[alpha1]

Originally Posted by Sutripta

What would be the Cd of a teardrop moving in reverse?

And that of the shape of two teardrops with heads superposed. Essentially both ends being 'tails'.

So what gives Cd = 1.0?

From what I recollect, what matters in the fore side is to have a shape that allows gentle splitting of the fluid into separate streams without creating vortices. Extending the leading edge from a sphere to a needle point will definitely improve the Cd.

But the effect is more pronounced in the aft side where your aim is to not let the boundary layer get separated from the body. Hence the long trailing edge offers more returns per meter compared to a long leading edge.

Regarding the point about Cd.
Your question does makes sense since I see a close resemblance with the kinetic energy equation. (In fact I can even derive the drag force equation from kinetic energy equation!)

Originally Posted by MegaWhat

I don't have exact numbers right now, but a teardrop moving in reverse will have a Cd similar to a sphere. With a teardrop having their heads superimposed, I think it'll be close to the teardrop shape itself.

If you can please run a simulation, our doubts will be put to rest at once.

[Jeroen]

Originally Posted by MegaWhat

In the case where there's no or less wind and the car is going along a straight road at decent speeds, the side force is close to zero and only the force acting along the car and upwards (or downwards) become more significant. That's lift and drag. That's the situation I'd say 65-70% of the time. Normally, we design to be efficient at the condition that the car is going to operate in for the maximum time, and then we just verify that the forces along the other axes are still manageable. We look at the other axes in detail only if the forces seem out of control. This validation can be done by CFD or a wind tunnel test. Do correct me if I'm wrong here.

I would like to think there is a bit more science to dealing with side wind. It is not just natural wind, but also buffeting when you for instance drive into a tunnel, or when a large truck comes on to you on the other side of the (narrow) road.

We have almost forgotten. These days most cars tend to be pretty stable and they are not that much influenced by side winds. But anybody who has ever driven say a VW Beetle at more than 40 km/h whilst being overtaken by a truck, or traversing a bridge with a bit of cross wind, will know what I am on about. These days we would call such handling atrocious if not murderous!

For any car its longitudenal stability is susceptible to cross winds one way or the other. How it deals with it is probably more down to suspension, weight distribution and so than perhaps aerodynamics. Although I guess the shape and size of the car side surface might play a role as well

Jeroen

[Thermodynamics]

Thank a ton for writing in layman language on one of the most advanced arts of technology. 5 stars isn't enough for this article, because this is not only a compilation of literature but also a proper simulation project. You have chosen an already well designed car and further optimized it, Kudos

Would you like to comment on aerodynamics of SUVs. Is there any scope for improvment or are they just like helicopters without rotors. Maybe it is too much to ask for, I would appreciate, if you can compare any popular car model from our market and suggest improvement, because people can relate to it.

[MegaWhat]

Originally Posted by alpha1

But the effect is more pronounced in the aft side where your aim is to not let the boundary layer get separated from the body. Hence the long trailing edge offers more returns per meter compared to a long leading edge.

Spot on. That is why the airfoils you see on most subsonic aircraft will be like an elongated teardrop. Or you could imagine it like two teardrops with their heads superimposed, but the the sharp point at the front is slightly rounded and not kept too pointy (refer image below). This makes a difference for aircraft where not just the lift and drag, but the rate of change of lift and drag with change in angle of the aircraft plays a major role. There is a lot more aerodynamics involved in aircraft beyond this though.

A symmetric airfoil

Profiles with sharp front and sharp aft edges are more efficient at supersonic speeds or close to Mach1. That has more to do with shock waves and area-rules. The wings on most fighter aircraft will have a slightly sharper front edge (also called leading edge) as compared to an aircraft that flies well below the speed of sound.

Originally Posted by Jeroen

I would like to think there is a bit more science to dealing with side wind. It is not just natural wind, but also buffeting when you for instance drive into a tunnel, or when a large truck comes on to you on the other side of the (narrow) road...

...How it deals with it is probably more down to suspension, weight distribution and so than perhaps aerodynamics. Although I guess the shape and size of the car side surface might play a role as well

Oh yeah I had forgotten about the buffeting. That effect can be mad! Basically it is just your car getting sucked into the low pressure region behind a truck, or your car getting pushed by the high pressure region ahead of a truck. I'm not sure what the car companies do to account for it though. I'd think it comes down to suspension as well. A tunnel might not have much of an effect on a car I think, considering the relative sizes. We might not see a car entering a small tunnel at extremely high speeds, and the big tunnels are quite big. A train is another thing. The Japanese Shinkansen train was actually inspired by the Kingfisher (which goes from air into water without much loss of speed) and they have designed the front of the train to give low drag at high speeds and enter tunnels without issues.

Originally Posted by Thermodynamics

Would you like to comment on aerodynamics of SUVs. Is there any scope for improvment or are they just like helicopters without rotors. Maybe it is too much to ask for, I would appreciate, if you can compare any popular car model from our market and suggest improvement, because people can relate to it.

I actually started out looking for decent virtual models (CAD models) for Indian cars, but unfortunately did not find anything that I could work with. There are models of a few international vehicles, but nothing that can be seen on Indian roads. If someone here can help with scanning a car and generating that model, nothing like it . I'll still keep looking though.

Coming to SUVs, I think its a matter of what is the design intent for that vehicle. SUVs in general are not supposed to go that fast. And if we put a front lip, it will adversely affect it's off-road capabilities. Coming to Land rovers and other SUVs that CAN go fast, I only see a basic spoiler, but I'd like to explore that as well. Will update here If I can get my hands on a good CAD model.

[Sutripta]

Originally Posted by MegaWhat

Oh yeah I had forgotten about the buffeting. That effect can be mad! Basically it is just your car getting sucked into the low pressure region behind a truck, or your car getting pushed by the high pressure region ahead of a truck.

Buffetting - Anything to do with vortex shedding?

Everyone knows of slipstreaming to improve speed etc. Never found discussed what happens to the object in front.

Sutripta

[Jeroen]

Originally Posted by Sutripta

Buffetting - Anything to do with vortex shedding?

Everyone knows of slipstreaming to improve speed etc. Never found discussed what happens to the object in front.

I was looking into this the other day when I was discussing slipstreaming as practiced by cyclist racing. I came across this explanation:

This is all about drag. The current answer by xpda is the correct idea, but I thought it might be worth going into a little more detail, to explain exactly why there is "no suction to speak of pulling back on it" (in fact, I wouldn't say there is "no suction"...rather, "less suction" would be more apt).

Drag results from a difference in pressure between the regions of air in front and behind the moving object in question. As air hits a single cyclist (let's say a female), it stagnates (slows down, resulting in high pressure) and moves around her. At real-world velocities and scales (where the Reynolds number is many orders of magnitudes greater than unity), the air is travelling too fast to remain attached over the cyclist's non-streamlined body. The air on each side is therefore not able to "meet-up" again at the trailing edge of the cyclist, as would be the case on a more streamlined object like an airfoil. Instead, the air on each side separates, forming shear layers (narrow streams of high-gradient velocity) downstream either side of the cyclist. Immediately after separation, these are separate from one another, but not far downstream they begin to interact (due to opposite-signed vorticity drawing them together). This results in a "wake" region behind the cyclist, where the flow is turbulent, eddies are plentiful, and the pressure is low.

When a second cyclist (let's say a male) is placed behind the first, the wake is affected. The presence of the new cyclist disrupts the interaction of the shear layers. A small amount of air will be drawn inward to form a mini-wake between the two cyclists, but the majority will continue to travel past the second cyclist (and may even re-attach to him before separating again), before interacting and forming a wake behind him. It is clear then, that the end result is mutually beneficial for both: the front cyclist has an unchanged high pressure region in front of her, but a "less negative" low pressure region behind her; the second cyclist has a far lower pressure region in front of him, but an (essentially) unchanged pressure region behind him. The difference in pressure between front and rear for each cyclist is therefore significantly lower for both. They both experience a lower drag than they would alone, and are more efficient together than the sum of their parts! Note the true picture has many more three-dimensional and transient wake effects in play (many are still yet to be fully understood and much research is going into this as we speak!), but the general idea holds.

However, the reduction in drag for the lead cyclist is likely to be less than that experienced by the rear cyclist. The lead cyclist still has unchanged stagnation pressure to deal with - which is a pressure coefficient of, say, around +0.5 (averaged over her front body). The pressure coefficient acting on the rear, however, rarely approaches negative pressures of that magnitude - it might be somewhere around -0.2 (these are all very rough estimates, but I feel the point is made better with numbers involved). With the cyclist behind her, that may rise to around -0.1. So the lead cyclist will be experiencing a pressure difference of around 0.6, as opposed to 0.7 normally. The second cyclist however, will have that stagnation pressure in front of him cut significantly from +0.5 to around, say, +0.1. Let's assume the pressure behind him is the same as if he were alone - around -0.2. The second cyclist is therefore now experiencing a pressure difference of only 0.3 - around half the pressure difference (aka half the drag!) of the lead cyclist! This rough calculation (I've simplified things significantly to get the point across) shows that it still pays to be the second cyclist!

Here is the link: https://bicycles.stackexchange.com/q...the-lead-rider

It has some more information and somebody conducting some interesting experiments and coming to the same conclusion.

But the Internet does not agree to this conclusion!

The lead rider in a pace line does not benefit from the drafting process. icists have suggested that a region of increased pressure forms ahead of the drafting rider, and this increased pressure region may assist the lead rider. To date, however, no measurable effect of drafting on a lead rider has been identi- fied.

https://performancecondition.com/wp-...Slipstream.pdf

I sort of think there might be an positive effect, but it is likely to be more theoretical than practical if it exists at all. But then again, I know very little about aerodynamics.

We are doing some work using 5G to allow trucks/cars to “platoon”. I am trying to get some information on this effect for the lead truck. Not sure we are looking at this aspect, rather than total fuel saved / congestions minimised.

Jeroen

[Sutripta]

^^^
Now we are getting into really esoteric territory like the flight of wild geese in formation!

Sutripta

[MegaWhat]

Originally Posted by Sutripta

Buffetting - Anything to do with vortex shedding?

Everyone knows of slipstreaming to improve speed etc. Never found discussed what happens to the object in front.

Sutripta

I wouldn't say vortex shedding, it's more about riding in the low pressure, low speed wake created by the object in the front.
If a car is being driven at 90kmph, the approaching air hits it at 90kmph and a corresponding high pressure region is created. Now, if the same car is being driven close behind another car, the air that hits it is at a much lower speed (thanks to the wake created by the car in front). So the region of high pressure created in front of the trailing car is lower leading to lower drag.

On the question of what happens to the leading car, I can think of the following:

1. If the trailing car is of the same size as the leading car, the trailing car will have significant advantage, while the leading car will have a minuscule - almost negligible - advantage. This is because the high pressure region in front of the trailing car slightly fills into the low pressure region of the leading car, reducing the pressure difference that the leading car experiences.
2. If the trailing car is much smaller than the leading car, the trailing car will have significant advantage, while having no effect on the leading car.
3. If the trailing vehicle is much larger than the leading vehicle - say a truck behind a nano - then the drag experienced by the leading car will reduce significantly as it's rear is surrounded by the high pressure region of a much larger body. Whereas the drag on the trailing vehicle will probably stay the same. I'm not sure about this, I'll have to read up a bit more. I think for the Nano, it'll be like riding a wave created by the truck behind it. Whereas the truck creates that wave irrespective of the presence of the Nano.

Originally Posted by Jeroen

I was looking into this the other day when I was discussing slipstreaming as practiced by cyclist racing. I came across this explanation...


...I sort of think there might be an positive effect, but it is likely to be more theoretical than practical if it exists at all. But then again, I know very little about aerodynamics.

We are doing some work using 5G to allow trucks/cars to “platoon”. I am trying to get some information on this effect for the lead truck. Not sure we are looking at this aspect, rather than total fuel saved / congestions minimised.

Nice link! Thanks! I agree. There could be a positive benefit for the lead truck, but practically negligible. In your convoy of trucks I don't think there's any additional cost/fuel consumption that the lead truck will have to pay.

Originally Posted by Sutripta

^^^
Now we are getting into really esoteric territory like the flight of wild geese in formation!

Oh man that's fun. I don't know how the birds evolved to identify that as the most efficient formation to fly long distance. There were some studies done on this and it seemed like each trailing bird uses the vortices generated by the flapping of the leading bird. The distances - sideways and in front - are key. Good topic to read up on - weekend fodder for me, thanks! .

[venkyhere]

Excellent thread, MegaWhat.

Perhaps you can include the following as well into your simulation model and show us the results :

1) importance of flat underbody - reduces Cd and Cl in one stroke
2) why the side running board areas are extended as low as possible - Gordon Murray's road scraping F1 car can be used as an extreme example
3) how front airdam is different from front splitter
4) wheel air curtains
5) wheelwell air removers
6) separation edges in rear bodywork

[MegaWhat]

Originally Posted by venkyhere

Perhaps you can include the following as well into your simulation model and show us the results :

1) importance of flat underbody - reduces Cd and Cl in one stroke
2) why the side running board areas are extended as low as possible - Gordon Murray's road scraping F1 car can be used as an extreme example
3) how front airdam is different from front splitter
4) wheel air curtains
5) wheelwell air removers
6) separation edges in rear bodywork

Firstly, apologies for such a delayed response @venkyhere. Work has been keeping me occupied more than usual lately.
Secondly, thanks for these points! I read up about these design aspects in detail only thanks to your post! I won't be able to run simulations for each of these situations at this point; maybe sometime later. But I'll share my comments based on some basic reading.

1. Importance of flat underbody.
A rough/cluttered underbody basically makes the flow under the car rather chaotic. It's easier for the air to flow over a smooth and flat surface as compared to around pipes, ridges, tubes and suspensions. The effect on drag is rather straightforward. Smoother flow over a smooth flat underbody will end up reducing drag (as it disturbs the air less). Coming to lift though, it's got two steps.
First, as the oncoming air is effectively "pinched" into the underside of the car, it's speed increases. This has something to do with the conservation of mass. For example, whatever mass of water goes into a pipe, must also come out - even if the outlet diameter of the pipe is lesser than the inlet diameter.

Say we have a one-way, 4 lane road and a camera mounted on the roadside at a fixed point. Let's assume that the cars on the road are distributed uniformly across all lanes. Say the camera counts that in one minute, there are 50 cars that cross it. Now, if the road becomes a one-way 2 lane road, and if there still have to be the same 50 cars per minute "traffic flow rate", every car will have to speed up to meet that "traffic flow rate". If we have a situation where a 4 lane road converts into a 2 lane road, the maximum "traffic flow" is governed by the choking at the 2 lane road.

So when the shape of the car forces a fairly big chunk of air through a small gap like the underside, the speed of the air increases. For a smooth underside, this increased speed leads to lower pressure (under the car) and hence, a reduction in net lift. If the underside is cluttered with pipes, these present an obstruction to the air which reduces it's speed instead of increasing it. That's why a smooth underside can help reduce lift by a measurable margin.

2. why the side running board areas are extended as low as possible
As mentioned above, the pressure under the car is slightly lower than the ambient pressure due to the faster air. Due to this, the air on the sides of the car can tend to flow towards this underside (flowing from high pressure to low pressure region) leading to some more vortices, disturbance and hence higher drag. The side skirts/running boards help in isolating this low pressure region from the high pressure region.

Here's a good representation from a paper:

(Ref: https://www.researchgate.net/figure/...fig9_228616843) Effect of side skirt to ground gap clearance on vehicle’s total downforce coefficient. (From Wright 1983.) (Note that the underbody diffuser is called “venturi” in this sketch.)

3. how front airdam is different from front splitter
An air dam basically blocks the air from going under the car and routes it over the car instead. The front splitter actively produces downforce due to the high pressure region created by the air dam on top of the splitter and a lower pressure region under the splitter. Here's a website that explains it rather well: https://nasaspeed.news/tech/aero/air...reduce%20drag.:

An air dam can be one of those magic improvements, increasing downforce and reducing drag at the same time. It can also improve engine cooling by increasing the air flow rate through the radiator. An air dam reduces drag by reducing the rate of air flow under the car, which reduces drag caused by all of the protrusions and cavities under the car. Everything that you see under the car is a drag source, so hiding that mess behind an air dam is a simple way to reduce drag.

An air dam can increase downforce by reducing the average air pressure under the car. Maximizing downforce from an air dam requires its lower edge to be very near the pavement, so it has to be flexible to survive in the real world.

A basic air dam

A splitter is a horizontal shelf mounted under the nose of the car, or to the bottom of an air dam, with the top side of the splitter sealed to the body. A splitter produces downforce from the difference in air pressure on the top and bottom surfaces of the splitter area. Because airflow over the top is blocked by the body or air dam, the local airspeed is low and the air pressure on top is high. Because air can flow under the splitter freely, the local airspeed under it is high and the air pressure on the bottom side is low.

More splitter area produces more front downforce, but only up to a point. The effectiveness of adding splitter area falls rapidly as it extends away from the body. That is because the difference in airspeed between the top and bottom sides decreases with distance away from the body. The useful length is 4 to 5 inches. The downforce that a splitter creates changes with height from the ground. A small increase in downforce can be had by extending the splitter behind the air dam or nose, and gently curving the trailing edge up to create a short but wide ground effect tunnel.

4. wheel air curtains
The airflow around rotating wheels is quite chaotic and can lead to a lot of drag by itself. In addition, this disturbed air can slow down the air that is attached to the sides of the car and lead to an early separation there as well (leading to more drag). A wheel air curtain basically takes some high pressure air (from the front) and routes it around the wheel so as to "energise" the flow that eventually attaches on to the side of the car. This helps reduce drag by ensuring the air stays attached on the car door. I found a good explainer on youtube for this. This guy has some pretty neat videos and performs a few tests with tuft visualizations to understand the impact of various aerodynamic features. It is worth a watch.

https://www.youtube.com/watch?v=piWgv60DTSY

5. wheelwell air removers
At first glance, it seems to be a way to evacuate the chaotic air behind the wheel, but I'm going to need to look into this in a little more detail. Do you have a particular image or something that you can share as an example?


6. separation edges in rear bodywork
These are essentially sharp edges used to do the following:

a. Ensure clean separation of flow at the same point on the car at different speeds
b. Help to control and minimise the size of the wake

Here's another video that explains this well:

https://www.youtube.com/watch?v=MtJs...w5fDz&index=21

[venkyhere]

Originally Posted by MegaWhat

5. wheelwell air removers
At first glance, it seems to be a way to evacuate the chaotic air behind the wheel, but I'm going to need to look into this in a little more detail. Do you have a particular image or something that you can share as an example?

Yes, the vents 'above/behind' the wheel wells remove the air that is 'churned' by the rotation of the wheel, particularly at high speeds. Why is this 'discordant mess' of air inside the wheel well so bad, that it has to be removed ?

It creates a high pressure zone within the wheel well, whilst above the wheel well there is a low pressure zone due to fast flowing 'unhindered' air. This creates needless lift - or in other words, sullies the effect of the splitters in the front, which are busy trying to provide downforce in the front.
This problem gets especially magnified, if there are 'cooling vents' for the front brakes on the bumper, because now even more air is in this 'churning zone' between the tyres and the wheel well.

Which kind of cars have this kind of 'air removal from wheel well vents' installed most commonly ?
WRC.
Take a look at this very educative post , some excellent pictures are shown

[MegaWhat]

Got a chance to run a car simulation after an extremely long time. Turns out, the procrastinator in me takes over, as the amount of work that needs to be done increases .

This time - thanks to @Shreyans_Jain and @Thermodynamics for their request

Originally Posted by Shreyans_Jain

...Will be great if you can do a similar study with an SUV. The differences will be very interesting...

Originally Posted by Thermodynamics

..Would you like to comment on aerodynamics of SUVs. Is there any scope for improvement or are they just like helicopters without rotors...

- I scouted for a model of an SUV. Unfortunately (again) I did not find a good enough model for an actual SUV that is widely sold in India. However, I thought maybe a compact SUV might do for now, just for comparison sake. Thankfully, I did find a decent model of a fourth generation Kia Sportage that I could use without much of a hassle. And although it isn't a car that is currently sold in India, we could see it in the market sometime soon. In any case, it seems like a good SUV-esque geometry to compare the Tesla's aerodynamic performance to.

The fourth generation Kia Sportage

Simulation setup:

The simulation for the Sportage was setup exactly like that for the Tesla Model S. This is done to minimize the variables and assumptions that go into CFD simulation, thereby maintaining an acceptable level of accuracy and the ease of doing an apples to apples comparison.

Comparing the models for Sportage and Model S

Results:

Let's jump into the results.

• The Sportage has a drag coefficient (Cd) of 0.41 and a lift coefficient (Cl) of 0.09.
• The Tesla Model S (with an OEM type trunk lip spoiler attached) has a Cd of 0.25 and a Cl of 0.18.

This means that the Sportage experiences ~64% higher drag and ~49% lower lift as compared to the Model S with the trunk lip spoiler.

Here are some of my thoughts:

On lift:

Cl vs. speed comparison for the Sportage and Model S



Comparing streamlines along the length for both cars

1. The Sportage has less than half the lift as that of the Model S, and this is a good thing. We do not want a compact SUV to feel any more floaty at high speeds than what it already does.
2. It is positively surprising to see such a low Cl despite a sloping roofline and that contoured spoiler. This implies that both these design features are meant for reducing drag and for aesthetics - or maybe something else - but definitely not to reduce lift.
3. If we look at the streamlines in the figure above, we clearly see that the air tends to flow downwards as it flows over the Model S, whereas it does not tend to shift as down with the Sportage. The chaotic flow in the wake of the Sportage is quite evident.

On drag:

Cd vs. speed comparison for the Sportage and Model S



Drag creating regions around the Sportage



Comparison of drag creating regions in the wake of the Sportage and Model S

1. A sedan by itself tends to have lower drag as compared to an SUV. This is because the shape of the rear glass and the boot of a sedan help in keeping the air attached to the car, thereby reducing the volume of separated air. As separated air is essentially lost energy, an SUV - which has a rather flat, non streamlined rear - ends up experiencing higher drag than a sedan, simply because it generates a larger volume of separated air.
2. As discussed in previous posts, lift and drag scale as the square of velocity. This means, that the drag experienced by the Kia is going to scale up quite rapidly with speed. For example, at 120 kmph, the Sportage experiences a drag of 64 Kg, while the Model S experiences a drag of 41 Kg. Speed it up to 150 kmph, and the Sportage experiences 100 Kg of drag while the Tesla undergoes 63 Kg.
3. If we look carefully at the drag generating areas on the Sportage, we see that the wheels and the A-pillar contribute a fair chunk to the overall drag as well and it is not just the wake at the rear. In fact, the design of the fog lamps - where the lamps are embedded into a small cavity within the bumper, right in front of the wheels - end up disturbing the air right before it interacts with the wheels, leading to compounding of those energy losses and more drag.
4. The mirror has it's own fairly big wake.

On power required to overcome drag:

Power required to overcome drag vs. speed for Sportage and Model S

1. As Cd for the Sportage is 64% more than the Model S, at all speeds, it will consume 64% more power than the model S.
2. As the power required to overcome drag is fucntion of the cube of the velocity, it scales up significantly as the speeds increase. For example, the Sportage needs 28 HP to overcome drag at 120 kmph while the Tesla needs 18 HP. Increase the speed to 150 kmph, and the Sportage needs 55 horses to overcome drag as compared to 35 HP needed by the model S.
3. However, the base petrol model of the Sportage had 120 HP, which is more than enough to pull the car through air at the speeds it is designed for.

Other observations:

Let's look at the three dimensional streamlines over both cars.

Vortices in wake of both cars

It is interesting to see that the Model S has a total of two vortices in it's wake - one to the left and another to the right of the car. The Sportage has a total of 4 vortices in it's wake - two on each side. I'm guessing this is also good in a way because 4 small vortices will tend to consume less energy than two large vortices, but I could be wrong here by a large margin.


Overall, the Sportage is designed to be a mini SUV and not an ultra aerodynamic electric sedan. The aerodynamic requirements of this car are significantly different than that of a Model S and it seems to address them well enough. For example, even if the Cd is quite high, the car is capable of pulling through even at high speeds. The power requirement here is going to be mostly governed by many other factors and not aerodynamics. It is also good to see the Sportage have a rather low Cl, despite the sloping roof. It seems the makers have attained a sufficiently good balance between Cl and Cd for the Sportage.

Assumptions for the simulation:

1. Speed = 90 kmph. Cl and Cd are calculated based on forces experienced at this speed. Once calculated, Cl and Cd are more or less independent of speed. If we simulate the car at 120 kmph and recalculate Cl and Cd at that speed, these will come out to be same because the forces also scale up with speed accordingly.
2. The ground under the car is assumed to move back at 90 kmph as the car moves ahead at that speed.
3. Wheels do not rotate. This is a big assumption. Rotating wheels usually reduce the drag by a small amount.
4. Wheels are assumed to be cylinders. A rim will have a different impact on drag.
5. No flow through the grille.
6. The underbody region is completely flat. No details at all.
7. Panel gaps not simulated.

I'll keep adding some runs as and when I find the time and models to do so. Fingers crossed I do it frequently.

[venkyhere]

Originally Posted by MegaWhat

Got a chance to run a car simulation after an extremely long time. Turns out, the procrastinator in me takes over, as the amount of work that needs to be done increases .


I'll keep adding some runs as and when I find the time and models to do so. Fingers crossed I do it frequently.

I have one request to make, w.r.t your tesla v/s sportage CFD sims.
It is regarding the ORVMs.
To your existing models (as is), how hard is it for you to make an "edit" and add some shapes/objects to the existing bodywork ?

If it's easy, can you try the following :

tiny hemispheres of 2/3 mm radius, stuck to the outer boundary of the ORVM casing, the gap between neighboring hemispheres being some 4/5 mm (like a series of halogen bulbs around a film celebrity's make up mirror) around the periphery of the casing, right at the edge where air leaves the ORVM surface and goes berserk in its wake.

I am particularly interested in the difference (between pressure levels revealed by simulations) in "waves of air" hitting the front window glasses, both before & after the mod suggested above. The intensity and frequency of these "waves of air coming off the orvm wake" and hitting the front window glasses, is what I think is the major contributor to wind noise (in stock form, without any rain visor etc installed).

Kindly help me with this.

[Hayek]

What a fantastic thread!! The first three posts are a master class in how to explain a complex topic in a manner that is comprehensible for a layman.

Was surprised that the difference in drag coefficient between a Model S and a Kia SUV is as little as what you have found. I somehow had the impression that an SUV would have twice the drag of a streamlined sedan. Was also surprised to learn that the power needed to overcome drag rises with the cube of the speed. I had always thought of power in terms of acceleration based on Newton’s laws - which rises with the square of the speed. This explains why fuel efficiency in SUVs tails off so quickly with speed.

To your point on the value of smooth underbodies, I wonder what the impact of the difference between a typical Indian road (usuallly cement concrete with loads of patches and undulations) is compared to smooth Tarmac. So we probably suffer a double negative impact - from the larger ground clearance needed to tackle our road surfaces and the lack of smoothness of the roads.

Look forward to seeing more simulations from you on this thread.

[Thermodynamics]

Thanks for the analysis, indeed it justifies why some of our SUVs have a decent fuel efficiency.

Hayek has aptly said all what I had to say. Amazing thread!. Really appreciate your efforts, I guess it takes days to run every scenario.

What next, a convertible ?