scholarly journals Transition near the edge of a rotating disk

2013 ◽  
Vol 737 ◽  
Author(s):  
Benoît Pier
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Large Data ◽  
Rotating Disk ◽  
Finite Radius ◽  
Objective Criterion ◽  
Data Set ◽  
Wide Range ◽  
Stabilizing Effect ◽  
Strong Source

AbstractThe rotating-disk boundary layer is generally considered as an example of a flow that displays a robust transition from laminar to turbulent régimes. By taking into account disks of finite radius, Healey (J. Fluid Mech., vol. 663, 2010, pp. 148–159) has predicted a stabilizing effect of the boundary condition, but Imayama et al. (J. Fluid Mech., vol. 716, 2013, pp. 638–657) were unable to confirm this prediction experimentally. Following these contradictory results, the present experimental investigation revisits the rotating-disk boundary layer, without any artificially imposed excitation, and studies in further detail the dynamics prevailing in the region closely surrounding the edge of the disk, as well as the flow beyond the disk. Azimuthal mean velocities and fluctuation amplitudes are recorded with small steps in radial and axial directions for a wide range of disk sizes. An objective criterion is used to define the onset of fluctuations consistently over a large data set. Two distinct mechanisms for the onset of fluctuations are identified. In particular, it is found that the flow over the edge of the disk acts as a strong source of fluctuations. Explanations and suggestions for a possible reconciliation of previous studies are given.

Conversaziones 1955

1956 ◽  
Vol 12 (1) ◽  
pp. 9-20
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Rotating Disk ◽  
National Physical Laboratory ◽  
Turbulent Fluctuations ◽  
Research Activities ◽  
Physical Laboratory ◽  
Wide Range ◽  
Dimensional Boundary ◽  
Swept Wings

Conversaziones were held on 19 May and 28 June 1955. At the first there were thirty-six exhibits, covering a wide range of research activities. An acoustical demonstration of the instability of the laminar boundary layer on a rotating disk was given by Mr N. Gregory, Mr J. T. Stuart and Mr W. S. Walker, of the Aerodynamics Division, National Physical Laboratory. The rotating disk illustrates a phenomenon which also occurs in the flow over the swept wings of modern aircraft, the instability in the latter case being due to the growth of self-amplifying vortices in the three-dimensional boundary layer over the nose of the wing. By using a stethoscope the vibrations produced by the vortices and by the random turbulent fluctuations at the edge of the disk can clearly be heard.

MODEL SELECTION AND AVERAGING OF HEALTH COSTS IN EPISODE TREATMENT GROUPS

2016 ◽  
Vol 47 (1) ◽  
pp. 153-167 ◽  
Author(s):  
Shujuan Huang ◽  
Brian Hartman ◽  
Vytaras Brazauskas
Keyword(s):  
Random Forest ◽  
Model Selection ◽  
Bayesian Model ◽  
Bayesian Model Averaging ◽  
Cost Model ◽  
Model Averaging ◽  
Large Data ◽  
Data Set ◽  
Treatment Groups ◽  
Wide Range

Episode Treatment Groups (ETGs) classify related services into medically relevant and distinct units describing an episode of care. Proper model selection for those ETG-based costs is essential to adequately price and manage health insurance risks. The optimal claim cost model (or model probabilities) can vary depending on the disease. We compare four potential models (lognormal, gamma, log-skew-t and Lomax) using four different model selection methods (AIC and BIC weights, Random Forest feature classification and Bayesian model averaging) on 320 ETGs. Using the data from a major health insurer, which consists of more than 33 million observations from 9 million claimants, we compare the various methods on both speed and precision, and also examine the wide range of selected models for the different ETGs. Several case studies are provided for illustration. It is found that Random Forest feature selection is computationally efficient and sufficiently accurate, hence being preferred in this large data set. When feasible (on smaller data sets), Bayesian model averaging is preferred because of the posterior model probabilities.

scholarly journals Experimental duration and predator satiation levels systematically affect functional response parameters

2017 ◽  
Author(s):  
Yuanheng Li ◽  
Björn C. Rall ◽  
Gregor Kalinkat
Keyword(s):  
Food Web ◽  
Functional Response ◽  
Large Data ◽  
Handling Time ◽  
Set Covering ◽  
Parameter Estimates ◽  
Functional Responses ◽  
Predator Satiation ◽  
Data Set ◽  
Wide Range

AbstractEmpirical feeding studies where density-dependent consumption rates are fitted to functional response models are often used to parametrize the interaction strengths in models of population or food-web dynamics. However, the relationship between functional response parameter estimates from short-term feeding studies and real-world, long-term, trophic interaction strengths remains largely untested. In a critical first step to address this void, we tested for systematic effects of experimental duration and predator satiation on the estimation of functional response parameters, namely attack rate and handling time. Analyzing a large data set covering a wide range of predator taxonomies and body sizes we show that attack rates decrease with increasing experimental duration, and that handling times of starved predators are consistently shorter than those of satiated predators. Therefore, both the experimental duration and the predator satiation level have a strong and systematic impact on the predictions of population dynamics and food-web stability. Our study highlights potential pitfalls at the intersection of empirical and theoretical applications of functional responses. We conclude our study with some practical suggestions how these implications should be addressed in the future to improve predictive abilities and realism in models of predator-prey interactions.

scholarly journals The importance of vehicle emissions as a source of atmospheric ammonia in the megacity of Shanghai

2016 ◽  
Vol 16 (5) ◽  
pp. 3577-3594 ◽  
Author(s):  
Yunhua Chang ◽  
Zhong Zou ◽  
Congrui Deng ◽  
Kan Huang ◽  
Jeffrey L. Collett ◽  
...  
Keyword(s):  
Urban Area ◽  
Urban Areas ◽  
Vehicle Emissions ◽  
Crop Residues ◽  
Chemical Properties ◽  
Large Data ◽  
Ambient Air ◽  
Urban Atmosphere ◽  
Data Set ◽  
Wide Range

Abstract. Agricultural activities are a major source contributing to NH3 emissions in Shanghai and most other regions of China; however, there is a long-standing and ongoing controversy regarding the contributions of vehicle-emitted NH3 to the urban atmosphere. From April 2014 to April 2015, we conducted measurements of a wide range of gases (including NH3) and the chemical properties of PM2.5 at hourly resolution at a Shanghai urban supersite. This large data set shows NH3 pollution events, lasting several hours with concentrations 4 times the annual average of 5.3 µg m−3, caused by the burning of crop residues in spring. There are also generally higher NH3 concentrations (mean ± 1 σ) in summer (7.3 ± 4.9 µg m−3; n = 2181) because of intensive emissions from temperature-dependent agricultural sources. However, the NH3 concentration in summer was only an average of 2.4 µg m−3 or 41 % higher than the average NH3 concentration of other seasons. Furthermore, the NH3 concentration in winter (5.0 ± 3.7 µg m−3; n = 2113) was similar to that in spring (5.1 ± 3.8 µg m−3; n = 2198) but slightly higher, on average, than that in autumn (4.5 ± 2.3 µg m−3; n = 1949). Moreover, other meteorological parameters like planetary boundary layer height and relative humidity were not major factors affecting seasonal NH3 concentrations. These findings suggest that there may be some climate-independent NH3 sources present in the Shanghai urban area. Independent of season, the concentrations of both NH3 and CO present a marked bimodal diurnal profile, with maxima in the morning and the evening. A spatial analysis suggests that elevated concentrations of NH3 are often associated with transport from regions west–northwest and east–southeast of the city, areas with dense road systems. The spatial origin of NH3 and the diurnal concentration profile together suggest the importance of vehicle-derived NH3 associated with daily commuting in the urban environment. To further examine vehicular NH3 emissions and transport, sampling of the NH3 concentration was performed in (from the entrance to the exit of the tunnel) and out (along a roadside transect spanning 310 m perpendicular to the tunnel) of a heavily trafficked urban tunnel during the spring of 2014. NH3 concentrations in the tunnel exit were over 5 and 11 times higher than those in the tunnel entrance and in the ambient air, respectively. Based on the derived mileage-based NH3 emission factor of 28 mg km−1, a population of 3.04 million vehicles in Shanghai produced around 1300 t NH3 in 2014, which accounts for 12 % of total NH3 emissions in the urban area. Collectively, our results clearly show that vehicle emissions associated with combustion are an important NH3 source in Shanghai urban areas and may have potential implications for PM2.5 pollution in the urban atmosphere.

scholarly journals User perceptions of long-term costs and benefits of MDMA use: Findings from a large online sample

2021 ◽  
Author(s):  
James Elsey ◽  
Vivi Anna Felicia Wuestman ◽  
Anouk Fieten
Keyword(s):  
Online Survey ◽  
Drug Effects ◽  
Large Data ◽  
Future Research ◽  
Long Term Effects ◽  
Negative Effects ◽  
Data Set ◽  
User Perceptions ◽  
Wide Range

Background. Previous research has focused largely on risks associated with non-clinical (‘recreational’) MDMA use. Potential benefits produced through MDMA-assisted psychotherapy raise the question whether recreational users might report some positive long-term effects of MDMA use.Aim. 1) To assess the perceptions of recreational MDMA users regarding a wide range of both positive and negative long-term effects of MDMA use. 2) To generate a large data set including other correlates of interest to highlight targets for future research. Methods. Eight-hundred and eighty-six MDMA users gave valid responses to an online survey, covering perceived long-term effects, acute and subacute effects, use motivations, contexts of use, and polydrug use. We present descriptive results for motivations, acute, subacute, and long-term effects, and exploratory relationships between long-term effects and these variables.Results. User perceptions of MDMA’s long-term effects were far more positive than negative, with a positive overarching perspective on having used MDMA and endorsement of several more specific effects. Negative/unpleasant acute and subacute effects of MDMA use were positively related to negative long-term effects. Positive/pleasant acute and subacute effects were positively related to positive long-term effects. User motivations were also significantly associated with perceived long-term effects. A minority of users reported negative effects (e.g., on concentration/memory and mood), which should not be ignored.Conclusions. Factors beyond the anticipation of a short-term high likely play into many users’ decisions to use MDMA. A nuanced understanding of user perceptions can inform drug safety communication, our understanding of drug effects, and drug policy.

scholarly journals Rainfall-Runoff Modelling Considerations to Predict Streamflow Characteristics in Ungauged Catchments and under Climate Change

Water ◽  
2018 ◽  
Vol 10 (10) ◽  
pp. 1319 ◽  
Author(s):  
Francis Chiew ◽  
Hongxing Zheng ◽  
Nicholas Potter
Keyword(s):  
Climate Change ◽  
Flow Characteristics ◽  
Large Data ◽  
Low Flow ◽  
Rainfall Runoff ◽  
Ungauged Catchments ◽  
Objective Criterion ◽  
Data Set ◽  
Daily Streamflow ◽  
Parameter Values

This paper investigates the prediction of different streamflow characteristics in ungauged catchments and under climate change, with three rainfall-runoff models calibrated against three different objective criteria, using a large data set from 780 catchments across Australia. The results indicate that medium and high flows are relatively easier to predict, suggesting that using a single unique set of parameter values from model calibration against an objective criterion like the Nash–Sutcliffe efficiency is generally adequate and desirable to provide a consistent simulation and interpretation of daily streamflow series and the different medium and high flow characteristics. However, the low flow characteristics are considerably more difficult to predict and will require careful modelling consideration to specifically target the low flow characteristic of interest. The modelling results also show that different rainfall-runoff models and different calibration approaches can give significantly different predictions of climate change impact on streamflow characteristics, particularly for characteristics beyond the long-term averages. Predicting the hydrological impact from climate change, therefore, requires careful modelling consideration and calibration against appropriate objective criteria that specifically target the streamflow characteristic that is being assessed.

Joint High Speed Sealift (JHSS) Segmented Model Test Results

2012 ◽  
Author(s):  
Dominic Piro ◽  
Thomas C. Fu ◽  
Kevin J. Maki
Keyword(s):  
High Speed ◽  
Large Data ◽  
Torsional Stiffness ◽  
Data Set ◽  
Segmented Model ◽  
Test Matrix ◽  
Operational Conditions ◽  
Wide Range ◽  
Vibrational Characteristics ◽  
Water Test

The Joint High Speed Sealift segmented model (Model 5663) tests performed in 2007, in the Maneuvering and Seakeeping Basin at the Naval Surface Warfare Center, Carderock Division, were designed to provide a large data set for validation of numerical simulations. Model 5663 is a segmented structural ship model that has scaled longitudinal bending and torsional stiffness. The scaled stiffness is obtained by building a backspline into the model that accounts for the bending stiffness and cutting the shell in several places, segmenting it to isolate the stiffness to the backspline. The alternative way to obtain structural loads would be to build a model with scaled plates and stiffeners; however, this would be very difficult and expensive. The backspline allows the stiffness to be scaled properly while using reasonable materials and simple construction. The hull segments of the model are connected with silicone to maintain a watertight connection. The model is self propelled and steered during data collection. The test matrix spans a wide set of wave conditions, including regular and irregular seas, with heading angles spanning the possible range. A wide range of speeds are also included, with Froude numbers ranging from 0 to 0.43. This test matrix, which includes about 2000 runs, allows for validation of codes from still water test, through operational conditions, to extreme design load determination. Different aspects of the data have been studied, but much is still left to be considered. An aspect of the model tests that has not previously been considered in detail is the hydroelastic response of the vessel. Hydroelastic phenomena are a subset of fluid-structure interaction problems where the elasticity of the structure is important. The vibrational characteristics of the model are determined. The main phenomena of interest are springing and whipping, and an analysis of the springing response and the whipping response in head seas is also discussed.

Steady and unsteady modelling of the float height of a rotating air hockey disk

2015 ◽  
Vol 778 ◽  
pp. 39-59 ◽  
Author(s):  
Patrick D. Weidman ◽  
Michael A. Sprague
Keyword(s):  
Reynolds Number ◽  
Stokes Equations ◽  
Rotating Disk ◽  
Navier Stokes ◽  
Large Reynolds Number ◽  
Finite Radius ◽  
Navier Stokes Equations ◽  
Similarity Reduction ◽  
Disk Rotation ◽  
Wide Range

A similarity reduction of the Navier–Stokes equations for the motion of an infinite rotating disk above an air-bearing table yields a coupled pair of ordinary differential equations governed by a Reynolds number $Re=Wh/{\it\nu}$ and a rotation parameter $S=\sqrt{2}h{\it\Omega}/W$, where $h$ is the float height, $W$ is the air levitation velocity, ${\it\Omega}$ is the disk rotation rate, and ${\it\nu}$ is the kinematic viscosity of air. After deriving the small- and large-Reynolds-number behaviour of solutions, the equations are numerically integrated over a wide range of $Re{-}S$ parameter space. Zero-lift boundaries are computed as well as the boundaries separating pure outward flow from counter-flow in the gap. The theory is used to model the steady float height of a finite-radius air hockey disk under the assumption that the float height is small relative to the diameter of the disk and the flow is everywhere laminar. The steady results are tested against direct numerical simulation (DNS) of the unsteady axisymmetric Navier–Stokes equations for the cases where the disk rotates at constant angular velocity but is either at a fixed height or free to move axially. While a constant shift in the gap pressure conforms closely to that found using steady theory, the interaction of the radial jet emanating from the gap with a vertical transpiration field produces vortex rings which themselves propagate around to interact with the jet. Although these structures diffuse as they propagate up and away from the gap, they induce a departure from the steady-flow assumption of atmospheric pressure at the gap exit, thus inducing small irregular axial oscillations of the floating disk.

Heat transfer from an air-cooled rotating disk

1974 ◽  
Vol 336 (1607) ◽  
pp. 453-473 ◽  
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Numerical Solutions ◽  
Rotating Disk ◽  
Experimental Results ◽  
Boundary Layer Equations ◽  
Nusselt Numbers ◽  
Wide Range ◽  
The Mean ◽  
Radial Outflow

This paper describes a combined theoretical and experimental investigation into the heat transfer from a disk rotating close to a stator with a radial outflow of coolant. Experimental results are obtained from a 762 mm diameter disk, rotating up to 4000 rev/min at axial clearances from 2 to 230 mm from a stator of the same diameter, with coolant flow rates up to 0.7 kg/s. Mean Nusselt numbers are presented for the free disk, the disk rotating close to an unshrouded stator with no coolant outflow, the disk rotating close to a shrouded and unshrouded stator with coolant outflow, and for the unshrouded stator itself. Numerical solutions of the turbulent boundary layer equations are in satisfactory agreement with the experimentally determined mean Nusselt numbers for the air-cooled disk over a wide range of conditions. At large ratios of mass flow rate/rotational speed the mean Nusselt numbers for the air-cooled disk are independent of rotation, and both the numerical solutions and experimental results become asymptotic to an approximate solution of the boundary layer equations.

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