23,144 research outputs found
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
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
Peter M. Mach
No full textabstract: Peter was 12 years old when he heard gunshots and bombing at his village.
“Lost Boys Found” is an ongoing, interdisciplinary project that is collecting, recording and archiving the oral histories of the Lost Boys/Girls of Sudan. The collection is a work-in-progress, seeking to record the oral history of as many Lost Boys/Girls as are willing, and will be used in a future book.Age: 28Region: Upper NileThis picture and bio was donated to the Lost Boys Found project from The Arizona Lost Boys Cente
Modelling of all-optical symmetric Mach-Zehnder switch with asymmetric coupler
No full textUltra high-speed optical network is developing rapidly as growing capacity demand in telecommunication system is increasing. In these networks, it is desired to carry out switching, routing and processing in optical domain to avoid bottlenecks of optoelectronic conversions. Optical time-division multiplexing (OTDM) technique is one option to implement all optical networks. It provides a single data stream at a very high rate (>100Gbits/s) using a single wavelength. These networks will be based on optical packet switching. The success of these networks depends on how well switching and routing are being done at this very high speed. An all optical switch based on symmetric Mach-Zehnder (SMZ) with asymmetric coupler (60:40) is proposed. Its characteristics and switching window profiles will be investigated. The results show that symmetric Mach-Zehnder (SMZ) with asymmetric coupler gives a better contrast ratio rather than symmetric Mach-Zehnder (SMZ) with normal 50:50 coupler
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
Cure monitoring of a UV cured epoxy resin using a long period grating Mach- Zehnder interferometer
No full textA cascaded long period grating Mach-Zehnder interferometer is used to monitor
the change in refractive index of a UV cured epoxy resin over a cure cycle.
Fourier techniques are used to calculate the phase shift and frequency spectral
amplitude of the associated fringe pattern during the cure. The results are
compared with the refractive index change during cure calculated using a Fresnel
reflection based technique
Mach Number Estimation and Pressure Profile Measurements of Expanding Dense Organic Vapors
No full textThis paper describes an experiment conducted within the nozzle test section of the Organic Rankine Cycle Hybrid Integrated Device (ORCHID) aimed at providing accurate data for the validation of NICFD flow solvers [5]. A supersonic flow of the dense vapor siloxane MM established in the nozzle of the setup was characterized by means of the schlieren technique and by pressure taps along the nozzle profile. The nozzle inlet conditions corresponded to a stagnation temperature and pressure of T0=253∘C and P0=18.36bara. At these inlet conditions, the compressibility factor of the fluid is Z0= 0.58. The nozzle backpressure was equal to Pb=2.2bara. The experimental data-set includes: 1) the average mid-plane local Mach number, which was derived from the schlieren images by estimating the angle of the Mach waves originating from the roughness of the upper and lower nozzle surfaces, 2) the angle of a shock wave generated by a 5∘ wedge placed at the nozzle exit, also detectable in the schlieren images, and 3) the static pressure distribution along the flow expansion acquired with a Scanivalve DSA3218 pressure scanner device. The Mach number at the nozzle exit estimated based on the schlieren images is M= 1.95 ± 0.05, very close to the expected value of M= 2 according to the design conditions of the experiment. The static pressure measurements have a maximum absolute uncertainty amounting to ± 1.80 kPa in the initial stages of the expansion. This information was used to assess the capability of the open-source SU2 flow solver in evaluating the NICFD effects in a supersonic flow of MM when the fluid thermodynamic properties are modeled with a cubic equation of state. For this purpose, two-dimensional Euler simulations were carried out with SU2 for the operating conditions achieved in the experiment. The numerical results are in good agreement with the experimental data. The largest deviation between the simulation and experiment is observed in the nozzle uniform region, where two dips in the Mach number occur due to a slight local decrease in flow velocity owing to two weak shock waves. The shock wave generated by the wedge located at the nozzle outlet propagates with two different angles, namely, βabove= 37. 6∘± 0.86, and βbelow= 31. 6∘± 0.64, due to the axial misalignment of the wedge with respect to the flow.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Facility Aerodynamics LaboratoryAerodynamicsFlight Performance and Propulsio
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
Component Analysis of TBCC Propulsion for a Mach 4.5 Supersonic Cruise Airliner
No full textThis paper describes the status of the study on component analysis for the different Variable TBCC
cycle configurations. The paper investigates different Variable Cycle TBCC configurations and compares
them with an advanced turbojet for the generic configuration of a Mach 4.5 supersonic passenger
airliner.
One VCE engine variant and the turbojet are preliminarily designed and their mass including airintake
and nozzle is estimated. The air-intake has been preliminarily sized and pressure recovery and
mass flow in design and off-design conditions is estimated. The intake's dimension and airflow data
have been subsequently delivered for further analyses. Engine weight analysis is also conducted both
for existing engines and proposed LAPCAT turbo engines
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