Benchmark table
Table 1: Benchmarking Table
Nasa SuperSonic
Test 1 (Source)
Nasa SuperSonic
Test 2 (Source)
National
Aerospace Labs
Report (Source)
Reynolds Number
Riblet Height (in.)
0.003
Skin Friction
Reduction (%)
4-8%
Total Drag
Reduction
-
2-3.4 million
3.6-6 million
0.0013
4-15%
-
3.5-6 million
(Re/ft)
0.033-0.152 (mm)
5-8%
2-3%
A drag calculation was performed in order to approximate the drag force that a bicycle frame section
(specifically the head tube) would experience. A NACA 0024 airfoil was used to approximate the shaped
of an aerodynamic head tube and no riblets were considered in this analysis. The profile of the NACA
0024 aifoil was analyzed using XFoil in order to determine the profile drag coefficients and lift
coefficients at different angles of attacks (-10° to 10°). This data was used to calculate the induced drag
coefficient (to account for edge effects) by using the equation 𝐶𝐷,𝑖 =
𝐶𝐿2
𝜋𝑒𝐴𝑅
and the drag force by using
1
the equation 𝐹𝐷 = 2 𝜌𝐶𝐷 𝐴𝑉 2 . The analysis was performed three times to study the effect of changing
the angle of attack (Drag Coefficient), wind speed and frontal area (size of section). The maximum and
minimum values for each analysis are shown in the Figure 1 below. A final analysis was performed to
determine the maximum drag force that may be seen by considering a large section (4.5”x1.5’) at the
maximum speed of 39 m/s. This analysis is part of the benchmarking section and feasibility section since
it gives insight into what to expect as well as the feasibility of the project.
Figure 1: Results from Drag Analysis
Potential Concepts
Concept #1
Prototype Production
A mold could be built that created the shape of the riblets. The mold would be adjustable so that it
could make different riblet thicknesses, shapes and so that the riblets could be placed in different
positions. The material that is being molded would be a thermoplastic.
Figure 2: Riblet Design 1
Figure 3: Prototype Production 1
Riblet Design
A design that has seen some success in other applications is the combination of triangular riblets with an
aspect ratio of h/b=1. The riblets would be applied to the entire portion of the bike section from leading
to trailing edge. The riblets would have a uniform height through the whole section cross section.
Concept #2
Figure 4: Protoype
Production 2
Prototype Production
A process that creates the riblets separately from the airfoil would allow the airfoil to be almost any
material. This could be achieved by using rubber to make the riblets so that it would be flexible enough
to wrap around an airfoil.
Riblet Design
Another feasible design for the riblets would be to taper the riblets. The riblets would start out smaller
in both height and width and would get taller and wider as they approached the trailing edge of the
airfoil.
Figure 6: Riblet Design 2
Engineering Analysis
Area of Concern #1 – Drag from Frontal Area
The drag force on an object is proportional to the frontal area. We can perform an analysis to get a quick
idea of what to expect from only the change in frontal area on drag force. A standard frontal area will be
determined and the area will be calculated when riblets are both subtracted and added to the standard
frontal area.
The results of the analysis show that the addition or subtraction of the riblet area only changes the drag
force by 0.45%. This shows that the results of other tests were not due solely to a change in frontal area
and that the riblets somehow change the drag coefficient.
Area of Concern #2 – Reynolds Number
One of the most important characteristics when considering aerodynamics is the Reynolds number of a
flow. The Reynolds number of a typical bicycle head tube was calculated using a standard tube diameter
of 1.5” (taken from Cannondale Claymore) and an average cyclist speed of 15.5 mph (Livestrong.com
sites 15.5 mph as the speed that bicycle lanes are designed for). The current record for unpaced flat
land speed on a bicycle (recumbent bike with carbon fiber shell) is 82.819 mph. This value will be used
as the absolute maximum speed of the bike.
Area of Concern #3 – Dimensional Analysis
One way to test a riblet design would be to use a tow tank in water. In order to compare a result from
testing in water a scaling analysis must be performed. Dynamic similarity will be achieved if the Reynolds
number in the air is equal to the Reynolds number in water (there are other considerations but this is
the most important). Using this constraint the required velocity in water was calculated in order to see if
that value is a reasonable velocity to achieve in the tow tank (1 m/s is essentially the limit of the tow
tank).
The results show that using an the average speed of a cyclist (15.5 mph) the tow tank would require a
velocity of 0.467 m/s which is within the tow tanks capabilities. If the maximum cyclist speed is used
(82.8 mph) then the tow tank will be insufficient. The graph shows how the scale of the model can be
used to fall within the velocity capabilities of the tow tank.
Area of Concern #4 – Safety (Force concentration from riblets)
A model can be created involving a crash scenario in which the rider comes into contact with the bike
frame. This can be analyzed for a bike with riblets and without riblets. The bike with riblets would be
expected to create force concentrations at the peak of the riblets.
Engineering Analysis Work
References
Cannondale Claymore 1 Mountain Bike
http://www.cannondale.com/2013/bikes/mountain/overmountain/claymore
Livestrong.com article
http://www.livestrong.com/article/421140-the-average-bicycle-speed/
Bicycle Speed Record
http://www.wisil.recumbents.com/wisil/whpsc2009/results.htm
Nasa Riblets Paper
http://www.nasa.gov/centers/dryden/pdf/88256main_H-1774.pdf
National Aerospace Labs report
https://edge.rit.edu/websvn/filedetails.php?repname=R13902&path=%2Fweb%2Fpublic%2FHW_Deliverables%2FGroup+Subm
issions%2FResearch+Articles%2FRiblets%2FAerodynamics+of+Riblets.pdf