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
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.