Characteristics of a high power ultrasonic array transmitter in air
A high-power ultrasonic array transmitter with 144 transmitting elements for 3D measurements in air has been developed. The sound pressure level of 126 dB was obtained at a distance of 5 m in the air with an ultrasonic frequency of 40 kHz. This value is approximately 32 dB higher than that obtained using a single transmitting element. The characteristics of the array transmitter were studied and compared with simulation results. The experimental results were in good agreement with the simulation results.
Three-dimensional imaging sensor system using an ultrasonic array sensor and a camera
This paper describes a three-dimensional (3D) imaging sensor system that uses an ultrasonic array sensor and a camera. Ultrasonic array sensors can be used to measure the 3D positions of an object. However, they do not have high resolutions. To overcome this limitation, the proposed system uses the camera to measure the shape of an object and combines this information with 3D position data obtained using the ultrasonic array. The relationship between the ultrasonic array sensor data and the image obtained by the camera was clarified. A prototype system was constructed using a camera and an ultrasonic array sensor that has 16 receivers. The theory proposed in this paper was confirmed experimentally. 3D images were obtained that had higher shape resolutions than those obtained using only an ultrasonic array sensor.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.