Luksa Luznik

Distribution of Energy Spectra, Reynolds Stresses, Turbulence Production, and Dissipation in a Tidally Driven Bottom Boundary Layer

Abstract Seven sets of 2D particle image velocimetry data obtained in the bottom boundary layer of the coastal ocean along the South Carolina and Georgia coast [at the South Atlantic Bight Synoptic Offshore Observational Network (SABSOON) site] are examined, covering the accelerating and decelerating phases of a single tidal cycle at several heights above the seabed. Additional datasets from a previous deployment are also included in the analysis. The mean velocity profiles are logarithmic, and the vertical distribution of Reynolds stresses normalized by the square of the free stream velocity collapse well for data obtained at the same elevation but at different phases of the tidal cycle. The magnitudes of 〈u′u′〉, 〈w′w′〉, and −〈u′w′〉 decrease with height above bottom in the 25–160-cm elevation range and are consistent with the magnitudes and trends observed in laboratory turbulent boundary layers. If a constant stress layer exists, it is located below 25-cm elevation. Two methods for estimating dissipatio...

PIV Measurements in the Atmospheric Boundary Layer within and above a Mature Corn Canopy. Part I: Statistics and Energy Flux

Particle image velocimetry (PIV) measurements just within and above a mature corn canopy have been performed to clarify the small-scale spatial structure of the turbulence. The smallest resolved scales are about 15 times the Kolmogorov length scale ( 0.4 mm), the Taylor microscales are about 100, and the Taylor scale Reynolds numbers range between R 2000 and 3000. The vertical profiles of mean flow and turbulence parameters match those found in previous studies. Frequency spectra, obtained using the data as time series, are combined with instantaneous spatial spectra to resolve more than five orders of magnitude of length scales. They display an inertial range spanning three decades. However, the small-scale turbulence in the dissipation range exhibits anisotropy at all measurement heights, in spite of apparent agreement with conditions for reaching local isotropy, following a high-Reynolds-number wind tunnel study. Directly calculated subgrid-scale (SGS) energy flux, determined by spatially filtering the PIV data, increases significantly with decreasing filter size, providing support for the existence of a spectral shortcut that bypasses the cascading process and injects energy directly into small scales. The highest measured SGS flux is about 40% of the estimated energy cascading rate as determined from a5/3 fit to the spectra. Terms appearing in the turbulent kinetic energy budget that can be calculated from the PIV data are in agreement with previous results. Evidence of a very strong correlation between dissipation rate and out-of-plane component of the vorticity is demonstrated by a striking similarity between their time series.

Effect of Finite Spatial Resolution on the Turbulent Energy Spectrum Measured in the Coastal Ocean Bottom Boundary Layer

The effect of finite spatial resolution on the measured energy spectrum is examined via a parametric study using in situ particle image velocimetry (PIV) measurements performed in the bottom boundary layer on the Atlantic continental shelf. Two-dimensional (2D) box spatial filters of various scales are applied to the data, and these filtered distributions are used to compute 1D energy spectra in both frequency and wavenumber domains. It is found that energy levels are attenuated by more than 15% at all length scales that are smaller than 10 times the scale of the filter. Filtering both in the direction of the spectrum as well as perpendicular to it contributes to the extent of attenuation, the latter via implicit integration over all wavenumbers. At scales larger than that of the filter, Gaussian, nonlinear Butterworth, and median filters attenuate less energy than the box filter. When frequency spectra are converted using Taylor’s hypothesis, wave energy appears in wavenumber space at a location different than its true physical scale, which is much larger than the filter sizes. Consequently, wave energy is not attenuated and dominates over the turbulence through this spectral range. Because wave energy and turbulence respond differently to the filtering, modified spectral slopes at the transition between wave- and turbulence-dominated regions occur, resulting in inordinately steep spectral slopes. Finally, removal of the pressure-coherent part of the velocity signal is not sufficient to reveal the turbulence within the wave peak spectral range. Remaining energy in this range is still dominated by much larger scales.

Hydrodynamic performance of a horizontal axis tidal turbine under steady flow conditions

Performance characteristics are presented for a two-bladed horizontal axis tidal turbine, representing a 1/25th scale operational turbine. The tests were conducted in a 116 m long tow tank facility at the United States Naval Academy. The performance characteristics of power and thrust coefficient are measured for the turbine for a range of tip speed ratios. The results of the model test are applicable to full scale due to Re number independence of the rotor blades for the tested conditions. A full uncertainty analysis is performed and major sources of uncertainty are identified. Uncertainty levels are 4% and 1% for power and thrust coefficient respectively, and 3% for the tip speed ratio.

USNA Ship Air Wake Program Overview (Invited)

This paper provides an overview of a multi-year research project that involves the systematic investigation of ship air wakes using an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The objective is to validate and improve Computational Fluid Dynamics (CFD) tools that will be useful in determining ship air wake impact on naval rotary wing vehicles. This project is funded by the Office of Naval Research and includes extensive coordination with Naval Air Systems Command. Currently, ship launch and recovery wind limits and envelopes for helicopters are primarily determined through at-sea in situ flight testing that is expensive and frequently difficult to schedule and complete. The time consuming and potentially risky flight testing is required, in part, because computational tools are not mature enough to adequately predict air flow and wake data in the lee of a ship with a complex superstructure. The top-side configuration of USNA YPs is similar to that of a destroyer or cruiser, and their size (length of 108 ft and above waterline height of 24 ft) allows for collection of air wake data with a Reynolds number that is the same order of magnitude as that of modern naval warships, an important consideration in aerodynamic modeling. A dedicated YP has been modified to add a flight deck and hangar structure to produce an air wake similar to that on a modern destroyer. Three axis acoustic anemometers, fog generators and an inertial measurement unit have been installed. Repeated testing on the modified YP is being conducted in the Chesapeake Bay, which allows for the collection of data over a wide range of wind conditions. Additionally, a 4% scale model of the modified YP has been constructed and tested in the 42×60×120 inch USNA wind tunnel. The project involves USNA midshipmen who are participating in test planning, collecting and analyzing data, and in CFD modeling, providing the midshipmen with valuable professional and research experience. Comparison of YP in situ data with similar data from wind tunnel testing and CFD simulations shows reasonable agreement for a headwind condition and for wind 15° off the starboard bow.

Influence of the Atmospheric Surface Layer on a Turbulent Flow Downstream of a Ship Superstructure

AbstractThis paper describes a set of turbulence measurements at sea in the area of high flow distortion in the near-wake and recirculation zone behind a ships superstructure that is similar in geometry to a helicopter hangar/flight deck arrangement found on many modern U.S. Navy ships. The instrumented ship is a 32-m-long training vessel operated by the United States Naval Academy that has been modified by adding a representative flight deck and hangar structure. The flight deck is instrumented with up to seven sonic anemometers/thermometers that are used to obtain simultaneous velocity measurements at various spatial locations on the flight deck, and one sonic anemometer at bow mast is used to characterize inflow atmospheric boundary conditions. Data characterizing wind over the deck at an incoming angle of 0° (head winds) and wind speeds from 2 to 10 m s−1 obtained in the Chesapeake Bay are presented and discussed. Turbulent statistics of inflow conditions are analyzed using the Kaimal universal turbu...

Observations of turbulent flow fields in the Chesapeake Bay estuary for tidal energy conversion

This paper presents preliminary results from a two-week long set of field experiments performed to characterize flow structure and turbulence in the mid-water column during tidal flows in the Chesapeake Bay estuary. At the deployment location, in the vicinity of Kent Island peak flood/ebb tidal flows range from +/-40 cm/s in the along-channel direction and secondary flow in the cross-channel direction reaches peak of +/-10 cm/s during the slack parts of a tidal cycle. Conditionally sampled mean velocity profiles indicate asymmetry in the flood/ebb tidal flows, which is attributed to the tidal straining. Temporal velocity spectra are presented and show modifications of the turbulence spectra due to presence of surface gravity wave field. Time history of turbulent kinetic energy dissipation rate shows variations on a tidal time scale with magnitudes ranging from 10-6 to 10-3 m2/s3. Results are discussed in the context of tidal turbine design and performance.

Velocity measurements in a ship airwake with crosswind

In situ air velocity measurements in the near wake of a Navy training ship are presented for an inflow of 15 to starboard. This data is required for the validation of ship airwake simulations, which are used to determine the launch and recovery envelopes for shipborne rotorcraft and for use in piloted flight simulations. The measurements are taken primarily above an aft flight deck, which sits immediately behind a step-like hangar structure. A description of the mean flow structure is included, as well as the Reynolds stresses at numerous points along the ship centerline. Comparisons are made between the present 15 case and the case of a direct headwind, presented previously. Compared to the 0 inflow condition, the flow symmetry is clearly broken with a cross-wind. The port and starboard sides of the deck have very different mean flow profiles and turbulent stress components. An updraft is visible over much of the starboard side of the flight deck, which is not found on the port side, or on either side under a headwind. Along the centerline, the streamwise normal component of the turbulent stresses are much larger in the cross-wind case than in the headwind case, while the shear components have similar magnitudes. This suggests that the wake turbulence is similar, but that in the cross-wind case the flight deck is more heavily burdened by inflow fluctuations from the atmosphere.

Near wake flow field measurements of a marine current turbine: Preliminary results

More advanced simulation tools are required to predict the loads experienced by a turbine in situ due to the surrounding environment including the effects of inflow turbulence, the impact of turbines operating upstream, the effect of the free surface, and the impact of surface waves, to name a few. These advanced models require detailed data sets for validation. At this time, few studies have focused on the near wake in an effort to provide such detailed data. It is critical to understand this region because it is where much of the high-energy phenomena occur that is most likely to impact turbine performance and reliability. To this end, a towing-tank particle image velocimetry (PIV) system was designed, built, and used to provide measurements of the flow in the near wake of a scale-independent marine current turbine. Included are preliminary results including a representative case and turbulent statistics for an ensemble of realizations.

Effects of waves on BEM theory in a marine tidal turbine environment

Blade Element Momentum (BEM) theory is a well understood and proven method for modeling blade loads and determining steady state performance characteristics of wind turbines. Recently this theory has successfully been applied to horizontal axis marine current turbines when incorporating modifications that lead to better predictions of turbine performance for a range of operating conditions. Relatively little work exists in the implementation of BEM theory in a marine environment with surface gravity waves. A better understanding of the effects of waves on tidal turbines is necessary to predict fatigue loading that can eventually lead to blade failure. This paper presents a BEM numerical model that incorporates the unsteady velocities due to the presence of waves and assesses the effects of waves on tidal turbine performance. Numerical results of the coefficient of power (CP) and coefficient of thrust (CT) match closely with experimental results for the mean Cp and CT for a range of tip speed ratios. The model is also able to predict the instantaneous amplitudes of the performance characteristics as compared with the measured values except for the BEM calculated thrust which shows a 20% reduction in amplitude.