Engine Nacelle Aerodynamics

Figure 1: Vortex system from an aircraft engine nacelle-wing-pylon junction. Image source Ref [12].

2231 words / 11 minutes read

Introduction

The aviation industry accounts for 2% of global greenhouse gas emissions. With an annual increase of around 4.8% in air passenger transport, greenhouse gas emissions are very likely to go up unless some drastic measures are taken to curb them. In order to reduce the environmental footprint of the aviation industry, various governing bodies around the world are coming up with ambitious goals to cut carbon dioxide and Nox emissions by as high as 75 to 95% by 2050.

The solution to this challenge lies in lower fuel consumption and increasing aircraft efficiency. One promising approach the aircraft industry is currently pursuing is the development of Ultra-high bypass ratio (UHBR) engines. UHBR engines, as the name suggests, maximises the air mass flowing through the bypass duct, thereby reduce thrust-specific fuel consumption. The ratio of the amount of air that is allowed to bypass compared to that entering the engine core is called bypass ratio. Since 1975, the bypass ratio has increased from 6 to 12 and the next generation of engines are expected to have a bypass ratio even higher, ranging from 15 to 21.

Ultra-high bypass ratio (UHBR) engines

Figure 2: Bypassing flow. a. Schematic diagram. b. CFD computations. Image source Ref [8, 5].

A larger bypass ratio means larger diameter engines. Fitting a larger diameter engine nacelle below the wing becomes a major challenge as compliance with ground clearance regulations requires a close coupling of nacelle and wing. This necessitates either to cut off a large chunk of the slat in order to avoid collision with nacelle during landing and take-off or fit the engine nacelle to the upper surface of the wing or embedded into the fuselage. Figure 3 shows, some of the different ways to mount engines in aircraft.

Different positions to install aircraft engines: UHBR - Ultra-High Bypass Ratio under wing engines, OWN - Over Wing Nacelle configuration, Fuselage embedded engines.

Figure 3: a. UHBR – Ultra-High Bypass Ratio under wing engines. b. OWN – Over Wing Nacelle configuration c. Fuselage embedded engines. Image source – arstechnica, air.one, Ref [10].

Each approach has their own set of problems and challenges. For example, removing a segment of the slat leads to the creation of premature separation on the main wing leading to early stall and reduced maximum achievable lift.

In this article, we limit ourselves to understanding the issues in the under-wing installation of large bypass ratio engines and their complex flow physics.

Video 1: Why bypassing of air necessary?

Before we get into the details of under-wing installed engine nacelles, let’s start our appreciation of the nacelles from ground zero.

Aircraft engine nacelles

Nacelles are nothing but the housing for the aircraft engines as they protect the gas turbine from foreign object ingestion(FOI). They are designed with the objective of delivering air efficiently and with minimum distortion to the fan and also expand the gases in the exhaust system with maximum efficiency. Figure 4a shows the different parts of an engine nacelle.

Though they are designed to ensure good engine performance, their presence leads to a drop in lift and an increase in drag by a large percentage. Optimisation of nacelle design is very essential as high drag-generating flow phenomena like flow separation, shock waves and wake may develop during flight. A thorough analysis is needed to find the right engine location on the wing that provides the best integration of engine and airframe. This integration not only depends on the design of the engine nacelle and wing individually but also on the resulting interference effects.

Propulsion system integration is considered quite complex as it dramatically affects both the aircraft and the propulsive system performance. With bigger and larger engines, the propulsive system is becoming highly coupled with the airframe. Therefore, for correct evaluation of the performance of both systems, it is essential to take into account the installation effects. In other words, the airframe design and propulsion system design cannot be considered as two separate tasks but their designs need to evolve considering the interference effects they have on each other as well.

Engine nacelle parts
Figure 4: a. Parts of a typical engine nacelle. b. Engine Position. Image source Ref [9].

Parameters affecting interference phenomena

When a nacelle is being installed onto the airframe, a large number of influencing factors need to be taken into consideration such as engine positioning, the shape of the nacelle, pylon and wing, etc. Studies have shown that about half of the overall lift loss can be attributed to pylon shapes as it alters the lower wing pressure distribution. Another reason for the loss in lift is due to the intersection of the pylon with the fan cowl, as the flow tends to stagnate on it and then later accelerate over the top of the structure reaching supersonic velocities.

Along with the airframe shapes, engine positioning has a large influence on the interference phenomena. The position of an engine can be varied in a multitude of ways by moving it up/down, fore/aft, spanwise, and also by changing the pith and toe angle.

If the vertical distance is reduced, the shock on the upper surface gets shifted more upstream, resulting in loss of lift. Simultaneously, on the lower wing surface, the flow is less accelerated resulting in pressure gain. The loss in lift on the upper surface and gain in pressure on the lower, almost negate each other and thereby reduce the vertical positioning influence on lift and drag.

However, horizontal positioning strongly influences the wing performance, as moving the engine downstream results in a downstream shift of the shock leading to a loss in lift. Spanwise shifting mainly influences the lower surface by modifying the virtual flow channel between the inboard side of the pylon and the wing causing a more accelerated flow in the case of inboard engine placement.

Interestingly, the pitch angle influences both the upper and the lower wing surfaces and modifies the total drag and lift. Toe angle, on the other hand, has an effect similar to spanwise position, as it modifies the shape of the virtual flow channel around the pylon, influencing mainly the wing lower surface.

The presence of these many influencing parameters make the nacelle installation on the wing a daunting task. A large amount of critical analysis is needed, as a bad installation can increase the total drag by about 4.2 percent, which, in a transport aircraft is equivalent to 1000 kg of payload.

Engine nacelle installation effects

Figure 5: Installation effects of engine nacelle: a. On the upper surface of the wing. b. On the lower surface of the wing. Image source Ref [9].

Now, that we have understood the factors to be considered in nacelle-airframe integration, let’s try to have a birds-eye-view of the flow physics developing because of this integration.

Below wing nacelle integration – cruise condition

The flow field development during the cruise is largely controlled by the adverse interference in junction regions such as the wing-pylon and nacelle-pylon junctions. The presence of an engine modifies the location of the stagnation point on the wing and reduces the angle of attack at the wing-pylon junction. This results in an upstream displacement of the shock front on the upper surface. Further, the reduction in incidence also increases the pressure on the suction side.

On the lower wing surface, a virtual flow channel between the inboard side of the pylon and the wing develops, causing the flow to accelerate. This later leads to flow separation. Additionally, the flow acceleration causes buffeting – a shock boundary layer interaction that causes the shock wave to oscillate which in turn causes, oscillation of lift and pitching moment. This is a major concern, as buffeting at transonic conditions, limits the speed at which an aircraft can cruise.

Lastly, a form of drag called blowing drag or jet effect is generated because of the reduction in wing circulation as the exhaust jet induces a higher velocity which is against the direction of natural circulation. Further, additional losses can occur, if the jet-induced velocity exceeds sonic speed, resulting in shock formations and possible flow separation.

Aircraft engine nacelle up wash flow
Figure 6: High lift propulsion system integration aerodynamic effects: up-wash flow. Image source Ref [9].

High lift propulsion system integration

Just as in cruise conditions, nacelle in high-lift conditions during landing and takeoff has the same effect of increased drag and reduced lift. However, the effects are more damaging during high-lift conditions where there are severe interactions between the engine nacelle and the wing flow field, especially at high angles of attack. The maximum lift (Cl_max), the key design parameter in high-lift configurations could get severely compromised during their integration with engines.

What happens is that, in order to accomplish the engine installation under the wing, a segment of the slat needs to be cut out to accommodate the pylon and as a consequence, a part of the precious lifting surface is lost. Apart from the reduced lift, the exposed adjacent part of the wing profile now faces a higher alpha flow and increases the probability of early flow separation.

The sheer physical presence of nacelle generates an up-wash flow as shown in Figure 6. This up-wash flow interacts with the low-pressure flow field on the upper surfaces of the wing, pylon, and the slat cut-outs resulting in multiple vortices.

Flow topology around wing-pylon-nacelle junction.

Figure 7: Flow topology around wing-pylon-nacelle. 1 – outboard slat vortex, 2- outboard leading-edge vortex, 3- nacelle vortex, 4- pylon shoulder vortex, 5-strake vortex, 6- inboard leading-edge vortex, 7- inboard slat vortex. Image source Ref [11].

The vortex system

The up-wash deflecting more flow to the upper surfaces at high incidences is responsible for generating 6 vortices, namely, the pylon vortex, two slat vortices, two leading edge-pylon vortices and the nacelle vortex. If vortex generators called nacelle strakes are mounted, then another pair of vortices are generated. These vortices which actively interact with each other play a major role in controlling the boundary layer separation and strongly dictate the maximum achievable lift.

The nacelle vortex is generated when the flow on the slat interacts with the nacelle up-wash flow. When compared to a simple wing-body configuration, the flow angle as seen by the slat in high-lift propulsive configuration is higher due to the presence of nacelle. The upper slat flow’s direction, especially from the inboard slat side, due to its close proximity to nacelle may be pitched in the opposite direction to that of nacelle flow, resulting in reduced local velocity. This may lead to flow separation, paving the way for the formation of the nacelle vortex.

Next, the slat vortex generation is something similar to that of the wing-tip vortex. The slat cut-out creates two vortices one on either side of the slat gap, due to the pressure difference between the slat’s suction side and pressure side.

Further, as a cumulative effect of the presence of the nacelle vortex, the slat vortex, and the upstream positioning of the inboard slat, a pressure difference can set in between the two sides of the pylon, triggering a flow displacement from one side to the other. As this happens, a flow recirculation on the pylon upper surface gets established which subsequently develops as a pylon vortex.

Furthermore, the slat-cut out portion exposes the adjacent part of the main wing profile to a higher angle of attack flows, thereby subjecting them to early flow separation. These flow separation ultimately culminates as vortices at the main wing-pylon junction.

Vortex system due to wing-nacelle-pylon junction

Figure 8: Iso-vorticity surfaces with underlined high lift installation vortices at an AOA of 17°. Image source Ref [9].

What one should realize is that the strength and position of the vortices are directly dependent on the nacelle, pylon, slat and main-element wing geometries and their installation. Geometric optimisation of these components will tremendously aid in reducing the installation penalties.

Now that we came to know how these vortices are generated, let us now try to understand what happens due to the presence of these vortices. If the engine is close to the wing, the nacelle vortices attach themselves to the wing’s upper surface under the influence of the low-pressure zone at the leading edge. This interaction is beneficial as these vortices supply additional energy to the particles in the boundary layer to resist the adverse pressure gradients and prevent flow separation. In a way, this flow phenomenon mitigates the side effect of nacelle installation by decreasing the loss in lift.

Although the installation vortices are generally favourable since they originate in a zone of low kinetic energy (wing-pylon junction), they tend to have a low axial velocity and as a consequence, they are eventually bound to breakdown and cause flow separation when faced against a high-pressure gradient, especially at higher alphas. In severe cases, flow separation can happen both on the inboard and as well as on the outboard sides of the main wing as shown in Figure 9.

Nacelle wake flow separation

Figure 9: Cfx distribution with skin friction streamlines: 14 to 18.5 degrees. Image source Ref [2].

Further, since the inboard side of the slat compared to the outboard, is forward positioned relative to the nacelle, the inboard vortex is more exposed to higher pressure fields. This means they are more susceptible to breakdown, leading to easier flow separation.

All these flow interactions have a detrimental effect on the total lift and drag of the aircraft. Figure 10 shows the comparative plots of lift and drag polars for a wing-body configuration with and without nacelle-pylon. It can be observed that nacelle introduction reduces the Clmax and stall angle. While the stall angle reduces from 32 degrees to 21 degrees, the lift at alpha 21 degrees reduces by nearly 12%. This degrading effect can be seen even at low angles of attack. For example, at alpha 6 degrees, the lift is reduced by about 2%.

Lift and drag polar for wing-body and wing-body-nacelle-pylon configuration

Figure 10: Lift and drag polar for a Wing-Body (WB) and Wing-Body-Nacelle-Pylon configuration. Image source Ref [9].

Engine Nacelle strakes

To reduce the negative impact of the vortex system on the aerodynamic performance of the wings, Engineers came up with the idea of mounting a pair of strakes, popularly called chimes, to generate two additional strong vortices to regulate the flow separation on the wings.

nacelle double strake induced vortices

Figure 11: a. Double chime strakes vortices. b. Lift and drag polar for WB (configuration 1), WBNP (configuration 2) and WBNP with strakes (configuration 3). Image source Ref [9].

As can be seen in the lift and drag polars in Figure 11, strake vortices have a positive impact, as they aid tremendously in energizing the boundary layer and prevent flow separation. Appreciable recovery of lift happens with the introduction of engine nacelle strakes.

Active flow control technique by pulsed jet blowing
Figure 12: Active flow control technique: Pulsed jet blowing. Image source Ref [4].

Parting Remarks

However, nacelle strakes are not good enough to overcome the adverse pressure gradients in the flow field around Ultra-high bypass ratio engines. The energy supplied by them is not sufficient enough to compensate for the losses in lift due to the missing slat section. Currently, researchers are looking towards active flow control techniques like pulsed blowing and synthetic jet actuation to control flow separation. Studies so far have shown them to be quite successful in counteracting the setbacks caused by extended stat cut-outs.

Ultra-High Bypass Ratio engines are pitched as the power plant for future commercial transport aircraft. Increasing environmental and economic requirements are pushing the aviation industry to embrace such newer technologies. This is a positive step going forward. Though they pose immense engineering challenges, the industry is able to develop technologies that help to push the envelope and make air transport more affordable, cleaner, and less noisier.

Further Reading

  1. Influence of vortex generators in aircraft aerodynamics
  2. Role of Vortex Generators in Diffuser S-Ducts of Aircraft
  3. A Whale of an Idea in Wing Design

References

1. “Reynolds number and wind tunnel wall effects on the flow field around a generic UHBR engine high‑lift configuration”, Junaid Ullah et al, CEAS Aeronautical Journal (2020) 11:1009–1023.
2. “Simulations of an Aircraft with Constant and Pulsed Blowing Flow Control at the Engine/Wing Junction”, David Hue et al, HAL Id: hal-01721678, 2 Mar 2018.
3. “Optimal Design and Installation of Ultra High Bypass Ratio Turbofan Nacelle”, Andrey Savelyev et al, ICMAR, Oct 2016.
4. “Active Flow Control Applied at the Engine-Wing Junction”, Sebastian Fricke et al, CEAS 2015 paper no. 249.
5. “DLR TAU-Code uRANS Turbofan Modeling for Aircraft Aerodynamics Investigations”, Arne Stuermer et al, Aerospace 2019, 6, 121.
6. “Overview on nacelle design”, Jesuíno Takachi Tomita et al, 18th International Congress of Mechanical Engineering, November 6-11, 2005, Ouro Preto, MG.
7. “CFD Study of an Over-Wing Nacelle Configuration”, Steven H. Berguin et al, Georgia Institute of Technology, Atlanta, October 5, 2018.
8. “Aerodynamic Evaluation of Nacelles for Engines with Ultra High Bypass Ratio”, Andreas Petrusson, Master’s thesis 2017:02, Chalmers University of Technology.
9. “Modelling the aerodynamics of propulsive system integration at cruise and high-lift conditions”, Thierry Sibilli, PhD Academic Year: 2011-2012, Cranfield University.
10. “Fan noise due to boundary layer ingestion in novel aircraft architectures“, CEAS-ASC Workshop ‘Future Aircraft Design and Noise Impact’, 6-7 Sep 2018, Amsterdam.
11. “Application of active flow control on aircraft – state of the art“, Ahmad Batikh1 et al, AST 2017, February 21–22, Hamburg, Germany.
12. “CFD Prediction for High Lift Aerodynamics”, Jeffrey Slotnick, Technical Fellow, Boeing Commercial Airplanes, RAeS Conference on Aerodynamics Tools and Methods in Aircraft Design, 15 October 2019.

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