This paper is submitted in partial fulfillment of the requirements for the title of Master of Science in Mechanical Engineering. The optimal angle position for the mirror arm is required resulting in the least aerodynamic forces and acoustics.
Introduction
Overview
The lower the value of the drag coefficient, the less energy is needed to move forward, since the vehicle has less air resistance that hinders its movement. However, cars must stay on the ground, and the more negative the lift coefficient, the more stable the vehicle.
Statement of the problem
As for the exhaust pipe, the acoustics can be handled by placing a muffler and not much optimization can be done. However, the aerodynamic effect of the position of the exhaust pipe on the whole vehicle must be studied and the optimum position for either the lowest drag and/or lift coefficient is concluded and discussed.
Research objectives
Exhaust noise reaches both passengers and nearby pedestrians and can cause hearing problems if used for long periods of time. Therefore, this thesis focuses mainly on the aerodynamic forces aspect of the tailpipe, where different positions are tested to measure the effect on drag and lift coefficient while the tailpipe emits gas at a certain mass flow rate.
Relevant literature
- Looks versus practicality
- Aeroacoustics
- Vehicle side mirror
- Airflow numerical models
- Mesh analysis
- Rear exhaust effect on aerodynamics
A bionic blade can reduce up to 3 dB of generated noise as mentioned by (Liu et al., 2021). The transformation of the structure due to the change in Reynolds numbers is examined (Mahato. et al., 2020).
Structure of the thesis
- Side mirror base part 1 geometry design
- Side mirror base part 2 geometry design
- Vehicle rear exhaust geometry design
For example, in terms of the side mirror base study, the design is made to ensure that the projected area remains the same and independent of the orientation of the base. Furthermore, in terms of the exhaust pipe modeling, the pipe is inserted into the DrivAer model rear section; therefore, a similar cut-off region between different positions is desired to ensure a fair comparison. To test mirror base placed at different angles from 0 to 90, a different design is needed which ensures that the same projected area is kept at different orientation of the mirror base.
This is achieved by a circle, where it has a constant radius and at each mirror base angle the dimensions of the base remain constant. The mirror base depends on the design of the rest of the vehicle, specifically the side of the car. This design isolates the mirror to measure its effect without the interference of the A-pillar.
At the corner of the mirror there will be a semi-circle to guide the base surface, with a radius of 0.05 m. An investigation into the effect of rear tailpipe height on aerodynamic forces is set up. Removing these components from the exhaust simulation therefore isolates the effect of the exhaust flow at different heights, allowing more concentration on the effect of the movement of the exhaust pipe on aerodynamic forces.
CFD analysis
- Material properties
- Governing equations and model assumptions
- Numerical setup
- Boundary conditions
The drag results are in the average of the other models, similar to the pressure coefficient results. In terms of computational effort, this model is the most expensive two-equation turbulence model. An explanation of the working model of the two gradient schemes is presented and the difference between the two is shown.
In Fluent software, flow variables such as temperature, pressure, and velocity are stored at the center of the cell. For example, if the temperature is the desired value, the temperature at the center of the face is taken and then multiplied by the area and the normal vector, then sum those contributions for all cell faces. Finally, the resulting value is divided by the cell volume, and this results in the centroid gradient, as shown in the following equation.
𝑇𝑇) 𝑝𝑝 (2.30) where the new cell center of gravity is calculated, the value of the former neighbor (𝑇𝑇𝑃𝑃) is added together with the multiplication of the spacing vector (𝑑𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑑𝑃𝑇 cents with the gradient 𝑇𝑇) 𝑝𝑝. First, an estimate of the volumetric amount of fuel consumed per 𝑚𝑚/𝑙𝑙]). The setup used in ANSYS is mentioned for each case with the governing values for the equation where applicable.
Model validation
In the next chapter, after the results are presented, an experimental validation is linked through the literature or shown in the work.
Results and Discussion
Geometry meshing and convergence
- Side mirror base meshing
- Side mirror base part 2 geometry meshing
- Different exhaust pipe positions mesh model
This section is divided into three subsections, each of which discusses the mesh refinement process for the respective case. The reason for choosing aluminum as the body material because it is the current trend in the automotive industry. The Mesh size should be chosen to give the best results without taking long calculation time.
For the mesh refinement process, ANSYS fluent first generates an automatic mesh, then the solver is run and the results are recorded. At each step of the mesh refinement process, the solver is run and data is recorded. In the setup phase, the mesh is switched to polyhedral, changing the mesh size as shown in Table 14.
BLS requires fine mesh around the object that has no-slip conditions, and the best mesh method that BLS represents is the puff layer. In the boundary layer, the no-slip condition creates a fluid boundary layer (𝛿𝛿99) which is the BLS thickness value from the wall, and this value is usually very small, so very small mesh elements are required to simulate this result. Where, k is the turbulent energy, U is the inlet velocity, I is the turbulent intensity, ′ is the mean square of the turbulent velocity fluctuations, l is the turbulent length scale, 𝜔𝜔 is the specific dissipation rate, is a turbulent model constant of 0.09, and 𝑣𝑣𝑤𝑤 is the turbulent viscosity.
CFD simulation and results
- Side mirror base part 1 results
- Side mirror base results and discussion
- Exhaust pipe at different positions results and
The location of the highest noise for the horizontal position is about 5.868E-03 meters closer to the mirror than the angular position. The aerodynamic lift and drag forces at each speed for the mirror and the side body of the vehicle are presented in Tables 19 and 20. The obtained results are shown in Figure 36 for the drag force and Figure 37 for the lift force with respect to the different velocity air intakes for the angular and horizontal base position .
Generated results are used to simulate airflow around the mirror for the velocity streamlines as well as the pressure contour. In Figure 46, the aerodynamic forces for the mirror part (mirror and mirror base) are obtained and plotted with respect to the mirror base angle. Simulation of various positions at the rear for the exhaust pipe to be placed is performed, and at each position the aerodynamic forces are reported.
The exhaust pipe is positioned at the rear of the DrivAer model, as shown in Figure 62 and previously in Figure 25. The longer vacuum is explained by position 4 having a central position all the way to the rear of the vehicle. In terms of experimental validation, the model used is the DrivAer model with the addition of the exhaust pipe at the rear.
Therefore, a numerical validation is valid by solving for the aerodynamic forces of the DrivAer model and comparing the values obtained with the available experimental data. However, in terms of lift coefficient, the value obtained at position 6 is the most negative value and therefore the optimum position for the exhaust pipe if the lowest lift coefficient is desired.
Conclusions and Future Work
Conclusions
In this thesis, the effect of changing a mirror base on aerodynamic forces and acoustics generated is studied. Further, after the mirror base conclusion, further studies are performed to determine the optimal position that results in least generated aeroacoustics. Finally, the tailpipe is simulated at various locations on the rear of the vehicle to determine the height to the ground that results in the least aerodynamic forces.
Furthermore, as long as the projected surface area is constant, the aerodynamic forces are not affected by changing the base orientation. As results show that if the bottom of the mirror is located 85±3.75 degrees from the horizontal axis, this results in the least noise emission compared to all other angles investigated. Furthermore, the aerodynamic force analysis shows almost constant drag but changing lift, as increasing the angle increases the lift until it reaches 90 degrees, where the force decreases significantly, but is still not as small as 0 degrees.
Although there is a change in lift force, the maximum change is around 15 newtons and this small increase does not affect the stability of the vehicle so much as it does the noise reduction effect. degree angle, we notice a significant difference in noise emission, which reaches up to 32 dB. The traditional position of the exhaust pipe at the bottom of the rear negatively affects the aerodynamics of the vehicle and reduces overall efficiency. However, a vertical height increase of 100 mm results in the lowest value of the drag coefficient, and a further increase of 25 mm results in the lowest value of the lift coefficient.
Future work
Numerical simulation of a generic car side mirror using large vortex simulation with polyhedral meshes. A numerical study of the flow past a generic wing mirror and its impact on sound generation. The state of the art of hybrid RANS/LES modeling for the simulation of turbulent flows.
Numerical simulation of aeroacoustic sound generated by generic bodies placed on a plate: Part I - Prediction of aeroacoustic sources. Influence of bed proximity on the three-dimensional characteristics of the wake of a sharp-edged bluff body. On the aerodynamics of a closed-wheel racing car: An assessment and proposal of add-on devices for a fourth, high-performance configuration of the DrivAer model.
Wavenumber-frequency analysis of the pressure fluctuations in the wall in the wake of a car side mirror. Effect of the proximity of walls on the flow over a cube and the implications for the sound emitted. Numerical investigation of the flow over beams with different aspect ratios and the emitted sound.
