# Shock Boundry Layer: Definition & Aircraft Response

• scott_alexsk
In summary: Shock Boundary Layer: A thin region of non uniformity in the Euler Equations that slows down the flow due to viscosity. The boundary layer has a non dimensional thickness, which is \delta \sim O(M^2/\sqrt{Re}), where M is the Mach# and Re is the Reynolds#. Inside the boundary layer there is a Shock Layer, which is decreasing the flow velocity.
scott_alexsk
Hello,

In some papers I have come across this term. There are no formal definitions online, but I have a rough idea that it deals with a change in the nature of air in the boundry layer of an aircraft surface after reaching supersonic speeds. About this topic I have several questions. What is the formal defintion of the shock boundry layer? Also does anyone know anything about a responsive aircraft surface to variable speeds?

Thanks,
-scott

When looking at the supersonic flow over a slender body, there are two main regions. Exclude for a moment what happens at the nose, and focuse on one of the sides of the body. There the bulk flow is also mainly supersonic, even though the leading shock wave has decreased a little bit its velocity (the leading shock wave is a weak Mach wave at this point). Even though the bulk flow is mainly inviscid and isentropic (assuming ionization and dissociation of air negligible at this mach #) and so the Euler Equations are applied there, there is an small region of non uniformity of the Euler Equations, where the flow is slowed down because of the viscosity. This thin region is call boundary layer, and has a non dimensional thickness $$\delta \sim O(M^2/\sqrt{Re})$$, where M is the Mach# and Re is the Reynolds#. In that region the flow has to go from supersonic speeds to subsonic speeds, and as you may know there is no physical way of slowing down a supersonic flow to subsonic unless a shock is produced. Inside the boundary layer there should be a Shock Layer, in which a shock (more or less of the same shape than the boundary layer) is decreasing the flow velocity. This shock layer has a great importance, because it is related to the Entropy Layer. This shock layer is one of the most important causes of the drag of a hypersonic aircraft. And as you may imagine by yourself, the state of the surface of the body is extraordinarily important, because $$\delta<<1$$, and the shock layer may interact with the local compressions-expansions of the fluid caused by a non uniform surface. That is why those planes are also extraordinarily polished.

Last edited:
Thanks Clausius2,

Also what is there to be gained from having an aircraft with a 'variable' surface that can respond to the shock-boundry layer? Has anyone heard of mesoflaps?

Thanks
-scott

## What is a shock boundary layer?

A shock boundary layer is a layer of compressed air that forms on the surface of an aircraft when it travels at supersonic speeds. This layer is caused by the shock waves that are generated by the aircraft's high speed, and it can affect the aircraft's performance and stability.

## How does a shock boundary layer affect aircraft response?

A shock boundary layer can cause changes in the airflow around an aircraft, which can affect its aerodynamic properties. This can result in changes in the aircraft's lift and drag, as well as its stability and control. In extreme cases, it can even cause the aircraft to lose control.

## What factors influence the formation of a shock boundary layer?

The formation of a shock boundary layer is influenced by a number of factors, including the shape and speed of the aircraft, the atmospheric conditions, and the altitude at which the aircraft is flying. Additionally, the design of the aircraft's wings and other surfaces can also impact the formation of the shock boundary layer.

## Can a shock boundary layer be beneficial to aircraft performance?

In some cases, a shock boundary layer can actually be beneficial to aircraft performance. For example, it can reduce drag and increase lift, allowing the aircraft to fly more efficiently at high speeds. However, this is highly dependent on the specific design and conditions of the aircraft.

## How do scientists study shock boundary layers?

Scientists study shock boundary layers through a combination of theoretical models, computational simulations, and experimental testing. This involves using specialized equipment and techniques to measure and analyze the airflow around an aircraft, as well as its response to different conditions and design changes.

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