Osirisf16

I think we overdid it a bit but whatever

So, i found some formulas, which are:

1) D=Cd*r*V^2*A/2

Cd: drag coefficient
V: Velocity
r: air density
A: Based area
D: Drag
From the (1) formula we have: Cd=(2*D)/(r*V^2*A)

We did this because we must find the Cd, and unfortunately we must expirement [&o]

As you know, when throwing...for example...a ball from really very high, the ball comes down in very high velocity. Why? Because of the force the earth is acting on it, so the ball wants to go down. That force is the W (weight). On the other hand, there is another force applied to the ball. That's the D (drag). The D is opposite of W, so the that force wants to take the ball up again but of course can't do that.

The other fact is that when throwing the ball from very high, the velocity of the ball changes all the time BUT that acceleration stops somewhere. That "somewhere" is when the W (weight) equals to D (Drag). I think that's how we are going to calculate the speeds and all that we want to find in the end. How? Throught he Newton's laws of motion. Actual, we are going to use one of them. The second (F=m/a).

Now imagine a ball. One arrow above of the ball that points up, and another arrow below of the ball points down. The arrow which is up is the D, and the below arrow which point down is the W.

W=m*g

W: weight
m: mass
g: gravitational acceleration which usually is around 9.8 m/s^2 (the m/s^2 is not a formula, is a unit of measurement)

That formula, we will use it while in the end.

Then, we are going to do a scenario. We have the usual story with the ball. It falls, it falls quicker, it falls really quicker.....what? It slow down!!! Oh my god, the ball stops accelerating. That ball now has a steady velocity which we can use it. So now we have:

W=D
m*g=Cd*r*v^2*A/2

Then, you want to know what is next right? Well, the fact is that...I DON'T KNOW!!!

That's the reason why i made this thread. GYNAIKAS, a little halp here my physics buddy.
That cosφ you told me, i think i understand now. It's the A (BASED area). That's why i named it "based", because a windshield for example is not either vertical, neither horizontal. It has an angle. That angle we will use it for find the A, right?

So you now know how desperate i am. I made a whole thread and spent the day reading NASA. Of course they are using aircrafts, so i think it will be easier for as with the cars, right?

So, i started the thread, BUT YOU will finish it!!!

PS: I please the moderators not to lock or delete that thread. It's someway out of topic but indeed it has something to everybody win from that thread.

PS2: Maybe someone here didn't understand the formulas at all, right? Well, the "/" means divide, the "*" means times, and my favourite "^" means how many times the unit that is on the left of that symbol must time. For example, if i have 5^2, that means 5*5. Another examples:

e^4 = e*e*e*e
4^4 = 4*4*4*4
5^e = 5(1)*5(2)*5(3)*5(4).....5(e)

superjimis

wow[X(][X(][X(]

GYNAIKAS

Well the coefficient Cd contains all the information about the surface compound (how slippery it is for the air to move on it, and if a certain amount of air can pass through te material how much this amount is) and also the surfaces flexibility. Cd is a constant that should be measured experimentaly. You measure several times and once you have a big enough sample of measurements then you calculate the mean value and that's the final value for Cd. But how are you gonna measure the drag? Well uncle Gynaikas is here...simply you don't need to! Cd=1 is the standart medium precision measuremet value that you should use.
There's also a coefficient CL that contains all the info of the spape and curves that the object has. It is also measured experimentaly. But you should separate the whole object to normalized surfaces (discoid surfaces, trapezoid surfaces etc.) and concider the Cd=1. This will make things a lot more easier without ruining your precision more than 20% I believe. Also the NASA guys apparently have forgoten the case of non static air, I mean the wind! So I found a very very useful example of aerodynamics in one of my books. Here you go, enjoy:

PS: Cd(Ï€/2)=1. If the wind blows then find its drection and calculate the coefficient of its speed that has the same direction with your car... xreiazetai trigwnometria megale!

Osirisf16

I didn't understand the second image. What exactly does it shows?
You got a very old book there. I have also one about electrical engineering somehow (there are not ALL the chapters inside)

Greek: Mporeis na mou eksigeis kai ligo se ellinika? Kathomoun sxedon 5 wres simera kai diavaza SINEXEIA aerodynamika, ilektromagnitismo, mexri kai ligo piriniki fysiki eida. Eida kai kati gia PLASMA se microwave, poli dynati fasi

Next time, i will post a picture of a car (drew by me of course ) and we will see what is going

GYNAIKAS

I bet that you haven't yet the mathematical base in order to understand electromagnetism. But aerodynamics is a piece of cake isn't it? To Vw einai to dianysma taxythtas tou anemou. To katw velaki einai to dianysma taxyttas tou aytokinhtou sou. To v' prokyptei apo ton kanona tou parallhlogrammou kai einai h synistwsa tou Vw pou epidra ston axona pou einai sygrammikos me to dianysma taxythtas. Sthn ousia otan epidra plagios anemos to v' einai afto pou epidra ki oxi ololhro to Vw. Sorry for the Greek if anyone wants translation let me know and I'll do it at once.

pollapellos

physics i hate them lol

robbio4422

so.. im just barely able to understand this... (your formulas)

and to think im sitting in my physics class ATM

good job at explaining it. but what was the point of all of this? am i missing something? or was this for a class?

GYNAIKAS

This thread is something like an "add-on" for the previous top speed threads I've made. It explains how the air resistance coefficients get into the problem under solution. In practice it's really a detail compaired to other factors but I really enjoy it. If you know physics well not only you can explain how everything work together but also predict how a machine, object etc will bahave under specific circumstances. By the way that's why I love physics! Relax I'm not a freak sitting all day and solving equations wearing big glasses...always there's a reason when I hit these numbers!

PS: I'm sure that if I explain it to you you're gonna say "That was all? Now where's the hard part?"

Osirisf16

Yeah, i agree with you, but first i must understand. I'm a little hard-learning guy. Heres a picture i told you:





For me, this is the angle that the windshield forcing the air upside. (φ)

Now we can talk seriously [>:]



PS: And a little history. George Cayley is recognized as the founder of aerodynamics. If you want his biography, [link=http://www.encyclopedia.com/doc/1E1-Cayley-S.html]here[/link]

GYNAIKAS

Well you seem a bit confused...that's not what I meant. Here's something that may help you a lot more:

Osirisf16

I'm starting to understand now, but that's a "oloklirwma", right? Why we use that? Ok, the equation, but the S(total)...

Greek: Re megale, milame eisaste tixeroi ekei stin Ellada. Koita kefaleia na mathei o anthropos ki emas mas mathainoun mikropragmata gia kolorwpes kai malakies. Varethika. Afta einai fysiki. pes mou ki alla. Re esy. Mporw na vrw vivlia, les? Tha mou xrisimevan poly.

GYNAIKAS

You really need to be patient. The torque chapters are gold...see what we did in my thread with the top speeds? Aerodynamics are also based in prefixed formulas...there's nothing special with them. You only need to know "dianysmata kai trigwnometria" for the rest. For now you shouldn't get any extra books. If you want get the serway physics (4books). There's all the classic physics in the world and they're made for the amateur-intermediate learner. But Isuggest one thing: GOOGLE. Just type the name of your problem and you'll find anything you want. If you want to learn mathematics, physics, mechanics and of course ELECTROLOGY (I believe that it's the only thing that allows you to do simply ANYHTING) then join our club: Study much and come to Patras electrical enginnering department. Mechanical design, electrical design, circuits, technical mechanics, structures, programing, physics, mathematics, alternative energy sources, energy managment, robotics, systems of automated control, signal processing, digital control, analog control, artificial intelligence, circuits protection and many many more are some of the 72courses you need to pass in order to get the master degree (5-years). Any questions?

Osirisf16

NO Back to our topic then...

Well, we were there about "oloklirwmata". I know that we use them to calculate an area WHEN the lines are not straight but curves. Lets disguss it that there aren't curve windshields, just straight parts of the front of a car. The only i didn't understand well is there with the speeds. That (V-u)^2. The formula i gave is correct but you change the V. What or whose velocity we are trying to find? I thought that the "V" in the formula was only the speed of the car. You speaking for wind too? I thought we were going to talk for calm situations first.

GYNAIKAS

Yes I'm speaking of wind to. You don't need to calculate the "oloklhrwmata". You just have to separate the whole surface to small ones that are not curves but trapezoids so their surface will be calculated as X*Y (width*length) and then according to their angle multiply by sinφ to find the vertical equal surface in order to suppose Cd as 1.

Osirisf16

Well done, that's what i'm talking about. I thought about that surface of an angled windshield. As the air comes horizontal, it wants a vertical to be calculated so for the windshield, you just calculate the windshield widht and then do a sinφ if the angle you take is the one up in the windshield or a cosφ if you take the other angle down the windshield. Am i right?

Now i almost understand but i have another question. The Cd. Why is it 1? I know that you take it for datum, of course is not exactly 1 but around that number. But the 1 is only for windshields? Or it's the same thing? I can't understand. What ever you do, you will find experimentally a number around 1?

GYNAIKAS

Cd is approximately 1 for surfaces vertical to the air flow (Ï€/2) so once you calculate the normalized vertical surfaces instead of the real ones it's ok to make this proximity. The right thing would be to calculate the surface with its own Cd but that's not possible with the means you got. By the method we follow we simplify the problem A LOT without ruining the precision significantly...it's a trick nothing more.

Osirisf16

Yeah but the air flow on a vertical part and the airflo on an angled part is different. On the angled part, the airflow slips somehow. If i understand, that's why we must take the V'?

Greek: megale, an mporeis se parakalw na kaneis tipota panw sto sxima pou edwsa (to aftokinito. Einai kai ferrari ).

GYNAIKAS

I will once I get home today...cu around. [8D]

Osirisf16

Alright you guys, my school business is over for 2 years. Finally, SOME peace As i can see now here, i think i must forgot all of them almost but i will read it once again and see where we left. Anyway, i think i told Gynaikas to do something as i read his last post here

GYNAIKAS

Yes you did but unfortunately I forgot it...well I'm really busy with university exams this period...anyway like I said you have to divide every surface in linear (grammikes) srfaces and calculate the drag of each one of them with the method I suggested before. Then simply add them all. Keep in mind that slip steam, vortex and gap filling effects change the whole thing. But you need ansis software in order to calculate these things...and that means several thousands of euros and complex tutorials by experts.

Osirisf16

Ouuuuu [X(] Great, i have that in mind I guess i will stick with the intermidiate grade after all. When you finished with your university we can have a little talk, right? Ok dude, c u around

GYNAIKAS

Anytime once examination period is over.

Anomie

GREAT thread. You guys are really dipping into some deep calculations!

I work with a couple of NASA aerospace engineers that specialize in the physics of flow and fluid mechanics, and if there are questions to which you cannot find sufficient info, let me ask one of them. They really love this stuff!

Also, I have to say that the bilingual posts are cool. As someone who grasps only one language, it's fascinating to see the Greek.

Osirisf16

I like very much physics. Some people sees it like a lesson in the school which you must be forced to read and pass the final examinations. On the other hand, i enjoy learning this stuff because i can understand many things in the science method and is wicked cool I'm trying to learn as much as i can. I like science

GYNAIKAS

Well I like physics electrics and mechanics. Next year I hope to finally get my master on electrical engineering. We've also done some great work on transmission gear ratiosand top speed related to engine's rpm and wheel diameter. Right now I'm programming a microcontroler in order to convert the rpm of the wheels (measured using a photoresistor and LED pair) into 8-bits digital word. Then I'll work on RF wireless signal transmission and display the car's speed and RPM on a 4x7 segment diplay or LCD screen up to 100m away using receiver-transmitter modules. When this is done I'll do the same for engine's temperature measurement in order to have a complete telemetry unit...cool isn't it?[8D]