[Jeroen]

Thanks very much for all your detailed work, really outstanding and much appreciated.

A few questions. Why is drag/lift expressed in kilograms? Should that not be Newton’s?

On the graphs on lift/drag vs vehicle speed. Is the nett effect the drag minus lift or is that too simple way of looking at it?

Just wondering because I assume that on the graph aerodynamic power vs vehicle speed, I assume both drag and lift are included? Or am I making a mistake here?

The aeroydnamic power of both these cars at 100 km / h is very low! Only about 16-18 HP! Other than rolling resistance and drive train losseswhat else should be taken into consideration to get to the BHP required from the engine to keep these cars going at 100 km /h?

I always thought that the drag, at speed, would be considerable larger than other forces that the engine needs to overcome. So the total would not be that much higher as what your required aerodynamic power requirement shows. So it seems a very low number, which should give you an incredible efficient cruising speed of 100 km / h, or range for the Tesla.

Thanks again for all your efforts!

Jeroen

[venkyhere]

taking a stab at some of these Qs

Originally Posted by Jeroen

Why is drag/lift expressed in kilograms? Should that not be Newton’s?

as you can guess, it's just a convention to provide a relatable image in the reader's mind. The 'X kg force' implicitly means X * 9.8 m/s2 Newtons.

Originally Posted by Jeroen

On the graphs on lift/drag vs vehicle speed. Is the nett effect the drag minus lift or is that too simple way of looking at it?

Just wondering because I assume that on the graph aerodynamic power vs vehicle speed, I assume both drag and lift are included? Or am I making a mistake here?

are you referring to the lift influencing drag ? ie, with lift, overall GC will increase, increasing the quantity of air flowing (if there was no lift) which might affect drag in turn ? are you asking whether the CFD simulation independently measures lift and independently measures drag, compared to when both are measured simultaneously in a wind tunnel ? In other words is your question "since the two forces are at 90 degrees, can we trust superposition principle to hold good ?"

Originally Posted by Jeroen

The aeroydnamic power of both these cars at 100 km / h is very low! Only about 16-18 HP! Other than rolling resistance and drive train losseswhat else should be taken into consideration to get to the BHP required from the engine to keep these cars going at 100 km /h?

I always thought that the drag, at speed, would be considerable larger than other forces that the engine needs to overcome. So the total would not be that much higher as what your required aerodynamic power requirement shows. So it seems a very low number, which should give you an incredible efficient cruising speed of 100 km / h, or range for the Tesla.

my back of the envelope calculations used to reveal almost the same - that for a decently aerodynamic car (unlike a brick like SUV) , for it to cruise at a steady 100 kph, somewhere around 25hp will be the engine output at crank, from which, some 80-85% will be translated to actual power needed to fight the aero drag. The real usage of bhp/torque is to accelerate the weight forward. Unless it is something like 150+ kph steady speed, the engine doesn't need to generate much power to keep the car moving at constant speed.

[MegaWhat]

Originally Posted by venkyhere

...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...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.

That should be an interesting run! The mirror surely has a role to play with noise, but I think (especially in the Sportage case) the flow near the A-pillar is going to play a much bigger role. If you see the drag generating areas on the Sportage, you'll see a fairly strong local vortex created at the A-pillar. This will lead to a lot of air buffeting against the front windows which is going to drown out the noise generated by the mirror.

In general, when it comes to component level analysis like hemispheres on the mirror, we tend to perform a component level simulation and focus only on the mirror. This is done because the size of the car is significantly big as compared to the size of the hemispheres, so the difference in performance caused due to the hemispheres is tougher to identify on a car level simulation. It can be done, but it simply needs a lot more computational power. I'll see what I can do, but can't commit .

Originally Posted by Hayek

Was surprised that the difference in drag coefficient between a Model S and a Kia SUV is as little as what you have found.

Thanks a lot for your kind words! Although the Cd is rather close between the two cars, the Cd for the Sportage is still 1.64 times that of the Model S. The Sportage is rather a mini-SUV than a full fledged SUV. I'm sure that the drag for an Endeavour or a Fortuner is much more than that for the Sportage. But it comes down to what each of these cars is designed to do. Also, the Sportage is a relatively streamlined mini-SUV; in the sense that Kia seems to have actively given a thought to it's drag coefficient.

Was also surprised to learn that the power needed to overcome drag rises with the cube of the speed.

If you dig only a little bit deeper, you'll see that the Power being proportional to cube of the speed is quite in line with Newtons laws for rigid bodies. For example, according to Newtons laws, the energy (E) taken to move a body by a distance 's' is given by E = F * s. Where F is the force it takes to move the body by that distance. Power (P) is defined as energy consumed (or generated) per unit time (t). So in this equation, P = E/t = F * (s/t). Now, 's/t' is distance divided by time, which is speed (V). So P = F*V. In the aerodynamic sense, the force here that provides a resistance to motion is drag (D). Hence Power needed to overcome this resistance (Pd) is Pd = D*V. And drag itself scales as a square of velocity. Hence, power turns out to be proportional to V^3.

...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...

The roughness of the road will not make much difference because the road is stationary relative to the air. Only the car is moving, so the air is moving over the car, while the car moves ahead (relative to the air and the road). In fact, in the simulations, we model the road to move backwards (in direction of the air) because the car is moving relative to the road as well.

Originally Posted by 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 ?

Indeed, it's good to see a fairly acceptable Cd for a car of this size. It takes a a few days to prepare the model, run the simulation and post-process the results. Lately, the bigger question is of finding those few days .

Originally Posted by Jeroen

A few questions. Why is drag/lift expressed in kilograms? Should that not be Newton’s?

@Venkyhere got it right. It's only a matter of units. I used Kg because I assumed that non-engineering audiences can relate to Kg more easily as compared to Newtons. But equation wise, yes, all equations are written in the SI form.

On the graphs on lift/drag vs vehicle speed. Is the nett effect the drag minus lift or is that too simple way of looking at it?

Just wondering because I assume that on the graph aerodynamic power vs vehicle speed, I assume both drag and lift are included? Or am I making a mistake here?

Theoretically, the lifting force is acting perpendicular to the direction of motion and does not resist motion as such and hence does not consume any power. Which is unlike aircraft, where we have a lift induced drag component in the total drag. In general, for a car, the lift does not take much power to overcome. If we look at dynamic phenomena like @Venkyhere mentioned, there surely is a coupling between the two. But that should not make much of an impact when it comes to driving design decisions on a regular passenger car. For F1, I'm sure they must be taking it into account.

The aeroydnamic power of both these cars at 100 km / h is very low! Only about 16-18 HP! Other than rolling resistance and drive train losses what else should be taken into consideration to get to the BHP required from the engine to keep these cars going at 100 km /h?

Indeed! 100 kmph is - aerodynamically speaking - rather slow. So the drag forces are also relatively low. It is just that the power needed to cruise at higher speeds increases quite rapidly as one goes faster in their cars. Again as @Venkyhere mentioned, most of the energy goes in accelerating the vehicle. At cruise, the power requirement is quite low - unless you are going quite fast.

To add another point, the simulations I have run are on relatively clean geometries. No underbody pipes/manifolds, no flow through the hood, stationary wheels, no panel gaps and what not. The real case Cd will be slightly different than what is estimated here.

Originally Posted by venkyhere

taking a stab at some of these Qs

...The real usage of bhp/torque is to accelerate the weight forward. Unless it is something like 150+ kph steady speed, the engine doesn't need to generate much power to keep the car moving at constant speed.

Thanks a lot for your inputs @Venkyhere!

[Pam81]

First of all, a big thank you for taking time and getting serious with a intricate but very time consuming subject.

Originally Posted by MegaWhat

Theoretically, the lifting force is acting perpendicular to the direction of motion and does not resist motion as such and hence does not consume any power. Which is unlike aircraft, where we have a lift induced drag component in the total drag. In general, for a car, the lift does not take much power to overcome. If we look at dynamic phenomena like @Venkyhere mentioned, there surely is a coupling between the two. But that should not make much of an impact when it comes to driving design decisions on a regular passenger car. For F1, I'm sure they must be taking it into account.

Very apt explanation. I think, from a theory perspective, the passenger car traction limits usually do not get challenged by the lift they will be subjected to pertaining to their geometrical proportions (spoilers / disruptions / diversion of flow patterns). Design decisions are usually for max speeds considering Indian conditions, a spoiler, most of the times is kind of a beautification only.
For a high speed racing car or Porsche alikes, the lift @ speeds excess of certain speeds over 100 kmph, does produce a loss in traction with road. The keep it planted, sometimes the spoilers are even having raise up design which increases the downward weight at the rear and limits the reaction of lift.
you can see it the adaptive aerodynamics here if you are interested https://www.autoblog.com/2019/02/21/...ve-aero-video/

Originally Posted by MegaWhat

Indeed! 100 kmph is - aerodynamically speaking - rather slow. So the drag forces are also relatively low. It is just that the power needed to cruise at higher speeds increases quite rapidly as one goes faster in their cars. Again as @Venkyhere mentioned, most of the energy goes in accelerating the vehicle. At cruise, the power requirement is quite low - unless you are going quite fast.

well said. upon overcoming inertia and having gained momentum, if a steady state speed (cruising) is what one is after, the engine will be smoothly operating in a lazy zone. Much of mechanical stuff happens when you push it for more torque which is where resistances from every aspect need to overcome (air, traction, weight, gear ratios, etc...).

Vehicle Dynamics is indeed an extremely interesting subject. Thanks again MegaWhat for your hobby (if you'd allow me to call it that way) and posting the results here!!!!