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EuroBrake 2022: Thermal simulation session overview

The Thermal simulation session will take place on Tuesday May 17th and will be chaired by Joachim Noack of ZF Group and Yannick Desplanques of University of Lille.

Topics and speakers to include:

Robust disc and pad temperature estimation model – a machine learning / artificial intelligence approach

Anthony Ohazulike, Hitachi Astemo Brake Systems

Brake disc and pad temperatures are an important element in determining the clamping force needed to stop or hold a vehicle from moving or rolling off the hill. Accurate disc and pad temperature estimation is key to parking brake system. Further, real-time accurate estimation of the brake discs temperature is vital to finding the precise clamping force needed to stop an autonomous vehicle, which needs to decelerate or stop by itself.

Installing physical sensors is expensive for mass production units and impacts the ecological footprint. To deal with these, Brake Thermal Models (BTMs) which estimate the discs/pads temperature using physical models have been in development and used for several years. Due to very high dimensionality of the brake disc/pad heating and cooling phenomenon, these thermal models fail to achieve a satisfactory accuracy level. BTMs tend to accumulate errors over time, leading to an enlarged error gap during driving. Brakes are unarguably one of the most important safety systems of a vehicle, and hence there is need for a more accurate model.

Hitachi Astemo Brakes Systems in collaboration with Hitachi Europe Corporate R&D has developed a very robust and accurate virtual brake disc/pad temperature estimation model based on readily available vehicle data. We expanded the vehicle data using mathematical relations. Our solution is driven by data, and the model is driven by Machine Learning and Artificial Intelligence (AI). The model is trained on real driving data in real conditions, and the trained model is also tested on real data in varying real driving conditions. The robustness of the model comes from training and combining several machine learning / AI models such that the models support one another in making the final temperature estimation (the hybrid approach). The model error does not accumulate over time and easily recovers from erroneous previous prediction(s). The developed model by Hitachi requires minimal effort and results show that it outperforms a given BTM.

Methodology for conjugate heat transfer analysis of brake discs

Balaji Ravindranath, Wabtec

Brakes are critical safety devices of any equipment in operation and for its control. Disc Brakes (DB) are ubiquitous with the present-day transportation industry for the many advantages it brings in through design, performance, operation, reliability, serviceability, and manufacturing. Many solutions that prove to work for the brake systems in other domains prove inefficient and cost constrained for implementation in the rail industry. Starting from a requirement of better thermal performance in both directions of rotation, the rail DB equipment establishes its own set of challenges.

The thermal performance of the DB may have to be compromised by the design requirements of life and service, on account of the constraints, in applications such as the rail. The advent of new manufacturing methods, processes, assembly tools and materials, provide a path for improving the performance of the DB in question.

Industry is accustomed to Computational Fluid Dynamics (CFD) as an efficient alternative to physical tests, detailed calculations, and one-dimensional simplifications. The rail industry solicits such an established practice in CFD to achieve standard, high-fidelity results for the development of the DB system.

The design and performance parameters of system (Disc Brake System) like the calipers, actuators, braking fluid lines, layout and supporting equipment also require input from such simulations - for them to work in unison. It is an established requirement hitherto, that the methods, models, assumptions, and differences of such simulations be documented and debated, which defines the scope of this publication.

This effort is aimed at discussing a method of modelling a DB using Conjugate Heat Transfer (CHT) principles of CFD to accurately assess the heat transfer from the disc to the air that flows through. As solids and the fluid domains are coupled for heat transfer – the metal temperatures can be predicted more accurately. The models used for turbulence, the material properties, the geometric simplification in simulating the DB, help in predicting the local heat transfer coefficients, metal, and bulk fluid temperatures as primary output.

Cloud-based thermal simulation of brake disc cooling performance

Christian Taucher, ICON

To automate and make simulation set-up easy and simple ICON has set up a cloud-based platform. This platform allows the user to set up, run and manage multiple design iterations to find the optimum solution. For this an App has been developed for efficient thermal CFD simulation of the brake disc cooling flows including its thermal effects, especially the prediction of the heat transfer coefficients. This allows analysis of the thermal performance of different cooling designs in short timeframes and increases the accuracy of thermal prediction to optimize the design’s cooling potential at a minimum dimension of the disc.

Once a design has been defined, ICON’s novel BAF boundary conditions can be applied with advanced thermal models to integrate into a full vehicle model, to further refine the design in the final environment. This boundary condition allows to model the thermal distribution of the disc heat fluxes, while rotating this on a static CFD mesh. This helps to keep the run time of a simulation small while the accuracy remains high and thus makes it possible to run more design optimizations in the same timeframe. Moreover, using conjugate heat transfer not only the disc itself, can be assessed, but furthermore the domain can be expanded to predict the temperatures for example of the calliper, brake fluid, pistons or any heat shields surrounding the disc. The simulation model considers heat conduction, convection as well as radiation.

With the help of ICON’s advanced tools, the set-up of this complex thermal simulation can be made simple and enables simulating minutes of real time disc heat up and cool down cycles on a full vehicle simulation in a matter of hours. To demonstrate these methods on a non-confidential geometry, ICON has used the AeroSUV model provided by ECARA and modified it to include a realistic brake cooling system and front corner geometry.

Identification and quantitative modelling of thermal localization mechanisms associated with low frequency vibration

Maxime Cathelineau, University of Lille

The current rapid evolution of automotive vehicles is pushing brake disc designers to evolve their design for reasons of performance, weight reduction, etc. Low-frequency vibration issues (cold and hot judder) are re-emerging that need to be considered for multi-criteria design optimization.

Even if this problem has been solved in the past, the origin of these low frequency vibrations has always been controversial. Thus, even if it is known that these vibrations are caused by hot or cold residual deformations of the disc and that it is possible to attenuate them by acting on some key parameters, such as the out-of-plane deformation amplitude of the disc, the origin of these localizations is still debated, without predictive modelling.

The objective of this work is to consider the origin of these low frequency vibrations, i.e., the associated thermal localization mechanisms and to propose a quantitative approach to the resulting out-of-plane deformations.

Firstly, a classification of the hot spot mechanisms was carried out, showing that different physical explanations are possible, and depend on the brake geometry and the level of stresses applied. The study then focused on ventilated discs with a study of the influence of the disc design (2 geometries considered). The results show that this design is influential, which is a known result, but also that the complete design of the brake and in particular the choice of the brake pad. This is shown through thermomechanical modelling of the brake system, which provides quantitative deformation information. It is thus shown that the vibration arises from the thermal localizations associated with the out-of-plane deformations which are themselves dependent on the radial localizations (hot bands). This was made possible by considering the evolution of the disc/pad contact pressure during the transient braking situation.

The simulation results have been confronted to experiments with successful judder measurements to validate the modelling and to propose solutions for vibration mitigation. This modelling is integrated into the multi-criteria methodology for defining disc brakes. The interest of predictive modelling is also to identify the key influencing parameters.

Thermomechanical modelling of frictional contact localization as wear and emissions sources sites

Valentin Bruant, University of Lille

Experimental observations of brake and clutch systems reveal that thermal localisations within contact area are source of increased wear and friction induced emission, judder, plastic deformation, and cracks. Non-uniform contact areas come from wear and thermomechanical phenomenon such as hot bands and several type of hot spots on sliding bodies. The aim of the study is to predict macroscopic evolving thermal localisation process with stability and transient analysis.

The develop strategy consist first in classifying thermal localisation and associated phenomenon. As a classical way of thermoelastic instabilities, a stability numerical approach has been developed to predict occurrence of unstable mode of the system. Even if results of stability numerical approach are close to theorical approach for simple 2D geometry, it remains strong assumptions in such modelling: full contact, material elastic behaviour, etc…. Another approach is to consider transient effects with time-domain numerical model which can take account more complex mechanical behaviour and mechanisms.

Numerical simulation may predict thermal localisations, which depend on brake and clutch geometry, system induces contact pressure and type of thermomechanical loading: more or less straight hot bands, on surface or depth phenomenon, hot spots, etc…. Associated contact level of stress and strains may be so quantified and used for thermomechanical-tribological coupling studies in contact areas.

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