EXPANDING CORNERS FOR SUBSONIC WIND TUNNELS GRADUATION PROJECT. Hasan Emre UŞDU. Department of Astronautical Engineering

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1 ISTANBUL TECHNICAL UNIVERSITY FACULTY OF AERONAUTICS AND ASTRONAUTICS EXPANDING CORNERS FOR SUBSONIC WIND TUNNELS GRADUATION PROJECT Hasan Emre UŞDU Department of Astronautical Engineering Thesis Advisor: Prof. Dr. Fırat Oğuz EDİS JUNE 2018

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3 ISTANBUL TECHNICAL UNIVERSITY FACULTY OF AERONAUTICS AND ASTRONAUTICS EXPANDING CORNERS FOR SUBSONIC WIND TUNNELS GRADUATION PROJECT Hasan Emre UŞDU Department of Astronautıcal Engineering Thesis Advisor: Prof. Dr. Fırat Oğuz EDİS JUNE 2018

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5 Hasan Emre Uşdu, student of ITU Faculty of Aeronautics and Astronautics student ID , successfully defended the graduation entitled EXPANDING CORNERS FOR SUBSONIC WIND TUNNELS, which he prepared after fulfilling the requirements specified in the associated legislations, before the jury whose signatures are below. Thesis Advisor : Prof. Dr.Fırat Oğuz EDİS... İstanbul Technical University Jury Members : Prof. Dr.N.L. Okşan ÇETİNER YILDIRIM... İstanbul Technical University Yrd.Doç.Dr. Kemal Bülent YÜCEİL... İstanbul Technical University Date of Submission: 28 May 2018 Date of Defense: 13 June 2018 v

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7 To my family, vii

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9 FOREWORD I sincerely would like to thank my supervisor Prof. Dr. Fırat Oğuz Edis for trusting me with this thesis project. It did not only allow me to gain my experience in the field of Computational Fluid Dynamics, but it was also a great opportunity to gain knowledge about wind tunnels. I also would like to thank my darling, Selin Balı for his invaluable support during this thesis project. Finally, I would like to thank my mother, Şafiye Uşdu and my father, Hüseyin Uşdu for their moral support even though they are thousands of miles away. June 2018 Hasan Emre UŞDU ix

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11 TABLE OF CONTENTS Page FOREWORD... ix TABLE OF CONTENTS... xi ABBREVIATIONS... xiii LIST OF TABLES... xv LIST OF FIGURES... xvii SUMMARY... xix 1. INTRODUCTION Purpose of Thesis Types of Wind Tunnels Subsonic Wind Tunnel High Speed Wind Tunnel THE WIND TUNNEL DESIGN Selection of the Wind Tunnel MTL Wind Tunnel Parts of the MTL Wind Tunnel Description of the parts Pressure Loss FLOW ANALYSIS STUDIES Boundary Conditions CFD Subsonic Wind Tunnel with Five Airfoils CFD Results for Subsonic Wind Tunnel with Five Airfoils Subsonic Wind Tunnel with Ten Airfoils CFD Results for Subsonic Wind Tunnel with Ten Airfoils Subsonic Wind Tunnel with Fifteen Airfoils CFD Results for Subsonic Wind Tunnel with Fifteen Airfoils CONCLUSIONS AND RECOMMENDATIONS REFERENCES xi

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13 ABBREVIATIONS M : Mach number MTL : Minimum turbulence level m/s : Meter per second m : Meter m 2 : Square meter kw : Kilowatt : Degree e : Expansion ratio Pa : Pascal Mpa : Megapascal CFD : Computational fluid dynamics xiii

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15 LIST OF TABLES Page Table 2.1: MTL wind tunnel specifications... 6 Table 2.2: Numbering of the sections... 7 xv

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17 LIST OF FIGURES Page Figure 1.1: Open circuit wind tunnel[1] Figure 1.2: Closed circuit wind tunnel[1]... 2 Figure 1.3: The Blowdown Tunnel[1]... 3 Figure 2.1: The MTL wind-tunnel layout[2]... 5 Figure 2.2: The Parts of the MTL wind tunnel[2]... 6 Figure 2.3: The configuration of the wire diamater, d, and mesh width, M[3]... 9 Figure 3.1: The shape of the airfoil Figure 3.2: General layout of wind tunnel with five airfoils Figure 3.3: The mesh view of wind tunnel with five airfoils Figure 3.4: Pressure distribution of wind tunnel with five airfoils Figure 3.5: General layout of wind tunnel with ten airfoils Figure 3.6: The mesh view of wind tunnel with ten airfoils Figure 3.7: Pressure distribution of wind tunnel with ten airfoils Figure 3.8: General layout of wind tunnel with fifteen airfoils Figure 3.9: The mesh view of wind tunnel with fifteen airfoils Figure 3.10: Pressure distribution of wind tunnel with fifteen airfoils xvii

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19 EXPANDING CORNERS FOR SUBSONIC WIND TUNNELS SUMMARY The area of use of wind tunnels is very common today. In this thesis, the types of wind tunnels are explained and the reason why the subsonic wind tunnel is used is mentioned. The wind tunnels are divided into two, high speed and subsonic. High-speed wind tunnels are classified as transonic, supersonic, and hypersonic, according to Mach number. Subsonic wind tunnels are divided into open and closed. In this study, closed wind tunnel was used and the pressure changes were analyzed at the expanding corners. In accordance with this study, the MTL wind tunnel with a closed wind tunnel was used. MTL is a tunnel type suitable for subsonic analysis with wind tunnel. The tunnel used has 7 different section. The section used is the corners. The expanding ratio at the corners was taken as Five, ten and fifteen airfoils were placed in the corners with expansion in this direction, respectively. As a result, it is aimed to analyze the pressure differences at the inlet and outlet. Analyzes were performed in two dimensions. The type of airfoil used is NACA The entrance area is 50 cm. According to the expanding ratio, the exit area is taken as 66 cm. The fluid used in the analyzes is air and the density is kg/m 3. The rate of arrival of the fluid to the entrance of the tunnel was determined to be 10 m/s. After designing, pressure distribution of the wind tunnel was obtained by CFD analysis. The distance between airfoils in a wind tunnel using five airfoils is ten centimeters. This distance is seven centimeters for ten airfoils and four centimeters for fifteen airfoils. The pressure change on five and ten airfoil corners yielded the same result. The maximum pressure of the five-airfoil corner is higher than ten-airfoil corner. In the fifteen-airfoil corner, the result was extreme. As a result of, the use of expanding corners to create a pressure differential. xix

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21 SESALTI RÜZGAR TÜNELLRİNDE GENİŞLEYEN DÖNÜŞ KÖŞESİ KULLANIMI ÖZET Rüzgâr tünellerinin kullanımı alanı günümüzde oldukça yaygındır. Özellikle rüzgâr tünellerinin uçak kanadı analizlerindeki yardımı çokça faydalıdır. Rüzgâr tünelleri ses altı ve yüksek hız olmak üzere iki gruba ayrılabilir. Yüksek hızlı rüzgâr tünelleri mach sayısının değerlerine göre transonic, supersonic ve hypersonic olarak sınıflandırılır. Bu tezde kullanılan sesaltı rüzgâr tünelleri ise kapalı ve açık devre olmak üzere ikiye ayrılır. Bu tezde kapalı rüzgâr tüneli referans alınmıştır. Eğer bir rüzgar tünelindeki hava akış hızı 180 m/s nin altındaysa sesaltı, üstündeyse yüksek hızlı rüzgar tüneli olarak adlandırılır. Sesaltı rüzgâr tünellerinin diğer bir ismi ise düşük hız rüzgar tünelidir. Transonic rüzgâr tünelleri uçak endüstrisinde kullanımı yaygındır. Supersonic rüzgâr tüneli ise askeri uçak ve jet motorlarında kullanılır. Son olarak hypersonic rüzgâr tünelinin kullanım alanı da roketler ve uzay araçlarıdır. En avantajlı yüksek hız rüzgâr tüneli blowdown rüzgâr tünelidir. En büyük avantajı basit ve daha ekonomik olmasıdır. Bu tezde kullanılan sesaltı rüzgâr tüneline gelicek olursak açık devrede hava tünele girer ve çıkar. Bu tipte köşe yoktur. Ancak kapalı devrede hava sürekli akış halindedir. Bu akış fan ve motorlar yardımıyla sürekli olarak devam ettirilir. Deneylerde rüzgâr tüneli kullanmak çok avantajlıdır çünkü rüzgâr tünelleri sayesinde deneyler kontrollü yapılabilmektedir. Rüzgâr tünellerinde farklı hızlarda hava akışları yaratmak mümkün olmaktadır. Bu sayede farklı hızlarda farklı veriler alınarak deney sonuçları iyileştirilebilir. Sonuçta deneyin doğruluğu oldukça önemlidir. Bu tezde analiz yapmak için kapalı devre çeşidinden sesaltı rüzgâr tüneli olan MTL rüzgâr tüneli referans alınmıştır. MTL rüzgâr tüneli 80 lerin ortasında tasarlanmıştır. Bu rüzgâr tünelinde hava akışını devam ettirebilmek için gereken enerji oldukça xxi

22 azdır. Bu rüzgar tünelinin yedi farklı bölümü vardır. Bunlar test odası, difüzörler, köşeler, sürüş ünitesi, ısı değiştirici, durgunluk odası ve kontraksiyondur. Test bölümü, yüksek serbest akış hızı olmayan yüksek Reynolds sayılarıyla çalışmak için uygun boyutlara sahiptir. Üst ve alt duvarları ayarlanabilir olduğundan, akış yönünde basınç gradiyenti negatif ile pozitif değerler arasında değiştirilebilir. Difüzörler, sınır tabaka ayrımı olmaması için konservatif olarak tasarlanmıştır. Difüzörler sayesinde, rüzgar tüneli daha uzun bir dönüş devresine sahiptir. Akış 90 derece döner, çünkü köşelerin kademeli airfoilleri vardır. Uygun şekilde tasarlanmazsa, büyük türbülans seviyesine ve yüksek toplam basınç kaybına yol açan büyük sınır tabaka ayrışması meydana gelebilir. Sürüş ünitesinde fan ve motor bulunur. Motor 85 kilowatt gücünde olup tristör kontrol ünitesi fanın hızını kontrol eder. Durgunluk odası veya çökeltme bölmesi, büyük bir kesit alanına sahiptir, böylece rüzgar tüneli devresindeki akış düşük hızdadır. Durgunluk odası üç farklı parçaya sahiptir: petek, elekler ve gevşeme kanalı. Daralma düşük hız değişmesine yol açar. Daralmanın şekli mükemmel ise, duvar sınır tabakası ayrımı olmaz. Bu tezin analizlerinde köşeler kullanılmıştır. Analiz 2 boyutlu olarak yapılmıştır ve analiz için Ansys Fluent 16.0 kullanılmıştır. Köşelerde akış 90 dereceyle dönmektedir. Köşenin genişleme oranı olarak alınmıştır. Giriş alanının genişliği 2 boyutlu olarak 50 santimetredir. Genişleme oranına göre de çıkış alanın genişliği 66 santimetre olarak hesaplanmıştır. Analiz için köşelere airfoliller yerleştirilmiştir. Değişken olarak aifroil sayısı referans olarak alınmıştır. Beş, on ve on beş airfoil sayısında analizler teker teker yapılmıştır. Analizlerde amaç basınç farklarını bulmak olarak belirlenmiştir. Kullanılan airfoiller NACA 7408 olarak seçilmiştir. Ses altı rüzgar tünelinden geçen akışkan hava olup özkütlesi kg/m 3 tür. Havanın köşeye giriş hızı 10 m/s dir. Köşeye beş airfoil yerleiştirildiğinde airfoiller arası uzaklık 10 santimetre seçilmiştir. Bu uzaklık on airfoilli köşede 7 santimetreyken, on beş airfoilli köşede 4 santimetre olarak alınmıştır. xxii

23 Analiz yapılırken viscous modelde k-epsilon denklemleri seçilmiş olup iterasyon sayısı 500 olarak belirlenmiştir. Ansys Fluentte sınır koşulları ve model belirlendikten sonra CFD analizine geçilmiştir. CFD, hareketli veya sabit bir nesne etrafındaki hava akışı nedeniyle oluşan reaksiyonları tanımlamak için matematiksel ve hesaplamalı modelleri kullanır. Analizin sonucunda beş airfolli köşede maksimum basınç 146,79 paskal, minimum basınç ise -42,56 paskal çıkmıştır. On airfolli köşede ise maksimum basınç 117,89 paskal iken minmum basınç -66,05 paskaldır. Analizin son kısmında on beş airfoilli köşeye gelindiğinde sonuçlar çok fazla aşırı çıkmıştır. Sonuç olarak ses altı rüzgar tünellerinde genişleyen köşelerle bir basınç farkı yaratmak mümkündür. Eğer rüzgar tünelindeki hava akışının hızı değiştirilirse ya da kullanılan airfoliller farklı seçilirse daha farklı sonuçlar almak mümkündür. xxiii

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25 1. INTRODUCTION Wind tunnels are useful for different flow types. Using wind tunnels in experiments is more advantageous because these experiments can be better controlled. Wind tunnels generate airflows at different speeds so that different data is obtained at different airspeeds. This is very important for the accuracy of the experiment. When the design of an aircraft starts, the wind tunnels are used to experiment so that enough data is gathered before the stage of the production. 1.1 Purpose of Thesis The aim of this thesis is to measure the pressure difference at the inlet and outlet according to different airfoil numbers in the expanding corners. 1.2 Types of Wind Tunnels There are two types of wind tunnels depending on their velocity range. These are low speed or subsonic and high-speed tunnels. Low speed or subsonic wind tunnels are the most used type and research in this paper depends on this type Subsonic Wind Tunnel Test section speed in subsonic tunnels is less than 180 m/s[1]. The subsonic wind tunnels are divided into two categories: open circuit tunnels and closed circuit tunnels. The airflow firstly enters to effuser then passes test-section and continues to diffuser. The air does not return in open circuit tunnels as shown in Figure 1.1[1]. There are no corners in this type. 1

26 Figure 1.1: Open circuit wind tunnel[1]. The airflow is in continuous flow with fan and motor. The air continues in a loop in closed circuit wind tunnel as shown in Figure 1.2[1]. This type needs less power because it does not out airflow. Figure 1.2: Closed circuit wind tunnel[1] 2

27 1.2.2 High Speed Wind Tunnel Test section speed in high-speed wind tunnel is more than 180 m/s [1]. The highspeed tunnels classified by the test section M range as follows. 0.8 < M < 1.2 Transonic Tunnel 1.2 < M < 5 Supersonic Tunnel 5 < M Hypersonic Tunnel Transonic wind tunnels are generally used in the aircraft industry. Supersonic wind tunnels are used to research on military aircraft and jet engines. Hypersonic wind tunnels are used to investigate on the area of rockets and space vehicles. Three of these tunnels are significant for various fields. The high-speed tunnels are also divided into two categories like supersonic wind tunnels: open circuit tunnels and continuous return circuit tunnels. This classification is based on their operation type. The blowdown type wind tunnels are the most advantegous type of high-speed wind tunnels because of being the simplest and the most economical as shown in Figure 1.3[1]. Figure 1.3: The Blowdown Tunnel[1] 3

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29 2. THE WIND TUNNEL DESIGN 2.1 Selection of the Wind Tunnel MTL wind tunnel is suitable for the experiments that investigate expanding corners. The MTL wind tunnel was designed in the mid 80s. Its design purpose is creating to suit experiments in basic transition and turbulence research. This wind tunnel is initiator in this situation[2]. 2.2 MTL Wind Tunnel The type of the MTL wind tunnel is closed circuit. Less energy is needed to experiment in the MTL wind tunnel. There is a small gap between the test section and the first diffuser, therefore the static pressure in the test section is almost equal to the atmospheric presssure as shown in Figure 2.1[2]. Figure 2.1: The MTL wind-tunnel layout[2] It is possible to experiment in high Reynold numbers without high speed thanks to the dimensions of the test section. The specifications of the MTL wind tunnel is listed in Table 2.1[2]. 5

30 Table 2.1: MTL wind tunnel specifications Type of Tunnel Length of Test Section Closed Circuit 7 m Cross Section Area of Test Section 1.2 x 0.8 m² Maximun Velocity in The Test Section 69 /s Parts of the MTL Wind Tunnel The parts of the MTL wind tunnel are test section, diffusers, corners, driving unit, heat exchanger, stagnation chamber and contraction as shown in Figure 2.2[2]. Figure 2.2: The Parts of the MTL wind tunnel[2] The parts of the MTL wind tunnel that shown in the Figure 2.2 is listed in Table

31 Table 2.2: Numbering of the Sections Number of the Sections Names of the Sections 1 Test section 2 Diffusers 3 Corners 4 Driving unit 5 Heat exchanger 6 Stagnation chamber 7 Contraction Description of the parts The test section has appropriate dimensions to work with high Reynolds numbers without high free stream velocity. Due to its top and bottom walls are adjustable, the streamwise pressure gradient can be modified between negative to positive values. The diffusers are conservatively designed so that there is no boundary layer separation. Owing to the diffusers that are conservative, they have a longer wind tunnel return circuit. The flow turns with 90 because the corner has a cascade of airfoil. If it is not properly designed, it can gain to large boundary layer seperation that lead to large turbulence level and high total pressure loss. The driving unit includes the fan and motor. The motor has power of 85 kw and the thyristor control unit controls to the speed of the fan. The stagnation chamber or settling chamber has large cross section area so that there is low velocity of the flow in the wind tunnel circuit. The stagnation chamber has three different parts: the honeycomb, the screens and the relaxation duct. The contraction leads to low velocity variation. If the shape of the contraction is perfect, there is no wall boundary layer separation. 7

32 2.2.3 Pressure Loss The dynamic pressure in the test section is measured according to the variation of fan rpm so that the power factor of the wind tunnel can be estimated. The power factor is a measure of the total pressure loss of the wind tunnel circuit[3]. λ = P tot n f q 1 = P mn m A 1 U 1 q 1 (2.1) where λ is the power factor of the wind tunnel circuit, P tot is the total pressure loss of the wind tunnel circuit, P m is the power input from the motor, U 1 is the test section flow speed, A 1 is the test section cross sectino area, q 1 is fan rpm, n f and n m are the fan and motor efficiency factors respectiely. The relation of the inflow angle, Ѳ, with the outflow angle, Ø, is Ø = αѳ (2.2) where α is a constant between 0 and 1. α can be written as the local pressure loss coefficient, K 0. α = K 0 (2.3) The local pressure loss coefficient, K 0, can be written as the wire diameter Reynolds number, Re d, and the solidity of the screen, α = 1 β. K 0 = f (Re d ) 1 β2 β 2 (2.4) The porosity, β, is defined as, (2.5) β = (1 d MS )2 where d is the diameter of the wire, MS is the mesh size as shown in Figure 2.3[3]. 8

33 Figure 2.3: The configuration of the wire diamater, d, and mesh width, M[3] The decrement in the streamwise mean flow changing in terms of α and K 0 is defined as, (2.6) u 2 u 1 = 1+α αk 0 1+α+K 0 where u 1 is the changing in streamwise velocity upstream and u 2 is the changing in streamwise velocity downstream. The total pressure variation is defined as, P(y,z) q 1 = P 1(y,z) P 0 q 1 (2.7) where P(y,z) is total pressure, P 0 is the total pressure in the stagnation and q 1 is the dynamic pressure at a fixed position. 9

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35 3. FLOW ANALYSIS STUDIES When the corner of the wind tunnel was designed, the exit area was taken as times the entrance area for the difference in cross-sectional area between the inlet and outlet. This area difference is called the expansion rate. The analysis parameter was determined as the number of airfoils. Airfoil numbers were used as five, ten and fifteen respectively. It was aimed to analyze the pressure drop between the inlet and outlet areas by drawing in ANSYS Fluent. Expansion ratio is defined as the ratio of inlet and outlet areas. e = The Area of Outlet The Aera of Inlet (3.1) The inlet area is 50 cm; the outlet area is 66 cm according to the expansion ratio. NACA 7408 [4] is selected as the airfoil. Figure 3.1: The shape of the airfoil 3.1 Boundary Conditions The fluid used is air and its density is kg/m 3. The speed of the inlet was chosen as 10 m/s. 11

36 3.2 CFD After making drawings with desing models, CFD used to measure the pressure distribution of the wind tunnel. CFD uses mathematical and computational models to describe the reactions that occur due to the airflow around a moving or stationary object. 3.3 Subsonic Wind Tunnel with Five Airfoils The distance between the airfoils placed in the corners is 10 cm. Figure 3.2: General layout of wind tunnel with five airfoils Figure 3.3: The mesh view of wind tunnel with five airfoils 12

37 3.3.1 CFD Results for Subsonic Wind Tunnel with Five Airfoils Figure 3.4: Pressure distribution of wind tunnel with five airfoils CFD analysis showed that the maximum pressure is Pa and the minimum pressure is Pa. 13

38 3.4 Subsonic Wind Tunnel with Ten Airfoils The distance between the airfoils placed in the corners is 7 cm. Figure 3.5: General layout of wind tunnel with ten airfoils Figure 3.6: The mesh view of wind tunnel with ten airfoils 14

39 3.4.1 CFD Results for Subsonic Wind Tunnel with Ten Airfoils Figure 3.7: Pressure distribution of wind tunnel with ten airfoils CFD analysis showed that the maximum pressure is Pa and the minimum pressure is Pa. 15

40 3.5 Subsonic Wind Tunnel with Fifteen Airfoils The distance between the airfoils placed in the corners is 4 cm. Figure 3.8: General layout of wind tunnel with fifteen airfoils Figure 3.9: The mesh view of wind tunnel with fifteen airfoils 16

41 3.5.1 CFD Results for Subsonic Wind Tunnel with Fifteen Airfoils Figure 3.10: Pressure distribution of wind tunnel with fifteen airfoils CFD analysis showed that the maximum pressure is MPa and the minimum pressure is MPa. 17

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43 4. CONCLUSIONS AND RECOMMENDATIONS Three different parameters were determined for analysis. These are the number of airfoils placed in the corners. These aifroil numbers are five, ten and fifteen, respectively. The distance between the five-airfoil corners is 10 centimeters, while the distance between the ten airfoils corner is 7 centimeters and the distance between the fifteen airfoils corner is 4 centimeters. these intervals have been determined so that the airfoils spread evenly across the corners. As a result, the pressure differents between five and ten airfoil corners were almost equal. Only the results of fifteen airfoils corner are very extreme. it is possible to create a different pressure between the corners that expand in subsonic wind tunnels. If the velocity of the airflow in the wind tunnel is changed, or if the airfoils used are different, it is possible to obtain different results. 19

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45 REFERENCES [1] Rathakrishnan, E. (2007). Instrumentation, measurements, and experiment in fluids. Boca Raton: CRC Press [2] Lindgren, B., & Johansson, V. (2002). Evaluation of the flow quality in the MTL windtunnel. Retrieved from KTH Royal Institute of Technology website: [3] Lindgren, B., & Johansson V. (2002). Design and evaluation of a low-speed wind-tunnel with expanding corners. Retrieved from KTH Royal Institute of Technology website: [4] Url-1 < date retrieved