공기 역학적 항력 및 감소 기술 이해

공기 역학적 항력을 이해하는 것은 엔지니어가 설계를 최적화하고 항력을 최소화하는 데 매우 중요합니다.

### 공기 역학적 항력 이해
* ✈ 유체가 물체를 지나가거나 물체가 정지된 유체를 통과할 때, 유체는 물체에 힘을 가합니다. 이 힘을 **유체의 흐름과 같은 방향으로 작용하는 힘(항력)**과 흐름 방향에 수직으로 작용하는 힘(양력)의 두 가지 요소로 나눌 수 있습니다.
* 🛡 대부분의 경우 항력은 **차량의 연료 소비와 성능에 큰 영향**을 미칠 수 있기 때문에 바람직하지 않습니다. 엔지니어들은 이를 최소화하기 위해 많은 노력을 기울입니다.
* 🦈 "항공 업계는 난류 경계층을 수정하고 마찰 저항을 줄이는 **인공 상어 피부를 기반**으로 한 항력 감소 혁신 기술을 사용하여 **연간 수백만 달러의 연료비를 절감**할 수 있습니다."
* 🌬 압력 항력은 무딘 몸체에서 가장 중요하며, **흐름 분리가 발생하여 큰 항력이 발생**하면 크게 증가합니다.
* 🌀 "난류는 흐름의 분리를 지연시키고 압력 항력을 감소시키기 때문에 **골프공에 보조개**가 있고 일부 비행기 날개에 **작은 와류 발생기**가 있는 것입니다."
* ✈ 유체를 통과하는 몸체는 일반적으로 **흐름 분리의 영향을 최소화**하기 위해 **물방울 모양**으로 유선형으로 설계됩니다.
* 🦈 "연구에 따르면 상업용 여객기에 **인공 상어 가죽을 입히면 총 항력을 2%까지 줄일 수 있어** 업계에 막대한 연료 절감 효과를 가져올 수 있습니다."

Thanks to CuriosityStream for sponsoring this
video. Watch thousands of documentaries and get access
to Nebula for free when you sign up using the link in the description. When fluid flows past an object, or an object
moves through a stationary fluid, the fluid exerts a force on the object. We can split the force into two components
- one acting in the same direction as the fluid flow, which is called drag, and one
acting perpendicular to the flow direction, which is called lift. If the fluid is a gas, like air, we call these
aerodynamic forces, and if it's water or any other liquid, we call them hydrodynamic forces. This video is going to focus on the drag force,
and I'll cover lift in a separate video. Most of the time drag forces are undesirable. They can have a huge effect on the fuel consumption
and performance of vehicles, for example. And so engineers go to great lengths to minimise
them. We'll explore some interesting ways of reducing
drag later on in the video, including how

the airline industry could save millions of
dollars a year in fuel costs by using a drag-reducing innovation based on artificial shark skin. But to optimise the design of objects affected
by drag forces, we first need to understand where these forces come from, so let's start
by covering the basics. Drag forces are caused by two different types
of stress, which act on the surface of an object. First, we have the wall shear stresses. These stresses act tangential to the object's
surface and are caused by frictional forces that arise because of a fluid's viscosity. Then we have the pressure stresses. They act perpendicular to the object's surface
and are caused by how pressure is distributed around the object. The drag force is the resultant of these two
stresses in the direction of the flow. So if we know exactly how the stresses are
distributed over the surface of our object, we can integrate them to obtain the resultant
drag force. The component of drag caused by the shear
stresses is called friction drag, and the

component caused by the pressure stresses
is called pressure drag, or form drag. Pressure drag is most significant for blunt
bodies like this sphere. It is essentially caused by a difference in
pressure between the front and rear of an object. Pressure drag increases significantly if flow
separation occurs, which is when the fluid boundary layer detaches from the body, creating
a wake of recirculating flow. This creates an area of low pressure behind
the body, called the separation region, and results in a large drag force. If you're trying to reduce drag forces, you'll
want to minimise flow separation at all costs. Flow separation can also cause vortex shedding,
which can generate unwanted vibrations and instability. To understand why flow separation occurs, let's
look at flow on the upper surface of the sphere. As the fluid passes over the surface of the
sphere, it is initially accelerating, and so pressure is decreasing in the direction of
the flow. This is called a favorable pressure gradient.

Beyond a certain point, the flow then begins to decelerate, and so pressure in the flow direction is increasing. This increase in pressure is called the adverse pressure gradient, and it has a significant effect on the flow close to the wall. If the pressure increase is large enough, the flow will reverse direction, and since it can't travel backwards because of the oncoming fluid, it detaches from the surface, resulting in flow separation. Flow separation occurs at around 80 degrees for a smooth sphere in laminar flow. If the boundary layer is turbulent instead of laminar, it's better able to remain attached to the surface, and flow separation is delayed until around 120 degrees, which reduces the pressure drag significantly. This is because turbulence introduces a lot of mixing between the different layers of flow, and this momentum transfer means the flow can sustain a larger adverse pressure gradient without separating. This is why golf balls have dimples instead of being perfectly smooth - the dimples generate

turbulence, which delays flow separation,
reduces drag, and allows the ball to travel further. This idea of using turbulence to delay flow
separation and reduce pressure drag is also why some airplane wings have small vortex
generators protruding from them. Bodies that travel through fluids, like plane
wings or submarines, are usually designed to be streamlined in a teardrop shape to minimise
the effect of flow separation. For very streamlined bodies like this airfoil
at a shallow angle of attack, pressure drag is small because flow separation is significantly
delayed, or doesn't occur at all. For bodies like these it's the wall shear
stresses which contribute most to the total drag force. The drag component caused by these stresses
is called friction drag. Friction drag increases with the viscosity
of the fluid, and is most significant for bodies which have a large surface area aligned
with the direction of flow. We saw earlier that turbulence delays flow
separation, which reduces the pressure drag.

But for friction drag, it has the opposite
effect. Laminar and turbulent boundary layers have
very different velocity profiles. The velocity gradient at a wall is steeper
in turbulent boundary layers than in laminar ones, and so turbulence produces larger shear
stresses. So to reduce friction drag, you want to delay
the transition to the turbulent regime and maintain laminar flow for the largest possible
distance around the object. It has been estimated that obtaining laminar
flow over the wings and fin of commercial aircraft could reduce the total drag force
by around 10 to 15 percent, but this is very difficult to achieve. Techniques like Hybrid Laminar Flow Control
have had some success - it involves using suction to pull air through small holes into
the wing, which delays the onset of turbulence. Research has also focused on minimizing the
friction drag associated with a turbulent boundary layer. When trying to minimize drag, engineers very
often look to nature for inspiration.

Sharks are of particular interest because of the unique microstructure of their skin. Shark scales contain microscopic ridges, which are aligned with the direction of flow. These ridges modify the turbulent boundary layer in the near-wall area, and this has the effect of reducing friction drag. Research indicates that coating a commercial airliner with artificial shark skin of similar microstructure could reduce its total drag force by 2%, which would result in massive fuel savings for the industry. This approach has yet to be widely implemented on commercial aircraft, partly due to challenges associated with manufacturing, but this could change as the technology improves. We've seen that the magnitude of pressure and friction drag depends on the geometry of a body relative to the direction of flow. The most obvious example of this is a flat plate. If we position the plate at 90 degrees to the flow, it is a blunt body. Flow separates easily, creating a separation region, and so the pressure drag is large.

But the friction drag is almost zero, since
shear stresses aren't aligned with the drag direction. If we rotate the plate by 90 degrees, we now
have a very streamlined body. The pressure drag is small since there's no
separation region behind the body, but the friction drag is now much more significant. This logic also applies to airfoils, where
the angle of attack has a large influence on the drag force. At high angles of attack, separation occurs,
which significantly increases the drag force. When streamlining a body to reduce drag, it's
important to remember that the friction drag will increase as the pressure drag reduces,
and so these two aspects need to be carefully balanced. The shape that has the smallest total drag
force won't necessarily be the one that is most streamlined. I mentioned earlier that we can integrate
the pressure stresses and the wall shear stresses to obtain the total drag force. The problem is that in the vast majority of
cases, it's pretty much impossible to know

the detailed distribution of these stresses, and so we usually represent the total drag
force using the drag equation instead. The C-D term is the drag coefficient. It captures all of the hard-to-measure parameters,
like the effect of the geometry of the object or the effect of the flow regime, and can
be determined either experimentally, using a wind tunnel for example, or by running numerical
simulations. The other terms in the equation are the fluid
density, Rho, the free-stream velocity, V, which is usually assumed to be steady and uniform,
and A, which is a reference area that will depend on how the drag coefficient was determined. For airfoils and other streamlined bodies,
A is usually the planform area, and for blunt bodies, it's usually the projected
frontal area. The drag coefficient can vary quite substantially
with Reynolds number. Let's look at how it varies for a few different
two-dimensional shapes. For a flat plate oriented at 90 degrees to
the flow, the drag coefficient doesn't vary.

significantly with Reynold's number, because
flow separation will always occur at the edge of the plate, and so, although it is a blunt
body, it isn't affected by whether flow is laminar or turbulent. For blunt shapes like this disk, we see a large
decrease in the drag coefficient at the transition between laminar and turbulent flow, because
flow separation is delayed when the boundary becomes turbulent, reducing the drag force. And for streamlined bodies, the drag coefficient
reduces gradually as Reynolds number increases, since viscous forces are less significant
at higher Reynolds numbers. But the drag coefficient begins to increase
after the transition to turbulent flow, because as we saw earlier, a turbulent boundary layer
produces larger shear stresses. For a sphere, the drag coefficient graph looks
like this. One interesting thing about this graph is
that it's a straight line for Reynolds numbers less than 1, and the line is defined as 24
divided by Reynolds number.

At these very low Reynolds numbers, flow separation
doesn't occur, even for very blunt bodies like the sphere, and so all of the drag comes
from friction drag. Plugging this equation for C-D into the drag
force equation gives us an interesting expression. This is called Stokes' Law, and it is an exact
solution for the drag force acting on a sphere for Reynolds numbers less than 1. It's one of few cases where we have an analytical
solution for calculating the drag force, and it has some very useful applications. We can use it to easily calculate the terminal
velocity of a sphere falling in a fluid, so long as Reynolds Number is low enough. As a sphere falls through a fluid, its velocity
will increase, and so will its drag force. Terminal velocity is reached when the weight
of the sphere perfectly balances the drag force, so that the sphere stops accelerating. The drag force is defined by Stokes' Law, and the weight of the sphere is easy to calculate
based on its volume and density.

We just need to remember to subtract the density
of the fluid as well, to account for the buoyancy force. And so, based on Stokes' law, we can obtain this
equation for the terminal velocity of the sphere. We can apply this equation to create a viscometer,
which is used to measure the viscosity of a fluid. To do this, a sphere is dropped into a tube
of liquid, which is long enough that the sphere will reach terminal velocity. The terminal velocity can be measured by timing
how long it takes the sphere to pass between two points marked on the tube, and so the
viscosity of the fluid can be calculated using the equation we just derived. We know that pressure stresses and shear stresses
are the two fundamental causes of drag. But in some cases, specific components of the
drag force are named because of how they're caused, even though they're just different
forms of pressure or friction drag. In aviation, for example, three important sources
of drag are induced drag, wave drag, and interference drag.

If you'd like to learn more about these sources
of drag, I've covered them in the extended version of this video, which is available
now over on Nebula. For those who don't know, Nebula is a video
streaming service built entirely by educational creators. It's home to some of the best independent
creators out there, including Mustard, Real Engineering, and Practical Engineering. It's a place where creators can experiment
with new formats and longer videos. So in addition to their normal content, which
is completely ad-free on Nebula, you can explore exclusive Nebula Originals that you won't
be able to watch anywhere else, or dive a little deeper with bonus content,
like the extended version of this video where I've replaced this ad with a look at induced
drag, wave drag, and interference drag. To make Nebula even better, we've partnered
up with CuriosityStream. CuriosityStream is the streaming service for
high-quality documentaries.

It has thousands of incredible titles for you to explore, covering engineering, aviation, history, and more. Their new Engineering the Future series, in particular, is excellent. It investigates how the latest innovations in engineering are likely to shape our future, with hour-long episodes on wind energy, nuclear fusion, and one on aviation, which takes a fascinating look at how electric aircraft and new aircraft designs could revolutionize the industry. If you sign up to CuriosityStream using this link, you'll get a 26% discount on the annual plan, AND you'll get Nebula for free. That's CuriosityStream AND Nebula for less than 15 dollars a year. It really is the best deal in streaming. So to get the bundle deal and 26% off, head over to curiositystream.com/efficientengineer, or click on the link in the description. You'll be supporting not only this channel, but a load of other creators in the educational

community as well! And that's it for this review of aerodynamic
drag. Thanks for watching!

Understanding Aerodynamic Drag

The Efficient Engineer is a channel aimed at mechanical and civil engineers. The mission is to simplify engineering concepts, one video at a time!

The Efficient Engineer

유체가 물체를 지나갈 때 항력과 양력을 가합니다.

벽 전단 응력과 압력 응력으로 인한 항력을 최소화하여 연료 소비를 줄이고 차량 성능을 개선할 수 있습니다.

무딘 물체에서 중요한 압력 항력은 흐름 분리에 따라 증가하여 큰 항력과 잠재적인 불안정성을 유발합니다.

흐름 분리를 최소화하는 것은 항력을 줄이고 원치 않는 진동을 방지하는 데 매우 중요합니다.

압력 구배와 난기류가 흐름 동작에 미치는 영향

압력 구배는 흐름 분리를 유발할 수 있지만 난류는 이를 지연시켜 압력 저항을 줄일 수 있습니다.

골프공의 딤플과 비행기 날개의 와류 발생기는 난류를 생성하고 흐름 분리를 최소화하여 항력을 줄이도록 설계되었습니다.

유선형 물체는 흐름 분리가 지연되거나 전혀 일어나지 않아 압력 항력은 감소하지만 마찰 항력은 증가합니다.

난류는 압력 항력에는 긍정적인 영향을 미치지만 마찰 항력을 증가시킵니다.

항공기 날개에 층류를 구현하면 항력을 크게 줄일 수 있지만, 이를 달성하기는 어렵습니다.

마찰 항력을 최소화하고 전체 항력을 줄이기 위해 하이브리드 층류 제어 및 자연 모방과 같은 기술이 연구되고 있습니다.

상어 스킨이 드래그 포스에 미치는 영향

상어 가죽의 미세 구조는 마찰 항력을 감소시켜 상업용 여객기의 총 항력을 2% 감소시킬 수 있는 잠재력을 제공합니다.

흐름 방향과 관련된 기체의 형상은 압력 및 마찰 항력에 큰 영향을 미치며, 유선형은 항력 균형에 영향을 미칩니다.

총 항력은 일반적으로 항력 방정식을 사용하여 표현되며, 항력 계수는 물체의 기하학적 구조와 흐름 체계에 영향을 받습니다.

레이놀즈 수가 항력 계수에 미치는 영향

무딘 형상의 경우 층류에서 난류로 전환할 때 항력 계수가 급격히 감소하여 흐름 분리가 지연되고 항력이 감소합니다.

유선형의 경우, 항력 계수는 레이놀즈 수가 증가함에 따라 점차 감소하지만 난류로 전환한 후에는 전단 응력이 커지기 때문에 다시 증가합니다.

스토크의 법칙은 낮은 레이놀즈 수에서 구체의 항력에 대한 정확한 해를 제공하며 말단 속도를 계산하는 데 사용되어 유체 점도를 측정하기 위한 점도계를 만드는 데 적용됩니다.

항공에서 유도 항력, 파동 항력, 간섭 항력은 특정 원인에 따라 이름이 붙여진 전체 항력의 중요한 구성 요소입니다.

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