In a vehicle, the drag force is generally caused by the difference in pressure between the front and the rear of the vehicle. This pressure difference causes more than 70% of the total drag force to be compression drag force and the remainder of the drag force is due to viscous drag. To understand the effect of adding aerodynamics devices such as spoiler, diffuser, and bed bump to reduce drag and lift forces, we will investigate the flow field and local pressure distribution.
4 − 1 Validation
The flow around "Ahmed's body" has been studied. This geometry and its details are illustrated in Fig. 5. The results of the present numerical simulation are compared with the experimental data of Ahmed et al . Figure 6 depicts the drag coefficient at various angles of the back of the body. As shown in this figure, the results of the numerical solutions are well-matched with the available experimental data.
4 − 2 Effect of the diffuser and its angle
In this section, we will study the effect of adding a diffuser and changing the diffuser angle from 0 to 50 degrees; in this case, the diffuser length is fixed at 230 mm; moreover, examining the length of the diffuser relative to the length of the entire vehicle is considered and in this case, the diffuser angle is assumed to be 10°. Diffuser length is considered from 129 mm to 629 mm with a 100 mm increment. all of the cases in this section are 6 different cases. The ratio of this diffuser length to the total length of the vehicle is 0.025 to 0.125 with a 0.02 increment. The diffuser angle is measured between the diffuser and the horizon line. The simulated pickup is illustrated in Fig. 7.
From Fig, 8 as the diffuser angle increases to 30 degrees, the drag coefficient increases to over 0.45 but then it falls from 40 degrees to almost 0.44, and then with an increase in diffuser angle to 50 degrees, the drag coefficient raise to approximant 0.45 again; on the other hand, as the diffuser length increases to 329 mm with the same trend the amount of drag raise to over 0.45, which is the highest drag amount. However, in the opposite trend, the drag amount falls to less than 0.45 with an increase in diffuser length up to 629 mm.
The reason for alter in drag amount when the diffuser angle increase can be explained in Fig. 9. This figure shows, that with increasing the diffuser angle, the pressure difference between the front of the car and its back increases; consequently, it causes to increase in the drag coefficient. Except for pickup truck cases with 40 degrees the diffuser angle has fluctuated pattern shown in Fig. 9. This is because while for all cases with increased spoiler degree, these difference increases, this case has the opposite influence. Therefore, the drag amount decreased and this caused a fluctuating pattern.
Figure 10 shows the rise in the diffuser length, the difference between front pressure and pressure at the end of the pickup truck with the same trend increased. As a result of this, the drag coefficient rises. Although, this pattern changed after the 329 mm diffuser length. This means that the pressure difference decrease with an increase in diffuser length, therefore, the drag coefficient drops with length increase.
As you can see from Fig. 11 as the diffuser angle is increased to 30 degrees, the negative lift coefficient increases, and the stability of the vehicle increases but as the angle increases, the negative lift coefficient decreases. On the other hand, as the diffuser length increases, the negative lift coefficient increases as the separation zone expands, and the stability of the vehicle increases.
Figure 12 illustrates that the diffuser attached to the rear end of the car raises the upper surface local air pressure which effectively increases the downward force that is known as negative lift; moreover, the rear diffuser not only changes the pressure on the top of the vehicle, which causes a pressure increase but the pressure on the underside is decreased. Therefore, with an increase in diffuser angle to 30 degrees, upper surface air pressure increases while at 40 degrees this amount decreases. This means a more negative lift in the 30-degree case than the 40-degree case. In addition, with an increase in diffuser length, upper surface air pressure increases this case raise in the negative lift.
4 − 3 Spoiler length and Double spoiler
To investigate the effect of the spoiler’s length, a zero-degree spoiler is placed behind the pickup cab. We increase the spoiler length from 200 mm to 600 mm with a difference of 100 mm; moreover, to study the simultaneous impact of two spoilers, insert a zero-degree ceiling spoiler at the back end of the pickup cabin and another spoiler at the downstream end of the pickup bed. Increase the spoiler angle at the end of the pickup room from negative 10 degrees to positive 40 degrees with a 10-degree angle difference. The modeled pickup truck with spoiler and double spoiler are depicted in Fig. 13.
it can be seen from Fig. 14 as the spoiler angle increases to 30 degrees, the drag coefficient increases to over 0.5 but then it falls in 40 degrees to almost 0.48. On the other hand, as the spoiler length increases to 500 mm with the opposite trend the amount of drag decrease to almost 0.42, which is the lowest drag amount. However in the opposite trend, the drag amount rais to over 0.42 with the increase in spoiler length up to 600 mm.
Figure 15 illustrated, that with an increase in the spoiler length, the pressure difference between the front of the pickup truck and the end decrease, this is less than pressure diffraction in the case without a spoiler. As a result, this causes a dropping pattern in drag amount. However, with an increase in spoiler degree, the pressure difference increases even more than the case without a spoiler. Therefore, the drag amount has the same increasing trend
As you can see from Fig. 16 by adding a spoiler and increasing its length behind the cabin roof, the negative lift coefficient decreases. On the other hand, Fig. 14 shows that by adding a spoiler behind the tailgate and changing its angle in the presence of a ceiling spoiler whose length and angle are constant, the negative lift coefficient increases with increasing angle.
Figure 17 illustrates that the spoiler attached to the car raises the upper surface local air pressure which effectively increases the downward force known as negative lift; moreover, using the rear spoiler and another spoiler behind the cabin roof together not only changes the pressure on the top of the vehicle, which causes a pressure increase; but the pressure on the underside is decreased. Therefore, an increase in rear spoiler angle causes to increase in the upper surface air pressure. This means more negative lift to be created. In addition, with an increase in spoiler length, upper surface air pressure increases and performs to raise in the negative lift.
4–4 Tailgate bumps
An aerodynamic device has been added to the top surface of the tailgate. This is a bump on the tailgate. To investigate the effect of this device on the drag coefficient and lift coefficient, we have increased its height from 65 mm to 225 mm by a ratio of 50 mm, and the results are compared with the no-bump model. The pickup truck model and tailgate bumps are depicted in Fig. 18.
Figure 19 shows the drag coefficient graph for the pickup, with no bumps and with bumps as you see up to 115 mm height, the addition of bumps reduces drag coefficient while with an increase in bumps height drag raise. Moreover, the case with 215 height of the bumps has more drag than the case without a bumper. On the other hand, Fig. 16 shows that adding bumps behind the tailgate and changing its height causes the negative lift coefficient to decrease.
As we can see from Fig. 20, with an increase in spoiler height, the pressure difference between the front of the pickup truck and the end increase which at the beginning is less than pressure diffraction in the case without tailgate bumps. As a result, this causes an increasing pattern in drag amount. In addition, with an increase in tailgate bumps height to 215, the pressure diffraction increased more than tailgate bumps less case. Therefore, this has more drag amount than a simple case.
Figure 20 illustrates that attaching tailgate bumps to a car decrease the upper surface local air pressure which effectively decreases the downward force known as negative lift; moreover, using tailgate bumps not only changes the pressure on the top of the vehicle, where it causes a pressure decrease but the pressure on the underside is increased. Therefore, with an increase in tailgate bumps height, cases upper surface air pressure decrease. This means less negative lift to be created.