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What happens to the normal shock that helped the oblique shock form, in the first shock of a lambda shock? Oblique shock waves form because :

"An oblique shock wave is a shock wave that, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses." - Wikipedia.

(Over this whole question, I'm referring to the first shock in a lambda shock, just to make an example)

For the oblique shock to form, there has to be a normal shock first, I think. The normal shock makes a separation bubble which makes the flow turn, making an oblique shock. My question is, what happens to that normal shock when the oblique shock forms? Does it disappear? If so, why? This answer and its comments has some really good info, but (for me) it didn't explain this part.

Also, if the flow behind a normal shock is always subsonic, how would the oblique shock form? I thought flow had to be supersonic to form an oblique shock?

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  • $\begingroup$ Does this answer answer your question? $\endgroup$
    – ROIMaison
    Commented Feb 13 at 14:56
  • $\begingroup$ @ROIMaison Thanks for the link, but it doesn’t really talk about how the oblique shock I was asking about forms, unless I missed something. $\endgroup$
    – Wyatt
    Commented Feb 13 at 15:57
  • $\begingroup$ The way I read it, initially there is a normal shock, which disrupts the flow and causes separation. This separation bubble presents an obstacle to the flow, which means it needs to generate an oblique shock to get around it. $\endgroup$
    – ROIMaison
    Commented Feb 13 at 16:11
  • $\begingroup$ @ROIMaison That’s actually how I thought of it too, but my question was what happens to that initial normal shock? $\endgroup$
    – Wyatt
    Commented Feb 13 at 16:13
  • $\begingroup$ @ROIMaison (what happens to it when the oblique shock is formed of course) $\endgroup$
    – Wyatt
    Commented Feb 13 at 19:16

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I found a schematic of the situation with a separation bubble in the paper Capsule aerodynamics and shock-wave boundary layer interaction (SBLI) in supersonic and transonic flow : enter image description here

This shows that the normal shock is still there, but the surface to which it's normal is not the airfoil, but the displaced surface of (airfoil + separation bubble + boundary layer).

This makes sense, to the flow, the separation bubble + boundary layer is also an obstacle that it has to navigate.

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  • $\begingroup$ I see, thanks. So if the air behind a normal shock is subsonic, how do the oblique shocks form? I thought flow had to be supersonic for an oblique shock to form. $\endgroup$
    – Wyatt
    Commented Feb 14 at 16:34
  • $\begingroup$ Is it because by the time the normal shock creates enough separation for the oblique shock to form, the flow is still supersonic? $\endgroup$
    – Wyatt
    Commented Feb 14 at 23:42
  • $\begingroup$ In case of a normal shock: Supersonic-shock-subsonic (look at the value of M2 in the tables wiki Normal shock In case of an oblique shock: it depends on angles and speeds, see how this image has two solutions, one for a strong shock M2<1 and one for a weak shock M2 >1. Oblique shock graph The oblique shock solution was also explained to you here $\endgroup$
    – ROIMaison
    Commented Feb 15 at 8:23
  • $\begingroup$ Ah I see. I think I understand it more. So for an oblique shock to form the flow has to be supersonic, so in some fashion the normal shock is still able to form the oblique shock with supersonic flow. (Probably doesn't make sense how I explained it) $\endgroup$
    – Wyatt
    Commented Feb 15 at 20:18
  • $\begingroup$ It's the other way around. Supersonic flow leads to shocks. The flow is supersonic and needs a method to get back to subsonic. The shocks are the way to do that. If the flow is going straight, you get a normal shock, if the flow needs to turn, you get an oblique shock. Then there are two options, either there is a strong shock that directly makes the flow subsonic, or there is a weak shock, which leaves the flow supersonic. This then is followed by another shock to return the flow to subsonic. That is how you get the double shock pattern, the lambda. $\endgroup$
    – ROIMaison
    Commented Feb 16 at 8:28

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