|Simulation and Measurement Tools to Study the Environment and Amphibious Vehicle Motion in the Surf Zone|
7210 Pindell School Road
Fulton, MD 20759
Dr. Georges L. Chahine
(301) 604 3688
(301) 604 3689
Critical Navy operations rely on accurate predictions of vehicle maneuvering, simulation of the flow field around vessels, determination of propulsion system behavior, and proper characterization of the environment. These require accurate modeling and simulation of non-linear dynamic free surface flows, their interactions with nearby structures, and solution and diagnostics of two-phase flow problems. These are encountered with cavitation at high speeds or in off-design flow conditions, or with the detonation of underwater explosions.
Dynaflow has developed several complementary tools and products to address these problems. Among them is 3DynaFSTM, a general purpose simulation tool for non-linear dynamic free surface flows and their interactions with submerged or floating structures. Under an ONR SBIR program, we are using 3DynaFSTM to model the wave dynamics and the motion of the Advanced Amphibious Assault Vehicle (AAAV) in the surf zone. 3DynaFStm has been developed and used for several Navy applications (for ONR and NSWC) including modeling and simulation of underwater explosions (UNDEX), cavitation inception and dynamics, noise generation in propulsors, non-conventional fish-like propulsion systems, etc.
Off-line simulations using 3DynaFStm provide a database that is used in our ship simulator, DynaSimtm, to account for the interaction of breaking waves with a moving vehicle. DynaSimtm is a real-time physics-based Windows NT ship simulator, which incorporates on-screen control and input of water depth, waves, currents, bottom, nearby wall effects, etc.
Cavitation, noise generation, and UNDEX effects are due to the explosive growth and collapse of extremely dynamic bubbles. These are highly nonlinear free surfaces that can be simulated quite accurately by 3DynaFStm. Such simulations have been shown to correspond very well to Navy UNDEX data, and to Dynaflow data obtained in our spark-generator facility and in our wave tank.
Ocean background noise and propeller cavitation inception depend strongly on the presence of micro-bubbles (nuclei) in the water. Accurate predictions and extension of laboratory results to full-scale require knowledge of the nuclei size distribution. In order to address this need, we have developed the ABS Acoustic Bubble Spectrometer®, a PC-based device that measures bubble size distributions and void fractions in liquids. The underlying acoustic technique is very sensitive to bubbles but effectively insensitive to particulate matter. The ABSÒ can be used in a wide variety of two-phase flow applications where knowledge of the bubble size distribution and the volume fraction and/or area of contact between the gas and the liquid is important.
The above-described Dynaflow products respond to a number of growing industry trends including:
The potential markets for DynaSimtm and 3DynaFStm include training centers for mariners, firms designing ship and ship autopilots, naval architects, designers of waterways and harbors, ocean navigators and planners of fleet operations, propeller and turbomachinery designers, and ship protection and torpedo designers. Particular beneficiaries include:
The potential markets for the ABS Acoustic Bubble SpectrometerÒ include propeller and turbomachinery designers, biomedical centers, environmental agencies and marine life management agencies, chemical engineers, drilling fluid monitoring, power plants flow monitoring, etc. Particular beneficiaries include:
Most current Computation Fluid Dynamics codes are based on variations of finite difference or finite element methods, which are highly developed for modeling single-phase flows. They are not specialized to study cases where the details of liquid/gas interfaces are important. These codes are also computationally intensive and require supercomputers running for many days to simulate 3D problems.
Most ship simulators are full mission simulators, equipped with a full size ship bridge, and able to display "virtual reality" scenes. They are very impressive but very expensive and complex. The mathematical models used in the simulators are based on the simplifying assumption of linear small free surface deformation, and are not adequate to model vehicle behavior in the surf or transition zones.
Several techniques exist to evaluate the bubble size distribution in liquids. Low-tech methods include light and sound attenuation and photography. High-Tech approaches consist of optical technologies (scattering, fiber-optic probes, holography), acoustic technologies (scattering, attenuation, dispersion), and others (electrical impedance, cavitation susceptibility). Acoustic methods take advantage of the fact that the acoustic cross-section of a bubble is three orders of magnitude greater than its geometrical cross-section. However, they are inverse methods, and have the difficulty solving the resulting ill-posed problem.
3DynaFStm is a nonlinear free surface hydrodynamics code based on the computationally efficient Boundary Element Method. 3DynaFStm has been used extensively and validated for the simulation of cavity dynamics due to cavitation and to underwater explosions. It is able to accurately describe non-spherical bubble dynamics in the presence of nearby structures, boundaries, vortical flow fields, and free surfaces. It is presently being extended to handle free surface breaking waves and cavitation on lifting surfaces. It is being implemented into a general-purpose propulsor design code. In addition to its being coupled with the Navier Stokes code of Mississippi State University, UNCLE, to the hydrocode, DYSMAS, and to an in-house Vortex Element Method code, it has been coupled to the structural dynamics codes Dyna3D and Nike2D developed by Lawrence Livermore National Laboratories to enable simulation of fluid-structure interaction effects. For surf zone wave simulation and fluid-structure interaction, 3DynaFStm has the following major features:
Results from 3DynaFStm are fed as a database to DynaSimtm, the desktop ship maneuvering simulator. DynaSimtm enables training and self-study for mariners. Existing ship simulators are primarily large facilities that are developed for particular ships. Due to its compactness and low cost, DynaSimtm can be installed on board a ship or on a student’s laptop. It uses a GUI layout allowing for arbitrary manipulation of the control/steering devices, and a graphic display of multiple ships and sea environments. It can also be used as a decision-making aid:
DynaSimtm can also be further developed as an autopilot for marine vehicles. An additional important feature of DynaSimtm is its incorporation of a multi-objective system identification technique to determine the vehicle parameters. This would enable the autopilot to adjust the vehicle parameters based on feedback from the ship response to the pilot commands.
The ABS Acoustic Bubble SpectrometerÒ is a PC-based device that measures bubble size distributions in liquids. It uses a mathematical model to regularize the ill-posed inverse procedure of retrieving bubble populations from acoustic phase velocity and attenuation of various frequency acoustic bursts. The advantages of the ABS Acoustic Bubble SpectrometerÒ over optical techniques are its high sensitivity to bubbles, ability to operate without requiring a transparent liquid or container, and ease of use.
The device consists of a set of two hydrophones connected to boards resident on a PC, which controls signal generation by the first hydrophone and reception by the second. Short bursts of sound at different frequencies are transmitted and received after passage through the liquid. These signals are analyzed utilizing copyrighted algorithms to obtain the attenuation and phase velocities of the acoustic waves, and, from these, the bubble size distribution. A user-friendly Graphical User Interface (GUI) allows for easy control and measurement using a series of dialog boxes. The results are displayed graphically in real time and can be stored or printed.
Initial versions of 3DynaFStm are complete and in use. Various upgrades are in progress. A preliminary version of DynaSimtm is available. Further development is planned. Surf Zone simulation software is in the middle of a Phase II SBIR for the USMC via NAWC/TSD (Orlando) that should be completed in mid 2001.
A first version of the ABS Acoustic Bubble SpectrometerÒ is complete. Several units have been shipped and are in use by customers. Current efforts are directed at further improving both the software and hardware to increase its versatility and ease of use. Continuing development efforts seek to provide the capability of bubble population measurements in non-Newtonian media such as blood, tissue, and visco-elastic/visco-plastic fluids and for larger void fractions.
Sponsors and Program Managers for the work described above include:
Documents that provide more information include:
Dynaflow, Inc. has been serving the industrial and military market since 1988. The company has established a reputation for quality R&D and testing work. The company attempts to follow an inter-disciplinary approach to problems, using a combination of scientific tools to achieve results. This fact is reflected in the diversity of research at Dynaflow, Inc. Other than contracts on hydrodynamics and CFD research from federal agencies (ONR, NSWC, NASA) and consulting and testing jobs from industry, Dynaflow, Inc. has developed under internal funds and SBIR awards, several products that it is seeking to market. These include a ship simulator; a multi-purpose boundary element code suite for the study of fluid dynamics, gas/liquid interfaces, and fluid structure interactions (3DynaFStm); an acoustic tomography instrument for bubble size measurements (ABS Acoustic Bubble SpectrometerÒ); cavitating jet systems and nozzles for cleaning, cutting, drilling, and for oxidation of organic compounds (DynaJets®); electrical impedance tomography software; improved systems for trace gas concentration; and a cross-flow filtration system (DynaPerm®).
Dynaflow, Inc. has extensive experimental facilities for fluid dynamic investigations including flow visualization, high-speed photography, spark generated bubble dynamics for simulation of underwater explosions and cavitation, high pressure water jets, a wave tank, and an erosion and high impact materials testing laboratory.
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