1. Status Report of New Issues TF –CFD* sub wg
WLTP-25-14e
app1
Status Report of New Issues TF –CFD* sub wg
*CFD: Computational Fluid Dynamics method, method to simulate aero drag
12th April, 2019
WLTP CFD sub wg

2. WLTP CFD software validation for simulation of aero drag
Motivation
Current requirement for flow-field validation in Annex 7, § (a) to GTR 15 doesn’t specify the measurement method and criteria
We propose to add ΔcD·A correlation approach (in analogy to wind tunnel method validation in Annex 4, § 6.2)
We also propose to replace the requirement for flow-field validation with the proposed ΔcD·A correlation approach
Premise
CFD simulation in WLTP is intended for calculation of optional equipment (not absolute values, only ΔcD·A).
General idea of the ΔcD·A correlation approach
Compare the results (ΔcD·A) from the WT and the simulation software for one or more vehicle types/body-styles (depending on model potrfolio and intended simulation usage) and a specific number of optional equipment parts (e.g. several types of wheels).
Large enough set of compared variants can assure the validity of the CFD method and indirectly prove the validity of the flow-field.
Car selection
Use one or more representative types/body-styles (e.g. sedan, hatchback, SUV), approve the selection by technical authority (TA) or Technical Service (TS).

3. WLTP CFD software validation for simulation of aero drag
Equipment selection　(See following pictures for details)
Select representative groups of equipment (eg. covers, wheels, spoilers) intended for CFD simulation, approve the selection by TA/TS
Calculate the effect (ΔcD·A) of equipement from each group for the selected representative types/body-styles (where applicable)
Number of variants　(See following pictures for details)
Option 1: Minimum total number of variants
Option 2: Minimum number of variants per group
Option 3: Option 1 + 2
Accuracy
Error ε = Δ(ΔcD·A) is difference of ΔcD·A obtained by simulation and by WT measurement
[xx% of the test-cases must have error ε ≤ 0,015m2] -> discussion about what to do in case of a failed certification attempt

4. WLTP CFD software validation for simulation of aero drag
Validity of simulation process
Recertification of a previously approved CFD simulation process is required in case of … e.g. Any change made to the mathematical model or to the software likely to invalidate the validation report shall be brought to the attention of the approval authority which may require that a new validation process is conducted. (From Page 10 of WLTP-26-11e_app3)
Road Load Families (RLF)
Clarify the wording of Annex 7, § (c)
1. Number of certifications:
Certification doesn’t have to be performed for each RLF
The same certification acquired by the described process then needs to be submited for each RLF
2. Why is the condition different for CFD simulation and measurement method?

5. GTR 15 Annex 7, §
Alternative method for determination of aerodynamic influence of optional equipment At the request of the manufacturer and with approval of the responsible authority, an alternative method (e.g. simulation, wind tunnel not fulfilling the criteria in Annex 4) may be used to determine Δ(CD×Af) if the following criteria are fulfilled: (a) The alternative method shall fulfil an accuracy for Δ(CD×Af) of ±0.015 m² and, additionally, in the case that simulation is used, the CFD method should be validated in detail such that the actual air flow patterns around the body, including magnitudes of flow velocities, forces, or pressures, are shown to match the validation test results; (b) The alternative method shall be used only for those aerodynamic-influencing parts (e.g. wheels, body shapes, cooling system) for which equivalency was demonstrated; (c) Evidence of equivalency shall be shown in advance to the responsible authority for each road load family in the case that a mathematical method is used, or every four years in the case that a measurement method is used, and in any case shall be based on wind tunnel measurements fulfilling the criteria of this UN GTR; (d) If the Δ(CD × Af) of a particular item of optional equipment is more than double the value of the optional equipment for which the evidence was given, aerodynamic drag shall not be determined by the alternative method; and (e) In the case that a simulation model is changed, a revalidation shall be necessary.