Study of the aerodynamic sampling effects of a holographic cloud droplet instrument

Abstract Computational fluid dynamics and particle tracing simulations are presented for a cloud droplet sensor. Airspeeds and streamlines around the sensor are calculated at several wind speeds and their effect on the droplet sampling are examined. Particle tracing is used to study the effect of different wind speeds and droplet sizes on the sampling of the cloud droplets. Simulated droplet concentrations are confirmed by comparing them with measured wind tunnel data. Results demonstrate clear sampling effects that are functions of both wind speed and droplet size. Optimal compromise between maximal measurement volume and sampling effects is found and a simple approximation for sensor’s sampling bias is presented. The results show that CFD simulations can give valuable information about the sampling of droplets in an ideal environment with known droplet concentrations. Even in a wind tunnel, the true test conditions are often impossible to accurately determine. Thus by simulating the sampling effects in different conditions, the sensor can be calibrated for a wide range of naturally occurring cloud conditions

Predicting scattering properties of fiber suspensions using Mie theory and probabilistic cross-sectional diameter of fibers

Abstract Scattering of visible light by micrometer-scale natural wood fibers is usually treated by assuming fibers to be perfect long cylindrical scatterers. In industrial processes, however, fibers experience deformations and are far from ideal cylinders. Variation in fiber morphology affects their scattering properties and it poses a challenge for reliable process measurements. In this paper, we have studied experimentally scattering of both deformed natural and ideal artificial non-absorbing fibers in aqueous suspension and their response to mass concentration of fibers. Experimental results are compared with the predictions of the Mie theory which is combined with cross-sectional diameter probability distribution of fibers. It is shown that the diameter distribution of the fibers together with Mie theory provides results that agree with experiments in case of both natural and ideal fibers

The effect of drop shape, sensing volume and raindrop size statistics to the scattered field on 300 GHz

Abstract Scattering and attenuation properties of rain using various drop models are calculated using Mie theory, the T-matrix method and numerical technique respectively. Ellipsoidal and oblate drops use the axis ratio of raindrops reported in literature. Scattering are presented both for small volumes containing a single drop and for large volumes with multiple drops in which the statistics of drop size distribution need to be taken into account. The angular dependence of scattering, absorption and scattering cross sections and polarization ratios are investigated. For single scattering the angular dependence and cross sections of nonspherical drops differed from those of a spherical drop. Differences between ellipsoid and oblate drops were minimal. Drop shape affected the polarization mostly at 40–140 degree detection angles. The averaged linear attenuation and rain induced cross channel signal were studied. Attenuation was close to the ITU-R.838 model and in the same order of magnitude as the attenuation due to atmospheric moisture. The cross channel signal was calculated as a function of rain rate and transmitter/receiver angle. Vertical polarization was observed to produce a higher cross channel signal than horizontal polarization. It was concluded that drop shape is not an important factor at 300 GHz due to small drops dominating the scattering signal. Overall, the results showed that in single scattering, the ellipsoid is a good approximation for raindrops and that for large volumes, a spherical drop approximation and a Mie solution may be sufficient at frequencies of 300 GHz and higher

Rain induced co-channel interference at 60 GHz and 300 GHz frequencies

Abstract Results at mm wave frequencies have shown that it is important to evaluate co-channel interference between directional beams that cross each other. Terahertz frequency bands, especially around 300 GHz, has been considered for future wireless communications. As far as we know, no work has considered the rain induced interference at THz frequencies. In this work, we evaluate the rain induced interference at 300 GHz. Results are calculated also at 60 GHz in order to make a comparison. We combined bistatic radar equation, first order multiple scattering approximation and full Mie scattering calculations to existing drop size distribution models to estimate interference due to rain. Considered effective path lengths between transmitter and receiver are 100 m and 500 m and the effect of selected drop size distribution is studied. Overall interference levels was observed to be approximately 20 dBm smaller at 300 GHz than at 60 GHz and angular dependency of interfering power was much more forward oriented for 300 GHz frequency. The results show that rain induces interference has significantly different behaviour at THz as compared to lower frequencies

An integrated 9x9 SPAD array with a 10-channel TDC and a CMOS laser diode driver for a wearable time-domain diffuse optics optode

Abstract A 9x9 Single-Photon Avalanche Photodiode (SPAD) array with a 10-channel time-to-digital converter and an integrated laser diode driver fabricated in 0.35 μm HV- CMOS technology were used to carry out time domain diffuse optics measurements of optical phantoms. The average optical power of 20 μW was achieved by the laser diode transmitter at the repetition rate of 500 kHz with a pulse width of 80 ps. The resolution of the TDC and the measured instrument response function (IRF) of the system were ~65 ps and ~190 ps, respectively. The power consumption of the proposed system was 180 mW. Measurements showed that with the proposed system a perturbation with a different absorption property than in surrounding material at the depth of 15 mm could be recognized. In addition, in in vivo measurement with the proposed components, hemodynamic changes in volunteer’s arm during a venous occlusion were detected

Compensation of aerodynamic sampling effects of a cloud droplet instrument

Abstract Precise sampling is a crucial part of the aerosol measurement processes that ideally requires perfectly isokinetic conditions in which particles in the sampling volume move exactly the same way as they would in an undisturbed flow. Such conditions might be difficult to achieve in practical measurement situations where the direction and speed of the air stream may change continuously. We propose a novel method avoiding sampling errors due to moderately disturbed particle flow in case of an imaging cloud droplet instrument. It is shown that despite the non-isokinetic and non-isoaxial conditions accurate droplet density can be obtained by rejecting part of the measurement volume in post processing. The adjustment of the sampling volume is easily applied using a holographic imaging method, which offers multiple well-defined image planes to accurately set the boundaries of the sampling volume. To verify the hypothesis, aerodynamic sampling effects of a holographic cloud droplet instrument are studied using computational fluid dynamics (CFD) and particle tracing simulations and by comparing them with wind tunnel experiments. We found out that changes in the airflow affected the particle density mostly near the walls of the probe. It was observed that the error in droplet density could be kept under 10 % by limiting the cross-channel depth of the measurement volume to two-thirds of the full wall-to-wall distance. Further improvement was achieved by using simulation results to formulate a relation between sampled and ambient droplet concentration as a function of droplet diameter and air speed. Less than 1 % deviation in droplet density was achieved in this case compared to simulated values. Orientation of the instrument’s inlet relative to the direction of airflow was found out to have the strongest effect on the achievable accuracy. Results show that the droplets can be reliably sampled also in a non-isoaxial case if the measurement volume was further reduced. Reasonable accuracy was achieved with 10-degree deviation within limited air speed and droplet diameter range