Quantifying the Airflow Distortion over Merchant Ships. Part II: Application of the Model Results

2006 ◽  
Vol 23 (3) ◽  
pp. 351-360 ◽  
Author(s):  
Bengamin I. Moat ◽  
Margaret J. Yelland ◽  
Anthony F. Molland
Keyword(s):  
Wind Speed ◽  
Leading Edge ◽  
Bulk Carrier ◽  
Flow Distortion ◽  
Velocity Gradients ◽  
Large Velocity ◽  
General Cargo ◽  
Dynamics Models ◽  
Accelerated Flow ◽  
Relative Wind

Abstract Wind speed measurements obtained from ship-mounted anemometers are biased by the presence of the ship, which distorts the airflow to the anemometer. Previous studies have simulated the flow over detailed models of individual research ships in order to quantify the effect of flow distortion at well-exposed anemometers, usually sited on a mast in the ship's bows. In contrast, little work has been undertaken to examine the effects of flow distortion at anemometers sited on other merchant ships participating in the voluntary observing ship (VOS) project. Anemometers are usually sited on a mast above the bridge of VOS where the effects of flow distortion may be severe. The several thousand VOS vary a great deal in shape and size and it would be impractical to study each individual ship. This study examines the airflow above the bridge of a typical, or generic, tanker/bulk carrier/general cargo ship using computational fluid dynamics models. The results show that the airflow separates at the upwind leading edge of the bridge and a region of severely decelerated flow exists close to the bridge top with a region of accelerated flow above. Large velocity gradients occur between the two regions. The wind speed bias is highly dependent upon the anemometer location and varies from accelerations of 10% or more to decelerations of 100%. The wind speed bias at particular locations also varies with the relative wind direction, that is, the angle of the ship to the wind. Wind speed biases for various anemometer positions are given for bow-on and beam-on flows.

scholarly journals Using global reanalysis data to quantify and correct airflow distortion bias in shipborne wind speed measurements

2020 ◽  
Vol 13 (6) ◽  
pp. 3487-3506
Author(s):  
Sebastian Landwehr ◽  
Iris Thurnherr ◽  
Nicolas Cassar ◽  
Martin Gysel-Beer ◽  
Julia Schmale
Keyword(s):  
Wind Speed ◽  
Computational Fluid Dynamic ◽  
Wind Direction ◽  
Fluid Dynamic ◽  
Reanalysis Data ◽  
Correction Factors ◽  
Flow Distortion ◽  
Near Surface ◽  
Wind Speeds ◽  
Relative Wind

Abstract. At sea, wind forcing is responsible for the formation and development of surface waves and represents an important source of near-surface turbulence. Therefore, processes related to near-surface turbulence and wave breaking, such as sea spray emission and air–sea gas exchange, are often parameterised with wind speed. Thus, shipborne wind speed measurements provide highly relevant observations. They can, however, be compromised by flow distortion due to the ship's structure and objects near the anemometer that modify the airflow, leading to a deflection of the apparent wind direction and positive or negative acceleration of the apparent wind speed. The resulting errors in the estimated true wind speed can be greatly magnified at low wind speeds. For some research ships, correction factors have been derived from computational fluid dynamic models or through direct comparison with wind speed measurements from buoys. These correction factors can, however, lose their validity due to changes in the structures near the anemometer and, thus, require frequent re-evaluation, which is costly in either computational power or ship time. Here, we evaluate if global atmospheric reanalysis data can be used to quantify the flow distortion bias in shipborne wind speed measurements. The method is tested on data from the Antarctic Circumnavigation Expedition onboard the R/V Akademik Tryoshnikov, which are compared to ERA-5 reanalysis wind speeds. We find that, depending on the relative wind direction, the relative wind speed and direction measurements are biased by −37 % to +22 % and -17∘ to +11∘ respectively. The resulting error in the true wind speed is +11.5 % on average but ranges from −4 % to +41 % (5th and 95th percentile). After applying the bias correction, the uncertainty in the true wind speed is reduced to ±5 % and depends mainly on the average accuracy of the ERA-5 data over the period of the experiment. The obvious drawback of this approach is the potential intrusion of model biases in the correction factors. We show that this problem can be somewhat mitigated when the error propagation in the true wind correction is accounted for and used to weight the observations. We discuss the potential caveats and limitations of this approach and conclude that it can be used to quantify flow distortion bias for ships that operate on a global scale. The method can also be valuable to verify computational fluid dynamic studies of airflow distortion on research vessels.

scholarly journals Using global reanalysis data to quantify and correct airflow distortion bias in shipborne wind speed measurements

2019 ◽  
Author(s):  
Sebastian Landwehr ◽  
Iris Thurnherr ◽  
Nicolas Cassar ◽  
Martin Gysel-Beer ◽  
Julia Schmale
Keyword(s):  
Wind Speed ◽  
Computational Fluid Dynamic ◽  
Wind Direction ◽  
Fluid Dynamic ◽  
Correction Factors ◽  
Flow Distortion ◽  
Near Surface ◽  
Wind Speeds ◽  
Surface Turbulence ◽  
Relative Wind

Abstract. At sea, wind forcing is responsible for the formation and development of surface waves and represents an important source of near surface turbulence. Therefore, processes related to near surface turbulence and wave breaking, such as sea spray emission and air-sea gas exchange are often parametrised with wind speed. Shipborne wind speed measurements thus provide highly relevant observations. They can, however, be compromised by flow distortion due to the ship's structure and objects nearby the anemometer that modify the airflow, leading to a deflection of the apparent wind direction and positive or negative acceleration of the apparent wind speed. The resulting errors in the estimated true wind speed can be greatly magnified at low wind speeds. For some research ships, correction factors have been derived from computational fluid dynamic models or through direct comparison with wind speed measurements from buoys. These correction factors can, however, loose their validity due to changes of the structures nearby the anemometer and thus require frequent re-evaluation, which is costly in either computational power or ship time. Here we evaluate if global weather forecast model data can be used to quantify the flow distortion bias in shipborne wind speed measurements. The method is tested on data from the Antarctic Circumnavigation Expedition (ACE) on board the R/V Akademik Tryoshnikov, which are compared with ERA-5 reanalysis wind speeds. We find that, depending on the relative wind direction, the relative wind speed and direction measurements are biased by −37 % to +20 % and −13° to +15°, respectively. The resulting error in the true wind speed is +11 % on average but ranges from −5 % to +40 % (5th and 95th percentile). After applying the bias correction, the uncertainty in the true wind speed is reduced to 5 % and depends mainly on the average accuracy of the ERA-5 data over the period of the experiment. The obvious drawback of this approach is the potential intrusion of model bias in the correction factors. We show that this problem can be somewhat mediated when the error propagation in the true wind correction is accounted for and used to weight the observations. We discuss the potential caveats and limitations of this approach and conclude that it can be used to quantify flow distortion bias for ships that operate on a global scale. The method can also be valuable to verify Computational Fluid Dynamic studies of airflow distortion on research vessels.

The Influence of Boundary Layer Velocity Gradients and Bed Proximity on Vortex Shedding From Free Spanning Pipelines

1984 ◽  
Vol 106 (1) ◽  
pp. 70-78 ◽  
Author(s):  
A. J. Grass ◽  
P. W. J. Raven ◽  
R. J. Stuart ◽  
J. A. Bray
Keyword(s):  
Boundary Layer ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Combined Effects ◽  
Maximum Increase ◽  
Velocity Gradients ◽  
Large Velocity ◽  
Layer Velocity ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The paper summarizes the results of a laboratory study of the separate and combined effects of bed proximity and large velocity gradients on the frequency of vortex shedding from pipeline spans immersed in the thick boundary layers of tidal currents. This investigation forms part of a wider project concerned with the assessment of span stability. The measurements show that in the case of both sheared and uniform approach flows, with and without velocity gradients, respectively, the Strouhal number defining the vortex shedding frequency progressively increases as the gap between the pipe base and the bed is reduced below two pipe diameters. The maximum increase in vortex shedding Strouhal number, recorded close to the bed in an approach flow with large velocity gradients, was of the order of 25 percent.

Study on the Relation Between Side Ratios of Rectangular Cross Sections and Secondary Vortices at Trailing Edge in Motion-Induced Vortex Excitation

2017 ◽  
Author(s):  
Kazutoshi Matsuda ◽  
Kusuo Kato ◽  
Kouki Arise ◽  
Hajime Ishii
Keyword(s):  
Wind Speed ◽  
Wind Tunnel ◽  
Cross Sections ◽  
Leading Edge ◽  
Trailing Edge ◽  
Wind Speeds ◽  
Smoke Flow ◽  
Flow Visualizations ◽  
Institute Of Technology ◽  
Secondary Vortices

According to the results of conventional wind tunnel tests on rectangular cross sections with side ratios of B/D = 2–8 (B: along-wind length (m), D: cross-wind length (m)), motion-induced vortex excitation was confirmed. The generation of motion-induced vortex excitation is considered to be caused by the unification of separated vortices from the leading edge and secondary vortices at the trailing edge [1]. Spring-supported test for B/D = 1.18 was conducted in a closed circuit wind tunnel (cross section: 1.8 m high×0.9 m wide) at Kyushu Institute of Technology. Vibrations were confirmed in the neighborhoods of reduced wind speeds Vr = V/fD = 2 and Vr = 8 (V: wind speed (m/s), f: natural frequency (Hz)). Because the reduced wind speed in motion-induced vortex excitation is calculated as Vr = 1.67×B/D = 1.67×1.18 = 2.0 [1], vibrations around Vr = 2 were considered to be motion-induced vortex excitation. According to the smoke flow visualization result for B/D = 1.18 which was carried out by the authors, no secondary vortices at the trailing edge were formed, although separated vortices from the leading edge were formed at the time of oscillation at the onset wind speed of motion-induced vortex excitation, where aerodynamic vibrations considered to be motion-induced vortex excitation were confirmed. It was suggested that motion-induced vortex excitation might possibly occur in the range of low wind speeds, even in the case of side ratios where secondary vortices at trailing edge were not confirmed. In this study, smoke flow visualizations were performed for ratios of B/D = 0.5–2.0 in order to find out the relation between side ratios of rectangular cross sections and secondary vortices at trailing edge in motion-induced vortex excitation. The smoke flow visualizations around the model during oscillating condition were conducted in a small-sized wind tunnel at Kyushu Institute of Technology. Experimental Reynolds number was Re = VD/v = 1.6×103. For the forced-oscillating amplitude η, the non-dimensional double amplitudes were set as 2η/D = 0.02–0.15. Spring-supported tests were also carried out in order to obtain the response characteristics of the models.

The Importance of Relative Wind Speed in Estimating Air–Sea Turbulent Heat Fluxes in Bulk Formulas: Examples in the Bohai Sea

2020 ◽  
Vol 37 (4) ◽  
pp. 589-603 ◽  
Author(s):  
Xiangzhou Song
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Wind Speed ◽  
Bohai Sea ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Surface Currents ◽  
The Bohai Sea ◽  
Turbulent Heat Fluxes ◽  
Relative Wind

AbstractSea surface currents are commonly neglected when estimating the air–sea turbulent heat fluxes in bulk formulas. Using buoy observations in the Bohai Sea, this paper investigated the effects of near-coast multiscale currents on the quantification of turbulent heat fluxes, namely, latent heat flux (LH) and sensible heat flux (SH). The maximum current reached 1 m s−1 in magnitude, and a steady northeastward current of 0.16 m s−1 appeared in the southern Bohai Strait. The predominant tidal signal was the semidiurnal current, followed by diurnal components. The mean absolute surface wind was from the northeast with a speed of approximately 3 m s−1. The surface winds at a height of 11 m were dominated by the East Asian monsoon. As a result of upwind flow, the monthly mean differences in LH and SH between the estimates with and without surface currents ranged from 1 to 2 W m−2 in July (stable boundary layer) and November (unstable boundary layer). The hourly differences were on average 10 W m−2 and ranged from 0 to 24 W m−2 due to changes in the relative wind speed by high-frequency rotating surface tidal currents. The diurnal variability in LH/SH was demonstrated under stable and unstable boundary conditions. Observations provided an accurate benchmark for flux comparisons. The newly updated atmospheric reanalysis products MERRA-2 and ERA5 were superior to the 1° OAFlux data at this buoy location. However, future efforts in heat flux computation are still needed to, for example, consider surface currents and resolve diurnal variations.

scholarly journals High-resolution, near-IR spectroscopy and imaging of the Egg and Rotten Egg nebulae (AFGL 2688 and OH 231.8+4.2)

1999 ◽  
Vol 191 ◽  
pp. 431-436
Author(s):  
Joel H. Kastner ◽  
LeeAnn Henn ◽  
David A. Weintraub ◽  
Ian Gatley
Keyword(s):  
Near Infrared ◽  
Polar Axis ◽  
Central Star ◽  
Infrared Images ◽  
Near Ir ◽  
Velocity Gradients ◽  
Mira Variable ◽  
Model Combining ◽  
Large Velocity ◽  
Spherical Expansion

Velocity-resolved 2.12 μm H2 maps of the Egg Nebula (AFGL 2688), obtained with the NOAO Phoenix spectrometer, elaborate on previous observations of large velocity gradients in molecular emission both along and perpendicular to the polar axis of the system. The measured gradients along the polar axis support the notion that the polar H2 emission regions are formed in shocks as fast, collimated winds collide with material ejected while the central star was still on the AGB. The kinematics of H2 emission along the equatorial plane, meanwhile, are consistent with a model combining spherical expansion with rotation about the polar axis, although a model invoking multiple outflow axes cannot be ruled out.We also obtained the first direct near-infrared images of QX Pup, a Mira variable embedded within the evolved bipolar nebula OH 231.8+4.2. The inferred absolute K magnitude of the central Mira appears “normal” given its ∼ 700 day period, which is remarkable in light of its position at the heart of such an unusual object.

scholarly journals Field Measurements of Wind Characteristics Using LiDAR on a Wind Farm with Downwind Turbines Installed in a Complex Terrain Region

Energies ◽  
2020 ◽  
Vol 13 (19) ◽  
pp. 5135
Author(s):  
Tetsuya Kogaki ◽  
Kenichi Sakurai ◽  
Susumu Shimada ◽  
Hirokazu Kawabata ◽  
Yusuke Otake ◽  
...  
Keyword(s):  
Wind Speed ◽  
Wind Farm ◽  
Field Experiments ◽  
Wind Farms ◽  
Field Measurements ◽  
Horizontal Wind ◽  
Flow Distortion ◽  
Horizontal Wind Speed ◽  
Vertical Profiling ◽  
Wind Characteristics

Downwind turbines have favorable characteristics such as effective energy capture in up-flow wind conditions over complex terrains. They also have reduced risk of severe accidents in the event of disruptions to electrical networks during strong storms due to the free-yaw effect of downwind turbines. These favorable characteristics have been confirmed by wind-towing tank experiments and computational fluid dynamics (CFD) simulations. However, these advantages have not been fully demonstrated in field experiments on actual wind farms. In this study—although the final objective was to demonstrate the potential advantages of downwind turbines through field experiments—field measurements were performed using a vertical-profiling light detection and ranging (LiDAR) system on a wind farm with downwind turbines installed in complex terrains. To deduce the horizontal wind speed, vertical-profiling LiDARs assume that the flow of air is uniform in space and time. However, in complex terrains and/or in wind farms where terrain and/or wind turbines cause flow distortion or disturbances in time and space, this assumption is not valid, resulting in erroneous wind speed estimates. The magnitude of this error was evaluated by comparing LiDAR measurements with those obtained using a cup anemometer mounted on a meteorological mast and detailed analysis of line-of-sight wind speeds. A factor that expresses the nonuniformity of wind speed in the horizontal measurement plane of vertical-profiling LiDAR is proposed to estimate the errors in wind speed. The possibility of measuring and evaluating various wind characteristics such as flow inclination angles, turbulence intensities, wind shear and wind veer, which are important for wind turbine design and for wind farm operation is demonstrated. However, additional evidence of actual field measurements on wind farms in areas with complex terrains is required in order to obtain more universal and objective evaluations.

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