Development of a Numerical Investigation Framework for Ground Vehicle Platooning

This paper presents a study on the flow dynamics involving vehicle interactions. In order to do so, this study first explores aerodynamic prediction capabilities of popular turbulence models used in computational fluid dynamics simulations involving tandem objects and thus, ultimately presents a framework for CFD simulations of ground vehicle platooning using a realistic vehicle model, DrivAer. Considering the availability of experimental data, the simulation methodology is first developed using a tandem arrangement of surface-mounted cubes which requires an understanding on the role of turbulence models and the impacts of the associated turbulence model closure coefficients on the prediction veracity. It was observed that the prediction accuracy of the SST k−ω turbulence model can be significantly improved through the use of a combination of modified values for the closure coefficients. Additionally, the initial validation studies reveal the inability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to resolve the far wake, and its frailty in simulating tandem body interactions. The Improved Delayed Detached Eddy Simulations (IDDES) approach can resolve the wakes with a reasonable accuracy. The validated simulation methodology is then applied to the fastback DrivAer model at different longitudinal spacing. The results show that, as the longitudinal spacing is reduced, the trailing car’s drag is increased while the leading car’s drag is decreased which supports prior explanations of vortex impingement as the reason for drag changes. Additionally, unlike the case of platooning involving Ahmed bodies, the trailing model drag does not return to an isolated state value at a two car-length separation. However, the impact of the resolution of the far wake of a detailed DrivAer model, and its implication on the CFD characterization of vehicle interaction aerodynamics need further investigations.

Numerical Computation of Low Reynolds Number Viscous Flow Past Bluff Bodies

This article presents a two-dimensional steady viscous flow simulation past circular and square cylinders at low Reynolds numbers (based on the diameter) by the finite volume method with a non-orthogonal body-fitted grid. Diffusive fluxes are discretized using central differencing scheme, and for convective fluxes upwind and central differencing schemes are blended using a ‘deferred correction’ approach. A simplified pressure correction equation is derived, and proper under-relaxation factors are used so that computational cost is reduced without adversely affecting the convergence rate. The governing equations are expressed in Cartesian velocity components and solution is carried out using the SIMPLE algorithm for collocated arrangement of variables. The mesh yielding grid-independent solution is then utilized to study, for the very first time, the effect of the Reynolds number on the separation bubble length, separation angle, and drag coefficients for both circular and square cylinders. Finally, functional relationships between the computed quantities and Reynolds number (Re) are proposed up to Re = 40. It is found that circular cylinder separation commences between Re= 6.5-6.6, and the bubble length, separation angle, total drag vary as Re, Re−0.5, Re−0.5 respectively. Extrapolated results obtained from the empirical relations for the circular cylinder show an excellent agreement with established data from the literature. For a square cylinder, the bubble length and total drag are found to vary as Re and Re−0.666, and are greater than these for a circular cylinder at a given Reynolds number. The numerical results substantiate that a square shaped cylinder is more bluff than a circular one.

Steady separated flow past a circular cylinder at low Reynolds numbers

The steady two-dimensional laminar flow around a stationary circular cylinder has been investigated via a stabilized finite-element method. The Reynolds number Re is based on the cylinder diameter and free-stream speed. The results have been presented for 6 ≤ Re ≤ 40 and the blockages between 0.000125 and 0.80. The blockage B is the ratio of the cylinder diameter to the domain width. There is large scatter in the value of Res, reported in the literature, marking the onset of the flow separation. From the present study the Res is found to be 6.29, approximately for B = 0.005. The effect of the blockage on the characteristic flow parameters is found to be insignificant for B ≤ 0.01. The bubble length, separation angle and Res exhibit non-monotonic variation with the blockage. It is for the first time that such a behaviour for the separation angle and Res is being reported. Two types of boundary conditions at the lateral walls have been studied: the slip wall and towing tank. In general for high blockage, the results from the slip boundary condition are closer to the ones for the unbounded flow. In that sense, the use of the slip boundary condition as opposed to the towing tank boundary condition on the lateral walls is advocated. The bubble length, separation angle, base suction, total drag, pressure drag, viscous drag and maximum vorticity on the cylinder surface for the steady flow are found to vary as Re, Re−0.5, Re−1, Re−0.5, Re−0.64, Re−0.60 and Re0.5, respectively. The extrapolated results for the steady flow, for higher Re, are found to match quite well with the other results from the literature.

Chromatographic Length Separation and Photoluminescence Study on DNA-Wrapped Single-Wall and Double-Wall Carbon Nanotubes

Water-soluble DNA-wrapped single-wall and double-wall carbon nanotubes (DNA-SWNTs, DNA-DWNTs) have been well separated by length incorporating size-exclusion high-performance liquid chromatography (HPLC). The morphology and electronic properties of the size- (length-) separated DNA-SWNTs and -DWNTs are investigated by atomic force microscopy (AFM), photoluminescence (PL), and Raman spectroscopy. By using length-separated DNA-SWNTs and -DWNTs, we have found that PL intensity of the DNA-SWNTs varies sensitively depending not only on the chirality (or diameter) but more importantly on the length of the hybrids.

Sample Preparation for Precise and Quantitative Electron Holographic Analysis of Semiconductor Devices

Wedge polishing was used to prepare one-dimensional Si n-p junction and Si p-channel metal-oxide-silicon field effect transistor (pMOSFET) samples for precise and quantitative electrostatic potential analysis using off-axis electron holography. To avoid artifacts associated with ion milling, cloth polishing with 0.02-μm colloidal silica suspension was used for final thinning. Uniform thickness and no significant charging were observed by electron holography analysis for samples prepared entirely by this method. The effect of sample thickness was investigated and the minimum thickness for reliable results was found to be ∼160 nm. Below this thickness, measured phase changes were smaller than expected. For the pMOSFET sample, quantitative analysis of two-dimensional electrostatic potential distribution showed that the metallurgical gate length (separation between two extension junctions) was ∼54 nm, whereas the actual gate length was measured to be ∼70 nm by conventional transmission electron microscopy. Thus, source and drain junction encroachment under the gate was 16 nm.