Wave Tank Facility Research Articles

Flexible Porous Wave Barrier for Enhanced Wetlands Habitat Restoration

A flexible porous wave barrier is proposed to increase the effectiveness of wetlands habitat restoration projects by protecting cordgrass seedlings from wave action during the critical initial stages of growth following planting. The barrier consists of a vertical porous membrane anchored to the sea bed and kept under tension by a small buoyant float. The structure is easily constructed, requires little maintenance, and may be deployed from a small boat without the need for specialized equipment or trained personnel. A numerical model is developed to assess the effectiveness of this structure as a barrier to wave action. The fluid motion is idealized as linearized, two-dimensional potential flow and the equation of motion of the structure is taken to be that of a one-dimensional beam of uniform mass per unit length subjected to a constant axial force. The boundary integral equation method is applied to the fluid domain, and the dynamic behavior of the structure is also described through an appropriate Green function. Numerical results are presented that illustrate the effects of the various wave and structural parameters on the wave reflection characteristics of the structure. Small-scale physical model tests have also been carried out in the University of Houston wavetank facility to validate the predictions of the numerical model. In general, good agreement was obtained between the theoretical and experimental reflection coefficients. It is found that for certain structural parameter combinations a porous tensioned membrane structure may provide a reasonably effective barrier in the short- to mid-wavelength range.

Dispersant Testing at Ohmsett: Feasibility Study and Preliminary Testing

ABSTRACT The Minerals Management Service (MMS), U.S. Department of the Interior, operates a wave tank facility in Leonardo, New Jersey known as OHMSETT (Oil and Hazardous Material Simulated Environmental Test Tank), which is used primarily for testing oil spill booms and skimmers. This paper summarizes two studies undertaken to examine the feasibility of testing dispersants at the facility as well. The first study included: (1) interfacial tension laboratory tests, (2) turbidity tests, (3) laboratory tests to evaluate filtering materials for removing dispersant and chemically dispersed oil, and (4) full-scale evaluation testing at OHMSETT. The results indicated that dispersant testing at OHMSETT could be done with good success if the testing program were carefully designed and implemented. It was determined that a number of dispersant tests could be conducted over several days, after which the tank water would have to thoroughly cleaned to remove dispersed oil (with a cellulose-based filter) and dispersant (with an activated carbon system). Following the feasibility study, the project moved to the second study, namely the design and validation of an experimental protocol for dispersant effectiveness testing at the facility. Full-scale tank work was conducted in April 2000. Preliminary results, provided in this paper, indicate that OHMSETT is an attractive facility for determining dispersant effectiveness.

Behavior of a chemically-dispersed oil and a whole oil on a near-shore environment

To investigate the use of dispersants as an oil spill chemical countermeasure in the surf-zone, a simulated oil spill was conducted at the Shoreline Environmental Research Facility (SERF), formerly known as the Coastal OilSpill Simulation System (COSS), a wave tank facility in Corpus Christi, Texas. Sand was added to each tank to establish a beach with a prescribed slope of 10 degrees. Natural seawater flowed continually through the system to emulate alongshore currents. The replicated experimental treatments included pre-mixed oil plus dispersant (three tanks), oil only (three tanks), and unoiled controls (two tanks). Known amounts of either whole oil or dispersed oil were added to the respective tanks. Both the sediment and water column were periodically sampled during the 10-day experiment, and a materials balance on the oil was determined for both oil treatments. The environmental compartments where oil accumulated were sediments, water column, and non-aqueous-phase layer. The discharge from the tanks was presumed to be the primary sink, as water was drawn from the tanks at a known and constant flow rate. Tidal cycles were simulated by varying the computer-controlled influent rate. The oil mass (measured as total petroleum hydrocarbons) for each compartment/sink was calculated using data from four time points. At the experiment’s conclusion, approximately 49% of the applied oil for the oiled treatment remained in the tanks sorbed to sediments or other surfaces. The rest of the oil was removed via the effluent. In the chemically-dispersed oil treatment, all of the oil was flushed from the tanks; no oil (≪1%) remained on the sediments. These studies indicate that a timely dispersant application to spilled oil can reduce residual oil accumulation on beach substrates.

Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Measurements of the damping of small gravity and gravity‐capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave‐wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8–9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three‐scale) composite‐surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite‐surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind‐wave and wave‐wave interactions.

Lineshape analysis of breaking-wave Doppler spectra

Based on analyses of spectral densities of microwave backscatter from ocean waves, it has previously been observed that the Doppler spectra could be decomposed into different components, with Bragg scatterers represented by Gaussian lineshapes and faster-than-Bragg scatterers represented by Lorentzian and/or Voigtian lineshapes. It has been conjectured that the faster-than-Bragg scatterers are due to scattering from breaking waves, and established that the scattering mechanisms are non-Bragg in nature. In order to test the conjecture regarding the decomposition of spectral lineshapes which represent meaningful scattering processes, experiments have now been conducted at a wave tank facility. The lineshapes of breaking-wave Doppler spectra are analysed. It is found that for mechanically-generated breaking waves in the absence of wind, the co-polarised (HH and VV) spectra have predominantly Voigtian or Lorentzian shape and that the cross-polarised (VH and HV) spectra have exclusively Voigtian shape.

Scattering from breaking gravity waves without wind

Scattering experiments from breaking gravity waves conducted at a wave tank facility at small grazing angles in the absence of wind are analyzed. Breaking gravity waves are studied using a fully plane polarimetric horizontal (HH), vertical (VV), vertically transmitted and horizontally received polarization (VH), and horizontally transmitted and vertically received polarization (HV) pulse-chirped X-band (8.5-9.6 GHz) radar in conjunction with optical instruments: the plane polarimetric optical specular event detector (OSED) and side-looking camera (SLC). Spatially and temporally resolved radar backscatter has been measured and temporally correlated to the data obtained from the optical diagnostics. The experiments yield the following results: (1) enhanced scattering compared to Bragg scattering levels occurs throughout the evolutionary process of wave-breaking, i.e., the radar scatters strongly from both the unbroken and broken surfaces; (2) an explanation is found for the observation that the scatterer Doppler frequency is slightly less than the Doppler frequency corresponding to the fundamental wave phase speed; (3) a representative non-Bragg cross section of a breaking wave can be obtained; and (4) a breaking wave surface is found to be an efficient depolarizer.

Higher Order Wave Loading on Fixed, Slender, Surface-Piercing, Rigid Cylinders

Abstract The use of Morison's equation together with the linear wave theory is considered a first approximation to evaluate the inline wave forces on a surface-piercing cylinder. Significant second-order forces are expected to arise from the waterline and dynamic pressure effects, even when a wave is described by the linear theory. Experiments have been carried out at the MUN (Memorial University of Newfoundland) wave tank facility to identify these second-order forces for various wave frequencies and for various cylinder diameters. A strain gage force transducer has been used for this purpose. First and second-order force components have been identified using a Fast Fourier Transform. Theoretical evaluation of wave forces involved computing components from Morison’s equation using second-order Stokes theory. The waterline forces and convective acceleration forces which contribute toward the total second-order force have also been evaluated. First-order results are in acceptance with previously established data. Theoretical considerations for second order are satisfactory. Scatter in second-order experimental results were observed. Different approaches to the second-order inertia force are compared. It is expected that the inclusion of second-order forces will lead to a better representation of wave loading on offshore structures.