F1 Fuel Tank Surging; Model Validation

Transcription

1 F1 Fuel Tank Surging; Model Validation Luca Bottazzi and Giorgio Rossetti Ferrari F1 team, Maranello, Italy SYNOPSIS A Formula One (F1) car can carry more than 80 kg of fuel in its tank. This has a big influence on the car s behaviour, not only due to the additional weight, but also due to the fuel surging inside the tank. A vehicle simulator and a CDF model were used to asset the impact of fuel surging on the car s performance. The accuracy of the CFD model was verified using an experimental tank, installed in a road car. A 6 axis balance was used to mount the tank and measure the forces. The forces and momentums measured, on the track, were compared with the results of the CFD calculation. The results of the test showed that the CFD calculation can predict accurately the behaviour of the fuel inside a F1 car fuel tank, and could be used as a useful optimization tool in the design process of the tank itself. 1. INTRODUCTION A Formula One car is subject to large accelerations that cause rapid movements of the fuel. The dynamic energy of the fluid is transmitted by the tank to the car, affecting its performance. The tank is divided into many compartments; if the number of the compartments is increased, the surging is reduced, but this leads to a heavy tank and a difficult refuelling operation. To optimize the tank design, a time dependent volume of fluid (VOF) computational fluid dynamic (CFD) surging model was developed together with ANSYS, Inc. The accuracy, of the model, was verified by undertaking a test on a race prepared Ferrari 360 Modena. The fuel was substituted with water for safety reasons. 2. TANK COMPARTMENT CFD MODEL The shape and the dimensions of the experimental tank were chosen to be representative of the simplest compartment of the F1 car tank, and to reduce the complexity of the experimental equipment. It has the shape of a parallelepiped, and it was possible to mesh the model with 96,000 hex cells. Because of the relatively low speed of the fluid, the motion is considered to be laminar. To reproduce the effects of the car s acceleration, a source term is added in the equilibrium equation as follows: ( v) ( vv) p v v t T 2 v I g. a 3 Where a is the acceleration of the car. The acceleration time history, measured on the car, is recorded in a data file and passed to the solver by a user defined function developed by ANSYS, Inc. The time step used for the simulation is 0.01 sec., with a convergence threshold of 10-3 and a maximum number of iterations per time step of

2 The calculation outputs, processed by the UDF are: - Components of the shear forces on each face - Components of the pressure forces on each face - Coordinates of the pressure center on each face - Average pressure on each face - VOF primary and secondary phases in selected points - Total kinetic energy The shear forces are at least 1000 times smaller than the pressure ones, so they can be neglected. The resultant forces and momentums are then calculated using a visual basic macro defined in an excel sheet. 3. EXPERIMENTAL EQUIPMENT The experimental equipment is made up of five main parts (see figure 1): the tank itself (5) is made from the same material used for the formula 1 car. Since it s not rigid, is contained in an external structure (4) that is directly connected with a six axis balance (3), normally used in the wind tunnel to measure the aerodynamic loads on the car. The balance is then mounted to a plate (2) that is designed to be connected directly with the chassis of the car (1), replacing the passenger s seat (see figure 2). Figure 1: experimental equipment Many pressure sensors were fitted on the internal faces of the tank, in a way that the fluid movement was not disturbed, to verify the values calculated by the CFD model. The data was acquired with the electronic control unit used on the formula one car, and then downloaded at the end of the test. 276

3 Figure 2: car installation of the experimental tank 3-1. CALIBRATION OF THE BALANCE AND CHARACTERIZATION OF THE EQUIPMENT The core of the balance, is a steel structure, subject to deformation under load; the deformation is measured using extensometers and, using a calibration matrix, it is possible to obtain the loads (three forces and three momentums). This means that the measured load is influenced by the weight of the steel structure. In the wind tunnel application, that is the standard one for the balance, this problem can be easily overcome, because once the balance is fitted in position, the balance s weight is just an offset. In the application described in this paper, the solution is not that simple. Because the direction and the magnitude of the accelerations are not constant, it is necessary to define a dynamic offset that can be calculated once the mass and the centre of gravity of the steel structure are known. First, the balance was positioned with the Z axis in vertical position, as fitted on the car, but without the tank and the support structure and it was reset to zero. Second, it was positioned with the X axis in vertical position. This allowed the mass and the centre of gravity of the steel structure to be found. To reduce the error, the procedure was repeated in four different positions, rotating the balance 90 with respect to the Z axis, and then the results averaged (see table 1). Position Mass [kg] X G [mm] Y G [mm] Z G [mm] X up Y up X down Y down Avarage Table 1: mass and centre of gravity of the scale 277

4 Once the balance was characterized, it was possible to measure the mass ( kg) and the centre of gravity of the equipment (support structure, tank, sensors, see table 2). The difference between the theoretical values and the measured ones are due to the sensors and their cables not being included in the CAD model. CAD MEASURED X G [mm] Y G [mm] 0 0 Z G [mm] Table 2: centre of gravity of the equipment 4. RESULTS Three different manoeuvres were carried out on the Fiorano test track: - High speed braking, from 200 kph to the complete stop of the car - Start/stop, first gear acceleration until the rev limiter activated, followed by heavy braking - Tyre warm-up, a series of sudden direction changes at constant speed The tank was filled with six kg of water. The free surface of the fluid was assumed to be horizontal in the initial condition of the model. This of course is not completely true, but the manoeuvres were realized so as to be as close as possible to this ideal condition. The comparison between the calculation and the experimental results is presented in the following paragraphs HIGH SPEED BRAKING HIGH SPEED BRAKING 1 ACCELERATIONS [g] ax ay az Time [s] Figure 3: high speed braking, accelerations time history 278

5 Figure 4: high speed braking, resultant forces Figure 5: high speed braking, resultant momentums 279

6 4-2. START/STOP Figure 6: start-stop, accelerations time history Figure 7: start-stop, resultant forces 280

7 Figure 8: start-stop, resultant momentums 4-3. TYRE WARM-UP Figure 9: tyre heat-up, accelerations time history 281

8 Figure 10: tyre heat-up, resultant forces Figure 11: tyre heat-up, resultant momentums 282

9 4-4. AN APPLICATION ON THE F1 CAR: THE ASCARI CHICANE IN MONZA The forces and momentums generated by the fuel surging can be seen as external loads that affect the handling of the car. The effects of these loads are: - Change in longitudinal load transfer (Fx and My) - Change in lateral load transfer (Fy and Mx) - Change in vertical load (Fz) - Different amount of longitudinal and lateral force needed to be generated by the tyres for the same values of longitudinal and lateral acceleration - Change in diagonal weight transfer (Mz) - Increase in vertical load variation The first four of these mentioned effects result in different tyre slip angles, whilst the load variation causes a general reduction in grip. The strongest of these effects are the longitudinal and lateral forces generated by the tyres and the vertical force. This means that for the same acceleration, the tyres will need more longitudinal and lateral slip or, in other words, for a given potential grip of the tyres, the limit of the car performance will be lower. Note that the loads due to the fuel surging are not synchronized with the car s acceleration because of the time needed by the fuel to move from one side of the tank to the other. This delay on the car s behaviour, as well as the absolute value of the load, is not negligible. For example, the load generated by 10 kg of fuel in a manoeuvre that reproduces the Ascari chicane is reported in figure 12. Figure 12: loads due to the surging in the Ascari chicane 283

10 5. CONCLUSIONS The experimental test has shown that the VOF CFD model can reproduce, with a very good accuracy, the effects of the fuel surging, and can be considered a useful tool in the design of a formula one car tank. 284