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I'm exploring a question concerning whether or not boundary layer airflow presents physical limitations that are too great for current technologies to overcome (I'm holding the position that it is in fact doable) but I've seem to only be able to find one main method of active flow control in the inlet of an embedded engine.

Specifically, pulsating air jets for active manipulation of the air ingested into the engine. As to create uniformity along any modeling 3D or otherwise for future flow sims I am using the basic frame of the Boeing BWB 450-1U concept with the proposed GE58 F2/B1 UEET turbofans.

I would like to know if there are any aerodynamic phenomena that would decrease or eliminate a pressure gradient that would naturally build up on the face of a Boundary Layer Ingestion.

As previously noted, I am aware of active control measures, but I would prefer a passive solution like vortex generators (as they present a minimal detriment in aerodynamic quality yet a significant advantage in efficient solution to the given lift problem)

P.S. I have checked all related posts/answers and none are either recent enough nor complete enough for my question in particular, so please don't disregard it as such.

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    $\begingroup$ Now we know what you are working on, but we don't know what you are asking. Could you please be more clear? And in case you expect a complete list of all boundary layer manipulation techniques: We are not here to write your paper. $\endgroup$
    – Peter Kämpf
    Jul 19, 2018 at 5:46
  • $\begingroup$ @PeterKämpf I apologize for not being clearer and want to say that I in no way am trying to cut corners or anything. To better phrase my intent, I would like to know if there are any aerodynamic phenomenae that I haven't identified that could reduce or eliminate a pressure gradient that would build up naturally along a BLI. I want to let it be known that I could only find the air pulse as a physically and practically viable solution. It was referenced and studied in several reports; one by ERAU and two by NASA/NACA and other methods were simply too problematic for consideration. $\endgroup$
    – Jihyun
    Jul 19, 2018 at 6:06
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    $\begingroup$ You know what I'm just going to edit my question to better reflect my intent. Thanks @PeterKämpf for your brutally candid but constructive insight ;) $\endgroup$
    – Jihyun
    Jul 19, 2018 at 6:07

2 Answers 2

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First, let me explain the context of your question: It is about a concept for a future airliner:

Boeing Blended-Wing-Body Model 450-1U

Boeing Blended-Wing-Body Model 450-1U, taken from NASA/CR-2006-214534.

In the artist impression above the engines are podded and sit on struts in order to achieve a uniform flow field at the intake face. However, the strut and nacelle surface area will contribute drag that could be avoided if the engines would be mounted closer to the rear fuselage. The picture below, taken from the same source, shows a CAD rendering of a possible geometry with semi-buried engines:

Semi-buried engines for the BWB concept

The disadvantage of this concept is the ingestion of boundary layer flow such that the speed profile over the intake face shows a speed gradient (BMI = boundary layer ingestion). This will result in distorted flow at the compressor and cyclic pressure variations for the compressor blades, which in turn will require a more robust and less optimized compressor geometry or run the risk of compressor stalls and early blade failure. Also, the energy loss within the boundary layer results in lower pressure recovery ahead of and inside the intake, which will increase specific fuel consumption.

Or to quote the NASA page on inlets:

As the air is brought from free stream to the compressor face, the flow may be distorted by the inlet. At the compressor face, one portion of the flow may have a higher velocity or higher pressure than another portion. The flow may be swirling, or some section of the boundary layer may be thicker than another section because of the inlet shape. The rotor blades of the compressor move in circles around the central shaft. As the blades encounter distorted inlet flow, the flow conditions around the blade change very quickly. The changing flow conditions can cause flow separation in the compressor, a compressor stall, and can cause structural problems for the compressor blades. A good inlet must produce high pressure recovery, low spillage drag, and low distortion.

The question now is: What can be done to keep the semi-buried engine concept but to avoid the intake flow distortion from boundary layer ingestion?

Solution 1:

As it turns out, the same NASA paper proposes a simple diverter ahead of the engines which is supposed to push the slow boundary layer to the side:

Boundary layer diverter

Solution 2:

An even better solution is to use a splitter plate; after all, the semi-buried engine has an intake very similar to the side-mounted intakes of trainer and combat aircraft. Below is an example from another subsonic aircraft, the Czech L-39 Albatros jet trainer:

Subsonic splitter plate intake on the L 39

Subsonic splitter plate intake on the L 39 (picture source)

Solution 3:

Less commonly used, boundary layer suction can be employed to remove the slow-moving air close to the surface and to re-establish a more uniform speed profile. Here my example is the Eurofighter EF-2000 intake, where boundary layer suction is used to remove the boundary layer that has accumulated on the splitter plate itself:

"Smiling intake" of the EF-2000

EF-2000 intake, here abused as a baggage compartment. The grid of holes is used to suck away the boundary layer.

A similar arrangement was used on the intake cone of the SR-71, but I did not find a good picture. Boundary layer suction, however, is not the kind of passive solution you desire. But it has the advantage of being adaptable to the specific flight situation.

Solution 4:

Also not passive, and not being used in existing airplanes, would be a moving surface ahead of the inlet. This has been tried with rotating cylinders at flap breaks, and there is some literature covering the topic. The moving surface will reverse the effect of the wing ahead of the intake and re-energize the boundary layer right ahead of or inside the intake.

In all cases, you need to check whether the newly incurred losses of the boundary layer manipulation will be lower than the gains in engine efficiency. I did not go into vortex generators: They might help to equalise the speed loss or at least to reduce the speed gradient towards the surface, but they incur new and intense losses all by themselves and would most likely reduce efficiency a lot. If well placed, they will reduce intake distortion, but at the price of a much reduced pressure recovery.

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    $\begingroup$ Wow... this was so comprehensive thank you so much @PeterKampf! However, I must have misstated my question, cuz I understand the concept of BLI, but I'm not necessarily to the BWB 450 concept. I got it from the MANY NASA reports as the standard basis for the BLI engine, so if I wanted to do further testing it would produce data I could probably benchmark with the NASA reports. As for the splitter plates... I never thought of diversion rather than manipulation. Regardless of the design I prob wouldn't choose a supersonic frame anyways. $\endgroup$
    – Jihyun
    Jul 19, 2018 at 17:56
  • $\begingroup$ Sorry one more question: I'm not too well versed on diverterless supersonic inlets, but the Wikipedia page for DSI says that it could possibly divert boundary layer airflow. Given that it has supersonic applications, could the bump still provide adequate diversion of the boundary layer while introducing less form drag than splitter plates? $\endgroup$
    – Jihyun
    Jul 19, 2018 at 18:04
  • $\begingroup$ @Jihyun: Diverterless supersonic inlets are rather new, so I am not so familiar with the details. The Wikipedia article sounds overly enthusiastic about their merits - after all, why did designers use splitter plates for half a century, and only moved to DSI when stealth mandated it? The main difference between sub- and supersonic inlets would be the intake lip (rounded vs. sharp), because most of the boundary layer in supersonic flow is moving at subsonic speed. The bump will have less surface friction than a splitter plate, but will not nearly be as effective. What is adequate is up to you. $\endgroup$
    – Peter Kämpf
    Jul 19, 2018 at 19:40
  • $\begingroup$ Right, I will probably modify my preexisting BWB model and do a CFD analysis on both. Thanks for the extra info. $\endgroup$
    – Jihyun
    Jul 19, 2018 at 21:16
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Current trend... just accept it and make a fan/compressor that is capable with working in the boundary layer. NASA's latest concept on this (here is a link https://www.nasa.gov/feature/aviation-renaissance-nasa-advances-concepts-for-next-gen-aircraft) is that by ingesting the boundary layer they might need more power, but they also reduce drag. Think of it this way, an engine already needs to decelerate high speed air that is coming in, doing this wastes some energy (most is converted to pressure though), the boundary layer is already decelerated, so if it can be used, then you are not wasting energy on deceleration twice.

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  • $\begingroup$ Yes I understand the concept man ;). Think of the issue this way, the air hitting the top of the inlet is fundamentally at a different velocity and pressure than the bottom, creating a gradient that at speed becomes unstable and triggers vibrations and more importantly stalls inside of the engine, leading to damage and possibly thrust reversal. How to fix this? According to the word of the aero-lord PeterKampf splitter plates, I previously thought air pulse jets. All this goes to say, I know what I'm talking about, this answer doesn't answer my question though... thanks for explaining anyways? $\endgroup$
    – Jihyun
    Jul 20, 2018 at 4:54
  • $\begingroup$ The boundary layer type of deceleration turns speed into temperature while ram effect deceleration turns it into pressure. Big difference! Super brief info on jet intake flow: Pressure = good, heat = bad, gradient = very bad. $\endgroup$
    – Peter Kämpf
    Jul 20, 2018 at 6:18
  • $\begingroup$ Yes, adverse pressure gradients are bad. Yes, it is of benefit to take advantage of the velocity to pressure transfer in the ram effect. If possible however, you want to ingest as much of the boundary layer as possible to get maximum effect, and by rejecting the worst flow, you also ignore the part that has the most to gain from BLI. Even though it is harder, you want to capture as much of the boundary layer as possible, so the current goal is to make a fan blade design that can handle it. Check the last section of the link link $\endgroup$
    – XRF
    Jul 21, 2018 at 9:02

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