深度技术贴，美国网友看图断定J20进气口DSI遇到麻烦重新 ...

原文在keypublishing自己查一下即可。每次我贴链接都被删掉。

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网友“Jō Asakura”发表大作描述进气口附近出现的两个6楞通气孔的原因。可怜我的知识欠缺只能尽量试着翻译。各位大神请喷。

该网友在试图猜测新出现的两个6楞通气孔的原因。没有对j20特别夸奖或者贬低。是否有意义各位自己判断。不代表本人观点
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他们一定是碰到问题了。（DSI附近新的两个6楞通气孔）对亚音速性能提供一定好处但是对超音速性能伤害大。相比之下F35用DSI但是没有通气孔。JF17用的DSI鼓包上有很多通气孔

并且DSI冲激波和通气孔互相作用会带来乱流，特别是和s进气道结合使用的情况下。

。。。

他们这么改一定是为了增加亚音速和超音速的气流量。进气口看起来重新设计了。原因可能是原设计进气口边缘对于增加“阻矩”有反作用。这是DSI常见问题。

靠前的6楞通气孔的作用是增强初震波（initial oblique shockwave）靠后的6楞通气孔的作用是处理因为初震波和鼓包压缩产生的残余附面层（residual boundary layer separation）。这个总压力流（total pressure flow i）降低的问题在超音速状态产生。。。。F35也有类似的问题但是LM重新设计了进气口，但是可能对（超音速）性能还有负面影响。。。。。

。。。
。。。
最可能的情况是他们用计算流体力学的算法设计了进气口，但是在测试整个飞行曲线时发现现实中老的设计不能达标。
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Then they must be having problems with the DSI. The F-35 doesn't have a bleed system, not even a passive bleed system incorporated into the 'bump' itself (see porous elements on JF-17 below), this may have limited benefits for the subsonic regime but performance will degrade significantly in the supersonic.

Besides, interaction of a DSI shockwave and bleed system will result in significant flow distortion, particularly in the S-duck.
It has the opposite effect of what conventional splitter-plate diverter type fixed inlets at supersonic speeds, where the passive bleed system on the splitter plate offers significant benefits by stabilising the downstream (after-terminal-shockwave) boundary layer.

Having said that, they may be attempting to increase the bleed mass flow rate to improve the intakes' characteristics in both sub & supersonic regimes. It certainly appears the intake has undergone a redesign which would imply it was a problem area for increased drag. This is a common problem with DSIs.

The first bleed vent (L) serves to strengthen the initial oblique shockwave, the second (R) is to deal with residual boundary layer separation due to the intensity of the initial shock wave (and ingested BL) downstream of the compression bump surface. The problem arises in the supersonic regime when total pressure flow is degraded due to the interaction of the bleed system and shock waves. However, as these two vents increase the bleed flow rate, then the severity of these low pressure gradients is reduced.

The F-35 also suffered from similar problems but LM instigated (at least one) redesign of the duct itself. Even so it is probable that the duct design will still, to some extent, inhibit the F-35's performance. If Mr. PLAwolf is trying to sell these J-20 duct modifications as some sort of highly evolved hybrid of the DSI, he's most welcome to- but his assertions would be wrong. After all the whole idea of the DSI is a simple, elegant high pressure shock solution devoid of mechanisms and gizmos.

The passive porous elements (above) employed on the JF-17 are almost certainly due to address the flow separation imparted from the forward fuselage, remember the DSI was retrofitted. One would need very accurate dimensional data on the J-20 before one can confirm the root cause of it's DSI problems, but I would suggest they are for similar reasons- namely in the absence of a 'conventional' splitter plate and BL cavity, the design and dimensions of the fuselage forebody are not conducive to a pressure gradient that efficiently deals with flow separation, not only trans & supersonic, but @ various AoA.

There's no question of "re-engineering" the intake duct. They are a central, design-defining feature of all fighter designs, particularly 5G.

What most likely happened was that they utilised CFD code & algorithms to design the duct. However, when they opened up the envelope they found that the real-World data fell short of the predictions. I retract the comment "a step backwards", what they've actually done is applied solutions to address these shortcomings, that they appear more fundamental than the F-35's maybe an indicator of the differing requirements of the J-20, as well as the steep learning curve it represents for Chengdu et al.

So long as the engine-duct performance squares favourably with the aerodynamic/kinematic design and capabilities of the airframe (see Andraxxus' posts on the other thread) and later engine upgrade, then they'll have achieved their aim.




原文在keypublishing自己查一下即可。每次我贴链接都被删掉。

=================================================================================================
网友“Jō Asakura”发表大作描述进气口附近出现的两个6楞通气孔的原因。可怜我的知识欠缺只能尽量试着翻译。各位大神请喷。

该网友在试图猜测新出现的两个6楞通气孔的原因。没有对j20特别夸奖或者贬低。是否有意义各位自己判断。不代表本人观点
=================================================================================================

他们一定是碰到问题了。（DSI附近新的两个6楞通气孔）对亚音速性能提供一定好处但是对超音速性能伤害大。相比之下F35用DSI但是没有通气孔。JF17用的DSI鼓包上有很多通气孔

并且DSI冲激波和通气孔互相作用会带来乱流，特别是和s进气道结合使用的情况下。

。。。

他们这么改一定是为了增加亚音速和超音速的气流量。进气口看起来重新设计了。原因可能是原设计进气口边缘对于增加“阻矩”有反作用。这是DSI常见问题。

靠前的6楞通气孔的作用是增强初震波（initial oblique shockwave）靠后的6楞通气孔的作用是处理因为初震波和鼓包压缩产生的残余附面层（residual boundary layer separation）。这个总压力流（total pressure flow i）降低的问题在超音速状态产生。。。。F35也有类似的问题但是LM重新设计了进气口，但是可能对（超音速）性能还有负面影响。。。。。

。。。
。。。
最可能的情况是他们用计算流体力学的算法设计了进气口，但是在测试整个飞行曲线时发现现实中老的设计不能达标。
。。。
。。。
。。。


=================================================================================================
Then they must be having problems with the DSI. The F-35 doesn't have a bleed system, not even a passive bleed system incorporated into the 'bump' itself (see porous elements on JF-17 below), this may have limited benefits for the subsonic regime but performance will degrade significantly in the supersonic.

Besides, interaction of a DSI shockwave and bleed system will result in significant flow distortion, particularly in the S-duck.
It has the opposite effect of what conventional splitter-plate diverter type fixed inlets at supersonic speeds, where the passive bleed system on the splitter plate offers significant benefits by stabilising the downstream (after-terminal-shockwave) boundary layer.

Having said that, they may be attempting to increase the bleed mass flow rate to improve the intakes' characteristics in both sub & supersonic regimes. It certainly appears the intake has undergone a redesign which would imply it was a problem area for increased drag. This is a common problem with DSIs.

The first bleed vent (L) serves to strengthen the initial oblique shockwave, the second (R) is to deal with residual boundary layer separation due to the intensity of the initial shock wave (and ingested BL) downstream of the compression bump surface. The problem arises in the supersonic regime when total pressure flow is degraded due to the interaction of the bleed system and shock waves. However, as these two vents increase the bleed flow rate, then the severity of these low pressure gradients is reduced.

The F-35 also suffered from similar problems but LM instigated (at least one) redesign of the duct itself. Even so it is probable that the duct design will still, to some extent, inhibit the F-35's performance. If Mr. PLAwolf is trying to sell these J-20 duct modifications as some sort of highly evolved hybrid of the DSI, he's most welcome to- but his assertions would be wrong. After all the whole idea of the DSI is a simple, elegant high pressure shock solution devoid of mechanisms and gizmos.

The passive porous elements (above) employed on the JF-17 are almost certainly due to address the flow separation imparted from the forward fuselage, remember the DSI was retrofitted. One would need very accurate dimensional data on the J-20 before one can confirm the root cause of it's DSI problems, but I would suggest they are for similar reasons- namely in the absence of a 'conventional' splitter plate and BL cavity, the design and dimensions of the fuselage forebody are not conducive to a pressure gradient that efficiently deals with flow separation, not only trans & supersonic, but @ various AoA.

There's no question of "re-engineering" the intake duct. They are a central, design-defining feature of all fighter designs, particularly 5G.

What most likely happened was that they utilised CFD code & algorithms to design the duct. However, when they opened up the envelope they found that the real-World data fell short of the predictions. I retract the comment "a step backwards", what they've actually done is applied solutions to address these shortcomings, that they appear more fundamental than the F-35's maybe an indicator of the differing requirements of the J-20, as well as the steep learning curve it represents for Chengdu et al.

So long as the engine-duct performance squares favourably with the aerodynamic/kinematic design and capabilities of the airframe (see Andraxxus' posts on the other thread) and later engine upgrade, then they'll have achieved their aim.