[Stabum]

Hi guys,
I would like to know something about THIS.


Every explanation (from a fluid dynamic point of view) is much more than appreciated.

Thanks and have a wonderful CFD day!

[Stabum]

I can't believe nobody has ever get curious looking at the TRENT 1000 rear side, where the outer streams meet the "cold stream" generated by the fan. Its shape is so strange, so unique...

[wyldckat]

Greetings Stabum,

Since apparently you wanted an answer within a 24h time frame, then I suggest you show effort in getting such an answer

For example, can you describe in at least some detail as to why today's commercial jet engines for commercial flights use the multi-layered air flow intakes and exhausts?
Depending on how detailed is your explanation, you will more likely get a detailed answer.
Better yet, you can perform some CFD simulations with such a geometry and get back to us with some possible explanations

Otherwise, the answer to your question seems somewhat obvious, which would explain why no one has chosen to answer your question on a weekend, when there are some many others asking for help on more pertinent questions

Best regards,
Bruno

[Stabum]

Originally Posted by wyldckat

Greetings Stabum,

Since apparently you wanted an answer within a 24h time frame, then I suggest you show effort in getting such an answer

For example, can you describe in at least some detail as to why today's commercial jet engines for commercial flights use the multi-layered air flow intakes and exhausts?
Depending on how detailed is your explanation, you will more likely get a detailed answer.
Better yet, you can perform some CFD simulations with such a geometry and get back to us with some possible explanations

Otherwise, the answer to your question seems somewhat obvious, which would explain why no one has chosen to answer your question on a weekend, when there are some many others asking for help on more pertinent questions

Best regards,
Bruno

My dearest Bruno,
you will excuse me if I reply "per points" but -you must have more than a clue by now- english is not my mother language and I don't want you get me wrong.

1) "You wanted an answer within 24h"
False. I just noticed that in 24 hrs 40 persons have read my post, without leaving a single comment. The "object" of the question is clearly visible to everyone. Doesn't seem curious that no one has nothing to say?

2) My question was purposely generic, every kind of answer would have been welcome. Simple question, simple answers. Sounds good to me.

3) "You can perform some CFD simulations and get back with some possibile explanations". How should I get it???

4) "The answer to your question seems somewhat obvious"
Do you mean the question is too stupid for deserving an answer? Are you really saying that?

5) I posted my question in the "Lounge" section of the Forum, so I don't really understand what does "more pertinent questions" mean.

Have a marvellous week-end, Bruno; for all the others, if you guys have an answer, even the most obvious, please reply. Thanks.

[wyldckat]

Hi Stabum,

My apologies for my writing sounding like I'm a grumpy old man, but unfortunately I tend to get grumpy when I have over 20 posts I would like to answer to, some of which are waiting for an answer for over 6 months... all because it takes 1 to 2 hours to answer each one. The grumpy part comes from not having enough time to spare on weekends to answer to those posts
Also, my apologies for not sending my previous post as a private message, since it was a bit out of context.

Originally Posted by Stabum

1) "You wanted an answer within 24h"
False. I just noticed that in 24 hrs 40 persons have read my post, without leaving a single comment. The "object" of the question is clearly visible to everyone. Doesn't seem curious that no one has nothing to say?

From my experience here on the forum and from personal experience, even after a week and 500 views, not getting an answer can relate to people not having the time to write an answer. And if the question is too generic, it usually leads to more questions to the answer given... which people don't always have time for it.

And 40 views can easily be from people that are not registered, because this forum is open to the public!

Originally Posted by Stabum

2) My question was purposely generic, every kind of answer would have been welcome. Simple question, simple answers. Sounds good to me.

My apologies for misunderstanding, but when you wrote:

Every explanation

it sounded to me like you wanted detailed explanations... which takes a lot of time.

Originally Posted by Stabum

3) "You can perform some CFD simulations and get back with some possibile explanations". How should I get it???

It's somewhat simple to create a similar 3D model. It doesn't need to be accurate, for diagnosing what's happening in the flow around the tail fins.

Originally Posted by Stabum

4) "The answer to your question seems somewhat obvious"
Do you mean the question is too stupid for deserving an answer? Are you really saying that?

Again, sorry for not being detailed. What I meant to say is that if you want a detailed answer to this, it would help if you also detailed what you already know on this topic. Otherwise, it would take a few hours to explain everything...

Originally Posted by Stabum

5) I posted my question in the "Lounge" section of the Forum, so I don't really understand what does "more pertinent questions" mean.

It felt strange that people expect answers on the Lounge forum within a 24h time frame, or even in the whole forum for that matter.
I see almost every day someone on the forum writing "please help me", "urgent help needed", some of which are from people who have been waiting for an answer for over a week.

----

Back to your original question, if you don't mind a quick and simple answer: It's a way of mixing the air flow at the end of the jet. This way it induces a lot more mixing than a conventional airliner jet engine. It can also have to do with how the engine is mounted on the plane... for which more details are needed for diagnosing.

Best regards,
Bruno

[Stabum]

Dear Bruno,
thanks for your answers. I want to thank you for all the time you (and all the other moderators and users as well) spend in organizing, managing and give yourself to this wonderful forum. The amount of time you devote in answering posts must be daily and enormous; moreover the subject of the questions is often extremely complex, so your commitment is incredibly huge. Thanks so much again for what you do.

Probably the reason of my lack of patience is... you guys! Because everytime I've posted a thread with "help me!" or "I'm in trouble!" in the title, I've received answers within FEW HOURS litteraly. That impressed me so much! And, moreover, it's not an obvious thing to add that the advices I received have always been correct and have helped me out and problem solving. Maybe if I have had waited longer in the past, I would have not been so impatient.That's why, please forgive my behaviour. I promise I'll be definitely more patient in the future.

And thanks for your answer about the "tail fins" (now I know how I have to call them!). I have to design a bladed nozzle through which I have to eject water-in-water at very high speed (~50 m/s). All the nozzles I've designed and simulated so far show a high amount of turbulent kinetick energy all around the outlet edge (which means water spray and noise?), and for this, I'm not satisfied yet. At the same time, since I'm very fond in aerodynamics, I'm costantly thinking that the Dreamliner has these "feature" (you know what I'm talking about), so I was curious to know if I could use it in order to solve my problem. But I'm not modeling it, or simulating it until I'm sure it can be the right direction to head to, because my knoledge of CFD techniques is still quite poor and the time available before the deadline is so few, and getting smaller and smaller, so I'm not that free to test every single geometry my imagination is offering to me...

Many thanks, again, for the time you have spent for me, Bruno!

If you like, and have time to, I'd be glad to know something about you: are you a student, a professional in CFD? You can PM me any time.
Have a pleasant sunday!

[wyldckat]

Hi Stabum,

The mixing done at the end the example jet probably isn't for reducing turbulence, it's for improving the mixing between the outer flow around the jet engine and the outer ring flow that assists in reducing the noise generated by the engine. And that engine seems to be 1/4th covered by a wall/object on the vehicle/plane, which would explain why they would need to increase the mixing ability, in order to reduce the noise emitted by the air jet.

As for the water jet: the energy scale is completely different to that of air. Specially at 50 m/s. What you're seeing probably is as if there is a water hammer being constantly thrown against the water. Using fins like those on the air jet, but in the water, would probably result in a lot more turbulence and bubble generation, which is possibly not what you're looking for.

A possible solution for your water jet, would likely be related to giving it rotational flow, similarly to how the inside of a high speed projectile launching tunnels/pipes... I had to look it up, it's called "rifling": http://en.wikipedia.org/wiki/Rifling - that and using an outer ring as the one on the air turbo-jets, near the end, so that it can do some pre-mixing of the water flow.

Originally Posted by Stabum

are you a student, a professional in CFD?

Neither and nor both. I'm regularly studying OpenFOAM, as it's part of my job, but I'm not a professional in CFD itself, since I have not yet studied it properly
I only know some details and have connected the dots in many others, when it comes to CFD and fluid flow. Ironically, you'll find more details about what I know and don't know on this blog post of mine: Read this before emailing/PM me

Best regards,
Bruno