Turbulent Velocity and Pressure Fields in Boundary-Layer Flows Over Rough Surfaces

This paper is a discussion of recent measurements of the statistics of the turbulent velocity and wall pressure fields in rough-wall boundary layers. These measurements, made in part by the author, have been performed over a variety of walls covering a wide range of roughness sizes and configurations. The various measurements are compared in order to determine the structure and scaling parameters of the turblulent field convected at speeds near the mean velocity of the boundary layer. The mean square turbulent velocities, their one-dimensional spectral densities, and their longitudinal and vertical microscales are compared for different walls. The velocity fields are shown to be similar when described in terms of local mean velocity, friction velocity, and displacement thickness. Turbulent production and dissipation rates, which are derived from the measurements, are also discussed. Recent measurements of wall pressure fluctuations are also reviewed. The wall pressure spectrum levels on rough walls are shown to increase with local mean wall shear through a dependence on the vertical component of turbulent velocity by a mean shear-turbulence interaction. Finally, the dependence of the high frequency convected pressure field on the insitu roughness size is discussed in terms of the mean shear-turbulence interaction

Periodic and Random Excitation of Streamlined Structures by Trailing Edge Flows

This paper is an examination of semi-empirical techniques which are shown to be effective in predicting the response of hydrofoils to flow excitation. Examples are given for buffeting by inflow turbulence, excitation by the boundary-layer pressures on the hydrofoil, and linear and non-linear excitation by vortex street formation in the wake. Co-ordinated aerodynamic and hydrodynamic measurements are used in Reynolds number - scaled experiments to determine both the flow induced forces and the hydro-elastic behavior on cantilever struts. An analytical formulation based on normal mode theory is used to combine the results

CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?

We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star xi Per, together with contemporaneous H alpha observations. The X-ray spectrum of this star is similar to other single O stars, and not pathological in any way. Its UV wind lines are known to display cyclical time variability, with a period of 2.086 days, which is thought to be associated with co-rotating interaction regions (CIRs). We examine the Chandra and H alpha data for variability on this time scale. We find that the X-rays vary by about 15% over the course of the observations and that this variability is out of phase with variable absorption on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV absorption associated with CIRs). While not conclusive, both sets of data are consistent with models where the CIRs are either a source of X-rays or modulate them.Comment: Accepted by MNRAS. 9 pages, 9 figure

Determining the Spectral Signature of Spatial Coherent Structures

We applied to an open flow a proper orthogonal decomposition (pod) technique, on 2D snapshots of the instantaneous velocity field, to reveal the spatial coherent structures responsible of the self-sustained oscillations observed in the spectral distribution of time series. We applied the technique to 2D planes out of 3D direct numerical simulations on an open cavity flow. The process can easily be implemented on usual personal computers, and might bring deep insights on the relation between spatial events and temporal signature in (both numerical or experimental) open flows.Comment: 4 page

On Recognizing Transparent Objects in Domestic Environments Using Fusion of Multiple Sensor Modalities

Current object recognition methods fail on object sets that include both diffuse, reflective and transparent materials, although they are very common in domestic scenarios. We show that a combination of cues from multiple sensor modalities, including specular reflectance and unavailable depth information, allows us to capture a larger subset of household objects by extending a state of the art object recognition method. This leads to a significant increase in robustness of recognition over a larger set of commonly used objects.Comment: 12 page

Making graphene visible

Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of silicon wafers with a certain thickness of silicon dioxide. We study graphene's visibility and show that it depends strongly on both thickness of silicon dioxide and light wavelength. We have found that by using monochromatic illumination, graphene can be isolated for any silicon dioxide thickness, albeit 300 nm (the current standard) and, especially, approx. 100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, we quantitatively describe the experimental data without any fitting parameters.Comment: Since v1: minor changes to text and figures to improve clarity; references added. Submitted to Applied Physics Letters, 30-Apr-07. 3 pages, 3 figure

Spatially Resolved Spectroscopy of the E+A Galaxies in the z=0.32 Cluster AC114

We present spatially resolved intermediate resolution spectroscopy of a sample of twelve E+A galaxies in the z=0.32 rich galaxy cluster AC 114, obtained with the FLAMES multi-integral field unit system on the European Southern Observatory's VLT. Previous integrated spectroscopy of all these galaxies by Couch & Sharples (1987) had shown them to have strong Balmer line absorption and an absence of [OII 3727] emission -- the defining characteristics of the``E+A'' spectral signature, indicative of an abrupt halt to a recent episode of quite vigorous star formation. We have used our spectral data to determine the radial variation in the strength of Hdelta absorption in these galaxies and hence map out the distribution of this recently formed stellar population. Such information provides important clues as to what physical event might have been responsible for this quite dramatic change in star formation activity in these galaxies' recent past. We find a diversity of behaviour amongst these galaxies in terms of the radial variation in Hdelta absorption: Four galaxies show little Hdelta absorption across their entire extent; it would appear they were misidentified as E+A galaxies in the earlier integrated spectroscopic studies. The remainder show strong Hdelta absorption, with a gradient that is either negative (Hdelta equivalent width decreasing with radius), flat, or positive. By comparing with numerical simulations we suggest that the first of these different types of radial behaviour provides evidence for a merger/interaction origin, whereas the latter two types of behaviour are more consistent with the truncation of star formation in normal disk galaxies. It would seem therefore that more than one physical mechanism is responsible for E+A formation in the same environment.Comment: 15 pages, 10 figures, accepted MNRA