What is this flange on the engine?

Question

Flying home from the Denver ComiCon, on a 737, I saw an angled flange on the engine. Could you describe its purpose?

^{Photos by CGCampbell licensed CC0}

Answer 1

This is called a "strake" or "chine" (Aerodynamic fan cowl strake/chine, Nacelle strake/chine).
They allow the aircraft to generate more lift at lower speeds, which entails such positive consequences as lower stall speeds, lower landing speeds, lower take-off speeds and shorter runways.

Strakes should be used if the nacelles are mounted closely under the wing of the aircraft, see also Why don't all aircraft use/need nacelle chines?. Nacelles are usually mounted like that in order to

• maximize the distance between the lower nacelle side and the runway, so that the engine/fan cowl/nacelle doesn't hit the ground during violent landings when the wings bend down, and in order to
• minimize the risk for runway debris being sucked into the engine.
• Source: Flugzeugtriebwerke, Bräunling Willy J.G., VDI-Buch, Springer Vieweg, 2015

From US Patent 20100176249: Engine Nacelle Of An Aircraft Comprising A Vortex Generator Arrangement:

With an optimal arrangement and at high angles of attack, such vortex generators, which are known as nacelle strakes or chines, generate a powerful vortex that flows over the wing, where on a slat in front of said wing it delays airflow separation until the aircraft flies at greater angles of attack.

The idea dates back to 1971 (US Patent 3744745: Liftvanes):

in operational conditions wherein high angles of attack are encountered, such as in landing or takeoff, the vanes oppose the strong upwash around the nacelle, reducing the flow separation on its upper areas, and providing a strong downwash marked by marginal trailing vortices

From R.S. Shevell. Aerodynamic Bugs: Can CFD spray them away? 1985

DC-10 wind tunnel tests showed a significant loss in maximum lift coefficient in the flap deflected configurations, with landing slat extension, compared to predictions. This resulted in a stall speed increase of about 5 knots in the approach configuration. The initial wing stall occured behind the nacelles and forward of the inboard ailerons. The problem was traced by flow visualization techniques to the effects of the nacelle wake at high angles of attack and the absence of the slat in the vicinity of the nacelle pylons. The solution was developed in the NASA Ames Research Center 12 ft. pressurized tunnel and turned out to be a pair of strakes mounted forward on each side of the nacelles in planes about 45 degrees above the horizontal. The final strake shape was optimized in flight tests. The strakes are simply large vortex generators. The vortices mix the nacelle boundary layer air with the free stream and reduce the momentum loss in the wake. The vortices then pass just over the upper surface of the wing, continuing this mixing process. The counterrotating vortices also create a downwash over the wing region unprotected by the slat, further reducing the premature stall. The effect of the strakes is to reduce the required takeoff and landing field lengths by about 6%, a very large effect.

Answer 2

Adding to user2168's answer, here are some pictures visualizing the principle of nacelle chines, also called nacell strakes or aerodynamic fan cowl strakes.
Please note that the vortices are always there, but they can only be seen in special conditions, see the end of this answer for an explanation.

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• YouTube-Video New Boeing 747-8 Intercontinental Majestic First Flight by user Altumkell, see the left-side engines of the plane.

As was stated above, some conditions have to be true if vortices are to be seen:

• To be more precise, the humidity, pressure and temperature of the ambient air has to "match" the pressure drop induced by the nacelle chines/strakes (this pressure drop is due to the flow acceleration, i.e. a higher flow velocity, caused by the nacelle chines/strakes). For passenger aircraft, this is usually the case on humid days not too far above the ground.

• Let's assume this is true. In addition, we assume that the air is incompressible, which is usually true for velocities $\mathit{Ma} \leq 0.3$ - which is the case for take-off and landing.
  Now, the increase in flow velocity $c$ caused by the nacelle chines/strakes leads to the decrease of static pressure $p$, which leads to the decrease of static temperature $T$ (the total/stagnation pressure $p_0 \approx p_t$ and total/stagnation temperature $T_0 \approx T_t$ approximately stay constant, $p_0 \approx p_t \approx \mathit{const}$, $T_0 \approx T_t \approx \mathit{const}$).
  If the temperature drops below the local dew point, condensation occurs: the gaseous water vapor transforms into liquid water droplet clouds - this is what can be seen in pictures.
  As a side note: During condensation, the "enthalpy / latent heat of condensation" is released.

• Similar effects and explanations can be found e.g. in How does this vortex form inside a jet engine?, Why does condensation form on the wing especially during take-off and landing? and What caused a fluctuating cloud to form in a jet engine intake on a humid day?.

Image sources:

• Nacelle chine vortex 1

• Nacelle chine vortex 2 from C. Frank Starmer's "Photo Adventures with Curiosity and Learning" -- "April 1, 2010:From SIN to EWR with Yen Ping and Kok Mun"

Answer 3

I do not believe the teaching in the patent is correct.

Instead of the downwash from the strake suppressing the wing upwash (and remember, the other side of the vortex adds to the wing upwash!) we believe that the vortex is well organized and can withstand the adverse pressure gradient of the wing better than the 'junk' wake of the nacelle without the strake.

The size and location of the strake determine when the vortex will burst, and hence govern when the inboard wings stalls. Ideally you would like this to be at the same time as the outboard wing. Too late and you get unacceptable pitch up. Too early and you lose low speed performance (higher stall speeds equated to higher speed schedules and longer takeoff field lengths).

A side note, the outboard strake is of little to no value at all, which is why Boeing airplanes only have strakes on the inboard side.