SDF Aerospace and Aerodynamics Corner

MiG-29 said:

On the video they have an internal compression system see


Internal compression intakes
The entire system of shocks in this case is located inside the intake duct. A
simple example of such a system is a convergent-divergent nozzle with reversed flow
(refer Fig. 1.2). Compression in this case is isentropic up to the throat after which a
normal shock, whose location is dictated by the back pressure, decelerates the flow.
This is followed by diffusion in the divergent portion of the intake.This configuration is however unstable with regards to fixing the position
(anchoring) of the normal shock. A system of oblique shocks too can be used to the
same effect.
The main advantage offered by internal compression intakes is the lower cowl drag,
due to the fact that flow turning is contained within an almost parallel sided duct.
Some of the major problems associated with internal compression intakes include
boundary layer ingestion (shock boundary-layer interaction) and the problem of
‘starting’. The later is discussed in detail in the next chapter. These problems
complicate the design of these intakes.

Another problem associated with internal and mixed compression intakes is
that of ‘starting’. In the ‘not-started’ condition, the mass flow and total pressure
recovery of the intake are very low leaving no room for sub-critical performance. In
such a situation, catastrophic failures due to combustor over-heating and compressor
surge have a high probability of occurrence. To avoid such a situation, the ‘start’ and
‘unstart’ limits of an intake need to be defined clearly at the design stage.

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MiG-29 said:

External compression intakes
In these, the entire system of shock waves is located outside the intake duct. A
simple example of such a system is a divergent duct having a normal shock at entry
do these intakes have starting?


These intakes do no suffer from problems like ‘starting’. Also, this
configuration allows shortening the length of the intake as a whole. There
where is located the shock system on the video?


Internal compression intakes
The entire system of shocks in this case is located inside the intake duct

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Engineer said:

Whether an inlet is internal, external, or mixed compression is determined by the position of the oblique shock waves. This is illustrated in the book, which I reproduce here as a screen capture.

The video shows what happen as compressible flow (that is air) goes through the throat region. The throat is inside the inlet, not outside. A normal shock is created near the throat due to air being compressible, and due to Bernoulli's principle. Rules of aerodynamic are the same regardless of what type of inlet it is.

Your claim that variation in throat area regulates air flow in the inlet is your opinion, and it is wrong. The premises that you have created to support this opinion of yours are pseudo-aerodynamic theories that you made up, where they only occur in your fantasy world and does not apply in reality. All variable-geometry does is move the terminal shock, and it is said so in the video

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. Mass flow is fixed because of choke, said so in video

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. Density at the throat changes as area of the throat changes because of fixed mass flow, said so

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.

Mass flow is the same at the inlet mouth and at the throat, and this is called conservation of mass:

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Engineer said:

Nope. The video shows what happen inside the inlet as compressible flow goes through a throat. Two effects are observed. One is that of Bernoulli's principle, where air speeds up as it flows from a wider region to the throat. The second effect is compressibility, which is observed as shock waves.

Mass flow rate is fixed. All variable-geometry does is move the terminal shock, and it is said so in the video

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. Mass flow is fixed because of choke, said so in video

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. Density at the throat changes as area of the throat changes because of fixed mass flow, said so

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.

.

The terms internal, external or mixed compression refer to the location of oblique shock waves. For an internal compression inlet, the oblique shocks are inside the inlet. For an external compression inlet, the oblique shocks are outside the inlet. For mixed compression, the oblique shocks are inside and outside of the inlet.

The location of oblique shock waves of internal, external, and mixed compression inlet does not change the rules of aerodynamic.

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Engineer said:

Nope. The video shows what happen inside the inlet as compressible flow goes through a throat. As the air flows from a wider point to narrower point, the velocity picks up due to Bernoulli's principle. At the same time, compression occurs, as evident by the normal shock. Mass flow rate is conserved before and at the throat region, showing your claim that varying the throat area causes change to flow rate as false. This is also explained in the book as shown in the screen capture below:

The symbol mi is the mass flow the inlet mouth, and ms is the mass flow at the throat. The two values are equal, thus narrowing of the throat region does not regulate air flow, meaning your claim is incorrect.

The terms internal, external or mixed compression refer to the location of oblique shock waves. For an internal compression inlet, the oblique shocks are inside the inlet. For an external compression inlet, the oblique shocks are outside the inlet. For mixed compression, the oblique shocks are inside and outside of the inlet.

The location of the oblique shock waves does not alter how compressible fluid behaves within an inlet. Here is a video showing how compressible flow behaves:
[video=youtube;JhlEkEk7igs]http://www.youtube.com/watch?v=JhlEkEk7igs[/video]

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MiG-29 said:

haha the video shows an internal compressor, with starting or running the shock wave, it is a supercritical condition inside the duct. it can not get subcritical condition or spilling

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MiG-29 said:

the formula only stablishes As=A0i, or in other words the air in the intake is equal to the air that enters the throat in fact As/As=1 multiply that for A0i/A1 means some air is spilled, while A0i/A1 means the spilling so far you are wrong completly there is conservation of mass from the air entering the intake but spills do exist and that air never enters the intake and remains free stream and that is called the mass flow rate

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MiG-29 said:

Aoi/A1 means flow is spilled or mass flow rate, sink analogy stands, intakes and sinks can be obverflowed.

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MiG-29 said:

the video has an intake starting, external compressor never start the shock wave, in fact DSI is external compression

this paper

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proves my point and the video says shock running and starting

External compression intakes
In these, the entire system of shock waves is located outside the intake duct. A
simple example of such a system is a divergent duct having a normal shock at entry




do these intakes have starting?
These intakes do no suffer from problems like ‘starting’. Also, this
configuration allows shortening the length of the intake as a whole. There


where is located the shock system on the video?
Internal compression intakes
The entire system of shocks in this case is located inside the intake duct

in the video at minute 22 they say engine started or running and in minute 26 the say the inlet design is for mach numbers up to mach 3, which are the SR-71 and Xb-70 intake types

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Engineer said:

Nope. The video shows how compressible flow behaves inside the duct of an inlet. Whether the compression is internal, external, or mixed does not alter the aerodynamic principles of the inlet. Oblique shock waves reduce the speed of supersonic flow, and a final normal shock turns the supersonic flow into a subsonic one. Mass flow rate remains the same upstream and downstream of the throat.

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Engineer said:

Nope. As is the throat cross sectional area, whereas A0i is the cross sectional area of the free stream flow, these two variables do not equate. The two variables that equate are mdoti and mdots, referring to mass flow rate through the inlet mouth and throat respectively.

Spill air does not enter the inlet. The remaining air that does have its mass conserved as its go from the inlet mouth to the throat. Mass is conserved, there is nothing wrong with what I've said. Furthermore, spillage occurs on fixed inlet and DSI, without the need of variable-geometry.

All variable-geometry does is move the terminal shock, and it is said so in the video

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. Mass flow is fixed because of choke, said so in video

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. Density at the throat changes as area of the throat changes because of fixed mass flow, said so

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.

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