Multi-camera digital holographic PIV: Tomographic DHPIV

The wide scale application of digital holographic particle image velocimetry (DHPIV) as a three-component three-dimensional (3C-3D) velocity field measurement tool is current restricted by the limited size and resolution of commercially available CCD arrays, resulting in a elongation of particle is the direction normal to the hologram plane. This elongation can be over an order of magnitude greater than the true particle diameter and posses significant problems for the cross-correlation analysis used in particle image velocimetry (PIV). In this paper we discuss a multi-camera method of tomographic digital holographic particle image velocimetry (Tomo-DHPIV) to reconstruct a 3D intensity field without a loss of resolution in the hologram normal direction. Application of this reconstruction technique is provided along with Monte Carlo simulations of the effects of various operating parameters

Algebraic Reconstruction Techniques for Tomographic Particle Image Velocimetry

Tomographic particle image velocimetry (Tomo-PIV) is a technique for three-component three-dimensional (3C-3D) velocity measurement based on the tomographic reconstruction of a volume intensity field from multiple two-dimensional projection. As such the performance and accuracy of this technique is highly dependant on the algorithm used for reconstruction. This paper presents an evaluation of four different tomographic reconstruction algorithms, namely multiplicative algebraic reconstruction techinique (MART); adaptive algebraic reconstruction technique (AART); improved iterative algorithm for sparse object reconstruction (IIASOR); and simultaneous iterative reconstruction technique (SIRT). Results indicate that the MART and AART algorithms provide considerably better particle field reconstructions for fewer iterations

Characterisation of a Low Reynolds Number Turbulent Boundary Layer using PIV

An investigation by 2D-2C particle image velocimetry was carried out in the far downstream section (greater than 70 downstream boundary layer thicknesses, δ99 measured in the region of interest) of a nominally zero pressure gradient 0.5m square re-circulating water tunnel seeded with 11μm Potter’s hollow glass spheres. The use of a PCO4000 camera (4008 by 2672 pixel2) in combination with a Nikon Nikkor 200mm f4 IF MF lens and a PK-13 lens extension provides a measurement spatial resolution (in the streamwise-wall normal plane) of between 2 and 25 wall units for momentum Reynolds number between 200 and 2000 respectively. The paper will present characteristic flow quantities with results from the measurements non-dimensionalised by the skin friction velocities that are estimated by the Clauser method

An Investigation of a Zero-Net-Mass-Flux Jet in Cross Flow using PIV

Zero-Net-Mass-Flux Jets in cross flow can be used in a number of engineering applications, such as combustion in a gas turbine engine and the cooling of turbine blades. ZNMF-Jets have been seen to create two distinct flow regimes when impinging into a flow, either singular or multiple trajectories as characterised by the Strouhal Number. This paper shows two cases for a ZNMFJet in Cross-flow, when the flow is singular and when the flow shows multiple trajectories, using Digital Particle Image Velocimetry (DPIV). Time and Phase-averaged experiments were conducted in a vertical water tunnel, where the ZNMF-Jet was created using a stepper motor scotch-yoke piston arrangement of cylinder diameter 20mm to force fluid through a 10mm circular jet orifice. The Strouhal Number was taken at 0.11 and 0.56 with the Reynolds Number held constant at 1066 and the velocity ratio between the free stream and the r.m.s. fluctuating velocities of the ZNMF - Jet also held constant at 2. The depth of penetration of the ZNMF–Jet into the cross flow decreases with an increase in Strouhal Number, while the strength of the vortical structures increases with increasing Strouhal Number

Cylinder Wake – Boundary Layer Interaction in the Near Field

The interaction between the wake generated from separation off a cylinder and turbulent structures evident in a boundary layer are of significant importance in understanding the flows for cooling towers, submerged and semi-submerged vessels. This investigation was conducted on a wall mounted circular cylinder 25.4mm in diameter in the near wake region using MCCDPIV (multi-grid cross correlation digital particle image velocimetry) using a PCO4000 CCD array with full resolution of 4008 x 2672 pixels2 per image. The investigation was conducted for (ReD ≡ DU / ν) of 3600 and 5400. It was seen in the mean after taking the ensemble average of the instantaneous results that a stagnation line was formed between x / D = 1 and 1.5 downstream of the cylinder. The region of the cavity wake was highly turbulent having the largest velocity fluctuations in this region. The shape of the stagnation line was also seen to change, where for the ReD = 3600 the stagnation line position from the cylinder changed for a given height above the flat plate surface at y / D = 0

The Pliocene Mediterranean infilling of the Messinian Erosional Surface: New biostratigraphic data based on calcareous nannofossils (Bajo Segura Basin, SE Spain)

The Bajo Segura Basin (eastern Betic Cordillera) is a Mediterranean marginal basin where the Messinian Erosional Surface (MES), formed during the Messinian Salinity Crisis sea-level fall, is well developed. Overlying this major discontinuity the lower Pliocene transgressive sediments record the reflooding of the Mediterranean and the return to an open marine environment, the continental shelf being rebuilt after the Messinian erosion. The stratigraphic and biostratigraphic study of seven sections allow two transgressive-regressive sequences filling the MES to be distinguished, correlated with the previously distinguished Mediterranean offshore seismic units. Ten calcareous nannofossil bioevents have been identified. The lower sequence can be dated according to nannofossil biozones NN12 to NN14 and the upper sequence by NN15 to NN16. The boundary between both lower Pliocene sedimentary sequences is dated by the First Common Occurrence (FCO) of Discoaster asymmetricus in the uppermost sediment of the lower sequence and the First Occurrence (FO) of Discoaster tamalis in the lowermost part of the upper sequence. This discontinuity can be calibrated at between 4.1 and 4.0 Ma ago

Comparison of high spatial resolution stereo-PIV measurements in a turbulent boundary layer with available DNS dataset

In the present contribution, the aptitude of Stereoscopic Particle Image Velocimetry (SPIV) and of Direct Numerical Simulations (DNS) to investigate coherent structures of near wall turbulence is evaluated. For this purpose, the general properties and constraints of the two techniques are first reviewed. Then, data obtained from stereo-PIV experiments in a boundary layer and DNS in a channel flow are considered. Some statistics of the velocity fields are computed, and the results obtained from the two approaches compared

Scaling and aging in the homogeneous cooling state of a granular fluid of hard particles

The presence of the aging phenomenon in the homogeneous cooling state (HCS) of a granular fluid composed of inelastic hard spheres or disks is investigated. As a consequence of the scaling property of the $N$-particle distribution function, it is obtained that the decay of the normalized two-time correlation functions slows down as the time elapsed since the beginning of the measurement increases. This result is confirmed by molecular dynamics simulations for the particular case of the total energy of the system. The agreement is also quantitative in the low density limit, for which an explicit analytical form of the time correlation function has been derived. The reported results also provide support for the existence of the HCS as a solution of the N-particle Liouville equation.Comment: 17 pages, 3 figures; v3 revised version (minor changes, corrected typos, v2=v1 due to a submission error)accepted for publication in J. Phys. A: Math. Theo

Recovery of the Historical SN1957D in X-rays with Chandra

SN1957D, located in one of the spiral arms of M83, is one of the small number of extragalactic supernovae that has remained detectable at radio and optical wavelengths during the decades after its explosion. Here we report the first detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with \chandra. The X-ray luminosity (0.3 - 8 keV) is 1.7 (+2.4,-0.3) 10**37 ergs/s. The spectrum is hard and highly self-absorbed compared to most sources in M83 and to other young supernova remnants, suggesting that the system is dominated at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The high column density may be due to absorption within the SN ejecta. HST WFC3 images resolve the supernova remnant from the surrounding emission and the local star field. Photometry of stars around SN1957D, using WFC3 images, indicates an age of less than 10**7 years and a main sequence turnoff mass more than 17 solar masses. New spectra obtained with Gemini-South show that the optical spectrum continues to be dominated by broad [O III] emission lines, the signature of fast-moving SN ejecta. The width of the broad lines has remained about 2700 km/s (FWHM). The [O III] flux dropped precipitously between 1989 and 1991, but continued monitoring shows the flux has been almost constant since. In contrast, radio observations over the period 1990-2011 show a decline rate inf the flux proportional to t**-4, far steeper than the rate observed earlier, suggesting that the primary shock has overrun the edge of a pre-SN wind.Comment: 28 pages, including 3 tables and 7 figures, accepted for publication in Ap

Hydrodynamic modes, Green-Kubo relations, and velocity correlations in dilute granular gases

It is shown that the hydrodynamic modes of a dilute granular gas of inelastic hard spheres can be identified, and calculated in the long wavelength limit. Assuming they dominate at long times, formal expressions for the Navier-Stokes transport coefficients are derived. They can be expressed in a form that generalizes the Green-Kubo relations for molecular systems, and it is shown that they can also be evaluated by means of $N$-particle simulation methods. The form of the hydrodynamic modes to zeroth order in the gradients is used to detect the presence of inherent velocity correlations in the homogeneous cooling state, even in the low density limit. They manifest themselves in the fluctuations of the total energy of the system. The theoretical predictions are shown to be in agreement with molecular dynamics simulations. Relevant related questions deserving further attention are pointed out