scholarly journals A Comparison of Methods for Estimating Reynolds Stress from ADCP Measurements in Wavy Environments

2011 ◽  
Vol 28 (11) ◽  
pp. 1539-1553 ◽  
Author(s):  
Anthony R. Kirincich ◽  
Johanna H. Rosman
Keyword(s):  
Low Frequency ◽  
Acoustic Doppler Current Profiler ◽  
Spatial Filtering ◽  
Wave Climate ◽  
Coastal Ocean ◽  
Wave Band ◽  
Reynolds Stresses ◽  
Spectral Fitting ◽  
Quadratic Drag ◽  
Adcp Measurements

Abstract Turbulent Reynolds stresses are now routinely estimated from acoustic Doppler current profiler (ADCP) measurements in estuaries and tidal channels using the variance method, yet biases due to surface gravity waves limit its use in the coastal ocean. Recent modifications to this method, including spatially filtering velocities to isolate the turbulence from wave velocities and fitting a cospectral model to the below-wave band cospectra, have been used to remove this bias. Individually, each modification performed well for the published test datasets, but a comparative analysis over the range of conditions in the coastal ocean has not yet been performed. This work uses ADCP velocity measurements from five previously published coastal ocean and estuarine datasets, which span a range of wave and current conditions as well as instrument configurations, to directly compare methods for estimating stresses in the presence of waves. The computed stresses from each were compared to bottom stress estimates from a quadratic drag law and, where available, estimates of wind stress. These comparisons, along with an analysis of the cospectra, indicated that spectral fitting performs well when the wave climate is wide-banded and/or multidirectional as well as when instrument noise is high. In contrast, spatial filtering performs better when waves are narrow-banded, low frequency, and when wave orbital velocities are strong relative to currents. However, as spatial filtering uses vertically separated velocity bins to remove the wave bias, spectral fitting is able to resolve stresses over a larger fraction of the water column.

Extracting Reynolds Stresses from Acoustic Doppler Current Profiler Measurements in Wave-Dominated Environments

2008 ◽  
Vol 25 (2) ◽  
pp. 286-306 ◽  
Author(s):  
Johanna H. Rosman ◽  
James L. Hench ◽  
Jeffrey R. Koseff ◽  
Stephen G. Monismith
Keyword(s):  
Reynolds Stress ◽  
Simple Procedure ◽  
Acoustic Doppler Current Profiler ◽  
French Polynesia ◽  
Reynolds Stresses ◽  
Variance Method ◽  
Beam Velocity ◽  
Current Profiler ◽  
Adcp Measurements ◽  
Velocity Variances

Abstract Surface waves introduce velocity correlations that bias and often dominate Reynolds stress estimates made using the traditional variance method for acoustic Doppler current profilers (ADCPs). This analysis shows that the wave bias is the sum of a real wave stress and an error due to instrument tilt, both of which have a large uncertainty. Three alternative extensions to the variance method for calculating Reynolds stress profiles from ADCP measurements in wavy conditions are analyzed. The previously proposed variance fitting method (Variance Fit) is evaluated and two more general methods that use along- and between-beam velocity differencing with adaptive filtering (Vertical AF and Horizontal AF) are derived. The three methods are tested on datasets containing long-period monochromatic swell (Moorea, French Polynesia) and shorter-period mixed swell (Santa Barbara, California). The Variance Fit method leaves a residual wave bias in beam velocity variances, especially for intermediate waves, but gives physically reasonable Reynolds stress estimates because most of the residual wave bias cancels when the variance method is applied. The new Vertical AF method does not produce inherent wave bias in beam velocity variances, but yields comparable Reynolds stresses to the Variance Fit method. The Horizontal AF method performs poorly for all but monochromatic waves. Error remaining after one of the above methods is applied can be attributed to residual wave error, correlation of turbulence between points chosen for differencing, or correlation between waves and turbulence. A simple procedure is provided for determining the minimum bin separation that can be used.

The Dynamics of Towed Arrays

1989 ◽  
Vol 111 (3) ◽  
pp. 208-213 ◽  
Author(s):  
G. S. Triantafyllou ◽  
C. Chryssostomidis
Keyword(s):  
Iterative Procedure ◽  
Cross Flow ◽  
Low Frequency ◽  
Harmonic Excitation ◽  
Fluid Forces ◽  
Damped System ◽  
Equivalent Linear ◽  
Quadratic Drag ◽  
Linear Damping ◽  
Towed Arrays

A procedure for calculating the response of an array to a harmonic excitation applied at the upstream end is presented. The fluid forces on the array are modeled following the slender-body approximation and the cross-flow principle. An equivalent linear damping is used to replace the quadratic drag due to cross-flow separation. The equivalent linear damping is determined using an iterative procedure. Numerical and asymptotic solutions are derived, and the response of a typical long array is calculated. It is found that, when the separation drag is included, the array exhibits the behavior of an over-damped system, responding only to low-frequency excitations.

scholarly journals A Method for Processing Acoustic Doppler Current Profiler Velocity Data from Towed, Undulating Vehicles

2008 ◽  
Vol 25 (9) ◽  
pp. 1710-1716 ◽  
Author(s):  
Jiayi Pan ◽  
David A. Jay
Keyword(s):  
Surface Waters ◽  
High Frequency ◽  
Acoustic Doppler Current Profiler ◽  
Small Time ◽  
Ocean Current ◽  
Near Surface ◽  
Vertical Range ◽  
External Reference ◽  
Current Profiler ◽  
Adcp Measurements

Abstract The utility of the acoustic Doppler current profiler (ADCP) for sampling small time and space scales of coastal environments can be enhanced by mounting a high-frequency (1200 kHz) ADCP on an oscillating towed body. This approach requires both an external reference to convert the measured shears to velocities in the earth coordinates and a method to determine the towed body velocities. During the River Influence on the Shelf Ecosystems (RISE) project cruise, a high-frequency (1200 kHz) and narrowbeam ADCP with mode 12 sampling was mounted on a TRIAXUS oscillating towfish, which steers a 3D path behind the ship. This deployment approach extended the vertical range of the ADCP and allowed it to sample near-surface waters outside the ship’s wake. The measurements from a ship-mounted 1200-kHz narrowbeam ADCP are used as references for TRIAXUS ADCP data, and a method of overlapping bins is employed to recover the entire vertical range of the TRIAXUS ADCP. The TRIAXUS vehicle horizontal velocities are obtained by removing the derived ocean current velocity from the TRIAXUS ADCP measurements. The results show that the method is practical.

scholarly journals Divergence-Free Spatial Velocity Flow Field Interpolator for Improving Measurements from ADCP-Equipped Small Unmanned Underwater Vehicles

2012 ◽  
Vol 29 (3) ◽  
pp. 478-484 ◽  
Author(s):  
Jamie MacMahan ◽  
Ross Vennell ◽  
Rick Beatson ◽  
Jenna Brown ◽  
Ad Reniers
Keyword(s):  
Flow Field ◽  
Signal To Noise Ratio ◽  
Acoustic Doppler Current Profiler ◽  
Unmanned Underwater Vehicles ◽  
Velocity Magnitude ◽  
Velocity Flow ◽  
Current Profiler ◽  
Adcp Measurements ◽  
Measured Flow ◽  
Divergence Free

Abstract Applying a two-dimensional (2D) divergence-free (DF) interpolation to a one-person deployable unmanned underwater vehicle’s (UUV) noisy moving-vessel acoustic Doppler current profiler (MV-ADCP) measurements improves the results and increases the utility of the UUV in tidal environments. For a 3.5-h MV-ACDP simulation that spatially and temporally varies with the M2 tide, the 2D DF-estimated velocity magnitude and orientation improves by approximately 85%. Next the 2D DF method was applied to velocity data obtained from two UUVs that repeatedly performed seven 1-h survey tracks in Bear Cut Inlet, Miami, Florida. The DF method provides a more realistic and consistent representation of the ADCP measured flow field, improving magnitude and orientation estimates by approximately 25%. The improvement increases for lower flow velocities, when the ADCP measurements have low environmental signal-to-noise ratio. However, near slack tide when flow reversal occurs, the DF estimates are invalid because the flows are not steady state within the survey circuit.

scholarly journals Wave Climate at Shallow Waters along the Abu Dhabi Coast

Water ◽  
2018 ◽  
Vol 10 (8) ◽  
pp. 985 ◽  
Author(s):  
Waleed Hamza ◽  
Letizia Lusito ◽  
Francesco Ligorio ◽  
Giuseppe Tomasicchio ◽  
Felice D’Alessandro
Keyword(s):  
Shallow Water ◽  
Numerical Models ◽  
Grid Point ◽  
Arabian Gulf ◽  
Acoustic Doppler Current Profiler ◽  
Wave Climate ◽  
Abu Dhabi ◽  
Water Conditions ◽  
Key Factor ◽  
Offshore Wave

High-resolution, reliable global atmospheric and oceanic numerical models can represent a key factor in designing a coastal intervention. At the present, two main centers have the capabilities to produce them: the National Oceanic and Atmospheric Administration (NOAA) in the U.S.A. and the European Centre for Medium-Range Weather Forecasts (ECMWF). The NOAA and ECMWF wave models are developed, in particular, for different water regions: deep, intermediate, and shallow water regions using different types of spatial and temporal grids. Recently, in the Arabian Gulf (also named Persian Gulf), the Abu Dhabi Municipality (ADM) installed an ADCP (Acoustic Doppler Current Profiler) to observe the atmospheric and oceanographic conditions (water level, significant wave height, peak wave period, water temperature, and wind speed and direction) at 6 m water depth, in the vicinity of the shoreline of the Saadiyat beach. Courtesy of Abu Dhabi Municipality, this observations dataset is available; the recorded data span the period from June 2015 to January 2018 (included), with a time resolution of 10 min and 30 min for the atmospheric and oceanographic variables, respectively. At the ADCP deployment location (ADMins), the wave climate has been determined using wave propagation of the NOAA offshore wave dataset by means of the Simulating WAves Nearshore (SWAN) numerical model, the NOAA and ECMWF wave datasets at the closest grid point in shallow water conditions, and the SPM ’84 hindcasting method with the NOAA wind dataset used as input. It is shown that the best agreement with the observed wave climate is obtained using the SPM ’84 hindcasting method for the shallow water conditions.

Measurements of Form and Frictional Drags over a Rough Topographic Bank

2014 ◽  
Vol 44 (9) ◽  
pp. 2409-2432 ◽  
Author(s):  
H. W. Wijesekera ◽  
E. Jarosz ◽  
W. J. Teague ◽  
D. W. Wang ◽  
D. B. Fribance ◽  
...  
Keyword(s):  
Drag Coefficient ◽  
Low Frequency ◽  
Multiple Time ◽  
Linear Wave ◽  
Frictional Drag ◽  
Form Drag ◽  
Quadratic Drag ◽  
Roughness Elements ◽  
Roughness Lengths ◽  
Surface Generate

Abstract Pressure differences across topography generate a form drag that opposes the flow in the water column, and viscous and pressure forces acting on roughness elements of the topographic surface generate a frictional drag on the bottom. Form drag and bottom roughness lengths were estimated over the East Flower Garden Bank (EFGB) in the Gulf of Mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates. The EFGB is a coral bank about 6 km wide and 10 km long located at the shelf edge that rises from 100-m water depth to about 18 m below the sea surface. The average frictional drag coefficient over the entire bank was estimated as 0.006 using roughness lengths that ranged from 0.001 cm for relatively smooth portions of the bank to 1–10 cm for very rough portions over the corals. The measured form drag over the bank showed multiple time-scale variability. Diurnal tides and low-frequency motions with periods ranging from 4 to 17 days generated form drags of about 2000 N m−1 with average drag coefficients ranging between 0.03 and 0.22, which are a factor of 5–35 times larger than the average frictional drag coefficient. Both linear wave and quadratic drag laws have similarities with the observed form drag. The form drag is an important flow retardation mechanism even in the presence of the large frictional drag associated with coral reefs and requires parameterization.

scholarly journals Low Frequency Gravitational Waves in Cosmology

1974 ◽  
Vol 63 ◽  
pp. 299-315
Author(s):  
G. Dautcourt
Keyword(s):  
Gravitational Waves ◽  
Gravitational Radiation ◽  
Low Frequency ◽  
Wave Band ◽  
Thermal Background

An intense non-thermal background of cosmic gravitational radiation in the Megaparsec wave band could be detected by its influence on many astrophysical processes. In particular, it may give an explanation of the so-called redshift anomalies.

scholarly journals GRB 030329: 3 years of radio afterglow monitoring

2007 ◽  
Vol 365 (1854) ◽  
pp. 1241-1246 ◽  
Author(s):  
A.J van der Horst ◽  
A Kamble ◽  
R.A.M.J Wijers ◽  
L Resmi ◽  
D Bhattacharya ◽  
...  
Keyword(s):  
Blast Wave ◽  
Gamma Ray ◽  
Ambient Medium ◽  
Low Frequency ◽  
Blast Waves ◽  
Physical Parameters ◽  
Late Time ◽  
Wave Band ◽  
Radio Telescopes ◽  
Relativistic Phase

Radio observations of gamma-ray burst (GRB) afterglows are essential for our understanding of the physics of relativistic blast waves, as they enable us to follow the evolution of GRB explosions much longer than the afterglows in any other wave band. We have performed a 3-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes and the Giant Metrewave Radio Telescope. Our observations, combined with observations at other wavelengths, have allowed us to determine the GRB blast wave physical parameters, such as the total burst energy and the ambient medium density, as well as to investigate the jet nature of the relativistic outflow. Further, by modelling the late-time radio light curve of GRB 030329, we predict that the Low-Frequency Array (30–240 MHz) will be able to observe afterglows of similar GRBs, and constrain the physics of the blast wave during its non-relativistic phase.

Interactions between second mode and low-frequency waves in a hypersonic boundary layer

2017 ◽  
Vol 820 ◽  
pp. 693-735 ◽  
Author(s):  
Xi Chen ◽  
Yiding Zhu ◽  
Cunbiao Lee
Keyword(s):  
Boundary Layer ◽  
Parametric Resonance ◽  
Low Frequency ◽  
Frequency Mode ◽  
Linear Stability Theory ◽  
Hypersonic Boundary Layer ◽  
Reynolds Stresses ◽  
Second Mode ◽  
Low Frequency Mode ◽  
Low Frequency Waves

The stability of a hypersonic boundary layer on a flared cone was analysed for the same flow conditions as in earlier experiments (Zhang et al., Acta Mech. Sinica, vol. 29, 2013, pp. 48–53; Zhu et al., AIAA J., vol. 54, 2016, pp. 3039–3049). Three instabilities in the flared region, i.e. the first mode, the second mode and the Görtler mode, were identified using linear stability theory (LST). The nonlinear-parabolized stability equations (NPSE) were used in an extensive parametric study of the interactions between the second mode and the single low-frequency mode (the Görtler mode or the first mode). The analysis shows that waves with frequencies below 30 kHz are heavily amplified. These low-frequency disturbances evolve linearly at first and then abruptly transition to parametric resonance. The parametric resonance, which is well described by Floquet theory, can be either a combination resonance (for non-zero frequencies) or a fundamental resonance (for steady waves) of the secondary instability. Moreover, the resonance depends only on the saturated state of the second mode and is insensitive to the initial low-frequency mode profiles and the streamwise curvature, so this resonance is probably observable in boundary layers over straight cones. Analysis of the kinetic energy transfer further shows that the rapid growth of the low-frequency mode is due to the action of the Reynolds stresses. The same mechanism also describes the interactions between a second-mode wave and a pair of low-frequency waves. The only difference is that the fundamental and combination resonances can coexist. Qualitative agreement with the experimental results is achieved.

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