How does a diffuser/ride-height create drag?

[ringo]

You got to be careful with your equation application. The force you got there has to be in the direction of the velocity.
A diffuser is basically a pipe, just a flared one, what i was saying is that a diffuser does not consume work, just as the same as a pipe with water flowing through it does not consume work.

Does the diffuser takes this 10hp and turn it to heat energy? motion? electrical?
No moving parts inside the diffuser so no work being done.

To illustrate, a house roof can behave like a diffuser when wind blows over it, but it does not consume power because it doesn't move.

If you increase the system size that considers the whole car and some volume around it, then it's possible to say work is being done by the air to compress the springs and lower the car by a certain distance.

from wiki

A diffuser is the mechanical device that is designed to control the characteristics of a fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the fluid's velocity and to enhance its mixing into the surrounding fluid. In contrast, a nozzle is often intended to increase the discharge velocity and to direct the flow in one particular direction.

Flow through nozzles and diffusers may or may not be assumed to be adiabatic. Frictional effects may sometimes be important, but usually they are neglected. However, the external work transfer is always assumed to be zero.

[hecti]

PDEs are where the fun starts

I have my PDE Exam on the 28th.... its not gonna be pretty

[xxChrisxx]

Diffusers are better thought of as 'downforce enhancers' as opposed to downforce generators. As a system it doen't make sense to simply look at the diffuser and ignore all other aero appendages.

They amplify the effect of wings. So a layman answer to 'why do diffusers create drag' is induced drag.

Endplates work as they atifically increase the aspect ratio of a wing. Wings with larger aspect ratios are more efficient. They also have a double whammy effect that it stops the high and low pressures on either side on an aerofoil from flowing round the edge and interacting.

And thats pretty much all I can remember from my aero lectures. So if some minor detail, or indeed all of it is wrong don't bitch at me.

[xpensive]

@ringo, Everything moving consumes energy, even the wind blowing over that rooftop does. Power is volumetric flow times pressure differential, that will never change.

Give me a hand kilcoo?

[BreezyRacer]

OK, here's the deal in simple form .. DF is NOT drag .. turbulence is drag.

A body traveling thru air creates turbulence, especially behind it (the wake).

A diffuser, at low angles REDUCES wake turbulence, by introducing airflow from the bottom of the car into the wake, thus reducing drag.

A diffuser at high angles, creates more DF but also increases turbulence, in the form of vortexes. These vortexes are extremely powerful and increase diffuser created downforce significantly, so they are desirable for DF creation. But there is a drag penalty for them.

So you can have a low angle diffuser on something like a Toyota Prius and will reduce drag. It will create very little but some DF, and no vortexes. Then, you can also have a diffuser like on an F1 car, and that will certainly contribute to drag thru large vortexes. It will also be quite effective at producing DF relative to drag.

Understand though, diffuser downforce is quite low in drag, when compared to rear wing DF to drag. Much of the increase in drag is due to increased frontal area, but also the same vortex creation as large angle diffusers create.

And like a diffuser, front wings also have very little drag relative to DF, since they do not increase frontal area. They do of course create vortexes, and thus some increase drag, all other things being equal, which they almost certainly never are.

Class over ..

[xpensive]

I disagree, downforce from the floor comes from an increased air-speed, nothing else, while you can't get incresed air-speed under the car from nothing, where power is still volumetricc flow times pressure differential there has to be a power source somewhere.

Kilcoo?

[BreezyRacer]

I disagree, downforce from the floor comes from an increased air-speed, nothing else, while you can't get incresed air-speed under the car from nothing, where power is still volumetricc flow times pressure differential there has to be a power source somewhere.

Kilcoo?

The power source you make reference to is already created and paid for, drag-wise, in the form of vehicle wake. The wake energy (low pressure you created when you pushed the car thru the air) is what speeds the airflow under the car, when linked to underbody airflow, which is what a diffuser does .. it links the low pressure wake energy to the underbody airflow.

[Just_a_fan]

So you have to have a turbulent wake to increase DF, well what do you know.

All vehicles have a turbulent wake of some form anyway. The diffuser just makes use of that wake to do something useful.

[kilcoo316]

I disagree, downforce from the floor comes from an increased air-speed, nothing else, while you can't get incresed air-speed under the car from nothing, where power is still volumetric flow times pressure differential there has to be a power source somewhere.

Kilcoo?

The simple equation for turbojet thrust is MF * (V2-V1)

MF = Massflow rate (kg/s)
V2 = Outflow speed (m/s)
V1 = Inflow speed (m/s)


Normally (for aircraft engines) V2 is greater than V1.


In the case of a diffuser, V2 will be less than V1. That is a decent start for approximating the drag of the diffuser itself.

Now, to (approx) estimate the drag of the floor as a whole, you would let V1 = Vfreestream and keep V2 = diffuser exit speed. (Note speeds and not velocities - neither speed nor axial velocity gives the 100% correct answer, but speed is closer for this quick and dirty estimate)

[ringo]

Heh heh, i think you guys are using the wrong equations. Try the same thing with a pipe or an air compressor nozzle and see what happens.
Air compressor speeds are very high and are require power to displace the air. The power is coming from the compressor pump. The nozzle you put at the end of the hose does not consume energy in itself. The only form of irreversibility will be from friction due to the roughness of the nozzle material and the viscosity of the air.
The fact it increases the air speed does not mean it adds power to the air, where is it getting the power from?

This is why i asked if a diffuser takes power then a nozzle must supply power. A diffuser slows down and a nozzle speeds up.
Does speeding up the air mean a nozzle powers the air? That's the bombshell

[kilcoo316]

The fact it increases the air speed does not mean it adds power to the air, where is it getting the power from?

By back pressure.


It reduces the massflow rate that would have been ejected through a non convergent nozzle .


Don't forget, energy cannot be created or destroyed. KE = 1/2 * MF * V^2

KE = Kinetic energy
MF = massflow
V = velocity

[ringo]

There are viscous and friction losses but a diffuser cannot do work, and it cannot be worked on.

[PlatinumZealot]

Diffuser?

IT's just like a special pipe fitting. Coincidentally I am sizing a pump for a Cleaning in place system now and I have to find the system head loss using friction factor for various fittings.

An Ideal diffuser with an Ideal fluid won't have any power loss. There is a gain in static pressure.. but The total pressure and energy is the same. Diffuser and Nozzles don't do work.

For a real diffuser. There are frictional losses and viscous losses in the fluid. So the fluid itself loses energy. You have to experimentally determine the friction factor from the diffuser and then the behaviour of the fluid travelling through the diffuser will determine the loss in total pressure.

So it is the fluid that loses energy. If you make area around the diffuser your control volume. No work is done. The only energy is lost due to irreversibility in the fluid. to put more energy in the fluid you use a pump or something else (drive the car faster).

So you cannot calculate a power loss without doing an experiment to get the friction factor. And you can not use static pressure, you have to use total pressure because the velocity of the fluid is important to the diffuser. A normal comparison of cross sectional area won't give you the frictional power loss. An ideal diffuser with an Ideal fluid has no power loss. So equations without total pressure and terms for the loss due to friction and viscous losses won't work.

So you have the frictional and viscous power loss in the fluid. (not the car). How does this translate to a power loss for the car (other than skin friction)?
I think this is dependent on whatever pressure is behind the car vs the pressure in front of the car. Simple drag. So the diffuser itself causes the fluid to lose energy along with some surface drag. But I would say the major power loss is from the induced lower pressure behind the car.

Also the diffuser does it's best to bring the pressure back up. So a flat floor would cause more drag than a diffuser.
I may be wrong...just some thoughts.

[kilcoo316]

There are viscous and friction losses but a diffuser cannot do work, and it cannot be worked on.

It won't add or remove energy, but it will trade it to make the system more efficient.


It can accelerate and decelerate the flow.


You are almosty trying to say that a con-di nozzle on the back of a jet engine doesn't add anything.


Massflow is traded for velocity... or visa versa depending on what you do with the nozzle (which is what a diffuser is in this case - a divergent nozzle).

[xpensive]

Like kilcoo is xplaining in a little more advance way than I'm able to;
Speeding up the air under the car to 20 m/s more than the surroundings costs energy, one way or another, in this case a linearly moving diffuser will provide it.