BOUNDARY LAYER SEPARATION IN AIRCRAFT
MCHE332 – Fluid Mechanics 2
Aya Houssein Chames, 201602278
Engineer Atif El-Khatib
Mechanical Engineering Department
Faculty of Engineering
Beirut Arab University
TABLE OF CONTENTS
Table of Contents ii
List of Figures iii
List of Tables iv
Chapter 1. Literature Review 5
1.1. Abstract 5
1.2. Literature Review 5
1.2.1. Assessment of leading-edge devices for stall delay on an airfoil with active circulation control 6
1.2.2. Engine nacelle of an aircraft comprising a vortex generator arrangement 6
1.2.3. Initial virtual flight test for a dynamically similar aircraft model with control augmentation system 7
1.2.4. Conclusion 7
Chapter 2. Aircraft 9
2.1. Overview 9
2.2. Control Methods 9
2.3. Advantages of Boundary Layer in Aircraft 10
LIST OF FIGURES
Figure 1: The synthetic jet concept 10
LIST OF TABLES
No table of figures entries found.
CHAPTER 1. LITERATURE REVIEW
Boundary Layer is the layer of air over the surface which is slower moving in relation to the rest of the slipstream is called the boundary layer. The initial airflow on a smooth surface gives evidence of a very thin boundary layer with the flow occurring in smooth laminations of air sliding smoothly over one another. Therefore, the term for this type of flow is the laminar boundary layer. As the flow continues back from the leading edge, friction forces in the boundary layer continue to dissipate the energy of the airstream, slowing it down.
Boundary layer separation is the laminar boundary layer increases in thickness with increased distance from the wing leading edge. Some distance back from the leading edge, the laminar flow begins an oscillatory disturbance which is unstable. Waviness occurs in the laminar boundary layer which ultimately grows larger and more severe and destroys the smooth laminar flow. Thus, a transition takes place in which the laminar boundary layer decays into a turbulent boundary layer.
1.2. LITERATURE REVIEW
Boundary layer control devices are additional means of increasing the maximum lift coefficient of section. Boundary layer control devices for high-lift applications feature various devices to maintain high velocity in the boundary layer and delay separation of the airflow. Control of the boundary layer’s kinetic energy can be accomplished using slats and the application of suction to draw off the stagnant air and replace it with high-velocity air from outside the boundary layer. Boundary Layer can be controlled by using:
1. Slat and Slots
2. Vortex generator
3. Stall strip
1.2.1. Assessment of leading-edge devices for stall delay on an airfoil with active circulation control
Slats are movable control surfaces attached to the leading edge of the wing. When the slat is closed, it forms the leading edge of the wing. When in the open position (extended forward), a slot is created between the slat and the wing leading edge. Thus, high-energy air is introduced into the boundary layer over the top of the wing. This is known as “boundary layer control.” At low airspeeds this improves handling characteristics, allowing the aircraft to be controlled laterally at airspeeds below the otherwise normal landing speed.
1.2.2. Engine nacelle of an aircraft comprising a vortex generator arrangement
A vortex generator is a complementary pair of small, low aspect ratio (short span in relation to chord) airfoils mounted at opposite angles of attack to each other and perpendicular to the aerodynamic surface they serve. Like any airfoil, those of the generator develop lift. In addition, like any airfoil of especially low aspect ratio, the airfoils of–the generator also develop very strong tip vortices. These tip vortices cause air to flow outward and inward in circular paths around the ends of the airfoils. The vortices generated have the effect of drawing high-energy air from outside the boundary layer into the slower moving air close to the skin. The strength of the vortices is proportional to the lift developed by the airfoils of the generator.
1.2.3. Initial virtual flight test for a dynamically similar aircraft model with control augmentation system
An airplane wing stall progressively from the root out to the tip, if a wing does not naturally have this stall progression characteristic, it is possible for the manufacturer to place a small triangular strip of metal on the leading edge of the wing in the root area. At high angle of attack this triangular stall strip break the airflow over the root section, create turbulence and restore the smooth flow of air over the entire wing.
Benefits of vortex generator
• Improved low-speed handling characteristics
• An added safety margin for low-speed flight
• Significantly improved low-speed aileron control
• Improved cross wind handling at low speeds
• Increased safety margin in the event of an engine
• Reduced take-off distance, improving short field
• Reduced take-off from water for float equipped
• Reduced tire and brake wear due to landing at
lower speeds, resulting in less maintenance
Benefits of stall strips
Stall strips begin working when your wing is at a high angle of attack. Because the stagnation point is on the underside of the wing, air flows up and around the leading edge, making its way over the top of the wing. With no stall strip, airflow can stay attached to the wing as this happens. ?
CHAPTER 2. AIRCRAFT
A laminar boundary-layer forms when a real (viscous) fluid flows past a solid body. The boundary layer separation represents a major problem which constrains the design of aircraft device. It is characterized by a loss of kinetic energy near solid surface and it depends on the flow regime, which is either laminar or turbulent. Thus, it is necessary to have a flow separation control. The approaches for separation control can be splitted into: Passive control (Vortex Generators, Flaps/Slats, Absorbant Surfaces, Riblets) and Active control (Mobile surface, Planform control, Jets, Advanced controls). The jets can be splitted into: jets with additional flow mass – forced jets (blowing, suction, blowing-suction) and jets without additional net mass flow – named “dynamic forcing” (synthetic jets).
2.2. CONTROL METHODS
A synthetic jet is a concept consisting of an orifice or neck driven by an acoustic source in a cavity. At high level of excitation by the acoustic source, a mean flow is generated in the vicinity of the orifice with no net mass flux added to the main flow. At sufficiently high levels of excitation by the acoustic source, a mean stream of flow has been observed to emanate from the neck. Because there is no mass added to the system, the mean streamlines must form a closed recirculation. The control of boundary layer with synthetic jet is shown in Fig. 1, where the synthetic jet is embedded in the wall of a boundary layer for which separation control is desired. On the in-stroke of the neck velocity, vertical momentum is impacted to the flow causing the neck to preferentially ingest approaching low axial momentum of the incoming boundary layer (without external flow, the instroke would pull in flow from all directions). On the out-stroke due to the curved neck, the fluid particles are re-accelerated and injected with positive axial momentum into the wall region of the boundary layer. Hence, both the in-stroke and out-stroke of the cycle increase the ability of the boundary layer to resist separation. In the time-averaged sense, the synthetic jet provides a decreasing step in the shape factor of the boundary layer.
Figure 1: The synthetic jet concept
2.3. ADVANTAGES OF BOUNDARY LAYER IN AIRCRAFT
Control volume and one-dimensional analyses are used to illustrate two major features of boundary layer ingestion: reduction of jet mixing losses due to decreased jet kinetic energy from reduced velocity of flow entering the propulsor and, to a lesser extent, reduction of airframe wake mixing losses. Embedded boundary layer ingestion propulsion systems can also enable nacelles with reduced surface area and associated weight and drag, further decreasing the aircraft propulsive power requirement. The required propulsor flow power is shown to decrease with increases in both the amount of boundary layer ingested and the propulsor mass flow, and there is thus no unique way to compare boundary layer ingestion and non-boundary-layer-ingestion systems. Using the ideas presented, however, the benefit can be assessed for any given comparison. The analysis is applied to an advanced civil transport aircraft concept with 40% of the fuselage boundary layer ingested, yielding a reduction in required propulsor mechanical power of 9% relative to a non-boundary-layer-ingestion configuration with the same propulsors, in agreement with computational fluid dynamics calculations and wind tunnel experiments.
Boundary Layer Ingestion (BLI) Reduces Wasted Energy
Flow control of separating boundary layer on the – Studylib.net – http://studylib.net/doc/8731736/flow-control-of-separating-boundary-layer-on-the
A Method for the Control of the Coanda Effect – IEEE Conference Publication – Ieeexplore.ieee.org – https://ieeexplore.ieee.org/document/8326823/
SYNTHETIC JETS | Annual Review of Fluid Mechanics – Annualreviews.org – https://www.annualreviews.org/doi/abs/10.1146/annurev.fluid.34.090501.094913
Benefits of vortex generators – PNW Aero – Pnwaero.com – http://www.pnwaero.com/benefits_of_vgs.htm