Kennard P. Watson

Application of a panel method to hydrodynamics of underwater vehicles

A low-order singularity panel method based on Greens formulation is used to predict the hydrodynamics characteristics of underwater vehicles. The low-order modeling employs constant strength sources and doublets, and the body surface is modeled by quadrilaterals. The method is first applied to predicting the force and moment coefficients of underwater vehicles for the body-alone and finned configurations. Hydrodynamic coefficients of added mass and added moment of inertia are also calculated by modifying the code. Results for several two and three-dimensional bodies show the usefulness of the method for predicting the added mass and added moment of inertia.

Added mass coefficients for submerged bodies by a low-order panel method

The added mass coefficients for two and three-dimensional submerged bodies were calculated using a low-order panel code. The source and dipole strengths, and the panel surface area for each panel, were used to compute the integrals needed for added mass in all six degrees of motions. Several applications of this method were used in comparing the results with the theoretical, when available, experimental or other numerical results. The method was found to be successful in predicting the added mass coefficients using relatively low numbers of panels.

Development of a 6-DOF Model for Mine Clearing Darts

A six degree-of-freedom (6-DOF) model was developed to predict the dispense patterns of darts from air-delivered systems to clear mines in the surf and beach zones in support of an amphibious assault. The payload consists of small darts dispensed from a delivery vehicle above the target minefield. An aircraft or Naval gun delivers a parent vehicle which flies a guided trajectory that brings it into a vertical orientation above the target, at which point hundreds or thousands of darts are released at supersonic speeds several hundred feet above the target. The simulation terminates upon impact of all the darts with the ground or water surface. A major objective is to incorporate improved aerodynamic models into the 6-DOF. The effort uses a simplified multiple-store interference model and Computational Fluid Dynamics simulations to develop the interference modeling terms. Comparison of 6-DOF predictions with flight test data shows that accurate prediction of the dart dispense pattern on the target requires knowledge of initial conditions at time of dispense and the aero interference forces during dispense when the darts are in close proximity.

Wind Tunnel Measurements of Transonic Aerodynamic Loads on Mine Clearing Darts

Wind tunnel tests were performed to measure the freestream and interference loads on multiple 6-inch long darts at speeds of Mach 0.8 to 1.2. The darts are intended for use in new mine clearance technology being developed by the U.S. Navy to support military amphibious operations. Freestream data were collected for a single dart at angles of attack up to 42 degrees, and interference data were collected for a dart in the presence of dart clusters. The interference results show the presence of significant interference loads when the bodies are in close proximity at transonic speeds. In addition to well-known drafting effects when the darts are placed behind each other, they also experience significant transverse interference forces when placed side-by-side. The force and moment data are in generally good agreement with semi-empirical and computational fluid dynamics models. This study fills a gap in published data for aerodynamic loads on multiple bodies in proximate flight at transonic speeds.

Modeling of underwater bomb trajectory for mine clearance

The falling of a Joint Direct Attack Munition (JDAM) through a water column was modeled using a six degrees of freedom model (called STRIKE35), which contains three components: hydrodynamics, semi-empirical determination of the drag/ lift/torque coefficients (depending on the Reynolds number and the angle of attack), and water surface characteristics.To validate and verify this model, three underwater bomb trajectory tests were conducted in the Naval Air Warfare Center, Weapons Division (NAWC/WD) in the middle of Indian Wells Valley, California. During the test, several JDAMs were dropped from an airplane into two frustum ponds with the same bottom diameter of approximately 30.5 m, different surface diameters (61 m, 79 m), and different depths (7.6 m, 12.1 m). High-speed digital cameras with light/pressure sensors, and a global positioning system were used to record the location and orientation of JDAMs. Model—data inter comparison shows the capability of STRIKE35, which may lead to a new approach (breaching technology) of sea mine clearance in very shallow water (water depth less than 12.2 m, i.e. 40 ft).

Prediction of submersible maneuvering performance at high incidence angles

A model is described to predict the hydrodynamic loads on an underwater vehicle at high incidence angles. The model has been implemented into a trajectory program to allow the prediction of highly nonlinear maneuvers such as low speed hovering and vertical rise and descent. Two key features of the model are the ability to predict out-of-plane loads due to asymmetric hull vortex shedding and fin loads through the linear, stall and post-stall flow regimes. Model validation was performed by comparing predicted total force and moment coefficients with missile and airship data.<<ETX>>

An Engineering Method to Predict Propeller Loads on Maneuvering Underwater Vehicles

An engineering method is presented for calculating propeller shaft loads on a maneuvering underwater vehicle. The method predicts time-averaged loads on apropeller in nonuniform, inclined flow. The computational affordability of the method makes it ideal for use in preliminary vehicle design. Comparisons of measured and predicted data for propeller alone and propeller-hull configurations show that the method accounts for the effect of nonuniform and inclined flow on shaft loads. INTRODUCTION A propeller influences the hydrodynamic performance of an underwater vehicle through direct and indirect loads. Direct loads, consisting of thrust, torque, lift, and side forces are transmitted to the hull through the shaft. Indirect loads are caused by the propeller-induced flow on the stem and tail fins. Because the propeller operates in the nonuniform flow field of the vehicle, propeller loads typically contain time-averaged and harmonic components. This paper considers direct, time-averaged loads only. Methods to predict direct propeller loads range from empirical to advanced numerical methods. The Hydrodynamic Analysis Techniques (HYDAT) program is a reliminary design tool for towed and free-swimming submersibles? HYDAT predicts propeller loads using an empirical method based on open water performance data for a particular propeller series. Further development of this and other empirical methods has been hampered by the lack of data. Potential flow-ba~edmethods~~~ are capable ofpredictingpropellerloads, given the geometry and inflow conditions. These methods have been successfully used to design propellers, but they are large and difficult to run on a production basis. Semi-empirical methods are available to predict propeller loads, given the open water performance characteristics of the propeller. McCarthy4 developed a method to predict the thrust and torque of an arbitrary propeller operating in a nonuniform but noninclined flow. McCarthys method accounts for the substantial effect of the axisymmetric hull boundary layer on propeller thrust and torque. G~tsche~developed amethodto predictthe thrust, torque, andinplane normal force of a propeller in a uniform, inclined flow. A method developed by Perkins and Mendenhal16 combines the McCarthy and Gutsche methods, providing a means to compute direct loads in a nonuniform, inclined flow. This method is capable of predicting the three forces and three moments acting on a propeller attached to a maneuvering vehicle. For use in preliminary vehicle design, semi-empirical methods offer the most flexible and affordable approach for propeller load prediction. This paper presents a semi-empirical method based on the work of Perkins and MendenhalL6 The following sections describe the procedure for calculating the propeller inflow and load components. Predictions are compared with measured shaft loads on propeller alone and propeller-hull configurations. TECHNICAL APPROACH The propeller model, designated PROPLD, computes direct, timeaveraged propeller loads on a maneuvering submersible. Typical execution times on a VAX 8810 computer are less than 10 seconds. Propeller input for PROPLD consists of the noninclined open water curves for thrust and torque and geometrical data consisting of the propeller diameter, hub radius, and the number of blades. The method is currently restricted to non-skewed propellers but is capable of modeling skew. Steady maneuver conditions may consist of combinedangle of attack and rotation. Figure 1 defines the coordinate system and sign conventions. The technical approach is summarized below. Changes made to the propeller inflow model of Pe rk id are described in detail. Other aspects of the propeller model, including the mathematical expressions for the load calculations, may be found and are not repeated here.

Development of a collision model for mine clearing darts

This paper describes the development of a collision model for mine clearing darts and its implementation in a six-degree-of-freedom (6-DOF) code for predicting dart trajectories from dispense to ground impact. Previous 6-DOF simulations did not include dart collisions but only mimicked its effects by initializing the darts with random tipoff angles. These simulations provided good predictions of dart impact pattern size and density distribution when compared with flight test data. However, the impact velocity distribution was poorly predicted. To improve results for impact velocities, a collision model is implemented and presented here. The collision detection is based on the algorithm of [1] for collisions between finite circular cylinders. By treating the darts as cylinders, geometric features can be exploited for quicker detection, which is very important for keeping the simulation time to a reasonable level. Additional efficiency is achieved by dividing the darts into overlapping rectangular bins. The collision search is performed for dart pairs in individual bins instead of searching the entire dart cloud. With collision modeling implemented, the 6-DOF now provides excellent agreement with flight test data for impact pattern size, density and velocity distribution.