar to local convection results
Variation around rotor face is slightlylarger with local CFD but relatively small
Temperature around circumference of rotor contact face
290
295
300
305
310
315
320
Elem
45600
Elem
46938
Elem
47322
Elem
45822
Elem
46991
Elem
46213
Elem
45341
Temperature(degC)
Standalone (RadTherm convection)
Reference temp (CFD convection)
Reference temp (part-averaged CFD convection)
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Comparison of Convection Methods:
Average convection flux Part convection fluxes very similar between localCFD and part-average CFD
Part convection flux
-16000
-14000
-12000
-10000
-8000
-6000
-4000
-2000
0
2000
75 275 475 675 875
Time (s)
Netconvectionflux(W/m^2)
Caliper (part-average CFD)
Caliper (local CFD)
Rotor (part-average CFD)
Rotor (local CFD)
Rotor contact face (part-average
CFD)
Rotor contact face (local CFD)
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Simulation performance comparison
CFD Runtime
(12 processors)
Thermal Runtime
(1 processor)
Total
Standalone
(RadTherm
convection)
0 2 hours 2 hours
Reference-
temperature CFD
convection
66 hours 2 hours 68 hours
(2.8 days)
Fully coupled
CFD convection
(estimated)
175 hours 14 hours 189 hours
(7.9 days)
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Coupling of results to FEA model
Temperature results from solid and shellelements can be exported to FEA tools for
structural analysis including heat loads /thermal stresses
Abaqus .odb file (RadTherm v11.0)
Nastran file with temperatures at vertices (otherFEA codes)
Mesh similar to or same as thermal model
could be used Mixed shell/solid hexa & tetra
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Validation of standalone thermal model
Rotor Face
Rotor Edge
Hub
Adapter
Bearing RaceAverage
Peak (C)
Average
Trough (C)
Sim 736 631
Test 716 648
Difference 20 -17
Sim 649 622
Test 653 642
Difference -4 -20
Final (C)
Sim 332
Test 290
Difference 42
Sim 138
Test 136
Difference -2
TC1&3
TC 2
TC4
TC5
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Conclusions
One-way reference-temperature based CFDcoupling shown to be
effective at capturingdetailed flow effects
Efficiency is improvedcompared to full coupling,
although lower thanstandalone model
Choice of convectionmethod and couplingapproach will be dictatedby needs of specificanalysis