03-03-2010, 11:21 PM
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ABSTRACT
VORTEX GENERATORS
This topic provides basic working principle of a vortex generator suitable for automobiles. Vortex generator has a vital role in increasing the fuel economy and aerodynamic stability. This enables them reduce the drag force acting on it.
There are several methods for positioning the vortex generators over the vehicles. The key results of a simple experiment conducted on Toyota prius exemplifying the advantages of vortex generators discussed here.
Submitted By: Sarun Sagar
1. INTRODUCTION
This topic provides basic working principle of a vortex generator suitable for automobiles. Vortex generator has a vital role in increasing the fuel economy and aerodynamic stability. This enables them reduce the drag force acting on it.
There are several methods for positioning the vortex generators over the vehicles. The key results of a simple experiment conducted on Toyota prius exemplifying the advantages of vortex generators discussed here.
Over the first three parts of this seminar we will see how vortex generators work, looked in detail at their use on the Mitsubishi Evo Lancer, and found where they can be bought for aftermarket use. And now™s the time to put them to the test “ to see if they can be used to improve fuel economy by reducing drag, or used to make a car more aerodynamically stable.
2. VORTEX FLOW
Vortex flow is defined as the flow of a fluid along a curved path or the flow of rotating mass of fluid is known as vortex flow.
There are two types of vortex flow namely and
1. free vortex.
2. forced vortex.
In free vortex flow no external torque is required to rotate the fluid mass.
In forced vortex flow some external torque is required to rotate the fluid mass.
Figure: Vortex Generator.
3. BOUNDARY LAYER FLOW
When a real fluid flows past a solid body or a solid wall, the fluid particles adhere to the boundary and conditions of no slip occurs. This means that the velocity of the fluid close to the boundary will be same as that of the boundary. If the boundary is stationary, the velocity of the fluid at the boundary will be zero. Further away from the boundary the velocity will be higher and as a result of this variation of velocity gradient will exist.
The velocity of fluid will increase from zero velocity on the stationary body to free stream velocity in direction normal to the boundary takes place in a narrow region in the vicinity of the solid boundary. This narrow region of fluid is called boundary layer.
The theory dealing with boundary layer is called boundary layer theory.
3. CHANGING FLOW DIRECTION
Vortex generators are used to put energy back into the boundary layer, so causing the airflow to better stick to the body on the transition from the roof to the rear window. As a result, there is increased pressure on the rear glass (good for reducing drag and lift) and also better airflow to the rear wing.
Figure: Vortex Generator.
So, can commercially available vortex generators perform the same function on other Ëœthree boxâ„¢ sedans Using a bunch of AirTabs, we decided to find out.
The guinea pig car was one that is already highly aero-efficient - an NHW10 Toyota Prius. The first step was to track the airflow pattern over the rear window, using the very effective (and cheap!) approach of wool-tufting. Unlike the many other occasions where weâ„¢ve used similar wool-tufting techniques at AutoSpeed, this time we photographed the wool tufts from the side of the road, using a telephoto lens and a fast shutter speed. (On other occasions weâ„¢ve used a second car to carry the photographer.)
This is the airflow pattern over the rear window of the Prius at about 50 km/h. As can be seen, there is attached flow across the transition from roof to rear window (ie the wool tufts all nicely line up). The attached flow continues down the window at both ends of the rear glass, however, in the lower middle area (circled) there is turbulence. In other words, a separation bubble forms at the middle/base of the rear window which would adversely affect the flow onto the boot-lid.
To see if the separation bubble at the base of the rear glass could be eradicated, four AirTab vortex generators where centred at the trailing edge of the roof. (Theyâ„¢re hard to see because theyâ„¢re clear/white and thereâ„¢s glare on the top of the roof.) With the four vortex generators in place, the difference in airflow was immediately apparent. This time, the airflow down the middle of the rear window remained attached to the glass (circled). This change in flow pattern is directly downstream of the vortex generators. However, either side of this path of influence, the turbulence remained.
Another two vortex generators were added, giving a total of six centred on the trailing edge of the roof. Again, the difference was obvious. As can be seen here, the airflow pattern is completely transformed, with no separation bubble forming at all. However, with such good airflow, any turbulence becomes more visible and some can be seen at the base of the window at each extreme end.
With eight vortex generators placed on the roof, the separation at the lower edges of the rear glass remains “ perhaps caused by airflow wrapping around the C-pillars. This may be able to be addressed by fitting a pair of vortex generators part way down the rear glass, one each side. However, we decided to go with six vortex generators and run with the small edge turbulence remaining.
So, was there a fuel economy gain (indicative of a deacrease in drag) from the fitting of the six vortex generators Unfortunately, we don™t know. The NHW10 Prius doesn™t run a trip fuel consumption display and we simply don™t think that filling the tank each time is a sufficiently accurate way of checking the fuel consumption “ not when we™re talking a car where just 250ml difference in tank fill volume would dramatically change the results. This is one case where the fuel consumption records would need to be accurately kept over a long period if valid data were to be gained.
However, one thing is clear from this test “ the AirTab vortex generators can certainly energise the boundary layer, so promoting attached flow where previously there was turbulence.
But what about the idea that vortex generators can pull extra air into the low pressure wake, increasing its pressure and so decreasing drag As far as weâ„¢re aware, there is no independent scientific evidence for this idea (as opposed to the energising of boundary layers, where there is 50+ years of experience on aircraft!). But as it happened, we had available a vehicle and a daily route that was perfect for checking out this Ëœfilling-the-wakeâ„¢ idea.
5. REDUCING DRAG BY FILLING THE WAKE
The test car in this case was a Honda Insight. Each week for four weeks it travelled an identical morning route of 86 kilometres, comprising mostly freeway travel at 80 “ 110 km/h. The car runs a trip fuel consumption display which was reset each day. The fuel consumption was incredibly consistent over this distance, every trip being 2.9 litres/100 km except for one that was 2.8 litres/100km and another that was 3.0 litres/100km. (Yes, those figures aren™t mistakes: this is the most fuel-efficient car in the world!)
Five AirTab vortex generators were placed across the trailing edge of the rear hatch.
After fitting the vortex generators, the measured trip fuel consumption immediately rose to 3.0 - 3.1 litres/100 km, a 3-7 per cent increase over the 2.9 litres/100km average. No change in the feel of the car could be felt - there was no apparent improvement in stability, reduction in wake noise or any other positives. Simply put, the vortex generators made things worse.
On the basis of the scientific evidence that is available, and the results of this test, we very much doubt whether vortex generators fitted to the trailing edges of vehicles will reduce drag.
6. FRONT UNDERTRAY
A further test was performed on the Honda.
The Honda runs a short front undertray with a slightly raised section in the front bumper flowing air to it. Further back under the car there are some in-fill panels, but the underside is certainly not a smooth, flat and continuous surface.
I thought that if the boundary layer of air under the car was energised, the flow might better ˜jump the gaps™ on the underside. Additionally, it might accelerate air past the frontal undertray, reducing lift. Or, to be honest, fitting the vortex generators to the undertray might do something “ I doubt if anyone in the world could say what the outcome would be without first trying it!
Four AirTab vortex generators were placed at the leading edge of the undertray.
Immediately noticeable was improved aerodynamic stability. The Insight is not an aerodynamically stable car at speed “ it is discombobulated by, especially, the bow waves of trucks. This can be felt when slowly passing a truck heading in the same direction on a multi-lane road. As the Honda draws adjacent to the front of the truck, the very light car is pushed away from the truck. (This affect, once recognised, can be felt to a degree in lots of cars “ but it™s quite clear in the Honda.) With the undertray vortex generators in place, the affect of the truck bow waves was diminished. The bow wave affect could still be felt, but it needed less steering correction.
OK, so the car was experiencing better stability, probably through reduced frontal lift. But would there be a trade-off in extra drag, resulting in increased fuel consumption The answer to that is “ no!
With the undertray vortex generators in place, the fuel consumption averaged 2.9 litres/100. In fact, on one trip the car equalled its best-ever consumption at 2.8 litres/100km “ however, the traffic flow that day was particularly kind and so I don™t think that the vortex generators can be said to have helped in achieving this. But they certainly weren™t harming fuel consumption...
7. CONCLUSION:
Some very important conclusions can be drawn from the four parts of this series.
1. Vortex generators on cars can achieve measurable, scientifically proven improvements in car aerodynamics “ reducing both lift and drag. The Lancer Evo is probably the shape of things to come “ expect vortex generators (of whatever design) to be used on new cars, especially to improve the flow around corners towards the rear of the body.
2. Commercially available (and cheap) vortex generators like AirTabs can achieve a provable change in car aerodynamic behavior. Even a single vortex generator will alter local flow behavior, something to keep in mind when considering airflow into bonnet scoops and at specific problem areas.
3. Positive gains can be achieved only by practical experimentation. That™s the downside “ the upside is that the vortex generators are easy to temporarily stick into place with masking tape, and just as easily removed if they are not achieving the desired results. They™re also cheap enough that buying ten or so for experimentation is a good investment.
3. The use of vortex generators under cars has zero visual impact and looks to have excellent potential for reducing lift without increasing drag.
8. REFERENCES
8.1 Books and Journals;
Fluid Mechanics & Hydraulic machines (2002), Dr. R.K. Bansal.
8.2 Related Websites
airtab. airtab.com.
Autospeed autospeed.com
