Simulation Helps Reduce Sloshing in Vehicles
Phillip Oberdorfer December 9, 2015
When a vehicle rapidly accelerates or brakes, the liquid within its tanks moves back and forth, producing dynamic forces and splashing — a process known as sloshing. Using COMSOL Multiphysics, a team of simulation engineers at IAV found that these forces could be efficiently reduced through the numerical optimization of the internal wave breakers, without impeding the fluid supply. Today, we’ll explore this force reduction approach and the principles behind it with our own tank geometry example.
Designing Tranquilized Fuel Tanks with Simulation Support
Over the last few decades, the automotive industry has come to rely more and more on simulation-based techniques for fostering design advancements. These analyses enable designers and engineers to study a wide range of components within vehicles, from the smallest screw to the engine system as a whole, while saving on both time and costs.
With the growing complexity of vehicles and reduced development time, the demand for simulation-based design has grown particularly high. While many variables are important points of focus within this segment of the automotive industry, one of the most requested details concerning noise, vibration, and harshness (NVH) optimization is reducing sloshing forces in car tanks.
Sloshing is a term that refers to liquid moving inside of another object. Modern vehicles contain a whole string of fluids in tanks and, when a vehicle accelerates (or brakes) rapidly, inertia forces cause movement in all of them. Such movement of liquids produces transient longitudinal and transversal forces that disturb vehicle dynamics and cause splashing, which can in turn generate unwanted noise. The latter problem grows even more important as other sources of noise such as the vehicle’s combustion engine or tires become quieter.
Rather than building and testing a variety of prototypes, simulation engineers at IAV, one of the world’s leading engineering companies in the automotive industry, created a numerical model and used it to develop and design a solution to this problem. The purpose of their study was to find out how internal walls in the tank can best reduce the sloshing of fuel. This process involved solving a two-phase flow problem in COMSOL Multiphysics. The design that IAV ultimately developed with this method has been rather successful and is easily applicable to a variety of sloshing problems in automotive tank systems.
Using an arbitrary tank geometry that we designed, let’s take a closer look at the principles behind their force reduction approach.
Reducing Sloshing in Vehicles: An Analysis of a Two-Phase Flow Problem
The situation presented here starts after the vehicle’s braking process, when the free fluid surface is inclined and about to return to a horizontal position because of gravity. It can take several seconds for the fluid inside of the tank to settle down.
The initial distribution of liquid and gas in the tank.
In the series of animations below, we can see the impact that the use of internal walls has on the movement of the fluid in the tank. When no wall is used, the fluid moves noticeably more freely and takes longer to settle. Partially equipped approaches (one uses only the boxy structure with gaps or one uses only the two vertical double walls) somewhat help to calm down the movement of the fluid. A fully equipped approach that utilizes both of these internal components is the most effective at damping the wavy movement of the fluid. We can conclude that these modifications to the tank’s internal geometry have a large influence on the fluid system.
Animations illustrating the impact of internal walls on sloshing in a fluid tank design.
The graph shown below compares the fluid’s kinetic energy over time. Without the use of the walls and the boxy structure, the initial peak of kinetic energy is significantly higher with damped, but lasting, oscillations due to the sloshing liquid. The peak and the oscillations are damped most effectively when all of the internal wall components are included in the tank’s geometry. Through this analysis of the liquid’s kinetic energy, we can quantify and assess the impact of modifications to the tank’s geometry at an early stage in the design process.
Kinetic energy of the sloshing fluid.
A Single Model with Applications in Many Different Vehicle Types
The majority of fluid tanks in vehicles are designed to fit into free spaces between other important components. This can often result in complex tank geometries that vary greatly depending on the vehicle type in which they are used.
Here, we have presented a model of a sloshing tank that offers a solution for all conceivable designs. Designers and engineers simply need to modify the tank geometry to fit their needs and recompute the solution, allowing them to easily compare different design configurations and identify the one that is optimal.
If you have any questions pertaining to your own simulations, please contact your COMSOL support team.