Quantitatively Measuring In situ Flows using a Self-Contained Underwater Velocimetry Apparatus (SCUVA)

The overall goal of the following experiment is to measure the motion of fluid induced by moving or by stationary bodies in a natural field environment. This is achieved by using scuba, which is deployed by a scuba diver within safe diving limits. First, the diver identifies and swims to a target.

After aligning the apparatus relative to the target and positioning the laser sheet, the diver illuminates and starts video recording to capture consecutive particle fields over time after the completion of the dive, in order to yield velocity fields surrounding a target, the videos are downloaded and processed. Results of the analysis are obtained. That show flows in the natural field environment after fluid transport by swimming animals.

The main advantage of using scuba over laboratory measurement techniques like digital particle image, FLOS symmetry, or DPIV, is that you can now measure motions of fluid in a natural field environment. And what this does is allows us to answer key questions in the fields of oceanography and biology related to animal swimming performance, predator, prey interactions, and ecology. This method can be used to measure flow around swimming animals or coral pulps or benthic interactions.

It can also be used to measure flows around manmade structures. Visual demonstration of this method is critical as the field measurements are difficult to learn, requiring knowledge of measurement techniques and comfort with nighttime scuba diving. To begin this procedure, obtain high definition or HD recording tape.

Ensure that all S scuba components have sufficient battery power and are functioning properly. Prepare the laser and camera housings for use by cleaning the O-ring grooves and O-rings with the clean towel or wipe spread manufacturer provided O-ring grease evenly on the O-rings and replace them in the housing grooves. In addition, clean the laser and camera housing apertures to prevent laser sheet deformation and marks on the camera housing lens.

Next, prepare for a pressure test by placing disposable paper moisture strips into the housings. Check the O-ring seals by placing the empty housings in a tub full of water. Weighted objects should be placed on top of the housings to submerge them.

Since the housings float when empty after five to 10 minutes, remove the housings from the tub and towel. Dry the outside. Check whether there is any moisture inside the housings by inspecting the moisture strips.

Once the housings pass the pressure test place scuba components inside the housings. Attach high intensity discharge light pods to the camera housing. Ensure that the lights are oriented in such a way that they illuminate the area directly ahead of the camera and operator, but do not interfere with maintaining grip on handles and operation of camera controls.

In a low light environment, a temperature sensitive sheet of paper can be used to determine laser sheet orientation. When the laser beam is properly aligned relative to the optical lens installed in the laser housing, the laser lens combination will create a vertical sheet of light that is oriented perpendicular to the camera housing, using scuba attachments and the rigid or extendable arm, connect the laser housing and the camera housing to each other. Ensure that the housings are firmly attached and that the housings cannot rotate with respect to each other.

It is critical that the laser sheet remains oriented perpendicular to the camera's field of view. Due to the current capabilities of scuba measurement, dives can only be conducted in low light locations or at nighttime to prevent natural light interference with laser light. Therefore, it is recommended to be dusk or later before entering the water.

Turn on the camera housing before entering the water. The camera housing has a built-in electronic moisture sensor that provides visual warnings in case of moisture in the camera housing. The sensor only works when the camera housing is on.

The next step is to enter the water and begin set up of the equipment and experiment as described in the written protocol. Once the equipment has been set up, calibrate scuba by recording an object with known dimensions in the laser sheet. After the dive, a calibration constant can be extracted to convert the field of view size from units of pixels to centimeters upon finding a target.

The environmental bulk flow properties need to be determined. If present, the current direction will dictate apparatus and diver positioning relative to the target. During measurements, the direction of bulk flow surrounding the target can be inferred by observing bubbles exhaled from the diver and noting their lateral motion.

A small quantity of fluorescent dye can also be released to determine the current direction. Since diver generated flow can be a source of DPIV measurement error, the diver should not be located upstream of the target position. The laser sheet parallel to the direction of current so as to maximize particle residence time within the laser sheet, thereby minimizing DPIV errors.

Then position S scuba to illuminate and record the fluid motions surrounding a target. Begin recording as the target moves through the camera's field of view, refrain from rotational and out of plane motions during video recording. After turning off all components of scuba and restoring the laser arm to its retracted position, remove scuba from the water, detach the camera and laser housings from the arm.

Rinse or soak the apparatus in fresh water before drying to prevent rusting of the apparatus. After the housings are dried, remove components from the housings, recharge and replace batteries if needed. For another dive, connect the video camera to a computer and extract video from the HD tape by using a HD video software package.

Subsequent to video extraction, determine the range of video to be converted into a series of images for DPIV analysis. Ensure that the pixel aspect ratios and extracted image sizes match the HD video settings. Import these images to A-D-P-I-V processing program from the pre dive scuba calibration.

Enter the calibration constant select image capture parameters, which are prompted from the DPIV software package. Then generate the velocity fields from consecutive particle images. Additional post processing steps, depending on the quality and types of measurements, can also be applied when imaging is performed correctly.

The particle images surrounding the target should be sharp and easy to distinguish as shown in this in situ particle field surrounding aurelia in combination with A-D-P-I-V processing software package, the velocity fields of flow surrounding the target are revealed here. Yellow vectors in the velocity field indicate magnitude and direction of the local flow velocity. If sufficient video is collected to provide a time series of images, a time series of velocity fields can also be determined.

Improper imaging can result in unclear in cju particle fields. Shown here is a particle field surrounding missi. The red arrow indicates a region of high reflectivity, which results in saturation of the image, making it difficult to distinguish between particles and the target.

Another example of improper imaging is apparent in this particle field surrounding so misses here, the red arrow indicates a region of streaking that results when the flow rate is not sampled at a high enough frequency. Upon mastering this procedure, the measurements can be completed between one to two hours, depending on dive duration while doing this measurement technique, however, it's really important to maintain proper neutral buoyancy and also have sufficient knowledge of local diving conditions prior to entering the water. Don't forget that working with lasers is extremely hazardous, so the proper precautions, such as alignment of the laser should always be followed when working with this procedure.

Following this procedure, MATLAB post-processing can be used to answer additional questions such as energetics performance and flow rates.