A PofF feedback question!! Help needed

[RowleyUK]

When comparing a normal and an oblique shockwave, the normal shckwave:

A) has a higher total pressure
B) has a higher total temperature
C) has a lower static pressure
D) has a lower total pressure


Answer= D

Is there any way of remembering which has the higher:

total pressure
static pressure
temperature

I have never come across any of this info in my studying time and any help would be greatly appreciated!!

[empty pockets]

will tell you whan i get home if no one else does.

this is worrying, but i'm doing an aero engineering degree and i can't remember any of this stuff. gonna have to go through it all again for the atpl's - hope it comes back to me!!:confused:

[FlyingForFun]

Got my PofF exam in Feb :eek:, so if this is wrong, I really need someone to tell me!

First of all, a normal shockwave is present at Mdet. An oblique shockwave is present at speeds higher than Mdet.

Once the local airflow is supersonic (i.e. at or above Mdet), increased speed results in increased temperature and increased pressure. Hence, the oblique shockwave will have a higher total pressure and temperature, or, put the other way around, the normal shockwave will have the lower temperatuer and pressure - answer D.

But, actually, from the answers given, I think you could work it out by a process of elimination. Static pressure doesn't change no matter what speed you pass through the air at (otherwise your altimeter would do strange things), so that rules out C. In most cases where something causes the temperature to rise, the pressure will want to rise as well, and vice versa, so if A was true B would have to be too, and vice versa - hence neither A nor B could be correct. So, with very little knowledge about trans-sonic flight or shockwaves or anything, you could probably make a reasonable guess at D.

Someone please correct me if any of my logic is wrong!

FFF
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[Jagbag]

Hi Rowley,

Just a small brief on your question, hope it helps....

Firstly, there is a direct relationship between the pressures 'ahead' of the shock wave and the strength of the shock wave itself. The stronger the shock wave the greater the energy loss, but also the lower the pressure 'ahead'. as I tried to explain when replying to your previous post.

Secondly the shockwave is a very narrow region 1/10000 inch thick.

Thirdly, the flow behind an oblique shockwave has a reduced MNo but if the wave angle is<70 degrees, it will still be supersonic, however direction of flow is turned thru' an angle equal to the deflection angle of the obstruction giving rise to the shockwave. Hence the flow is broken into two components, X normal to the shock wave which goes subsonic and Y parallel to the shockwave which remains unaffected. In addition at some distance the supersonic flow will encounter an adverse pressure gradient. The decreasing pressure will force the flow to decelerate. This would normally happen thru' a normal shockwave.

Fourthly, in a weaker or normal shockwave, the energy loss is less because the X component is less and the Y component is more, the wave angle is >70 degrees, and the flow behind the shockwave is probably subsonic.

What I am attempting to compare is an oblique shockwave ahead/attached to the leading edge and a normal shockwave at some distance behind the LE.

A rough way of remembering could be:
-All forward facing surfaces causing disturbances to form oblique shockwaves around an above Mdet experience greater pressure (than atmospheric).
-All rearward facing surfaces (where normal or close to normal shockwaves form) experience less pressure (than atmospheric).

I must emphasise that the above may be incorrect as it is from bookish knowledge so I would very much accept any valid correction to my logic.

Cheers!:)

[RowleyUK]

This might help you chaps who are sitting the exams soon!


O N E
V= D D I
D= I I D
P= D D =
T= I I D


V=speed
D= Density/static pressure
P= total pressure
T= Temp.


The above shows what happens when the air flows through through either an Oblique, Normal, Expansion wave.ie when the air goes through a normal shock wave then the Temp increases!


Does anyone know of a table like this that works for the total;

V
D
P
T

????????

[Jagbag]

Sorry Rowley...
couldnt think of one as yet, but if you know of some such table than please let us all in.:confused: