# Tag Info

56

The idea that force can only be transferred/propagated at the speed of sound is a simplification. It's a very reasonable simplification, which works in a very wide array of cases, but it's a simplification. A fundamental concept in gas mechanics is the "mean free path length." This is how far a molecule can travel on average before it collides with ...

33

It is only the pressure wave that can propagate at the speed of sound. This means that a molecule of "air" that is ahead of a subsonic aircraft can get pushed out of the way without hitting that aircraft. It gets a push from another molecule, which is pushed by chain of molecules until it get to the one which is the one that hit the aircraft. The aircraft ...

30

After passing through the shock wave, the air still has to evade the body. If the shock-wave touched the body, the air would have to escape through infinitely thin space at infinitely high pressure and it obviously can't do that. So behind the shockwave there is a boundary layer (called shock layer) in which the air is moving with the body and flows, under ...

10

In case of an infinestimally thin body in supersonic flow, the disturbances created propogate as Mach waves. However, as the the thickness of the body increases, the flow has to physically turn away from the object after the shock wave. For example, if a wedge at an angle $\theta$ is placed in supersonic flow, the fluid has to turn by that angle after ...

9

Normally, the flow speed at the wing is about the same as the speed ahead of the bow shock. Only if the fuselage is still conical and expanding at the wing station would the local flow speed be smaller. In order to be subsonic, you would need an absurdly blunt fuselage at Mach 1.5. But the wing is in a way still subsonic if its leading edge sweep is ...

8

Supersonic aircraft do not push the air out of the way in front of the aircraft. They only push it sideways out of the way after the aircraft has passed the air (for the air beside the aircraft), or at the very moment the air meets the aircraft (for the air directly in front of the very centre of the aircraft, i.e. directly in front of the nose spike). That’...

8

Where is the camera relative to these target aircraft, and what's behind them? Straight from NASA themselves: NASA flew a B-200, outfitted with an updated imaging system, at around 30,000 feet while the pair of T-38s were required to not only remain in formation, but to fly at supersonic speeds at the precise moment they were directly beneath the B-200 ...

7

Perhaps, at the root of your question, there is a misunderstanding of the nature of a shock wave. A shock wave is a surface, an ensemble of points, where the properties of the gas change discontinuously (it is not the only surface with this characteristic, but the other class, that of contact discontinuities is absolutely unstable, and quickly disappears). ...

7

As of the date of this answer, the X-59 has not yet flown, but according to the designers... The ground noise is expected to be around 60 dB(A), about 1/1000 as loud as current supersonic aircraft. This is achieved by using a long, narrow airframe and canards to keep the shock waves from coalescing. It should create a 75 Perceived Level decibel (PLdB) thump ...

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 ...

6

The Osprey's engines drive the rotors at 412 RPM in heli mode or 333 RPM in forward-flight mode, according to this article. With a rotor radius of 5.8m, that puts the tip velocity at 250m/s, or about 75% the speed of sound, at the higher RPM. So they aren't near supersonic yet because the blades are short enough. But there is a possibility of reducing blade ...

5

Very Interesting results. I must say I have never before seen such a peculiar case. Yes, theoretically it seems possible to have this situation, but it's difficult to say if this is possible in reality or not. I found another similar behavior mentioned here: One of the three aerofoils has dual shock. It's difficult to say whether it's a physically correct ...

5

The shock observed is effectively the same as the one over a wing. Compression waves are normally considered of infinitesimally small thickness, although due to the non-continuous nature of gases made of particles, this normally makes the shock on the same order of magnitude as the mean free path of a gas molecule (See here for the values at different ...

5

Yes, it is possible . The shockless transition from supersonic to subsonic. It has been tested both experimentally and numerical solution. Read this AIAA Paper. But in your case I would suggest you to be a bit cautious and recheck your mesh and boundary condition in CFD solver.

5

Neglecting friction does indeed lead to inaccuracies. Those, however, are small for weak shocks, so using the equations for inviscid shocks gives a good approximation. Especially in the times before electronic computers, this was a major simplification while the error was tolerably small. Stronger shocks should be avoided and replaced by a cascade of weaker,...

4

Flow conditions are not so different, only the reaction of the flow to an obstacle is. By shaping the intake appropriately, supersonic flow is handled easily, and shocks will be helpful in decelerating and compressing the flow. However, supersonic flow demands sharp intake lips which are hit by the flow at the right angle, otherwise drag rises quickly to ...

4

An adiabatic expansion lowers temperature, an adiabatic compression raises temperature, analogous to the way a refrigerator and an air conditioner work. Or a bicycle pump, it gets quite hot while you're pumping air. A shock wave compresses air - at supersonic speeds the object travels faster than the pressure information, and when the object arrives at a ...

4

Supplementary answer: As the question mentions this NASA page shows a diagram of traditional Schlieren photography and it's the idea I had in my head. I assumed you'd need a carefully prepared light beam and couldn't figure out how this was done. Than's to @ymb1's comment, here are some slides from the NASA Armstrong Flight Research Center presentation ...

4

Regardless of the shape of the wing, ramjets only outperform turbojets at supersonic airspeed. Ramjets work because the speed of the incoming airflow is such that there is no rotating compressor/turbine required for keeping the jet thrust going. So "I think ramjets are a really fuel-hunger type of turbine.." is a misconception, ramjets have no ...

3

Continue reading, I have found the answer. Look at problem 9.3, page 663: And here is the author's solution:

3

All pilots who transited into supersonic speed on older designs which were not perfected for transsonic flight reported that suddenly all that shaking and vibrating stopped and flight became smooth again as soon as they went over Mach 1. So the answer is no. With supersonic speed the buffeting stops.

3

After the rear shock leg, the subsonic flow inside the boundary layer decelerates further, which causes an expansion of the streamtubes and therefore reattachment. Note that with the presence of the supersonic tongue, the deceleration into the subsonic regime happens somewhat "slower" inside the boundary layer than outside (especially above the slip line).

3

The simplest answer is that a shock wave for a boundary layer means a sharp adverse pressure gradient. You probably know already that adverse pressure gradient will thicken a boundary layer and thick boundary layers are more prone to separation. Also, if the adverse pressure grad. is so strong, it can immediately produce a detachment of the flow from the ...

3

Looking at the Wikipedia article on ramjets, we have: Although ramjets have been run as slow as 45 metres per second (160 km/h), below about Mach 0.5 (170 m/s; 610 km/h) they give little thrust and are highly inefficient due to their low pressure ratios. So supersonic speed is not necessary for ramjets to work, although Due to the stoichiometric ...

2

The wind tunnel design is not providing clean smooth airflow. Turbulence at supersonic speed does not travel upstream so the turbulence must be created by the wind tunnel design.

2

After a lot of digging around, I found this. ...the main noise-reduction technique is to redesign the aircraft to reshape the acoustic signature. A sonic boom consists of a double bang on the ground that is caused by two large pressure pulses. As the random shockwaves travel away from the aircraft at the speed of sound, they pile up and coalesce. ...

2

As you know, wall shear stress $\tau_w$ (and thus friction drag) can be described as: $$\tau_w = \mu \frac{\partial u}{\partial y} \vert_{y=0}$$ where $\mu$ is the dynamic viscosity of the fluid and $\frac{\partial u}{\partial y} \vert_{y=0}$ is the fluid velocity gradient at the wall. The shock-wave boundary-layer interaction (SBLI) can change these two ...

2

Why is that the ramjets do not require [variable geometry] to maximize pressure recovery? Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide ...

2

With a higher flight velocity (still subsonic), the supersonic pocket on the upper side of the wing starts earlier and takes longer until it collapses into the shock. That is quite normal, as the speed over the whole upper wing is increased with increasing flight speed. Think of what happens at the extreme case of accelerating flight speed towards Mach 1: ...

2

Your airfoil is traveling supersonic at around Mach 2. First the flow encounters a bow shock. Away from the leading edge, the bow shock morphs into an oblique shock. You can clearly see Mach number rapidly decreasing after the shock. After passing through the thickest point of the airfoil, the expansion wave begins due to gradually increasing expansion ...

Only top voted, non community-wiki answers of a minimum length are eligible