Diffusers non flat underneath.
Discussion
Mr Whippy said:
monthefish said:
Mr Whippy said:
I'm not sure a flat floor is essential to the diffuser concept really.
It is.monthefish said:
In basic terms, for the diffuser to work correctly the airflow that approaches it needs to be fairly laminar (smooth/uninterrupted) and this can only really be achieved by a flat floor.
It will be pretty challenging to get good pressure recovery by trying to accelerate a load of separated air with low total pressure, which is what you would have if you didn't have a flat floor. I imagine you could get some effect without a flat floor, but it would be far less effective than with a flat floor.RenesisEvo said:
monthefish said:
In basic terms, for the diffuser to work correctly the airflow that approaches it needs to be fairly laminar (smooth/uninterrupted) and this can only really be achieved by a flat floor.
It will be pretty challenging to get good pressure recovery by trying to accelerate a load of separated air with low total pressure, which is what you would have if you didn't have a flat floor. I imagine you could get some effect without a flat floor, but it would be far less effective than with a flat floor.
I worked for an OEM as a Design Engineer responsible for the Exterior Trim (Spoilers, Bumpers etc etc ).
We looked at incorporating a diffuser onto one of our vehciles but as we did not have a fully flat floor (only the front 2/3 was covered by undertrays), the simulations showed that the effect of the diffuser would have been negligable, and we were only prepared to incorporate one if it was fully functional.
monthefish said:
Mr Whippy said:
monthefish said:
Mr Whippy said:
I'm not sure a flat floor is essential to the diffuser concept really.
It is.You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Note how many diffusers have fins in them to aid laminar flow, I guess to reduce drag. But it's not until the diffuser that you generate a negative pressure vs your surroundings. Up until the diffuser the air could be a real mess, and you tidy it up... or anything.
Not meaning to be really pedantic over it... just not essential in my view.
Dave
kambites said:
Pft, you asked for example of feeding a diffuser with air from somewhere other than under the car. The bit you quoted said nothing about the floor of the car. 
Fair enough, but I did ask for a production example, as in a regular car. (on one-offs anything is feasible/possible)Mr Whippy said:
But it's not really essential.
You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Note how many diffusers have fins in them to aid laminar flow, I guess to reduce drag. But it's not until the diffuser that you generate a negative pressure vs your surroundings. Up until the diffuser the air could be a real mess, and you tidy it up... or anything.
Not meaning to be really pedantic over it... just not essential in my view.
Dave
It's the floor that generates the downforce, not the diffuser. Use air solely from the top of the car and you'll achieve sweet FA.You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Note how many diffusers have fins in them to aid laminar flow, I guess to reduce drag. But it's not until the diffuser that you generate a negative pressure vs your surroundings. Up until the diffuser the air could be a real mess, and you tidy it up... or anything.
Not meaning to be really pedantic over it... just not essential in my view.
Dave
monthefish said:
Fair enough, but I did ask for a production example, as in a regular car. (on one-offs anything is feasible/possible)
Give it a year or so and we'll have road cars blowing their exhaust gas into the diffuser. It wont do anything, but it'll be fashionable because they did it in F1. PhillipM said:
It's the floor that generates the downforce, not the diffuser. Use air solely from the top of the car and you'll achieve sweet FA.
I don't think that's strictly true - a diffuser in itself can generate down-force as long as it has air moving under it. A situation where the air is flowing from a confined space into a wider one is going to develop increasingly low air density and hence increasingly low pressure underneath whatever is diffusing the air. kambites said:
I don't think that's strictly true - a diffuser in itself can generate down-force as long as it has air moving under it. A situation where the air is flowing from a confined space into a wider one is going to develop increasingly low air density and hence increasingly low pressure underneath whatever is diffusing the air.
Yes, but the air pressure change is pretty low, hence the way to get it working a decent amount is to use the diffuser to develop that under the entire surface area of the floor, which, being so large, makes a small change much, much more effective.A diffuser on it's own as a single entity isn't particularly useful.
Feeding the diffuser with exhaust gas is mainly to accelerate the main flow under the floor, and to seal the sides of the diffuser against the floor to give a greater expansion ratio and more downforce from the floor...
Edited by PhillipM on Tuesday 2nd August 14:27
monthefish said:
Mr Whippy said:
You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Not being argumentative, but I'd be intersted to see a (production) example of this...Who makes a diffuser but doesn't care about ground effect too?
I guess the perfect example is the good old blowing air between two pieces of paper. They close together because as the air velocity increases, and the paper prevents air from filling the void the air leaves behind it, the pressure drops, and so the air that tries to fill the void presses on the paper to try get to that low pressure area.
So, take it a step further...
Enclosed sides, pivot at red point, top rotates about pivot, bottom is fixed.
Dark blue is a jet of compressed air.
The jet of compressed air is not laminar, but it will tend towards laminar flow as it exits the funnel. Also, vanes in the tunnel would encourage laminar flow.
I'm fairly certain the door would have a tendency to close down as you pumped the air through the nozzle faster, to a point where the pressure exceeds the exhaust capability of the nozzle.
All a diffuser is, is an air accelerator, within a reasonable enough void (air can't get in to fill the space easily), to generate low pressure.
A flat bottom to a car isn't essential to that process. What is near essential is the ability to not leak air INTO the diffuser once it starts opening up.
Dave
monthefish said:
I worked for an OEM as a Design Engineer responsible for the Exterior Trim ...the simulations showed that the effect of the diffuser would have been negligable, and we were only prepared to incorporate one if it was fully functional.
Apologies for thread derailment, but do you or anyone you know have substantial experience of modelling ground effect aerodynamics? What I'm interested in, specifically, is modelling the interference/interface of a body operating in ground effect where the 'ground' surface is, in fact, water (...so the airflow being deflected by the body will in turn deflect the surface of the water to some degree).
Obviously the normal moving road wind tunnel has limitations in that the road surface is 'hard' and cannot be deflected by the airflow impinging upon it, so I'd be interested in any suggestions/experience as to how this can be more accurately modelled.
PM me if you prefer not to drift off-topic or respond on-forum.
PhillipM said:
Mr Whippy said:
But it's not really essential.
You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Note how many diffusers have fins in them to aid laminar flow, I guess to reduce drag. But it's not until the diffuser that you generate a negative pressure vs your surroundings. Up until the diffuser the air could be a real mess, and you tidy it up... or anything.
Not meaning to be really pedantic over it... just not essential in my view.
Dave
It's the floor that generates the downforce, not the diffuser. Use air solely from the top of the car and you'll achieve sweet FA.You can feed a diffuser with air from above the car, from a fan, or duct. It's just convenient to use air from under the car.
Note how many diffusers have fins in them to aid laminar flow, I guess to reduce drag. But it's not until the diffuser that you generate a negative pressure vs your surroundings. Up until the diffuser the air could be a real mess, and you tidy it up... or anything.
Not meaning to be really pedantic over it... just not essential in my view.
Dave
I thought it was air pressure differentials.
A flat floor isn't a pre-requisite for a pressure differential. If you can explain why it is a pre-requisite then I'm all ears.
Lets also re-iterate, we are talking diffusers here, not ground-effect?
Dave
I may have confused or over looked something, but, AIUI:
The diffuser aims to accelerate the air out from beneath the car, creating a region of low pressure under the car; given the higher pressure on the upper surfaces of the car, the net result is a force acting downwards - downforce.
With a flat floor, you have a large expanse of perfectly smooth surface, which not only gives nice clean flow to the diffuser, but also a nice big area to reduce the pressure over - a slight pressure drop, over a large area, adds up. Without a flat underside, the air ends up tripping over all the lumps and bumps, becoming turbulent and separated, which means the dynamic pressure will be much lower, so the effect of the diffuser won't be nearly as great, as the pressure recovery starts from a worse place. You can achieve a pressure reduction under a rough underside, and this will give downforce, but without a flat floor you are limited to what you can achieve because of the losses.
Perhaps the confusion is with regards to racing cars, which are often said to be using the 'floor' to generate downforce. In my experience the 'floor' refers to the collection of panels that mount beneath the tub/powertrain, which form both the top and underside of the floor. These surfaces may incorporate a diffuser. The aim is to get as high a pressure on the top of the floor, and as low a pressure on the bottom of the floor as possible, to give the biggest difference in pressure, therefore the most downforce. The more floor area you have, the more downforce you can generate.
The diffuser aims to accelerate the air out from beneath the car, creating a region of low pressure under the car; given the higher pressure on the upper surfaces of the car, the net result is a force acting downwards - downforce.
With a flat floor, you have a large expanse of perfectly smooth surface, which not only gives nice clean flow to the diffuser, but also a nice big area to reduce the pressure over - a slight pressure drop, over a large area, adds up. Without a flat underside, the air ends up tripping over all the lumps and bumps, becoming turbulent and separated, which means the dynamic pressure will be much lower, so the effect of the diffuser won't be nearly as great, as the pressure recovery starts from a worse place. You can achieve a pressure reduction under a rough underside, and this will give downforce, but without a flat floor you are limited to what you can achieve because of the losses.
Perhaps the confusion is with regards to racing cars, which are often said to be using the 'floor' to generate downforce. In my experience the 'floor' refers to the collection of panels that mount beneath the tub/powertrain, which form both the top and underside of the floor. These surfaces may incorporate a diffuser. The aim is to get as high a pressure on the top of the floor, and as low a pressure on the bottom of the floor as possible, to give the biggest difference in pressure, therefore the most downforce. The more floor area you have, the more downforce you can generate.
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