The necessity for boundary corrections in a standard practice for the open-jet wind tunnel testing of automobiles. (August 2017)

• Record Type: Journal Article
• Authors: Cooper, Kevin R; Mercker, Edzard; Müller, Jürg
• Other Names: Passmore Martin guest-editor.; Howell Jeff guest-editor.; Dominy Robert guest-editor.
• Is Part Of: Proceedings of the Institution of Mechanical Engineers. Volume 231:Number 9(2017:Sep.)
• Page Start: 1245
• Page End: 1273
• Publication Date: 2017-08
• Subjects: Surface vehicles -- aerodynamics -- wind tunnels -- open-jet boundary corrections -- recommendations for standard practice; Mechanical engineering -- Congresses; Transportation engineering -- Congresses; 629.2
• DOI: 10.1177/0954407017701287
• Languages: English
• ISSNs: 0954-4070

Abstract

This paper is intended to provide a summary of the necessary adjustments required for road-representative open-jet wind tunnel measurements on automobiles. The open-jet wind tunnel provides accurate measurements, but they are made in a finite-sized jet that differs from the unconfined open-road conditions. Furthermore, measurements on a given automobile made in different open-jet wind tunnels disagree with each other, and with measurements in closed-wall wind tunnels that were corrected for the influences of their solid boundaries. There appears to be reticence at some company levels to making 'corrections' to open-jet measurements. Perhaps non-specialist managers think that the need for a 'correction' means an erroneous measurement. It does not! Any high-quality wind tunnel measurement is accurate, but it needs to be 'calibrated' to on-road conditions through an appropriate set of procedures. Closed-wall wind tunnels measure higher drag coefficients, in comparison with those in an unconstrained on-road flow. Open-jet wind tunnels frequently measure a lower value. The closed-wall adjustments lower the drag coefficient to the unconstrained value. Open-jet adjustments should also adjust the drag coefficient to the same unconstrained value. This paper explores the range of effects from the finite jet and elucidates the effectiveness of a two-measurement correction procedure. It is shown that not every data point must be measured twice, only a small selected subset. Since approximately 20% of tunnel occupancy is in the fan-on condition, then the additional cost of correct accurate on-road-equivalent data is low.