the application of an atmospheric boundary … · the application of an atmospheric boundary layer...

THE APPLICATION OF AN ATMOSPHERIC BOUNDARY LAYER TO EVALUATE TRUCK AERODYNAMICS IN CFD

“A SOLUTION FOR A REAL-WORLD ENGINEERING PROBLEM”

Ir. Niek van Dijk

DAF Trucks N.V.

CONTENTS

• Scope & Background

• Theory: the atmospheric boundary layer

• Cause: the earth’s roughness

• Turbulence in the atmospheric boundary layer

• Modelling of an atmospheric boundary layer in STAR-CCM+

• Problem analysis

• Numerical effects on the atmospheric boundary layer

• Improving numerical settings

• Conclusions & Recommendations

INTRODUCTION

Scope

• Results shown are part of a graduation project performed at DAF Trucks NV

• Project focus: Effects of the atmospheric boundary layer on truck aerodynamics

• Presentation focus: Highlights of the numerical aspects

Background

• Significant drag differences are observed between on-road testing methods and

computational fluid dynamics (CFD) simulations

• The atmospheric boundary layer is not present in CFD simulations

Vtruck

3

Vwind

THE ATMOSPHERIC BOUNDARY LAYER

What is the atmospheric boundary layer (ABL)?

• The atmospheric boundary layer describes the

wind profile over the earth's surface as varying

over height due to a certain roughness

Aerodynamic roughness

4

• Cause for an ABL is aerodynamic roughness (𝑧0)

• Defined by terrain type, rougher terrain results

in a higher aerodynamic roughness

𝑉𝑤,𝑙𝑜𝑔 𝑧 = 𝑉𝑟𝑒𝑓𝑙𝑜𝑔 ⋅ln𝑧 + 𝑧0𝑧0

ln𝑧𝑟𝑒𝑓𝑙𝑜𝑔 + 𝑧0

𝑧0


• Velocity variation can be approximated with a logarithmic profile

• Variation depends on aerodynamic roughness 𝑉𝑤𝑖𝑛𝑑 = 𝑉𝑙𝑜𝑔(𝑧, 𝑧0)

5

• “Low roughness” profile

(simulating highway

conditions) used in

numerical investigations

3 𝑚/𝑠 at 𝑧 = 2

Aerodynamic roughness


• Turbulent intensities and length scales in the ABL are typically higher than in wind

tunnels and the traditional CFD approach

Turbulence in the atmospheric boundary layer

6

• High turbulence levels used in simulations

• TI= 5% and TL = 5 m

Turbulent intensity (TI)

Tu

rbu

len

t le

ng

th (

TL

)

MODELLING OF AN ABL IN STAR-CCM+

Inlet boundary conditions:

• 3 𝑚/𝑠 at 𝑧 = 2 𝑚

• Wind angle 𝜙 = 0°

• TI = 5% and TL = 5 m

(constant over height)

• The truck will not experience the profile specified at the domain boundaries without

surface roughness and mesh modifications due to unwanted development of the

profile

• In the first part this unwanted development is analysed for an empty domain

Initial analysis

7


Velocity inlet

Pressure outletSymmetry planes

No-slip floor with 𝑽 = 𝟐𝟓𝒎

𝒔

without surface roughness

Initial analysis: empty domain

8

Red plane used to monitor the ABL development

𝟏𝟔𝟎𝐦

𝟑𝟎𝐦

𝟕𝟓𝐦


Solver settings

• All simulations are performed with a steady state RANS modelling approach in

combination with the K-Omega SST turbulence model

Initial analysis

9

Velocity magnitude profile is not constant through the domain

Front of truck

Truck height


The velocity profile development with “standard” CFD settings at position of the truck

Initial analysis

10

1

2

1. Due to the symmetry plane as a top boundary, a zero normal velocity is “forced”

2. Aerodynamic roughness is not included


Friction velocity shows a rapid change in the first few meters of the domain

Initial analysis

11

𝚫𝟏𝟔𝟎𝐦

𝚫𝟎. 𝟏𝟓 𝐦

𝚫𝟑𝐦

Floor (top view)

Near-ground velocity profile: changes rapidly in 3 m from inlet


The observed unwanted development is caused by absence of aerodynamic roughness

Goal: to make flow profile at position of the truck equal to the input

1. To avoid change of the ABL profile in the top part of the domain

Change top boundary to a velocity inlet with only a parallel velocity component

Numerical effects on the ABL: top part (1)

12


Preventing development in the lower part of the profile

Literature:

1. Match the aerodynamic roughness with the surface roughness of the no-slip floor

Surface roughness: 𝑘𝑠 = 30𝑧0 → first cell layer > 2𝑘𝑠2. Y+ values > 30, to ensure wall functions are applied (which can be modified)

Literature & STAR-CCM+ user guide:

3. Specific combinations of numerical settings in order to prevent development of the

profile

Result: an error of more than 10% close to the ground w.r.t. to the input

− Unknown effects on the flow characteristics

− Wind profile develops into a constant (over height) wind profile

Numerical effects on the ABL: lower part (2)

13


• Alternative approach: modifying the floor boundary condition upstream of the truck

• Two alternatives: “slip floor” or a “velocity plane”

• This prevents adaption of the ABL profile to the near wall region and the creation

of a boundary layer on micro scale, which cannot be neglected downstream of the

truck

1. Slip floor

2. Velocity plane

No-slip floor

Truck position

Numerical effects on the ABL: alternative approach

14


1. Slip floor

• The air profile close to the ground can be influenced since the flow is forced to have

a zero normal velocity

• This allows control via mesh settings and the use of the “undershoot”

2. Velocity plane (velocity inlet with only a parallel velocity component)

• Both velocity and turbulence quantities can be specified

• Less sensitive to mesh settings

ABL input

Slip floor

Numerical effects on the ABL: alternative approach

15

≪ 𝟏𝐦


• Both the slip and velocity plane

floor give very good fits of the

ABL profile

• Errors within 0.5% for both

methods

Numerical effects on the ABL: results

16


Differences in mesh of both alternatives

“Volumetric cells”: 0.5 𝑚

1. Slip floor

• First layer = 1 ⋅ 10−5 𝑚

• 25 prism layers

Larger mesh required for the “slip floor”

2. Velocity plane

• First layer = 1 ⋅ 10−2 𝑚

• 15 prism layers

Numerical effects on the ABL: mesh details

17


Effects of turbulent intensity and turbulent length scale profiles

Numerical effects on the ABL: turbulence

18

Turbulent intensity (TI)

Turbulent length (TL)

Initial conditions: profiles through the domain with constant TI and TL specified at the inlet

• Various tests performed to match the turbulent intensity

and turbulent length scale profiles to the profiles found

in literature

• Both a “realistic” profile for the TI and TL increase the development of the ABL profile

(compared to the input) at the position of the truck

• The Realizable K-Epsilon model did increase the development even more

• A decrease in development is obtained with modifying turbulence model parameters

• Undesired: since the effects on truck aerodynamics are unknown

• With initial settings: turbulence quantities at truck position are still within the range

found in literature without turbulence profiles

• Conclusion: TI and TL are kept constant over height (5% and 5m)

MODELLING OF AN ABL IN STAR-CCM+Numerical effects on the ABL: turbulence

19


• Optimal numerical settings: achieved with the “slip floor”

• Based on wind angle variation

• “Velocity plane” seemed to be very sensitive to mesh refinement while the “slip floor”

achieved the same accuracy without changing the mesh

Slip floor

No-slip floor

Vtruck

ABL

Yaw angle

Vair

Optimal numerical settings

20

CONCLUSIONS

• Standard CFD settings result in an unwanted development of the ABL in an empty

domain

• As a result, the standard CFD settings are not useful to evaluate the effects of the

ABL on truck aerodynamics

• Appropriate CFD settings have been found to keep the development of the ABL to

a minimum

• Proposals shown in literature proved to be insufficient

• Defining a slip floor in front of the truck in combination with a very fine mesh showed

best results

• Numerical settings shown only apply to a specific ABL

• A different aerodynamic roughness requires, most likely, other mesh settings

21

RECOMMENDATIONS

• The ABL approach might be useful as a alternative of the traditional CFD approach

to match real world conditions

• Significant differences in the flow field analyses were found for all yaw angles

• The definition of the drag coefficient is ambiguous because the air velocity is not

constant

• An averaging method can be applied to calculate a single unique reference velocity

and yaw angle

• When specifying the ABL profile via a table at the boundaries, a perfect match should be

ensured with the cell centroids of the mesh to avoid unnecessary interpolation

22

Ir. Niek van Dijk

[email protected]

SUPPLEMENTARY SLIDES

CALCULATION OF THE DRAG COEFFICIENT

• Drag force in driving direction

𝐹𝑥 =1

2𝜌𝑉𝑟𝑒𝑓

2 𝐴𝑟𝑒𝑓𝐶𝑥 𝐶𝑥 = 𝑓(𝛽)

• How to formulate the reference velocity to calculate the drag coefficient for the

atmospheric boundary layer

• Both air velocity and yaw angle vary over height

CALCULATION OF THE DRAG COEFFICIENT

• Reference velocity for the atmospheric

boundary layer

• Average air velocity over truck height

• This averaging method is one of

many methods

• Most accurate method

• Corresponding yaw angle

• Average yaw angle over truck height

the application of an atmospheric boundary … · the application of an atmospheric boundary layer...

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