Boundary Layer Wind Tunnel Experiments - An Introduction

Introduction

Written by Adam Kahraman

Ever since the invention of the wind tunnel, engineers have found countless practical uses for it. What began as a tool for aerodynamic research eventually became an important part of aircraft development, before finding applications in civil engineering. This includes studying wind loads, structural movements, environmental effects, and testing conditions relevant to building code requirements.

This article will provide a background on boundary layer wind tunnel (BLWT) studies, followed by some of the challenges that engineers have faced when simulating natural wind. The article then looks at the technologies and techniques available today that make atmospheric boundary layer studies possible.

Table of Contents

1. Introduction
2. Boundary Layer Wind Tunnel Experiments in Civil Engineering
   
3. What is an Atmospheric Boundary Layer?
   
4. Boundary Layer Experiment Design
   1. Wind Effects on Structures
   2. Environmental Effects on Structures
5. How does a Boundary Layer Wind Tunnel Work?
6. How to Generate an Atmospheric Boundary Layer with a Wind Wall?
7. Conclusion

Figure 1: 3D mockup of a WindShaper being used to test scale model buildings

Boundary Layer Wind Tunnel Experiments in Civil Engineering

Some of the earliest recorded experiments studying the effects of air on objects date back to 1742 and 1759. Scientists performed these tests by attaching objects to rotating arms and spinning them through the air. A diagram of this test setup is shown in the figure below. It took over 150 years for this early technology to develop into the modern tools and techniques used in aerospace and civil engineering (source).

Figure 2: An early diagram of a whirling arm system, driven by a falling weight to test airflow on objects (source)

Early wind tunnels were mainly used to simulate stable, steady airflow. This made them useful for aircraft and airfoil design in aerospace, as well as for automotive applications.

It seemed reasonable to assume that wind tunnels could also be used in civil engineering applications to simulate real wind conditions and evaluate how structures would behave before construction. However, engineers quickly realized that the smooth and steady airflow produced by existing wind tunnels did not realistically represent the more chaotic and turbulent behaviour of atmospheric wind.

For civil engineering applications, the airflow in the tunnel needed to reproduce key characteristics of the atmospheric boundary layer, which was much more difficult to recreate. To understand why, we first need to look at how the atmospheric boundary layer behaves.

What is an Atmospheric Boundary Layer?

In fluid mechanics, a boundary layer is the region next to a surface where the flow is influenced by its interaction with that surface. In a general boundary layer, the flow may be laminar or turbulent. The Reynolds number, a dimensionless value that describes the relative influence of inertial and viscous forces, is one factor that affects this behaviour.

Lower Reynolds numbers generally favour laminar flow, while higher Reynolds numbers increase the likelihood of transition to turbulent flow. Surface roughness, disturbances, geometry, and other factors can also influence this transition. The difference between laminar and turbulent boundary layers is illustrated in the figure below.

Figure 3: Boundary layer demonstration in laminar and turbulent flow (source)

The atmospheric boundary layer is the lowest part of the atmosphere, where wind is directly affected by the Earth's surface.

The flow of atmospheric wind over the Earth’s surface is similar to the flow within a turbulent boundary layer over a relatively flat surface. As wind flows over the ground, terrain, buildings, trees, and other surface features, the flow is disrupted, which can generate turbulence. Turbulence is also produced by thermal effects, as the Sun heats the Earth’s surface and causes warmer air to rise.

Additional turbulence can be generated by atmospheric convection associated with clouds. An illustration of the atmospheric boundary layer is shown below.

Figure 4: Structure of the atmospheric boundary layer (source)

Reproducing these atmospheric conditions in a wind tunnel is complex. This is why boundary layer wind tunnel technology appeared later in the history of wind tunnels.

Common use cases studied with atmospheric boundary layer experiments include:

• Chaotic wind loads on tall buildings
• Snow accumulation simulation on roofs
• Wind flow around industrial buildings
• Pedestrian wind flow in urban areas
• Wake effects and flow velocity in wind farms
• Aerodynamic performance of drones and UAVs

Boundary Layer Experiment Design

For civil engineering applications, these experiments can broadly be grouped into two categories: wind and environmental effects on structures.

Wind Effects on Structures

One of the most common uses of a boundary layer wind tunnel is to study how buildings and structures are expected to move under atmospheric wind conditions. Wind-induced motion can be tracked using cameras, while load cells at the base of the model can measure shear forces and moments acting on the building. An example of a boundary layer wind tunnel with a structure used to test wind loads is shown below.

Figure 5: U.S. Air Force Memorial in Arlington, VA with a spire as tall as 270 ft (82 m) being tested in a BLWT (source)

Today, BLWT testing has become an established part of evaluating how buildings respond to wind. In Ontario, for example, any building exceeding 60 meters, or with a height more than four times its minimum effective width, must be tested in a BLWT before development.

BLWTs are also used to study aeroelastic models that experience wind-induced deformation or other motion that increases with wind loads. These structures are usually flexible, low in mass, and lightly damped, such as:

• Chimneys
• Masts
• Long-span bridges
• Transmission lines

Environmental Effects on Structures

The second use case for BLWT in civil engineering is to study environmental effects. In colder climates, the accumulation and transport of snow on buildings with large roof structures are studied. This allows designers to see where snow accumulates on buildings and to design the structure for the resulting snow loads and plan for snow removal where necessary. An image of a scale model used to study snow accumulation in a wind tunnel is shown below.

Figure 6: Scale model of a building covered in snow-like material to simulate snow accumulation (source)

Closely spaced buildings can create areas of accelerated wind around pedestrian zones. Prediction and measurement of mean and peak wind gusts in these areas allow for better site development and a better experience for pedestrians. Cities such as Boston, Toronto, New York and San Francisco require BLWT test data on pedestrian-level winds with and without the new structures prior to approval for development.

In industrial areas, emissions from building vents, industrial stacks, and automobiles are among the most prevalent sources of atmospheric pollutants, and their dispersion and location after release can be difficult to trace and predict. Physical modelling in a BLWT allows researchers to estimate how pollutants disperse and identify where elevated concentrations may occur.

How does a Boundary Layer Wind Tunnel Work?

Researchers wanted to adapt existing wind tunnel infrastructure for structural testing under more realistic wind conditions. They found that the wind tunnel airflow could be adjusted to simulate the atmospheric boundary layer. Researchers achieved this by adding flow-adjustment devices, such as spires, vortex generators, and fences that could replicate the atmospheric boundary layer.

These devices work by disrupting the smooth airflow to create turbulent vortices that mix the air as it moves downstream. This mixing creates turbulent flow and changes the velocity profile so that wind speed increases gradually with height, similar to the vertical wind speed profile found in the atmospheric boundary layer. An example of this kind of wind tunnel system can be seen in the figure below.

Figure 7: Traditional wind tunnel simulating atmospheric boundary layer conditions with a scale model of buildings (source)

How to Generate an Atmospheric Boundary Layer with a Wind Wall?

A wind wall like the WindShaper is made up of counter-rotating fan arrays that are individually controllable, which gives the operator the ability to generate realistic uniform or turbulent wind conditions.

The atmospheric boundary layer can be reproduced with the WindShaper by controlling the wind-speed and turbulence profiles across the test section. The fan speed can be set lower in the bottom rows and increased progressively in the higher rows. In addition, the counter-rotating fans in each fan module allow the WindShaper to generate turbulent flow without requiring a long test section with additional flow-adjustment devices. An illustration of a test setup with the WindShaper and a model city block is shown below.

Figure 8: 2D side-view of the WindShaper generating atmospheric boundary layer conditions

Wind shear is another condition relevant to civil engineering wind studies. It describes a change in wind speed and direction over a given distance. We made a video that demonstrates how wind shear can be simulated with the WindShaper and measured using the WindProbe.

Researchers from UCLouvain have installed a WindShaper that allows them to reproduce the atmospheric boundary layer for testing wind turbines and wind farm dynamics. The tests use scale models of wind turbines, which allow researchers to collect data faster and at a much lower cost than full-scale field trials. The university also plans to use the wind wall to test drones, helicopters, biomimetic flight, and environmental weather conditions such as rain and snow.

Conclusion

Boundary layer wind tunnel testing has become an important part of civil engineering, supporting applications ranging from structural wind-load studies to snow accumulation and pollutant dispersion.

These experiments depend on the equipment’s ability to reproduce the wind-speed profiles and turbulence characteristics found in the atmospheric boundary layer.

As testing methods continue to improve, technologies like the WindShaper are expanding how atmospheric wind conditions can be reproduced, controlled, and studied in the laboratory.

What other application do you think could benefit from atmospheric boundary layer wind tunnel testing? Let us know in the comments.