Time-averaged Response Research Articles

Gain stabilization of scintillation and Cerenkov spectrometers at high counting rates

A photomultiplier gain stabilization system, capable of operation at rates well over 5×10 4 pulses/s, is described. Pile-up and dead-time effects, which hamper conventional systems at such rates, are minimized through the use of a fast-gated charge sensitive integrator directly coupled to the photomultiplier anode. Deviations in the time-averaged response to a reference light pulser are fed back to control the high-voltage supply. Rapid fluctuations due to variations in average dynode current are reduced through the use of active voltage divider chains. Stability, tested with a large NaI(Tl) crystal viewed by five photomultiplier tubes, is better than 0.7% over 24 h, and for counting rates charging from a few hundred counts/s to over 2×10 5 counts/s.

A Study of the Response of NCAR GCM Climatological Statistics to Random Perturbations: Estimating Noise Levels

General Circulation Models (GCM's) of the earth's atmosphere have been frequently used to test different mechanisms for climate change. Typically, these tests involve comparing the statistics of the model with a prescribed change of a variable, system parameter or boundary condition, to the statistics from an unperturbed control case. However, it is not uncommon for the prescribed change experiment statistics to be comparable to statistics from a random perturbation experiment. Consequently, it is essential to determine the inherent “noise climatology” of GCM's in order to distinguish between signal and noise in climate experiments. We have examined the time-averaged response of the NCAR GCM to random perturbations in the initial conditions, while leaving all boundary conditions fixed. The dependence on time averaging interval of the noise level of a number of globally- and zonally-averaged GCM variables has been computed and gives an indication of how long to time average in order to reduce noise levels by a given amount. Additionally, we have found that it is important to delay for several simulated weeks the process of compiling climatological statistics from perturbed runs. Furthermore, the global distribution of noise levels reveals that certain regions are more prone to inherent variability for a given variable than are other areas. Also, we show that analysis of noise characteristics can be a useful diagnostic tool. However, different random perturbations do not reproduce the same noise level distribution, which implies that a Monte Carlo approach with more independent samples may he necessary for a more definite determination of the noise levels of GCM-generated statistics. Unfortunately, generating more samples means using more computer time, and that can be a fairly imposing barrier to the use of a GCM in climate experiments.

Response of Pitot probes in turbulent streams

The response of a Pitot probe in a uniform laminar stream is commonly expressed in the form\[ P_s = P+{\textstyle\frac{1}{2}}C_{\rho} U^2, \]wherePsis the probe signal pressure,Pis the stream static pressure,Uis the stream speed and ρ is the fluid density. It has been found that for ordinary sphere-nosed, round-nosed and square-nosed probes\[ 1-C = K(\sin^2\theta)^m = K(U^2_n/U^2)^m, \]where θ is the angle between the velocity vector and the probe axis, andUn≡Usin θ is the transverse velocity component. The parametersmandKare functions of the probe geometry. These formulae also describe the performance in a turbulent stream when the probe is small compared with the turbulence scale. The evaluation of the time-averaged response is treated, and an answer is developed to the question of what it is that a Pitot probe measures in a turbulent stream. In a turbulent shear flow having the properties of a boundary layer, the reference pressure is best taken to be the static pressure at the shear-layer edge. It is shown that round-nosed probes withDi/D≃0·45 and square-nosed probes withDi/D≃0·15 then detect${\textstyle\frac{1}{2}}\rho\overline{U}^2_x$with good accuracy, whereDi/Dis the ratio of the inside and outside diameters of the Pitot tube. When measurements are made with two probes of dissimilar geometry, the differential response can be used to find the mean-square level of the transverse velocity fluctuations. Turbulence levels so measured agree closely with results from hot-wire anemometry.

A diffusion chamber radon dosimeter for use in mine environment

A diffusion chamber dosimeter for measurement of radon gas exposure in a mine environment was developed and tested. This dosimeter is a small rugged device of simple design. It can be conveniently and economically utilized by miners. Cellulose nitrate plastic track detectors are used to measure the 4He-particle emissions resulting from 222Rn decay. Calibrations were made for a series of exposure times in a laboratory chamber containing measured concentrations of radon gas. The time-averaged response was found to rise for short exposure times due to the slow dissipation of radon from the sensitive volume of the diffusion chamber. For longer exposure times the response is flat. The measured response varied from a high of 0.0654±0.0033 tracks/cm 2 per pCi-h/l for shortest exposure time of 0.5 h, to 0.0431±0.0005 for longest exposure time of 8 h. The decrease after exposure times of 2 h is about 14%.

Effect of Correlated Phase Fluctuation on Array Performance

It is shown that the expectation of the mean-square time-averaged response for a configuration of receivers is immediate from known statistical considerations.

An analysis of unstable combustion of premixed gases

The time-averaged heat release rates of two-dimensional laminar premixed flames subjected to axial oncoming flow perturbations are theoretically studied in this paper. The proposed models of both the two-dimensional conical flame and V-shape flame are derived from the G-equation method. The weakly nonlinear perturbation analysis has been applied in the present work. The origins of time-averaged flame fronts consist of two parts: the unperturbed flame fronts caused by the balance between the laminar flame burning velocity and the incident normal velocity; the perturbed zero order time-averaged flame fronts caused by the second order nonlinear items in the perturbation analysis. For the conical and V-shape flame models, the analytical solution of the time-averaged flame heat release rate is derived based upon the perturbation method (analytical perturbation model) using different velocity perturbation models, in order to reveal the time-averaged flame heat release rate response mechanism to the axial disturbances. Results show that the velocity perturbation models and the flame aspect ratio have significant effects on the time-averaged flame heat release rates.