22

This is a broad question, hence a broad answer 😃 As high eng CFD now stands, it already is correct enough for, say, 95% of use cases. I am not aware of much utility in modelling complex turbulent flow to a high level of accuracy. CFD will proceed to become more and more accurate, but 100% accuracy will never be achieved, not at least if we are to assume ...


13

The mini stall is the stall that we normally talk about when considering the flight of an aeroplane, and is therefore the main stall. Below the AoA where the first stall occurs, the wing profile is reasonably aligned with the airflow, and flow at the upper wing surface can remain attached. When upper surface flow detaches: upper surface suction force ...


11

100% correct, no. That's because CFD (and a great many other computer applications) work on numerical approximations. There's either no exact solution* known, or the exact solution isn't computable. So the application uses numerical approximations, typically on a grid or mesh. To get a better approximation, you either make the grid smaller, or use a ...


8

Static stall is what we typically think of, when we think of stall. Slowly increasing AOA and then loss of lift as the flow separates. But pitch rate (rate of increase of AOA) has an impact too. At high pitch rates, the wing can go beyond normal stall AOA and still provide significant lift, but for just a short period of time. After that, the bottom falls ...


7

The answer is deeply rooted in the theory of compressible fluid dynamics, so for a fully satisfactory take on the matter you might want to refer to a textbook on that topic (such as Thompson or Shapiro). I will try to give a qualitative explanation. For our discussion here, let's consider a shockwave as the way to abruptly slow down a supersonic flow and ...


7

Beta lines are either linear or parabolic shaped lines that are conveniently placed and spaced between the surge line and the choke line of the compressor (or fan) characteristic (also known as compressor/fan map). Such lines are also found in the turbine characteristics maps. Basically, the Beta lines create a new axis system that prevent a solver from ...


7

First, the Millennium Challenge asks about the existence and uniqueness of a solution to the Navier-Stokes (NS) equations given any compatible initial/boundary conditions. This question is of significant interest from an applied mathematics perspective, but it doesn't* help with actually obtaining the numerical solution and, therefore, is of limited interest ...


6

What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers cause swirling of the fluid and reversal of the current, creating a turbulent flow region. It is this same principle that holds a ping pong ball stationary in a ...


5

Tripping the boundary layer refers to the action of artificially transitioning a laminar boundary layer into a turbulent one. It can happen intentionally (via turbulator) or unintentionally (via imperfect aero-smoothness, such as rivets, bolts, counter-sinks). At the point of boundary layer tripping, the surface protrusion destabilizes the laminar boundary ...


5

No. These are very different things. The shear centre (SC) is a purely mechanical (structural) feature of the wing design. It is a point along the chord such that application of force - no matter what force - causes the wing only to bend but not twist. Unlike the aerodynamic centre (AC) and centre of pressure (CP), it makes no sense to talk about a single ...


5

I think I have a partial answer that I have found in a paper [1]: The following is a quote from the introduction: It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two-dimensional flow the surface ...


4

Rectangular wings can get away without washout because that planform tends to have favourable root to tip stall progression as a characteristic of the rectangular shape. The stall strips aren't really to change the root to tip stall behaviour as a substitute for washout; their job is to generate a local flow separation, just prior to the natural stall, who'...


4

The airfoil always starts with a laminar boundary layer; at some point on the airfoil surface, the boundary layer may transition to turbulent. In a uniform free-stream, two modes of transitions dominate: Natural transition Forced transition a. Natural transition Natural transition is the transition mechanism that we typically associate with Reynolds ...


3

This is due to the Coandă effect. https://www.britannica.com/technology/fluidics#ref129652 In the 1930s Henri Coandă, a Romanian scientist, described what is now known as the Coandă effect, a major contribution to fluidic technology. He observed that as a free jet emerges from a jet nozzle the stream will tend to follow a nearby curved or inclined surface. ...


3

The subtitle of the video states: This video shows a high-fidelity CFD simulation of flow control applied to realistic wing profiles using PHASTA and ParaView Catalyst. Work done by Michel Rasquin from Argonne and Ken Jansen from UC Boulder. Realistic wing profiles sounds like: we're actually looking at a wing in a vertical orientation - or in a ...


3

Your answer is correct. The pressure distribution from the xfoil simulation on a NACA 0012 are shown below for AOA 0 and AOA 5. As you can see, the lower side pressure does increase. However, most of the lift is generated by the suction on the upper surface, which is typical of subsonic flight.


3

The illustrations under the time-invariant, inviscid, irrotational and incompressible (potential) description is a bit misleading, in my opinion. In the real world, assuming the airfoil starts from rest (no flow field) and you start accelerating it to some airspeed, viscosity should start generating vorticity as soon as airspeed becomes non-zero. Therefore, ...


3

Absolutely you can use a fabric for covering and airframe. I used to build large scale R/C aircraft much like they used to build real world aircraft. I used fine silk to cover the wings, elevator, tail, and fuselage. I used epoxy to adhere it to the wooden airframe and then later apply some kind of lacquer, which serves to shrink and pull tight the silk and ...


3

First, let me explain the context of your question: It is about a concept for a future airliner: Boeing Blended-Wing-Body Model 450-1U, taken from NASA/CR-2006-214534. In the artist impression above the engines are podded and sit on struts in order to achieve a uniform flow field at the intake face. However, the strut and nacelle surface area will ...


3

Excellent application inverted on the drag racer, which brings home the point perfectly. If you have enough excess thrust, devices like these can dramaticly increase Coefficient of Lift, but at the cost if loss of lift efficiency, or lower lift to drag ratio. In the case of the dragster, more propulsion efficiency is lost if tires break loose from the ...


3

On a flat plate the thickness of the boundary layer grows along the flow path without a tendency to separate. Therefore, thickness alone is not the culprit. However, when the pressure gradient in flow direction becomes positive (increasing pressure), the flow will slow down and thickness grows much faster than in non-decelerating flow. Now separation happens ...


3

For the reasons cited above, the answer definitely No, but one point to remember here is that high-end CFD does not have to be perfect to be immensely useful. Imagine one wanted to get 7-place precision out of a temperature prediction in CFD. This would be a fool's errand because in engineering practice, you can't measure free-stream temperatures to that ...


3

No, it will never be 100% accurate. Air or water are quite well described by the Navier-Stokes equations assuming the fluid is a continuum. Usually it is also assumed that they are Newtonian fluid. The Navier-Stokes equations for a turbulent flow must be solved numerically (for some laminar flows analytical solutions exist). Numerical solutions are never ...


2

6 degrees of washout means the angle of incidence is 6 degrees lower at the tip than the wing root, causing the wing root to stall first. Leading edge slats serve the same function, especially in combination with trailing edge flaps near the roots. Whether these strips would replace washout is debatable, trying to "save" an approach by pulling hard on the ...


2

This is a fascinating topic, and it really boils down to the mechanics of lift. Physically, upwash and downwash are generated as the air accelerates/decelerates and curves around the LE of the wing. At the wingtips, higher pressure air curves upward and generate the vortices we are familiar with. In fact, there are vortices generated at the trailing edge of ...


2

Both are different ways to express the boundary layer thickness in a deterministic way. The boundary layer according to wikipedia: a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. Since there can be debate about what significant ...


2

These may be regional terms, I have not heard of static vs dynamic stall, I am a pilot in the USA. Airplane flight (heavier than air, lifted mainly by reaction of wings not engine thrust.) is inherently dynamic and so a stall must be as well. I assume the question is actually asking about a typical low speed stall compared to an accelerated stall? An ...


2

14 CFR Part 60 regulates what constitutes a Flight Simulator Training Device (FSTD) for a particular aircraft model. FSTD is divided into two broad categories: Full Flight Simulators (FFS) and Flight Training Devices (FTD). A big difference between the two categories is FFS requires having motion cues, whereas motion cue is optional on FTD. Furthermore, the ...


2

As far as I can tell, there are two general approaches: Solid body dynamic simulation with lots of experimental coefficients. Most of the aerodynamic effects are simply summed for the whole aircraft, though the more advanced simulators need to calculate the wheel position and wing and fuselage flexing to get proper reactions mainly at landing. But it is ...


2

I'm pretty sure you were right. More angle of attack means more lift. More lift is caused by higher pressure under the wing and lower pressure on top. More lift causes more induced drag.


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