Plazma aktüatörlerin sağanak etkilerinin hafifletilmesi için potansiyel kullanımının araştırılması
Investigation of the potential use of plasma actuators for gust mitigation
Yazar: GÖKÇEN JURNAL
Danışman: PROF. DR. NURİYE LEMAN OKŞAN ÇETİNER YILDIRIM ; DR. ÖĞR. ÜYESİ CEM KOLBAKIR
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Savunma Teknolojileri Ana Bilim Dalı / Savunma Teknolojileri Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Havacılık ve Uzay Mühendisliği = Aeronautical Engineering ; Savunma ve Savunma Teknolojileri = Defense and Defense Technologies

Özet:
Büyük çaplı sağanakların oluşturduğu yükler, hava araçları üzerinde yıkıcı etkiler yaratmakta, bu nedenle yaklaşan sağanağın algılanmasıyla birlikte etkinin hızlı şekilde kontrol altına alınması gerekmektedir. Bu yükleri kontrol altına alabilmek için kullanılan akış kontrol yönteminin hızlı zaman yanıtı verebiliyor olması gerekmektedir. Son yıllarda güncel araştırma konusu olan plazma aktüatörlerine çalışma komutu verildiği an uygulandıkları yüzey etrafındaki akım yapılarını hızla düzenlemede etkili olduğu bilinmektedir. Bu nedenle, plazma aktüatörü kullanılarak büyük çaplı sağanakların kontrol edilmesi, gelecek vadeden bir konudur. Bu çalışmada; plazma aktüatörle büyük çaplı, girdap türü bir sağanağın kontrolüne yönelik deneysel bir ön çalışma gerçekleştirilmiştir. Çalışmada, büyük çaplı ayrılmayı oluşturmak için kanat hareketi yaklaşımı kullanılmış, sağanak etkisi ile hücum ve/veya firar kenarından oluşacak büyük çaplı ayrılmayı benzer şekilde oluşturabilmek için kanada 0°−45° ve 45°−0° arası yunuslama hareketi verilmiştir. Deneyler girdap türü sağanak etkilerinin incelenmekte olduğu su ortamında gerçekleştirilmiştir. Çalışmada amaç plazma aktüatörün etkisini görebilmek, havada kullanılan trafo ve dolayısıyla elektriksel değerlerle (gerilim, frekans ve akım değerleri) aktivasyonun sudaki durumunu ve akım ayrılmasına etkisini belirleyebilmektir. Bu amaçla uçları aynı eksende olan ve biri hava diğeriyse su ortamı içerisine karşılıklı konumlandırılan iki elektrot kullanılarak mevcut trafoyla su ortamında indirekt plazma deşarjı oluşumunun sağlanabilirliği incelenmiştir. Sonrasında, iki elektrot da su içerisinde bulunacak şekilde farklı aktüatör çeşitleri oluşturulmuş, deşarjın fiziki etkisi deneylerle incelenmeye çalışılmıştır. Deneylerde, Dijital Parçacık Görüntülemeli Hız Ölçüm Sistemi (DPIV) kullanılarak, durağan su ortamında yunuslama hareketi yapan NACA 0012 profiline sahip bir kanat etrafındaki hız ve girdaplılık alanları, plazma aktivasyonu olmadığı ve olduğu durumlarda incelenmiştir. 
Yunuslama hareketinin yüksek açılarda artan ve azalan değerler için yapıldığı denemede, büyük çaplı akım ayrılması görülebilmiştir. Ancak yunuslamanın çeyrek veterden ve durağan ortamda gerçekleşmesinden kaynaklı olarak büyük çaplı ayrılma firar kenarında ön planda olduğu gözlemlenmiştir.  Rampa çıkış hareketinin en başında plazma kapalı durumda emme yüzeyinde oluşan hem hücum kenarında yayılı negatif girdaplılığın, hem firar kenarında oluşan girdabın etkinliğinin rampa çıkış hareketinin en başında plazmanın etkisiyle bariz olarak azaldığı, basınç yüzeyinde ise plazma kapalı durumda gözlemlenen girdaplılığın plazmanın etkisiyle tamamen görülemez hale geldiği gözlemlenmiştir. Plazma kapalı durumda firar kenarında oluşan girdabın boyutlarının plazma aktive edildiği zaman azaldığı, ancak bu etkinin artan hücum açısıyla birlikte azaldığı ve yüksek hücum açılarında neredeyse yok olduğu gözlemlenmiştir. Rampa iniş hareketinde ise incelenen tüm hücum açılarında kanadın iz bölgesinde ve firar kenarında oluşan büyük çaplı girdap yapılarının etkinlik alanının indirgenmesi ve düzenlenmesi üzerinde plazma aktüatörün etkili olduğu gözlemlenmiştir. 
Yunuslama hareketinin servo kontrollü verildiği deneylerde, 1s veya 6s rampa iniş veya çıkış hareketleri için iki çeşit aktüatör çeşidi (PTPD, DBD) ve bu aktüatörlere 3 ayrı güç kaynağıyla enerji sağlandığı durumlardaki farklar incelenmiştir. Bu aşamanın sonucunda ise, PTPD aktüatörün DBD aktüatöre göre çoğu koşulda etkisinin fazla  olduğu gözlemlenmiştir. Ayrıca hareket hızı 1s olduğunda 6s'lik harekete göre deşarjın etkisinin daha yüksek olduğu görülmektedir.
Literatürdeki çalışmalarda, yunuslama hareketinde DBD plazma etkisinin rampa iniş hareketinde ön plana çıktığı ancak rampa çıkış hareketinde neredeyse etkisinin gözlemlenemediği belirtilmiştir. Mevcut çalışmada ise plazma aktivasyonun rampa iniş hareketinde oldukça yüksek, rampa çıkış hareketinde de kabul edilebilir düzeyde bir etkisinin olduğu gösterilmiştir. Bu durumun; havada kullanılan plazma aktivasyon yöntemlerinin (DBD) bu çalışmada etkin sonuç alınan PTPD yönteminden farklı olması, mevcut çalışmadaki aktivasyon ortamının hava yerine su olması ve kanadın hareketi dışında dış ortamda akışın olmaması dolayısıyla gerçekleştiği düşünülmektedir. Deşarjın yalnızca uygulandığı noktadan değil, kanadın diğer bölgelerinde etkili olabilmesi, çalışmanın bir diğer önemli sonucudur. İlerleyen çalışmalarda, akış içerisinde girdap türü sağanağın oluşturulacağı su kanalı testlerine geçilerek plazma aktüatörün iki boyutlu, daimi olmayan büyük çaplı ayrılmalı akış durumundaki etkisi kuvvet/tork sensörleri ve DPIV sistemi kullanılarak incelenecektir.

Abstract:
Plasma, which is used in many different areas such as propulsion, anti-icing, sound/vibration control, thrust, ignition/combustion in the aeronautics and astronautics industry, has been a subject of research that has attracted serious attention in recent years, especially in the field of flow control. Plasma actuators used for flow control offers many promising features, such as they can be easily applied to any surfaces, they are lightweight and have a small volume compared to other large control devices, they can be activated quickly when operated and do not cause excessive noise and vibration and they do not create a disruptive effect on the flow when they are not operated and they consume less energy than other active flow control methods. Plasma actuators, especially their fast time response properties, can be recommended for mitigating large-scale gusts that are vital to control. Control of gust loads with a plasma actuator has not been examined in the literature before, but there are many successful studies on the delay of static stall, as well as some studies on dynamic conditions. Based on these results, the following approach can be made: if encountering of a gust is detected quickly and with high accuracy, an effective flow control can be achieved by activating the plasma actuator system. The aircraft most exposed to large-scale gusts and their possible disruptive effects are micro aerial vehicles (MAV) or small unmanned aerial vehicles (UAV). These vehicles are widely used for various tasks today and efforts are being made to expand their operational limits. Since heavy and complex control mechanisms cannot be used on these small-scale aircrafts, it is important to investigate the use of light and low-volume plasma actuator systems under these conditions.
Three types of gusts are examined depending on whether the aircraft encounters a transient gust, namely transverse, streamwise and vortex gusts. Among the different types of gusts, the vortex gust is probably the best for representing the situations encountered in real life. However, experimentally, there are various difficulties in creating vortex type gusts that are well-defined and strong (the maximum gust velocity is in the order of the free stream velocity) and high encounter width (the gust velocity profile is in the order of chord or half chord length). However, experimentally for each gust types, in cases where the flow approaching the wing cannot be changed appropriately, the gust is represented by the wing movement. Although the encounter with the gust does not start from the leading edge, and therefore the entry into the gust is not spread throughout the entire wing, the wing movement approach instead of flow can be adopted to experimentally determine the gust loads, that is, the forces acting on the wing. The equivalent wing movement for the vortex type gust can be examined by pitching. Although the control of gust loads with plasma actuator systems has not been examined in the literature, there are a number of studies on its effectiveness on the wing performing pitching motion.
Since the vortex gust encounter is modeled in the water medium for experimental research, there was a need to examine the possibility of plasma flow control in water in this preliminary study. The use of plasma actuators for the control of the flow around the bodies which are immersed in water has not been found in the literature. Although there is no study conducted in water flow, plasma activation in water is used in many areas of industry (agriculture and food, biology and medicine, water treatment, etc.) and it is thought that this situation can be adapted to aeronautics applications. Because, there are many different types of plasma actuators (corona and DBD discharges, sliding discharge, local arc filament, spark jet etc.) which are used for flow control. Additively, it is known that by changing the states/combinations of actuator parameters, optimum control can be achieved according to the flow situation examined. Thanks to these situations, it is possible to simulate plasma actuators in water and air flow and provide flow control in water.
The purpose of this study is to see the effect of the plasma actuator and to determine the state of the activation in water and its effect on flow separation with the transformer used in the air and therefore the electrical values (voltage, frequency and current values). For this purpose, the feasibility of creating an indirect plasma discharge in the water with the existing transformer was examined by using two electrodes with the ends on the same axis and positioned opposite to each other in the air and the other in the water. Afterwards, the wing model with two electrodes positioned opposite to each other on its upper surface was used, with both electrodes in water, and the physical effect of the discharge was tried to be examined throughout the experiments. In these experiments, with the use of a 2D2C Digital Particle Imaging Velocimetry System (DPIV), the velocity and vorticity fields around a model with NACA 0012 profile, rotating between the angle of attack ranges of 0°−45° and 45°−0° in a still water environment, were determined with and without plasma activation. In this way, the effect of plasma actuators on controlling the flow structures formed around the wing and especially the control of large-scale flow seperations were examined by the wing movement approach.
The experiments were carried out in a water tank (located in the Trisonic Laboratory of ITU Faculty of Aeronautics and Astronautics) with the dimensions of 50 cm x 50 cm x 50 cm (depth x width x height) and filled to 40 cm deep, with still water. The models used were produced from PMMA (Polymethyl methacrylate). The first used model was produced with a span of 30 cm and the second model was produced with a span of 20 cm. The chord length of the models were 10 cm. Geometrically, the wing span ratios are AR=3 (for 30 cm span) and, AR=2 (for 20 cm span) but since the free surface is considered as the symmetry plane, the effective span ratios are respectively considered as AR=6 and AR=4. For plasma generation, two types of actuators namely PTPD (point-to-point discharge) and DBD (dielectric barrier discharge) were used. Regardless of where the actuator position is on the chord, a 10 cm section in the middle of the wingspan is not insulated and a discharge zone is created. Three types of power source were used to apply voltage to the actuators, these are: 15kV-3kHz and 15 kV-DC high voltage and 15V-DC low voltage power supply. The experiments were repeated for closed and opened plasma conditions. 
For DPIV measurements, the water was seeded with polyamide particles with an average diameter of 50μm. The mid-section plane of the wing model was illuminated using an Nd-YAG laser. Flow field images were taken with a camera perpendicular to the illuminated plane with a resolution of 1600 × 1200 pixels. Experimental data were recorded as a single image. Adaptive Correlation process was applied to sequential image pairs such as 1-2, 2-3, 3-4 using the Dantec Dynamics Adaptive PIV algorithm. 
When the experimental results are examined, during α = 0°−45° pitch-up motion and α = 45°−0° pitch-down motion: It has been observed that large-scale flow separation is effective at the trailing edge because the first model performs pitching motion in the still water environment from the position close to the leading edge. Large-scale flow separation could be observed as pitching motion was accomplished for increasing and decreasing values at high angles. The effect of the actuator was observed more clearly during the pitch-down motion, that is, at the very beginning of the movement, when the intensity of the large-scale flow separation was highest. In the experiments where the second model was used, it was observed that the PTPD actuator was more effective than the DBD actuator in most conditions. The effect of the DC low voltage power supply could not be observed under most of the conditions, and it was also observed that the high voltage AC power supply was more effective than the high voltage DC power supply. In addition, a higher effect is observed in the pitching motion, which takes 1s, compared to the movement, which takes 6s. 
Plasma activation was more effective on the velocity and vorticity fields in the pitch-down compared to that in the pitch-up. In the literature, for the results obtained for flow control of pitching motion with DBD plasma actuator in the air environment, it is stated that the effect of plasma activation is generally observed to be low or not at all on the pitch-up, while the effect is high on the pitch-down. These results in the literature are similar to those in the current study; because the effect of plasma activation on flow separation is higher in the pitch-down state and lower in the pitch-up state. However, in the current study, the effect of plasma activation during pitch-up could be observed more clearly than other studies in the literature. It is thought that this is due to the different actuator type, the ionization tendency of the neutral particles around the wing when high voltage is applied is higher than that applied in air because the medium is water, and the movement occurs in quiescent medium.. In order to create a high effect in the air environment similar to that in the water environment, the activation strategies used in the air environment (actuator type or electrical parameters of the power supply) can be changed. As a result, the actuators can also provide energy to points far from where the actuators are placed on the wing and effectively prevent separation.
In this study, it was examined how and to what extent plasma actuators can control the effects of large-scale gusts on the NACA 0012 airfoil. Its effect in the case of separated flow will be examined using force/torque sensors and DPIV system, and servo motors will be used to control the movement of the wing and vortex generator. The aim of future studies is to provide plasma activation in the water flow where large-scale vortex-type discrete gust is created in a controlled manner, and to reduce the gust loads and large-scale flow separations. The goal of using plasma actuation is to reduce the maximum deviation in the lift force by 50% (NATO AVT-347: "Large-amplitude gust mitigation strategies for rigid wings" research group target value).