NISMO explains how its use of CD-Adapco CFD software helped it refine the design of its GT300 and GT500 racers to ultimately claim the Super GT series crown
Introduction
Nissan Motorsports International Co., Ltd. (NISMO), located in the Tsurumi district of Yokohama, is sacred ground for Nissan GT-R enthusiasts. NISMO is the development site of Super GT and other premier racing cars, as well as production and development of automobile parts. Launched in 1984 as the Nissan works team, NISMO has risen to the challenges of a long list of races, compiling an illustrious track record along the way.
After three decades of legendary performance, success and distinction over its core competitors, NISMO uncovers how the unique capabilities of CFD helps the aerodynamic engineers improve their most recent aero-packages during the development process. Yoshitaka Yamamoto, the chief aerodynamicist of the development division of NISMO GT-R GT cars, believes that “CFD serves as an indispensable tool in development aimed at bringing visibility to the invisible dimension of flow.”
SUPER GT Series
The SUPER GT, a FIA-sanctioned international race, contains two different regulations known as the “GT500” for the top class racing category and the “GT300” for the privateer category.
GT500 class: Consists primarily of vehicles developed and manufactured by the big three Japanese automakers Nissan, Toyota and Honda, along with their affiliated companies GT300 class: Marked by a trend toward amateurs, with the majority of participating teams comprised of privateers
Super GT fields both GT300 and GT500 cars at the same time. The speed differences translate into even more complex and exciting race conditions. Cars fight tooth and nail to outstrip each other as they roar around the circuit, making the race very attractive.
The NISMO team develops and produces vehicles for both of these categories. In particular, the GT500 class vehicle, on which the company has truly staked its reputation as an automaker, is formed based on the GT-R. In 2013, the GT regulations became more aligned with those of the German DTM series, which stimulated new mechanical and aerodynamic changes for 2014 Nissan GT-R NISMO GT500. Utilized in its development is STAR-CCM+ the flagship product of CD-adapco. In his role as chief aerodynamicist at the NISMO Development Division, Yoshitaka Yamamoto oversees the aerodynamic development for the GT500 GT-R, and shares his knowledge and experience in simulation through this article.
Juggling the Use of Wind Tunnel Tests and CFD in Aerodynamic Development
In race car development, the traditional approach is to run wind tunnel tests on scale models, with visualization realized through pressure measurements at specified points, smoke, tuft, PIV, and other methods. However, with these methods, it is almost impossible to visualize the entire domain of the aerodynamic flow. On the other hand, the recent advances in CFD software, hardware, and computing capabilities, has enabled engineers to simulate a complete race car in a highly detailed CFD model from scratch, and to gain deeper insight into their designs, which would not be viable through any other means. CFD substantially helps with understanding the phenomena involved in fluid flows, permitting accurate display and analysis of the information, with a level of detail that is hard to provide experimentally. It enables engineers to test the car virtually prior to any wind tunnel session, so as to pre-evaluate various configurations and “what if” scenarios, and submit to test only the most promising solutions.
This makes CFD a widely accepted tool for the design and development of racing cars, complementing wind tunnel tests. Alongside this rapid growth of computing resources, it can be hoped that CFD can hold the promise of offering itself as the “digital wind tunnel” that can replace the physical testing in coming years as the science behind CFD improves and computers become even more powerful. Yoshitaka Yamamoto explains the contributions of CFD and the advantages this method offers in comparison to wind tunnel testing in aerodynamic development: “When attaching small parts to the car, for example, wind tunnel testing alone is not sufficient to determine the impact of these parts on the backside, especially where the effects occur, and whether or not downforce has been obtained. CFD however offers valuable insight into the flow behavior. GT cars are growing more complex every year. The number of intricate devices is also on the rise, making it increasingly tough to get the job done on the strength of experience alone. This is where CFD becomes necessary.”
How CFD Inspires NISMO
Every aerodynamic engineer in racecar development has two major concerns; the creation of downforce to help push the car’s tires onto the track and keeping it from sliding off in corners due to centrifugal forces; and minimizing the drag that is caused by turbulence and slows the car down. The harder and faster you drive, the more low pressure air (higher speed) goes underneath the car, and the more downforce will be created.
On the other hand, with increase in speed comes an increase in drag which is not desirable. The ideal setup is normally to get the maximum amount of downforce for the smallest amount of drag generated. However, the decision on either to create an aero-package which is balanced or leaning towards one of these two forces is highly dependent on the track and condition. A track with tight turns requires a car with higher downforce configuration to navigate the turns. But on tracks with long straightway, wide and banked turns, such as speedways, less downforce is required; higher speed can be negotiated, and hence the drag reduction is of greater importance. When Yoshitaka Yamamoto re-joined NISMO in 2011, he started developing an aero-package with low drag specification that improves the high speed, while prior to this year, all the improvements were carried out to increase downforce while maintaining the drag level. One of the initiatives behind selecting the low drag specification was to prepare an aero-package specifically designed for the Fuji speedway in 2012. Because of its long straight course, lap time could be decreased significantly by reducing the drag.
Under the low drag specifications for 2013, the front fender assumed a configuration along the lines of a steep wall (below).
That resulted in an extremely distinctive front mask for the vehicles. Yoshitaka Yamamoto’s team conducted an initial CFD simulation using STAR-CCM+ to find the potential areas for drag reduction, and they found out that the pressure on the mentioned fender section has lowered comparing to their previous design (below).
Yoshitaka Yamamoto explains: “As a result of the CFD analysis, we realized that the front fender is an effective area regarding drag reduction. Under the regulations, the only means of expanding that area was to widen it vertically. When we attempted that, the results were favorable as expected.” Prior to that, they had attempted to eliminate the drag by rounding the configurations, but with the help of CFD, the NISMO team understood that it was possible to reduce such drag without streamlining the shape in that way. They featured this so-called approach as “aerodynamic harnessing modulation” in their 2014 model. This refers to raising performance through the adept use of pressure differences a step that cannot be taken without CFD. Yoshitaka Yamamoto highlights the impact of CFD in their new achievement, as he continues: “Measurements may be carried out in wind tunnels as well, but the visualization that becomes necessary is difficult to achieve over the entire vehicle. CFD is an effective tool to view the body as a whole.”
Although CFD has led Yamamoto’s team to a better design by offering a valuable insight on the entire design continuum, it has been used until now in a supplementary role rather than as a replacement for the work in the wind tunnel. As he explains: “One of the areas that still has room for improvement lies in evaluating portions that bear the wake of the parts in the front. As a case in point, I still regard assessments of tire wake as posing a stiff challenge. In addition to the wake, we have also failed to get 100% satisfactory results with the rear diffuser and other reverse pressure gradients. If we can achieve that, I feel confident that the time will arrive when we no longer need wind tunnels.”
CFD and Development of Visionary Rear Fender Turning Vane
The regulation for the second Fuji race in 2012 stipulated that engineers were at liberty to tweak the design of the rear fender. From that time onward, this has become a standard measure for all F1 races. Following the regulation, NISMO attached turning vanes that had winged cross-sections mounted on the rear fender (Figure 5). According to Yoshitaka Yamamoto, the primary goal was to create an eye-catching external appearance that could be presented in the F1 series.
In addition to its distinctive appearance, it also contributed positively to the rear wing performance. As can be seen in the graphic below, CFD results showed that the air flow churned up from the wheel arch lip on the front fender alleviated the breaking away at the rear fender and top surface of the trunk lid. While, the addition of the rear fender turning vane changed the flow pattern and improved the flow attachment towards the trunk lid.
This ultimately helped reduce the drag and improve the efficiency of the rear wing. But it is notable that the upside-down winged cross section profile of the turning vane generated lift in specific locations near the front, back, and center of the vehicle. Therefore, the turning vanes were not devices to improve the downforce. But the aerodynamic team approved its addition since it had drag reduction effects, and hence improved the overall performance of the rear fender. However, the ruling authorities (GTA) had a different interpretation of the newly mounted turning vanes. While the rules stipulated only ‘one wing,’ GTA considered the rear fender turning vane as the second wing. Therefore, it was shelved after having been installed for only ten minutes during the open inspection, and became little more than an illusion.
