Established Transfer Function Research Articles

An experimental assessment of the effect of current on wave buoy measurements

Wave measurement buoys provide characterisation of wave climates that forms the basis for the design of offshore systems. These buoys are commonly subjected to currents which affect the resulting wave measurements, and if not accounted for will result in errors in the estimated sea state parameters. The present work provides results and observations from experiments aimed at assessing the impact that currents have on wave buoy measurements, thereby informing processing techniques to more accurately include this effect. Through scaled testing (circa 1:15) in a combined wave–current test tank, buoy motions (diameter, D = 0.24 m) are recorded in current only, waves only, and combined wave–current including oblique conditions. From these, the wave-induced motions are extracted and compared against three prediction methods based on established transfer function approaches as well as a frequency-domain hydrodynamic coefficient (HC) model based on potential flow. The scaled buoy was observed to have large, complex, irregular oscillatory vortex-induced motions (VIM) exceeding the buoy diameter. Both the magnitude and frequency of these oscillations was found to be significantly altered by the mooring stiffness and configuration whilst the addition of collinear waves was found not to affect the magnitude of VIM. Furthermore, due to the lack of VIM heave response and a large difference between the frequencies of the vortex-induced and wave induced horizontal motions, it was found that the VIM did not significantly alter the interpretation of the wave climate for the tested conditions. The HC model was found to accurately capture the observed modified hydrodynamics for opposing wave–current conditions, where larger horizontal motions than (typically) predicted are observed for all frequencies. This behaviour is concluded to result from increased excitation forces owing to the higher wavenumbers. The experiments highlight the potential effects of VIM on wave measurement performance of wave buoys, along with the complex and mooring-dependent nature of the response. Altered dynamics in the presence of currents are described which must be accounted for to avoid errors and the presented prediction methods provide a mechanism to account for these effects in wave processing methodologies which can subsequently reduce uncertainty in our understanding of the offshore environment.

Study on the test load for the structural test of the flight-load condition of the external fuel tank for fixed-wing aircraft

In this study, for the purpose of conducting the structural tests for the verification of structural soundness of the flight-load conditions of the external fuel tank for the fixed-wing aircraft, the flight load acting on the external fuel tank was converted to test load and the suitability of the converted loads was verified. The loads imposed on the external fuel tank were expressed as the combination of the inertial load (based on the acceleration in the translational direction) and the tangential direction inertial load (based on the angular acceleration of the moment). To calculate the test load, the transfer function table was generated by calculating the shear load and moment based on the unit load. For this purpose, a transfer function table was established by dividing the external fuel tank into a few sections and calculating the shear load and moment generated by the unit shear load and unit moment in each section. In addition, the test load for each section was calculated by computing the established transfer function table and flight-load conditions. However, in actual structural tests, it is often not possible to impose a load in the same position as the point at which the shear load and moment are calculated. For this reason, the actual test-load positions had to be determined and the calculated test loads were redistributed to those positions. Then, the final test load plan was established by applying a whiffle tree to increase the efficiency of the test while also making it easier to apply the actuators. Finally, the suitability of the established test load plan was confirmed by comparison with the flight-load conditions.

Effect of skull thickness and conductivity on current propagation for noninvasively injected currents

Objective. When currents are injected into the scalp, e.g. during transcranial current stimulation, the resulting currents generated in the brain are substantially affected by the changes in conductivity and geometry of intermediate tissue. In this work, we introduce the concept of ‘skull-transparent’ currents, for which the changing conductivity does not significantly alter the field while propagating through the head. Approach. We establish transfer functions relating scalp currents to head potentials in accepted simplified models of the head, and find approximations for which skull-transparency holds. The current fields resulting from specified current patterns are calculated in multiple head models, including MRI heads and compared with homogeneous heads to characterize the transparency. Experimental validation is performed by measuring the current field in head phantoms. Main results. The main theoretical result is derived from observing that at high spatial frequencies, in the transfer function relating currents injected into the scalp to potential generated inside the head, the conductivity terms form a multiplicative factor and do not otherwise influence the transfer function. This observation is utilized to design injected current waveforms that maintain nearly identical focusing patterns independently of the changes in skull conductivity and thickness for a wide range of conductivity and thickness values in an idealized spherical head model as well as in a realistic MRI-based head model. Experimental measurements of the current field in an agar-based head phantom confirm the transparency of these patterns. Significance. Our results suggest the possibility that well-chosen patterns of current injection result in precise focusing inside the brain even without a priori knowledge of exact conductivities of intermediate layers.

Depth of Interaction Measurements Based on Rectangular Light Sharing Window Technology and Nine-Crystals-to-One-SiPM Coupling Method

Depth of interaction (DOI) decoding capability shows great application potential for positron emission tomography (PET), especially for high-resolution small-animal PET or long axial field of view PET systems. On the basis of GATE optical simulation results, we presented a novel $9\times 9$ lutetium–yttrium oxyorthosilicate array with 10-mm rectangular light-sharing windows (RLSWs) and assembly the detector with nine crystals coupled to one silicon photomultiplier (SiPM) method; this caused DOI-related position shifts in the crystal decoding spots of the flood map. Homogeneous radiation was used to establish transfer functions to convert spot shifts into DOI measurements. The conventional collimated radiation experiments were used to assess the accuracy of the DOI measurements. Flood map results show that 81 segmented crystals can be clearly resolved, including 16 crystals adjacent to two RLSW windows (M-type), 40 crystals adjacent to one RLSW window (E-type), and 25 crystals not adjacent to any RLSW window (C-type). The mean absolute error of all DOI-capable crystals, M-type crystals, and E-type crystals are 2.15 ± 0.61, 1.95 ± 0.50, and 2.23 ± 0.63 mm, respectively. The novel nine-crystals-to-one-SiPM coupling technology is a low cost approach for constructing high-performance DOI detector modules. Detectors using nine-crystals-to-one-SiPM coupling technology are good candidates for high-resolution preclinical PET and for total-body PET.

Improvement of the signal-to-noise ratio of the constant-temperature hot-wire anemometer using the transfer function

In the present paper, a compact low-noise constant-temperature hot-wire anemometer (CTA) was developed to improve the signal-to-noise (S/N) ratio in hot-wire measurements. The CTA circuit was small enough to be attached to the support stem directly, which minimized the reactance of the sensor cable, maximized the electromagnetic shielding effect and simplified the circuit’s transfer function. White noise as well as a square wave were used to examine the dynamic responses of the CTA in detail. Three parameters in the transfer function were successfully determined by the square-wave test. The transfer function with experimentally determined parameters was found to describe the CTA responses quite well over the wide frequency range, from 1 Hz to 250 kHz. It was demonstrated for residual free-stream turbulence in a wind tunnel that the established transfer function allowed the separation of electric noise from the measured data, improving the S/N ratio in the power spectrum of the velocity fluctuations.

Estimating Net Photosynthesis of Biological Soil Crusts in the Atacama Using Hyperspectral Remote Sensing

Biological soil crusts (BSC) encompassing green algae, cyanobacteria, lichens, bryophytes, heterotrophic bacteria and microfungi are keystone species in arid environments because of their role in nitrogen- and carbon-fixation, weathering and soil stabilization, all depending on the photosynthesis of the BSC. Despite their importance, little is known about the BSCs of the Atacama Desert, although especially crustose chlorolichens account for a large proportion of biomass in the arid coastal zone, where photosynthesis is mainly limited due to low water availability. Here, we present the first hyperspectral reflectance data for the most wide-spread BSC species of the southern Atacama Desert. Combining laboratory and field measurements, we establish transfer functions that allow us to estimate net photosynthesis rates for the most common BSC species. We found that spectral differences among species are high, and differences between the background soil and the BSC at inactive stages are low. Additionally, we found that the water absorption feature at 1420 nm is a more robust indicator for photosynthetic activity than the chlorophyll absorption bands. Therefore, we conclude that common vegetation indices must be taken with care to analyze the photosynthesis of BSC with multispectral data.

Improved averaged model and stability analysis of voltage-mode controlled positive output super-lift Luo converter

Positive output super-lift (POSL) Luo converter, which has some particular good features: such as its power switch being grounded, high voltage gain and positive polarity output, is a good topology for overcoming the drawbacks of the conventional Buck and Boost converters to obtain high output voltage and power for satisfying the requirements in practical engineering. In this paper, based on the averaging method and taking into account the abrupt changing of the voltage across the energy-transferring capacitor, the improved reduced order averaged model and the corresponding small signal model of the POSL Luo converter are established, and its transfer function from the duty cycle to the output voltage is derived and analyzed. By combining the derived transfer function from the duty cycle to the output voltage of the POSL Luo converter, with that for the voltage compensator and that for the pulse width modalation (PWM) generator, the transfer function from the reference voltage to the output voltage of the voltage-mode controlled POSL Luo converter is also derived. And then, the stability of the voltage-mode controlled POSL Luo converter is identified by calculating the poles of its transfer function from the reference voltage to the output voltage, so the corresponding stability boundaries are obtained. The power electronic simulator (PSIM) software is applied to simulate the POSL Luo converter in time domain and frequency domain to preliminarily confirm the effectiveness of the established transfer function from the duty cycle to the output voltage of the POSL Luo converter, and to simulate the voltage-mode controlled POSL Luo converter to preliminarily verify the theoretical analysis about its stability. Finally, the hardware circuits for the POSL Luo converter and the voltage-mode controlled POSL Luo converter are designed, and the circuit experimental results in time domain from the digital oscilloscope and in frequency domain from the impedance/gain-phase analyzer are presented for further validation. Theoretical analysis, PSIM simulations and circuit experiments are in basic agreement with one other, and all of them demonstrate that it is effective to use the improved average model to analyze the performance of the POSL Luo converter and the stability of the voltage-mode controlled POSL Luo converter.

Construction and verification of a linear-rotary microstage with a millimeter-scale range

This paper presents a second-generation linear-rotary (Z-θZ) micro-stage driven by impact friction. The micro-stage was constructed by miniaturizing a previously developed linear-rotary stage. The moving element of the micro-stage, which was a steel cylinder, could be actuated along and around the Z-axis in millimeter-scale range. The steel cylinder was supported by two driving units. Each unit consisted of a permanent magnet and two piezoelectric actuators (PZTs) for generating the impact friction motions in the Z- and θZ-directions. The size of the micro-stage was made to be 11.0 mm (L) × 11.0 mm (W) × 5.7 mm (H). For enhancement of the stage velocity, the transfer function of the micro-stage with the voltage applied to the PZT as the input and the PZT displacement as the output, was established. An improved waveform of the input voltage was then obtained based on the established transfer function for getting a triangular-shaped PZT displacement, which was an ideal waveform for the impact friction motion. The characteristics of the stage velocity before and after the improvement of the input voltage were verified and the effectiveness of the improved input voltage for enhancement of the stage velocity was confirmed.

Meteorological observations 2005 and in a longer perspective at the Sermilik Station, Ammassalik Island, Southeast Greenland

Geografisk Tidsskrift, Danish Journal of Geography 107(2):59–68, 2007 Meteorological stations have been in operation since 1993 at Sermilik (65°40'N, 38°10'W), Ammassalik Island, southeast Greenland. This note presents meteorological observations for the year 2005 from the two meteorological stations: Station Nunatak (515 m a.s.l.) and Station Coast (25 m a.s.l.). The year 2005 was dominated by a temporal break down of Station Nunatak and defect instrument recordings at both stations. Average yearly measurements at both meteorological stations: Station Nunatak (1994–2004) and Station Coast (1998–2005) are presented to illustrate the climatic variability within the Mittivakkat Glacier catchment, and further compared with simultaneous standard synoptic meteorological DMI station climate observations (1994–2004) at the town Tasiilaq (Ammassalik) in order to establish transfer functions (linear regressions) and describe the spatial variability of climate variables.

Application of cardioid beamforming in both impedance tube and free‐field measurements of ground impedance

Measurements of the specific acoustic impedance of ground covered by institutional grass were performed using both a cardioid beamforming method and the established transfer function method [J. Y. Chung and D. A. Blaser, 907–921 (1980)]. The cardioid method steers a beampattern to separate the incident and reflected waves. Both methods were applied to impedance tube and free‐field measurements. With proper calibration, the cardioid and transfer function methods are mathematically identical and produce similar experimental results. Comparison of the impedance tube and free‐field measurements suggest that the latter approach produces more reliable results. However, due to practical limitations of the source/receiver geometry free‐field measurements at low frequencies are more complicated in that the plane wave assumption is not valid.