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

Transcription

1 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.

2 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

3 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 V wind V truck 3

4 4 THE ATMOSPHERIC BOUNDARY LAYER Aerodynamic roughness 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 Cause for an ABL is aerodynamic roughness (z 0 ) Defined by terrain type, rougher terrain results in a higher aerodynamic roughness V w,log z = V reflog ln z + z 0 z 0 ln z reflog + z 0 z 0

5 5 THE ATMOSPHERIC BOUNDARY LAYER Aerodynamic roughness Velocity variation can be approximated with a logarithmic profile Variation depends on aerodynamic roughness V wind = V log (z, z 0 ) 3 m/s at z = 2 Low roughness profile (simulating highway conditions) used in numerical investigations

6 Turbulent length (TL) 6 THE ATMOSPHERIC BOUNDARY LAYER Turbulence in the atmospheric boundary layer Turbulent intensities and length scales in the ABL are typically higher than in wind tunnels and the traditional CFD approach High turbulence levels used in simulations TI= 5% and TL = 5 m Turbulent intensity (TI)

7 7 MODELLING OF AN ABL IN STAR-CCM+ Initial analysis 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 Inlet boundary conditions: 3 m/s at z = 2 m Wind angle φ = 0 TI = 5% and TL = 5 m (constant over height)

8 8 MODELLING OF AN ABL IN STAR-CCM+ Initial analysis: empty domain Symmetry planes Pressure outlet 30 m 160 m Velocity inlet 75 m No-slip floor with V = 25 m s without surface roughness Red plane used to monitor the ABL development

9 9 MODELLING OF AN ABL IN STAR-CCM+ Initial analysis Solver settings All simulations are performed with a steady state RANS modelling approach in combination with the K-Omega SST turbulence model Velocity magnitude profile is not constant through the domain Front of truck Truck height

10 10 MODELLING OF AN ABL IN STAR-CCM+ Initial analysis The velocity profile development with standard CFD settings at position of the truck Due to the symmetry plane as a top boundary, a zero normal velocity is forced 2. Aerodynamic roughness is not included

11 11 MODELLING OF AN ABL IN STAR-CCM+ Initial analysis Friction velocity shows a rapid change in the first few meters of the domain Δ160 m Floor (top view) Near-ground velocity profile: changes rapidly in 3 m from inlet Δ3 m Δ0. 15 m

12 12 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: top part (1) 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

13 13 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: lower part (2) 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: k s = 30z 0 first cell layer > 2k s 2. 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

14 14 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: alternative approach 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 No-slip floor 1. Slip floor 2. Velocity plane Truck position

15 15 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: alternative approach 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 1 m ABL input Slip floor 2. Velocity plane (velocity inlet with only a parallel velocity component) Both velocity and turbulence quantities can be specified Less sensitive to mesh settings

16 16 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: results Both the slip and velocity plane floor give very good fits of the ABL profile Errors within 0.5% for both methods

17 17 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: mesh details Differences in mesh of both alternatives Volumetric cells : 0.5 m 1. Slip floor 2. Velocity plane First layer = m First layer = m 25 prism layers 15 prism layers Larger mesh required for the slip floor

18 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: turbulence Effects of turbulent intensity and turbulent length scale profiles Initial conditions: profiles through the domain with constant TI and TL specified at the inlet Turbulent intensity (TI) Turbulent length (TL) 18

19 19 MODELLING OF AN ABL IN STAR-CCM+ Numerical effects on the ABL: turbulence 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)

20 20 MODELLING OF AN ABL IN STAR-CCM+ Optimal numerical settings 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 V air ABL No-slip floor V truck Slip floor Yaw angle

21 21 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

22 22 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

23 Ir. Niek van Dijk

24 SUPPLEMENTARY SLIDES

25 CALCULATION OF THE DRAG COEFFICIENT Drag force in driving direction F x = 1 ρv 2 ref 2 A ref C x C x = 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

26 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