118 research outputs found
Assessing the sensitivity of turbine cascade flow to inflow disturbances using direct numerical simulation
Identification of large coherent structures in supersonic axisymmetric wakes
Direct numerical simulation data of supersonic axisymmetric wakes are analysed for the existence of
large coherent structures. Wakes at Ma ¼ 2:46 are considered with results being presented for cases at
Reynolds numbers ReD ¼ 30; 000 and 100,000. Criteria for identification of coherent structures in freeshear
flows found in the literature are compiled and discussed, and the role of compressibility is
addressed. In particular, the ability and reliability of visualisation techniques intended for incompressible
shear-flows to educe meaningful structures in supersonic wakes is scrutinised. It is shown that some of
these methods retain their usefulness for identification of vortical structures as long as the swirling rate is
larger than the local compression and expansion rates in the flow field. As a measure for the validity of
this condition in a given flow the ‘vortex compressibility parameter’ is proposed which is derived here.
Best ‘visibility’ of coherent structures is achieved by employing visualisation techniques and proper
orthogonal decomposition in combination with the introduction of artificial perturbations (forcing of
the wake). The existence of both helical and longitudinal structures in the shear layer and of hairpin-like
structures in the developing wake is demonstrated. In addition, elongated tubes of streamwise vorticity
are observed to emanate from the region of recirculating flo
Factors regulating recruitment from the sediment to the water column in the bloom-forming cyanobacterium Gloeotrichia echinulata
All-sky search for long-duration gravitational wave transients with initial LIGO
We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10-500 s in a frequency band of 40-1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10-5 and 9.4×10-4 Mpc-3 yr-1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves. © 2016 American Physical Society
All-sky search for long-duration gravitational wave transients with initial LIGO
We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10-500 s in a frequency band of 40-1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10-5 and 9.4×10-4 Mpc-3 yr-1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves. © 2016 American Physical Society
Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background
The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society
THE RATE OF BINARY BLACK HOLE MERGERS INFERRED FROM ADVANCED LIGO OBSERVATIONS SURROUNDING GW150914
A transient gravitational-wave signal, GW150914, was identi
fi
ed in the twin Advanced LIGO detectors on 2015
September 2015 at 09:50:45 UTC. To asse
ss the implications of this discovery,
the detectors remained in operation with
unchanged con
fi
gurations over a period of 39 days around the time of t
he signal. At the detection statistic threshold
corresponding to that observed for GW150914, our search of the 16 days of simultaneous two-detector observational
data is estimated to have a false-alarm rate
(
FAR
)
of
<
́
--
4.9 10 yr
61
, yielding a
p
-value for GW150914 of
<
́
-
210
7
. Parameter estimation follo
w-up on this trigger identi
fi
es its source as a binary black hole
(
BBH
)
merger
with component masses
(
)(
)
=
-
+
-
+
mm
M
,36,29
12
4
5
4
4
at redshift
=
-
+
z
0.09
0.04
0.03
(
median and 90% credible range
)
.
Here, we report on the constraints these observations place on the rate of BBH coalescences. Considering only
GW150914, assuming that all BBHs in the universe have the same masses and spins as this event, imposing a search
FAR threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a
90% credible range of merger rates between
–
--
2
53 Gpc yr
31
(
comoving frame
)
. Incorporating all search triggers that
pass a much lower threshold while accounting for the uncerta
inty in the astrophysical origin of each trigger, we estimate
a higher rate, ranging from
–
--
13 600 Gpc yr
31
depending on assumptions about the BBH mass distribution. All
together, our various rate estimat
es fall in the conservative range
–
--
2
600 Gpc yr
31
Efficient parallel computing with a compact finite difference scheme
This paper proposes an efficient parallel computing approach based on a high-order accurate compact finite difference scheme in conjunction with a conventional domain decomposition method and MPI libraries. The proposed parallel computing approach consists of two major features: (a) a newly developed compact finite difference scheme with extended stencils containing halo points around subdomain boundaries, and (b) a predictor–corrector type implementation of a compact filter that effectively suppresses spurious errors from the subdomain boundaries. The current work employs three halo cells for the inter-node communication, based on which the coefficients of the new compact scheme at the subdomain boundaries are optimized to achieve as high level of resolution and accuracy as the interior compact scheme provides. Also, an optimal set of cut-off wavenumbers of the compact filter that minimizes spurious errors is suggested. It is shown that the level of errors from the proposed parallel calculations lies within the same order of magnitude of that from the single-domain serial calculations. The overall accuracy and linear stability of the new parallel compact differencing-filtering system are confirmed by grid convergence tests and eigenvalue analyses. The proposed approach shows a substantial improvement with respect to existing methods available
Tandem cylinder flow and noise predictions using a hybrid RANS/LES approach
The performance of a novel hybrid RANS/LES methodology for accurate flow and noise predictions of the NASA Tandem Cylinder Experiment is investigated. The proposed approach, the modified Flow Simulation Methodology (FSM), is based on scaling the turbulence viscosity and the turbulence kinetic energy dissipation rate with a damping function. This damping function consists of three individual components, a function based on the Kolmogorov length-scale ensuring correct behaviour in the direct numerical simulation (DNS) limit, a function ensuring that FSM provides the correct damping in large-eddy simulation (LES) mode, and a shielding function that forces the switch from Reynolds-Averaged Navier–Stokes (RANS) to LES to occur outside the boundary layer. The FSM is proposed for the k ??-SST two-equation model (FSM-SST) and for an Explicit-Algebraic-Stress-Model (FSM-EASM), which is better suited to resolve anisotropy and non-equilibrium of the unresolved scales and the strain and rotation-rate dependent coefficients introduce a dynamic response of the model to the resolved flow field. Simulations are performed on a relatively coarse grid and the FSM data are compared with results obtained from the Scale-Adaptive-Simulation (SAS) and IDDES approaches. Acoustic predictions are obtained using an acoustic analogy approach based on Curle’s theory. The FSM-SST approach was found to predict the hydrodynamic field in very good agreement with reference data, whereas the FSM-EASM did not improve the predictions. The acoustic spectra predicted show good agreement with experimental results at various microphone positions, with some deficiencies in capturing the broadband noise levels at high Strouhal numbers
Compressible direct numerical simulation of low-pressure turbines: part II - effect of inflow disturbances
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