Coanda Effect

Flow Phenomenon and Applications

Noor A Ahmed

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781032090320
• eBook ISBN: 9780429805431
• Topic: Physical Sciences
• Subtopic: Civil Engineering

Coanda effect is a complex fluid flow phenomenon enabling the production of vertical take-off/landing aircraft. Other applications range from helicopters to road vehicles, from flow mixing to combustion, from noise reduction to pollution control, from power generation to robot operation, and so forth. Book starts with description of the effect, its history and general formulation of governing equations/simplifications used in different applications. Further, it gives an account of this effect's lift boosting potential on a wing and in non-flying vehicles including industrial applications. Finally, occurrence of the same in human body and associated adverse medical conditions are explained.

1

Basic Concepts

1.1 Historical Background

The beginning of the 20th century saw two momentous events occur in rapid succession; one happened in 1903, and took the world by storm, while the other happened in 1904, and almost went unnoticed. The two events, however, were necessary for the discovery and widespread popularity of what is now widely known as the “Coanda effect.”

The event of 1903 marked the beginning of the conquest of air because on December 3 of that year, the first ever successful manned flight in human history by the Wright brothers took place [1]. This fired up people’s imaginations, and accelerated developments in aviation began to occur. Within a few years, at the October 1910 Paris Salon de l’Aeronautique show, a young, 24-year-old engineer by the name of Henri Coanda was attracting the crowd’s attention, displaying a new bi-plane aircraft he had designed to fly with a novel piston engine. But things did not go as planned. The aircraft was burned during its engine warm up before it could take off the ground [2]. Coanda was naturally disappointed. But there was a silver lining to the disaster when Coanda noticed while his engine burned, which, by 1934, led to the recognition of a new flow phenomenon that now bears his name as the “Coanda effect,” a term coined by Theodore von Karman in his honor [2].

Coanda had observed that the burned gases which exhausted from the engine showed a tendency to remain very close to the fuselage. He was confident that he had discovered a new phenomenon and immediately began working on it to find practical applications. His initial objective was to deflect the exhaust gases, such as those of a radial piston engine away from the fuselage to protect the fuselage from getting burned. Later, he also experimented to apply the concept to turn flow occurring at corners of many devices, such as turning vanes of wind tunnels, thrust augmenters, pumps, and so forth. Coanda eventually managed the incredible feat of turning a flow through 180° deploying a series of deflecting surfaces, with each surface at a sharper angle to the previous one.

Historically speaking, however, Thomas Young was probably the first who had provided the first account of the flow phenomenon that produces the Coanda effect in a lecture delivered to The Royal Society in 1800 [3], where he stated:

The lateral pressure which urges the flame of a candle towards the stream of air from a blowpipe is probably exactly similar to that pressure which eases the inflection of a current of air near an obstacle. Mark the dimple which a slender stream of air makes on the surface of water. Bring a convex body into contact with the side of the stream and the place of the dimple will immediately show the current is deflected towards the body; and if the body be at liberty to move in every direction it will be urged towards the current.

In a paper [4] seventy years later, Osborne Reynolds observed a similar flow phenomenon when he discovered that a ball can be held in suspension by a jet of fluid. But it was Henri Coanda who had actually realized the practical importance of the effect and conducted detailed investigations to realize its potential. In 1934 he obtained a patent in France in which he described the effect as the “deviation of a plain jet of a fluid that penetrates another fluid in the vicinity of a convex wall.” His two patents [5] filed in 1936 and granted in 1938 [Figure 1.1] also describe the Coanda effect quite clearly in the following text:

FIGURE 1.1
Henri Coanda’s 1936 patent ([5].)

The present invention relates to propelling devices in which there is produced a suction zone in front of the body in motion on which the propeller is mounted, this suction being such that the body in motion is propelled under the influence of the atmospheric pressure existing at the rear of the propeller.

The second historic event took place in 1904 when Ludwig Prandtl, at that time, a young, 29-year-old Professor of Mechanics at the Technical Institute of Hannover, Germany, delivered a ground-breaking paper [6] in which he explored the role of very small friction or vanishing viscosity in a fluid flow.

Viscosity is a major cause of drag produced on a body in motion, and the topic “drag” has received continued attention for a long time, from as far back as the times of Aristotle [7]. It was in the 18th century, nearly 2,000 years later that Navier and Stokes provided a mathematical formulation of a fluid motion that included the effects of viscosity. Unfortunately, the partial differential nature of the equations meant that they were difficult to use and remained unsolved until Prandtl paid attention to them.

Prior to Prandtl, every order-of-magnitude analysis to simplify the Navier–Stokes equations had resulted in potential flow equations suggesting that the flow was determined by the normal velocity component at the surface leaving no role for the viscous no-slip boundary condition, which at infinite Reynolds number would be exactly zero.

Prandtl also managed to eliminate, through a process of logical and intuitive deductions, the less significant terms from the Navier–Stokes equation and reduced them into simpler forms with some significant differences. He argued that to satisfy the no-slip condition, there must be at least one term that would retain the effect of viscosity. He postulated that to compensate for the effect of vanishing viscosity, there must be an equivalent increase in the strain rate at the surface. Using this argument and from the corresponding simplified equations that he derived, he was able to deduce the viscous effects on a moving body in a stationary fluid and vice versa. He demonstrated that the viscous effects were dominant very close to the surface only. He also showed that from a point normal to the surface of the body, the static pressure remained constant within the viscous layer.

Prandtl’s findings were highly significant. Now, for the first time, a viable and effective method for calculating the effects of viscosity near the surface of a moving body became possible. The scientific community did not take much notice of his work until a few years later, in 1908, when Blasius [8] proved the validity of Prandtl’s hypothesis through physical experimentati...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Preface
• Author
• 1 Basic Concepts
• 2 Tools of Investigation
• 3 Coanda Effect in Aeronautical Applications
• 4 Miscellaneous Applications of Coanda Effect
• 5 Coanda Effect in a Human Body
• Supplemental Reading List
• Index