110,110 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
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
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
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
Experimental Investigation on the Influence of Yaw Angle on the Inlet Performance at Mach 7
No full textThis paper presents the results of an extensive measurement campaign that was conducted with an two-dimensional hypersonic inlet in the Hypersonic Windtunnel H2K at the German Aerospace Center in Cologne in order to study the effects of angle of yaw on the inlet performance. Both the external and internal flow paths of the inlet have been analyzed. Conducted measurements include static and Pitot pressure measurements, shadowgraph imagery, IR-thermography, Mach number and mass flow measurements. To simulate the behaviour of the inlet at different operating points, a throttle was used to impose different backpressures on the inlet
3D planar velocity measurements, using Mach-Zehnder interferometric-filter-based planar Doppler velocimetry (MZI-PDV) and imaging fibre bundles
No full textThree component planar flow-field measurements are made using imaging fibre
bundles to port different views of the measurement plane, defined by a laser
light sheet, to a single imaging head. The Doppler frequency shifts of light
scattered by particles entrained in the flow are transduced to intensity
variations using a Mach-Zehnder interferometric filter. The free spectral range
of the filter can be selected by adjusting the optical path difference of the
interferometer. This allows the velocity measurement range, sensitivity and
resolution to be varied. Any laser wavelength may be used. A phase locking
system has been designed to stabilise the interferometric filter
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
Low-mach number effects and late-time treatment of Richtmyer-Meshkov and Rayleigh-Taylor instabilities
No full textThe Richtmyer-Meshkov instability appears when the mixing between two fluids is triggered
by the passage of a shock wave. It occurs in a range of different applications,
such as astrophysics, inertial confinement fusion and supersonic combustion. Due to
the extreme complexity of this phenomenon to be reproduced in a controlled environment,
its study heavily relies on numerical methods. The presence of a shock wave
as a triggering factor requires the use of compressible solvers, but once the shock has
started the mixing process, the flow field freely decays and becomes incompressible.
The dynamics of this instability is still to be fully understood, especially its long-time
behaviour. One of the hypothesis is that the mixing layer achieves a self-similar development
at some point during its evolution. However, the low-Mach flow at late-times
does not always allow to push compressible simulations so far in time and when it is
possible, they become extremely demanding from a computational point of view. In
fact, it is known that standard compressible methods fail when the Mach number of
the numerical field is low and moreover they lose time-marching efficiency.
In this thesis, a new approach to the study of the very late-stage of the instability
through the use of ILES is presented. The technique consists in starting the simulation
by using the compressible model and to initialise the incompressible solver when
the compressibility of the numerical field becomes sufficiently low. This allows to bypass
the issues previously mentioned and study the very late-stage of the instability
at reasonable computational costs. For this purpose, a new incompressible solver that
employs high-resolution methods and which is based on the pressure-projection technique
is developed. A number of different Riemann-solvers and reconstruction schemes
are tested against experiments using the incompressible, impulsive version of RMI as
test case. Two alternative methods are considered for triggering the mixing: velocity
impulse and gravity pulse. Excellent results were obtained by using the former, whereas
discrepancies were noticed when the latter was employed. Comparisons against numerical
simulations in the literature allowed to identify the inviscid nature of the solver as
the cause of these differences. However, this did not affect the capability of the solver
to correctly compute multi-mode cases, in which viscosity is negligible. A preliminary
study on the compressibility of the numerical field in time proved the feasibility of
the numerical transition and a switching criterion based on the Mach number was established.
The approach was therefore tested on a single-mode perturbation case and
compared against compressible simulation. Very good agreement was found in the prediction
of the growth of the instability and the analysis of the divergence of velocity of
the numerical field proved the incompressibility of the solution generated by the hybrid
solver. Finally, the approach was applied to multi-mode test cases. Excellent agreement
with the theory was found. The turbulent kinetic energy presented a modified
subinertial range and the growth exponent was very close to fully compressible predictions
and experiments. Deeper results analysis showed against compressible simulations
showed very good agreement on the flow physics. In fact, the instability settled to a
self-similar regime with the same time-scale predicted by compressible analysis, but
the simulated time reached by the hybrid solver was three times longer. The results
obtained proved the applicability of the approach, opening to new possibilities for the
study of the instability
Sounds Like Light: Einstein's Theory of Special Relativity and Mach's Work in Acoustics and Aerodynamics
No full textErnst Mach is the only person whom Einstein included on both the list of physicists he considered his true precursors, and the list of the philosophers who had most affected him. Einstein scholars have been less generous in their estimation of Mach's contributions to Einstein's work, and even amongst the more generous of them, Mach's great achievements in physics are seldom mentioned in this context. This is odd, considering Mach was nominated for the Nobel Prize in Physics three times. In this paper, I examine some of Mach's work in physics that bears conceptually on Einstein's 1905 paper on Special Relativity ("On The Electrodynamics of Moving Bodies"). Mach was the first to give the correct explanation of the Doppler Effect, and he presented it in a way that Einstein echoes in his 1905 paper: laying out two apparently contradictory principles and showing how both can be retained. It is also notable that Mach's explanation was explicit about not relying on the existence of a medium of transmission for the propagation of light waves. In his work on supersonic shock waves, Mach invokes the constancy of the velocity of sound (i.e., its independence of the motion of the sound source) , just as he had invoked the constancy of the velocity of light in his work on the Doppler Effect for Light. I examine the analogies between light and sound that were drawn upon by Einstein and Mach, as well as one analogy that Einstein could have, but did not make: Cherenkov radiation, or "singing electrons", i.e., cases in which the sound of light in the medium of transmission is exceeded, which results in an optical analogue of supersonic shock waves
High Pressure Hugoniot Measurements in Solids Using Mach Reflections
No full textShock compression experiments provide access to high pressures in a laboratory setting. Matter at extreme pressures is often studied by utilizing a well controlled planar impact between two flat plates to generate a one dimensional shock wave. While these experiments are a powerful tool in equation of state (EOS) development, they are inherently limited by the velocity of the impacting plate. In an effort to dramatically increase the range of pressures which can be studied with available impact velocities, a new experimental technique is examined. The target plate is replaced by a composite assembly consisting of two concentric cylinders and is designed such that the initial shock velocity in a well characterized outer cylinder is higher than in the inner cylinder material of interest. After impact, conically converging shocks are generated at the interface due to the impedance mismatch between the two materials and the axisymmetric geometry. Upon convergence, an irregular reflection occurs and the conical analog of a Mach reflection develops. This Mach reflection grows until it reaches a steady state, for which an extremely high pressure state is concentrated behind the Mach stem. The reflection is studied using a combination of analytical, numerical, and experimental techniques. Ideas from gas dynamics, such as shock polars, are connected to the classic treatment of one-dimensional shocks in solids to form a simple method for treating the oblique reflections in the Mach lens configuration. Numerical simulations provide detailed full-field solutions and illustrate a methodology for extracting EOS information. The technique is validated experimentally by studying the shock response of copper and iron. Two different confining materials, 6061-T6 aluminum and molybdenum, are used to drive the converging shock waves for which the high pressure state is measured through a combination of velocity interferometry and impedance matching techniques
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