Software Used: CFDesign V10 by Blueridge Numerics
Model analyzed: 2009 Honda Civic Si Sedan
Overview:
I have been very fond of Bill (NitrousG35) and his cowl induction idea. The ingenuity of it was mesmerizing and while I had heard of the technology, I had never thought to implement it myself on our vehicles.
Now...while the science is very sound, I was a bit skeptical of the actual performance of the system due to the shape of our vehicle. I figured I would run my own simulation to see how well this idea would actually work.
Problem Definition
- This program unlike many others generates its own mesh-tetrahydral type
- Input conditions: Velocity = 100 km/h
- Output conditions: Pressure = 0 Pa (while I know ambient pressure is usually 101.3 kPa, the pressure differential between the input and output is assumed to be 0)
- The model was traced from a picture of a stock FA5 and then scaled appropriately
- The model is 3D, however a 2D analysis was conducted due to the inaccuracy of the 3D model. The 2D model retains geometric accuracy over the midplane of the model
Sources of Error
- Underbody is perfectly flat. This does not explicitly affect the region of flow being analyzed. If you look closely at the pictures however, you will see that this reduces the overall amount of drag on the vehicle just aft of the model. (interesting by product to consider if our bottom panels were completely flat)
- The boundary layer over the surface of the vehicle was not defined by me. Rather, I relied on the program to generate it's own boundary layer mesh.
Analysis
I chose the input velocity of 100 km/h due to the fact that
1) Metric measurements are the way of the future (only you U.S people still work in imperial...lol)
2) It is only over 80 km/h that aerodynamic benefits can be felt on a vehicle.
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The first picture shows the meshing process I used. I let the program generate it's own mesh over the length of the volume being analyzed. I then defined a fine mesh where I wanted to accurately simulate the flow over that region. The fluid used was air (obviously) and an ambient temperature of 24 deg. C was used.
Without getting into the math of this too much, let me say that velocity and pressure are inversly proportional. This means that lower velocity regions will exhibit higher static pressure than higher higher velocity regions.
If we take a look at the velocity profile picture first: we can note there are two regions that demonstrate the extremity of velocity drop.
1) The bumper
2) Between the hood and the windshield - where the proposed cowl induction system would go
Now...the bluer the region, the slower the air. Comparing just the colours its very apperant that we have a slower region of air at the bumper than between the hood and the windshield.
We can attribute this to the stagnation point of the vehicle - the point at which the steam line of an air particle hits zero velocity.
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Now taking the Pressure profile picture into consideration: we can see that the static pressure is very high at the bumper and less so between the hood and the windshield. What this means is simply that while the cowl induction idea is very good....the area between the hood and the windshield (while relatively high in stative pressure) is not the primary region of high static pressure.
In order to gain maximum benefits of a ram air system, something like an M&M scoop would be much more useful because it is placed much closer to the primary region of high static pressure. This will in affect draw in the most amount of air, the quickest.
I realize this explanation was very quick and did not adequately explain the fundementals but if I get time when I am at home, I hope to do a better write up then (currently I am at work).
One thing I must mention is that Bill stated that our HVAC system draws air in from the region between the hood and the windshield...making use of this pressure zone. While this is true, I think that this is more a case of simplicity of ducting (being so close to the dash) and packaging of the system, rather than optimal performance of the HVAC system on our cars.
Also...the simulation also seems to prove why major aerodynamic companies like M&M have designed their intake ducts as such...and why cowl air induction is not as readily used on our vehicles. The scoop on the gruppe M induction system faces the front and not the rear because (in my opinion) there is still not great enough of a pressure differential created to produce a maximed state of ram air.
Just some thoughts...
Originally Posted by TbM
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