Arka gövde eklentileri ile C-130E uçağı basınç sürüklemesinin azaltılmasının sayısal olarak incelenmesi
Computational evaluation of C-130 aircraft base drag reduction with aftbody modifications
Yazar: HAKAN TELLİ
Danışman: DOÇ. DR. YASİN VOLKAN PEHLİVANOĞLU
Hava Harp Okulu Komutanlığı / Havacılık ve Uzay Teknolojileri Enstitüsü / Havacılık Mühendisliği Ana Bilim Dalı / Havacılık ve Uzay Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Havacılık Mühendisliği = Aeronautical Engineering ; Savunma ve Savunma Teknolojileri = Defense and Defense Technologies ; Uçak Mühendisliği = Aircraft Engineering

Özet:
Bu çalışmada, Lockheed Martin C-130E Hercules askeri kargo uçağında yapılacak arka gövde modifikasyonlarının uçağın toplam sürüklemesine olan etkisi, hesaplamalı akışkanlar dinamiği analizleri ile sayısal olarak incelenmiştir.
Yüksek bir rampa açısı ile daralan arka gövde geometrisine sahip C-130 uçaklarında, arka kargo kapısı bölgesindeki yüksek ters basınç gradyanları nedeniyle hava akışı düzensizleşerek zıt yönlü dönen ve uçağın gerisine doğru iletilen detaylı bir girdap çifti yapısına sahip olmakta; bu da arka gövde basınç sürüklemesini önemli oranda arttırmaktadır. Çalışma kapsamında, girdapların neden olduğu sürüklemedeki artış arka gövde eklentileri ile azaltılmak istenmiş; farklı ebat ve sayılarda çeşitli kanatçık konfigürasyonları oluşturularak bunların sürüklemeyi azaltma performansları karşılaştırılmıştır. Bu kanatçıklar, oluşturdukları küçük girdaplarla sınır tabakanın enerjisini arttırarak arka gövde bölgesindeki girdaplı akışı kontrol etmek, akım ayrılmalarını önlemek ve sürüklemeyi azaltmak amacıyla arka gövdenin her iki yanına akış çizgileri yönünde yerleştirilmiş "finlet" benzeri küçük yüzeylerdir. Bu kapsamda kanatçık ve mikro-kanatçık adı verilen 2 farklı tasarım oluşturulmuş ve farklı tasarım parametreleri kullanılarak en etkin tasarıma ulaşılmaya çalışılmıştır.  
Analizlerde Spalart-Allmaras'lı RANS türbülans modeli kullanılmıştır. Model ve metot, daha önce başka bir çalışmada yapılmış olan rüzgâr tüneli test verisi ile doğrulanmıştır. Çözüm ağı oluşturma aşamasında, ilk olarak düzensiz üçgen yüzey ağları atılmış, daha sonra tetrahedral akış hacim ağı ile bunların arasındaki prizmatik sınır tabaka katmanı oluşturulmuştur. Tüm analizler, yüksek performanslı Linux bilgisayar kümesinde daimi, 3-boyutlu ve sıkıştırılamaz RANS denklemleri çözen bir akış çözücüsüyle yapılmıştır. 
Eklentilerin boyut, şekil ve pozisyonu en doğru şekilde ayarlandığında, hem kanatçık hem de mikro kanatçık konfigürasyonları için C-130E uçağının sürükleme katsayısında önemli oranda azalma elde edilmiştir. Sürükleme katsayısındaki 0.0001 birimlik azalmanın 1 sürükleme birimine eşdeğer olduğu kabul edilirse, en uygun kanatçık konfigürasyonu ile sürüklemede 15.7 birimlik bir azalma görülürken, mikro-kanatçık yaklaşımı ile sürüklemedeki azalma 18.4 birime kadar çıkartılmıştır. Her iki yaklaşımın birlikte kullanıldığı melez konfigürasyonlarda ise sürüklemedeki azalma en fazla 17.71 br olmuştur. Sonuç olarak, yukarı doğru yüksek bir açı ile daralan arka gövde geometrisine sahip C-130 uçağında, arka gövde yanlarına eklenen eklentiler ile tüm uçağın sürüklemesinde % 4'lük bir azalma, yakıt tüketiminde ise 6 saatlik bir görev için yaklaşık 700 lb'lik bir tasarruf sağlanmıştır.

Abstract:
The Lockheed Martin C-130E is one of the most popular and widely used military transport aircrafts, which has also been used in the Turkish Air Forces since 1960s. There have been several studies so far to evaluate and improve the flight performance of the C-130. Aircraft drag-reduction research is one of the main aspects of these investigations. This thesis represents computational fluid dynamics analyses performed on the flow field around C-130E to investigate the effects of aft body modifications on the total drag. 
The overall study focuses on the possible increment in fuselage form drag due to generated vortices on the underside of the highly upswept aft region. Because of the rotation phase of take-off and the requirement of a main cargo door for prompt loading and airdrop operations, military transport aircrafts, like C-130 Hercules, differ from other aircrafts by highly up-swept aft fuselage portion. It has a high aft-body up-sweep angle, about 28º. During the flight, a complex flow field is initiated by the high adverse pressure gradient around this aft section, resulting in three-dimensional flow separation on the lower contour. Subsequently, the flow field behind the main cargo door has a detailed vortex structure, which is characterized by a pair of large counter-rotating vortices that convey far downstream. The vortices become stronger as they progress downstream and interact with the empennage. Accordingly, fuselage form drag due to vortices and possible flow separation become significant contributors of the overall aircraft drag. Therefore, need to increase the performance of C-130 by decreasing form drag; and so decreasing the fuel consumption is motivated this study.
Surface shapes are important to determine the paths of the created vortices. Vortices generally follow the surface of the aircraft but their paths are also depend on its size and being single or not. Vortices, which have greater size, have higher velocity components and may cause undesirable increase in drag by hitting other parts of the aircraft. Moreover, interactions between them may cause variation in size and should be taken into consideration. Therefore, creating desired vortices can be possible with correct size of vortex generators that have a carefully designed shape and good location.
The study consists of CFD analyses of the flow field around C-130E aircraft to evaluate the drag reduction performances of two types of add-ons, which are finlets and microvanes attached on the aft section of the fuselage. Finlets are 4 pairs of fins added to the sides of the lower surface of the upswept aft fuselage section. Similarly, microvanes are small and bumped-shaped devices installed on each side of the fuselage's aft section to reduce the drag. The operational mechanism of these attachments is similar to the vortex generators. They energize low energy boundary layer flow by creating mutually interacted line vortices near the edge of boundary layers; then prevent or delay separation in regions of high adverse pressure gradients on the afterbody, with consequent enhancement of the flow characteristics and reduction of the afterbody drag. By aft body modifications, it is hoped to reduce the drag by controlling the vortex flow around the aft fuselage region and decreasing the strength of the vortices. Various finlet and microvane configurations are created in different sizes and numbers. During the computational analyses, only a certain design parameter for each configuration is changed to understand its contribution to the drag reduction performance. By evaluating the flow field at the end of each analysis, design parameters of the next configuration are decided. Finally, optimum finlet and microvane configurations are found with required modifications and the results are compared. 
The CFD analyses are performed by using Reynolds-averaged Navier-Stokes methods with Spalart-Allmaras turbulence model. Using Detached Eddy Simulation or Large Eddy Simulation alone would be better to analyze the whole domain more correctly, especially for predicting separated flows; since more details of physical phenomena would be considered in turbulent flow. However, due to increasing computational cost and high number of required analyzes, RANS modeling is decided to be used and accepted as having enough accuracy. RANS models are the most economical way of computing complex turbulent flows. The Spalart-Allmaras model is a simple one-equation model that solves a modeled transport equation for the turbulent viscosity. The model is generated especially for wall-bounded flows and gives relatively good results for boundary layers subjected to adverse pressure gradients. Method and the results are validated with wind tunnel test results in a study carried out earlier. 
The simplified CFD model was generated using the geometry that had been scanned from the actual C-130E by TAI. In the meshing process, unstructured triangular surface grid is generated and then tetrahedral volume grid is constructed with the prism layers in between. In order to catch the vortices behavior correctly, fine grid is applied on the lower surface of the upswept aft body. To be able to understand the effect of the add-ons, only the mesh on the attachments is modified at each simulation by keeping the grids on other surfaces unchanged. For the volume mesh, grid convergence study is applied to the at the wake region in order to avoid mesh dependency. Also, the fine grid region is extended through the wake to capture the vortex structure behind the aircraft and prevent the early dissipation. The grid contains 25 prism layers with a geometric growth rate of 1.4 and an average y+ value of 0.1 which is required for good resolution of the velocity gradient in the boundary layer for Spalart-Allmaras turbulence model. The flow domain is established as a sphere having a radius of twenty times of the body length. Velocity inlet boundary condition is selected for the outer boundary. The final grid size is increased up to 15 Million cells. 
The simulations are made with a flow solver on a high-performance Linux cluster at the Turkish Air Force Academy Networks Laboratory. 5 nodes of that cluster are dedicated for the computations. Each node has a quad core Intel Core i7-2600K processor and 8 GB of shared memory. A commercial solver, which solves steady, three-dimensional, incompressible RANS equations, is used at all analyses. Full-scale model having 80 m/s cruise speed and 0º flight angle of attack at ISA sea level conditions is used in the analyses. Reynolds number is 1.16 × 108, based on the fuselage length.
Considering 1 drag count as 0.0001 of aircraft drag coefficient, 15.7 drag count of drag reduction is achieved with the optimum finlet configuration.  By the microvane approach, drag reduction is increased up to 18.4. Therefore, more than 4 % of the total C-130E aircraft drag is decreased with the aft body modifications.