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Dive into the research topics where Chris Elkins is active.

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Featured researches published by Chris Elkins.


Journal of Magnetic Resonance Imaging | 2003

Time-resolved three-dimensional phase-contrast MRI.

Michael Markl; Frandics P. Chan; Marcus T. Alley; Kris L. Wedding; Mary T. Draney; Chris Elkins; David W. Parker; Ryan B. Wicker; Charles A. Taylor; Robert J. Herfkens; Norbert J. Pelc

To demonstrate the feasibility of a four‐dimensional phase contrast (PC) technique that permits spatial and temporal coverage of an entire three‐dimensional volume, to quantitatively validate its accuracy against an established time resolved two‐dimensional PC technique to explore advantages of the approach with regard to the four‐dimensional nature of the data.


Journal of Fluids Engineering-transactions of The Asme | 2010

An Experimental Study of the Flow Around a Formula One Racing Car Tire

Emin Issakhanian; Chris Elkins; Kin Pong Lo; John K. Eaton

The wake of the front tires affects the airflow over the remainder of a fenderless race car. The tires can also be responsible for up to 40% of the vehicle’s drag. Prior experiments have used compromised models with solid, symmetric hubs and nondeformable tires. The present experiment acquires particle image velocimetry measurements around a 60% scale model of a deformable pneumatic tire fitted to a spoked Formula 1 wheel with complete brake geometry and supplementary brake cooling ducts. The results show reversed flow regions in the tire wake, asymmetric longitudinal vortex structures behind the tire, and a tire wake profile that is unlike previous experimental results and postulations. The flow through the hub of the wheel causes a shift of the wake inboard (toward the car) so that the outboard side of the wake does not extend past the outline of the tire.


Journal of Surgical Research | 2011

Hemodynamic comparison of differing anastomotic geometries using magnetic resonance velocimetry.

Richard F. Neville; Chris Elkins; Marcus T. Alley; Ryan B. Wicker

BACKGROUNDnHemodynamic factors at the distal anastomosis play an important role in prosthetic graft performance. A new magnetic resonance imaging (MRI) technique was used to determine the effect of anastomotic geometry on hemodynamic flow patterns.nnnMETHODSnFour dimensional (4D) magnetic resonance velocimetry (4D-MRV) is a noninvasive method of analyzing pulsatile flow in three dimensions (3D). End-to-side anastomotic models were constructed by suturing 6 mm polytetrafluoroethylene (ePTFE) grafts to silicone tubing (4 mm i.d.). The models included straight ePTFE, precuffed ePTFE, and patched ePTFE configurations in a pulsatile system, which created flow consistent with physiologic flow rates and pressures. Blood was simulated by a solution of 40% glycerol in distilled water with trace gadolinium. The different models were imaged using MRV techniques in a three-dimensional (3D) coronal slab (0.5 mm thick coronal slices, in-plane field of view (FOV) 18 cm.) The data were reconstructed, resulting in an interpolated resolution of 0.35 mm in each coronal plane. The 3D flow fields were represented as isosurfaces, visualizing the internal geometry of the models with streamlines tangent to the velocity vectors identifying the path of the fluid. Volumetric flow rates for each time phase were calculated by integrating the flow through cross sections of each anastomotic model. Analysis of the flow patterns focused on the anastomotic regions prone to the development of intimal hyperplasia and graft failure as identified in the literature; the toe, floor, heel, and hood.nnnRESULTSnConventional end-to-side geometry resulted in uniform flow with a low angle of impingement on the recipient vessel floor. A small vortex at the anastomotic heel created minimal recirculation. The precuffed geometry resulted in a large recirculation vortex of chaotic, low flow that increased throughout the pulsatile cycle. Regions of low flow velocity were noted in a substantial portion of the precuffed anastomotic configuration. Flow separation distal to the toe occurred in both geometries, but was more apparent in the precuffed configuration. The patch model had flow characteristics similar to the straight end-to-side geometry.nnnCONCLUSIONnMagnetic resonance velocimetry produces 3D, time varying velocity measurements with sufficient accuracy and resolution to analyze hemodynamics in anastomotic geometries. Flow structures in different graft configurations were effectively captured with marked differences noted between standard and precuffed anastomotic geometries. The findings support a conventional end-to-side anastomosis with a low incidence angle using a straight graft as producing favorable hemodynamics as compared to a cuffed configuration. The vein patch configuration closely approximates the conventional, straight anastomotic pattern. We believe the MRV technique has been sufficiently developed to warrant additional in vitro and in vivo studies providing insight into hemodynamic implications for the development of optimal prosthetic graft performance.


2005 Materilas Research Society Spring Meeting | 2005

'Three-Dimensional PEG Hydrogel Construct Fabrication using Stereolithography,'

Karina Arcaute; Luis Ochoa; F. Medina; Chris Elkins; Brenda K. Mann; Ryan B. Wicker

Layered manufacturing (LM) using stereolithography (SL) of aqueous polymer solutions was accomplished so three-dimensional (3D) tissue engineered scaffolds with complex distributions of bioactive agents could be produced. Successful LM with embedded channel architectures required investigation of hydrogel thickness or cure depth as a function of photoinitiator type and concentration, energy dosage, and polymer concentration in solution. Poly(ethylene glycol) dimethacrylate (PEG-dma) with an average molecular weight of 1000 in quantities of 20% and 30% (w/v) was prepared in distilled water. Different concentrations of two photoinitiators (PIs), Sarcure1121 (2-hydroxy-2-methyl-1-phenyl-1-propanone) and Irgacure 2959 (2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone), were used to vary gel thickness at select energy dosages by controlling the scan speed of the SL machines ultraviolet scanning system. Gel thickness was a strong function of PI type and concentration, energy dosage, and PEG-dma concentration, especially at the low PI concentrations required for implantation. The gel thickness curves were utilized to demonstrate LM for two construct geometries where different layer thicknesses were required to successfully fabricate the constructs. This work demonstrates the effective use of SL as a processing technique for complex 3D tissue scaffolds and addresses some practical considerations associated with the use of hydrogels in LM.


SAE World Congress & Exhibition | 2009

Computational and experimental investigation of the flow structure and vortex dynamics in the wake of a Formula 1 tire

John Axerio; Gianluca Iaccarino; Emin Issakhanian; Kin Pong Lo; Chris Elkins; John K. Eaton

The flow field around a 60% scale stationary Formula 1 tire in contact with the ground in a closed wind tunnel is examined experimentally in order to validate the accuracy of different turbulence modeling techniques. The results of steady RANS and Large Eddy Simulation (LES) are compared with PIV data performed within the same project. The far wake structure behind the wheel is dominated by two strong counter-rotating vortices. The locations of the vortex cores, extracted from the LES and PIV data as well as computed using different RANS models, show that the LES predictions are closet to the PIV vortex cores. All turbulence models are able to accurately predict the region of strong downward velociy between the vortex cores in the centerplane of the tire, but discrepancies arise when velocity profiles are compared close to the inboard and outboard edges of the tire, due to the sensitivity of the solution to the tire shoulder modeling. In the near wake region directly behind the contact patch of the tire, contour plots of inplane-velocity are compared for all three datasets. The LES simulation again matches well with the PIV data.


Journal of Geophysical Research | 2014

Local shear and mass transfer on individual coral colonies: Computations in unidirectional and wave‐driven flows

Sandy Chang; Gianluca Iaccarino; Frank Ham; Chris Elkins; Stephen G. Monismith

Flows through single coral colonies were simulated with an implementation of the Immersed Boundary (IB) method in Large-Eddy Simulation (LES). The method was first validated with magnetic velocimetry experiments, which demonstrated that computational results were within approximately 7% of flow measurements. The algorithm was then applied to simulate unidirectional and wave-driven flow conditions through two morphologically distinct coral colonies that naturally grow in very different hydrodynamic environments, with detailed analysis on spatial hydrodynamic and mass transfer variability. When the hydrodynamics of each corals native environment was simulated, the dynamics in the interior of both branching species appeared to converge, in spite of vast differences between the hydrodynamic conditions and morphologies. A correlation between local surface shear and mass transfer was derived from simulated data. The results suggest that the corals grew in such a way that mass transfer characteristics are similar despite of vast differences in their physical shapes and hydrodynamic conditions.


Journal of Turbomachinery-transactions of The Asme | 2013

Experimental-Based Redesigns for Trailing Edge Film Cooling of Gas Turbine Blades

Michael Benson; Sayuri D. Yapa; Chris Elkins; John K. Eaton

Magnetic resonance imaging experiments have provided the three-dimensional mean concentration and three component mean velocity field for a typical trailing edge film-cooling cutback geometry built into a conventional uncambered airfoil. This geometry is typical of modern aircraft engines and includes three dimensional slot jets separated by tapered lands. Previous analysis of these data identified the critical mean flow structures that contribute to rapid mixing and low effectiveness in the fully turbulent flow. Three new trailing edge geometries were designed to modify the large scale mean flow structures responsible for surface effectiveness degradation. One modification called the Dolphin Nose attempted to weaken strong vortex flows by reducing three dimensionality near the slot breakout. This design changed the flow structure but resulted in minimal improvement in the surface effectiveness. Two other designs called the Shield and Rounded Shield changed the land planform and added an overhanging land edge while maintaining the same breakout surface. These designs substantially modified the vortex structure and improved the surface effectiveness by as much as 30%. Improvements included superior coolant uniformity on the breakout surface which reduces potential thermal stresses. The utilization of the time averaged data from combined magnetic resonance velocimetry (MRV) and concentration (MRC) experiments for designing improved trailing edge breakout film cooling is demonstrated.


ASME 2011 Turbo Expo: Turbine Technical Conference and Exposition | 2011

Magnetic Resonance Imaging Studies of Flow and Mixing for Single-Hole Film Cooling

Emin Issakhanian; Chris Elkins; John K. Eaton

Magnetic resonance imaging (MRI) measurement techniques are used to reveal the coupled 3D velocity and coolant concentration fields for a single film cooling hole with L/D of 4, ejection angle of 60°, and blowing ratios of 0.5 and 1. The jet exits into a boundary layer with momentum thickness of 0.1D. Magnetic resonance velocimetry (MRV) measures 3 component mean velocity everywhere within the channel, cooling hole, and feed plenum. Magnetic resonance concentration (MRC) provides the coolant concentration distribution which is directly analogous to film cooling effectiveness. The coupled velocity and concentration show that high velocity ratios lead to a detached jet which lowers effectiveness. Vorticity from the feed hole creates a streamwise oriented counter rotating vortex pair which lifts the coolant stream from the surface and sweeps in main channel flow inducing a kidney-shape to the coolant jet cross-section. Without the need for optical access, MRV allows study of the flow inside the feed hole including the entrance separation and secondary flows. Cross-stream feeding of the cooling hole shows added spanwise asymmetry at the hole entrance, but this asymmetry is significantly reduced moving up the hole.© 2011 ASME


Archive | 2004

Multiple Material Micro-Fabrication: Extending Stereolithography to Tissue Engineering and Other Novel Applications

Ryan B. Wicker; Francisco Medina; Chris Elkins


Limnology and Oceanography | 2009

Flow inside a coral colony measured using magnetic resonance velocimetry

Sandy Chang; Chris Elkins; Marcus T. Alley; John K. Eaton; Stephen Monismitha

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Ryan B. Wicker

University of Texas at El Paso

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Michael Benson

United States Military Academy

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