GCM
Figure 8 below shows the graph for the drag coefficient of the GCM, with convergence after 1232 iterations. The drag coefficient value stabilised to 0.403.
Compared to the experimental value for CD of 0.397 found by Satran (2004), the CFD simulation result for CD was 1.244% higher. This discrepancy was due to the limitation of element quantities of the student version of Ansys Fluent as well as the computational limits of the solving computer. Examining the pressure distribution over the GCM, as shown in Fig. 9, high pressure zones can be seen at the nose, the intersection between the hood and windscreen, as well as in the frontal area of the trailer. Another high pressure zone can be seen in the geometry of the cab door recess, and the step created for easier driver entry into the cabin. Figure 10 shows the flow vorticity magnitude over the GCM, problem areas can be seen in the top of the tractor hood, the trailing edge of the tractor roof, the trailing edge of the tractor floor, and the leading edge of the trailer roof.
The GCM geometry creates highly turbulent flow on the front face of the trailer. The bluntness of the nose on the GCM as well as on modern trucks is a consequence of legislation on overall length as well having to accommodate the engine and cooling package/s of a modern diesel ICE into the design. It is evident that there are areas of the GCM which contribute significantly to the overall aerodynamic drag on the vehicle. The most prominent of these areas are the tractor blunt nose, the transition from the hood to the cabin windscreen, the tractor-trailer gap, as well as the transition of the floor of the tractor into the rear face of the tractor.
Concept Two – Revision A
The required geometry changes mentioned above are shown in Fig. 13. For the side radius, a non-linear radius was created. A large radius of 500 mm was added at the lower area, while a tighter 80 mm radius was added at the top. The reason for a tighter radius at the top was to avoid an abrupt transition of airflow from the tractor to the top front edge of the trailer. A linear 200 mm radius was added to the tractor floor to reduce the adverse aerodynamic effects.
Figure 14 below shows a comparison of the pressure coefficient between Concept Two (left) and Concept Two - Revision A (right). It is clear that the radius changes reduced the sharp pressure drop on the side of the vehicle – the strong blue region (left) has been dispersed evenly across the geometry (right).
Figure 15 shows the dramatic reduction in vorticity at the nose of the truck from Concept Two baseline (left) to Concept Two Revision A (right).
Concept Two – Revision D
The final design feature examined was a tapered trailer roof. This was to simulate the aerofoil profile as much as possible. A 1° taper of the roofline, measured from the front edge of the trailer roof was added. This modification can be seen in Fig. 21 below, highlighted in orange. This was the only concept which negatively impacts the practicality of the tractor-trailer as the total load volume has decreased.
The results of CD and the change reference to the GCM for all concepts are shown in Table 1.
Table 1
Comparison of CD values for all CFD simulated models.
3D CAD
|
Model name
|
CD
|
% change in CD relative to GCM
|
|
GCM
|
0.403
|
0.00
|
|
Concept One
|
0.680
|
+ 40.70
|
|
Concept Two
|
0.354
|
-12.16
|
|
Concept Two – Revision A
|
0.331
|
-17.87
|
|
Concept Two – Revision B
|
0.275
|
-31.76
|
|
Concept Two – Revision C
|
0.260
|
-35.48
|
|
Concept Two – Revision D
|
0.251
|
-37.72
|
A discussion of each simulation result is given below.
GCM – There are obvious areas in the GCM which contribute to increased drag, namely the very blunt nose, the abrupt angle change that is present in the transition of the tractor hood to the windscreen, the driver entry step, the cabin doors and the tractor-trailer gap. These areas also have higher flow vorticity magnitudes, and lower flow velocity magnitudes which result in increased aerodynamic drag.
Concept One – While the tractor for this concept represents the smallest frontal area exposed to airflow of all the models simulated, the frontal area of the tractor-trailer combination is approximately the same as all other models tested. As a result, the front face of the trailer presents a large perpendicular obstacle to the incoming airflow greatly increasing drag. The generated wake for this model is also greater than that of the GCM, due to the flow separation that occurs at the trailer front leading edges disrupting the flow along the length of the trailer.
Concept Two –The smooth transition from nose to the top of the trailer leading edge resulted in the fewest flow separation points compared to the two models discussed above. The transition from nose to floor created a flow separation point as well as a source for increased vorticity, resulting in disrupted flow along the floor of the truck. While the blended geometry that transitions from upper surface of the tractor to the left and right sides of the tractor was added to reduce the possibility of flow separation, this area also resulted in creating a pressure drop on the blended surface.
Concept Two – Revision A – The transition area was modified with an increased radius transition, as well as adding a curved transition to the floor. The vorticity magnitude at the nose lower surface was reduced by approximately 40% when compared to Concept Two. The addition of the curved floor transition also increased the velocity magnitude under the floor of the tractor by approximately 3%.
Concept Two – Revision B – Both the wake size and intensity was reduced in the concept when compared to Concept Two – Revision A. In both Concept Two – Revision A and Concept Two – Revision B, there are two vortex flows that occur at the rear of the trailer, one from the top surface of the trailer and one from the lower surface of the trailer. The vortex generated at the top is larger in magnitude than that of the one created at the bottom surface, due to the trailer wheels and axles disrupting the airflow and removing energy from the airflow. With the addition of the boat-tail, the upper vortex turbulent intensity was reduced by approximately 38% when compared to Concept Two – Revision A.
Concept Two – Revision C – Trailer skirts as well as a smoother tractor rear transition were added to this concept. The most significant aspect for adding the trailer skirts was limiting the amount of airflow flowing from the tractor sides to the underside of the trailer, as well as limiting the amount airflow from the sides of the trailer itself flowing to the underside of the trailer. The skirts resulted in lowering the turbulent intensity caused by the trailer leading wheel by approximately 65% compared to the absence of skirts. The overall turbulent intensity under the trailer was reduced with the addition of the skirts.
Concept Two – Revision D – The tapered roofline more closely approximates the profile of an aerofoil, which should theoretically provide a reduction in aerodynamic drag. The addition of a taper to the trailer roofline also resulted in reducing the turbulent intensity of the flow by approximately 5% compared to Concept Two – Revision C. The flow vorticity at the top rear of the trailer was reduced by approximately 32% when compared to that of Concept Two – Revision C.
Figure 22 and 23 show the pressure coefficient over the longitudinal centreline section of the GCM and Concept Two – Revision D, respectively. Inspecting the curves for the top surface of each model for the range 0m to 6m, it is clear how much smoother the curve is for Concept Two – Revision D. This is a reflection of fewer flow detachment areas, and lower turbulence for Concept Two –Revision D. Peak high and peak low values are also lower for Concept Two – Revision D. The curves for the bottom surface show similar trends between models, with higher peak values for the Concept Two – Revision D trailer. This is due to the influence of a rolling road, road boundary layer, the sealing effect of the skirts, and higher flow velocity under the Concept Two – Revision D trailer.
Figure 24 and 25 show the airflow turbulent intensity over Concept Two – Revision D and the GCM respectively. It is clear that the size, and particularly the turbulent intensity is greatly reduced in Concept Two – Revision D compared to the GCM. This difference is further emphasised when viewing the plan views of the GCM and Concept Two – Revision D.
Figure 26 and 27 show the pressure coefficient over Concept Two – Revision D and the GCM respectively. The multiple high pressure spots on the GCM tractor have been reduced to a single high pressure zone at the front of the Concept Two – Revision D tractor. The GCM high pressure zone between the rear of the tractor and the front of the trailer, as seen in Fig. 28, is dramatically reduced in Concept Two – Revision D. The GCM also had a localised high pressure zone under the trailer, whereas the pressure distribution was more evenly dispersed and of a lower magnitude in Concept Two - Revision D.