Abstract
A hybrid momentum theory of flat-plate boundary layer was proposed, to elucidate drag reduction on a van body surface under air film. The air mass and momentum before and after the outside air mixed with film air were compared, which may lead to the change of viscous force. A mathematical model for flat-plate boundary layer under air film was established based on the law of mass and momentum conservation, with selecting a unit of laminar boundary layer as analysis object and simultaneous solution of the momentum integral equation and the velocity distribution equation. The mass of air flow, momentum flux, loss of momentum, boundary layer thickness and surface viscous force were discussed to take a comparative analysis of flat boundary layer’s status change with and without an air film.
Highlights
In those schemes that reduce air resistance through air film, film air can be imported to the outside air in a variety of ways and different states
This paper provide a theoretically attempt to explore the drag reduction mechanism of flat surface under air film, present a hybrid momentum theory and establish a laminar flow friction calculation model of flat-plate surface under air film with theoretical assumptions
The air viscous force mainly concentrated in van body walls with large proportion of area, to simplify the calculations, an air viscosity problem of van truck can be simplified as the problem of van body walls
Summary
In those schemes that reduce air resistance through air film, film air can be imported to the outside air in a variety of ways and different states. The parameters of film, such as pressure, velocity, angle of incidence, etc. Gas in air sources was pumped outside into gas storage, ejected trough air vents in van body walls with a certain angle under the action of the air pressure, and in contact and mixed with the outside high-speed airflow on the car body surface. The van can be defined as under air film. Not all the air film has the effectiveness of drag reduction, it depends on the relationship between the external flow field characteristics of object itself and function parameters such as the viscosity of the air film, flow rate, pressure, incidence angle, and distribution scheme of air vents
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