EXECUTIVE SUMMARY
Vortex generators, aerotabs, located on the junction of the roof and rear window have no measurable effect on drag reduction for an NHW11 Prius. Although there is evidence of flow separation without the vortex generators, using these vortex generators show no measurable improvement in aerodynamic dominated, fuel consumption.
METHODOLOGY
The test course is I-565, a six to four lane, divided highway. The end points are given by:
East - 34d 42' 53.47" N, 86d 38' 37.45" W, 196m
West - 34d 38' 27.47" N, 86d 50' 38.37" W, 178m
During the test runs, May 9, 7:00-9:00 AM, the wind was from ~290 degrees, a right quartering headwind, increasing from 2-9 mph (3-13 kph.) This was evident in the data as the west-to-east runs had significantly improved MPG in spite of an 18 m increase in altitude. This suggests aerodynamic effects predominated in the data. The temperature ran 73-75F (23-24 C.)
The mileage came from the built-in display. The vehicle speed was set to 75 mph (120 kph) with a little under a mile, close to a km, run-up. The cruise control regulated at 75 mph (120 kph) and calibrated by using a Gramin nuvi GPS receiver for the true ground speed (indicated ~73 mph, oversized tires.) As soon as the overpass marking the start point was reached, the MPG was reset and the trip meter. Passing under the ending overpass, the mileage and trip distance were memorized and recorded.
DATA (comma delimited)
miles, MPG(E->W), MPG(W->E), vortex(0=no), comments
12.1, 38.5, 42.5, 1,
24.2, 37.1, 43.0, 0,
36.3, 35.1, 44.1, 1, 1st run required max pedal to avoid traffic
48.4, 36.5, 45.4, 1,
ANALYSIS
With the exception of the 3d run headed west when traffic forced maximum acceleration, all other runs are consistent with an increasing wind from the west. There is no evidence of a measurable improvement in MPG using the vortex generators.
I suspect the small, reverse flow behind the rear window probably serves the function of pulling the laminar flow air passing over the roof down. Mythbusters demonstrated this effect with pickup truck tailgate testing.
Mythbusters demonstrated that a stable vortex in the bed of the pickup with the tailgate up had a measurable improvement in mileage as it pulled the laminar flow down. Opening or taking the tailgate off eliminated the stable vortex and resulted in worse mileage.
LESSONS LEARNED
A better time to test would have been between 4:00 AM and 6:00 AM to minimize or eliminate the surface winds.
Testing at the posted speed limit plus 5 mph increased the aerodynamic drag, the primary force of interest, but it also resulted in over taking two slower vehicles running side-by-side and impending loss of lane. The maximum acceleration resulted loss of one run. Select better times than Saturday morning for testing.
Future aerodynamic enhancements need to focus on reducing velocity changes in larger masses of air. Likely areas are:
- bumper air inlet - has excess capacity to handle worst case, maximum power, slow speed climb, a rare condition. Significant mileage improvements are already known in cold weather so a temperature driven, variable air cover may be a better approach.
- tire air blocks - there are small, 2" deep, air blocks in front of each tire. A deeper or curved air block may reduce tire drag effects.
- underbody air cover - a proven technique, this should also reduce road noise although it may complicate maintenance.
- deeper front air dam and side skirts - a proven race track technique, a variable system makes more sense given the number of speed bumps and rough urban streets.
My thanks to "MetroMPG" who loaned me the aerotabs for this testing.
Bob Wilson