Development process
The wind tunnel method was already described in some existing standards. However, these standards rely on good engineering judgement and can therefore not be applied as a test procedure for the GTR as such. This meant that the text of the existing standards had to be completed in a much higher level of detail, e.g. by setting criteria and specifying requirements. This also included the need for a correlation program between on-road and wind tunnel tests.
During the process, the GTR text was developed within a small sub-group and in a very constructive manner to create a robust test procedure with guidance how to apply the method and to perform the testing.
The need for the inclusion of the wind tunnel method was acknowledged during phase 1a, but in the absence of sufficient validation data it was decided to postpone the adoption until phase 1b. This gave an opportunity to set up a validation test program, and use the results to develop the text for the GTR.
Within the taskforce on the wind tunnel method doubts were raised towards the validity of wind tunnel results, especially from Japanese side. As Japan has no concept of round robin comparison of wind tunnel results like in Europe, the concerns were well understood. Therefore the following precautions have been taken:
wind tunnel criteria have been scrutinised and tightened where possible,
the approval of the facilities via a correlation with on road testing was added, and
two validation studies were executed (by UTAC and by VW).
Having delivered the required validation data and a robust description of the method, IG agreed on adopting the wind tunnel method with the flat belt at the 10th IG meeting. Additional testing was needed for the correction function on the chassis dynamometer, so that part was adopted later at the 12th IG meeting.
The wind tunnel method is included in paragraph 6 of Annex 4.
Windtunnel criteria
It should be mentioned here that a windtunnel can be used for two purposes in the GTR:
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to determine the ‘delta Cd.A’ between options to the vehicle exterior and/or bodyshapes for the purpose of interpolation between vehicle L and H, and
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to determine the overall Cd.A of the whole vehicle to derive the target road load coefficients, i.e. the windtunnel method described in this paragraph.
The basic windtunnel criteria are laid down in paragraph 3.2 of Annex 4, but due to the differences between these purposes the criteria for the wind tunnel method are more stringent (see paragraph 6.4.1).
The reasons for these different criteria are as follows:
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The difference between the delta Cd.A of vehicle L and H is much smaller than the overall Cd.A of the whole vehicle. Therefore the absolute effect of an error in the determination of the delta Cd.A has less consequences.
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The sum of the delta Cd.A for the set of options on vehicle H is aligned by the Cd.A difference between vehicle L and H. This means that any error in the measurement is largely compensated.
For these reasons a bigger solid blockage ratio can be accepted for the wind tunnel used for the delta Cd.A determination, and a higher deviation is allowed between front and rear pressure coefficient. Also the blockage due to the vehicle restraint system has no influence, because its influence levels out during the delta Cd.A determination.
4.4.12Alternative delta Cd.A determination
For the interpolation method on CO2 as described in paragraph 4.4.1 there is a need to determine the variation in the value of C d .A for each vehicle option that has an influence on the aerodynamic performance of the vehicle. In the GTR this is referred to as the delta C d .A determination, which is an input for the calculation of the cycle energy for an individual vehicle. Examples of vehicle options which’ aerodynamic resistance would have to be determined are wheel rims and tires, spoilers, adjustable vehicle height system, grille shutters, etc.
It was acknowledged by the Annex 4 taskforce that:
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Variations in the delta Cd.A or vehicle options are in the same order of magnitude as the measurement tolerance. This makes it virtually impossible to determine an accurate value for the delta Cd .A by performing e.g. a coast-down with and without the option installed on the vehicle. Only the wind tunnel method may be sufficiently accurate to measure this due to the absence of uncontrollable influences.
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The determination of the delta Cd.A for all the options in a vehicle family may take a lot of effort in the windtunnel, and is therefore time consuming and costly. At the same time, not all manufacturers may have unlimited access to a windtunnel.
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There are simulation methods available which are able to accurately determine the influence on aerodynamic performance for different body styles and options installed at the vehicle exterior.
For this reason an alternative method was proposed which –under strict requirements- would allow the calculation of the delta Cd.A by e.g. computer simulations based on the method of Computational Fluid Dynamics (CFD). The basic principle for this alternative method is that it should always be validated by demonstrating equivalency with measured aerodynamic results. Therefore, the following requirements and restrictions were set to this method:
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The method may only be used after agreement by the responsible authority, and after fulfilling the other requirements and restrictions.
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It has to be demonstrated that the method has an accuracy of ±0.015 m2 delta Cd.A.
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The method has to be validated, not only by demonstrating the accuracy requirement, but also to yield similar flow patterns, air velocities, pressures and forces.
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It can only be used for those kind of aerodynamic influencing parts (e.g. wheels, body shapes, cooling system) for which equivalency was demonstrated.
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The evidence of equivalency is presented in advance to the responsible authority, for each road load family (if a simulation method is used) or by a correlation test programme (if a measurement method is used).
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Only the wind tunnel method is allowed to be used for the equivalency demonstration.
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The method may not be applied for vehicle options with a delta Cd.A that is more than 100% higher than the option for which equivalency was demonstrated.
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Whenever the simulation model is changed or updated, the validation needs to be re-demonstrated.
Note that the alternative delta Cd.A method may only be used to determine the difference in aerodynamic drag, it is not allowed to evaluate the absolute aerodynamic resistance of the whole vehicle. For the measurement of the overall aerodynamic resistance e.g. the wind tunnel method of paragraph 4.4.11 should be applied.
The alternative delta Cd.A method is described in paragraph 3.2.3.2.2.3. of Annex 7.
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