# Fluid Dynamics of an NX2000



## umdNX2k (Aug 20, 2003)

I'm a mechanical engineering student at the University of Maryland, hence the handle. In my fluid dynamics class we got to run two dimensional models of car shapes on a CFD (computational fluid dynamics) program. It basically takes equations and matrices and solves them for every node of a mesh over a model. Kinda cool. Anyway, I decided I'd spend some time and do the NX2000, since I love driving the car and it definitely has some nice Aerodynamics. Here's the results.

This is the mesh I used.









These are what the pathlines around the car look like.









These are the pressure contours. Blue means its pulling and red means its pushing.









This is the turbulence caused by the shape.









Pretty cool stuff although the class is a bitch. I was getting a coefficient of drag of around .6 which is a little high. Oh right. Just a disclaimer. This is what it looks like going reaaaaally slow. Like 5 mph. The reason is its been scaled down for a test wind tunnel. I'll do a full sized model soon. If I have time.


----------



## znamya (Sep 21, 2004)

Nice!! I am also a mechanical engineering student (PhD) at georgia tech. what you may want to do is to plot also C sub L which correspond to lift force on the body. For yourself add a baggage(make it sedan) and see the difference between sedan and hatchback. That will show you why you should buy hatchback by the way at 5 miles per hour speed, the friction from the road (tire rolling) precludes the wind drag. so go to speed of more than 55 miles per hour and you will more realistic results. :thumbup:


----------



## Tavel (Aug 21, 2004)

yum. do my sentra next!

my dad's an engineer for a plastics company, he does a bit of fluiddynamics, but nothing like that. and i'm an economics major...i dont know jack about fluiddynamics. j/k. i actually use fluid dynamics when arranging the fans in my computer cases. more fans does not mean better!

well i do have a real question related to fluiddynamics posed to either of you. is the "bean" shape of a ford taures more aerodynamic than the conventional shape of b13 and b14 sentras? more precisly, does the added ugliness have a real effect fuel efficiancy?


----------



## znamya (Sep 21, 2004)

Tavel said:


> well i do have a real question related to fluiddynamics posed to either of you. is the "bean" shape of a ford taures more aerodynamic than the conventional shape of b13 and b14 sentras? more precisly, does the added ugliness have a real effect fuel efficiancy?



The answer is YES. Take a look at the airplane design and the droplet that is falling from your tap. All have bean shape. Because bean shape (called streamlined body in fluid mechanics) has the least drag to the flow. It is as simply as this, when you leave a ball on a hill, it will go down to the least energy level. Same goes with the droplet. If you can produce the a droplet shaped body then you are in business. In my home country, my father has a 97 opel vectra 2.0. That car has a drag coefficient of 0.28, which is excellent. I really like the design of the car, it is very sporty looking. Take a look at their side mirror design it is how it should be for the least drag. 98 BMW 5 series has a drag coefficient of 0.26 for instance, the car doent have to be ugly to have low drag. But I mentioned, drag coefficient is important after 55 miles per hour(typically) before that most of the power from the engine is consumed by the tire friction. But there is no speed limit in the autobahns in my country so that may be more important across ther than the US.


----------



## lshadoff (Nov 26, 2002)

umdNX2k said:


> I'm a mechanical engineering student at the University of Maryland, hence the handle. In my fluid dynamics class we got to run two dimensional models of car shapes on a CFD (computational fluid dynamics) program. It basically takes equations and matrices and solves them for every node of a mesh over a model. Kinda cool. Anyway, I decided I'd spend some time and do the NX2000, since I love driving the car and it definitely has some nice Aerodynamics. Here's the results.
> <clip>
> Pretty cool stuff although the class is a bitch. I was getting a coefficient of drag of around .6 which is a little high. Oh right. Just a disclaimer. This is what it looks like going reaaaaally slow. Like 5 mph. The reason is its been scaled down for a test wind tunnel. I'll do a full sized model soon. If I have time.


Check out this page. Perhaps you can add some of the suggested enhancements and see the magnitude of the effect.

Lew


----------



## Tavel (Aug 21, 2004)

znamya said:


> the car doent have to be ugly to have low drag. .



the ford taures looks like a fish.


----------



## znamya (Sep 21, 2004)

Tavel said:


> the ford taures looks like a fish.


I think so

:hal:


----------



## lovemyser (Mar 14, 2003)

I'm having a hard time understanding this because of previous articles I've read in the past, My bro and I have info showing that a PT Cruiser has a coefficient drag of .38, and a ford windstar .32 http://wichita.bizjournals.com/wichita/stories/1997/12/15/newscolumn3.html

Are there different type drag rating scales or is a "coefficient Drag" pretty standard. Now I'm not saying your wrong and you did say you haven't done a full scale, but is this a accurate number?

Help !?!?!


----------



## znamya (Sep 21, 2004)

lovemyser said:


> I'm having a hard time understanding this because of previous articles I've read in the past, My bro and I have info showing that a PT Cruiser has a coefficient drag of .38, and a ford windstar .32 http://wichita.bizjournals.com/wichita/stories/1997/12/15/newscolumn3.html
> 
> Are there different type drag rating scales or is a "coefficient Drag" pretty standard. Now I'm not saying your wrong and you did say you haven't done a full scale, but is this a accurate number?
> 
> Help !?!?!



First of all there is on drag coefficient. But let me tell you, the drag coefficient and drag force is not dependent on the mass of the car. so Sentra may have a higher drag coefficient than Mercedes S class. The most important thing is the shape of the body, it is like how easily the body can go through the air in front of it. It certainly makes sense that Pt cruiser has a higher drag coefficient than Windstar because windstar has more streamlined shape(bean shape) and like the cars had 0.6 drag coefficient in 70s then 0.5 in 80's and then after the introduction of faster computers for CFD models, drag coefficients has decreased to as low as 0.20's. As PT cruiser's design is like the car in 60's. It is resonable to have higher drag. Like also VOlvo cars has high drag coefficient. Also in F-1,the cars are designed to have very low drag at very high speed like 200miles/hour but when its speed reduces to 150miles/hour, its drag coefficient increases to twice so that the driver can slow down much easier before turning, it is found that it gains time for this.

Also I am not an expert in fluid dynamics neither I am a big fan.


----------



## lovemyser (Mar 14, 2003)

Not to be critical, but I had a hard time understanding your response becaused of your sentence structure, for instance your first sentence doesn't make sense to me.

Se-r .net puts the NX2000's coefficient of drag at .32

http://www.se-r.net/about/nx2000/93_NX2000_stats.html

umdNX2k what kind of scale were you using to get this number? Also is this a average number or a specific speed?


----------



## znamya (Sep 21, 2004)

lovemyser said:


> Not to be critical, but I had a hard time understanding your response becaused of your sentence structure, for instance your first sentence doesn't make sense to me.
> 
> Se-r .net puts the NX2000's coefficient of drag at .32
> 
> ...



Thanks for your kind warningI am sorry. the first sentence has an 'on' in it. It should have been 1 (one). some warning that the drag coefficient is very misleading because it is speed dependant. for instance the guy who did this modeling took speed of 5 miles per hour and obtained drag coefficient of 0.6. I dont know what they take as velocity when calculating the drag coeffient of a car. Maybe 60 miles per hour. Interesting thing is the drag coefficient drops when speed increases as opposite to intution. BUT the drag force is proportional to that coef. times the velocity square. so actually when speed increases, drag coefficient decreases while the drag force(which is the important thing for fuel consumption) increases. 
To sum up, I dont know what they take as velocity when calculating for standart drag coefficient for a car, but 0.6 value that he found is not standart drag coef. comparison value. It is only a value corresponding to 5 miles per hour.


----------



## umdNX2k (Aug 20, 2003)

lovemyser said:


> umdNX2k what kind of scale were you using to get this number? Also is this a average number or a specific speed?


I put a disclaimer saying the number wasn't accurate. I'd stick with the numbers given by nissan posted on se-r.net. 

Actually, the project is to see if scalability can be kept even if the reynolds numbers weren't exactly scaled. Therefore the project asks for a test in a virtual (done on the computer) wind tunnel of .3m high x .7 meters long. To keep the blockage ratio low enough, to make sure the ceiling of the wind tunnel doesn't have an effect on the pressure distribution, the actually car height is limited to 5% of .7m. Also the other thing is they set a limit for the virtual wind tunnel, as if it was real, of 45 m/s maximum wind speed and the ground is set to moving at the same speed. The ground moving is important because otherwise the velocity under the car would equal zero at the ground and on the car creating a much slower velocity distribution The scaling of the velocity is not linear. Once you do some scalability calculations, you notice that it is actually in proportion with the Reynolds numbers of the model to the real life scale. I did the runs with these limitations so I could include the model in our report. If I have time, which these days I have none, I want to run the full scale model and see if I get close to .32. This is still a 2-dimensional model of the center of the car and its still not very accurate underneath anyway. Don't expect much, just ogle at the plots. Its fun.

Also, Coefficients are nondimensional for a reason. They should be equal for scaled models of all sizes. 

Hope you enjoyed the lesson. Oh and I love the engineers having some intelligent conversations, pretty neat stuff.


----------

