How to use wool tufts to analyse aerodynamics
Aerodynamics is one of the most challenging disciplines in the field of engineering. You can’t observe it with the naked eye and the underlying physics make numerical analysis difficult. As a result, it’s a common misconception that to understand the field aerodynamics a plethora of advanced tools are required. In fact, there are several simple techniques that offer effective ways to begin to understand the aerodynamic performance of a vehicle.
Aero baseline
Collins Limited were approached by VUHL, a Mexican sportscar manufacturer, to understand the full-scale aerodynamic performance of the VUHL 05.
To create a solid foundation for future aerodynamic development, should it be required or desired, it is helpful to measure basic single point values such as aerodynamic lift and drag along with the ratio on each axle across several vehicle attitudes. To be able to support digital development it is useful to characterise both the surface and volume flow fields through wool tuft and smoke wand physical tests. These primary tests create a strong basis for future full scale and digital aero development.
The VUHL 05 is a lightweight, track-focused two-seater sports car from Mexico City
The approach
Through our network we arranged a full-scale wind tunnel test at the world-class HORIBA MIRA facility in the UK.
‘We don’t have a single standard process that we apply to every problem’, explains Jilbruke, co-founder at Collins Limited. ‘Each problem gets its own bespoke solution package depending on the nature of the project and the requirements of the customer. For VUHL, it was critical to use our test time efficiently to gain as wide a picture of the aerodynamic characteristics as possible.’
To achieve this, the engineers decided to complement the force measurements from the wind tunnel with a comprehensive, yet simple, data gathering process. This involved utilising surface pressure taps, a smoke wand, multiple wall mounted cameras and wool tufts.
‘By combining several data capture methods, you can build a clearer picture of the airflow regime,’ highlights Jenner Collins, co-founder at Collins Limited. ‘The smoke wand helped us understand the global flow field, while the wool tufts showed the local flow field. The pressure taps provided finite pressure values which can be used to calculate the aerodynamic forces acting on the car.’
The value of wool tufts
Wool tufts are lengths of wool which conform to the direction of airflow. Placing several across a surface creates a powerful visual tool to describe local airflow behaviour. If a tuft is swept back and flush to the surface, it indicates clean, attached airflow. On the other hand, erratic flapping or a change in orientation is a sign of flow separation. Comparing the tuft behaviour in different parts of the car can offer valuable insight into where the aerodynamics are performing as expected.
For a new project such as this, we began by thoroughly covering the car with tufts at a spacing of 100mm. We identified areas of potential separation, one cause of drag, and increased the density of the tufts to 50mm. These included areas such as the front splitter, air inlets and behind both the front and rear wheel arches.
Uniform wool tufts indicate attached flow, erratic wool tufts indicate flow separation as seen here on the roll hoop
‘We wanted to have an accurate idea of where flow separation was occurring. With the tufts at a high density, we can effectively draw iso-lines on the body of the vehicle showing the separation line. This shows that everything inside the line is attached flow, and everything outside is separated. This information might give you an idea of where you could put an inlet duct, or, to correlate your CFD program for example.’
As you can see in the following image, the tufts on the bonnet are straight and unidirectional, whereas those on the blue section are folded over and randomly oriented. This highlights a transition zone between attached and separated flow on the front windshield.
Wool tufts located on the bonnet and windshield show that the flow is transitioning from attached to separated
‘Of course, one must remember that wool tuft data can only be captured when the wind is on,’ says Jilbruke. ‘Therefore, to obtain more reliable results, you need to set up a camera to record the behaviour of the tufts or take photographs. Then you can analyse these images and videos in more detail which will help draw more accurate conclusions.’
Wool tufts are an incredibly powerful tool to help understand aerodynamic behaviour. Today, they are utilised across a wide range of industries from automotive, aircraft and even spacecraft. With simplicity comes versatility – anyone with a camera and wool is fully equipped to carry out their own aerodynamic tests. How you analyse and make sense of the results is the next challenge!
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