Car-Ears AirStreamz vs. Wind Blocking
The importance of materials engineering cannot be overstated. Characteristics like texture, porosity, permeability (K), resilience, diffusion (D), etc. are evaluated / combined to meet internal and external flow requirements across a range of wind speeds and turbulence levels. In this regard, Cat-Ears is the only cycling related wind noise reduction (WNR) company to hold a utility patent (#US 9078482 - Sound Permeable Wind Noise Reduction Device).
Exhaustive Cat-Ears testing ensures that AirStreamz are the most effective products of their kind on the market. We use precision recording equipment, spectral analysis software, CT (hot-wire) anemometers / pressure probes, and high resolution imaging tools. In addition to stress testing in designed use environments, we incorporate laboratory testing in custom built facilities in Colorado (Wind Jet) and New Jersey (Wind Jet and Hydrodynamic Flume).
Wind flow patterns over the face / cheek and how it interacts with the pinna varies by speed and turbulence level.
Cat-Ears AirStreamz are made from a soft porous material that interacts with oncoming wind to reduce velocity and turbulence intensity. In this regard, porous material has benefits for flow-control by introducing a fluid permeable medium that modifies the boundary layer and wake characteristics. The unique surface texture of AirStreamz also contributes to vortex shedding at longer length scales. Furthermore, the movement of surface strands (mechanical bending and viscosity losses) are effective in reducing velocity / turbulence. Over the last several years, researchers have studied the flow over and through porous materials to reveal these flow control phenomenon.
Wind noise tends to be lower frequency. We evaluate materials that focus on the important frequency ranges.
Wind Flow Depiction
Passive control of wind using porous material vs. bluff body instabilities / buffeting (typically by the wake vortices).
Fig. 1: Depiction of wind flow interacting with porous pile material (AirStreamz) vs. bluff body wake buffeting.
Regarding base bleed, there is a relationship between base bleed pressure and wake / vortex formation (i.e. distance from flow separation). And turbulence (intensity) is an instability primarily generated by the free shear layer. With critical permeability, the free shear layer can be elongated significantly (away from / above the ear). In addition, by allowing some air to passively flow through, there is less higher velocity air / wind to divert over the ear.
Wind Flow Visualization
The wind flow / interaction characteristics noted above can be seen in the wind tunnel visualizations below:
Fig. 2: Flow visualization at 25 mph with smoke / laser sheet. Porous pile interaction vs. bluff / blocking turbulence.
It's important to remember that: "bluff bodies are characterized by a more or less precocious separation of the boundary layer from their surface, and by wakes having significant lateral dimensions and normally unsteady velocity fields." (Bluff Body Aerodynamics, Lecture Notes by G. Buresti, Dept. of Aerospace Engineering, University of Pisa, Italy)
Related Academic Research
Considerable academic research supports the use of pile materials to reduce wake turbulence / aerodynamic noise. As shown in the graphic below (hotwire measurements at 5 m/s wind speed): "It is remarkable that the near wall flow is very calm for the case of pile-fabric. We see that the separated shear layer is much thicker and thus much weaker for the case of pile-fabric compared with the smooth case. In fact, the streamwise position where the shear fluctuation becomes maximum is located at about x/d=2.5 for pile-fabric, while it is about x/d=0.4 for the smooth surface." -a)
Wind Turbulence Measurements
Using pressure sensors / hot-wires, the wind flow / interaction characteristics noted above are measured below (fig. 3). Velocity over the AirStreamz decreases due to the porous material interaction. In addition, a small amount of air flows though the product (under the strap) as base bleed - which helps stabilize the wake. Velocity over the Wind-Blox product remains strong and immediately contributes to wake instabilities / vortices into the concha. It should also be noted that turbulence increases with free stream velocity. Base bleed conveniently increases with velocity.
Fig. 3: a) Kurz hotwire anemometer probe placed adjacent to the WNR products. b) Pressure probes placed adjacent to the WNR products and at the rear of the concha. c) TrueRTA / oscilloscope to display turbulent pressure fluctuations.
In addition to the above measurements, we also use a differential pressure (pitot tube) manometer to measure static and dynamic pressure(s) around the products and outer ear. Measurements confirm that Wind-Blox create a significant low pressure (wake) zone on the leeward side. AirStreamz base bleed reduces this low pressure problem so turbulent (noise producing) wind flow stays farther away from the outer ear.
Wind Noise at 15 miles per hour
Wind Noise at 15 miles per hour with a Wind-Blox product
Wind Noise at 15 miles per hour using a patented Cat-Ears AirStreamz
Other Aero / Acoustic Considerations
Wind-Blox are relatively stiff / inflexible. They can produce their own aeroacoustic (self) noise at higher speeds.
Wind-Blox are designed with a relatively blunt trailing edge. A stronger wake is generated with this design flaw.
Wind-Blox incorporate known sound blocking (i.e. closed cell foam) material. This presents a possible problem.
All Cat-Ears / AirStreamz products are made from soft materials that don't generate aeroacoustic noise. In addition, our proprietary materials have been independently tested at Antioch University for acoustic transparency (i.e. they don't block the sounds that a cyclist needs to hear). At Cat-Ears / AirStreamz, we take cycling safety seriously.
Customer Satisfaction | Amazon Reviews
A comparison of AirStreamz vs. Wind-Blox Pro customer reviews is below. In our opinion, customer satisfaction is the ultimate performance measurement. In this regard, the reviews appear to contradict the Wind-Blox claim that their products have "the highest rating of any bicycle wind blocking alternative". A claim that lacks credible substantiation. In addition, Wind-Blox products are stiff / inflexible and customers give them mixed reviews (3.3) for comfort.
Related Research / References
The phenomenon of vortex shedding behind bluff bodies has been known since the days of Leonardo da Vinci...
(a- Kudo, T., Nishimura, M., Nishioka, M., Aerodynamic Noise Reducing Techniques by Using Pile-Fabrics., 5th AIAA/CEAS Aeroaoustics Conference., AIAA-Paper. (1999)
(a- Nishioka, M., Vorticity Manipulation as an Effective Means for Aerodynamic Noise Reduction., The Eighth Asian Congress for Fluid Mechanics., Shenzhen, China. (1999)
Nishioka, M., Aerodynamic Noise Suppression Technique Using Fur., Japan Society of Aeroacoustics and Astroautics, Osaka Prefecture University, Faculty of Engineering Dept. (2000)
Nishimura, M., Goto, T., Kobayashi, K., Effect of Several Kinds of Pile-Fabrics on Reducing Aerodynamic Noise., Aeroacoustics Conference Presentation., Monterrey, California. (2005)
Massaharu, N., Nishimura, M., Goto, T., Aerodynamic Noise Reduction by Pile Fabrics., Fluid Dynamics Research., Department of Mechanical / Aerospace Engineering, Tottori U., Japan. (2010)