Kanat planform şeklinin rampa hareketi yapan kanadın akım yapılarına ve kuvvetlerine etkisi
Effect of planform shape and pivot axis position on the flow structure and forces of an impulsively pitching flat plate
Yazar: CİHAD KÖSE
Danışman: PROF. DR. NURİYE LEMAN OKŞAN ÇETİNER YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı / Uçak ve Uzay Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Havacılık Mühendisliği = Aeronautical Engineering ; Uçak Mühendisliği = Aircraft Engineering
Dizin: Aerodinamik = Aerodynamics ; Akışkanlar mekaniği = Fluid mechanics

Özet:
Bu tez çalışmasında rampa yunuslama hareketi yapan çeşitli planform geometrilerine sahip düz plakanın iz bölgesine yakın akış alanları incelenmiştir. Kanat planform  şekli için esas olarak altı geometri ele alınmıştır. Eğrisel olmayan geometriler ise yine aynı konudaki bir çalışmadan esinlenilmiştir. Ancak bu çalışmada kuvvet ölçümleri ile birlikte üç düzlemde PIV verileri alınarak HKG'nın üç boyutluluk etkisi ile uç vorteks ile etkileşimi incelenmektir. Deneyler Re=10,000 için, keskin kenarlı levhalar kullanılarak ve 0°─45° genliği için gerçekleştirilmiştir. Modeller laboratuvarda bulunan lazer kesici kullanılarak pleksiglastan imal edilmiştir. Deneyler İstanbul Teknik Üniversitesi Uçak ve Uzay Bilimleri Fakültesinde bulunan Trisonik Laboratuvarındaki 1010 mm x 790 mm test kesitine sahip büyük ölçek su kanalında gerçekleştirilmiştir. 
Hava araçlarında ilgi sabit kanatlılardan ziyade çırpan kanatlılara yön değiştirmiştir. Düşük hızlarda uçabilen MHA (mikro hava araçları) için çeşitli alanlarda kullanımı nedeniyle verimliliklerinin artması için çırpan kanatlar kullanılabilir. Kanat çırpma hareketi yunuslama ve akıma dik yönde ötelenme hareketleri şeklinde kısımlara ayırarak incelenmektedir. AVT-202 araştırma grubunun raporunda (NATO-STO, 2016) rampa formunda yunuslama hareketi yapan kanatların incelenmesinin önemi vurgulanmıştır.
Bu çalışmanın ilk bölümünde tezin amacına yönelik giriş yapılmıştır. Daha sonra ise daha önce yapılmış literatürde geçen benzer çalışmalar incelenmektir. İkinci bölümde deney düzeneği ve kullanılan yöntemlerden bahsedilmektedir. En sonunda sonuçlar irdelenmekte ve sonuçlar ışığında özet bulgulara, ileriye yönelik tavsiyelere ve yorumlara yer verilmektedir.
Hücum kenarı dönme eksenli durumlarda, üçgen planformlu düz plakalar diğerlerinden farklıdır. Yamuk planformlarda olduğu gibi benzer geometrik planform şekilleri, benzer kuvvet farklılıkları ve akış topolojileri sergiler. Veter ortası dönme eksenli durumlar içinse hücum kenarı girdabının (HKG) gelişme safhasında, akış topolojisi incelenen dikdörtgensel, eliptik ve ikizkenar yamuk durumları için hızlı ve yavaş harekette planform şeklinden veya kanadın veter uzunluğundan bağımsızdır. HKG'nın kararlılığının korunması itki veriminde esas nokta olarak görülmektedir. Hücum kenarı dönme ekseninde çeyrek geometrik açıklıkta, uç girdabıyla etkileşimi HKG oluşumunu olumsuz etkilediği görüldüğü durum, üçgen kanatlar hariç, kanat ucunda fazlasıyla görülmüştür.
Veter ortası dönme ekseninde taşıma ve sürüklemedeki eksikliğin veter uzunluğunun azalmasından kaynaklanmasına rağmen eliptik planformlu kanat için girdaplı yapılar ve akış topolojisi çoğunlukla dikdörtgen planformlu kanada yakındır. Sonuç olarak, HKG kararlılığı için üç boyutluluk etkisi yani planform geometrisi önemli rol oynamaktadır.

Abstract:
2D2C Digital Particle Image Velocimetry (DPIV) method in conjunction with simultaneous force measurements are used to investigate the flow in the near wake of a flat plate with different planforms and pivot axes undergoing a pitch up ramp motion. The experiments are performed in the large-scale free-surface water channel of Trisonic Laboratories in Istanbul Technical University. Triangular, trapezoidal, rectangular, elliptic planform geometries are studied. The wings have sharp leading and trailing edges, pitch up to 45⁰ in 1s or 6s and the pivot axis is either the leading edge or the mid chord.
As known from the literature (Bernal et al., 2013), boundary layer separation at the leading edge yields the development of a leading edge vortex (LEV) which rests attached on the suction side of the airfoil and creates high lift. Later, the LEV detaches and the lift coefficient decreases reaching the steady state value after sometimes. Obviously, the growth of the LEV will interact with the growth of the wing tip vortex (TV) for limited aspect ratio wings and this interaction will be more evident for small aspect ratio wings. For pitch-up motion, aspect ratio is found to have an important consequence on lift and drag coefficients especially after the motion ends (Son and Cetiner, 2015). The growth and interaction conditions of those vortices will also be affected by the wing planform geometry. Bernal et al. (2013) studied five different wing planforms all having spanwise straight leading and trailing edges. They stated that lift and drag coefficients throughout the continuous pitch up part of the motion demonstrate strong interaction with the wing planform geometry and the pivot axis position. Moreover, they obtained that, for a given pivot axis, all flat plate planforms have analogous trend in force growth during the pitch-up stage. Grandlund et al. (2011) studied 2D and low-aspect ratio 3D plates in pitch-up motion and investigated both Zimmermann and rectangular plates. While the two AR=2 plates show significant dissimilarities in the leading edge vortex spanwise flow and tip vortex‒trailing vortex interaction, variances in their lift and drag accounts are minor. Even in steady flow situations, wing planform is very significant for MAV projects.
Experiments are performed in the close-circuit, free-surface, large scale water channel located in the Trisonic Laboratories at the Faculty of Aeronautics and Astronautics of Istanbul Technical University. The cross-sectional dimensions of the main test section are 1010mm × 790mm. The models are fixed in a upright cantilevered arrangement in the water channel about their leading edge as being the pivot point. The link rod attaches the models to the servomotor from their leading edge to run a pitch up motion by a coupling. The root chord length of the prototypes is 10 cm and prototypes have a span of 20 cm. The models are thin triangular, trapezoidal, rectangular and elliptic planform plates (t=0.5cm) with sharp edges. They are made of transparent plexiglas to let the laser illuminate both the suction and pressure sides. The experiments are carried out at a Reynolds number of 10,000 which matches to a flow velocity of U=0.1m/s.
Two 10-bit cameras with 1600×1200 pixels resolution are located under the water channel and are used to record flow structures around the models and hence to examine the vortical structures. The flow is illuminated by a dual cavity Nd:YAG laser (max. 120mJ/pulse) and the water is seeded with silver-coated hollow glass spheres with an average diameter of 10µm to obtain the flow fields around the wing and in its near wake. Two pictures from the two cameras are attached before cross-correlation using two marker points. Then, stitched PIV images are interrogated by a double frame, cross-correlation method with an interrogation window size of 64×64 pixels and 50% overlap in both directions. PIV images are acquired at three illumination planes, which are geometrically mid span, ¼ span and tip planes of the wings. 
A six-component ATI NANO-17 IP68 Force/Torque (F/T) sensor (ATI Industrial Automation, Inc.) is used to measure forces and moments acting on the model. The sensor is positioned on the rod between the model and the pitch servomotor with its tubular z-axis perpendicular to the pitch plane. The pitch-up motions of the airfoil are controlled with a Kollmorgen/Danaher Motion AKM33E servo motor. The models perform two kinds of motion: fast and slow. The model starts from 0° and reaches to its final angle of attack of 45° in 1 second for the fast motion and 6 seconds for the slow. Motor motion profiles are generated by a Labview VI (Virtual Instrument) for the set amplitude and period. The same VI triggers both the force acquisition and the PIV system. The beginning of the pitch motion starts 3 seconds after the start of force measurement; the synchronization with the PIV system is achieved via a National Instruments PCI-6601 timer device.
In fast motion, only triangular forms vary from the others. Though the typical differences are too alike, the flat plate with triangular planform and angled from the leading edge exhibits higher lift and drag. However, the flat plate with triangular planform and angled from the trailing edge performs worse, lift and drag are different compared to the previously mentioned flat plates with altered planform geometries. In general, the typical bump in the force curves observed for the rectangular wing is also present for a number of cases, for instance the flat plates having triangular leading edge and trapezoidal planforms exhibit the typical bump. The slow motion exposes well the dissimilarities and common features between the planform geometries. The lift and drag variations of flat plates with trapezoidal planforms are very similar particularly during the motion. The rectangular flat plate experiences at all times high lift and drag forces for the slow motion and the flat plate with triangular planform and angled from the trailing edge, generally, experiences lowest lift and drag forces for slow motion, the same observation is also valid for the fast motion.
The differences in force curves are clearly connected to the three-dimensionality of the flow and are thought to be clarified with the dissimilarities in vortex developments. It should be noted that the cross-section where the DPIV images are taken is at the half of the geometric span. 
The flow structures are in agreement with the force measurement results. For the duration of the growth phase of the leading edge vortex (LEV), the flow topology does not depend on the planform or the local chord length of the wing for all investigated cases, except for the flat plates with a triangular planform. Focusing on the first image representing the end of the motion, the flat plate with triangular planform and angled from the leading edge shows approximately a completely developed LEV. Quite the opposite, the flat plate with triangular planform and angled from the trailing edge exhibits weaker leading and trailing edge vortices and displays seperating shear layers long after the motion ends. A close look at the lift coefficient variation reveals the shift of the bump in time, in agreement with the flow topology.
Previous observations made for the same wings in fast motion are similar to those for slow motion. Based on the rectangular flat plate, the valley before the bump is again characterized by the dividing streamline being approximately at the half of the chord and the peak of the bump by the streamlines closing over the wing. 
In agreement with the force-time histories and with respect to previously planforms, triangular flat plates have weaker leading and trailing edge vortices and exhibit separating shear layers just after the motion ends. Amongst those two cases, the wing with triangular planform and angled from the trailing edge has low lift and drag forces in parallel with weak vortical structures. The presence of the bump is also revealed in the flow topology since the flat plate with elliptic planform displays streamlines closing over the wing. The flow topologies at the end of the motion and before the bump appears have similar vortical structures for the flat plates with trapezoidal planforms, consistent with their force-time histories.
In order to reveal the differences related to the three-dimensionality of the flow, the DPIV results are obtained for three different illumination planes. LEV is stronger for the two cross-sections not including the tip plane for the case of the flat plate with triangular planform and angled from the leading edge. Actually the chord length is zero for the flat plates with triangular planform and this is obvious in the pictures with absence of vortices on the tip plane. Eventhough the triangular flat plates have reduced planform area compared to the rectangular flat plate, the LEV extends to nearly entire chord length in both illumination planes. However, this is not valid at the tip plane for the flat plate with triangular planform and angled from the leading edge. The flow topology and vortical structures of the flat plate with triangular planform and angled from the trailing edge are typically similar to those of the rectangular flat plate, nonetheless the lack in the lift and drag originates from the decrease of the chord length in the direction from root to tip plane. 
Actually, the pitch-up lasts longer in slow motion, three illumination planes reveal the differences in relation with the three-dimensionality of the flow. The development of the LEV and its interaction with the tip vortex are considered for the rectangular plate. Even if the LEV is attached to the upper surface as seen on the two cross-sections apart from the tip plane, it interacts with the tip vortex, moves inboard and seperates at the future instant, as seen on the mid aspect ratio plane. The differences in the flow topology for the flat plate with triangular planform and angled from the leading edge are not obvious for in the slow motion, in agreement with the force-time history.
Both flow visualization and force measurement results, in agreement with each other, reveal the differences linked to the three-dimensionality of the flow. Wings with triangular planform differ from the others; while the angled from leading edge shows an improved performance in the fast motion, the angled from trailing edge shows decayed performance. Wings with triangular planform exhibit low performance in comparison with the others in the slow motion. Related planform shapes, for example trapezoidals, show similar force variations and flow topologies.