What is a Drag Coefficient?
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- Опубликовано: 24 авг 2024
- In this video, we will be discussing Drag Coefficients.
What are they? How can you calculate them? And most of all, how you can use them in your design process!
Theory
Drag coefficients are used to calculate the hydrodynamic (in water) or aerodynamic (in air) force on an object, given the density Rho (ρ), the speed (u) and the frontal area (A) of an object. So if you know the force on an object at a certain speed, for example after a wind tunnel test, you can calculate the drag coefficient yourself using this formula. (0:28)
Once you know the drag coefficient for a certain geometry, you can calculate the force for different object sizes or different velocities
That is very useful for example when you need to size engines, calculate required battery capacity, etc.
But keep in mind that the drag coefficient can vary in function of the Reynolds number. So be careful with large extrapolations to other speeds, sizes or densities.
Practical use
A drag coefficient allows you to analyze the aerodynamic efficiency of an object, irrespective of its size or velocity. That makes it possible to compare a cyclist for example to a building. They are quite different, but still, they have a normalized aerodynamic coefficient.
It is also quite useful within a design process: when you are looking at different concepts for example for your new project or new vehicle, you can rank them according to their drag coefficient. Or you could get inspired by aerodynamic shapes coming from a completely different sector (Mercedes once had a car design inspired by fish!).
Typical Values
A drop shape, which is quite efficient, can have a drag coefficient as low as 0,05, whereas a building typically has one above 1. Lower means more streamlined.
So if you are working on a drone that needs to fly as far as possible on a single charge or a cyclist that wants a higher top speed, you will want to reduce the drag coefficient as much as possible.
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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).
We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!
For more information, visit www.airshaper.com
very helpful. thanks
Thanks Spidervait, you're very welcome!
I think using D/q is a better practice than using Cd, even if you're consistent with the reference areas.
Are you referring to D/q as on this page:
www.grc.nasa.gov/www/k-12/airplane/dragco.html
So you'd leave out the A then, to not normalize by the area?
Looking forward to see more aerodynamics related videos .....
Hi Sarathy, there's a lot more new videos on our channel, or just visit www.airshaper.com/videos for the overview!
Thank you..
Thanks!
explain the relationship between drag coefficient and Reynolds number, please
Hi Nhor, some of our very first videos covered exactly those topics: if you visit our 'aero theory' channel you'll see them listed there. I hope you like them ☺️
Thanks for the amazing work Wouter, I have a question for you, how is Force calculated in a numerical solver?
Hi Vignesh, thanks for the compliment! In cfd, you calculate the flow by chopping up the air around the object in small blocks (see this video: ruclips.net/video/dyiREvdc4Gg/видео.html). This means that the surface of the object is represented by a high number of small elements (surface cells). By taking the vector product of normal pressure (perpendicular to the surface cell) and the wind direction, and then integrating it over the entire surface, you get the total "pressure force". By taking the vector product of the surface friction and the wind direction, and then integrating over the entire surface, you get the total "friction force". Hope this helps!
Hi Vignesh, in a numerical solver, the force is calculated by integrating both the normal force (perpendicular to the surface) and the friction force (parallel to the surface) over the entire surface of the object.
love it. thanks for the explanation
Thanks Rifki, I'm glad you like it!!
Thanks Rifki!
Hi, thank you for the presentation, can you explain a little more about wave drag.Regards.
Dear Laid,
I must admit I haven't worked on transonic / supersonic flows before and cannot answer your question (as it relates to shockwaves etc).
Great video... Straight to the point :)
Hi DailyPlanes, thanks a lot! I'm glad it was useful - are you getting into CFD simulations?
@@AirShaper I'm in my final year of school so not quite yet, but probably in the not to distant future. I'm writing a report on how cross-sectional area and the coeficient of drag has an affect on how long it takes for an object to fall a particular distance in a tube containing a medium like water or methanol.
@DailyPlanes That sounds cool - and indeed, taking theoretical Cd coefficients is a very good way to start. Don't forget to take the Reynolds number into account when using drag coefficient data (ruclips.net/video/sV8l8xd4MEQ/видео.html) - good luck!
Hi Sir, can you please explain what is P, u and lastly A in the drag coefficient formula.
P is the pressure, U is the velocity, A is the frontal surface area
I'm hoping to understand the difference in size on coefficient of drag.
If a particular vehicle has a Cd of 0.30
What would the same vehicle at 1/10th scale be?
Is it the same?
Yes, as long as the shape is identical, it will be the same (as far as I understand aerodynamics)
Thank so much .If possible Can I ask some question ?
I would like to know how to calculate Area . Area is project to Velocity vector or not . I'm confuse measure A for find Coefficient Drag of Car .I think Calculation of Area(A) for Aero Wing and Car is different right ?.
Indeed, they are different:
- Cars: frontal area is used, which is the area when projected on a surface perpendicular to the driving direction.
- Planes: planform area us typically used, which is the area when seen from above.
Woulter - I do have a question for you, please. I have always been given to understand that smooth bodywork on a vehicle was the best way to reduce drag, but, recently i was told that a matt surface is better as it holds air molecules and the passing air finds it easier to pass over the held boundary layer rather than the smooth surface... over to you good man!
Hi Graham, if you start from a drop-shaped object, where the goal is to have the flow nicely attached along the entire length, then yes, a smooth surface makes sense: it will reduce the friction drag. However, the friction drag is just a small portion of the total drag for road vehicles (just a few percent). So when you have an object that is not drop shaped, where you need to taper inwards faster (like for a car), then it might be beneficial to trip the flow a bit, to make the boundary layer turbulent. That allows you to keep the flow attached at larger negative angles of the body work. See our movie on DIY aerodynamics 1 and 2 (the ones with the beetle):
ruclips.net/video/CGmqdXKgBiQ/видео.html
ruclips.net/video/MC6woj6tsQY/видео.html
When it comes to the micro-structure of things, it is more difficult to analyse this using CFD. Sometimes people apply a rough surface to obtain the above effect to keep a flow attached to the suction side of a wing for longer, for example (bottom side of wings in F1 for example).
Whether a light amount of roughness could reduce the friction drag as well would be new to me, but I can imagine that some advances in tech might, for certain flow regimes, makes this possible!
I heard that the dimples on a golf ball provide laminar air flow and reduce drag. Also heard that jet fighter surfaces are painted with a special paint that has the same properties and holds a layer of air next to the surface and reduces the friction of the air that is passes through.
When you so frontal area, if I had a hemispherical parachute, do I only need the area of the circular base, or should I use whole inside of the parachute? ... / something about eddy currents.
The frontal area is the surface area after parallel projection onto a plane perpendicular to the winw/driving direction. Imagine taking a foto of the parachute, from a distance and seen from below, and then calculating the surface area on that photo. So yes, in short, the circular base and not the entire surface of the fabric 🙂
How and why laminar to turbulent flow is induced in a cross flow past a circular cylinder the value of the drag coefficient drops..
Dear Amit,
laminar flow detaches quite early on the circular cilinder (just a bit after the maximum thickness). When you have turbulent flow, this detachment point is postponed a bit more downstream, making the wake smaller. We did our best to explain this on our video on vortex generators, just have a look here:
ruclips.net/video/MC6woj6tsQY/видео.html
Hi Amit, check our "AirShaper meets Bioracer" video, there we explain this effect in more detail (with reference to golf ball dimples)!
Now another question, please Woulter. I'm driving UK to Nepal for a charity run and am trying to design boatails for the back of my large van - What would your view be in using shallow (15mm) vortex generators on these panels?? (I am thinking top panel at -5% from the roof line and approx 40cm deep. Side panels 30cm deep parallel with the sides of the van. I have no professional knowledge as I am a retired knife sharperner!)
Hi Graham, see also my previous answer: vortex generators only make sense if the angle of the shape your optimizing is too negative for normal flow to stay attached. The easiest path is to work with a smooth transition from flat to negative angle, indeed no more than 5-7% negative angle. With vortex generators you can go more negative, but it's not always beneficial, because it's a tricky tuning exercise (which can easily be offset when conditions like side wind and so on come into play). I would first try with a nice & smooth boatail design without vortex generators! Feel free to send us some pictures!
@@AirShaper Much gratitude - when I have done some more study I will try and find the confidence to show you my ideas Wouter.. (obviously) loving your channel and your style of presentation
@@CosmicSeeker69 You're welcome Graham, I'm curious to learn more about your findings! We can always learn ourselves at AirShaper, so feel free to share!
could you please explain the drag coefficient for a rectangular building?
Hi Martin, the flow around buildings is a complex & fascinating domain: for an isolated rectangular building (or a generic cube on the ground for that matter) you can find quite some interesting stuff to read online. Also nice is to study the horseshoe vortex that is sometimes created, like here: www.catalyzex.com/s/Kostas%20Karatzas. If you want a sample report of the flow around a more complex building, just contact me on wouter@airshaper.com and I'll send you one!
Thank you sir. I have a question if you don't mind me asking, how can we find drag coefficient since we don't know the Fd (drag force) ? I didn't understand that part
You would need to measure that drag force. For example through computer simulation, or through force sensors in a wind tunnel. Or via coast down testing - see this video: ruclips.net/video/Gip3iG2A2zE/видео.html
@@AirShaper Thank you so much sir :)
How to find drag coefficient of bullet
We don't do weapons / supersonic flows
Freak Xplanation sir
N yu luking damn gorgeous 😍
Do we consider frontal area for calculating downforce, if yes why?
Hi Prasad, very often the downforce is just considered as an absolute number (xxx kg or yyy Newton of downforce). But quite regularly, people also talk in terms of Cd and Cl, the drag and lift coefficient of a car. In that case, the lift is related to the frontal area (just have a look at our "drag coefficient" video and replace drag with lift in all formulae :) ). Just let me know if this is clear!
AirShaper sir i know about the formula but the actual doubt was why do we consider frontal area for downforce , i actually feel area seen from the top shouldbe considered for downforce,what do u think?
@@prasadchettiar6589 Yes, that would also make sense, and for planes, that is the way they do it (called the platform area for example). In general, it's a matter of choosing a reference and sticking to it. Also important is the lift/drag ratio for a car. When calculating that ratio, using Cl and Cd with the same reference area, it cancels out. So having a consistent reference area is important to be able to calculate other numbers!
AirShaper so this force actually doesnt act on the COP of the vehicle?
@@prasadchettiar6589 Ah, that is a different discussion all together :) the Cd and Cl values just provide a "normalized" value for lift & drag performance. They don't indicate where the force is acting!
I need a wind tunnel
What would you need it for? Testing a vehicle? To what accuracy level?
@@AirShaper yes it’s a née wing configuration for a supercub to gain cruise speed and efficiency but maintain stol characteristics
@@Mikesworld777 Then I would search online for "low speed wind tunnel" + "your location"
hmm
Can u please put the videos in an animated illustration so that it will be easier to understand...secondly can u put captions for English as well
Hi Sarathy, thank you for the feedback! You are right, as you will see with the newer video's, our editing skills are improving as we get more experience at it. This means more visuals! We are working towards adding the captions as well!
Kind regards,
The AirShaper team
@@AirShaper still a VERY good annunciation from a Non native speaker. I was very impressed. Really appreciating your uploads ( as I'm trying to make a project as efficient as possible
Could you please tell me what is volumetric drag coefficient and how is it different from the drag coefficient you explained in this video?
Hi Jeffy, normally the drag coefficient is calculated versus the frontal area (cars, ...) or planform area (planes, ...). The volumetric drag coefficient uses yet another definition of the reference area, calculated by taking the square of the cube root of the volume of the object (which results in a "surface" unit, like square meters or so). this link on Wikipedia explains it: en.wikipedia.org/wiki/Drag_coefficient
@@AirShaper thanks, I did check the Wikipedia page before but couldn't understand in which cases are we supposed to consider volumetric drag, like the value we get by using the formula, what does it signify exactly?
@@jeffybiju2667 Hi Jeffy, it depends on the application I guess. For cars, it makes sense to look at the frontal area, as that is directly related to vehicle size, passenger space, ... and a good indication of how it affects drag. For planes, the planform area of the wings (wing surface when viewed from above) is more relevant, as that is the main component causing drag (and friction drag is more important here - the force of the air sliding across the surface - and therefore a horizontal reference surface is more relevant to indicate drag). For ships, I can imagine that due to their long length, a frontal area alone would not be the best reference. My guess is that for that reason they chose to calculate an artificial surface area, which is directly related to the volume of the ship (and thus changes when width and height change, like on a car, but also when the length changes, like for wings). Apart from that, it remains an arbitrary reference, simply a tool to be able to compare different designs within the same category. So whichever industry you are in, there will probably be a most common reference area to use & to communicate values to others.
@@AirShaper Thanks a lot sir for this explanation. I have been searching the internet for an answer but didn't get any satisfactory explanation.