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DYNAMICS OF ACOUSTICS

DYNAMICS OF ACOUSTICS

TA News Bureau

An expert in mathematical modelling and high-performance computing, Dr Abhilash J Chandy’s research is helping tyre engineers design safer and more efficient tyres having lower tyre/pavement noise levels. Using computational fluid dynamics and chemical kinetics solution algorithms, he has developed models to reduce tyre noise. A winner of the Firestone Research Award, Dr Chandy is a tenure-track Assistant Professor in the department of Mechanical Engineering of University of Akron, Ohio. He has authored several research papers, including those on industrially-relevant topics such as manufacturing and tyre acoustics. In this interview he speaks on issues relating to tyre noise levels

One of the major challenges facing tyre engineers is to design products that reduce tyre/pavement noise levels. With tougher tyre labelling and noise reduction measures that are being enforced by governments worldwide, tyre manufacturers are compelled to produce tyres that conform to these regulations.
It is here that the research work of Abhilash J Chandy’s is assuming great importance. His work at University of Akron on mathematical modelling and high-performance computing is contributing to a wealth of research on tyre acoustics.
The winner of the Firestone Research award, Dr Chandy – who is a tenure-track Assistant Professor in the department of Mechanical Engineering – in an interview to Tyre Asia, explains his current research on reducing tyre noise levels.
He says although the US Department of Transportation and National Highway Traffic Safety Administration (NHTSA) is primarily responsible for developing standards for vehicles and their components, including those for vehicle noise, it has not developed anything like some of the tough tyre noise standards mandated in Europe.
“In Europe for instance, the new European Union (EU) tyre label, which is mandatory for all new tyres sold in Europe after 2012, helps in comparing tyre standards in terms of fuel efficiency, wet grip and noise,” he points out.
“The EU rating measures external noise emissions for tyres in decibels, and classifies them as 1 black wave (quiet), 2 black waves (moderately noisy) and 3 black waves (noisy).”
The NHTSA has proposed new tyre labels, which are similar to those now being used in Europe with regard to fuel efficiency, wet traction and tread durability, but not noise, at least so far.
“Nevertheless, due to the dearth of published literature in tyre acoustics, particularly airborne-related, the US tyre industry only stands to benefit from any research on this topic,” he explains emphasising the importance of his research field.
Commenting on the major measurement techniques that are being used to characterise tyre/road noise radiations, he said that there are multiple methods that are used to measure this. The statistical pass-by (SPB) method measures the speed and sound pressure levels of a number of vehicles and obtains an average noise level that is characteristic of the surface the vehicles were on.
The close proximity (CPX) measurement method on the other hand measures the tyre/road noise in close proximity to the interface of tread and road, typically via a setup with microphone mounted on a trailer attached to the vehicle.
Then there is the on-board sound intensity (OBSI) method, which measures tyre-pavement noise at the source using microphones in a sound intensity probe configuration mounted to the outside of a vehicle, near the tyre-pavement interface.

Traffic noise

Dr Chandy thinks that in order to reduce traffic noise, one will definitely need to take into consideration all the different factors associated with tyre/road interaction noise, which depends on a variety of aspects such as vehicle speed, tyre tread size and material, and pavement.
“There are several complexities involved with studying these phenomena as whole. In addition, there have not been very many investigations on airborne tyre noise in the past and hence, there is really not a whole lot of literature on this topic.”
Therefore, it is useful and extremely important to isolate each mechanism and gain an understanding of it, so that it can be controlled within limits, and then moving forward to investigating traffic noise reduction holistically. The computational fluid dynamics (CFD) tools that his research team has developed are in a good position to do exactly that.
“As far as I know, there is no single parameter that can quantify the tyre dynamic behaviour in terms of reducing tyre/road noise while not affecting rolling resistance,” he says.
However, several different parameters have to be taken into consideration, while coming up with new groove designs with the intent of reducing tyre noise. For instance, one has to keep in mind that the rolling resistance and hydroplaning characteristics are not affected significantly for new designs.
On his current research in computational fluid dynamic simulations that can be employed by tyre designers, Dr Chandy says it is applied to develop numerical tools capable of predicting air-pumping noise generation, where the rotation of the tyre is represented using mesh motion and deformation techniques.
“Deformations of the slot within the tyre are either prescribed by empirical-based custom functions or obtained in a more sophisticated manner from structural solvers. Near-field and far-field acoustic characteristics are predicted using fluid dynamic equations and acoustic models.”
The use of various spectral analysis tools have shown that CFD is capable of predicting the high frequency air-pumping noise while also predicting other air-related mechanisms such as pipe resonance, Helmholtz resonance, and rotational turbulence.
“Our goal in this area of research is to improve the understanding of the various air-related noise generation mechanisms, so that one can use numerical models in order to predict tyre acoustics economically and effectively, and possibly design ‘quieter’ tyres in the future.
The issue of tyre noise is gaining importance in the context of increasing market penetration of electric vehicles and body light-weighting initiatives.
Tyre/pavement noise represents 75 to 90 per cent of the total noise generated by passenger vehicles, and it could be even larger for larger vehicles.
Reductions in tyre/pavement noise levels could reduce the overall traffic noise levels substantially, thereby bringing down the cost for expensive noise-attenuating measures in traffic. The contribution of tyre/road noise increases with lightweight vehicles or electric vehicles, which have very little to no engine sound.
“So yes, absolutely, the importance of tyre dynamics, particularly in terms of tyre noise will be even greater in electric vehicles for instance, which I believe are going to be the norm of the automotive future. This is exactly why research, both experimental and computational, on this topic can prove to be valuable, because unless you understand the problem substantively, you cannot control it,” Dr Chandy emphasised.

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