322,362 research outputs found
Stabilized fiber-optic Mach–Zehnder interferometer for carrier-frequency rejection
No full textWe have demonstrated stabilization of a fiber-optic Mach–Zehnder interferometer, with a centimeter-scale path difference, to the transmission minimum for the carrier wave of a frequency-modulated laser beam. A time-averaged extinction of 32 dB, limited by the bandwidth of the feedback, was maintained over several hours. The interferometer was used to remove the carrier wave from a 780 nm laser beam that had been phase modulated at 2.7 GHz
Transition Between Regular Reflection and Mach Reflection in the Dual-Solution Domain
No full textA study of the shock-reflection domain for steady flow is presented. Conditions defining boundaries between different possible shock-reflection solutions are given, and where possible, simple analytic expressions for these conditions are presented. A new, more accurate estimate of the steady-state Mach stem height is derived based on geometric considerations of the flow. In particular, the location of the sonic throat through which the subsonic convergent flow behind the Mach stem is accelerated to divergent supersonic flow is considered. Comparisons with previous computational and experimental work show that the theory presented in this thesis more accurately predicts the Mach stem height than previous theories. The Mach stem height theory is generalized to allow for a moving triple point. Based on this moving triple point theory, a Mach stem growth rate theory is developed. This theory agrees well with computational and experimental results. Numerical computations of the effects of water vapor disturbances are also presented. These disturbances are shown to be sufficient to cause transition from regular reflection to Mach reflection in the dual-solution domain. These disturbances are also modeled as a simple energy deposition on one of the wedges, and an estimate for the minimum energy required to cause transition is derived.
Experimental results using an asymmetric wedge configuration in the Ludwieg tube facility at the California institute of Technology are presented. A Mach 4.0 nozzle was designed and built for the Ludwieg tube facility. This Mach number is sufficient to provide a large dual-solution domain, while being small enough not to require preheating of the test gas. The test time of the facility is 100ms, which requires the use of high-speed cinematography and a fast motor to rotate one of the two wedges. Hysteresis in the transition between regular to Mach reflection was successfully demonstrated in the Ludwieg tube facility. The experiments show that regular reflection could be maintained up to a shock angle approximately halfway between the von Neumann condition and the detachment condition.
Energy deposition studies were performed using an Nd:YAG laser. Triggering transition in this manner is found to depend on the location of the energy deposition. This finding is consistent with the numerical work presented in this thesis. Experiments were also performed to measure the Mach stem height and its growth rate. These results are compared with the theoretical estimates presented in this thesis. Excellent agreement between the steady-state Mach stem height and the theoretical estimates is seen. Comparisons of Mach stem growth rate with theoretical estimates show significant differences, but do show good agreement regarding the time required to reach the steady-state height.</p
Accounting for convective effects in zero-Mach-number thermoacoustic models
No full textThis paper presents a methodology to account for some mean-flow effects on thermo-acoustic instabilities when using the zero-Mach-number assumption. It is shown that when a computational domain is represented under the M=0 assumption, a nonzero-Mach-number element can simply be taken into account by imposing a proper acoustic impedance at the boundaries so as to mimic the mean flow effects in the outer, not computed flow domain. A model that accounts for the coupling between acoustic and entropy waves is presented. It relies on a “delayed entropy coupled boundary condition” (DECBC) for the Helmholtz equation satisfied by the acoustic pressure. The model proves able to capture low-frequency entropic modes even without mean-flow terms in the fluctuating pressure equation
Low Mach Asymptotic Preserving Scheme for the Euler-Korteweg Model
No full textWe present an all speed scheme for the Euler-Korteweg model.
We study a semi-implicit time-discretisation which treats the terms, which are stiff for low Mach numbers, implicitly and thereby avoids a dependence of the timestep restriction on the Mach number.
Based on this we present a fully discrete finite difference scheme.
In particular, the scheme is asymptotic preserving, i.e., it converges to a stable discretisation of the incompressible limit
of the Euler-Korteweg model when the Mach number tends to zero
Mid-infrared photonics devices in SOI
No full textIn this paper we present silicon photonics devices designed for the 3-4µm wavelength region including waveguides, MMIs, ring resonators and Mach-Zehnder interferometers. The devices are based on silicon on insulator (SOI) platform. We show that 400-500nm high silicon waveguides can have propagation losses as low as ~4dB/cm at 3.8µm. We also demonstrate MMIs with insertion loss of 0.25 dB, high extinction ratio asymmetric Mach-Zehnder interferometers, and SOI ring resonators. This combined with our previous results reported at 3.4µm confirm that SOI is a viable platform for the 3-4µm region and that low loss mid-infrared passive devices can be realized on it. Keywords: Mid-infrared, silicon, silicon-on-insulator, Mach-Zehnder interferometer, multimode interference, ring resonator
Optisch-akustische Versuche. Die spectrale und stroboskopische Untersuchung tönender Körper
No full textvon E. Mac
Ernst Mach interprete di Pierre Duhem. Valore e limiti della sperimentazione mentale
No full textLo stereotipo interpretativo dominante che concerne la critica di Duhem alle "experiences fictives" — secondo cui questo autore, in La théorie physique, avrebbe semplicemente rifiutato ogni genere d'esperimento mentale nel senso di Mach — è insostenibile per più ragioni. Esso, infatti, è in conflitto 1) con le dichiarazioni e i silenzi di Mach; 2) con ciò che Duhem disse e ciò che non disse riguardo ai punti di accordo e di disaccordo fra la propria critica della sperimentazione mentale e la concezione di Mach; 3) con una lettura priva di preconcetti delle poche pagine dedicate da Duhem alla critica delle experiences fictives. Possiamo quindi condudere che è stato commesso un errore fondamentale nell'interpretare Duhem nel senso di un rifiuto indiscriminato della sperimentazione mentale. Il fatto che Duhem stesso non abbia tracciato esplicitamente e chiarito la distinzione fra le espressioni "expérience fictive" e "expéri-mentation mentale", è stata probabilmente la principale ragione che ha favorito il sorgere dello stereotipo tradizionale che ho cercato di criticare, cui hanno però forse contribuito anche le difficoltà connesse alla traduzione dell'aggettivo "fictif-, ve" in lingue filosoficamente rilevanti, le quali hanno fatto andare perduta una connotazione importante della scelta di Duhem, e cioè quella che concerne una sperimentazione mentale che trascura o sottovaluta i limiti della convenzionalità delle sue assunzioni fondamentali, contrabbandando per merce scientifica ciò che è o uso logico-matematico della ragione o, addirittura, uso "metafisico" nel senso negativo in cui Mach e Duhem usavano questo termine
Lattice Boltzmann method for compressible flows with high Mach numbers
No full textIn this paper we present a lattice Boltzmann model to simulate compressible flows by introducing an attractive force. This scheme has two main advantages: one is to soften sound speed effectively, which greatly raises the Mach number (up to 5); another is its relative simple procedure. Simulations of the March cone and the comparison between theoretical expectations and simulations demonstrate that the scheme is effective in the simulation of compressible flows with high Mach numbers, which would create many new applications
A Mach-uniform algorithm: coupled versus segregated approach
No full textA Mach-uniform algorithm is an algorithm with a good convergence rate for any level of the Mach number. In this paper, the severe time step restriction for low speed flows is removed by treating the acoustic and diffusive terms implicitly. After identification of these terms in the conservative set, we end up with a semi-implicit system. The way to solve this system can be chosen. Three different solution techniques are presented: a fully coupled algorithm, the coupled pressure and temperature correction algorithm from [K. Nerinckx, J. Vierendeels and E. Dick. Mach-uniformity through the coupled pressure and temperature correction algorithm. Journal of Computational Physics, 206(2) (2005) 597-623], and a fully segregated pressure-correction algorithm. We analyse the convergence behavior of the considered algorithms in some typical flow problems. Moreover, a Fourier stability analysis is done for each of the algorithms. For inviscid flow, the fully segregated and the fully coupled algorithm need about as much time steps to reach steady state. Therefore, the more segregation is introduced, the cheaper the calculation can be done. In case of heat transfer, the fully segregated pressure-correction algorithm suffers from a diffusive time step limit. This is not the case for the semi-segregated coupled pressure and temperature correction algorithm. Finally, when the gravity terms play an important role, only the fully coupled algorithm can avoid an additional time step restriction
Sound radiation in turbulent channel flows
No full textLighthill’s acoustic analogy is formulated for turbulent channel flow with pressure as the acoustic variable, and integrated over the channel width to produce a two-dimensional inhomogeneous wave equation. The equivalent sources consist of a dipole distribution related to the sum of the viscous shear stresses on the two walls, together with monopole and quadrupole distributions related to the unsteady turbulent dissipation and Reynolds stresses respectively. Using a rigid-boundary Green function, an expression is found for the power spectrum of the far-field pressure radiated per unit channel area. Direct numerical simulations (DNS) of turbulent plane Poiseuille and Couette flow have been performed in large computational domains in order to obtain good resolution of the low-wavenumber source behaviour. Analysis of the DNS databases for all sound radiation sources shows that their wavenumber–frequency spectra have non-zero limits at low wavenumber. The sound power per unit channel area radiated by the dipole distribution is proportional to Mach number squared, while the monopole and quadrupole contributions are proportional to the fourth power of Mach number. Below a particular Mach number determined by the frequency and radiation direction, the dipole radiation due to the wall shear stress dominates the far field. The quadrupole takes over at Mach numbers above about 0.1, while the monopole is always the smallest term. The resultant acoustic field at any point in the channel consists of a statistically diffuse assembly of plane waves, with spectrum limited by damping to a value that is independent of Mach number in the low-M limit
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