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Mohammad Kermani December 3, 2000 01:33

Inlet Buzz??
 
Hi dear All,

I wonder if someone can simply explain what is inlet buzz?

Is that a phenomena of the viscous world, i.e. real world? Or it could happen in inviscid world as well?

Thanks

M

John C. Chien December 3, 2000 05:26

Re: Inlet Buzz??
 
(1). I am sure that this is not a CFD question. (2). Inlet buzz is something related to the operation of an engine inlet, more precisely, a jet engine inlet of a supersonic aircraft.(or supersonic air breathing rocket engine.) (3). It should be discussed in most propulsion text books.

A. Taurchini December 5, 2000 04:50

Re: Inlet Buzz??
 
If you make such a question you should be involved with supersonic airbreathing propulsion, so you should be able to follow me even without a figure. Suppose you have a mixed compression inlet, with two external ramps (for external compression). The properties of the two shock waves generated by those ramps depends form may factors as Mach, inclination of the ramps, Temperature ... ; but surely the second shock wave is steeper than the first one. We say that an inlet is operating in ideal condition if the two shock impinge on the cowl lip. Now suppose that those two shock waves impinge themselves just before and down the cowl lip (Mach number below the optimal), from this interaction will be generated two different zones of flow entering the inlet with different velocity. This difference produces a vortex that makes the flow unstable and could even lead to a burnout of the engine. Of course this mode does exist only in viscous flow. This operational mode have to be absolutely avoided within the operational range of the engine. If you need further information (eventually a fig.) e-mail me. Bye

Mohammad Kermani December 5, 2000 07:21

Re: Inlet Buzz??
 
Hi.

1) OK. The two ramp combination makes two velocity profiles, which they are parallel (becuase they are having same pressure) and in fact these two streams make a slipline and as you have mentioned that makes a vortex sheet. But how these vortex sheet (or slipline) can cause instability?

2) The scenario U just pictured and i just repeated here in # 1 in different wordings, are also possible in inviscid world as well. Unless someother things only related to viscous flow field will occur downstream of the slipline. That would be much benefitial if you could possibly e-mail me some pictures. My e-mail address is: mkermani@mae.carleton.ca

Thanks alot. M

A. Taurchini December 5, 2000 10:38

Re: Inlet Buzz??
 
You're neglecting the role of viscosity : in an inviscid flow two layer of flow with different velocity just slip one over another and no vortex sheet is generated ! Zone with vortex make possible (probable!) the flow to reverse producing a suddenly reduction of mass flow directed to the engine ; and this lead to an increase in spillage outside the inlet 'til the normal shock go outside the forward facing hole and so unstarting the inlet , but as soon as the shock is located outside the subcritical condition is renstablished and the mass flow start to grow again til the previous situation reoccurred. This phenomenon (oscillation at high frequencies of the normal shock wave) is called Buzz. I'm sorry but currently I'm not at home (or office) so I won't be able to send you some images. I hope to send them as soon as possible.

Mohammad Kermani December 5, 2000 13:38

Re: Inlet Buzz??
 
Hi.

With my understanding from what you say:

1) The inlet buzz is ONLY a phenomena of viscous world. Because the vortex sheet is only allowed in viscous flow, which makes local reverse flow => reducing the mass flow => pushing the shock outside the cowl lip (called unstrat condition). Now I have questions in this regard:

2) Is vortex sheet (or better to say local reverse flow) the only reason that can cause inlet buzz?

3) If by any means we can avoid the geneartion of reverse flow, can we say the inlet buzz 100% is avoided? Going back to the original picture of double ramp you made. If we have just a single ramp, then flow behind the shock will be only a single zone (uniform velocity, i.e. no vortex sheet), therefore, no means to reduce the mass flow rate.

4) Reverse flow could also be generated by strong shock impingement to the wall. Will this make inlet buzz. Is this a general statement saying "If Any reverse flow is existing in the flow field of the inlet, that will cause inlet buzz for sure"? In other word, i am asking we can not have a stable (time invarient) reverse flow (like a time invarient separation bubble close to a wall) which can introduce no instability to the flow regime?

Thanks alot.

M


John C. Chien December 5, 2000 14:12

Re: Inlet Buzz??
 
(1). "The operation of a supersonic inlet" means that the flight Mach number and the mass flow rate can change for active or passive reasons. (2). If for some reasons, the engine is taking only small mass flow rate, then the spilage will be higher (portion of the flow move around the inlet cowl). As a result, the flow behind the last shock has to go through higher diffusion (and cause flow separation in the inlet.) At the same time, the inlet becomes more like a blunt body (more solid behavior because of higher spilage) which will push the last shock further away from the inlet. (3). Now you see the complexity of the "operation" of a supersonic inlet. (in most cases, the inlet ramps are adjustable, they are not fixed by design.)

A. Taurchini December 6, 2000 05:40

Re: Inlet Buzz??
 
3)Unfortunately inlet theories, specially for certain mode of operation, are not so developed. Anyway, we could safely (pay attention when you say "safely"!) say that if there's no vortex sheet entering the inlet then buzz is avoided (...which are the means you wish to use to avoid a central reverse flow?!). Moerover it is clear that a single external ramp will never produce buzz, but a single external ramp is not so good in terms of pression recovery. 4)You're right a strong shock impinging the wall produces a separation, but this is another problem not correlated to buzz phenomenon. Infact usually this kind of separation is restricted just to a little bubble near the wall, were the presence of the boundary layer make the flow subsonic and this does not produce buzz ! You can find further information on books like: 1)Practical Intake Aerodynamic Design (Aiaa Education) by E. L. Goldsmith(Editor), J. Seddon(Editor) 2)Inlets for Supersonic Missiles (Aiaa Education Series) by John J. Mahoney


Mohammad Kermani December 14, 2000 18:50

Re: Inlet Solution
 
Hi,

I am testing an inlet diffuser, say inlet of Ramjet, with inviscid code based on Roe scheme.

The specification of test case:

The altitude I choose is 6000 meter, which corresponds to p_inf=2.8 kpa. For a specified back pressure equal to p_back=28 kpa, I can make the code to converge and it gives a couple of oblique shocks with a terminating normal shock (the converged solution is a correct physical solution, as far as I could check). The normal shock for this back pressure is not located at the throat and it is further downstream. So what i do, I slightly increase the back pressure to 30 kpa within some iterations. Then what happens is as follows:

My problem: The normal shock moves upstream until it goes out of the inlet, making a shock like a bow shock, then the shock is swallowed by the inlet. This procedure is repeated as long as i have not stopped the code. I wonder how i can check with this specified back pressure and this geometry, there is any existing physical solution at all.

P.s. I am just dealing with inviscid flow now.

Do you know any physical solution for a given geometry that I can check weather the code works properly. I really have no idea where the source of instability is. Any help is appreciated.

Thanks alot.

M

I have checked the


A. Taurchini December 15, 2000 05:55

Re: Inlet Solution
 
I'm sorry but I have not enough infos (inlet design parameters)... but I suppose you've reached the chocking condition (subcritical operational range) and a certain amount of mass flow has to be spilled : the inlet might be in unstarted condition.

Mohammad Kermani December 15, 2000 06:37

Re: Inlet Solution
 
Hi.

Do you mean, the specfied back pressure is too high, therefore, it causes unstart condition?

Do u still have those pictures about inlet buzz?

Thanks

M

A. Taurchini December 15, 2000 06:52

Re: Inlet Solution
 
Hi Mohammad, this is exactly what I meant, but without geometrical parameters (throat area, capture area ...) I can't say if this is right ! For those pictures, I'm sorry but I'm still not home (Hard work !!!) Bye


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