Software Tool Suite for Bubble Wake Signature of Waterjet Propelled Ships

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1 LCS-1 "Approved for public release; distribution is unlimited" Software Tool Suite for Bubble Wake Signature of Waterjet Propelled Ships Dynaflow, Inc J Iron Bridge Road Jessup, Maryland Contact: Dr. Georges L. Chahine Phone: (301) Fax: (301) Website: LCS-2 Command: NAVSEA Topic: N06-T022 PROBLEM STATEMENT Developing the capability to defend surface ships against attack by wake-homing torpedoes is a high priority for the U.S. Navy. While significant expertise and capability has been developed in understanding the risks of acoustic signatures of wakes from conventionally propelled surface ships, little headway has been made for the risk of acoustic signatures of wakes from waterjet propelled ships such as the Littoral Combat Ship (LCS). The ability to predict the density of bubbles and acoustic signature of wakes of these ships would enable the Navy to better assess their vulnerability and enhance their design during the ship engineering phase. In addition, the ability to model wakes under varying operating conditions would enable the Navy to better develop defenses against wakehoming torpedoes. Bubble generation and entrainment in the transom region of a surface ship, along its hull and in breaking following waves are major contributors to the ship signature. For advanced ships, such as the LCS and the Joint High Speed Vessels (JHSV), waterjet propulsion adds another source of air entrainment through the waterjet and free surface interactions in the transom region and beyond. 1 In littoral warfare, such bubbly wakes provide a means to home torpedoes enabling them to find their target because of large acoustic cross section of the bubbles. 1 Development of Software Tool Suite for the Prediction of Bubbly Wake Acoustic Signature of Waterjet Propelled Surface Ships, Report 2M6024 ONR STTR 1, March Dynaflow, Inc. 1

2 There is not a proven method for predicting the acoustic signature of wakes for waterjet propelled ships The method for conventionally propelled ships is to use simulation to model ship vulnerability to torpedo attack combining both hull and propulsor bubbly wake field metrics. These simulations have been validated for conventionally propelled ships. In contrast, current models for waterjet propulsors are based upon a very small number of water measurements and do not appear to simulate actual waterjet propulsors well. The limited data seems to indicate that waterjet powered ship wakes can be either much shallower or much deeper and also more intense than those of conventionallypropelled ships. 2 A software suite that enables ship designers and builders to model and predict bubbly wakes for different ship and waterjet propulsion designs would contribute to the minimization of ship acoustic cross sections and thereby increase ship and warfighter safety. Similar considerations and challenges of minimizing acoustic signatures from waterjet propulsion also extend to the design and operation of other surface and underwater vehicles including submarines and submersibles. WHO CAN BENEFIT? The Littoral Combat Ship (LCS) will also perform MIW, SUW and ASW missions as well as Special Operations Forces (SOF) support, high-speed transit, Maritime Interdiction Operations (MIO), Intelligence, Surveillance and Reconnaissance (ISR), and Anti-Terrorism/Force Protection (AT/FP). While complementing capabilities of the Navy's larger multi-mission surface combatants, LCS will also be networked to share tactical information with other Navy aircraft, ships, submarines, and joint units.. The software tool suite can also be of interest to the Joint High Speed Vessel (JHSV) program managed by PMS 385. It is a Navy led acquisition of a platform intended to support users in the Department of the Navy and Department of the Army. The JHSV program is a cooperative effort for a high-speed, shallow draft vessel intended for rapid intra-theatre transport of medium sized cargo payloads. JHSV is expected to reach speeds of 35~45 knots and allow for the rapid transit and deployment of conventional or Special Forces as well as equipment and supplies. Waterjets are a major choice of propulsion for this high speed. The Sea Base Connector Transformable-Craft (T-CRAFT) Prototype Demonstrator Program could be another application opportunity. The Office of Naval Research, beginning in FY06, has been embarking on an effort to develop Game Changing Innovative Naval Prototypes (INPs) for Seabasing. The total INP Seabasing effort is currently programmed to run from FY06 through the end of FY11 with a total of $295M programmed. Other Seabasing INP BAAs to be released include the Sea Base Transformational Package and Ordnance Rapid Transfer System (TransPORTS) 2 STTR Technology Transition Plan for N06-T022 - Waterjet Wake Characterization Suite, October Dynaflow, Inc. 2

3 Prototype Demonstrator, the Personnel Transfer At-Sea Prototype Demonstrator, and the Sea Base Intermediate Transfer Station (ITS) Prototype Demonstrator. Other potential users in DoD are Naval intelligence agencies and Special Operations Command (SOCOM). Immediate application could be made to high speed Special Operations Command (SOCOM) Combatant Crafts such as the MK V Special Operations Craft (using two Kamewa K50S waterjets), the Rigid Inflatable Boats (RIB; some types use waterjets), and the Riverine Crafts. BASELINE TECHNOLOGY While there have been significant successful R&D efforts to understand and observe bubble generation and entrainment in breaking waves, little has been done so far about other means of bubble generation and entrainment. 3 The Office of Naval Research already has responded to such a need and has an on-going R&D program with DYNAFLOW on the bubble entrainment in propeller flows. Much experimental and computational work has been done in studying these bubbly wakes with the objective of reducing the vulnerability of naval vessels mostly on traditional propeller driven ships 4 such the CVN-75. However, much less effort has been made towards non-conventional waterjet driven ships such as the Littoral-Combat-Ship class, which is of primary interest in this STTR. TECHNOLOGY DESCRIPTION DYNAFLOW has teamed with the Applied Research Laboratory (ARL) at the Pennsylvania State University to develop software for modeling the waterjet driven ships bubbly wakes and has conducted experiments to validate and refine the models. In Phase I, the software components were validated against laboratory experiments at DYNAFLOW. In the experimental study, the bubble entrainment, population and size distribution were measured with optical and acoustic means as well as using laser light attenuation. The observed bubble information and understanding of the entrainment physics lead to the development and evaluation of bubble entrainment models. In Phase I, the efforts were directed to submerged waterjets. However, since the actual LCS waterjets operate both submerged and above water depending on the ship speed, in Phase II the efforts were to develop bubble entrainment models for ships propelled by waterjets and to demonstrate that the models predict properly the bubbly wake signature and enable engagement simulations integrating the software components with the Technology Requirements Model (TRM). 3 Development of Software Tool Suite for the Prediction of Bubbly Wake Acoustic Signature of Waterjet Propelled Surface Ships, Report 2M6024-ONR-STTR-1, March Dynaflow, Inc. 3

4 Using experimental laboratory observations, we have developed bubble entrainment models for waterjet free surface interaction and for cavitation in the jet shear layer of the bubbly wake. We have incorporated bubble generation models and wave breaking models into ship hydrodynamics flow field computations. Through comparisons against laboratory experiments, the validity of the developed codes will be demonstrated. We will develop an interface with the Technology Requirements Model (TRM) and demonstrate its capabilities. We will also conduct experiments to verify the results on a model scale of the LCS and at sea on the LCS involving wake acoustic monitoring and bubble measurements to provide further validation and help improve the modeling. We will then use the Technology Requirements Model (TRM) engagement simulation to predict ship vulnerability to torpedo attack. Table 1: Features, Advantages, and Benefits Features Advantages Benefits Avoid using moving grids to track the free surface deformations Uses Level Set Two- Phase Model Uses Air Entrainment Physical Models Provides instantaneous bubble size spatial distribution Interfaces to the ARL Technology Requirements Model (TRM) Capability for varied operating conditions and parameters Air entrainment and subsequent bubble creation in waterjet-induced free surface breaking are modeled based on understanding of the physical mechanisms with the help of experimental observations Utilizes a Discrete Bubble Tracking Model which enables time-accurate prediction and analysis. Provides input of waterjet propulsors generated wake to the Technology Requirements Model enabling modeling of the engagement between platforms (ships and submarines) and weapons and/or countermeasures Ability to simulate across full range of conditions Better for large free surface deformations /breakup into bubbles and droplets Reliable and accurate physicsbased model Can be integrated with Technology Requirement Model (TRM) to achieve acoustic signature analysis Characterize wake acoustics and assess threat performance for various operational configurations in order to predict ship vulnerabilities. Better ability to predict ship vulnerability and develop defenses against wake homing torpedoes Dynaflow, Inc. 4

5 CURRENT STATE OF DEVELOPMENT The current software model of the waterjet plume has been completed and prediction performance has been validated in local test tank resulting in a TRL of 4 as of November We are planing to conduct towing tank test to acquire a reliable data for validation with Phase II Option 1 and following by an Option II for at sea testing. Major milestones in Phase II are summarized in the following table. Milestone TRL Risk- Test Measure of success Validation of bubbly wake 3 low Predictions and for submerged water jet measurements agreement Two-Phase Flow Model for 4 low Implemented model is plunging jet able to capture free surface breaking Waterjet Air Entrainment 4 medium Implemented model is Model able to generate bubbles from free surface breaking Co-Flow Integration in 5 medium Reliable data is required Towing Tank for validation At Sea Testing 6 medium Reliable data is required for validation TRL date April 2006 (complete) Oct 2009 (complete) Feb 2010 Aug 2010 Feb 2011 Following Phase II development and data measurement in towing tank and sea testing, we will focus our efforts toward transition. These steps required to transition the technology subsequent to completion of the Phase II are summarized below: Required Tests, Demos, TRL and next steps 6 Applications to actual ships (LCS) 7 Mission scenario demonstration 8 Matured capability at mission scenario level 9 Fully integrated software system Organizations to Target date be involved 2011 DYNAFLOW, ARL, ONR, NUWC 2012 DYNAFLOW, ARL, ONR, NUWC 2013 DYNAFLOW, ARL, ONR, NUWC 2013 DYNAFLOW, ARL, software users REFERENCES Technical Point of Contact: (401) CSC Mr. Thomas F. Owen Senior Engineer Dynaflow, Inc. 5

6 Alion Science and Technology (540) NSWC ONR Dr. Mark Hyman Naval Surface Warfare Center Panama City (850) Dr. Patrick Purtell Office of Naval Research (703) ABOUT THE COMPANY DYNAFLOW, INC. was founded in 1988 by Dr. Georges L. Chahine, a leading expert in the dynamics of interfaces such as in air cavities, free surfaces, and hydrodynamics. The company has undergone a controlled and steady period of growth. Starting from a staff of one assistant, the company has grown to a staff size of 20, with 10 PhDs. Revenues for 2008 totaled $1.7 million. The company has established a reputation for quality R&D and testing work, and follows an interdisciplinary approach to problems, using a combination of scientific tools to achieve results. DYNAFLOW has leading research scientists in the fields of mechanical, civil, chemical engineering, laboratory facilities including fluid dynamics, jet technology, chemical, materials testing, microbiology, environmental testing, and almost 20 years of experience in bubble dynamics, cavitation, ship hydrodynamics, jets, material testing, etc. The company also has acquired several patents in waterjets and filtration technology and has commercialized these products along with its fluid dynamics software. DYNAFLOW s state of the art technology in bubble dynamics and ship hydrodynamics and customer-minded flexible interdisciplinary approaches will bring the best solutions to customers with the benefit of reduced cost for development. DYNAFLOW s major customers include the Office of Naval Research, Naval Surface Warfare Center (NSWC) Indian Head and Carderock Divisions, NASA, the National Institute of Health (NIH), the National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation (NSF). Dynaflow, Inc. 6