question about rear structure of cars

[jaydosu]

why are the backs of the cars open, particularly around the 'coke bottle' area of the back. why is the bodywork not fully enclosing around the exhaust? has there been a regulation change that prompted this?

[F1NAC]

why are the backs of the cars open, particularly around the 'coke bottle' area of the back. why is the bodywork not fully enclosing around the exhaust? has there been a regulation change that prompted this?

hot air from engine needs to go somewhere, and it's placed higher so to not obstruct the flow over top of the diffuser.

[jaydosu]

is there a reason for why hot gasses aren't necessarily suited to the diffuser? is it due to the temperature changing the pressure?

[F1NAC]

is there a reason for why hot gasses aren't necessarily suited to the diffuser? is it due to the temperature changing the pressure?

hot air isnt dense as cooler air from surrounding thus it hinders preformance of wings, diffusers etc.

[godlameroso]

Hot air has more energy, and higher viscosity, and when mixed with cooler denser air causes a localized pressure differential.

[Vyssion]

Hot air has [...] higher viscosity [...]

Colder air has a higher viscosity. The reason why hot air may "feel" like it has a higher viscosity is because of the humidity in the air often being higher than cold air.

is there a reason for why hot gasses aren't necessarily suited to the diffuser? is it due to the temperature changing the pressure?

Most of the time the bodywork is open so that there is just room for struts and exhausts and other stuff that is around that busy area. But in terms of how the exhaust is used, there are two kinds of exhaust blowing that can be done to influence the diffuser:

Cold Blowing
Normally the engine will only produce enough exhaust gases when the driver is on the throttle. This means when the driver lifts off, the blown diffuser is suddenly robbed of the additional airflow. To improve the situation, some teams managed to realize that if the driver lifts off the throttle, they could tune the engine to throttle itself to 100% open and cuts all the fuel and ignition sparks. There is no drive from the engine, but all the air is still flowing through the engine which gives up to 75% of the exhaust pressure you would otherwise get when your foot was down. The exhaust is still ‘blowing’ into the diffuser, but that airflow is "cold" since no fuel or ignition are involved - its basically an air pump.

Hot Blowing
In hot blowing they cut the ignition when the driver lifts off the throttle, but continue to inject a small amount of fuel through the engine’s valves into the exhaust in order to increase the "energy" of the exhaust gas. The fuel isnt igniting inside the engine cylinders though; it ignites on the hot exhaust piping, increasing the mass flow, speed and temperature of the airflow exiting towards the diffuser. To do this, you have to retard the ignition and kill the torque, because if you don’t, then the engine will continue torquing even when the driver lifts off the throttle. Clever engine maps prevent the engine pushing the car on in these conditions by retarding the ignition by as much as 35-40% on the over-run. This way, the main problem of an exhaust blown diffuser whereby when a driver lifts off the throttle for a corner, the downforce goes missing when you most need it and the rear stability changes, is no longer an issue.

[godlameroso]

It's one of those counter intuitive things but the kinematic viscosity of air actually increases with temperature(as with most gases really). As bizarre as that seems, it's experimentally verifiable.

http://www.engineeringtoolbox.com/air-p ... d_156.html

Although air's density decreases by nearly half from room temperature to 400c, it's kinematic viscosity increases 4x. The effect is greater at higher temperatures still.

[Tommy Cookers]

there's absolute (dynamic) viscosity and kinematic viscosity (momentum diffusivity) - as their different naming suggests they are different things
viscosity changes little with density/pressure but a lot with temperature
for our purposes forget about absolute viscosity

passing air gets heated, yes as godl has shown its kinematic viscosity increases
so Reynolds No decreases, this can be beneficial
look for earlier threads, Olefud wrote about conceivable benefits
(heated airflow staying attached where seperation would otherwise occur, maybe helping to replace the banned exhaust blowing)


btw can anyone tell me (an old duffer) whether/how I can search for all the posts of eg Olefud by using that username ?

[godlameroso]

there's absolute (dynamic) viscosity and kinematic viscosity (momentum diffusivity) - as their different naming suggests they are different things
viscosity changes little with density/pressure but a lot with temperature
for our purposes forget about absolute viscosity

passing air gets heated, yes as godl has shown its kinematic viscosity increases
so Reynolds No decreases, this can be beneficial
look for earlier threads, Olefud wrote about conceivable benefits
(heated airflow staying attached where seperation would otherwise occur, maybe helping to replace the banned exhaust blowing)


btw can anyone tell me (an old duffer) whether/how I can search for all the posts of eg Olefud by using that username ?

click the magnifying glass and then advanced search tab, then you can search by user name