Average Window Size Research Articles

Modeling unsaturated flow in a layered formation under quasi-steady state conditions using geophysical data constraints

The identification of vadose zone flow parameters and solute travel time from the surface to the water table are key issues for the assessment of groundwater vulnerability. In this paper we use the results of time-lapse monitoring of the vadose zone in a UK consolidated sandstone aquifer using cross-hole zero-offset radar to assess and calibrate models of water flow in the vadose zone. The site under investigation is characterized by a layered structure, with permeable medium sandstone intercalated by finer, less permeable, laminated sandstone. Information on this structure is available from borehole geophysical (gamma-ray) logs. Monthly cross-hole radar monitoring was performed from August 1999 to February 2001, and shows small changes of moisture content over time and fairly large spatial variability with depth. One-dimensional Richards’ equation modeling of the infiltration process was performed under spatially heterogeneous, steady state conditions. Both layer structure and Richards’ equation parameters were simulated using a nested Monte Carlo approach, constrained via geostatistical analysis on the gamma-ray logs and on a priori information regarding the possible range of hydraulic parameters. The results of the Monte Carlo analysis show that, in order to match the radar-derived moisture content profiles, it is necessary to take into account the vertical scale of measurements, with an averaging window size of the order of the antenna length and the Fresnel zone width. Flow parameters cannot be uniquely identified, showing that the system is over parameterized with respect to the information content of the (nearly stationary) radar profiles. Estimates of travel time of water across the vadose zone are derived from the simulation results.

Polymerization and carbonization of high internal phase emulsions

AbstractHigh internal phase emulsions (HIPEs) of styrene and divinylbenzene (DVB), with 0–40 % vinylbenzyl chloride (VBC) in the oil phase, were polymerized to give porous polymers (pore volumes 84–92 %), which were sulfonated and carbonized at temperatures up to 700 °C to obtain macroporous carbon monoliths. On carbonization, overall sample dimensions decreased, whilst percentage pore volume, compressive strength and compressive modulus increased. Scanning electron microscopy (SEM) showed typical HIPE‐derived structures of cages (several µm across), interconnected by smaller windows, for polymer and carbon samples. Distributions of cage and window size were evaluated from electron micrographs. Average cage and window sizes decreased on carbonization approximately in proportion to the reduction in overall sample dimensions, and also decreased with increasing VBC content in the HIPE. Mercury intrusion porosimetry gave distributions of pore size in reasonable agreement with area‐distributions of window size determined from SEM images. Copyright © 2004 Society of Chemical Industry

The influence of dispersant concentration on the pore morphology of hydroxyapatite ceramics for bone tissue engineering

There is a clinical need for synthetic scaffolds that will promote bone regeneration. Important factors include obtaining an optimal porosity and size of interconnecting windows whilst maintaining scaffold mechanical strength, enabling complete penetration of cells and nutrients throughout the scaffold, preventing the formation of necrotic tissue in the centre of the scaffold. To address this we investigated varying slip deflocculation in order to control the resulting porosity, pore size and interconnecting window size whilst maintaining mechanical strength. Hydroxyapatite (HA) porous ceramics were prepared using a modified slip casting process. Rheological measurements of the HA slips were used to identify deflocculation conditions which resulted in changes in the cell and window sizes of the resulting ceramics. Sintered ceramics were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Pore and window size distribution was determined by SEM. XRD analysis confirmed that the crystal structure remained HA after the sintering process. SEM showed that HA porous ceramics presented a highly interconnected porous network with average pore sizes ranging from 391±39 to 495±25 μm. The average window size varied from 73±5 to 135±7 μm. Pore diameters obtained were controllable in the range 200–500 μm. Window sizes were in the range 30–250 μm. The use of dispersant concentration allows pore and window size to be modified whilst maintaining control over porosity demonstrated by a porosity of 85% for seven different dispersant concentrations. The advantage of this approach allows the correlation between the rheological conditions of the slip and the resultant sintered ceramic properties. In particular, optimising the ceramic strength by controlling the agglomeration during the casting process.

Improvement in GPR coherent noise attenuation using τ‐p and wavelet transforms

We present a new application of modern filtering techniques to ground‐penetrating radar (GPR) data processing for coherent noise attenuation. We compare the performance of the discrete wavelet transform (DWT) and the linear Radon transform (τ‐p) to classical time‐space and Fourier domain methods using a synthetic model and real data. The synthetic example simulates problems such as system ringing and surface scattering, which are common in real cases. The field examples illustrate the removal of nearly horizontal but variable‐amplitude noise features. In such situations, classical space‐domain techniques require several trials before finding an appropriate averaging window size. Our comparative analysis indicates that the DWT method is better suited for local filtering than are 2D frequency‐domain (f‐f) techniques, although the latter are computationally efficient. Radon‐based methods are slightly superior than the techniques previously used for local directional filtering, but they are slow and quite sensitive to the p‐sampling rate, p‐range, and sizes of the muting zone. Our results confirm that Radon and wavelet methods are effective in removing noise from GPR images with minimal distortions of the signal.

An analytical method for predicting surface soil moisture from rainfall observations

A simple analytical method for estimating surface soil moisture directly from rainfall data is proposed and studied. Soil moisture dynamics are represented by a linear stochastic partial differential equation [Entekhabi and Rodriguez‐Iturbe, 1994]. A diagnostic equation is derived from the soil moisture dynamics equation by eliminating the diffusion term. The derived daily soil moisture function is a time‐weighted average of previous cumulative rainfall over a given period (e.g., >14 days). The advantage of this method is that information on the initial condition of soil moisture, which is often not available at all times and locations, is not needed. The loss coefficient in the diagnostic equation for soil moisture can be estimated from land surface characteristics and soil properties. The method for determining the averaging window size, the loss coefficient, and the infiltration coefficient are described and demonstrated. The soil moisture data observed during three field experiments, i.e., Monsoon'90, Washita'92, and SGP'97, are compared to the calculated soil moisture. The results indicate that the proposed method is robust and has the potential for useful soil moisture predictions.

Stochastic modeling of TCP in networks with abrupt delay variations

An analytical model of TCP (Transport Control Protocol) over an end-to-end path with random abrupt round-trip time (RTT) changes is presented. Modeling the RTT as a stochastic process, we analytically quantify and compare between the degree of degradation of the steady-state average throughput and window size due to spurious retransmissions for the different versions of TCP (Reno/New Reno versus Tahoe). The modeling methodology in this paper is used for evaluating different design alternatives for TCP for highly dynamic networks.

Profitability of the CRISMA System: From World Indices to the Hong Kong Stock Market

Many studies show that the use of technical analysis can generate excess returns. We test the “CRISMA” technical trading rule introduced by [Pruitt and White J. Portfolio Managt. Spring, 1988, 55–58] on global equity indices and common stocks in Hong Kong. Out study shows that no excess returns could be found in indices except those in Asia. This validates the claims that the Asian stock markets are not as efficient as other stock markets and hence can be exploited by technical analysis. How does CRISMA perform on common stocks in Hong Kong? Generally speaking, CRISMA does not fair better than the buy and hold strategy. Further analysis reveals excess returns for stocks with very large turnover. This is consistent with other recent research on CRISMA conducted on US and UK stock markets. We also amend part of the original CRISMA rules to yield better performance: shrinking the moving average window sizes can increase both the number of trade signals and the excess returns. Therefore CRISMA can be made to work with some judicious choice of parameters, depending on the turnover.

TCP's role in the propagation of self-similarity in the Internet

This paper analyzes how TCP congestion control can propagate self-similarity between distant areas of the Internet. This property of TCP is due to its congestion control algorithm, which adapts to self-similar fluctuations on several timescales. The mechanisms and limitations of this propagation are investigated. It is demonstrated that if a TCP connection shares a bottleneck link with a self-similar background traffic flow, it propagates the correlation structure of the background traffic flow asymptotically, above a characteristic timescale. The cut-off timescale depends on the end-to-end path properties, e.g. round-trip time and average window size, and the receiver window size in case of high-speed connections. It is also shown that even short TCP connections can propagate long-range correlations effectively. In case when TCP encounters several bottleneck hops, the end-user perceived end-to-end traffic is also long-range dependent and it is characterized by the largest Hurst exponent. Through simple examples, it is shown that self-similarity of one TCP stream can be passed on to other TCP streams that it is multiplexed with. The limitations of propagation depend on network conditions. Propagation complements the widespread scaling phenomena reported in the literature. Our arguments are supported with a combination of analytic techniques, simulations and statistical analyzes of a number of wide area Internet traffic measurements.

On the propagation of long-range dependence in the Internet

This paper analyzes how TCP congestion control can propagate self-similarity between distant areas of the Internet. This property of TCP is due to its congestion control algorithm, which adapts to self-similar fluctuations on several timescales. The mechanisms and limitations of this propagation are investigated, and it is demonstrated that if a TCP connection shares a bottleneck link with a self-similar background traffic flow, it propagates the correlation structure of the background traffic flow above a characteristic timescale. The cut-off timescale depends on the end-to-end path properties, e.g., round-trip time and average window size. It is also demonstrated that even short TCP connections can propagate long-range correlations effectively. Our analysis reveals that if congestion periods in a connection's hops are long-range dependent, then the end-user perceived end-to-end traffic is also long-range dependent and it is characterized by the largest Hurst exponent. Furthermore, it is shown that self-similarity of one TCP stream can be passed on to other TCP streams that it is multiplexed with. These mechanisms complement the widespread scaling phenomena reported in a number of recent papers. Our arguments are supported with a combination of analytic techniques, simulations and statistical analyses of real Internet traffic measurements.

The influence of phase-averaging window size on the determination of turbulence quantities in unsteady turbulent flows

The phase-averaging window size is shown to affect the measurement of phase-averaged turbulence quantities in unsteady turbulent flows. The flow turbulence is usually estimated on the assumption of quasi-constant flow velocity during the duration of the phase-averaging window. The calculated turbulence level then consists of two parts: one due to the turbulent velocity fluctuations and the other due to the changes in the mean flow velocity. This second part is shown to be directly proportional to the averaging window size. In order to determine the true turbulence the averaging window size has to be made as small as possible, especially if the unsteady flow exhibits large temporal gradients and the flow turbulence itself is small.

Automatic deformation analysis by a TV speckle interferometer using a laser diode

A phase-shifting TV speckle interferometer is developed by using the frequency modulation capability of a laser diode and is applied to automatic deformation measurements. Temperature modulation is used for generating the frequency shift of a laser diode. We store four speckle interferograms with relative phase differences of pi/2 in a frame grabber by monitoring a phase-shift signal, which is obtained by integrating the fringe intensity over a local window. The optimum size of the local window is examined experimentally. Then, the optimum processing conditions for reducing the speckle noise in interferograms,including an averaging window size, are found. Experimental results of the automatic deformation analysis indicate a displacement accuracy of better than lambda/30.