906 research outputs found
The Screen representation of spin networks: 2D recurrence, eigenvalue equation for 6j symbols, geometric interpretation and Hamiltonian dynamics
This paper treats 6j symbols or their orthonormal forms as a function of two
variables spanning a square manifold which we call the "screen". We show that
this approach gives important and interesting insight. This two dimensional
perspective provides the most natural extension to exhibit the role of these
discrete functions as matrix elements that appear at the very foundation of the
modern theory of classical discrete orthogonal polynomials. Here we present 2D
and 1D recursion relations that are useful for the direct computation of the
orthonormal 6j, which we name U. We present a convention for the order of the
arguments of the 6j that is based on their classical and Regge symmetries, and
a detailed investigation of new geometrical aspects of the 6j symbols.
Specifically we compare the geometric recursion analysis of Schulten and Gordon
with the methods of this paper. The 1D recursion relation, written as a matrix
diagonalization problem, permits an interpretation as a discrete
Schr\"odinger-like equations and an asymptotic analysis illustrates
semiclassical and classical limits in terms of Hamiltonian evolution.Comment: 14 pages,9 figures, presented at ICCSA 2013 13th International
Conference on Computational Science and Applicatio
The screen representation of vector coupling coefficients or Wigner 3j symbols: exact computation and illustration of the asymptotic behavior
The Wigner symbols of the quantum angular momentum theory are related to
the vector coupling or Clebsch-Gordan coefficients and to the Hahn and dual
Hahn polynomials of the discrete orthogonal hyperspherical family, of use in
discretization approximations. We point out the important role of the Regge
symmetries for defining the screen where images of the coefficients are
projected, and for discussing their asymptotic properties and semiclassical
behavior. Recursion relationships are formulated as eigenvalue equations, and
exploited both for computational purposes and for physical interpretations.Comment: 14 pages, 8 figures, presented at ICCSA 2014, 14th International
Conference on Computational Science and Application
Definition of a benchmark for low Reynolds number propeller aeroacoustics
Experimental and numerical results of a propeller of 0.3 m diameter operated at 5000 RPM and axial velocity ranging from 0 to 20 m/s and advance ratio ranging from 0 to 0.8 are presented as a preliminary step towards the definition of a benchmark configuration for low Reynolds number propeller aeroacoustics. The corresponding rotational tip Mach number is 0.23 and the Reynolds number based on the blade sectional chord and flow velocity varies from about 46000 to 106000 in the operational domain and in the 30% to 100% blade radial range. Force and noise measurements carried out in a low-speed semi-anechoic wind-tunnel are compared to scale-resolved CFD and low-fidelity numerical predictions. Results identify the experimental and numerical challenges of the benchmark and the relevance of fundamental research questions related to transition and other low Reynolds number effects
Towards the definition of a benchmark for low Reynolds number propeller aeroacoustics
Experimental and numerical results of a propeller of 0.3 m diameter operated in
quiescent standard ambient conditions at 5000 RPM and axial velocity ranging from 0 to 20
m/s and advance ratio ranging from 0 to 0.8 are presented as a preliminary step towards the
definition of a benchmark configuration for low Reynolds number propeller aeroacoustics. The
corresponding rotational tip Mach number is 0.231 and the Reynolds number based on the blade
sectional chord and flow velocity in the whole radial and operational domain ranges from about
54000 to 106000. Force and noise measurements carried out in a low-speed semi-anechoic windtunnel are compared with scale-resolved CFD and low-fidelity numerical results. Results identify
the experimental and numerical challenges of the benchmark and the relevance of fundamental
research questions related to transition and other low Reynolds number effects
Creating a benchmark of vertical axis wind turbines in dynamic stall for validating numerical models
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PIV investigation of the 3D instantaneous flow organization behind a micro-ramp in a supersonic boundary layer
The flow field resulting from a single micro-ramp in a Ma=2.0 supersonic boundary layer is investigated using tomographic particle image velocimetry (Tomo-PIV). The measurements were carried out within two volumes behind the micro-ramp. Mean flow characteristics are analyzed, evidence of the streamwise vortex pair is given. In the instantaneous flow, a Kelvin-Helmholtz (K-H) instability developing in the wake of the element is revealed, and the K-H vortices are further identified. A conceptual model is provided to describe the instantaneous flow organization generated by a single micro-ramp
Low-order acoustic prediction tool for estimating noise emissions from distributed propeller configurations
A low-order numerical tool for estimating noise emissions from distributed propeller
configurations is presented. The paper describes the tool’s computational framework, which
uses Hanson’s near-field theory to calculate the thickness and loading noise components. The
formulation assumes steady blade loading, but an unsteady case can be handled numerically by
redefining the pressure distribution over the blade at each new time step. Two representative
cases are analyzed to validate the tool: an isolated propeller operating in uniform flow and
an array of three propellers in a side-by-side configuration under aerodynamic interference
caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to
high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed
propeller system. The low-fidelity tool provides accurate results for both cases, with less than a
1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise
predictions at an observer located 10 diameters away and at the propeller plane. When analyzing
the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the
propeller plane. However, this difference increases to 6 dB as the observer moves toward the
inflow direction. This difference is due to the dominance of broadband noise near the propeller
axis. The paper concludes with a noise analysis of the distributed propeller system, examining
the relative importance of aerodynamic interference in the noise emitted by a propeller. In this
case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the
propeller axis, while it had an insignificant effect at the rotor plan
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