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Images | Animations (Randy)

Analysis and design related to vehicle maneuvering, dynamics, and stability is a growing field in the area of Computational Fluid Dynamics (CFD). This is in part, due to the ability of CFD-based methods to yield accurate vehicle loads at all vehicle attitudes. This is contrary to the traditional methodology, of which only linear regimes could be analyzed numerically and the nonlinear are based on established experiments of similar shaped vehicles. Naval ship and vehicle design has yielded drastically different hull shapes and appendages, of which, creates design difficulties in these traditional experiments that are not longer valid. Without proper understanding of these novel concepts, has in the past, lead to the design of vehicles with poor performance, unstable maneuvering characteristics, amongst a variety of other issues.

ARL has in the past displayed that a CFD-based analysis enables the accurate prediction of the full scale loads, at any vehicle attitude, which enables the accurate prediction of the vehicle dynamics in any maneuver. This is prior to any model fabrication! This capability clearly improves the understanding of the design process.

The CM division encompasses this capability, and more, in various in-house codes (NPHASE, OVER-REL, TCURS, and UNCLEM). The vehicle prediction capabilities of these codes are summarized in Figure 1. One of the major enablers of this technology is with the Overset grid approach (see the Overset Grid Methods - OGM tab above). The tools used for this capability in our codes, as well as many other naval-focused CFD codes, are currently being developed at PSU-ARL (SUGGAR, DirtLIB, USURP). With this technology complex geometric features can be modeled, as well as design-relevant parametric studies can be examined in an efficient manner in the context of craft stability and dynamics.

In the past ARL has relied heavily on OVER-REL for vehicle and propulsion design. Recently, OVER-REL is being renovated with additional, state-of-the-art capabilities. This renovated code, TCURS (Tightly Coupled Unsteady RANS & SixDOF), is the latest in a vehicle maneuvering modeling capability. The CFD code is designed for the simulation of vehicle maneuvering integrated with a generalized controller. An outline of the methodology is described in Figure 2. This is a unique ability that enables a numerical evaluation of the controller, ensuring that the responses are as designed. This code can also handle an arbitrary number of bodies and control surfaces.

The multiphase code, UNCLEM, has a unique capability in that it can handle a wide variety of multiphase applications including natural and artificially cavitating flows. UNCLEM has shown to predict the dynamics of high speed supercavitating torpedoes, as well as the launching of these torpedoes. One truly unique factor is that the code is fully compressible; thus, the dynamics associated rocket thrusters can also be accurately analyzed. Fundamentally, the code can extend surface ship dynamics, especially in the high speed sense where cavitation is involved.

NPHASE is unique in that it enables an unstructured grid approach with adaptive refinement capabilities. This is useful in that grid design is less important and the flow features can be adequately resolved as they become apparent in the solution. The multiphase capabilities of NPHASE are also focused towards a multi-fluid approach, where more detailed species interactions can be investigated. This capability enables the modeling of micro-bubble drag reduction concepts.

In the past ARL has investigated supercavitating torpedo dynamics, submarine stability and dynamics, projectile dynamics, amongst many other vehicle- and body-dynamics related problems. Our capabilities include the accurate prediction of propeller loads with and without cavitation (assume there are figs. To ref), as well as moving control surfaces with and without cavitation (refs), and free-surface submerged control surfaces (Ref.).

The first proven methodology is the simulation of vehicle dynamics and maneuvers. This is more involved in the analysis of vehicle dynamics, rather than design. Here the aero/hydrodynamics on the body, which can include a full scale analysis, dynamic control surfaces, with an variety of physics, govern the predicted body motion throughout a maneuver. In short, this can simulate vehicle response to operator’s control inputs in various conditions, or examine motion while encompassing accurate loads with a minimum number of assumptions.

The first example is displayed in 6-DOF simulations from TCURS. Here the dynamics and mechanics of a non-lethal ring-airfoil projectile were investigated. This projectile had a muzzle exit velocity of 492 mph (220 m/s), and a rotation rate of 1500 rpm. The projectile was followed by a sabot, a device that maintains a seal between the projectile and the propellants through the muzzle, which was also analyzed in the context of the mechanics of the launching of the projectile. Multiple views of 6-DOF simulations with varied cross flow, or wind, speeds are displayed in Animations 1a, 1b, and 1c. Our predictions show the effect of varied wind speeds (0, 10, and 20 knots) to the path of the projectile. The effect of the varied wind speed is easily viewed from the bottom view, where the y-direction is inline with the cross wind, and the speeds clearly vary. The side and front views display the loss in altitude due to the increased cross-flow speeds. Figure 3 displays the predicted pressure field of the projectile in flight. The surface grid revealing the complicated geometric features, or ribs of the sabot, is displayed in the grid in Figure 4. Figure 5 displays the predicted flow field after the sabot exits the muzzle, of which enabled the prediction of the trajectory (shown in Figure 6). This was useful in predicting the flight path of the sabot in relation to that of the non-lethal projectile to ensure that the sabot was not becoming an additional, non-guided projectile.

Other applications of TCURS include body interactions, such as the simulation displayed in Animation 2. In this simulation the interactions between a deployment vehicle from a larger submerged vehicle are investigated. This in particular displays the flexibility of the Overset method and how it enables relative body motion.

ARL has also shown the capability to predict vehicle dynamics in complex flows. For example the 3-DOF predictions of a supercavitating torpedo performed in UNCLEM. Such a capability is crucial for supercavitating vehicle design where the control of such a vehicle is perhaps the greatest challenge. Animations 3 and 4 display videos of the predicted torpedo and cavity dynamics throughout the simulation. Animation 3 displays three different views: the top is a side view from a stationary viewpoint, the center is from the front, and the lower is the side view moving with the projectile. These videos display the dynamic behavior of the cavity, especially when body dynamics are considered, which significantly differ from experimental conditions. Animation 4 displays a representative simulation of the torpedo being fired from a submarine, and is predicted path throughout the launch. Figure 7 displays a 1-DOF, fully compressible calculation of a high speed, supercavitating projectile exiting a muzzle.

The other vehicle dynamics simulation method is based on force and moment computation at various conditions and configurations to provided forces at those conditions. This also enables the prediction of stability derivatives, thus, has proved to be useful in vehicle dynamics analysis. This is an essential component of the design for vehicle stability. Especially, as previously mentioned, when a novel not well understood shape is used. Without experimentation, CFD shows to provide a viable means to predict the loads at various attitudes, thus, enable more accurate dynamic simulations. Figure 8 displays a grid on the body examined, which includes a fully appended vehicle. In Figure 9 some of the highly nonlinear flow structures predicted is displayed at various vehicle attitudes and control fin orientations. Finally, Figure 10 displays the effect of the vehicle dynamic when they incorporate a CFD-based correction. This unstable mode predicted with this more comprehensive modeling approach was observed in experiments, while traditional models predicted a stable maneuver.

Images
View Image (24kb)	Figure 1: Summary of code capability.
View Image (20kb)	Figure 2: TCURS motion controller flow chart.
View Image (107kb)	Figure 3: Predicted pressure field around the non-lethal ring-airfoil projectile.
View Image (103kb)	Figure 4: Surface grid of a 6 degree portion of the sabot.
View Image (113kb)	Figure 5: Predicted flow structure when sabot flaps at 45degrees. Left: Front View Right: orthogonal view
View Image (19kb)	Figure 6: Predicted sabot trajectory.
View Image (101kb)	Figure 7: UNCLE-M results showing volume fraction of vapor and gun gas. Modeled gun gas is inserted at exit of muzzle. Projectile position computed based on the solution of the equations of motion for the rigid projectile given forces obtained from the CFD solution.
View Image (157kb)	Figure 8: Grid used to predict the forces on NEMO.
View Image (192kb)	Figure 9: Predicted flow-field at various attitudes. Note the separation patterns and vertical structures of which loads are best determined using experiment or CFD.
View Image (32kb)	Figure 10: Comparison to the flight path when using CFD method, compared to baseline, to achieve nonlinear stability derivatives. Note that the predicted unstable condition was observed in experiment.

Animations
View Animation (Flash Player 2.5mb)	Animation 1a: View from below of 6-DOF simulation with varied cross flow, or wind, speeds.
View Animation (Flash Player 2.5mb)	Animation 1b: Side view of 6-DOF simulation with varied cross flow, or wind, speeds.
View Animation (Flash Player 2.4mb)	Animation 1c: Front view of 6-DOF simulation with varied cross flow, or wind, speeds.
View Animation (Flash Player 744kb)	Animation 2: The interactions between a deployment vehicle from a larger submerged vehicle are investigated.
View Animation (Flash Player 913kb)	Animation 3: Predicted torpedo and cavity dynamics throughout a simulation. This animation displays three different views: the top is a side view from a stationary viewpoint, the center is from the front, and the lower is the side view moving with the projectile.
View Animation (Flash Player 6.8mb)	Animation 4: Predicted torpedo and cavity dynamics throughout a simulation. This animation displays a representative simulation of the torpedo being fired from a submarine, and is predicted path throughout the launch.
View Animation (Flash SWF file 5.5mb)	Animation 5: A notional non-body-of-revolution underwater vehicle executing a constant 20 degree rudder maneuver using Tightly-Coupled-Unsteady-RANS solver. Upper Left: The inertial reference frame of vehicle proceeding through the maneuver. Upper Right: X-Y inertial reference frame of vehicle trajectory Lower Left: X-Y-Z inertial reference frame of vehicle trajectory Lower Right: X-Z inertial reference frame of vehicle trajectory