Tunnel Noise Research Articles

Emulated Analysis of the Preview-LQG Controller for Micromachined Tunneling Gyroscope

Only when the tunneling gap is maintained near 10Å can the micromachined tunneling gyroscope operate regularly to generate tunneling current which is so weak that it would be submerged by the thermal-mechanical noise, 1/f tunneling noise and so on. At the meantime, the dynamic range of the device will be reduced drastically by the nonlinearity of the tunneling effect. So a controller must be designed for the miniature tunneling gyroscope to realize a 10 Å tunneling gap to decrease the nonlinearity and so as to enlarge the band-width of the device. At the other hand, the signal-to-noise ratio can be raised by the controller. Because of the anti-interference and robusticity of the LQG control theory and the time-varying characteristic of Coriolis acceleration, the preview-LQG control strategy is selected to design the closed-loop system. The simulation results suggest that the tunneling gap can be controlled near its nominal value and the measured input angular rate can be estimated accurately by the preview-LQG controller even if the input angular rate is a slowly time-varying random signal.

Aerofoil tones at moderate Reynolds number

AbstractIt is known experimentally that an aerofoil immersed in a uniform stream at a moderate Reynolds number emits tones. However, there have been major differences in the experimental observations in the past. Some experiments reported the observation of multiple tones, with strong evidence that these tones are most probably generated by a feedback loop. There is also an experiment reporting the observation of a single tone with no tonal jump or other features associated with feedback. In spite of the obvious differences in the experimental observations published in the literature, it is noted that all the dominant tone frequencies measured in all the investigations are in agreement with an empirically derived Paterson formula. The objective of the present study is to perform a direct numerical simulation (DNS) of the flow and acoustic phenomenon to investigate the tone generation mechanism. When comparing with experimental studies, numerical simulations appear to have two important advantages. The first is that there is no background wind tunnel noise in numerical simulation. This avoids the signal-to-noise ratio problem inherent in wind tunnel experiments. In other words, it is possible to study tones emitted by a truly isolated aerofoil computationally. The second advantage is that DNS produces a full set of space–time data, which can be very useful in determining the tone generation processes. The present effort concentrates on the tones emitted by three NACA0012 aerofoils with a slightly rounded trailing edge but with different trailing edge thickness at zero degree angle of attack. At zero degree angle of attack, in the Reynolds number range of$2\ensuremath{\times} 1{0}^{5} $to$5\ensuremath{\times} 1{0}^{5} $, the boundary layer flow is attached nearly all the way to the trailing edge of the aerofoil. Unlike an aerofoil at an angle of attack, there is no separation bubble, no open flow separation. All the flow separation features tend to increase the complexity of the tone generation processes. The present goal is limited to finding the basic tone generation mechanism in the simplest flow configuration. Our DNS results show that, for the flow configuration under study, the aerofoil emits only a single tone. This is true for all three aerofoils over the entire Reynolds number range of the present study. In the literature, it is known that Kelvin–Helmholtz instabilities of free shear layers generally have a much higher spatial growth rate than that of the Tollmien–Schlichting boundary layer instabilities. A near-wake non-parallel flow instability analysis is performed. It is found that the tone frequencies are the same as the most amplified Kelvin–Helmholtz instability at the location where the wake has a minimum half-width. This suggests that near-wake instability is the energy source of aerofoil tones. However, flow instabilities at low subsonic Mach numbers generally do not cause strong tones. An investigation of how near-wake instability generates tones is carried out using the space–time data provided by numerical simulations. Our observations indicate that the dominant tone generation process is the interaction of the oscillatory motion of the near wake, driven by flow instability, with the trailing edge of the aerofoil. Secondary mechanisms involving unsteady near-wake motion and the formation of discrete vortices in regions further downstream are also observed.

Prediction Method of Tunnel Blasting Work Vibration, Noise and infrasound

Prediction Method of Tunnel Blasting Work Vibration, Noise and infrasound

Crossover between ballistic and diffusive transport: the quantum exclusion process

We study the evolution of a system of free fermions in one dimension under thesimultaneous effects of coherent tunneling and stochastic Markovian noise. We identify aclass of noise terms where a hierarchy of decoupled equations for the correlation functionsemerges. In the special case of incoherent, nearest-neighbor hopping the equation for thetwo-point functions is solved explicitly. The Green’s function for the particle density isobtained analytically and a time scale is identified where a crossover from ballistic todiffusive behavior takes place. The result can be interpreted as a competitionbetween the two types of conduction channels where diffusion dominates on largetimescales.

Hypersonic Boundary-Layer Transition with Ablation and Blowing

Hypersonic boundary-layer transition is afiected by many factors, including Mach number, Reynolds number, geometry, roughness, and tunnel noise. The efiect of ablation or surface blowing is reviewed by summarizing the experimental data. Blowing generally moves transition upstream, with larger mass∞ow rates or lighter gases causing a larger efiect. Blowing that occurs farther upstream on the model generally also has a larger efiect. It may be feasible to estimate the efiect of blowing using semi-empirical stability-based methods such as e N : Experimental data suitable for comparisons to these methods are summarized, for blunt bodies and for slender bodies at zero and nonzero angles of attack.

Screening of a Luttinger liquid wire by a scanning tunneling microscope tip. II. Transport properties

We study the effect of an electrostatic coupling between a scanning tunneling microscope tip and a Luttinger liquid wire on the tunneling current and noise between the two. Solving the Dyson equations nonperturbatively for a local interaction potential, we derive the Green's functions associated to the wire and to the tip. Interestingly, the electrostatic coupling leads to the existence of new correlators, which we call mixed Green's functions, which are correlators between the bosonic fields of the wire and the tip. Next, we calculate the transport properties up to second order with the amplitude of the tunnel transfer; the profile of the tunneling current is strongly modified by the presence of screening. The zero-frequency noise is modified in a similar way but the Fano factor remains unchanged. We also consider the effect of the screening on the asymmetry of the finite-frequency nonsymmetrized noise and on the conductance.

バサバサ音の音源を風洞中で可視化する試み

バサバサ音の音源を風洞中で可視化する試み

Effect of Freestream Noise on Roughness-Induced Transition for the X-51A Forebody

A 20%-scale X-51A forebody model was tested in the Boeing/AFOSR Mach-6 Quiet Tunnel. Repolishing the nozzle throat has restored quiet flow at high Reynolds numbers. The effect of a smooth blank and two different trip strips on windward-forebody transition was measured using temperature-sensitive paint and hot-wire anemometry. Reducing freestream noise from conventional to quiet levels increased the smooth-wall transition Reynolds number by a factor of at least 2.2. In addition, the transition Reynolds number based on the distance from the trips increased by a factor of 2.4 for the smaller trips and by a factor of 1.7 for the larger trips. Thus, tunnel noise had a substantial effect on roughness-induced transition.

Phonon-assisted tunneling and shot noise in double barrier structures in a longitudinal magnetic field

The phonon-assisted resonant tunneling is studied for the double barrier structures in a longitudinal magnetic field. Using the scattering matrix approach with an appropriate one-particle Green's function we are able to calculate the current and the zero frequency shot noise power spectrum in a large range of the magnetic field and to any order of the electron–phonon interaction. Obtained results describe well the relevant experimental data and provide new suggestions for further examinations.

콘크리트 궤도 터널 내 KTX 차량의 실내소음 특성 분석

KTX trains show a high interior noise level in tunnel with concreted track at 300 km/h. Generally, the concreted track has higher sound emissions compared with ballasted track due to the reduced absorption and the major sources of interior noise for KTX are known as the aerodynamic noise and rolling noise. Therefore, It is necessary to find out noise source and noise components to affect interior noise in tunnel with concreted track. In this study, we measure the noise and vibration inside KTX in tunnel in order to find the cause of the interior noise of KTX. The analysis results show that the interior noise of KTX in tunnel with concreted track is increased sharply by a low frequency below 80 Hz. We know that the low frequency noise inside KTX in tunnel with concreted track is generated at the natural frequency of carbody by aerodynamic noise outside gangway and rolling noise. In order to reduce the noise level at 80 Hz, modification of mud-flap length between carbodys is suggested and the effect of noise reduction is examined in tunnel with concreted track.

Pair tunneling and shot noise through a single molecule in a strong electron-phonon coupling regime

We investigate the electronic transport through a single molecule in a strong electron-phonon coupling regime. Based on a particle-hole transformation, which is made suitable for a nonequilibrium situation, we treat the pair tunneling and cotunneling on an equal footing. We propose an experimental setup to enhance the visibility of pair tunneling, which has no Franck-Condon suppression. We also discuss the shot noise characteristics.

Vibration-mediated resonant tunneling and shot noise through a molecular quantum dot

Motivated by a recent experiment on nonlinear tunneling in a suspended Carbon nanotube connected to two normal electrodes [S. Sapmaz, {\it et al}., Phys. Rev. Lett. {\bf 96}, 26801 (2006)], we investigate nonequilibrium vibration-mediated sequential tunneling through a molecular quantum dot with two electronic orbitals asymmetrically coupled to two electrodes and strongly interacting with an internal vibrational mode, which is itself weakly coupled to a dissipative phonon bath. For this purpose, we establish rate equations using a generic quantum Langevin equation approach. Based on these equations, we study in detail the current-voltage characteristics and zero-frequency shot noise, paying special attention to the advanced or postponed of the appearance of negative differential conductance and super-Poissonian current noise resulting from electron-phonon-coupling induced {\em selective unidirectional cascades of single-electron transitions}.

Observing Majorana bound States inp-Wave Superconductors Using Noise Measurements in Tunneling Experiments

The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors should behave like Majorana fermions. We introduce a model Hamiltonian that describes the tunneling process when electrons are injected into such states. Using a nonequilibrium Green function formalism, we find exact analytic expressions for the tunneling current and noise and identify experimental signatures of the Majorana nature of the bound states to be found in the shot noise. We discuss the results in the context of different candidate materials that support triplet superconductivity. Experimental verification of the Majorana character of midgap states would have important implications for the prospects of topological quantum computation.

Structured energy distribution and coherent ac transport in mesoscopic wires

Electron energy distribution in a mesoscopic ac-driven diffusive wire generally is not characterized by an effective temperature. At low temperatures, the distribution has a form of a multistep staircase, with the step width equal to the field energy quantum. Analytic results for the field frequency high and low compared to Thouless energy are presented, while the intermediate-frequency regime is analyzed numerically. Manifestations in the tunneling spectroscopy and noise measurements are discussed.

Wind Tunnel Aeroacoustic Tests of Six Airfoils for Use on Small Wind Turbines*

Aeroacoustic tests of seven airfoils were performed in an open jet anechoic wind tunnel. Six of the airfoils are candidates for use on small wind turbines operating at low Reynolds numbers. One airfoil was tested for comparison to benchmark data. Tests were conducted with and without boundary layer tripping. In some cases, a turbulence grid was placed upstream in the test section to investigate inflow turbulence noise. An array of 48 microphones was used to locate noise sources and separate airfoil noise from extraneous tunnel noise. Trailing-edge noise was dominant for all airfoils in clean tunnel flow. With the boundary layer untripped, several airfoils exhibited pure tones that disappeared after proper tripping was applied. In the presence of inflow turbulence, leading-edge noise was dominant for all airfoils.

Feedback of the electromagnetic environment on current and voltage fluctuations out of equilibrium

A theory is presented for low-frequency current and voltage correlators of a mesoscopic conductor embedded in a macroscopic electromagnetic environment. This Keldysh field theory evaluated at its saddle-point provides the microscopic justification for our earlier phenomenological calculation (using the cascaded Langevin approach). The nonlinear feedback from the environment mixes correlators of different orders, which explains the unexpected temperature dependence of the third moment of tunneling noise observed in a recent experiment. At non-zero temperature, current and voltage correlators of order three and higher are no longer linearly related. We show that a Hall bar measures voltage correlators in the longitudinal voltage and current correlators in the Hall voltage. We go beyond the saddle-point approximation to consider the environmental Coulomb blockade. We derive that the leading order Coulomb blockade correction to the n-th cumulant of current fluctuations is proportional to the voltage derivative of the (n+1)-th cumulant, generalizing to any n the earlier results for n=1,2.

Aeroacoustic Measurements

9R7. Aeroacoustic Measurements. - Edited by TJ Mueller (Dept of Aero and Mech Eng, Univ of Notre Dame, 112 Hessert Center, Notre Dame IN 46556-5684). Springer-Verlag, Berlin. 2002. 313 pp. ISBN 3-540-41757-5. $99.00.Reviewed by MG Prasad (Dept of Mech Eng, Stevens Inst of Tech, Hoboken NJ 07030).It is well known that experimental work plays an important role in acoustics. In particular, the acoustical measurements in the presence of airflow is challenging due to test conditions. This book deals with this challenging and important topic in acoustics namely aeroacoustics. The five chapters of the book deal with various aspects aeroacoustic measurements with applications. Chapter 1, on “Microphone measurements in and out of air stream,” deals with the acoustic characteristics of wind tunnels. The discussions include criteria for simulation of aeroacoustic test environment with requirements such as propagation effect, scaling, low background noise levels, etc. Chapter 2 presents “Beam forming in acoustic testing.” The use of beam forming technique and its applications in the modeling of wind tunnel noise with non-acoustic hard walls and free jets are discussed. Several beam forming algorithms with techniques for removing interference is discussed in this chapter. Chapter 3 covers “Aeroacoustic phased array testing in low speed wind tunnels.” A comprehensive treatment of the design and application of acoustic phased array measurement capability for low speed wind tunnels is presented. Various aspects of operation such as calibration, data handling, etc, are discussed. Chapter 4 is on “Source characterization by correlation techniques.” The characterization of acoustical sources using correlation techniques is presented. The chapter discusses various mathematical aspects through cross-correlation and coherence functions. Various examples including correlation measurements between surface pressure and far field acoustic pressure field data are included. The last chapter is “An anechoic facility for basic aeroacoustic research.” The chapter presents the design, construction, and performance evaluation of an anechoic wind tunnel facility for low speed and low turbulence applications. Use of such a facility to experimentally investigate fluid-solid interaction is discussed. A specific wind tunnel facility is described. As an example the use of the facility for a marine propeller response to inflow disturbance is discussed. All chapters are well presented. In summary, Aeroacoustic Measurements is a nice integration of presentations of various recognized contributors. The book deals with important issues in design and performance evaluation of wind tunnel test facilities, measurement techniques, and mathematical aspects. Nice photographs and figures are included. Each chapter has a large number of references. The book is recommended for libraries and all graduate students, researchers, and professionals in acoustics. In particular, those involved in experimental aeroacoustics will find this book very useful.

Time dependence of tunnel statistics and the energy resolution of superconducting tunnel junctions

Multiple tunneling of quasiparticle charge carriers in a superconducting tunnel junction (STJ) enhances the signal generated by a photon absorption event. It is also an additional source of noise, responsible for a substantial degradation of the energy resolution. Although tunneling is a binomial chance process, governed by a constant tunneling probability, the resulting cumulative statistics of tunnelled quasiparticles depend on time. In particular, the variance of the total number of tunneled quasiparticles reaches a minimum after a finite integration time, corresponding to a minimum in the spectral linewidth. Since the intrinsic energy resolution of the present generation of STJs is mainly limited by the scatter on the number of tunneled quasiparticles, the improvement of the tunnel noise can be experimentally tested by variation of the pulse integration time. An analytical theory is developed that describes the relation between the tunnel noise and the transfer function of the pulse integration hardware for an STJ characterized by a quasiparticle tunnel and loss time in each electrode. We present experiments that demonstrate that the noise contribution from multiple tunnelling is not constant during the time that the quasiparticles are present in the STJ, and that by proper filtering of the STJ pulses the tunnel noise can be optimized at a level which lies well below the canonical tunnel limit.

Multiple-tunneling noise in superconducting tunnel junctions from partial current integration

Superconducting tunnel junctions can be used as high-resolution particle or photon energy spectrometers. A photon absorbed in a superconductor breaks Cooper pairs into quasiparticles. These quasiparticles tunnel through the junction barrier and are detected as a pulse of excess current. Many junction designs allow the quasiparticles to tunnel more than once, an exponentially mixed Poisson process. However, multiple tunneling increases the fluctuation in the measured charge. We calculate the significance of these fluctuations algebraically as a function of time during the current pulse. We also calculate the finite integration window that minimizes the contribution of this noise. In addition, we calculate the effects of a low-pass amplifier and a Gaussian-shaping amplifier on the tunneling noise. With certain filtering time constants, the tunneling noise can be reduced while still providing some gain.

Technology development for directional acoustic arrays

The need for noise source localization and characterization has driven the development of advanced sound field measurement techniques using microphone arrays; however, the cost and complexity of these systems limit their widespread use. One common application is the study of aircraft noise in wind tunnels. The arrays typically consist of moderate numbers (<40) of conventional condenser microphones and range in size from several inches to several feet. Arrays consisting of more microphones and/or the use of multiple arrays may be needed to analyze complex multipole noise sources. However, the feasibility of scaling the current technology to multiple arrays with large numbers of microphones is limited by the cost per channel, array mobility, and data handling efficiency. A reduced cost, mobile, high speed 16-channel array has been constructed to demonstrate key technologies to address these scaling issues. The system is based on a hybrid package that combines a silicon piezoresistive microphone and amplification circuitry. Each package is socketed into a printed circuit board, providing electrical connections and structural support. High speed data acquisition and signal processing hardware is used to generate the array response in near real time. The results of the initial array calibrations are presented.