SDF Aerospace and Aerodynamics Corner

Engineer · Jan 12, 2012

MiG-29 said:
the only wrong person is you intakes do spill.

Wrong. The only wrong person is you, as no one claimed that intakes do not spill except you. I have pointed out that spillage occurs with fixed, variable-geometry, and DSI. By claiming I am wrong, you are essentially disagreeing that spillage can occur, which makes you wrong.

MiG-29 said:
your argument has no validity simply because external compression intakes have subcritical conditions.

My arguments are perfectly valid, because I do not employ logical fallacies like you do.

When normal shock is outside of the inlet mouth, sub-critical condition occurs. This is what causes spillage, not variation in throat geometry. Spillage occurs on fixed inlet and DSI as well, and no variable-geometry is required.

MiG-29 said:
You think gases are infinitively compressable, so spilling is impossible, but no, they are not infinitively compressable as water they can overflow, in fact they do spill

what determines the position of the shock waves are the intake mass flow that is the reason on the video they regulate the flow with the valve to control the start of the shockwave

Nope, I made no claim that gases are infinitively compressible. I am pointing out that behavior of compressible flow is applied in inlets regardless of what type of inlet it is, which is a fact. We have seen this fact at work with the presence of shock waves in the video. Your water-in-a-sink example has no shock wave and water is not compressible, hence your analogy is flawed.

Engineer · Jan 12, 2012

MiG-29 said:
Obviously you do not understand what is an internal compression intake they only work with supercritical conditions in few words inside the duct, the external compression intake of the F-14 or F-15 are external compression, they work with critical condition they never start the intake shock niether run it along the diffuser. XB-70 and SR-71 have mixed compression

Obviously, you are the one who doesn't understand how intakes work. Sub-critical, critical and supercritical refers to the position of normal shock with respect to its desired location. Oblique shock waves forming inside the inlet is called internal compression, not supercritical as you have claimed.

What does internal compression mean? It means the compression effect (presence of shock waves) is inside the inlet's mouth. What does external compression mean? It means the compression effect is outside the inlet's mouth. What does mixed compression mean? It means the compression effect is both inside and outside of the inlet. Compressible flow occurs regardless of whether it is internal, external or mixed compression inlets.

MiG-29 · Jan 12, 2012

Engineer said:
Wrong. The only wrong person is you, as no one claimed that intakes do not spill except you. I have pointed out that spillage occurs with fixed, variable-geometry, and DSI. By claiming I am wrong, you are essentially disagreeing that spillage can occur, which makes you wrong.

My arguments are perfectly valid, because I do not employ logical fallacies like you do.

When normal shock is outside of the inlet mouth, sub-critical condition occurs. This is what causes spillage, not variation in throat geometry. Spillage occurs on fixed inlet and DSI as well, and no variable-geometry is required.

Nope, I made no claim that gases are infinitively compressible. I am pointing out that behavior of compressible flow is applied in inlets regardless of what type of inlet it is, which is a fact. We have seen this fact at work with the presence of shock waves in the video. Your water-in-a-sink example has no shock wave and water is not compressible, hence your analogy is flawed.

Hahaha to start spillage shows perfectly the sink analogy is correct, second what controls the position of the shock wave is mass flow and basicly the engine air needs.

Your position to start is irreal, variable geometry intake have better pressure recovery than fixed intakes.

Why?
simple they are smaller so the engine`s mass flow needs are closer to what the intake can provide thus the aircraft has a wider range of speeds where it can be used, why? simply by being smaller they bypass less air so pressure recovery is higher.
and the proof is F-15 and F-14 have better pressure recover than the DSI and F-35 is slower, SR-71 and XB-70 have variable geometry intakes and fly at Mach 3 the video says the intake they show is for a Mach 3 aircraft
The Variable throat is to capture more air at lower speeds thus eliminating the need for a bigger intake.

DSI are okay for stealth and price, however you still need bypass doors or a very good turbofan with good bypass ratio.

MiG-29 · Jan 12, 2012

Engineer said:
Obviously, you are the one who doesn't understand how intakes work. Sub-critical, critical and supercritical refers to the position of normal shock with respect to its desired location. Oblique shock waves forming inside the inlet is called internal compression, not supercritical as you have claimed.

What does internal compression mean? It means the compression effect (presence of shock waves) is inside the inlet's mouth. What does external compression mean? It means the compression effect is outside the inlet's mouth. What does mixed compression mean? It means the compression effect is both inside and outside of the inlet. Compressible flow occurs regardless of whether it is internal, external or mixed compression inlets.

haha i never claimed they are subcritical i simply said they work with supercritical shock waves inside the diffuser and the video has a internal compression intake because it is a duct therefore the video shows a internal cormpression intake without by pass doors where they control the mass flow with a fixed throat and a valve and they show you how to start or run the shock wave.

The video shows no spillage because it is a duct and it has no bypass doors so the air mass if fixed

Engineer · Jan 12, 2012

MiG-29 said:
Hahaha to start spillage shows perfectly the sink analogy is correct, second what controls the position of the shock wave is mass flow and basicly the engine air needs.

Nope. Air is a compressible fluid, while water is incompressible. The former can compress and form shock wave, and when normal shock wave lies outside of the inlet mouth, spillage occurs. Your water-in-a-sink example deals with incompressible fluid with no shock wave, hence it is flawed.

MiG-29 said:
Your position to start is irreal, variable geometry intake have better pressure recovery than fixed intakes.

Why?
simple they are smaller so the engine`s mass flow needs are closer to what the intake can provide thus the aircraft has a wider range of speeds where it can be used, why? simply by being smaller they bypass less air so pressure recovery is higher.
and the proof is F-15 and F-14 have better pressure recover than the DSI and F-35 is slower, SR-71 and XB-70 have variable geometry intakes and fly at Mach 3 the video says the intake they show is for a Mach 3 aircraft

DHI which bases on the same principle as DSI targets Mach 7 as the operation speed. Current hypersonic test vehicles employ fixed-inlets, not variable-geometry ones. Your focus on speed to argue variable-geometry inlets as better is a flawed argument.

Performance of an inlet is measured based upon its pressure recovery ratio, not speed. Looking at pressure recovery ratio, we see the ratio is higher than 0.91 at M1.8. At M2.0, the coefficient decreases to 0.87.

The value for F-4D variable-geometry inlet is only 0.89 at Mach 1.8. 0.91 is higher than 0.89, which means DSI has better performance. Your claim that variable-geometry inlet is always better is incorrect.

Adding more variable ramps to force your variable-geometry inlet would increase pressure recovery, but this increase comes at the cost of complexity and weight. Mach 3 is pretty meaningless when your example aircraft maneuver like an airliner, even when your example aircraft fly at low supersonic speed.

MiG-29 said:
The Variable throat is to capture more air at lower speeds thus eliminating the need for a bigger intake.

Variable throat can only makes the inlet smaller, not bigger. Variable-geometry inlets suffer at lower speed as a result. From

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:

MiG-29 said:
DSI are okay for stealth and price, however you still need bypass doors or a very good turbofan with good bypass ratio.

Variable-geometry inlets still need bypass doors, or a very good turbofan with good bypass ratio. None of what you say here is only applicable to DSI or fixed-inlet.

On the other hand, fixed-inlet and DSI do not need variable-geometry to regulate air flow. Fixed-inlet only need bypass doors. Recall what you have said:

MiG-29 said:
If you were right you only would need the by pass doors, not variable geometry throats.

Engineer · Jan 12, 2012

MiG-29 said:
haha i never claimed they are subcritical i simply said they work with supercritical shock waves inside the diffuser

Nope. Sub-critical, critical and supercritical refers to the position of normal shock. Oblique shock waves forming inside the inlet is called internal compression, not supercritical as you have claimed.

MiG-29 said:
and the video has a internal compression intake because it is a duct therefore the video shows a internal cormpression intake without by pass doors where they control the mass flow with a fixed throat and a valve and they show you how to start or run the shock wave.

Nope. What the video shows is the behavior of compressible flow, which is applicable to internal, external, and mixed compression inlets. Shock waves are formed because of compressibility of air, and shock waves are used on all three types of inlets. The rules of fluid dynamics do not change for different types of inlet, and no where in the video does it mention that compression effects are only applicable to internal compression inlets.

MiG-29 said:
The video shows no spillage because it is a duct and it has no bypass doors so the air mass if fixed

Spillage occurs when normal shock is outside of the inlet mouth, or your so called sub-critical condition. When an inlet operates at critical or supercritical condition, then there is no spillage.

MiG-29 · Jan 12, 2012

Engineer said:
Nope. Sub-critical, critical and supercritical refers to the position of normal shock. Oblique shock waves forming inside the inlet is called internal compression, not supercritical as you have claimed.

Nope. What the video shows is the behavior of compressible flow, which is applicable to internal, external, and mixed compression inlets. Shock waves are formed because of compressibility of air, and shock waves are used on all three types of inlets. The rules of fluid dynamics do not change for different types of inlet, and no where in the video does it mention that compression effects are only applicable to internal compression inlets.

Spillage occurs when normal shock is outside of the inlet mouth, or your so called sub-critical condition. When an inlet operates at critical or supercritical condition, then there is no spillage.

Consider a stand-alone internal compression intake. As the Mach number is
gradually increased in the supersonic regime, a bow shock is formed at the mouth of
the intake. At this point, flow entering the intake is subsonic. As the free-stream Mach
number is gradually increased, the bow shock is pushed closer to the intake. It is
observed that even when the free-stream flow is at the design Mach number, there still
exists a bow shock at the intake entry2. Under these circumstances, the mass flow rate
is lower than that for which the intake is designed and the pressure recovery too is
unacceptably low. This situation is referred to as the problem of ‘Starting the Intake’.

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can you tell me where is the oblique and normal shocks outside in internal compression?

To start you do not understand what is called supercritical condition, critical condition is the intake is matched to the engine needs of mass flow, supercritical is when the shock wave system is swallowed because the engine needs more air than what the intake can provide, an internal compression sistem has critical and subcritical conditions but in reality an internal compression system is a pitot tube gone supercritical and by starting the shockwave they achieve compression

MiG-29 · Jan 12, 2012

Engineer said:
Nope. Air is a compressible fluid, while water is incompressible. The former can compress and form shock wave, and when normal shock wave lies outside of the inlet mouth, spillage occurs. Your water-in-a-sink example deals with incompressible fluid with no shock wave, hence it is flawed.

DHI which bases on the same principle as DSI targets Mach 7 as the operation speed. Current hypersonic test vehicles employ fixed-inlets, not variable-geometry ones. Your focus on speed to argue variable-geometry inlets as better is a flawed argument.

Performance of an inlet is measured based upon its pressure recovery ratio, not speed. Looking at pressure recovery ratio, we see the ratio is higher than 0.91 at M1.8. At M2.0, the coefficient decreases to 0.87.

The value for F-4D variable-geometry inlet is only 0.89 at Mach 1.8. 0.91 is higher than 0.89, which means DSI has better performance. Your claim that variable-geometry inlet is always better is incorrect.

Adding more variable ramps to force your variable-geometry inlet would increase pressure recovery, but this increase comes at the cost of complexity and weight. Mach 3 is pretty meaningless when your example aircraft maneuver like an airliner, even when your example aircraft fly at low supersonic speed.

Variable throat can only makes the inlet smaller, not bigger. Variable-geometry inlets suffer at lower speed as a result. From

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:

Variable-geometry inlets still need bypass doors, or a very good turbofan with good bypass ratio. None of what you say here is only applicable to DSI or fixed-inlet.

On the other hand, fixed-inlet and DSI do not need variable-geometry to regulate air flow. Fixed-inlet only need bypass doors. Recall what you have said:

variable throat means it can expand or retract, become smaller or bigger at will.
At Mach 1.8 variable geometry gives you better pressure recovery and Sr-71 was never shot down it could fly at Mach 3.2

speed and variable goemetry go hand by hand
why? as intake mass flow grows, turbofan mass flow needs decrease, thus smaller intakes achieve better intake mass flow matching, that is the reason you have Mach 3 variable geometry intakes The problem is, as the static pressure and mass flow rate are increased, the
pressure recovery reduces while the drag increases. Hence an optimum design needs
to be arrived at.

5921d1325437119-sdf-aerospace-aerodynamics-corner-capture78.jpg

By the way DHI means divertless hypersonic intake and also needs mach number cowling or in few words sizing, they are not universal, they need to be optimised to different mach numbers,

Engineer · Jan 13, 2012

MiG-29 said:
Consider a stand-alone internal compression intake. As the Mach number is
gradually increased in the supersonic regime, a bow shock is formed at the mouth of
the intake. At this point, flow entering the intake is subsonic. As the free-stream Mach
number is gradually increased, the bow shock is pushed closer to the intake. It is
observed that even when the free-stream flow is at the design Mach number, there still
exists a bow shock at the intake entry2. Under these circumstances, the mass flow rate
is lower than that for which the intake is designed and the pressure recovery too is
unacceptably low. This situation is referred to as the problem of ‘Starting the Intake’.
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can you tell me where is the oblique and normal shocks outside in internal compression?

Nope. You are asking a
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, which is a logical fallacy that I have no obligation to answer.

The video illustrates the behavior of compressible flow which is applicable to internal, external and mixed compression inlets. Shock wave is one result of that compressible flow. The rules of fluid dynamics do not change just because the inlet design is different.

Why use the word compression for internal compression inlet? That's because compression effect (presence of shock waves) is inside the inlet's mouth. What does external compression mean? It means the compression effect is outside the inlet's mouth. What does mixed compression mean? It means the compression effect is both inside and outside of the inlet. All three types of inlet take advantage of compressible flow, so some effects seen in the video are just applicable in an external inlet as an internal inlet.

Engineer · Jan 13, 2012

MiG-29 said:
To start you do not understand what is called supercritical condition, critical condition is the intake is matched to the engine needs of mass flow, supercritical is when the shock wave system is swallowed because the engine needs more air than what the intake can provide, an internal compression sistem has critical and subcritical conditions but in reality an internal compression system is a pitot tube gone supercritical and by starting the shockwave they achieve compression

So what you are saying is that external compression inlet becomes internal compression inlet when shock wave is ingested. In other words, you yourself are admitting the same effects of compressible flow occur to inlets of different designs. What we see here is that the video is perfectly applicable, and all you do is deny, deny, deny and argue just for the sake of arguing.

The video shows how compressible flow behaves inside the duct of an inlet. In internal, external and mixed compression designs, oblique shock waves reduce the speed of supersonic flow, and a final normal shock occurs at the throat to turn the supersonic flow into a subsonic one. They are all based on the same principles, because the design of the inlets does not alter the rules of fluid dynamics.

[video=youtube;JhlEkEk7igs]http://www.youtube.com/watch?v=JhlEkEk7igs[/video]

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