Long ago, aerodynamicists stumbled upon the concept of net thrust when they were busy designing warbirds like the P51 Mustang. While the definition of net thrust can apply to a lot of things, what I'm talking about is getting thrust for free.
Before turbo jets, turbo fans, ram jets, scram jets and pulse jets, planes were powered by piston engines-similar to car engines, just a lot bigger. Since aerodynamics came first, most planes were either limited to air-cooled radial engines (like on most navy fighters) for serviceability on board a carrier. Or they had water-cooled in-line or narrow V-bank engines that fit inside the long, slender nose of something like the Mustang. Water-cooled engines obviously needed radiators, but because of the lines of the fuselage and turbulence from the prop wash, placing a sufficiently large radiator in front, automobile-fashion, doesn't work.
So designers took advantage of the fast airflow over the plane and placed heat exchangers under the body or in the wings. Even though fast-moving air through a heat exchanger is efficient, big flat surfaces like the face of a radiator is bad for drag, whether in a plane or in a car. In order to maximize cooling without adding massive drag, designers were forced to use a radiator with a smaller frontal area and a thick core. The problem there is that it takes pressure, not speed, to drive the airflow. Without adequate pressure, the front face of the radiator would become a stagnation zone.
To change flow velocity into pressure in a confined space, you have to slow down the flow. Bernoulli's equation states a proportional relationship between pressure, flow area and speed. Since flow rate doesn't change much, the inverse relationship between velocity and pressure means that if flow becomes slower within an enclosed area, the pressure would rise. The easiest way to reduce velocity is with a diverging nozzle. If the airstream enters through a small opening (small cross-sectional area) and exits a larger opening, the air velocity going out would be slower than what's going in, and the pressure would be higher at the outlet than the inlet.
A diverging nozzle or duct at the inlet of the radiator cuts the speed and increases pressure in front of the radiator. The pressure ensures that air has enough potential to push all the way through the resistance in the radiators' fins and tubing. Without that pressure, the flow would stop and all subsequent air would go around the radiator duct, making it useless.
On the outlet side, the opposite effect is desired. If there is little pressure on the back of the radiator, the flow in front will be sucked through. Dropping the pressure means using a converging nozzle and trading pressure for speed, so the outgoing flow is now accelerated. But that outgoing flow will never be as fast as the incoming flow, because of drag, and frictional losses induced by the radiator and turbulence generated off the converging and diverging nozzles. The net result is still drag.
Here's where net thrust comes in. Since the four-stroke internal combustion engine has a real-world thermal efficiency of under 50 percent, over half the fuel's energy is wasted as heat through the exhaust or sucked up by the cooling system. In a few instances, we scavenge some of this to pre-compress the intake charge, such as a turbocharger. But the majority is still dumped into the atmosphere.