THE APPLICATION OF AN ATMOSPHERIC BOUNDARY … · the application of an atmospheric boundary layer...
Transcript of 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
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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
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• 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
THE ATMOSPHERIC BOUNDARY LAYER
• Velocity variation can be approximated with a logarithmic profile
• Variation depends on aerodynamic roughness 𝑉𝑤𝑖𝑛𝑑 = 𝑉𝑙𝑜𝑔(𝑧, 𝑧0)
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• “Low roughness” profile
(simulating highway
conditions) used in
numerical investigations
3 𝑚/𝑠 at 𝑧 = 2
Aerodynamic roughness
THE ATMOSPHERIC BOUNDARY LAYER
• 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
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• 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
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MODELLING OF AN ABL IN STAR-CCM+
Velocity inlet
Pressure outletSymmetry planes
No-slip floor with 𝑽 = 𝟐𝟓𝒎
𝒔
without surface roughness
Initial analysis: empty domain
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Red plane used to monitor the ABL development
𝟏𝟔𝟎𝐦
𝟑𝟎𝐦
𝟕𝟓𝐦
MODELLING OF AN ABL IN STAR-CCM+
Solver settings
• All simulations are performed with a steady state RANS modelling approach in
combination with the K-Omega SST turbulence model
Initial analysis
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Velocity magnitude profile is not constant through the domain
Front of truck
Truck height
MODELLING OF AN ABL IN STAR-CCM+
The velocity profile development with “standard” CFD settings at position of the truck
Initial analysis
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1
2
1. Due to the symmetry plane as a top boundary, a zero normal velocity is “forced”
2. Aerodynamic roughness is not included
MODELLING OF AN ABL IN STAR-CCM+
Friction velocity shows a rapid change in the first few meters of the domain
Initial analysis
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𝚫𝟏𝟔𝟎𝐦
𝚫𝟎. 𝟏𝟓 𝐦
𝚫𝟑𝐦
Floor (top view)
Near-ground velocity profile: changes rapidly in 3 m from inlet
MODELLING OF AN ABL IN STAR-CCM+
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)
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MODELLING OF AN ABL IN STAR-CCM+
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)
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MODELLING OF AN ABL IN STAR-CCM+
• 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
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MODELLING OF AN ABL IN STAR-CCM+
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
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≪ 𝟏𝐦
MODELLING OF AN ABL IN STAR-CCM+
• 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
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MODELLING OF AN ABL IN STAR-CCM+
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
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MODELLING OF AN ABL IN STAR-CCM+
Effects of turbulent intensity and turbulent length scale profiles
Numerical effects on the ABL: turbulence
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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
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MODELLING OF AN ABL IN STAR-CCM+
• 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
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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
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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
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Ir. Niek van Dijk
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