Empirical Mappings of the Frequency Response of an Electron Ratchet to the Characteristics of the Polymer Transport Layer

Flashing electron ratchets oscillate a periodic asymmetric potential to rectify nondirectional forces and thereby produce directional transport of electrons with zero source-drain bias. The relationship between the oscillation frequency of the potential and the ratchet (short-circuit) current reflects microscopic mechanisms of charge transport within the device. This paper describes experimental mappings of the “optimal frequency(ies)” of the ratchet fpeak—the oscillation frequencies that produce the largest ratchet current—to the carrier concentration, nh, and to the linear field effect transistor mobility, μh, for a poly(3-hexylthiophene-2,5-diyl) (P3HT) transport layer. Measurements on multiple devices, multiple P3HT films per device, and a range of annealing and photoexcitation conditions yield the empirical relationships fpeak ∝ nh and fpeak ∝ μh2/3. Finite-element simulations suggest the sublinear relationship between mobility and peak frequency arises due to a combination of damped and inertial motion of the holes. This work also provides evidence that the frequency response of ratchets is sensitive to multiple length scales of asymmetry encoded within the periodic electrical potential. These multiple asymmetries cause changes in the polarity of the ratchet current at points within the frequency response, a long-mysterious characteristic of particle ratchets called “current inversion”, by encouraging transport in opposite directions in different frequency regimes

Spectral Oscillations, Periodic Orbits, and Scaling

The eigenvalue density of a quantum-mechanical system exhibits oscillations, determined by the closed orbits of the corresponding classical system; this relationship is simple and strong for waves in billiards or on manifolds, but becomes slightly muddy for a Schrodinger equation with a potential, where the orbits depend on the energy. We discuss several variants of a way to restore the simplicity by rescaling the coupling constant or the size of the orbit or both. In each case the relation between the oscillation frequency and the period of the orbit is inspected critically; in many cases it is observed that a characteristic length of the orbit is a better indicator. When these matters are properly understood, the periodic-orbit theory for generic quantum systems recovers the clarity and simplicity that it always had for the wave equation in a cavity. Finally, we comment on the alleged "paradox" that semiclassical periodic-orbit theory is more effective in calculating low energy levels than high ones.Comment: 19 pages, RevTeX4 with PicTeX. Minor improvements in content, new references, typos correcte

폐쇄형 스월 분사기에서 자체 불안정이 동특성에 미치는 영향 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 기계항공공학부, 2020. 8. 윤영빈.Injector is a device which can makes liquid to droplets. Injectors are used in various fields such as combustion, agriculture, cooling, etc. Injectors can be varied with atomization method, swirl-type, screw-type, impinging-type, shear-type, pintle-type, etc. Among them, owing to simple structure and high performance, closed type swirl injectors have been widely used, especially as rocket engine injector in Russia. However, when an injector is used for combustion, flow instability of injector becomes important. Combustion instability is occurred by coupling of acoustic characteristic of combustion chamber and heat release characteristics. At this time, heat release characteristics are changed with flow characteristics. If the propellant supply from the injector is incomplete or the atomization of the propellant becomes irregular, uniform heat release could not be generated. Therefore, it becomes combustion instability easy to occur. Thus, dynamic characteristics of spray flow such as periodical concentration of droplets or spray fluctuation should be investigated. Therefore, in this study, the reason of self-oscillation and characteristic of self-oscillation instability with varying experimental condition was investigated by measuring internal flow characteristic. Then, the effect of self-oscillation on swirl injector external flow was measured. Finally, the effect of self-oscillation on dynamic characteristic of swirl injector was investigated in with applying external pulsation. The result of experiments was as follows. First, the characteristics of self-oscillation instability was found from internal flow image of swirl injector. It was found that the amplitude of self-oscillation was inversely proportional to mass flow rate and self-oscillation frequency was proportional to mass flow rate. To find the characteristics of self-oscillation frequency in more detail, experiments with various swirl injector geometry was done. As a result, it was found that amplitude of self-oscillation was related to liquid momentum. On the other hands, it was found that frequency of self-oscillation was related to liquid axial velocity at the orifice. This phenomenon was same as Kelvin Helmholtz instability. Relation of frequency and velocity could be represented from Strouhal number, and in this study, constant Strouhal number, 0.236 was found in all experiment cases. Second, the effect of self-oscillation on swirl injector flow was investigated. It was found that the phenomenon of instability at external spray angle and internal air core diameter was same. The effect of the self-oscillation on external spray then investigated in more detail. At first, wave characteristic with sheet propagation was investigated from spray images. Self-oscillation frequency remained same with spray flow progression, and fluctuation amplitude was only increased. Effect of self-oscillation on breakup process was then examined. In this time, breakup occurred periodically, which have period same as inverse of self-oscillation frequency. Moreover, spending time between sheet progress and droplet conversion was different. Due to this difference, periodical concentration of droplet flow was formed. Finally, the effect of external pulsation on swirl injector flow and the relation between injector dynamic characteristic and self-oscillation was investigated. At this time, specific frequency could be applied at the feed line by using the hydraulic mechanical pulsator. Experimental condition which had 312 Hz self-oscillation frequency was used for this investigation. Dynamic gain was measured from air core diameter with respect to frequency, and the maximum gain found at the frequency same as self-oscillation frequency. Breakup length with respect to excitation frequency was then measured. Breakup length was minimum at the frequency same as self-oscillation frequency due to the high wave fluctuation. From the result, the fluctuation amplitude and breakup length were determined to be inversely related, and self-oscillation frequency was inferred to have some role on the dynamic characteristic of breakup length. Both the mean drop size and the SMD were smaller than those without pulsation. In addition, when the pulsation was performed at the injector self-oscillation frequency, an unusual phenomenon occurred for the SMD. The SMD was confirmed to become larger when excitation was performed at the injector self-instability frequency. This tendency seems to be due to the non-uniform distribution of droplets in the case of self-instability frequency pulsation. In conclusion, through this study, experimental data on the self-oscillation instability phenomenon of a single liquid swirl injector were obtained. Furthermore, the manifestation of unstable spray characteristics, when nonlinear large pulsation was applied, was experimentally confirmed. Self-oscillation with external pulsation was also confirmed to possibly change the characteristics of injector, especially breakup length and SMD, which is a combustion-related factor. Therefore, knowledge of self-oscillation frequency of injector is important for engine system design. Furthermore, frequency of pressure noise from feed system should be confirmed before entire engine system combining to prevent engine failure. Additionally, further research may be needed to clarify the relationship between self-instability, SMD, and combustion through an actual combustion test in the presence of such an instability.분사기는 액체를 미립화 시키는 장치로, 연소, 농업, 냉각 등 다양한 분야에서 사용되어 왔다. 분사기는 분무 방식에 따라 스월형, 스크류형, 충돌형, 전단형, 핀틀형 등 다양하게 분류되며, 그 중 폐쇄형 스월 분사기의 경우 구조가 간단하고 성능이 우수하기 때문에 러시아를 중심으로 로켓 엔진 분사기로서도 널리 사용되어 왔다. 하지만 분사기가 연소에 사용되는 경우에는 연소 불안정과의 관계로 인해 분사기의 유동 불안정성이 중요해진다. 연소 불안정은 연소실의 음향 특성과 열 방출 특성이 서로 결합하면서 발생하며, 이 때 열 방출 특성은 유동 특성에 따라 변경된다. 분사기로부터의 추진제 공급이 불완전하거나 추진제의 미립화가 불규칙하게 되면, 균일한 열 방출이 발생하지 못하며 연소 불안정이 발생하기 쉬워진다. 따라서 주기적으로 액체 입자 분포나 분무 안정성과 같은 분사기 유동의 동적 특성을 살펴봐야 한다. 따라서 본 연구에서는 다양한 실험 조건에 따라 내부 유동 특성을 측정하여 분사기의 자체 불안정 특성을 살펴보았다. 그리고 자체 불안정이 스월 분사기 외부 유동에 미치는 영향을 살펴보았다. 마지막으로, 스월 분사기의 동적 특성에 대한 자체 불안정과의 연관성을 살펴보기 위해 외부 가진을 적용하였을 때의 유동 특성을 살펴보았다. 실험 결과는 다음과 같다. 먼저, 분사기 자체 불안정 특성은 스월 분사기의 내부 유동 이미지로 부터 살펴보았다. 이로부터 자체 불안정의 진폭은 유량에 반비례하고 주파수는 유량에 비례함을 발견하였다. 자체 불안정의 특성을 더 자세히 살펴보기 위해 다양한 구조의 스월 분사기 형상을 사용하여 실험을 수행하였다. 그 결과, 자체 불안정 진폭은 액체의 운동량과 관련된 것으로 밝혀졌다. 한편, 자체 불안정의 주파수는 오리피스에서의 축방향 유동 속도와 관련된 것으로 밝혀졌다. 이 현상은 Kelvin-Helmholtz 현상과 유사한 것으로 확인되었다. 주파수와 축방향 속도의 관계는 Strouhal 수로 나타낼 수 있으며, 본 연구에서는 모든 실험 사례에서 0.236의 거의 일정한 Strouhal 수가 관찰되었다. 다음으로, 자체 불안정이 스월 분사기 유동에 미치는 영향을 살펴보았다. 외부 분무각 및 내부 에어 코어 직경에서의 불안정을 살펴본 결과 각각의 불안정 특성이 동일함이 밝혀졌다. 그리고 외부 유동에 대한 자체 불안정의 영향의 경우, 우선, 파동 형태의 액막 특성을 외부 유동 이미지로부터 살펴보았다. 자체 불안정 주파수는 유동이 진행하더라도 동일하게 유지되었고 불안정의 진폭만 증가하였다. 액막 분열 과정에서 자체 불안정의 영향을 살펴본 결과 주기적으로 액막 분열이 발생함을 확인 할 수 있었는데, 분열 주기는 자체 불안정 주파수의 역수와 같음을 확인하였다. 또한, 액막 진행과 액적으로의 변환 사이의 소요되는 시간이 달라 주기적으로 액적 분포의 밀집 현상이 나타남을 확인하였다. 이를 통해 분사기 자체 불안정이 분사기 내부 유동부터, 액막 형성, 분열, 그리고 액적 형성에 까지 전반적인 영향을 미치고 있음을 확인할 수 있었다. 마지막으로, 스월 분사기 유동에 대한 외부 가진의 영향 및 인젝터 동특성과 자체 불안정 간의 관계에 대해 살펴보았다. 이때 기계식 가진기를 사용하여 공급 라인에 특정 주파수의 압력 섭동을 가하였다. 실험에는 312Hz의 자체 불안정 주파수를 가지는 분사기 형상 및 실험 조건이 사용되었다. 주파수에 대한 에어 코어 직경으로부터 동특성을 살펴본 결과, 자체 불안정 주파수와 동일한 주파수에서 에어 코어 섭동이 가장 크게 발생함을 확인할 수 있었다. 그리고 가진 주파수에 대한 스월 분사기의 액막 분열 길이를 측정한 결과, 액막 분열 길이는 자체 불안정 주파수와 동일한 주파수의 가진을 가할 때 가장 빠르게 분열함을 확인할 수 있었다. 즉. 자체 불안정성 주파수는 분열 길이의 동특성에 중요한 역할을 함을 확인하였다. 평균 액적 크기와 SMD는 외부 가진이 있을 경우 더 작게 나타났고, 가진이 있는 경우 중에서도 인젝터 자체 불안정성 주파수와 동일한 주파수의 가진이 있는 경우, 그 이외 주파수로 가진한 경우에 비해 더 큰 SMD를 가짐을 확인하였다. 이는 자체 불안정 주파수 가진을 가했을 때는 액적 분포가 더 불균일해지기 때문으로 확인되었다. 결론적으로, 본 연구를 통해 액체 스월분사기의 자체 불안정 현상에 대한 실험 데이터를 얻었다. 또한, 큰 섭동이 있는 경우의 불안정한 분무 특성이 실험적으로 확인되었다. 자체 불안정 주파수를 가지는 외부 가진이 있는 경우 분열 길이나 SMD 같은 연소와 관련된 분무 특성에 영향을 미침을 확인하였다. 따라서 분사기의 자체 불안정 주파수에 대한 지식은 엔진 시스템 설계에 중요하다. 특히, 엔진에 발생하는 문제를 방지하기 위해 전체 엔진 시스템을 결합하기 전에 공급 시스템에서의 노이즈 주파수 확인이 중요함을 확인할 수 있었다. 추가적으로, 이러한 불안정성이 존재하는 경우 실제 연소 시험을 통해 자기 불안정성, SMD 및 연소 사이의 관계를 명확하게 하는 연구가 수행된다면 인젝터 동역학에서 중시하는 분사기를 통한 연소 불안정 제어에 한발짝 더 다가갈 수 있을 것으로 사료된다.CHAPTER1 INTRODUCTION 1 CHAPTER2 EXPERIMENT AND MEASUREMENT SYSTEMS 9 2.1 Swirl Injector Cold Test Facility 9 2.2 Method for Dynamic Analysis of Internal Flow Image 13 2.3 Method for Dynamic Analysis of External Flow Image 16 2.4 Hydraulic Mechanical Pulsator 18 CHAPTER3 CHARACTERISTICS OF SELF-OSCILLATION IN THE SWIRL INJECTOR FLOW 21 3.1 Objectives and Test Conditions 21 3.2 Relation between Internal and External Flow Characteristics 24 3.3 Main Parameters Affecting Self-excited Instability 27 3.4 Discussion on Self-excited Instability 39 CHAPTER4 EFFECT OF SELF-OSCILLATION ON THE SWIRL INJECTOR FLOW CHARACTERISTICS 41 4.1 Objectives and Test Conditions 41 4.2 Spray Characteritics with Self-oscillation Instability 43 4.2.1 Effect of Self-oscillation on Liquid Sheet Flow 43 4.2.2 Effect of Self-oscillation on Breakup Length 47 4.2.3 Effect of Self-oscillation on Ligament and Droplet Formation 50 CHAPTER5 RELATION BETWEEN SELF-OSCILLATION AND DYNAMIC CHARACTERISTICS OF SWIRL INJECTOR 55 5.1 Objectives and Test Conditions 55 5.2 Dynamic Characteritstics of Liquid Swirl Injector 57 5.3 Effect of External Pulsation with Self-oscillation Frequency 59 5.3.1 Effect of External Pulsation on Spray Angle 59 5.3.2 Effect of External Pulsation on Liquid Sheet Characteristics 62 5.3.3 Effect of External Pulsation on Liquid Sheet Characteristics 66 5.4 Comparison of Characteristics with and without External Pulsation 70 CHAPTER6 CONCLUSION 78 REFERENCES 82 ABSTRACT IN KOREAN 85Docto

Frequency pulling and mixing of relaxation oscillations in superconducting nanowires

Many superconducting technologies such as rapid single flux quantum computing (RSFQ) and superconducting quantum interference devices (SQUIDs) rely on the modulation of nonlinear dynamics in Josephson junctions for functionality. More recently, however, superconducting devices have been developed based on the switching and thermal heating of nanowires for use in fields such as single photon detection and digital logic. In this paper, we use resistive shunting to control the nonlinear heating of a superconducting nanowire and compare the resulting dynamics to those observed in Josephson junctions. We show that interaction of the hotspot growth with the external shunt produces high frequency relaxation oscillations with similar behavior as observed in Josephson junctions due to their rapid time constants and ability to be modulated by a weak periodic signal. In particular, we use a microwave drive to pull and mix the oscillation frequency, resulting in phase locked features that resemble the AC Josephson effect. New nanowire devices based on these conclusions have promising applications in fields such as parametric amplification and frequency multiplexing

The interaction between flow-induced vibration mechanisms of a square cylinder with varying angles of attack

This study examines the influence of angle of attack of a square section cylinder on the cylinder’s flow-induced vibration, where the direction of the vibration is transverse to the oncoming flow. Our experiments, which traversed the velocity–angle of attack parameter space in considerable breadth and depth, show that a low-mass ratio body can undergo combinations of both vortex-induced vibration and galloping. When the body has an angle of attack that makes it symmetric to the flow, such as when it assumes the square or diamond orientation, the two mechanisms remain independent. However, when symmetry is lost we find a mixed mode response with a new branch of vortex-induced oscillations that exceeds the amplitudes resulting from the two phenomena independently. The oscillations of this higher branch have amplitudes larger than the ‘upper branch’ of vortex-induced vibrations and at half the frequency. For velocities above this resonant region, the frequency splits into two diverging branches. Analysis of the amplitude response reveals that the transition between galloping and vortex-induced vibrations occurs over a narrow range of angle of incidence. Despite the rich set of states found in the parameter space the vortex shedding modes remain very similar to those found previously in vortex-induced vibration

The clockfront and wavefront model revisited

The currently accepted interpretation of the clock and wavefront model of somitogenesis is that a posteriorly moving molecular gradient sequentially slows the rate of clock oscillations, resulting in a spatial readout of temporal oscillations. However, while molecular components of the clocks and wavefronts have now been identified in the pre-somitic mesoderm (PSM), there is not yet conclusive evidence demonstrating that the observed molecular wavefronts act to slow clock oscillations. Here we present an alternative formulation of the clock and wavefront model in which oscillator coupling, already known to play a key role in oscillator synchronisation, plays a fundamentally important role in the slowing of oscillations along the anterior–posterior (AP) axis. Our model has three parameters which can be determined, in any given species, by the measurement of three quantities: the clock period in the posterior PSM, somite length and the length of the PSM. A travelling wavefront, which slows oscillations along the AP axis, is an emergent feature of the model. Using the model we predict: (a) the distance between moving stripes of gene expression; (b) the number of moving stripes of gene expression and (c) the oscillator period profile along the AP axis. Predictions regarding the stripe data are verified using existing zebrafish data. We simulate a range of experimental perturbations and demonstrate how the model can be used to unambiguously define a reference frame along the AP axis. Comparing data from zebrafish, chick, mouse and snake, we demonstrate that: (a) variation in patterning profiles is accounted for by a single nondimensional parameter; the ratio of coupling strengths; and (b) the period profile along the AP axis is conserved across species. Thus the model is consistent with the idea that, although the genes involved in pattern propagation in the PSM vary, there is a conserved patterning mechanism across species