Challenging experts: Stagnation Pressure

shiangyulee

LBSALE[100]LBSALEWhen we use the pitot tube to measure pressure, we all know that the pressre obtained is "total presure" or "stagnation pressure, if the tube is facing teh flow. What presesure should we get if the tube is facing the rare down stream?
Text books tell us that, if a cylinder is placed in an ideal flow stream, both front and rare stagnation points have the value of stagnation presure. Is that right?

junior

The answer should be ';yes';, but the total pressure will be changed if we need to take into account the energy lost.

yxjbuaa

下面引用由junior在 2006/07/01 04:30pm 发表的内容：
The answer should be ';yes';, but the total pressure will be changed if we need to take into account the energy lost.

Good question, and also smart answer!

kanhlbai

看问题也得花银子，这个年代，怎么这样拜金呢？？？/

shiangyulee

下面引用由kanhlbai在 2006/07/06 06:30am 发表的内容：
看问题也得花银子，这个年代，怎么这样拜金呢？？？/

Sorry about that, Mr kanhlbai,   
In the "Imperialist System", poor people don';t have any voice!
Junior,
I was stating it was in an "ideal fluid" (potential flow), so what could be the energy lose?

Let me pose the question differently:
If you have a semi-infinite rod with a semi-spherical end and retrieve it from an infinite space filld with fluids, what would be the pressure at the cneter of the end? If you apply Bernouli';s equation, what is the rpressure at infinity and what is the presure at the sphere? Should this pressure be the same as pushing the rod foward at the same speed?

flowermoon

[这个贴子最后由flowermoon在 2006/07/06 09:53am 第 1 次编辑]

下面引用由shiangyulee在 2006/06/29 02:08pm 发表的内容：
周华 站长
Thank you for your welcom emessage. I am very excited that we do have this forum coming from China. It is probably the first open discussion forum on Fluid Dynamics in the world. Let us use this platform as a community to work together and perhaps develop a comprehensive system of knowledge shared by all.

下面引用由shiangyulee在 2006/07/06 09:39pm 发表的内容：
Sorry about that, Mr kanhlbai,  
In the "Imperialist System", poor people don';t have any voice!

I sense a double standard here: taking the advantage of a "socialist system" built by many hard working people and then turning around to apply the "imperialist rule" to poor who like to advance by learning.

By the way, I visited Seattle a few times, a nice place with many good, kind people.

shiangyulee

Flowermoon,
That';s a nice "flower name", especially in Chinese.
Sorry that I can';t help the double standards: having lived in different systems, you first learn to survive, and then to exploit.
When you come to Seattle next time, look me up.
By the way, I spent a couple years of my life in Nan Jing, living on Tai Ping Non Lu, right next to Tze-Jing Hotel. When I visted in 2000, the place changed quite a bit, but still recognizable.

flowermoon

[这个贴子最后由flowermoon在 2006/07/11 02:15am 第 1 次编辑]

Chairman Mao taught us: “Where there';s exploitation, there';s rebellion.” Next time when you visit Nanjing, you had better watch out your wallet for a similar type of double-standard may also apply to you.
Thanks for your kind invitation.
Anyway, in terms of climate, Seattle is really a very good place to live though Seattle is notorious for its damp weather due to frequent rain. Actually, I, or most people from Nanjing, like rain in Seattle because it brings 凉爽. The over frequent rain in Seattle usually occurs in spring and often before dawn and in early morning. On the other hand, the rain season at Nanjing occurs in 梅雨 season when the air is always very hot and humid.
Washington State is also called “evergreen” state because its mountains and lands are mostly covered by trees, green grasses and flowerers. Frequent rains bring frequent freshness and keep everything clean.
California State is called “golden” state. It is so dry, especially in summer, that the open field is mostly in golden or yellow color with yellow dirt and grass fields. If one drives along I-5 (Interstate highway number 5) from California to Washington (or vice versa) at the moment, one can really experience the difference between two worlds the nature gives us. One most likely will feel a big relief once entering the Washington State from south and suddenly seeing all those mountains completely covered by fresh and green trees.

junior

下面引用由shiangyulee在 2006/07/06 09:39pm 发表的内容：
Junior,
I was stating it was in an "ideal fluid" (potential flow), so what could be the energy lose?

Let me pose the question differently:
If you have a semi-infinite rod with a semi-spherical end and retrieve it from an infinite space filld with fluids, what would be the pressure at the cneter of the end? If you apply Bernouli';s equation, what is the rpressure at infinity and what is the presure at the sphere? Should this pressure be the same as pushing the rod foward at the same speed?

well, the first question about the stagnation at the front and aft stagnation point , the answer should be ';yes';, because if you totally overwhelm the viscosity effect, both points will reach the stagnation state, therefore the pressure should be the same stagnation pressure.
the second question, if we use relative system, the result is p=p_a+0.5*density*v^2. if we use absolute system, the pressure at the front stagnaton point can not be derived out directly, because the air particles experincing a cause, in which they are accelerated from v=0 to v>< 0, therefore we don';t have a unified total pressure to apply the Bernouli theorem. is that right ?

kanhlbai

下面引用由shiangyulee在 2006/07/06 09:39pm 发表的内容：
Sorry about that, Mr kanhlbai,  
In the "Imperialist System", poor people don';t have any voice!

Please note that I won';t afford does not mean I have no money. And you shouldn';t reach the conclusion that I am a poor person.

shiangyulee

Let me illustrate with 3 diagrams. In the first case, there a piston placed in a cylinder, if we move the piston to the left, the pressure at  the piston must be lower then the ambient static pressure by 1/2rV**2. In the second case, if we remove the cylinder, the pressure should also be lower, at least, it should not be higher then static pressure. The third case is the problem pose in this discussion.
There shouldn';t be any difference between the third case and the other two and yet the standard potential flow solution gives us the Stagnation Pressure.
This is a paradox an the question is, can anyone give a convicing explanation that the potential solution is absolute truth?

coolboy

“There shouldn';t be any difference between the third case and the other two ......”
Why? Bernoulli’s theorem is derived by integrating the energy equation for steady flow of a frictionless fluid along a stream-tube. I do not see a stream-tube in the third case.

anyone

It is not nice to ask ppl 2 pay 2 answer ur question. Especially when u think u r superior to others. Maybe I am 2 rich 2 say that, Anyway money doesnt have a lot of use in this forum.
Back to ur question, i am not expert in potential flow. I think the case u post is an unsteady flow, steady Bernoulli’s theorem doesnt apply. If u fix the reference frame to the moving parts (which satisfy requirement of steady Bernoulli’s theorem ) the answer is quite obvious.
Seems flowermoon is in US 2? u r a pretty smart girl(?) study or work?

周华

关于前后驻点问题在空气动力学方面的教科书上有相关的讨论，目的就是解释为什么前驻点的压强很高，而后缘驻点的压强很低。其道理就在于，前驻点是来流未经机械能耗散而形成的驻点，而后驻点则是机翼表面的边界层在从前缘流到后缘大大耗散了机械能之后的驻点，二者之间相差一个机械能损失项，这就是原因。用无粘理论当然无法解释，按照无粘理论，前后驻点的压强确实相等，而这恰好是达朗贝尔疑题的主要内容。普朗特同志的伟大贡献就在于提出了边界层概念，并对这个问题进行了解释。因此，这个问题已经不再是悬案，而成为被解释过的问题。

shiangyulee

下面引用由周华在 2007/06/03 05:06am 发表的内容：
关于前后驻点问题在空气动力学方面的教科书上有相关的讨论，目的就是解释为什么前驻点的压强很高，而后缘驻点的压强很低。其道理就在于，前驻点是来流未经机械能耗散而形成的驻点，而后驻点则是机翼表面的边界层 ...

謝謝站長親自造訪並提出答覆，不過依您所說，好像所提這個問題己然是普通常識，不值一問了，但是筆者並沒有見過相關理論答案的正規數值，不知站長可否再度指點迷津。再者，就我所知，在圓柱体的后駐點，其壓差實驗值為 (-ρV^2 )，和粘性系數並無關係，果真如此，則 “二者之间相差一个机械能损失项”的說法亦不成立，不知應如何解釋?
個人對”伟大的普朗特同志”，也有幾分尊敬，但更怌疑他是一代”納粹學霸”，對他多項理論都存疑惑，例如他和其門徒的 “椭圆最低導引阻力機翼理論”從未被証明過，而証明其誤謬的實驗數據卻無人採信，又如早期哥亭根機翼實驗數據，一律採用”阻力極線”(Drag Polar) 方式表達，讓人難於看出其理論值的誤差，混淆視聽，我等應當仔細審查求証，以求”正本清源”。
Junior, 您的答案是正確的，但是和傳統理論結果相反，能夠加以解釋嗎?
Anyone, I think you are really “someone”! 一般流体力學，就是這樣處理問題，把座標移到運動中的物体之上，但是一般人習慣於尤拉(Euler)座標以后，就以為再也不能用絕對座標。您所提 “非穩定流”也是一發中的，有很多問提是需要用非穩定流來解答的，例如噴射推進發動機就是如此。我的第一篇論文(在本論檀有甚多討論，也可下載的) 就是採用 Milne-Thomson 的”半穩定流”(Quasi-Steady Flow) 渦流衝量理論來求解機翼昇阻力問題，但是很少人能夠接受。這項理論，我個人還未完全滿意，所以還沒有送交雜誌正式發表。