1,238 research outputs found
Modeling space-time correlations of velocity fluctuations in wind farms
An analytical model for the streamwise velocity space-time correlations in
turbulent flows is derived and applied to the special case of velocity
fluctuations in large wind farms. The model is based on the Kraichnan-Tennekes
random sweeping hypothesis, capturing the decorrelation in time while including
a mean wind velocity in the streamwise direction. In the resulting model, the
streamwise velocity space-time correlation is expressed as a convolution of the
pure space correlation with an analytical temporal decorrelation kernel. Hence,
the spatio-temporal structure of velocity fluctuations in wind farms can be
derived from the spatial correlations only. We then explore the applicability
of the model to predict spatio-temporal correlations in turbulent flows in wind
farms. Comparisons of the model with data from a large eddy simulation of flow
in a large, spatially periodic wind farm are performed, where needed model
parameters such as spatial and temporal integral scales and spatial
correlations are determined from the large eddy simulation. Good agreement is
obtained between the model and large eddy simulation data showing that spatial
data may be used to model the full temporal structure of fluctuations in wind
farms.Comment: Submitted to Wind Energ
A wavenumber-frequency spectral model for atmospheric boundary layers
Motivated by the need to characterize power fluctuations in wind farms, we study spatio-temporal correlations of a neutral atmospheric boundary layer in terms of the joint wavenumber-frequency spectrum of the streamwise velocity fluctuations. To this end, we perform a theoretical analysis of a simple advection model featuring the advection of small- scale velocity fluctuations by the mean flow and large-scale velocity fluctuations. The model is compared to data from large-eddy simulations (LES). We find that the model captures the trends observed in LES, specifically a Doppler shift of frequencies due to the mean flow as well as a Doppler broadening due to random sweeping effects
Spatio-temporal spectra in the logarithmic layer of wall turbulence: large-eddy simulations and simple models
Motivated by the need to characterize the spatio-temporal structure of
turbulence in wall-bounded flows, we study wavenumber-frequency spectra of the
streamwise velocity component based on large-eddy simulation (LES) data. The
LES data are used to measure spectra as a function of the two wall-parallel
wavenumbers and the frequency in the equilibrium (logarithmic) layer. We then
reformulate one of the simplest models that is able to reproduce the
observations: the random sweeping model with a Gaussian large-scale fluctuating
velocity and with additional mean flow. Comparison with LES data shows that the
model captures the observed temporal decorrelation, which is related to the
Doppler broadening of frequencies. We furthermore introduce a parameterization
for the entire wavenumber-frequency spectrum , where
, are the streamwise and spanwise wavenumbers, is the
frequency and is the distance to the wall. The results are found to be in
good agreement with LES data
Recruitment of childhood leukaemia patients to clinical trials in Great Britain during 1980-2007: variation by birth weight, congenital malformation, socioeconomic status and ethnicity.
OBJECTIVE: To assess recruitment of children to national clinical trials for acute lymphoblastic leukaemia (ALL) and acute myeloid leukaemia (AML) in Great Britain during 1980-2007 and describe variation by some factors that might influence trial entry. DESIGN AND SETTING: Records of leukaemia patients aged 0-14 years at diagnosis were identified in the National Registry of Childhood Tumours and linked to birth registrations, Children's Cancer and Leukaemia Group records, Hospital Episode Statistics and Medical Research Council clinical trial registers. Trial entry rates were compared between categories of birth weight, congenital malformation, socioeconomic status and ethnicity. RESULTS: 9147 ALL and 1466 AML patients were eligible for national clinical trials during 1980-2007. Overall recruitment rates were 81% and 60% respectively. For ALL, rates varied significantly with congenital malformation (Down syndrome 61%, other malformations 80%, none 82%; p4000 g 67%; p=0.001) and congenital malformation (Down syndrome 28%, other malformations 56%, none 63%; p<0.001). CONCLUSIONS: Although recruitment rates to clinical trials for childhood leukaemia are high, future trials should monitor possible variation by birth weight, ethnicity and presence of congenital malformations
The STOP COVID 2 study: Fluvoxamine vs placebo for outpatients with symptomatic COVID-19, a fully remote randomized controlled trial
BACKGROUND: Prior randomized clinical trials have reported benefit of fluvoxamine ≥200 mg/d vs placebo for patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
METHODS: This randomized, double-blind, placebo-controlled, fully remote multisite clinical trial evaluated whether fluvoxamine prevents clinical deterioration in higher-risk outpatients with acute coronavirus disease 2019 (COVID-19). Between December 2020 and May 2021, nonhospitalized US and Canadian participants with confirmed symptomatic infection received fluvoxamine (50 mg on day 1, 100 mg twice daily thereafter) or placebo for 15 days. The primary modified intent-to-treat (mITT) population included participants who started the intervention within 7 days of symptom onset with a baseline oxygen saturation ≥92%. The primary outcome was clinical deterioration within 15 days of randomization, defined as having both (1) shortness of breath (severity ≥4 on a 0-10 scale or requiring hospitalization)
RESULTS: A total of 547 participants were randomized and met mITT criteria (n = 272 fluvoxamine, n = 275 placebo). The Data Safety Monitoring Board recommended stopping early for futility related to lower-than-predicted event rates and declining accrual concurrent with vaccine availability in the United States and Canada. Clinical deterioration occurred in 13 (4.8%) participants in the fluvoxamine group and 15 (5.5%) participants in the placebo group (absolute difference at day 15, 0.68%; 95% CI, -3.0% to 4.4%; log-rank
CONCLUSIONS: This trial did not find fluvoxamine efficacious in preventing clinical deterioration in unvaccinated outpatients with symptomatic COVID-19. It was stopped early and underpowered due to low primary outcome rates.
CLINICAL TRIALS REGISTRATION: ClinicalTrials.gov Identifier: NCT04668950
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