Apparent Wind Modeling on a Road Vehicle

Matthew Anderson · July 6, 2023, 21:36

Suppose we have a road vehicle traveling forward with some headwind $-v$ under some wind $w$. The traditional apparent wind modeling approach says to add the headwind and wind vectors, and for a given $(v,w)$, we would give CFD an apparent wind speed and yaw angle. My concern is that this does not accurately account for the fact (?) that headwind and wind are not the same: headwind does not follow the same logarithmic-gradient that true wind does. Is this concern valid, and if so, what can be done to mitigate it? (If it is relevant, I typically use Star CCM and Ansys Fluent).

sbaffini · July 7, 2023, 15:59

I don't think I completely understood, but the matter is all about reference frames.

Start from your physical unsteady case (car moving + wind). A typically complex scenario that requires a moving mesh around the moving car. Then you recognize that there is a reference frame where you can keep the car fixed and just use that.

There is no other frame where the car is steady.

JBeilke · July 7, 2023, 17:55

There was a presentation about truck aerodynamics at a StarCD/CCM+ users meeting many years ago, which probably covered this topic. At least for trucks the atmoshperic boundary layer somehow influences the results.

Maybe someone on the CCM+ forum can help you to find the handouts.
https://community.sw.siemens.com/s/t...center-starccm

July 6, 2023, 21:36	Apparent Wind Modeling on a Road Vehicle	#1
Matthew Anderson New Member Matthew Anderson Join Date: Jul 2023 Posts: 1 Rep Power: 0	Suppose we have a road vehicle traveling forward with some headwind $-v$ under some wind $w$. The traditional apparent wind modeling approach says to add the headwind and wind vectors, and for a given $(v,w)$, we would give CFD an apparent wind speed and yaw angle. My concern is that this does not accurately account for the fact (?) that headwind and wind are not the same: headwind does not follow the same logarithmic-gradient that true wind does. Is this concern valid, and if so, what can be done to mitigate it? (If it is relevant, I typically use Star CCM and Ansys Fluent).

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July 7, 2023, 15:59		#2
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,190 Blog Entries: 29 Rep Power: 39	I don't think I completely understood, but the matter is all about reference frames. Start from your physical unsteady case (car moving + wind). A typically complex scenario that requires a moving mesh around the moving car. Then you recognize that there is a reference frame where you can keep the car fixed and just use that. There is no other frame where the car is steady.

July 7, 2023, 17:55		#3
JBeilke Senior Member Joern Beilke Join Date: Mar 2009 Location: Dresden Posts: 516 Rep Power: 20	There was a presentation about truck aerodynamics at a StarCD/CCM+ users meeting many years ago, which probably covered this topic. At least for trucks the atmoshperic boundary layer somehow influences the results. Maybe someone on the CCM+ forum can help you to find the handouts. https://community.sw.siemens.com/s/t...center-starccm