Wave Driver Research Articles

Modelling waves and currents at the LSTF and other laboratory facilities

The main objective of the paper is to use the detailed measurements of wave and current motions in the Large-scale Sediment Transport Facility (LSTF) at the Coastal and Hydraulics Laboratory (CHL), ERDC, Vicksburg, to compare with model simulations and particularly to investigate the accuracy of two wave models used for driving nearshore circulation computations. The major questions asked are how accurate are the models? And how do the inevitable inaccuracies in the prediction of the wave quantities influence the current predictions? The LSTF is one of a few large-scale facilities around the world designed to accurately reproduce the 2-D horizontal flow and sediment conditions in the nearshore region of a littoral coast. The measurements include all relevant wave data such as wave heights, setup variations, 2-D current distributions across the basin, and a detailed cross-shore array of measurements of the 3-D current structures. The two wave model formulations used for the comparisons are versions of the kinematic wave model, one using sinusoidal the other using non-sinusoidal wave shapes both before and after wave breaking, as wave drivers in the quasi-3-D model SHORECIRC (SC). The comparisons with the measurements are aimed at providing an overall picture of both the flow conditions and the model performance. The discussions provide insight into the mechanisms behind the complex nearshore dynamics and demonstrate the strengths and weaknesses of the two methods in a comprehensive comparison of available wave and current data. The comparisons also reveal that an essential requirement for a wave driver is its capability to represent the phase motion of the wave and its capability to correctly predict the phase speed of waves.

Numerical modeling of infragravity wave response during DELILAH

The nearshore circulation model SHORECIRC is applied to simulate the infragravity wave conditions at two intervals during the 1990 DELILAH campaign at Duck, N. C. The model has been extended to solve the energy equation for random short waves (on the timescale of the wave groups) which acts as a wave driver for the long‐wave model. To simulate the wave conditions an algorithm has been derived which synthesizes a directional offshore wavefield from measured data including the bound, directional low‐frequency components. The model‐data comparison shows that the mean short wave transformation and mean longshore current are predicted well. The structure of the infragravity spectrum as measured in the nearshore array is correctly represented, albeit that the energy levels are underpredicted in most locations. The 2‐D frequency longshore‐wavenumber spectra show that the infragravity wave energy consists mostly of edge waves, and that the computations qualitatively resemble the measurements. It is shown that for 1 day, bar‐trapped edge waves are present. The model is also used to show that the influence of the longshore variability is small for this particular beach and that the directional spreading in the offshore wave field is critical in predicting the nearshore infragravity wave conditions correctly. Edge waves which propagate against the current are generated by the directionally broad short‐wave forcing and to a lesser extent by back‐scattering. The nonlinear terms are relatively unimportant for the prediction of the infragravity wave climate. The cross‐shore gradient of the onshore‐directed radiation stress is the most important of the forcing terms.

40 Gb/s differential traveling wave modulator driver

In this letter a MMIC differential traveling wave driver for 40 Gb/s electro-absorption modulation driver is presented. The driver delivers 2.7 Vp-p or 2.4 V eye amplitude at each output into 50 /spl Omega/ load. The driver has high gain (>20 dB), a 3 dB bandwidth of 45 GHz, and rise/fall times of only 10/9 ps, respectively. The circuit uses a single -5.0 V power supply and consumes 1.8 W of dc power. The driver also features cross-point control and output amplitude control functions.

An unsteady wave driver for narrowbanded waves: modeling nearshore circulation driven by wave groups

In this paper, we derive an unsteady refraction–diffraction model for narrowbanded water waves for use in computing coupled wave–current motion in the nearshore. The end result is a variable coefficient, nonlinear Schrödinger-type wave driver (describing the envelope of narrow-banded incident waves) coupled to forced nonlinear shallow water equations (describing steady or unsteady mean flows driven by the short-wave field). Comparisons with experimental data show that good accuracy can be obtained for cases of nonbreaking wave transformation. Numerical simulations show that the interaction of wave groups with longshore topographic nonuniformities generates strong edge wave resonances, providing a generating mechanism for low-order edge waves.

Beats on a vibrating string

I showed Derrick Boucher’s acoustic demonstration of beats and interference to a colleague, and as we were talking I mentioned that the function generator could be hooked to mechanical wave drivers to obtain nice standing waves. Then it’s possible to verify that law of vibrating strings. However, we kept coming back to the concept of beats. Could we “see” beats on strings?

Square wave driver for extremely low resistanceand low frequency loads

A technique is presented for allowing efficient generation of high currents at low voltage and at frequencies ranging from DC to /spl sim/100 Hz.

Periodic and quasiperiodic ELF/VLF emissions observed by an array of Antarctic stations

This paper describes amplitude modulations in the frequency range 0–500 mHz of ELF/VLF (0.5–4.0 kHz) radio wave power recorded throughout 1993 and 1995 at Halley and South Pole stations, Antarctica, which lie in approximately the same magnetic meridian and at geomagnetic latitudes (Λ) of 61° and 74°, respectively. Data from the intermediate automatic geophysical observatories P2 and P3 (Λ = 70° and 72°, respectively) were also analyzed where available. In agreement with earlier work, spectrograms have revealed the frequent day‐time (typically 0700‐1700 MLT) occurrence of modulations lying almost entirely within the two period ranges: 10–60 s and 4–6 s. The first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two‐hop whistler mode echo period in the plasmatrough, and the events are termed periodic emissions (PEs). QP occurrence rates higher than some earlier studies (335 station‐days out of 667 examined) may be attributable to the sensitive spectral analysis technique. The type I QPs (i.e., those correlated with geomagnetic pulsations observed at South Pole and/or P2/P3) were consistent with an upstream wave driver, controlled by the IMF cone angle. Type II QPs (uncorrelated with magnetic pulsations) were always accompanied by PEs, suggesting a link between the two, reinforced by a frequently observed steady increase in period in both phenomena, especially during the morning, possibly associated with increasing densities due to upward flow of photoionized plasma from the ionosphere after dawn. Here we propose that type II QPs are driven by field line resonant ULF waves which in turn are generated by field‐aligned currents arising from PE induced electron precipitation.

Dynamics of plasma wave drivers

The working group on Dynamics of Plasma wave Drivers considered the evolution of lasers and particle beams used to drive plasma wakes for plasma accelerators. In addition, the group developed a comparison table of several laser and particle driven plasma accelerator designs. The table shows that differences between the schemes are associated largely with the plasma densities at which they typically operate. Designs with high plasma densities tend to have higher gradients and fewer stages but require repetition rates to achieve a given luminosity than designs at lower densities.

Growth and decay of field line resonances

The apparent growth rates of ULF field line resonances seen by the SuperDARN HF radars show a striking correlation with the azimuthal wave number m of the resonance. The observed high‐m (m>17) resonances have amplitudes that increase with time, indicating positive growth rates, while the low‐m resonances have decreasing amplitudes, indicating negative growth rates. The resonance growth rates and latitudinal phase shifts, a decrease for low‐m modes and an increase for high‐m modes, are found to be determined by the direction of the Poynting flux in the system. In the case of the low‐m resonance the Poynting flux direction is such that the energy flows from the fast wave driver into the shear Alfvén field line resonance and down into the ionosphere. In the case of the high‐m resonance an internal driver couples to the system and reverses the direction of the Poynting flux such that energy flows from the internal driver into the shear Alfvén field line resonance and out into the fast wave. There is evidence to suggest that the internal driver is in the form of a wave‐particle interaction.

Evaluating hysteresis in earth materials under dynamic resonance

A lumped parameter model is derived for studying hysteretic effects in resonant bar experiments on rock. The model uses equations of state obtained by approximating closed hysteresis loops in the stress‐strain plane by parallelograms. The associated approximate nonlinear state relations have a sound speed (modulus) that takes two values. Assuming hysteresis and discrete memory to be the primary nonlinear mechanisms, periodic solutions corresponding to these equations of state are obtained analytically for single‐frequency continuous wave drivers, and their frequency spectral densities are analyzed. In this simple approximation, if hysteretic contributions to the signal speed are a correction to the linear elastic signal speed (i.e., the parallelogram is narrow), the model predicts that the spectral density at even multiples of the source frequency is zero, and an approximate “pairing” of amplitudes is predicted for odd harmonic multiples. Comparison of the model spectrum with experimental data shows the model to be qualitatively correct. We conclude that hysteresis is an important mechanism in rocks. We consider the model to be a prototype.

Tokamak Power Systems Studies at ANL

A number of advances in plasma physics and engineering promise to greatly improve the reactor prospects of tokamaks. The following features, in particular, are examined: (a) large aspect ratio (A approx. = 6), which may ease maintenance; (b) high beta (..beta.. greater than or equal to 0.20) without indentation, which brings the maximum toroidal field down to about 7 T; (c) low toroidal current (I approx. = 5MA), which reduces the cost of the current drive and equilibrium field system; and (d) steady state operation with current density control via fast and slow wave current drive. The key to high beta operation with low toroidal current lies in utilizing second stability regime equilibria with the required current distributions produced by an appropriate selection of wave driver frequencies and power spectra. The ray tracing and current drive calculation is self-consistent with the actual magnetic fields produced in the plasma. In addition to matching desirable high-beta equilibria, this method is capable of producing a large variety of new equilibria, many of which look attractive. The impurity control activities in TPSS have emphasized the self-pumping concept as applied to using the entire first wall or ''slot'' limiters. The blanket design effort has emphasizedmore » liquid metal and Flibe concepts. The reference concept is a liquid lithium/vanadium, self-cooled configuration. Overall, there exists a number of major design improvements which will substantially improve the attractiveness of tokamak reactors.« less