A CFD study of Wind Tunnel Wall Interference

Wael Mokhtar, Grand Valley State UniversityMd Hasan, Grand Valley State University

2016 ASEE NCS Conference | Central Michigan University | March 19, 2016

OUTLINE

Introduction Wind Tunnel Testing

Overview Wall Interference Purpose of the Investigation CFD Study Simulation Results Future Scope Summary & Conclusion

Introduction

Wind tunnel test is an indispensable tool to predict the aerodynamic performance in automotive and aircraft industries.

In recent years, there is an interest in reduction of drag coefficient for commercial high blockage vehicles like buses and semi-trucks.

Flow over high blockage vehicles is complex and difficult to predict with accuracy.

Designers will therefore continue to rely on wind tunnel testing in their efforts to produce low-drag designs.

Efforts like minimizing wind noise, optimizing engine cooling, minimizing wind effects on vehicle handling etc. will also depend on wind tunnel testing.

Figure 1: Typical Wind Tunnel (courtesy: NASA)

Figure 2: 2008 Nissan GTR in Wind Tunnel1

Wind Tunnel Testing: An Overview

Three major classes of facility :

Full-scale: - Test actual vehicles at 100% scale

- Relatively few facilities available

NASA NFAC – Limited ground simulation NRC 9m – Limited yaw capability2

Wind Tunnel Testing: An Overview

Large-scale : – Test 25-50% scale truck models

- Relatively few available - Variable sophistication of ground simulation

Windshear – large RR GMAL - versatile

3

Wind Tunnel Testing: An Overview

Moderate-scale : – Test 10-15% scale models

- Relatively numerous; test sections in 710 ft range - Often feature sophisticated ground simulation - Reynolds numbers are a little low

Important contributions to aerodynamic R&D originate in “moderate scale” facilities – lower cost and good availability

ARC – 1/8th scale

Other examples: University of Maryland University of Washington NIAR (Wichita State) San Diego ASTC Texas A&M many others …

4

Wall Interference

It is a major concern for wind tunnel design, model shape and experimental techniques

Wind Tunnel Walls effect the free flow

conditions and needs to be corrected in measurements

It becomes most serious when the airflow began to choke in the transonic range.

Figure: Constraining effect of streamlines due to wall interference (Courtesy: NASA)

Blockage Ratio (BR) =

Models with higher blockage ratios create more wall interference

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Wall Interference (Contd.)

Efforts for Minimizing Wall Interference Effect:

Slotted Wall Wind Tunnel (NASA Langley 16 ft Transonic)

Adaptive Wall Wind Tunnel (DLR Göttingen-Germany)

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Purpose of the Study

CFD analysis of Wall Interference

effect for high blockage vehicle

Study of Flow structures

due to different blockage

ratios

Calculation of Drag coefficient

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CFD Analysis

CAD Model

Standard full scale model of 53 feet semi-trailer truck

The complex curvatures were avoided in order to simplify the model and computational effort.

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CFD Analysis (Contd.)

Computational Domain

Figure: Computational Domain(a) 1.875 % (b) 15 %

Figure: Blockage Ratios Two different blockage ratios (1.875 % and 15 %) 1.875 % blockage ratio represents relatively practical case where as worst case

scenario is 15 % Length of the computational domain is six times of the same of model. The ratio of width to height of wind tunnel is kept as 1:1. 9

CFD Analysis (Contd.)

Boundary Conditions The computational model was

simulated in 3D case for : Steady and Turbulent flow

For each blockage ratio, model was simulated for city speed (50 mph) and highway speed (70 mph)

Moving ground effect was also considered.

Moving ground

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CFD Analysis (Contd.)

Meshing Surface Remesher, Prism Layer

Mesher and Trimmer were considered as meshing model for creating around 3.5 million volume cells.

Mesh Block

Prism Layers 11Full Domain

Flow Structures BR= 1.875 %

50 mph

70 mph 12

Flow Structures (Contd.) BR= 15 %

50 mph

70 mph 13

Pressure Contours BR= 1.875 %

50 mph 70 mph

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Pressure Contours (Contd.) BR= 15 %

50 mph 70 mph

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Wall Pressure Signatures BR= 1.875 %

50 mph 70 mph

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Wall Pressure Signatures (Contd.) BR= 15 %

50 mph 70 mph

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Drag Co-efficient, Cd

2% 15%0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.6

0.85

0.58

0.84

Drag Co-efficient, Cd

50 mph 70 mph

Blockage Ratio

Drag

Co-

effici

ent,

Cd

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Summary

A brief overview of Wind tunnel testing

Wall Interference effect and Blockage Ratios

CFD study for two different blockage ratios

Comparison of Drag Co-efficient20

Conclusion

High pressure zone developed on the front face of the Truck for increased inlet velocity of the wind tunnel

Flow Vortex regions formed in the gap between truck and container, underbody and behind the truck (wake region)

There is a significant difference found in wall pressure signatures due to blockage effect.

Blockage ratio has drastic impact in the Drag co-efficient

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Future Scopes

CFD analysis for some intermediate blockage ratios

Use of Pressure Signature Method to calculate Wall Interference Correction

Experimental validation of the method

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Thank You !