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Constrained optimization control techniques with preview are designed in this paper to derive optimal velocity trajectories in longitudinal vehicle following mode, while ensuring that the gap from the lead vehicle is both safe and short enough to prevent cut-ins from other lanes. The lead vehicle associated with the Federal Test Procedures (FTP) [1] is used as an example of the achieved benefits with such controlled velocity trajectories of the following vehicle. Fuel Consumption (FC) is indirectly minimized by minimizing the accelerations and decelerations as the autonomous vehicle follows the hypothetical lead. Implementing the cost function in offline Dynamic Programming (DP) with full drive cycle preview showed up to a 17% increase in Fuel Economy (FE). Real time implementation with Model Predictive Control (MPC) showed improvements in FE, proportional to the prediction horizon. Specifically, 20s preview MPC was able to match the DP results. A minimum of 1.5s preview of the lead vehicle velocity with velocity tracking of the lead was required to obtain an increase in FE. The optimal velocity trajectory found from these algorithms exceeded the presently allowable error from standard drive cycles for FC testing. However, the trajectory was still safe and acceptable from the perspective of traffic flow. Based on our results, regulators need to consider relaxing the constant velocity error margins around the standard velocity trajectories dictated by the FTP to encourage FE increase in autonomous driving.

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The critical heat flux (CHF) for R-113 was measured in forced convection over a flat surface at various orientations for relatively low flow velocities corresponding to Reynolds numbers ranging between 3000-6500 in the test section. As expected, the CHF was found to depend upon the orientation of the buoyancy. Although the buoyancy force acting on the vapor generally dominates over the flow inertia in this flow range, the inertia would continue to be substantial if the gravity was to be reduced significantly. In the experiments of this study, the CHF was determined for heating surface orientations ranging from 0 to 360 deg, for flow velocities between 4-35 cm/s, and for subcoolings between 2.8-22.2°C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased. In addition, a simple model in which the effects of varying the buoyancy orientation in pool boiling is incorporated is proposed to correlate the CHF at low flow velocities, which finally leads to an analogy between the CHF under adverse gravity and that under microgravity conditions. Nomenclature c pl = specific heat of the liquid phasê BUOV = buoyancy forcê Drag = drag force g = gravitational acceleration h fg = latent heat of evaporation L c = characteristic length dimension q c = CHF q, = CHF in saturated pool boiling predicted by Zuber's model