As a vehicle moves forward, the external air flow along surfaces of the vehicle separates from the surfaces at the
aft end of the vehicle. Separation is most pronounced in vehicles with a substantial vertical aft face (e.g., trucks).
The separation of flow from the surface creates an area of low pressure behind the aft face of the vehicle.
The area of low pressure "drags" the vehicle backward in a phenomenon known as aerodynamic drag.
Aerodynamic drag on vehicles (e.g., trucks) significantly reduces fuel efficiency.
Mechanical (static) flow deflectors mounted on vehicles in various configurations are employed to control external
layer flow in an attempt to increase fuel efficiency. These mechanical flow deflectors are commonly referred to as
"flarings" or "deflectors."
Actual reduction in fuel consumption from aerodynamic drag reduction is influenced by other factors, including,
but not limited to, vehicle weight and energetic cost of implementing the reduction.
Following several years of study, a new Suction and Oscillatory
Blowing (SaOB) actuator Active Flow Control (AFC)
device was
invented at the Meadow Aerodynamics laboratory at Tel Aviv
University. The project is led by Professor
Avraham Seifert who
heads the laboratory.Laboratory site
Several sizes of actuators were developed, and small size devices,
all suitable for vehicle application with only
minor adjustments,
were also developed and validated.
The new SaOB AFC is a combination of a bi-stable fluidic oscillator,
connected downstream of an ejector (“jet pump”).
The purpose of the ejector is to create suction flow by increasing
the flow going into the valve. Suction is probably the
most efficient
flow separation control method, but it is difficult to generate efficiently.
It has been shown that the ejector
is indeed increasing the flow rate
by a factor up to three with its entrances unrestricted.
A presentation about this technology was held at the 46th American Institute of Aeronautics and Astronautics (AIAA) Meeting Jan. 2008. The paper is available at AIAA site
Active Flow Control (AFC) is a fast-growing, multidisciplinary science and technology thrust aimed at altering a natural flow state into a more desired flow state (or path). AFC typically refers to the use of time-dependent
(often periodic) disturbances that are introduced into the flow field by so-called "actuators." The technology is mainly implemented in military application for the moment.
AFC application includes:
- BOEING – Testing rockets equipped with Active Flow Control. Click here
- NASA- Compressor performance enhanced by Active Flow Control over stator vanes. click here
- Doors: Patent submitted regarding the solutions for door openings
- Installation: The installation of the unit can be done using Velcro, and therefore disassembled in just
a few minutes - Pressurized air system: The new suggested "add-on" device requires pressurized air for operation; and two solutions have been considered:
- Connecting the "add-on" to the truck’s existing pressurized air system
- Installing a designated pressurized air system that can be connected to the truck’s rear axis
AFC energy requirements were taken into account when calculating total savings.
