Momentum - April 2019 - 6

STUDENT
GENERATION

TURBULENCE ON THE TRACK
Upper body delta CP for turbulence intensity sweep.
IN MOTORSPORTS, AERODYNAMIC PERFORMANCE has
become extremely important, and this is reflected in the
amount of resources that are dedicated to wind-tunnel
testing and CFD simulations. In order to make the most
of these simulations, realistic racing conditions should be
matched as closely as possible. Unfortunately, conditions
that are constantly changing make it nearly impossible to
detect any trends in test data.

Under body delta CP for turbulence length scale sweep.

6 April 2019

Because of this, most of the ground vehicle wind tunnels take
extraordinary care to minimize the effects of freestream turbulence
during their tests, and CFD simulations have historically reflected
this. Despite this, many studies have been done to see the significant
effect that turbulent characteristics of the freestream flow have on
CFD simulation results. However, no previous studies have been done
on the effect of these characteristics on a CFD analysis of a realistic
racecar model.
Against this backdrop, the purpose of a study I participated in was to
systematically investigate the effect of different freestream turbulence
specifications on the CFD predictions of the aerodynamic characteristics
of a latest generation NASCAR Cup racecar model.
In the study, finite volume CFD simulations are carried out with: (a)
five turbulence intensities of 0.1%, 0.5%, 1%, 5%, and 10% with a fixed
turbulence length scale of 1 mm to examine the effect of the inlet
turbulence intensity variability, and (b) five turbulence length scales,
ranging from 0.1 mm to 10 mm, with a fixed turbulence intensity of 0.5%
to evaluate the effect of turbulence length scales.
To investigate the impact that these inlet turbulence specifications
have on the CFD predictions of the aerodynamic characteristics of a
full-scale NASCAR Gen-6 racecar, steady-state RANS simulations were
carried out.
From the results of these simulations, there were a few clear
conclusions that were seen. While both drag and total downforce of
the stock car racecar increase with increasing values of the freestream
turbulence intensity and length scale, skin friction drag is somewhat
insensitive to these specifications. However, this increase in both
the drag and downforce values appears to begin to level out at the
higher values of these two parameters. One phenomenon seen was a
decrease in the vehicle base as the freestream turbulence characteristics
increased, which is in accordance with previous studies done on other
bluff body shapes. A lower vehicle base pressure would ultimately cause
a larger pressure delta between the two ends of the car-effectively
increasing the vehicle drag.
It was also seen that the effects become stronger as one moves
towards the rear of the vehicle. Subsequently, while the prediction
of front downforce is somewhat independent of the freestream
turbulence specifications, the rear downforce predictions exhibit a
strong dependence on the inlet turbulence intensity and length scale
specifications. These turbulence specifications show a gradual decrease

MOMENTUM



Momentum - April 2019

Table of Contents for the Digital Edition of Momentum - April 2019

Momentum - April 2019
Contents
A better MOMENTUM
Briefs
Reliability overhaul: a lesson in resilience
Turbulence on the track
Full circle
Lap simulation tool shows the way
Baja SAE technical inspection
VW’s MEB platform: a modularity enabler
The F-22 Raptor gets its first metallic 3D-printed part
SAE 101: WCX 2019
Moon shot as metaphor for autonomous vehicle technology
DOSSIER: Greg Sawvelle
Momentum - April 2019 - Momentum - April 2019
Momentum - April 2019 - Cover2
Momentum - April 2019 - Contents
Momentum - April 2019 - A better MOMENTUM
Momentum - April 2019 - Briefs
Momentum - April 2019 - Reliability overhaul: a lesson in resilience
Momentum - April 2019 - 5
Momentum - April 2019 - Turbulence on the track
Momentum - April 2019 - 7
Momentum - April 2019 - Full circle
Momentum - April 2019 - 9
Momentum - April 2019 - Lap simulation tool shows the way
Momentum - April 2019 - 11
Momentum - April 2019 - Baja SAE technical inspection
Momentum - April 2019 - VW’s MEB platform: a modularity enabler
Momentum - April 2019 - The F-22 Raptor gets its first metallic 3D-printed part
Momentum - April 2019 - SAE 101: WCX 2019
Momentum - April 2019 - Moon shot as metaphor for autonomous vehicle technology
Momentum - April 2019 - 17
Momentum - April 2019 - DOSSIER: Greg Sawvelle
Momentum - April 2019 - 19
Momentum - April 2019 - 20
Momentum - April 2019 - Cover3
Momentum - April 2019 - Cover4
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