Links between Notchback Geometry, Aerodynamic Drag, Flow Asymmetry and Unsteady Wake Structure

Abstract

. (2011) 'Links between notchback geometry, aerodynamic drag, ow asymmetry and unsteady wake structure.', SAE International journal of passenger cars. Mechanical systems., 4 (1). pp. 156-165. Further information on publisher's website:http://dx.doi.org/10.4271/2011-01-0166Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTThe rear end geometry of road vehicles has a significant impact on aerodynamic drag and hence on energy consumption. Notchback (sedan) geometries can produce a particularly complex flow structure which can include substantial flow asymmetry. However, the interrelation between rear end geometry, flow asymmetry and aerodynamic drag has lacked previous published systematic investigation.This work examines notchback flows using a family of 16 parametric idealized models. A range of techniques are employed including surface flow visualization, force measurement, multi-hole probe measurements in the wake, PIV over the backlight and trunk deck and CFD.It is shown that, for the range of notchback geometries investigated here, a simple offset applied to the effective backlight angle can collapse the drag coefficient onto the drag vs backlight angle curve of fastback geometries. This is because even small notch depth angles are important for a sharp-edged body but substantially increasing the notch depth had little further impact on drag.This work shows that asymmetry originates in the region on the backlight and trunk deck and occurs progressively with increasing notch depth, provided that the flow reattaches on the trunk deck and that the effective backlight angle is several degrees below its crucial value for non-reattachment. A tentative mapping of the flow structures to be expected for different geometries is presented.CFD made it possible to identify a link between flow asymmetry and unsteadiness. Unsteadiness levels and principal frequencies in the wake were found to be similar to those for high-drag fastback geometries. The shedding of unsteady transverse vortices from the backlight recirculation region has been observed.

Influence of Short Rear End Tapers on the Unsteady Base Pressure of a Simplified Ground Vehicle

“…The long time bi-stable nature of the wake could also explain the asymmetry that characterizes the time averaged flow field past a simplified notchback geometry (for values of the effective backlight angle several degrees below the non-reattachment condition) investigated both experimentally and numerically by Sims-Williams et al [18] and Krajnović and Minelli [19] and could be responsible for the fluctuations in the aerodynamic load under steady-state conditions described by Kawakami et al [20] for a stock hatchback car model.…”

Investigations of the Rear-End Flow Structures on a Sedan Car

“…Looking into the literature shows that many notchback like geometry studies were done on simplified shapes and generic bodies [2,3,5,9,10,11,12]. These investigations show that the flow field is different, as for instance vortices are different in size and position, but on the other hand main features like saddle points and foci are present in a similar way for the different geometries.…”

“…Another interesting observation is the occurrence of symmetric and asymmetric flow pattern for different notchback vehicle shapes (for instance [8,9,14]). Sims-Williams et al [9] investigated the flow pattern for different generic notchback geometries and the connection between the backlight angle and the effective backlight angle.…”

“…The investigation of geometric parameters like the backlight angle, effective backlight angle, trunk deck length and the relation to each other give indications for an improved vehicle shape [9,10,11,15,16].…”

“…Sims-Williams et al [9] investigated the flow pattern for different generic notchback geometries and the connection between the backlight angle and the effective backlight angle. They found that the flow can be divided into three different groups: i) fully separated, ii) reattaching symmetric and iii) reattaching asymmetric.…”