Proceeding of the 2013 IEEE International Conference on Space Science and Communication (IconSpace), 1-3 July 2013, Melaka, Malaysia
Aircraft Trailing Vortices - Cirrus Cloud Interaction and Green Aircraft Technology: An Overview Harijono Djojodihardjo Aerospace Engineering Department Faculty of Engineering, Universiti Putra Malaysia 43400 Serdang, Selangor, Malaysia
[email protected] constitutes Radiative Forcing in the atmosphere; the aviation contribution is identified with + sign on the right hand side of the Figure (Lee et al [5]). Global aviation contributes to
Abstract—Anthropogenic solutions that can be offered by Aerospace Technology to address environmental changes known are known as green technology initiatives. Such initiatives, however, may only provide partial or temporary solutions, as learned from mankind experience through the ages. To this end, basic principles governing physical changes occurring in the earth’s environment will be reviewed. In the efforts to intervene such process and thus maintaining the sustainability of the earth’s environment, some scientific and technological guidelines to comprehend the natural phenomena and global changes are reviewed and discussed. In particular, efforts devoted to aircraft technology and aviation, will be elaborated in view of the goal of maintaining the sustainability of the earth’s environment. In addition to fuel burn, particular considerations are given to contrail and trailing vortices Keywords-component; Aircraft Trailing Vortices; Cirrus Cloud; Contrails; Climate Change; Green Aircraft Technology
Figure 1: Large Human Contributions to Radiative Forcing (Lee et al [5])
I. VISION FOR GREEN ENVIRONMENT AND AVIATION CONTRIBUTION TO GLOBAL WARMING
climate change by emissions of carbon dioxide (CO2), nitrogen oxides (NOx), water vapor, particles, contrails and cirrus changes. Carbon dioxide is the most important greenhouse gas. Its effect is independent of the altitude at which the emission occurs. Nitrogen oxides from aviation at
The world’s dream for socio-economic, intellectual and technological equity and networking for narrowing their gap should be accompanied by a vision, determination and proactive actions by all and everyone, in spite of diversity, distribution, growth and capacity of the population, in order to provide solutions and realization for mankind basic, intellectual and quality of life needs including natural and energy resources and sustainable environment. A vision for world’s “green” environment, energy and technology should be shared and contributed by all, and elaborated in [1 - 4]. Global aviation contributed to Radiative Forcing so far about 0.05 W/m2. These are about 3 % of the total (about 1.6 W/m2) radiative forcing from all anthropogenic effects. The largest uncertainty comes from aviation contributions to changes in cirrus clouds, which therefore are not included in the total. Including the presently know uncertainties, the aviation contribution is estimated within the range 2 to 8 %. Thus far, global aviation contributed to the observed global warming of 0.7°C about 0.02-0.03°C (ca. 3-4 %). Figure 1 is an example of anthropogenic contribution of greenhouse gases which
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Figure 2: Trends in Global Aviation Contributions in terms of GHG’s and Cirrus Formation to the observed Global Warming of 0.7°C about 0.020.03°C ( ~3-4 (Lee et al [7]).
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subsonic cruise altitudes enhance ozone formation and reduce methane; both are greenhouse gases. Water vapor and particles (soot etc.) emitted at altitudes near the tropopause can induce contrails and cirrus clouds, likely enhancing the greenhouse effect. Figure 2 exhibits the trends in Global Aviation Contributions in terms of GHG’s and Cirrus Formation to the observed Global Warming (Lee et al [5]). Figure 3 illustrates the close relationships between aviation emissions and aviation flight routes, particularly in the tropopauze which deteriorates global warming and prompted systematic efforts by aircraft industries, airlines and aviation authorities to provide acceptable and sustainable solutions (Lee et al [5], Eyers et al [6]).
Observations have also confirmed the close correspondence between contrails and cirrus formation in the troposphere. Cirrus enhances the greenhouse effect. In addition, soot may cause “soot-cirrus” [8].
Figure 4: Trends in aviation communities to reduce greenhouse gases, illustrated as the technology development to reduce fuel burn and combustion efficiency, thus the energy intensity [7].
Contrails and soot from cruising aircraft cause cloud changes which mostly contribute to global warming. Contrails are caused by water vapor emissions from aircraft flying in cold and humid air masses. Soot and other particles change contrails and cirrus properties. Line-shaped contrails are detectable from space. Figure 5 is a satellite image above North-Western France exhibiting line-shaped contrails evolving into “contrail-cirrus”.
Figure 3: (a) Global distribution of aviation emissions (Lee et al [5]) ; (b) Map of all flights over the world departing on August 2, 2002 (Eyers et al [6]).
Airlines have presented climate change proposals to heads of governments to improve carbon efficiency with a 1.5% average annual improvement in fuel efficiency to 2020, to stabilize emissions with carbon-neutral growth from 2020, and to reduce emissions reductions with a 50% absolute cut in emissions by 2050 compared to 2005 (Szodruch and Schumann[7]). Technologies are not available today. It is estimated that such goal requires the utilization of more than 50% biofuel. Figure 4 indicates the efforts that have been undertaken by aircraft industries, airlines and aviation authorities to commit themselves to such goal.
Figure 5 : Contrails and soot from cruising aircraft produce cloud changes which mostly contribute to global warming [7,8].
B. Cirrus Clouds. Extensive cirrus clouds have been observed to develop after the formation of persistent contrails. Increases in cirrus cloud cover (beyond those identified as line-shaped contrails) are found to be positively correlated with aircraft emissions in a limited number of studies. About 30% of the Earth is covered with cirrus cloud. On average an increase in cirrus cloud cover tends to warm the surface of the Earth. An estimate for aircraft induced cirrus cover for the late 1990s ranges from 0 to 0.2% of the surface of the Earth. For another scenario, this may possibly increase by a factor of 4 (0 to 0.8%) by 2050; however, the mechanisms associated with increases in cirrus cover are not well understood and need further investigation.
II. TRAILING VORTICES AND CIRRUS CLOUD INTERACTION A. Contrails Condensation trails due to aircraft trailing vortices (Contrails) from cruising aircraft and soot cause cloud changes.
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III. VISION FOR GREEN ENVIRONMENT AND AVIATION CONTRIBUTION TO GLOBAL WARMING
The contribution of vortices to lift and lift enhancement bring our attention to the second interest, i.e. efforts for induced drag minimization and accordingly, trailing vortices alleviation. This subject has received much attention and has prompted increased multidisciplinary research effort, and recent comprehensive account can be found in many recent review papers, among others Widnall [10], Spalart [11], and Breitsamter [12]. The prediction and control of trailing vortices presents both a technological challenge of importance in air traffic control and provide a myriad of instructive phenomena in fluid mechanics. Momentum of the flow that is deflected by a wing, or by another flow, develops into an organized vortex structures that are coherent and energetic. This phenomenon is also observed in the production of vortices by impulsive forces, as observed during animal flight, or in the jet turbulence. The mechanism underlying these phenomena, and its control for various applications, in particular for flight safety, has been the subject of intensive multidisciplinary field of research (Jacquin [14]). Wingtip vortices are associated with induced drag; this phenomenon is a direct consequence of the generation of lift due to viscous effect. Induced drag and wingtip vortices management by selecting the best wing planform for the mission is a classical problem and critically important in aerospace engineering.
A. Contrail Avoidance. The meteorological conditions required for the formation of contrails are fairly well understood; they persist only in regions of air which are supersaturated with respect to ice (Royal Aeronautical Society[9]). There is no technology in prospect to prevent contrail formation in such conditions. However, these regions are of finite extent – typically in the range 100-1000km in lateral extent and 1-5km deep – and are encountered on rather less than 50% of flights over Europe. Also in Europe, they are encountered only rarely at altitudes below 25,000ft. In principle they can be avoided by flying under or over or round them. The preferred option will depend on latitude and season. In every case it will be at the cost of increased fuel burn. Further, we cannot firmly conclude that contrail avoidance will be beneficial in all circumstances until we have a firmer understanding of (a) the effect of varying altitude on the radiative impact of other emissions (NOx, water and particulates) and (b) the radiative impact of the primary cirrus formed from contrails as distinct from secondary cirrus formed from particulate emissions. B. Induced Drag Minimization and trailing vortex alleviation. Encounters with lift-generated wake by a following aircraft during take-off and landing can pose wake vortex hazard to other aircrafts and objects in their vicinity, as illustrated in Figure 6. Efforts have been devoted in modern aircraft development to the enhancement of lift and reduction of drag as well in addition to minimizing weight and maximizing propulsion efficiency.
C. Classification and basic quantities. In an overview on wake vortex research including early studies, model and flight tests, numerical investigations and fundamental physical aspects and alleviation strategies, Breitsamter [12] identifies characteristic quantities for wake vortex analysis including typical length and time scales as well as turbulence quantities. The following features characterize the trailing vortex system in the near field (x/b ≤ 0.5, τ*≤0.01): due to aircraft surfaces discontinuity, i.e. 1. In approach configuration, a large transport aircraft with four-engine generates heterogeneous vortex systems where concentrated single vortices are embedded in the vortex sheet shed from the wing trailing-edge. Progressing from the wing tip to the wing root, the dominant single vortices comprise the wing tip vortex, the outboard flap vortex, the outboard and inboard nacelle vortices and the wing fuselage vortex. 2. The wing fuselage vortex has an opposite sense of rotation with respect to the other dominant wing vortices since the circulation gradient at the wing fuselage junction change sign. 3. The horizontal tail plane vortex is another dominant vortex, which is also counter-rotating with respect to the wing tip vortex due to the negative lift produced by the horizontal tail at trimmed flight. 4. Concentrated vortices of smaller scale are emanating at other wing surface discontinuities, such as at nacelle
Figure 6: Possible encounters with lift-generated wake by a following aircraft during take-off indicating wake vortex hazard (Breitsamter [12], Rossow [13]).
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following the wavelength range as follows: a. long wave instability (LW) – Crow Instability λ > 2π ρ0 b. medium-wave instability (MW) – Crouch Instability λ π 2π > > ρ0 2
strakes, and these are attributed to flow separation at slat horns, flap track fairings, etc. Judging from its downstream development, vortex wake can be divided into four regions, as depicted in Figure 7. (i) The near-field, x/b ≤0.5, (x/lμ < O(1)), which is characterized by the formation of highly concentrated vortices shed at all surface discontinuities. (ii) The extended near-field, 0.5 > 2 ρ0 4 d. ultra-short wave instability (USW) – vortex merging λ π < ρ0 4 Crow [15], Widnall [10] and Crouch [16] have pioneered in the analysis of vortex instabilities. The first two types concern the stability of a vortex pair and known as Crow and Widnall instabilities. These are: ¾ long-wavelength (or Crow) instability, with wavelength of the order of 5 to 10 times the separation distance of the two vortices. This type of instability can be frequently observed in the atmosphere when the vortex pair is visualized as the condensation trails of aircraft in cruise flight. ¾ the short-wavelength (or Widnall) instability with a wavelength of the order of the vortex core radius. E. Vortex modification and decay. As an alternative to active control, a number of methods have been suggested to modify the trailing vortices or to enhance their decay. These schemes are generally intended to weaken the strength of the vortices, or to enlarge the vortex cores. The vortices are modified by changing the wing span loading, and varying the axial flow using drag devices or mass injection. Various wing-tip devices have also been proposed.
Figure 7: Stages of wake vortex lifespan, showing: initial vortex perturbations, the intermediate development, and the far-field result of active control using conventional aircraft control surfaces adapted from Breitsamter [11].
F. Recent Patents On Trailing Vortices Alleviation. In addition to published research work, various inventions have been patented on trailing vortices alleviation, and selected recent patents have been elaborated in [3]; new techniques have been devised to take advantage, a well as to enhance, the state of our knowledge and its utilization in aircraft trailing vortices alleviation. Some of the patents reviewed utilize propellers or jets to introduce flow disturbance to reduce the induced drag, and required power activation. Some of the devices need additional power on top of the power already utilized by the aircraft to produce lift, and hence induced drag. The patents reviewed and the basic methods utilized are summarized in Table 1.
D. Vortex Pair Instability. Aircraft trailing vortices research efforts in the past fifty years have focused on the understanding of the structure and dynamics of trailing vortices, in particular on the interesting observation of their instability and break-up, and how these phenomena can be taken advantage of in tailoring the trailing vortices efficiently and effectively for various advantages. Breitsamter [9] observed and reproduced the relevant vortex pair instability taking place downstream of the transport aircraft model tested. In this conjunction, based on wavelength which includes both long and short wave phenomena, Breitsamter [11] classifies the instability mechanisms
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Table 1 – Summary of Selected Inventions on Trailing Vortex Alleviation (Djojodihardjo, 2011[4])
No.
Inventors
1
Brown; Garry L. and Nosenchuck, Daniel M.
2
Gerhardt, Heinz Adolf
3
Bilanin, Alan J. and Quackenbush, Todd R.
4
Crouch, Jeffrey D. and Spalart, Philippe R.
5
Greenblatt, David
6
Loth , John L.
7
Eaton, John K. and Matalanis, Claude
8
Delaplace, Franck
Methods or Innovation
Yr
Lifting Body With Reduced-Strength Trailing Vortices- US Patent 5492289 Wingtip Vortex Impeller for Drag ReductionVortex Cancellation - US Patent 5918835
1990 1999
System and Method of Vortex Wake Control Using Vortex Leveraging - US Patent 6,042,059 Active Control Method And Apparatus For Encouraging The Early Destruction Of Trailing Vortices - US Patent 6082679. Trailing Vortex Management via Boundary Layer Separation Control - Greenblatt, US Patent 2005/0103944 A Vorticity Cancellation At Trailing Edge For Induced Drag Elimination - US Patent US7134631 Translating Active Gurney Flap to Alleviate Aircraft Wake Vortex Hazard– US Patent 7,740,206 B2 Method and devices for reducing the wake vortices of an aircraft in the approach/landing phase. US7850125 B2
Wake vortices alleviation technique chord length variation specifying the lifting body with reduced trailing vortices strength Vortex Impeller for Drag Reduction-Vortex Cancellation
Passive/ active passive Active
2000
altering the generated initial vortex wake to make it vulnerable to rapid breakup; producing disturbances to this wake with secondary vortices using vortex leveraging tabs
Active
2000
actuating aileron and spoiler following active control system and scheme
Active
2005
varying the spanwise vortex-sheet strength via either passive or active boundary layer separation control
Active & passive
2006
upper and lower surface boundary layers control by suction to cancel opposing vorticity; and wing tip vorticity countering device
Active
2010
active flap that moves spanwise back and forth along the outboard section of the surface.
Active
2010
9
Sauvinet, Frederic
Method and Device for Generating an Optimum Aerodynamic configuration of an aircraft during a Flight, US2010/0219298 A1
2010
10
Lewis, Michael S.; Meseroke, Jere S., Dunn, Michael J., & Tillotson, Brian J.
Systems and method for tracing aircraft vortices, US Patent 2010/01133384 A1
2010
method and device for reducing the wake vortices of an aircraft during the approach/ landing phase by automatically controlling spoilers process and device for automatic flight optimization of aircraft aerodynamic configuration by determining and applying spoilers commands for optimum aerodynamic configuration. systems and methods for directing and tracing aircraft vortices, which include directing a tracer from an aircraft into a vortical flow generated by the aircraft flow.
Active
Active
Active
to be carried on flights of comparable distances leading to additional fuel savings.
IV. ENGINEERING SOLUTIONS: FUEL BURN REDUCTION A. Goals for Green Aviation. NASA (NASA-Green Aviation [17]) has a set of “Green Aviation” research goals which are related to mitigating environmental impacts of aviation. NASA’s goals represent world’s serious effort to achieve environmentally friendly aviation and aircrafts technology, such as to reduce aircraft fuel consumption, emissions and noise simultaneously, which is a much more difficult challenge than working to reduce them individually. Such effort has been envisioned in Europe as Joint Technology “Clean Sky Initiative”.
Table 2 – Summary of Selected green aircraft technology development efforts related to engineering solution for fuel burn reduction
SOME ENGINEERING SOLUTIONS FOR FUEL BURN REDUCTION Airframe technology
B. Fuel Efficiency. In 2008, major U.S. commercial air carriers burned 19.7 billion gallons of jet fuel, while aircraft owned and operated by the Department of Defense consumed another 4.6 billion gallons of jet fuel, with an estimated fuel cost of $73 billion. With such illustration in fuel consumption, the goal is to develop aircraft technology capable of reducing fuel burn significantly. This technology should enable the design of new aircraft that burn 33 percent less fuel than present airplanes by 2015, 50 percent less by 2020 and at least 70 percent less by 2025. It also should enable the adoption of new air traffic management operations that save up to 6 percent of annual commercial aviation fuel consumption by 2035. The improvement in the specific fuel consumption has furthermore reduced the necessary amount of fuel that has
Aerodynamic Technologies
Engine Technologies
Adaptive Structures Strctural Health Monitoring Al-Li Alloys Friction Stir Welding Nano-Tehnology Self-Healing Materials Glare Laser-Beam Welding Electron Beam Welding Ti-Alloys Intermediate Modulus Fibre Riblets Natural Laminar Flow Hybrid Laminar Flow Control Excessence Reduction Variable Camber Advanced Direct Online Turbofan, EPR 10-12 Geared Turbofan, EPR 10-20 OPEN Rotor, EPR ≈ 25
2015 2025 2020
The global efforts which are taking place in the research and development activities are well monitored by ICAO [18].
322
nd
Progress and trends in the fuel efficiency, airframe technology, Aerodynamic technologies for Fuel Burn reduction and aircraft engine technology development, which incorporate well known research interests such as airframe technologies, wing design, laminar and hybrid laminar, minimizing interference drag on aircraft, engine technologies (advanced direct drive turbofan, open rotor, geared turbofan), etc. Global research and technology development efforts in progress are elaborated in [18]. In this conjunction, Table 2 summarizes the green aircraft technology development efforts related to engineering solution for fuel burn reduction.
[3]
[4]
[5]
[6] [7]
V. CONCLUDING REMARKS Global warming is observed and largely caused by human drivers. Discussions in this paper started from aircraft trailing vortices and their relationships with shedding aircraft performance and safety of other aircrafts and objects. In addition, trailing vortices produce contrails which influence cirrus cloud formations. Both of these interactions have been the subject of environmentally friendly (“Green”) aircraft technology development, which include reduction of CO2 emissions and fuel burn. To this end, climate protection requires reductions of the total greenhouse gas emissions as well as contrails from aviation. The aviation share in CO2 emissions is presently about 2 %. Hence, increased fuel efficiency is important (for climatic as well as economic considerations). In addition, cruising aircrafts impact climate by NOx and contrails. The aviation share in radiative forcing is presently 3 % (range 2-8%) and show potential increase until 2050. Scenarios of aviation CO2 emissions show potential increase by factors of 3.3 to 5 until 2050. Hence mandatory efforts should be carried out for NOx and Contrails mitigation and increased fuel efficiency. Largest uncertainty and possibly largest contribution is derived from contrail cirrus. This particular contrail issue needs higher attention from researchers, authorities and aircraft industry to respond to Green Aircraft challenge.
[8] [9]
[10] [11] [12]
[13]
[14]
[15] [16] [17]
[18]
ACKNOWLEDGMENT The authors would like to thank Minister of Higher Education and Universiti Putra Malaysia (UPM) for granting Fundamental Research Grant Scheme (FRGS) Phase 1/2013, under which the present research is carried out. The author would also like to thank Universitas Al-Azhar Indonesia for the opportunity to carry out the present research at Universiti Putra Malaysia. REFERENCES [1]
[2]
H.Djojodihardjo (2010), Space Power System – Motivation, Review and Vision, in Solar Collectors and Panels, Theory and Applications, Sciyo Publishers. H.Djojodihardjo (2010), Aerospace and Green Technology: Progress and Outlook, World Engineering Congress 2010,
323
th
2 – 5 August 2010, Kuching, Sarawak, Malaysia, Conference on Aerospace and Mechanical Engineering. H.Djojodihardjo (2011), Review on Development and Recent Patents on Trailing Vortices Alleviation, Recent Patents in Engineering, Bentham Publisher. H.Djojodihardjo and R.Varatharajoo (2009), Space Power System Initiatives: Establishing World Vision And Capacity, paper IAC-09-C3.3.3, 60th International Astronautical Congress, 12 – 16 October 2009, Daejeon, Republic Of Korea. D.Lee, U.Lohmann, and U.Schumann (2009), Global Climate Change and Aviation - The Challenge, DLR, ETH Zürich, and Manchester Metropolitan University, CEAS Plenary. C.I.Eyers et al (2004), AERO2K Global Aviation Emissions Inventories for 2002 and 2005, December 2004, QinetQ Ltd. J.Szodruch and U.Schumann, (1989), DLR Climate Research and Aircraft Technologies, ICAS Program Committee Workshop, Amsterdam, September 2009. U.Schumann (2005), Formation, Properties And Climatic Effects Of Contrails, Comptes Rendus Physique 2005. Royal Aeronautical Society (2005), Air Travel – Greener by Design, Mitigating the Environmental Impact of Aviation: Opportunities and Priorities, Report of the Greener by Design Science and Technology Sub-Group, Published by the Royal Aeronautical Society. S.E,Widnall (1975), The Structure And Dynamics Of Vortex Filaments, Annual Review of Fluid Mechanics, 7; 141-165. P.R.Spalart (1998), Aircraft Trailing Vortices, Annual Review of Fluid Mechanics, 30:107–38. C.Breitsamter.(2011), Wake vortex characteristics of transport aircraft, Progress in Aerospace Science, vol. 47, no. 2, pp. 89-134. V.J.Rossow (1999), Lift-generated vortex wakes of subsonic transport aircraft, Progress in Aerospace Science, 35, pp 507660. L.Jacquin, D.Fabre, P.Geoffrey and E.Coustols.(2001), The properties of a transport aircraft wake in the extended nearfield, AIAA paper No. 2001-1038 S.C.Crow (1970), "Stability Theory for a Pair of Trailing Vortices," AIAA Journal, vol. 8, No. 12, pp. 2172-2179. J.D.Crouch (1997), Instability and transient growth for two trailing-vortex pairs. J. Fluid Mech. 350, 311-330, 1997. NASA (2012) , Green Aviation - A Better Way to Treat the Planet, green_aviation_fact_sheet_web-Accessed 2 December 2012 ICAO (2010), ICAO Environmental Report 2010, Chapter 2: Aircraft Technology Improvement, http://legacy. icao.int/icao /en/ env2010/Pubs/ EnvReport 2010/ ICAO-Env Report 10Ch2-en.pdf, accessed 30 March 2013.
