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Winglet takılmış kanatlar etrafında sayısal ağ üretimi ve viskoz akış analizi

Başlık çevirisi mevcut değil.

  1. Tez No: 75548
  2. Yazar: ORHAN KÜPÜŞOĞLU
  3. Danışmanlar: DOÇ. DR. A. RÜSTEM ASLAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Havacılık Mühendisliği, Aeronautical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Uzay Bilimleri ve Teknolojisi Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 41

Özet

Uzun süreden beri kanat ucuna monteli yüzeylerin kanat uçlarında oluşan girdap yapılarını azalttığı ve yaydığı bilinmektedir. Sonuç olarak indükleme sürüklemesinin azalmasına karşın elverişsiz etkileşimler ve viskoz etkiler elde edilen yararları sıfırlayabilmektedir. Winglet kavramı bu yaklaşımların en umut verici olanlarından biridir ve kanadın efektif açıklığını arttıran bir düzenek olarak düşünülebilir. Winglet kanat ucu girdabının içine yerleştirilmiş olan küçük bir kanattır. Winglet üzerindeki kaldırma kuvveti bir yan kuvvet olarak kendini göstermekte ve böylece akım yönünde bir itki komponenti ortaya çıkmaktadır. Gövde arkasına yerleştirilen kaplama yüzeylerinde (afterbody strakes) olduğu gibi girdap yapıları bir ölçüde yayılır. Buna karşın kanat kökündeki eğilme momentlerinde hem artan kanat yükleri hem de winglet yükleri nedeniyle bir artış görülür ve bu da wingletlerin sonradan ilave edilebilirliğini sınırlayabilir. Burun-aşağı bir moment de ağırlık merkezinin üstünde kalan itki nedeniyle oluşabilir ve bu da trimleme gerektirmesi dolayısıyla olumsuz etkilere yol açabilir. İlave olarak, yüzey sürüklemesi ve bileşke bölgede etkileşim sürüklemesi gibi diğer sürükleme biçimlerinde artışlar meydana gelebilir. Bu nedenle, tipik olarak sürüklemede yüzde üçle altı arasında azalma elde edilebilmesine karşın, benzer bir performansa bazı durumlarda kanat ucunun basit bir uzatılmasıyla da erişilebilir. Bu nedenle“winglet”in tercih edilmesi tasarımının optimizasyonuna bağlıdır. Bu çalışmada bir kanat-winglet konfigürasyonu etrafında sayısal ağ üretimi gerçekleştirilmiştir. Bunu takiben ilgili akım alanın sayısal analizi sonlu elemanlar yöntemi kullanılarak Navier-Stokes denklemlerinin parallel çözümü ile elde edilmiştir.

Özet (Çeviri)

In 1970s NASA aerodynamicist Richard Whitcomb, inventor of the Area rule, made two major contributions to aeronautical science: the GA(W) airfoil and the Whitcomb winglet [1]. Whitcomb, writing in 1976, had carefully documented the effect of winglets upon performance: a four- to eight-percent improvement in the lift/drag ratios of several large jet transports. The eight-percent figure applied to the KC-135, whose old-technology wing, dating back more than two decades, had the most to gain; five percent came to be the rule-of-thumb figure for the improvement to be expected [1,2]. It has long been recognized that the addition of tip-mounted surfaces to a wing can reduce and diffuse the vortex structures arising from the tips [3]. Induced drag reductions result, but these may be offset by unfavorable interference and viscous effects. The winglet concept is one of the most promising of these concepts and can be thought of as a device to increase the effective span of the wing. WINGLET LIFT Fig. 1.1 Winglets for drag reduction VIAs shown in Fig. 1. 1, the winglet is small wing mounted in the swirling flow at the wing tip. The lift on the winglet acts as a sideforce and, with proper positioning of the winglet, it will have a thrust component in the stream direction. As with the afterbody strakes, the structure of the vortices is somewhat diffused due to the winglets. However there will be an increase in wing root bending moment due to both the increased wing loading and the winglet loading and this may limit the utility of winglets as retrofittable devices. A nose-down pitching moment can also occur due to the above-center thrust location and this can lead to trimming penalty. In addition there are attendant increases in other forms of drag such as skin-friction drag and interference drag at the junction region. Thus, while typical drag reductions of the order of three- to six- percent may result, comparable performance can in some cases be achieved by a simple tip extension. For best performance, proper design of the winglets is critical and some specific design details are as follows [3]: 1. For good supercritical performance, the winglet should be tapered and swept aft. It should be mounted behind the region of lowest pressure of the main wing to minimize interference effects. 2. Some outward cant is desirable and helps to minimize interference at the junction. 3. Smooth fillets should be used between the wing tip and the winglet, or smaller drag-reduction benefits may result. 4. Some toe-out of the winglet is needed due to the inflow angles at the wing tip. This is also desirable since it reduces the likelihood of winglet stall during sideslip. 5. Although the drag reduction increases with winglet span, it is less than linear. Therefore, the optimal winglet height must be a trade-off between the improved aerodynamics and the increased moments due to the larger moment arms.6. In principle, winglets can be mounted above or below the wing, but operational requirements and ground clearances favor upper mounts. A smaller winglet below and ahead of the main winglet is desirable for preventing stall on the main winglet at high lift conditions (as installed to MDD/BoeingMD-11). It might also be mentioned that winglets confer other favorable characteristics, besides drag reductions, which might be important. Among these are the better control of the spreading and dispersal of particulates behind agricultural aircraft and improved hangar and ground maneuvering clearances for large aircraft. In certain integrated aircraft designs they can also act as aircraft control surfaces [3], Since current forecasts are that airline passenger traffic will double by the year 2010, on both sides of Atlantic, design teams are working on concepts for very large aircraft (VLA) to satisfy the anticipated need [4]. Contenders for this market segment, Boeing BWB-1, Aerospatiale FW and TsAGI FW-900, all contain very large winglets as essential design elements, because aspect ratio cannot be made arbitrarily large, even if structurally it is feasible, due to the size constraint of 80x80 m (262 ft) box. Proper design of the winglet depends upon the optimization of the winglet geometry. Thus, to obtain the optimum winglet design for a given wing requires the investigation of numerous winglets which differ in aspect ratio and in the angles which define how the winglet is mounted to the wing. Such a design approach is laborious and since it depends upon the use of wind tunnels it is very expensive. Fortunately the modern designer is provided with the facilities of a virtual wind tunnel. It is the Computational Fluid Dynamics, CFD, which gives the power of a wind tunnel and the ease of modeling the region of interest. Consequently, in the last two decades CFD has revolutionized the airplane design process, and in many ways it has modified the way we conduct modern aeronautical research and development [5,7]. Grid generation is a very important aspect of solving computational fluid dynamics problems where a set of partial differential equations has to be solved in a given domain. It is desirable for the grid to have good orthogonality properties, and the grid generator should be able to produce an arbitrary amount of clustering nearthe body surface when necessary. A grid is considered lo be structured if there is an order in the location of nodes. Similarly, a grid is said to be unstructured if there is no order in the location of nodes. Structured grids tend to be more efficient at using CPU times. Thus, structured grids take less time to solve a given problem. For complex geometries structured grids are preferred but their generation is more difficult. This a very important disadvantage of structured grids. Though unstructured grids have been in vogue for quite some time they are not yet as simple and easy to implement as structured mesh generation techniques. Nevertheless use of unstructured grids has gained increased popularity due to never-ceasing improvements in computer power and due to need for computations of complex geometries [15]. In this study a simple structured grid generation method is used. It depends on the use of stretching parameters. First, from a given airfoil a winglet-mounted wing is obtained. The tip of the winglet is closed and coordinates of the nodes at the leading and trailing edges are selected. The top surface of the grid and the bottom surface of the grid are formed. The middle surface contains the wing. From the middle surface to the top surface nodes are placed in the domain. Nodes are clustered near the wing surface. Similarly, from the middle surface to the bottom surface nodes are placed in the domain. Overall nodes are clustered near the wing surface. Connectivity of the nodes are given by the numbers of eight nodes which form a cell. A DXF output of the grid makes it possible to look at the grid in a CAD program. Flow enters the grid at one chord distance away from the leading edge. Flow exits from the grid at three chord distance away from the trailing edge. The normal direction extends approximately 1,4 chord distance in both directions. No previous computational analysis of a winglet-mounted wing could be found in the open literature. In this study first a grid around a simple winglet-mounted wing is generated. The airfoil of the wing is NACA 0012. A single airfoil consists of 34 nodes. The grid is coarse and is used in low Reynolds number calculations of Re=1000. A scalar parallel computation is performed using the domain decomposition technique. The table below specifies the grids used.Table 1.1. Grid Specifications In this thesis first a general introduction to the winglet concept is given. Then methods used for grid generation are discussed. Following the explanation of FEM and domain decomposition method employed for the numerical solution, results are presented in the form of graphics [18,19,20]. Finally, some conclusions are drawn and recommendations for future are given.

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