Enhanced driving comfort thanks to reduced tank noise

Acoustics in the vehicle interior have undergone a steady improvement in recent years: occupants are experiencing less and less disturbance from engine noise, airflow and rolling tyres. But when roars and whistles are reduced in one place, other noises become more noticeable. One of these is the tank contents sloshing back and forth. In the latest vehicles, you can occasionally hear the petrol splashing against the wall of the tank when starting off and braking. These sounds are significantly more noticeable to passengers in vehicles with start/stop functions or hybrid drives, due to the lack of engine noise. TECOSIM has studied this issue intensively and in a CIM research project (Centralized Innovation program, Mid-sized), it has developed an approach for optimising tank structures and built-in obstacles.

Reduced road noise makes for a more relaxing ride.

The basic starting point for the TECOSIM project team was an analysis of the acoustic properties of a representative tank geometry. Here, the specialists studied a variety of parameters, including the effects of installing and removing obstacles, known as 'baffle plates'. These are intended to minimise the kinetic energy of the fuel in the tank. In addition to this, they also analysed the effect of diesel and petrol of different densities and viscosities on the fluid-dynamic behaviour and simulated tank sloshing at three different fill levels (100%, 80% and 50%-full). In order to keep the scope of the research project manageable, a scenario was applied with moderate driving behaviour at a precisely defined speed, brake deceleration strength and measurement time length after stoppage.

At the beginning of the project, the engineers concentrated on the Computational Fluid Dynamics (CFD). However, they soon realised that this would not be adequate for an acoustic evaluation. It could in fact identify which baffle plate variants effectively reduced sloshing. Yet this did not permit any conclusions regarding the acoustic behaviour of the tank. This is because they also found that heavy sloshing does not necessarily result in an increase in noise emissions. On the contrary, the structure of the tank wall itself appeared to have a greater effect on noise emissions than initially assumed.

The TECOSIM engineers developed a multi-step simulation process for acoustic analysis.

Three-pronged approach to acoustic evaluation

The team also expanded the simulation process.

First off, the aforementioned time-dependent fluid simulation (CFD) was conducted. This analysed the local surface pressure distribution and the fluid dynamics in the tank and calculated the pressure and pressure gradients on the tank walls. The results showed the velocity, pressure and fluid distributions in the tank. The data were prepared for the subsequent FEM calculation using a fast Fourier transform.

Next, a frequency-dependent FEM calculation was performed: Here, the engineers analysed the deformations of the tank surfaces caused by the fluid pressure fluctuations.

The final step was a frequency-dependent acoustic analysis of the airborne noise (Sound Pressure Level (SPL)). This was conducted at a predefined distance from the tank at selected points.

The switch from a simulation over time to a simulation over frequencies has several benefits: The latter can be conducted separately for each frequency. Essentially, this enables selection of specific frequency ranges in the audible range or individual resonant frequencies. In addition to this, simulation by frequencies involves less expenditures compared to testing over time.

Identify interrelations

The various steps in the expanded simulation process identified important interrelations that influenced the subsequent steps. The CFD simulation provided information on the fluid forces at work. The subsequent structure simulation showed the deformation of the tank surface due to pressure in the FE model. Based on this, they were able to use the acoustic calculation to determine the airborne noise emissions, and thus also the perceptible noise. The TECOSIM specialists were able to use these results to install an obstacle with optimised geometry into the model tank. Installation of the baffle plate significantly reduced the fluid forces, and thus also tank sloshing, for all three of the fill levels studied. This had a demonstrable and positive effect on the acoustics.

A more effective and versatile approach to a solution

With the expansion of the simulation process, the TECOSIM team hit on a methodology for arriving at accurate conclusions on a tank's acoustic behaviour. The process developed is transferable to real tank geometries. In this way, the engineers managed not only to reduce noise emissions, but also to optimise the number and precise installation positions of the baffle plates, as installation of these obstacles is rather cost-intensive. For OEMs and manufacturers of plastic tanks, this method offers a clear advantage in the early stage of product development. Reducing noise emissions enhances comfort for vehicle passengers.

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