311 research outputs found
Large scale optimization of transonic axial compressor rotor blades
[First Paragraphs]
In the present work the Multipoint Approximation Method (MAM) by Toropov et al. (1993) has been applied to
the shape optimization of an existing transonic compressor rotor (NASA rotor 37) as a benchmark case.
Simulations were performed using the Rolls-Royce plc. PADRAM-HYDRA system (Shahpar and Lapworth 2003,
Lapworth and Shahpar 2004) that includes the parameterization of the blade shape, meshing, CFD analysis, postprocessing,
and objective/constraints evaluation. The parameterization approach adopted in this system is very
flexible but can result in a large scale optimization problem.
For this pilot study, a relatively coarse mesh has been used including around 470,000 nodes. The
parameterization was done using 5 engineering blade parameters like axial movement of sections along the engine
axis in mm (XCEN), circumferential movements of sections in degrees (DELT), solid body rotation of sections in
degrees (SKEW), and leading/trailing edge recambering (LEM0/TEMO) in degrees. The design variables were
specified using 6 control points at 0 % (hub), 20%, 40%, 60%, 80%, and 100% (tip) along the span. Thus the total
number of independent design variables N was 30. B-spline interpolation was used through the control points to
generate smooth design perturbations in the radial direction
Universal threshold for femtosecond laser ablation with oblique illumination
We quantify the dependence of the single-shot ablation threshold on the angle
of incidence and polarization of a femtosecond laser beam, for three dissimilar
solid-state materials: a metal, a dielectric and a semiconductor. Using the
constant, linear value of the index of refraction, we calculate the laser
fluence transmitted through the air-material interface at the point of ablation
threshold. We show that, in spite of the highly nonlinear ionization dynamics
involved in the ablation process, the so defined transmitted threshold fluence
is universally independent of the angle of incidence and polarization of the
laser beam for all three material types. We suggest that angular dependence of
ablation threshold can be utilized for profiling fluence distributions in
ultra-intense femtosecond laser beams.Comment: 4 pages, 5 figure
Experimental Tests of the New Paradigm for Laser Filamentation in Gases
Since their discovery in the mid-1990s, ultrafast laser filaments in gases
have been described as products of a dynamic balance between Kerr self-focusing
and defocusing by free electric charges that are generated via multi-photon
ionization on the beam axis. This established paradigm has been recently
challenged by a suggestion that the Kerr effect saturates and even changes sign
at high intensity of light, and that this sign reversal, not free-charge
defocusing, is the dominant mechanism responsible for the extended propagation
of laser filaments. We report qualitative tests of the new theory based on
electrical and optical measurements of plasma density in femtosecond laser
filaments in air and argon. Our results consistently support the established
paradigm.Comment: 4 pages, 4 figure
Ionization clamping in ultrafast optical breakdown of transparent solids
We formulate a multi-physics model to describe the nonlinear propagation of a
femtosecond, near-infrared, tightly focused laser pulse in a transparent
dielectric. The application of our model to the case of bulk sapphire shows
that even under extreme excitation conditions, ionization is universally
clamped at about one tenth of the electron density in the upper valence band.
The earlier estimate of ~10 TPa pressure that could be attainable through the
internal excitation of transparent dielectrics by tightly focused ultrafast
laser beams is shown to be off by two orders of magnitude
Measurements of fluence profiles in femtosecond laser filaments in air
We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our ap- proach is based on the dependence of single-shot laser abla- tion threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser fila- ments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the ex- ternal beam focusing. The limiting value of the peak inten- sity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f -number above âŒ500), is about 55 TWâcm2
A compact Airy beam light sheet microscope with a tilted cylindrical lens
We thank the UK Engineering and Physical Sciences Research Council under grant EP/J01771X/1, the âBRAINSâ 600th anniversary appeal and Dr. E. Killick for funding.Light-sheet imaging is rapidly gaining importance for imaging intact biological specimens. Many of the latest innovations rely on the propagation-invariant Bessel or Airy beams to form an extended light sheet to provide high resolution across a large field of view. Shaping light to realize propagation-invariant beams often relies on complex programming of spatial light modulators or specialized, custom made, optical elements. Here we present a straightforward and low-cost modification to the traditional light-sheet setup, based on the open-access light-sheet microscope OpenSPIM, to achieve Airy light-sheet illumination. This brings wide field single-photon light-sheet imaging to a broader range of endusers. Fluorescent microspheres embedded in agarose and a zebrafish larva were imaged to demonstrate how such a microscope can have a minimal footprint and cost without compromising on imaging quality.Publisher PDFPeer reviewe
Curved plasma channels: Kerr lens and Airy prism
We analytically calculate the transverse energy fluxes that would be
respectively induced in high-power Airy beams by the Kerr self-focusing and the
Airy profile itself if they were the only active process. In experimental
condition representative of laser filamentation experiments of high-power
ultrashort laser pulses in air and condensed media, the Kerr lens induces
transverse energy fluxes much larger than the Airy "prism" at the main peak. As
a consequence, the curved plasma channels in Airy beams are not only a plasma
spark on a curved focus, but indeed self-guided filaments, and their curved
trajectory appears as a perturbation due to the linear Airy propagation regime.Comment: 10 pages, 4 figures (6 panels
Effect of a plasma grating on pump-probe experiments near the ionization threshold in gases
Calculations are performed of the phase shift caused by the spatial
modulation in the plasma density due to interference between a strong pump
pulse and a weak probe pulse. It is suggested that a recent experiment [Loriot
et al., Opt. Express v. 17, 13429 (2009)] observed an effective birefringence
from this plasma grating rather than from the higher-order Kerr effect.Comment: 3 pages, 1 figure. Fix typos and correct number
Sending femtosecond pulses in circles: highly non-paraxial accelerating beams
We use caustic beam shaping on 100 fs pulses to experimentally generate
non-paraxial accelerating beams along a 60 degree circular arc, moving
laterally by 14 \mum over a 28 \mum propagation length. This is the highest
degree of transverse acceleration reported to our knowledge. Using diffraction
integral theory and numerical beam propagation simulations, we show that
circular acceleration trajectories represent a unique class of non-paraxial
diffraction-free beam profile which also preserves the femtosecond temporal
structure in the vicinity of the caustic
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