I have found a nifty free program that could do a flow simulation and show it graphically as a video clip in .avi format. It is called Flow Illustrator. It is not accurate to replace a full blown Computational Fluid Dynamics software but it is free. You will need to do your own model. This is simply done by using a side view of the model. You can do this by using an existing picture with non essential details such as background and people deleted. I use Microsoft paint to edit my picture. Please note that it is advisable for you to use the CUT function using SELECTION. Once the image you need is cut from the original, you crop the rest leaving with a totally white background. You can upload the picture and set three (3) variables; Reynolds number, time between frames and video length in seconds. I only change the setting for video length. If you want to do comparison of flow between two designs, you need to have the picture in the exact dimensions. The value of the Reynold’s number will be shown on the video.
You can see the comparison video on this link.
A screen capture of the comparison is shown below
Simulated flow on Satria Neon with Deflector
Simulated Flow over Satria Neo with Standard Spoiler
Notice I have cut out the wheels to indicate flow of air under the floorpan. Green coloured areas indicate high pressure/low velocity and Red indicates low pressure/high velocity. The comparison helps to show that the original design with standard spoiler has a bigger wake left in the trail of the car. The one with the deflector has a smaller wake as the height of the turbulence is reduced.
If you do another comparison with a sedan design, you would see that the wake is about the height of the boot. The deflector now looks like what we intended it to be. It is to reduce the wake by delaying air separation from the hatch. This partly answer the previous question of why the Fuel Consumption curve is not logarithmic. The deflector has therefore made the car more aerodynamic. It makes it easier to cut through the air. I would not be surprised if it has reduced the Cd value of the car.
Making the car aerodynamic has the added bonus of reducing drag and obtain faster acceleration and top speed.
Just a quick one as I do not want to give away too much. Finally managed to do up the fibre glass work with Chooped Mat and Roving. The Roving is better suited to hold the surface on the edges as it is stronger.
Needed to brace this securely on this working design. I would not need to get a really solid grip on the production model as it will totally be fibre glass. As it is now, wood is used as the main structural material. I would only need this working design to make some sets of mould. The finished product would therefore be much lighter and look nice.
Deflector mounted on car
Had a spin around Putrajaya. About 45km distance. First round registered 4.3 l/100km while the second registered 4.5l/100km. Will keep doing test and maybe try to add more surface area on the bottom. I am very happy as it is, there was little effort to try and clean it up. Not even fillers and it still meets the target. Probably I might get better results with the extension.
Posted in Aerodynamics, Fuel Economy, Spoilers
- Tagged Aerodynamics, Composite, Deflector, DIY, Fuel Consumption, Prototype, scratch buiding, spoilers, Woodwork
A day after posting Part 4. I did a calculation using a spreadsheet. I could either stick to my existing HITEC or get a slightly bigger one. There’s a local Arduino reseller that has lots of TowerPro MG945 which has a stall torque rating of 12kg/cm at 6VDC. I am definitely going to buy this. Although the gears are of metal, due to the power that’s required, I prefer Nylon to take in the additional service demand. I would also need to reduce the surface area by 30% while limiting the angle to not more than 30º. As preparation to accept this servo I had to install a 3 wire cable taken from a power cord extension. The length is about 1.3m which is sufficiently long to have the new controller next to the mid console close to the handbrake. I have routed this through holes that were meant for the rear water jet for the wiper and through one for the spoiler mounting. It’s a bit harder than just that, as I had to remove two panels and I had to make sure the cable was not interfering with the rear seat belt mechanism.
While at the same time, I found the bracket for the reverse sensor a bit too big for my liking. I therefore made holes to shave off 20g. It was not planned. Since it was hard to remove these panels, I would not be seeing it for many years to come.
A few more days and I finished my mechanical build of the spoiler and had it fitted on the car yesterday 27 May 2014. I had a spin in it around Putrajaya, in the most decent of 80km/h speed. It did not fell off even though all of the wooden part was held in place by nothing more than Dunlop Contact Adhesive and two screws. I had another spin today at much more livelier speed around the same track and it is still there on the roof. I might add more screws to keep it all sane. I used normal green coloured Garden Wire to secure the angle of attack. I do not want the angle to be too big that it would add drag and load onto the new assembly.
I am happy to report that on the first test, I recorded an 18% fuel consumption reduction. Initially the average at 80km/h is 5.3l/100km. Now it is 4.3l/100km. Tell me again how much that hybrid costs! This is amazing. Previously I could only make 4.0l/100km on a flat straight road. But with this spoiler. I could drive it normally without having to use hyper-milling techniques to get close to that. It goes to show that it does make sense to make the air separation as late as possible. The angle of the spoiler closely matches the angle of the hatch. I cannot be too sure if I had also reduced the wake.
With that I have completed the mechanical build and managed to clean the wet kitchen area of my house clear from tools and debris. This gives much joy to my wife. The next steps will be to finalize the electronics.