The Roles of Large Scale Structures in the Initial Development of Circular Jets

Experimental results are presented describing the flow field surrounding the potential core of a circular water jet in the Reynolds number region between 5,000 and 15,000. This field is divided into three regions: 1) the shear layer instability zone; 2) the vortex interaction region; and 3) the zone of higher order instability mode and turbulence generation. It is shown that the origin of the large scale structures just downstream of the potential cone can be traced to the upstream vortices, but their statistical behaviour (passage frequency, spatial coherence) seems to be independent of the initial shear layer instability

Initiation of Turbulent Spots in a Laminar Boundary Layer by Rigid Falling Particulates

A transitional laminar boundary layer is developed on a 1m wide km long flat plate in a 0.6m deep water channel with a freestream velocity of 15-50 cm/s. A particulate dispenser under computer control ejects individual particles having diameters of 1/3 delta into the free stream. The particulates are introduced with an initial velocity of U(sub infinity) in the direction of the free stream. They have differing specific gravities of 1.03-2.7 which introduces an additional non-dimensional parameter relating the time taken to traverse the boundary layer to the convective time scale. The particulates produce a wake in the upper region of the boundary layer as they sink towards the wall. Visualization data taken over the range 5 x 10(exp 4) less than Re(sub x) less than 5 x 10(exp 5) indicate that turbulent spots are produced by the disturbances due to the wake rather than by the particulates themselves. This suggests that the spot formation process in this case may be inviscid in nature and may not be strongly influenced by the presence of the wall

DOE Project on Heavy Vehicle Aerodynamic Drag

Class 8 tractor-trailers consume 11-12% of the total US petroleum use. At highway speeds, 65% of the energy expenditure for a Class 8 truck is in overcoming aerodynamic drag. The project objective is to improve fuel economy of Class 8 tractor-trailers by providing guidance on methods of reducing drag by at least 25%. A 25% reduction in drag would present a 12% improvement in fuel economy at highway speeds, equivalent to about 130 midsize tanker ships per year. Specific goals include: (1) Provide guidance to industry in the reduction of aerodynamic drag of heavy truck vehicles; (2) Develop innovative drag reducing concepts that are operationally and economically sound; and (3) Establish a database of experimental, computational, and conceptual design information, and demonstrate the potential of new drag-reduction devices. The studies described herein provide a demonstration of the applicability of the experience developed in the analysis of the standard configuration of the Generic Conventional Model. The modeling practices and procedures developed in prior efforts have been applied directly to the assessment of new configurations including a variety of geometric modifications and add-on devices. Application to the low-drag 'GTS' configuration of the GCM has confirmed that the error in predicted drag coefficients increases as the relative contribution of the base drag resulting from the vehicle wake to the total drag increases and it is recommended that more advanced turbulence modeling strategies be applied under those circumstances. Application to a commercially-developed boat tail device has confirmed that this restriction does not apply to geometries where the relative contribution of the base drag to the total drag is reduced by modifying the geometry in that region. Application to a modified GCM geometry with an open grille and radiator has confirmed that the underbody flow, while important for underhood cooling, has little impact on the drag coefficient of the vehicle. Furthermore, the evaluation of the impact of small changes in radiator or grille dimensions has revealed that the total drag is not particularly sensitive to those changes. This observation leads to two significant conclusions. First, a small increase in radiator size to accommodate heat rejection needs related to new emissions restrictions may be tolerated without significant increases in drag losses. Second, efforts to reduce drag on the tractor requires that the design of the entire tractor be treated in an integrated fashion. Simply reducing the size of the grille will not provide the desired result, but the additional contouring of the vehicle as a whole which may be enabled by the smaller radiator could have a more significant effect

Closed-loop separation control over a sharp edge ramp using Genetic Programming

We experimentally perform open and closed-loop control of a separating turbulent boundary layer downstream from a sharp edge ramp. The turbulent boundary layer just above the separation point has a Reynolds number $Re_{\theta}\approx 3\,500$ based on momentum thickness. The goal of the control is to mitigate separation and early re-attachment. The forcing employs a spanwise array of active vortex generators. The flow state is monitored with skin-friction sensors downstream of the actuators. The feedback control law is obtained using model-free genetic programming control (GPC) (Gautier et al. 2015). The resulting flow is assessed using the momentum coefficient, pressure distribution and skin friction over the ramp and stereo PIV. The PIV yields vector field statistics, e.g. shear layer growth, the backflow area and vortex region. GPC is benchmarked against the best periodic forcing. While open-loop control achieves separation reduction by locking-on the shedding mode, GPC gives rise to similar benefits by accelerating the shear layer growth. Moreover, GPC uses less actuation energy.Comment: 24 pages, 24 figures, submitted to Experiments in Fluid

October 1998 working group meeting on heavy vehicle aerodynamic drag: presentations and summary of comments and conclusions

A Working Group 1Meeting on Heavy Vehicle Aerodynamic Drag was held at NASA Ames Research Center, Moffett Field, California on October 22, 1998. The purpose of the meeting was to present an overview of the computational and experimental approach for modeling the integrated tractor-trailer benchmark geometry called the Sandia IModel and to review NASA� s test plan for their experiments in the 7 ft x 10 ft wind tunnel. The present and projected funding situation was also discussed. Presentations were given by representatives from the Department of Energy (DOE) Office of Transportation Technology Office of Heavy Vehicle Technology (OHVT). Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories (SNL), and NASA Ames Research Center. This report contains the technical presentations (viewgraphs) delivered at the Meeting, briefly summarizes the comments and conclusions. and outlines the future action items

DOE Project on Heavy Vehicle Aerodynamic Drag FY 2005 Annual Report

Class 8 tractor-trailers consume 11-12% of the total US petroleum use. At high way speeds, 65% of the energy expenditure for a Class 8 truck is in overcoming aerodynamic drag. The project objective is to improve fuel economy of Class 8 tractor-trailers by providing guidance on methods of reducing drag by at least 25%. A 25% reduction in drag would present a 12% improvement in fuel economy at highway speeds, equivalent to about 130 midsize tanker ships per year. Specific goals include: (1) Provide guidance to industry in the reduction of aerodynamic drag of heavy truck vehicles; and (2) Establish a database of experimental, computational, and conceptual design information, and demonstrate the potential of new drag-reduction devices