Timothy F. Miller

A next-generation AUV energy system based on aluminum-seawater combustion

This paper describes a hypothetical seawater-breathing autonomous underwater vehicle (AUV) energy system based on the reaction of powdered aluminum. The reaction of aluminum with seawater offers improvement in energy density that will revolutionize long duration underwater operation. Although the system is hypothetical, the critical components are currently undergoing active engineering research and development. Consequently, this paper also describes in more detail the critical components of the system. The technical maturity level of this component development suggests that the development of a next-generation aluminum-seawater combustion system for AUV applications is feasible.

A Fourier analysis of the IPSA/PEA algorithms applied to multiphase flows with mass transfer

Abstract The error amplification matrices for two variations of a pressure-based Eulerian–Eulerian multiphase algorithm are developed using the method of Fourier decomposition. The algorithms examined here include Spaldings IPSA/PEA, and a revised form proposed by Siebert and Antal appropriate for flows with mass transfer. The error amplification matrices are developed for a single spatial dimension, but for an arbitrary number of phases or fluids. The ramifications arising from these amplification matrices are explored in this article. For two-phase applications the revised form produces a broader theoretical range of convergent behavior for different interphase momentum and mass transfer rates. For four-phase applications both methods appear to be conditionally stable, and produce similar convergence behaviors. Large differences in interphase mass and momentum transfer rates between phasic pairs appear to adversely affect the algorithms stability range.

FOURIER ANALYSIS OF THE SIMPLE ALGORITHM FORMULATED ON A COLLOCATED GRID

Abstract The Fourier error amplification matrix has been developed for the SIMPLE algorithm formulated on a collocated grid using the Rhie-Chow approximation. The cause of and remedy for the checkerboard pressure effect associated with collocated grid systems is shown. Comparisons with observations from the technical literature suggest that Fourier analysis is capable of describing algorithm operation. This analysis also suggests that there is a conditional relationship between the momentum underrelaxation factor and the pressure-correction damping factor for convergence of the collocated form, and indicates that the collocated form should serve as a superior multigrid smoother.

A high-pressure, continuous-operation cyclone separator using a water-generated flow restriction

Abstract A novel high-pressure, continuous-operation separator for moderately laden, gas–solid flows was designed, built, and tested. The novelty of this device was its use of supplemental water to produce a virtual flow restriction that acted to maintain high-pressure and to prevent gas from escaping with the separated solids. The water–solid slurry thus produced may be pumped, and can be continuously and easily conveyed away. Over a range of practical operating points, the separator showed very high collection efficiencies (>90%), low-pressure drop, and robust operation. A minimum supplemental water requirement was determined to prevent gas loss with the separated solids stream.

Simulation of shock-wave reflection in a one-dimensional shock tube using smoothed particle hydrodynamics

Simulations of shock-wave reflection in a model one-dimensional shock tube have been made using a Lagrangian technique called smoothed particle hydrodynamics. Different boundary methods have been compared with regard to their ability to model a one-dimensional reflecting shock wave in a shock tube. Oar simulations show that the introduction of imaginary particles appears to be the most effective method of boundary simulation for transient shock reflection. The choice of artificial diffusion treatment is ambiguous. Use of flux-corrected transport minimizes shock smearing, but produces postreflection oscillations. Artificial viscosity produces better postreflection behavior, but greater smearing.

Morphing hull concepts for unmanned underwater vehicles

Morphing and adaptive structures are experiencing a surge in research due to their ability to optimize the operational envelopes of complex vehicles. Adaptive structures have been researched heavily and put to use extensively in the Aerospace Industry. This paper examines a possible use and benefit associated with morphing structures with Unmanned Underwater Vehicles (UUVs). The growing resume of applications associated with UUVs as well as their unique abilities make them an excellent subject of study for morphing concepts. This paper focuses on the possible uses of flexible hull morphing, specifically the benefits associated with such adaptability in terms of range, endurance, and speed. This research proposes a UUV system which has a flexible hull wrapped around a standard pressure hull. The annulus created between the pressure hull and the flexible hull is used for the storage of expendable energetics, in this case diesel fuel. Storing fuel in the newly created annulus also eliminates the need to waste space within the pressure hull storing fuel and thus the pressure hull may also be shrunk. This paper focuses on the benefits of morphing where the shape of the flexible hull remains the same but the diameter of the outer hull is shrunk in concert with the consumption of fuel thus decreasing the drag profile of the vehicle, resulting in increased vehicle range. Two different UUV missions are outlined: 1) slow speed scan and 2) slow speed scan with high speed dash. A commercial semi-submersible was used as a baseline. For the slow speed scan mission, semi-submersible heavy vehicles carrying in the range of 50,000 kg of fuel could realize increases of range close to 40% when compared to a similarly sized vehicle without the adaptive capabilities. Small vehicles on the other hand, show limited benefits of diameter morphing, gaining less than 5% in range over the non-adaptive system. Dash missions during which a scanning operation must be augmented with some sort of high speed maneuver show more significant benefits with a morphing system. The results show that diameter morphing is clearly not viable in all situations, yet the concept shows promise for certain long range/long endurance UUV missions, as well as missions that require sporadic high speed operation.

Linear Fourier and Iteration-Delay Analysis of a Computational Fluid Dynamics Problem During Execution

The basic equations for the Fourier error analysis are developed and then applied to the scalar conservation equation of a sample computational fluid dynamics (CFD) problem in which variables are continuously updated. The analysis helps explain basic features of numerical stability. When divergence and neutral stability are encountered, Fourier analysis provides insight into the emergence and location of the instability, but is not by itself found to be a sufficient indicator of the existence of numerical instability. Further analysis of central differencing cases is made using a variation on time-delay reconstruction, from chaos theory.

MODELING MULTIPHASE FLOWS SUBJECTED TO CENTRIFUGAL ACCELERATION WITH A MIXTURE-AVERAGED DRIFT-FLUX ALGORITHM

ABSTRACT A mixture-averaged multiphase flow model has been developed to predict local variations in individual phase concentrations arising from centrifugal accelerations in the fluid. This centrifugal acceleration can be caused by swirl or by flow streamline curvature. The model has been developed for turbulent, compressible, nonisothermal flows. Higher-order differencing was used to discretize the transport equations; however, the effect of differencing the particle acceleration model on model performance is shown. For a sample case, comparisons are made with results using a Eulerian/Eulerian two-phase model. Discrete phase mass fraction concentration profiles compare favorably between the modified, mixture-averaged model and a two-phase model.

Numerical modeling of two-dimensional device structures using Brandt's multilevel acceleration scheme: application to Poisson's equation

In expanding numerical modeling for electronic and optoelectronic devices from a single dimension to multiple dimensions, a large increase in machine storage space is required. Solution approaches based on relaxation techniques are typically used to minimize this increase, but they can be slow to converge. Presented is an adaption of Brandts multilevel acceleration scheme for control volume discretizations coupled with solvers based on either Stones strongly implicit method or the Gauss-Siedel (G-S) method to overcome this speed and storage space problem. This approach is demonstrated by solving Poissons equation in a two-dimensional amorphous silicon thin-film transistor structure. The structure has a generalized density of states function whose occupancy is computed using nonzero degree Kelvin Fermi-Dirac statistics. It is shown that the use of the multilevel acceleration algorithm gives more than an order of magnitude increase in the asymptotic rate of convergence for the potential distribution in this thin-film transistor. Numerical results of the analysis are presented. >

Simulating a shape memory alloy buoyancy heat engine for undersea gliders

Undersea gliders are autonomous undersea vehicles (AUVs) that travel the worlds oceans in a sawtooth climbdive pattern. These gliders are driven by buoyancy engines that typically use electrically powered pumps to displace water with an oil bladder and thus enable a buoyancy change. Battery capacity limits the endurance of these types of engines and vehicles. The Argo project uses similar electrical buoyancy engines to help monitor climate change [1]. Increasing the endurance of buoyancy engines could have an impact on sensors deployed in the worlds oceans by decreasing the maintenance cost of sensing missions and allowing for studies to be conducted over a longer period of time. Buoyancy heat engines driven by the naturally occurring thermocline that exists over a large expanse of the worlds oceans can drastically improve endurance. These engines harness environmental energy as opposed to consuming electrical energy to change vehicle buoyancy. Buoyancy engines have been developed that employ the volume change produced during the phase change of wax [2], but it may also be possible to use the oceanic thermocline to drive a buoyancy heat engine based on shape memory alloys (SMAs) (Fig. 1). Shape memory alloys can recover a large amount of applied strain, up to approximately 10% [3], when heated due to a solid state phase transformation in the alloy. The high strain energy of shape memory alloys can be leveraged to create a lighter and more efficient buoyancy heat engine, but the performance and capability of a solid state SMA based engine is unknown. Previous work has discussed the analysis, design, and testing of such an engine, called the shape memory alloy buoyancy heat engine (SMA-BHE) [4]. This paper will examine the dynamics of the SMABHE by coupling a lumped system heat transfer model and the Brinson model for one-dimensional SMA constitutive behavior to the kinematics of a notional undersea glider in an ocean environment. Including the Brinson model and other known SMA relations in the kinematic simulation of a notional glider led to a model that predicted successful AUV operation with a SMA-BHE at depths past 700 m.