Diffusers
The GT-R's underbody has two main diffusers to help drive the low-pressure flow beneath the car. The obvious one is the upswept duct at the rear and below the bumper. The not-so-obvious one is located directly behind the splitter leading into the front wheel wells. Aerodynamically, both of these diffusers achieve the same thing: minimizing pressure under the car. In addition to the rear diffuser, a number of vertical 'fences' are used so the airflow moves more efficiently underneath.
A rear diffuser helps drive the under-car flow by exposing it to the turbulent low-pressure wake region behind the car, using this low pressure to suck the flow out. In addition, the diffuser slows the air emerging from the underbody region by expanding it through a larger-area opening. They are effective in generating large amounts of downforce by increasing air speed underneath, thereby reducing pressure. Since this low-pressure region acts on a large surface area, plenty of downforce can be generated. Even if pressure below the diffuser is only half a psi lower than outside, over a 3x6-foot area, that equates to over 1000 pounds of downforce.
Vertical fences are installed within the diffuser channel to ensure that flow remains attached to the diffuser. Since the diffuser ceiling slopes upwards, airflow there is slowing down, resulting in increased pressure. Aerodynamicists call this type of region an adverse or unfavorable pressure gradient, since maintaining attached flow almost always requires that the flow speed increase throughout the region where it moves over a surface. Flow separation - and the resultant loss of flow velocity - would reduce downforce significantly if nothing were done to prevent it. The fences act as vortex generators to assist in energizing the flow through the diffuser, which help maintains attached flow and allows the air to fill in the wake.
Canards
Also known as dive planes or dive plates, since they resemble the winged appendages on submarines, canards help generate downforce in two different ways. First, the canard redirects the oncoming air's momentum upwards, which causes a downward force on the canard. This is only moderate, since the velocity near the skin is significantly slower than in the free stream. In addition, canards generate strong vortices that travel down the sides of the car and act as a barrier. If the canards are positioned correctly, these strong vortices act to keep high-pressure air around the car from entering the low-pressure underbody region, thus maintaining more downforce. If air was allowed to enter the underside, the pressure would inevitably rise, reducing downforce. Therefore, these strong vortices act like a virtual curtain or dam, restricting higher-pressure air around the car's sides from entering the underbody region. As a result, the low pressure under the car is maintained and downforce is maximized. Unfortunately, canards are not very efficient, since the strong vortices create a significant amount of drag. They are more useful for fine-tuning aerodynamic balance.
Side skirts
Side skirts are used to reduce the amount of air that goes under the car from the sides. If an air dam is used, air under the car is at a low pressure, which causes the higher-pressure air on the outside of the car to come rushing in. The effectiveness of the skirts depends primarily on how close to the ground the lower edge can be maintained. That edge should be less than a half-inch from the ground, otherwise the skirts' effectiveness diminishes rapidly as the gap increases.