How can boundary layer control reduce drag?

Boundary layer control is a technique that can improve the aerodynamic performance of an aircraft by manipulating the flow of air over the surface. It can reduce drag, increase lift, and enhance stability and control. In this article, you will learn what boundary layer is, how it affects drag, and what methods can be used to control it.

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1 What is boundary layer?

Boundary layer is the thin layer of air that adheres to the surface of an object moving through a fluid. It is formed by the friction and viscosity of the fluid, which slow down the air molecules near the surface. The boundary layer can be laminar, turbulent, or transitional, depending on the speed and shape of the object. Laminar boundary layer has smooth and orderly flow, while turbulent boundary layer has chaotic and irregular flow. Transitional boundary layer has both laminar and turbulent regions.

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2 How does boundary layer affect drag?

Drag is the force that opposes the motion of an object through a fluid. It consists of two components: pressure drag and skin friction drag. Pressure drag is caused by the difference in pressure between the front and rear of the object, while skin friction drag is caused by the friction of the fluid on the surface of the object. Boundary layer affects both types of drag, but in different ways. Laminar boundary layer has lower skin friction drag than turbulent boundary layer, because it has less friction and heat transfer. However, laminar boundary layer also has higher pressure drag than turbulent boundary layer, because it tends to separate from the surface earlier and create a larger wake behind the object. Turbulent boundary layer has higher skin friction drag than laminar boundary layer, because it has more friction and heat transfer. However, turbulent boundary layer also has lower pressure drag than laminar boundary layer, because it tends to stay attached to the surface longer and create a smaller wake behind the object.

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3 What are the methods of boundary layer control?

Boundary layer control is the process of modifying the boundary layer to achieve a desired aerodynamic effect. There are two main methods of boundary layer control: passive and active. Passive methods use the shape and texture of the surface to influence the boundary layer, while active methods use external devices or energy sources to manipulate the boundary layer. Some examples of passive methods are dimples, vortex generators, and riblets. Dimples are small depressions on the surface that create turbulence and delay separation. Vortex generators are small fins or tabs on the surface that create vortices and energize the boundary layer. Riblets are small grooves on the surface that reduce skin friction by aligning with the flow direction. Some examples of active methods are suction, blowing, and plasma actuators. Suction is the removal of air from the surface through holes or slots, which reduces the thickness and drag of the boundary layer. Blowing is the injection of air into the boundary layer through holes or slots, which increases the momentum and lift of the boundary layer. Plasma actuators are devices that use electric fields to ionize the air and create body forces that alter the boundary layer.

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Dr. Aleksandar Simic

Chairman of the Board of Directors at FTC Aviation SRL / FTC Business Jet Management AG
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Minimizing the boundary layer thickness with continuous suction or injection reduces the skin-friction drag considerably due to the increase of the laminar flow region. A systematic methodology is presented starting with the governing equations of the laminar boundary layer. After that, solutions are obtained using two numerical methods. Starting with Cebeci-Smith’s method, a Falkner-Skan transformation is done. After that, the system is linearized using Newton’s method. This procedure yields a tridiagonal matrix-vector system which is solved using the block-elimination method. The second approach for solving the boundary layer equation is to rewrite them in a discrete form.

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4 What are the benefits of boundary layer control?

Boundary layer control can have various benefits for an aircraft, depending on the method and the objective. For example, it can reduce drag by delaying separation, reducing wake size, and lowering skin friction, which can improve fuel efficiency, range, and speed. Additionally, it can increase lift by increasing the momentum, pressure, and circulation of the boundary layer, which can improve takeoff, landing, and maneuverability. Furthermore, boundary layer control can enhance stability and control by modifying the pressure distribution, vortex shedding, and flow separation; this can lead to improved handling, responsiveness, and safety.

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5 What are the challenges of boundary layer control?

Boundary layer control is a complex process that involves physics, engineering, and design. It often involves trade-offs between different aerodynamic parameters, such as drag, lift, noise, and weight. Additionally, it may require additional equipment or materials which can increase cost. Additionally, boundary layer control may be subject to regulation and certification by aviation authorities, limiting its application and innovation. Despite the challenges of boundary layer control, it can be a useful technique for improving the aerodynamic performance of an aircraft. By understanding boundary layer and what methods can be used to control it, you can appreciate the science and art of flying.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can boundary layer control reduce drag?

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1 What is boundary layer?

2 How does boundary layer affect drag?

3 What are the methods of boundary layer control?

4 What are the benefits of boundary layer control?

5 What are the challenges of boundary layer control?

6 Here’s what else to consider

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