Cross Winds and Transients: Reality, Simulation and Effects

Abstract

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. ABSTRACTThis paper provides a published counterpart to the address of the same title at the 2010 SAE World Congress.A vehicle on the road encounters an unsteady flow due to turbulence in the natural wind, due to the unsteady wakes of other vehicles and as a result of traversing through the stationary wakes of road side obstacles. This last term is of greatest significance.Various works related to the characterization, simulation and effects of on-road turbulence are compared together on the turbulence spectrum to highlight differences and similarities. The different works involve different geometries and different approaches to simulating cross wind transients but together these works provide guidance on the most important aspects of the unsteadiness.On-road transients include a range of length scales spanning several orders of magnitude but the most important scales are in the in the 2-20 vehicle length range. There are significant levels of unsteadiness experienced on-road in this region and the corresponding frequencies are high enough that a dynamic test is required to correctly determine the vehicle response. Fluctuations at these scales generate significant unsteady loads (aerodynamic admittance typically 0.6-1.4) and the corresponding frequencies can adversely affect vehicle dynamics.The generation of scales larger than the scale of the vehicle is impractical with passive grids and so active turbulence generation systems are preferred. These can be classified into lift and drag-based devices. Lift-based devices provide better control of the turbulence but can only just reproduce the smaller scales in the 2-20 vehicle length range. Different moving model approaches are also discussed. CFD offers real advantages through its ability to allow arbitrary time-varying boundary conditions.

Unsteady Aerodynamics of an Oscillating Fastback Model

“…The support beam was bolted under the wind tunnel floor and the model was supported with two bearings on the shaft. The driven crank produced a consistent and repeatable 1Hz oscillation with yaw angle amplitude of ±11º, which, when combined with a test speed of 40m/s, produces a reduced frequency, as defined by Sims-Williams [16] in equation 1, of 0.098, meaning the motion falls within the quasistatic approximation region of reduced frequency < 0.1.…”

Assessing the Aeroacoustic Response of a Vehicle to Transient Flow Conditions from the Perspective of a Vehicle Occupant

“…In the intermediate frequency range, scales of unsteadiness may exist that are sufficiently large and of sufficient energy to influence a vehicle, but not so large that they can be considered to be quasi-steady. This is discussed further in [7]. These effects could lead to transfer function values of greater than unity and these are sometimes associated with resonances of the vehicle suspension system in the case of vehicle forces.…”

“…These various sources of oncoming flow unsteadiness and their effects have been investigated by various researchers including [1,2,3,4,5,6] and are summarised by [7]. Previous work on unsteady on-road effects on aeroacoustics has been published by [8,9,10,11,12,13].…”

The Bandwidth of Transient Yaw Effects on Vehicle Aerodynamics

“…It is expected, as proposed by He [12] and Sims-Williams [3], that the effect of inlet turbulence on vehicle drag and stability will be curtailed by the quasi-steady limit, which this paper will define for a variety of inlet and geometry configurations. The use of idealized models, such as by Ryan [9] and Docton [8], allows a wide range of frequencies to be analysed with regard to the effect of delayed or encouraged separation and other consequential flow features that are apparent with simple geometries.…”

“…: as reviewed by [1], [2], [3]). The simulation of this time-varying airflow is now becoming possible in wind tunnels and in CFD (e.g.…”