Quote:
Originally Posted by Talos Woten
Ahh. If we are talking about abstract contiguous contours used as noses, then I completely agree with you that there are a wide range of different shapes that produce effectively equivalent drag.
But, if we are talking about real world cars, there is tremendous opportunity for improvement. The lead order of drag near the nose of a vehicle is the airflow through the radiator system. That has several distinct opportunities to decrease power loss:
1) The air inlet (say, reducing pressure / area that forces work over)
2) The radiator itself (say, by decreasing pressure drop / friction across it)
3) The airflow pathway (say, by using smooth ducting both in and out)
4) The air outlet (say, by matching steady state vector velocity and pressure)
Moreover, both theoretical calculations and actual mods racers make show this improvement is in excess of +20% fuel economy. That's equivalent to a well made and designed boat tail.
So I must disagree that the nose is already "saturated." It seems to me that you've mistaken the ideal airflow around a contour with the reality of airflow through a radiator. That's a big mess because air is being extracted, its properties changed, and then reinjected into the airflow through the vehicle envelope. Which further means comparisons to contours with sealed envelopes fails to capture the complexity of what's actually going on.
But the proof is in the pudding. I'll mod up the nose, post the results, and then we can see whether it was actually worth doing or not. Sound reasonable?
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The nose plays only a very minor role in the total aerodynamic drag.( ' Aero is more about the rear of the car than the front.' Freeman Thomas, Director, Strategic Design, Ford Motor Company, Los Angeles Auto Show, 2008 )
The 'ideal' nose was already, fully tested in the 1970s. It's DNA reaches back to 1922.
Very little leading edge radii is required to achieve attached forebody flow.
Once attached flow is achieved, no additional softening will result in additional drag reduction. By definition, the flow is 'saturated'.
The OEM Prius nose is for all intents and purposes, saturated for external flow.
I agree about cooling system drag, but that wasn't the topic.
And since I'm very familiar with the 2008 Prius, my observation would be that, there's no room inside the engine bay with which an ideal 2% racing cooling system could be realized. You want what Dr. Alberto Morelli designed for the CNR 'banana' car, but there's simply no space available. You'd have to move the AC condenser and radiator to the back of the car ( something which has been done for low-drag concept cars [ Ford Probe-IV, GM Precept ]).
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Streamlining can reduce cooling load and radiator size, but not air conditioner load condenser size. And it's preferred to keep heat exchangers in the vicinity of the forward stagnation point.
Heat exchangers are designed for 100% turbulent flow, to achieve optimum heat rejection. They MUST have some degree of flow resistance.( if you have a conventional gas water heater you'll find a turbulator stuck inside its chimney )
You might find a more expensive radiator, made from a more expensive metal, with a higher heat transfer coefficient, which would allow some down-sizing. Porsche has done this.
If the distance from the cooling inlet to the face of the heat exchangers is less than the radiator height, then extending the nose forwards to achieve that distance would be warranted ( I did this on the Chevette, CRX, Dodge pickup, and Toyota pickup ).
This reshaping itself doesn't reduce drag, but does enable cooling drag reduction which does.
I'm going to call you out on the 20% mpg improvement due to a cooling drag reduction. There's got to be some significant context attached to a claim like that.
The worst cooling system recently reported was Cd 0.048 by PhD Joseph Katz, in 2017, on a generic Cd 0.355 SUV. The 'rear surfaces' were Cd 0.085 ( 77% higher! ). Eliminating the cooling system entirely would be a 13.5% drag reduction. On the ancient 1970s metric, you'd only be looking at a 6.7% mpg improvement.
20% would be a Bonneville car, with a sealed nose like the Cd 0.2, HOT ROD Camaro Project Red Hat, using an ice-water bath to survive the long course. Not a 'passenger' car.