Sabit çıkış projeksiyon alanı ve lüle yüksekliğine sahip dairesel kesitli üç boyutlu hava yastıklarının mukayeseli etüdü

Abstract

ÖZET `Sabit çıkış projeksiyon alam. ve Hile yüksekliğine sahip dairesel kesitli üç boyutlu hava yastıklarının mukayeseli etüdü` ad lı bu tez, başlangıçta, hava yastıklı vasıtalar genel ad^ ile anılan hovercraftlarda, halen çok kullanılan iki yastık tipinin deneysel olarak incelenip karşılaştırılması şeklinde düşünülmüş idi. Bu maksatla, literatür araştırmalarını takiben kumlan bi?r deney tesisatında tecrübeler yapılmış, fakat, bu konuda vaz' edilmiş teorilerin gerçeğe pek uygun olmadıkları görülmüştür. Bu uyumsuzluğun bir sebebi, deney tesisatının tap ve mükem mel olmaması, bir sebebi de teorilerin hen> iki boyutlu olmaları ve hem de gerçeğe tam uymayan kabullere dayandırılmalarıdır, kanaatine varılmıştır. Bir ön inceleme ve gözlem safhası oiarak kabul edilen bu çalışmalardan sonra, deney tesisatı, bütün kaçakları önlenerek ` kuv vet ölçmelerinde son derece hassas hale getirilerek ıslah edilip sağ lıklı ve emin bir duruma getirilmiştir. Bu yeni tesisat ile yapılan ikinci safha deneylerde de, mevcut iki boyutlu teorilerin yine de gerçek ile iyi uyum göstermedi ği tesbit edilince, çalışmalar, uyumsuzluğun diğer sebebi üzerinde yo ğunlaştırılmıştır. Önce, teorilerdeki eksikliğin bir sebebi olan kabuller üze rinde durulmuştur. Hızlar ve basınçlarla ilgili kabuller, gerçeğe uy gun şekilde, ve yapılan deneylerde gözlenen durumlardan da ilham alı narak, değiştirilmiştir. Sonra da, uyumsuzluğun ikinci teorik sebebi olarak mütalaa olunan iki boyutluluk üzerinde durulup, `üç boyutlu akım` için yeni teoriler kurulup kurulamayacağı düşünülmüştür. Bu çalışmaların sonunda, çevresel jetli yastıklar için, te zin üçüncü bölümünde anlatılan, ıkiboyutlu iki yeni teori geliştiril miştir: İki boyutlu yeni ince jet teorisi ve ikboyutlu yeni kalın jet teorisi (vorteks teori)......IV Kurulan bu yeni teorilerin tahkiki için, ıslah edilmiş tesisatta deneyler tekrarlandığında, bu yeni teorilerin gerçeğe çok uygun oldukları görülmüştür. Çalışmanın bundan sonraki safhasında, halen en çok kullanı lan çan biçimli ve çevresel jetli yastıklar için bir seri deney ya pılmıştır. Bu deneylerde, her iki tip, muhtelif parametrelere bağlı olarak incelenmiş ve sonra da birbirleriyle mukayese edilmiştir. Tezde, bu iki yeni teoriye ilâve olarak, evvelki çalışma lar ve teorilerde rastlayamadığımız bir diğer husus üzerinde de du rulmuştur. Bu da bazı parametrelerin hakiki geçerlik sınırlarının tesbitidir. Bu konu da, hem teorik olarak incelenmiş ve hem de tecrü belerle tahkik edilmiştir. Böylece her iki tip yastığın pratik olarak anlam taşıdığı sınırlar belirlenmiştir. Teorik çalışmalarda akla gelmeyen ve görülmeyen, fakat deneylerde ortaya çıkan bazı kritik yüksekliklerin ve durumların bulun duğu görülmüştür. Ancak, çalışmayı çok uzatacağından ve bir doktora hacmini çok aşacağından, bunlar üzerinde daha detaylı incelemeye gi rilmemiş, fcesbit ve işaretle yetinilmiştir. Tez beş bölüm halinde kaleme alınmıştır. Birinci bölümde, hava yas tikli vasıtalar ve bunlara adını veren hava yastığı, bunların geliştirilmesi hakkında genel bilgiler verilmiştir. Sonra da, kara, deniz ve hava nakil vasıtaları ile hava yas tikli vasıtaların mukaye sesi yapılmıştır. İkinci bölüm, hava yastığı hakkındaki genel tariflere ve bunlar için geliştirilmiş, iki boyutlu bazı yaklaşık teorilere tah sis edilmiştir. Ancak, burada bir yenilik yapılarak, mevcut iki ince jet teorisi birleştirilmiş ve bütün ince jetler için geçerli genel bir çözüm haline dönüştürülmüştür. Üçüncü bölümde, çevresel jetli yastıklar için geliştirilen, birisi ince ve diğeri kalın jetlere mahsus iki boyutlu iki yeni teori anlatılmaktır. Bu teorilerde, basınç ve hız değişimleri için evvelce yapılan ve gerçek durum ile pek uyumlu olmayan eski kabullerin yeri-ne, gerçeğe çok dalla y<sk:«,n İcabü.;?..ev n?.?.j- -t e^: lm.rşl: i. Dördüncü bö lüK.de j, baş.1 »r-gj- ı:«,ndan £ -mi- i* l-ş^ar» kuruian de ney tesisatı, üzerinde -i- £»?l£.a ı??ai..t<v ^e c-isv-'-glerf r mcufc&j vfc ölçme teknikleri, bunların hassasiyet mertebeler:, ve modeller hakkında ge1- niş bilgi verilmiştik Sonuç bölüm! olam beşiacl bölüm ifss9. taiasjae-tt deney sonuçT larma ve bunlardan çıkarılan se 6i.ee- leî?e ayts.lffiigt*3,.?:a Teori ve deştey sonuçları, mukayese yapjjaaya imkS'n vermek malçsadiyles mü.a&ÜE ?oldügü ka dar boyutsuz büyüklüklere dönü ştüMİİsslş tür. 'Mevcut; iki boyutlu teorik ler, bu tezde geliştirilen iki boyııtlı* iki yeni teori ve deney sonuca lan grafikler üzerinde karşıla^txr5tlîiaiştır0- ¥e fee-r /birinin uygimlük dereceleri belirlenmiştir. En sonunda da, tesbit adilea yeni hus#&Iii*las bundan sonra, bu konuda hangi noktalarda derinliğine ve detaylı araştırma yapılabi leceği -bir tavsiye olarak- kaydedilmiştir.

SUMMARY `A COMPARATIVE STUDY OF THREE-DIMENSIONAL AIR CUSHIONS WITH CIRCULAR CROSS SECTION, CONSTANT EXIT PROJECTION AREA AND CONSTANT NOZZLE HEIGHT` This thesis consists of five chapters. The first chapter deals with air cushion vehicles (ACV). The subject matter is dealt with briefly with emphasis on its progressive development and entails a classification based on its various aspects. The basic functions of an air cushion, the most vital element of air cushion vehicles, are expounded, and brief information is given on the most-widely used types of cushions. To conclude this chapter, the characteristics of the low »medium, and high-velocity ACV Vis are presented in a detailed manner with a comparison of the existing land and sea transportation vehicles. The second chapter is devoted to a general description of air cushions and some related approximate theories.This chapter attempts to compare the plenum chamber and peripheral jet air cushions and puts emphasis on those characteristics of an air cushion upon which a comparative study can be based. Then, attention is focused on the calculation, for the plenum chamber type of cushions, of the augmentation ratio, which is the most important basis of comparison regardless of whether the air movement in the cushion is neglected or not. Later on, the section takes up the issue of four theories developed by various scholars on the peripheral jet air cushions» First, the general assumptions underlying all these theories are discussed and the differential equation of the. jetvıı flow is derived as the overall basis of these theories. Two of the four theories based on these assumptions and equations are for thinner jets and two for thicker jets. Here, the case of the thin jet refers to the situation in which the jet thickness is negligibly small in comparison with the cushion height. The thick jet, on the other hand, corresponds to the case where the jet thickness cannot be ignored. In one of the two theories for thin jets that assume the pressure distribution in the jet flow to be linear, the constant coefficient in this linear pressure variation expression is taken as f=0 (Chaplin-Boehler), while the- other theory uses f=l/2 (Stanton-Jones). In the present thesis, however, the theory for thin jets is presented in its most general form without assigning a numerical value to this coefficient. There is also a brief summary of the two existing theories for thick jets (The Exponential Theory developed by Stanton-Jones and the Vortex Theory by Fuller and Pinnes). The summary touches on the augmentation ratio and other def initions. The third chapter is reserved entirely for the two peripheral jet theories developed for this thesis. The preliminary experiments, conducted solely for observation and initial assessment on air cushions have shown that the theories developed for the two-dimensional flow (discussed in the second chapter) produce unsatisfactory results. This deficiency has prompted a novel approach in two regards. One involves the study of whether it would be possible to develop new theories for two-dimensional flow based on more realistic assumptions and the other deals with the improvement of the test equipment. It is thought that one reason for the existing theories to disagree with experiments is the fact that the assumptions involved are not realistic.vııı For this reason, a new model of air cushion is developed for practical use, and more realistic assumptions of inter-nozzle flow static pressure and velocity are made. This deficiency is corrected with the two theories, developed in this thesis. Thus by employing more realistic assumptions, a`new thin jet` theory is established for the two-dimensional flow.. The results of this theory are in improved agreement with the experiments. For the case of the two-dimensional flow, the assumption of constant jet velocity (equal to the flow velocity in the nozzle) made in the two-dimensional thick jet theory is discarded, and instead, a new vortex theory is developed by calculating the jet speed as a function of the nozzle flow and the jet radius of curvature. Especially for jets that are not excessively thin, this theory produces results that are in very good agreement with the experiments. The fourth chapter is devoted totally to the test equipment, its modification, the modification reasons, and the techniques applied. Starting from the first equipment, set up to undertake tests for observation purposes, the chapter discusses a whole range of equipment up to the most sophisticated, sensitive and reliable measurement devices. All the equipment in use has been described in full detail. The fifth chapter covers the tests and results. This chapter gives a comprehensive background on model assemblies, the models in use, the parameters taken into account, and the adjustments carried out before and after experiments. Later in the chapter, the effect of the volume flow rate is studied in the plenum, shaped cushions for four different inclination angles and for various cushion heights ranging from 2 nan to 60 mm.Above all, it is noted that, as expressed byIX Vcn-Mises and Chaplin, the discharge coefficient K is not only a function of the cushion exit slope but also of the flow rate and the cushion height. The variation of the discharge coefficient with tlsese parameters is determined. -3 3 Subsequently Q=.34,69.10 m /sec was taken as a maximum flow rate by the fan at a height of h=2mm (h/D=0,009).All of the experiments to obtain augmentation ratios were conducted at this constant flow rate. The nozzle thickness is taken as a third parameter, in addition to the inclination angle and height, in peripheral jet cushions and experiments were done on three different nozzle thicknesses. In. these experiments,v the flow rate was kept at the constat value mentioned above. The second part of this chapter is devoted to the results. From a comparison of the existing theories and experiments the following results are obtained, 1. The `K` discharge coefficient, which is only a function of the inclination angle `9` according to the formulations by Von-Mises and Chaplin, displays a smooth decrease with 9, although this variation has been observed, in reality, to exhibit fluctuations. 2. The discharge coefficient is not only a function of the inclination angle but also of the flow rate, and the cushion height. The discharge coefficient changes in direct proportion to the flow rate (Fig. 5.1). The discharge coefficient increases with che cushion height at a very slow pace. 3. The cushion heights up to 2Z carry a practical meaning. In other words, the plenum chamber cushion has a function only in locations close to the ground. Departure from the ground weakens its functions (Fig. 5. 4). 4. The plenum chamber cushion loses its stability as itX is detached from ground-level. The instability aetn in earlier far the larger value of the inclination angle. The^efoit. 2 the plenum chamber cushion is not a much- favoured device in regard te stability (Fig. 5.5, 5.6, 5.7 and 5.8), 5. Regardless of the flow rate two types of critical height have been determined in plenum chamber cuşhicas.. Th&re are -vo critical heights of the first type and both of them generally increase with 9. The first critical height corresponds to the height where the total lift changes from a positive to a negative value, and the second entails a recourse back to a positive value (Fig. 5.4). The second type of critical height is encountered only at 0*60°. In this inclination angle and for relative cushion... heights of h/D=0.024 and h/D=*0,137 it was noticed that: the total lift was zero regardless of the flow rate. 6. In plenum-chamber cushions, at the maximum flow capacity of the fan and at relative cushion heights approximately between 2% and 11%, it was observed that considerable suction took place in the reservoir. 7. In the peripheral cushion, in particular whan, the inclination angle was 8=60, it was seen that the cush5:on exit geometry influenced the total lift. The reason may be traced to the fact that the cushion surface and the ground form a diffuser. 8. Contrary to the plenum-chamber cushion, the peripheraljet cushion is extremely stable at all e) and h values. The pressure distribution at the ground level, from the centre towards the circumference, is nearly constant(Fig. 5.5, 5.6, 5.7.'nd 5.8). 9. In the peripheral-jet cushion, in sharp to the plenum-chamber cushion, the total lift is never negative(Fig. 5. 13, ? 5.14, 5.15).. 10. Although the plenum-chamber cushion loses its practical meaning for approximately h/D>2%, the peripheral jet cushion has practical significance at much greater heights from the ground.XI 11. Ir the new two-dimnjiiona^ chin, jet theory developed for peripheral je£Sr»~the theory-*experiipertt' agreement is good for large values of k(close to 1) and.* for small values of the nozzle thickness t.(Both of these variables appear in the expression V.= V +k (VK- V`) proposed for the jet velocity variation). Therefore, this new thin jet theory is a satisfactory tiheory0 12. The vortex theory developed for peripheral 'Jet. cushions also gives a very good agreement with experiments v especially at small inclination angles and as noz-sle thickness increases. For this reason, this new two-dimensional vortex theory is indeed a good theory. (Fig. 5.28,....5,39 and Table 5.2). 13. In this research t a meaningful limit is provided for nozzle thickness parameters described for peripheral jet cushion, X=l. The cushions have a practical function for X<1 values. For the X>1 value, the cushion loses its function. 14. The peripheral jet cushion's exterior geometry has an effect on lifting. It is helpful for the nozzle exterior part to resemble a dagger. It is note worthy to state here that the nozzle exterior part and the ground form a type of diffuser. 15. Very close to the ground, it was seen the peripheral jet cushion has virtually no effect on the total lift. Under such circumstances, both cushions may be regarded to be equivalent. 16. Especially at h/D>l% it is understood that the inclination angle, `0`, must be small to obtain the maximum possible efficiency from a cushion. ` 17. It has been observed that the operating noise levels of the peripheral jet cushion are lower than those of the plenum chamber cushion. 18. The following points are recommended to be considered in future investigations conducted on both types of cushions: VXll a) Variation of the discharge ^.coefficient '°K!3 in the plenuûi-chaıaibeÂ: cushion wish various parameters should be studied...,*?>?... b) Critical cushion height? established for plenum chamber* cushions should be investigated with more care and by taking measurements at smaller intervals c) The constant* zero lift situation that occurs at a 60° inclination angle and &.t all f low.ratas in plenum-chainbeV cushions deserves further consideration., with greater care. d) Since the plenum- chamber cushion at 60 is not a suitable cushion for use in a stationary (or slowly moving) vehicle, the mobile behaviour of this type of cushion should be investigated &? e) Practically significant end useful cushion heights for both cushion types should be determined as a result of a more detailed, investigation. f) Studies should be, carried out, for both cushion types, to determine the optimum cushion and reservoir profile shaped that would give minimum loss and maximum lift. g) Performances of both cushions in motion and their changes in due course should be studied. h) Obstacle-clearance abilities of both cushions should be reviewed. ? x i) The effect on the total lift on installing a flexible skirt in both cushions should be studied, j) Similarly, effects of flexible-skirt on obstacle clearance should be studied. k) Since a real-life ACV embodies several cushions, a research into their varying combinations is warranted. 1) It is predicted that, in cushions having large inclination angle, a geometry shaped like a fowler slot located close to the outlet will bear a positive effect on total lift. This subject requires further investigation, m) It` is also foreseen that in cushions with large inclination angle. Holes fixed on walls in clos,e proximity to the outlet will also bring about favourable consequences on totav. b.ft Th).s is ano,>;hec area worth a thorough study.