286,266 research outputs found
Assessing the Potential for Improved Scramjet Performance through Application of Electromagnetic Flow Control
Hypersonic flight using scramjet propulsion bridges the gap between turbojets and rockets. Recent efforts focus on magnetogasdynamic (MGD) flow control to mitigate the problems of high thermomechanical loads and low efficiencies associated with scramjets. This research is the first flight-scale, three-dimensional computational analysis of a realistic scramjet to assess how MGD flow control improves scramjet performance. Developing a quasi-one dimensional design tool culminated in the first open source scramjet geometry. This geometry was tested with the Air Force Research Laboratory\u27s electromagnetic computational code. To increase fidelity, an algorithm was developed to incorporate thermochemistry, resulting in the only open-source model of combustion in an MGD controlled engine. A control volume analysis with electron beam ionization was presented for the first time with this approach. Local MGD control within the inlet affected drag and heat transfer and was marginally successful in raising combustor inflow pressure. MGD acceleration to increase flow momentum was effective at improving flow into the scramjet\u27s isolator. Combustor-based MGD generators proved superior to inlet generators with respect to power density and engine efficiency. MGD acceleration was ineffective in improving performance with all the MGD engines having approximately 33% more drag than baseline and none of them achieving self-powered operation state
Mechanistic and pathological study of the genesis, growth, and rupture of abdominal aortic aneurysms
Postprint (published version
MGOS: A library for molecular geometry and its operating system
The geometry of atomic arrangement underpins the structural understanding of molecules in many fields. However, no general framework of mathematical/computational theory for the geometry of atomic arrangement exists. Here we present "Molecular Geometry (MG)'' as a theoretical framework accompanied by "MG Operating System (MGOS)'' which consists of callable functions implementing the MG theory. MG allows researchers to model complicated molecular structure problems in terms of elementary yet standard notions of volume, area, etc. and MGOS frees them from the hard and tedious task of developing/implementing geometric algorithms so that they can focus more on their primary research issues. MG facilitates simpler modeling of molecular structure problems; MGOS functions can be conveniently embedded in application programs for the efficient and accurate solution of geometric queries involving atomic arrangements. The use of MGOS in problems involving spherical entities is akin to the use of math libraries in general purpose programming languages in science and engineering. (C) 2019 The Author(s). Published by Elsevier B.V
Phase-field boundary conditions for the voxel finite cell method: surface-free stress analysis of CT-based bone structures
The voxel finite cell method employs unfitted finite element meshes and voxel quadrature rules to seamlessly
transfer CT data into patient-specific bone discretizations. The method, however, still requires the explicit
parametrization of boundary surfaces to impose traction and displacement boundary conditions, which
constitutes a potential roadblock to automation. We explore a phase-field based formulation for imposing
traction and displacement constraints in a diffuse sense. Its essential component is a diffuse geometry model
generated from metastable phase-field solutions of the Allen-Cahn problem that assumes the imaging data as
initial condition. Phase-field approximations of the boundary and its gradient are then employed to transfer
all boundary terms in the variational formulation into volumetric terms. We show that in the context of the
voxel finite cell method, diffuse boundary conditions achieve the same accuracy as boundary conditions
defined over explicit sharp surfaces, if the inherent length scales, i.e., the interface width of the phase-field,
the voxel spacing and the mesh size, are properly related. We demonstrate the flexibility of the new method
by analyzing stresses in a human femur and a vertebral body
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Hydraulic Model Study of Waller Creek Tunnel Project for the City of Austin, Texas
This report provides the results of a series of model tests designed to understand the fluid dynamics involved with the Waller Creek Tunnel.Waller Creek is centrally located within the City of Austin, Texas, and has one of the most densely developed watersheds in the locality. The main stem is approximately seven miles in length and generally flows from north to south. The total drainage area for Waller Creek is 5.74 square miles (3700 acres) and the watershed lies entirely within the jurisdictional boundaries of the City of Austin and Travis County as shown in Figure 1 and Figure 2.
Flood impacts to development adjacent to the creek have been a concern since the area was developed in the 1950’s. Central Texas (including the City of Austin) is prone to flooding, especially in creeks with highly impervious watersheds. The Waller Creek Tunnel Project (WCTP) will reduce the threat of flood damages to existing infrastructure and development, along the Lower Reach of Waller Creek. In addition to flood control, the proposed design will improve water quality, create ecological benefits, mitigate erosion problems, and provide safety to individuals and businesses located in the downtown Austin Waller Creek area.
At the request of Crespo Consulting Service Inc., Austin, Texas (Crespo), Alden Research Laboratory (Alden) conducted a hydraulic model study using Computational Fluid Dynamics (CFD) and physical model studies for the proposed WCTP for the City of Austin, Texas. The objective of the study included evaluation of the flow patterns approaching and in the various hydraulic structures. Additionally, the model study was used to: Establish a rating curve for the morning glory spillway, Evaluate the potential air entrainment in the tunnel and the need for any air bleed structures, Establish junction loss coefficients for the 4th Street and 8th Street lateral junctions, Determine the fluctuating pressures due to flow induced excitations at the tunnel portal to the recirculation pump intake when the valve is closed and, Obtain a rating curve for the outlet spillway/weir.
Inlet CFD Model -A CFD model of the inlet at Waterloo Park was used to design and evaluate the approach channel geometry, training wall and bar screens. The intake structure is comprised of a morning glory type inlet and includes six bar screen type trash racks which remove a portion of any debris before entering the vertical drop shaft and the underground tunnel. A training wall was designed to improve flow distribution approaching the structure. Model results show that 80% of the bar screen area has a velocity of less than 4 ft/s. The maximum velocity at any location on the screens is less than 5.5 ft/s.
To address concerns that debris may accumulate along the training wall, modifications were made to fill in the backside of the barbs (the barbs were developed as part of the design to improve flow distribution and conditions at the screens). With the modifications the model results show that two screens did not have 80% of the bar screen area velocities of less than 4 ft/s and one screen exceeded the 50 % flow variation from the target flow. The final alternative was evaluated in the physical model.
Lateral Junction CFD Models - Lateral junctions at 4th Street and 8th Street were evaluated using CFD models. The models were used to simulate flow conditions where the lateral flow is relatively large and the main conduit flow is relatively small. This condition results in the largest impact of the lateral junctions on the main conduit flow. Model results showed that the lateral junctions are not predicted to cause significant flow separations in the main conduit and cavitation potential is small.
Outlet CFD Model -A CFD model of the outlet structure was used to design the riser shaft from the tunnel to the surface, and a flip bucket at the toe of the spillway. Based on the CFD model results, Outlet connection 2 as shown in Figure 17 was selected. The final structure design, based on CFD results, shows uniform flow distribution over the spillway. The flip bucket at the toe of the dam decreased the water velocity near the bed of the discharge channel as compared to a no flip bucket condition. Water velocity near the end of the spillway with the 2.25 ft high flip bucket is not predicted to erode the spillway apron.
Inlet/Tunnel/Lateral Junctions/Outlet Physical Model - A 1:33 scale model of the Inlet, Tunnel, Lateral Junctions and Outlet of the Waller Creek Tunnel Project was constructed at Alden. The design flow for the model to simulate the friction losses and the expected Hydraulic Gradient Line (HGL) along the tunnel corresponded to the 100 year flood flow. The rating curves for the inlet spillway and outlet weir and the closed conduit flow loss coefficients for tunnel junctions were obtained from the model by testing a range of flows, as they were not affected by the tunnel HGL.
Upon initial model start up, air entrainment at the morning glory vertical shaft was observed. The measured average Volume Fraction of air (VFa) in the model was about 5% for 25 year flow and 4% for the 100 year flow. Maximum Volume Fraction of air (VFa) in the model for the 25 year and 100 year flows were 8% and 6%, respectively. The volume fractions obtained from the model data could be corrected for any scale effects on generation of air entrainment using correction factors available in the literature. Also, as the Volume Fraction of air, VFa, can be a function of pressure and temperature, corrections need to be applied taking into account the expected pressure (from HGL calculations) and temperature in the field.
The morning glory rating curve (Figure 64) was established in the physical model using inlet flows for the 2, 5, 10, 25, 50, 100 and 500 peak tunnel/ peak intervening events. During the 500 year event the morning glory spillway was submerged and an air drawing free-surface vortex was observed however it should be noted that 1) the building operations deck, which could interfere with vortex formation, was not included in the model and 2) the emergency spillway was not modeled which would result in lower water levels for the 500 yr event. Data was also recorded for two additional flows to determine the point at which the inlet weir becomes drowned out (approximately at a flow of 9,950 cfs at EL 483.2 ft water level). For the 100 year peak inlet flow condition (8,247 cfs) the average HGL was increased in the inlet shaft to above the morning glory crest elevation (474.0 ft) to elevations 478.2 and 479.7 ft to determine any effect on the inlet rating curve. For these submerged conditions, no change in the head on the morning glory spillway was observed. Therefore, the inlet rating curve is hydraulically disconnected from the inlet shaft tailwater up to at least 479.9 ft.
Testing was conducted to determine any fluctuating pressures due to flow induced excitations at the tunnel portal to the recirculation pump intake when the valve is closed using the 100 year peak tunnel/ peak intervening condition. A plot of the prototype pressure versus time fluctuation referenced to EL 427 ft is included in Figure 65. The predicted maximum, minimum and average pressures were 16.9, 15.5 and 13.4 psi, respectively. This range of fluctuating pressure was not of concern to the JV in terms of design criteria for the valve.
Tests were conducted using the model to determine the Minor Losses and corresponding junction Loss Coefficients at the 8th Street and 4th Street lateral junctions and the tunnel (tees combining main flow in the tunnel with flow from side inlet weir branch). Results indicating the Loss Coefficients determined from the model for the 100 year flood for both Peak Tunnel-Peak Intervening and Lagging Tunnel-Peak Intervening are shown below:
100 yr Peak Tunnel-Peak Int. 100 yr Lagging Tunnel-Peak Int. Loss Coefficient 8th St. 4th St. 8th St. 4th St. Tunnel: K2-3 0.3 0.2 0.3 0.3 Branch: K1-3 -0.6 -0.7 -0.4 -0.4
The branch loss coefficients (K1-3) are negative due to transfer of energy from the through flow in the tunnel to the flow from the branch as the branch flow is only about 10% or so of the tunnel flow. The outlet spillway rating curve was also established in the physical model using inlet flows for the 2, 5, 10, 25, 50, 100 and 500 year peak tunnel/ peak intervening events. The outlet spillway rating curve is shown in Figure 70.Waller Creek Working Grou
Arbitrary-Lagrangian-Eulerian discontinuous Galerkin schemes with a posteriori subcell finite volume limiting on moving unstructured meshes
We present a new family of high order accurate fully discrete one-step
Discontinuous Galerkin (DG) finite element schemes on moving unstructured
meshes for the solution of nonlinear hyperbolic PDE in multiple space
dimensions, which may also include parabolic terms in order to model
dissipative transport processes. High order piecewise polynomials are adopted
to represent the discrete solution at each time level and within each spatial
control volume of the computational grid, while high order of accuracy in time
is achieved by the ADER approach. In our algorithm the spatial mesh
configuration can be defined in two different ways: either by an isoparametric
approach that generates curved control volumes, or by a piecewise linear
decomposition of each spatial control volume into simplex sub-elements. Our
numerical method belongs to the category of direct
Arbitrary-Lagrangian-Eulerian (ALE) schemes, where a space-time conservation
formulation of the governing PDE system is considered and which already takes
into account the new grid geometry directly during the computation of the
numerical fluxes. Our new Lagrangian-type DG scheme adopts the novel a
posteriori sub-cell finite volume limiter method, in which the validity of the
candidate solution produced in each cell by an unlimited ADER-DG scheme is
verified against a set of physical and numerical detection criteria. Those
cells which do not satisfy all of the above criteria are flagged as troubled
cells and are recomputed with a second order TVD finite volume scheme. The
numerical convergence rates of the new ALE ADER-DG schemes are studied up to
fourth order in space and time and several test problems are simulated.
Finally, an application inspired by Inertial Confinement Fusion (ICF) type
flows is considered by solving the Euler equations and the PDE of viscous and
resistive magnetohydrodynamics (VRMHD).Comment: 39 pages, 21 figure
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