The U.S. military is developing low-noise drones that can be used in densely populated areas. It used to cost a lot of money, and it took a lot of time and effort to test, modify, and retest.
Miranda Costenoble, a researcher with the ARMY’s Combat Capability Development Command (DEVCOM), has found a way to use fluid dynamics software that takes noise directly into account during the design phase, dramatically reducing the time needed to develop a low-noise drone.
Small drones have a wide range of uses, such as delivering packages and landing directly in people’s backyards, which requires them to be very quiet or the community won’t accept drone delivery, Mr. Kostnoble said.
In the military, the U.S. military wants to design small, stable drones that can be used over a variety of terrain.
“Surveillance of adversaries is one of the uses that drones are often mentioned for. If the drone sounds like a thousand angry bees, it’s easy for your opponent to spot it and hide or shoot it down.”
The noise of a large rotorcraft comes mainly from the noise the rotors make in front of the observer as they rotate at high speed, but the noise of a small plane’s rotors is caused by the air around the observer being stirred by the rotors, Costernoble said. The frequencies of these noises vary from medium to high.
“We used a semi-empirical computer model. These models were developed 30 years ago for a particular wing, so they need to be updated to fit a variety of different wing shapes.”
These computer models take into account the parameters of the boundary layer close to the wing. A boundary layer, also known as a boundary layer, is a hydromechanical concept in which a layer of air is attached to an airfoil as it slides through the air. As air flows through an object, the surface is not perfectly smooth and the air is viscous, so the closer it is to the wing, the less it flows.
The flow in the boundary layer is divided into laminar flow and turbulent flow. Fluid is stratified flow and non-mixing, which is called laminar flow; When the velocity increases to large, the streamline is no longer clearly discernible, and there are many small vortices in the flow field, called turbulence.
Turbulence in the boundary layer is a major source of noise, Kostnoble says. Using the system for simulations, researchers can directly access information that previously would have required constant testing in a wind tunnel, saving a lot of time and labor.
The research was presented at the 76th annual forum of the Vertical Flight Association (VFS) recently.
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