Investigating NACA ducts through 3D Printing, CFD, and wind tunnel testing.
Disclaimer: I am not an aerodynamic professional. The findings highlighted in this article is for educational, exploration, and entertainment purposes only.
Now, being a young university student, what do you want to do when you are suddenly unveiled to the world of endless research possibilities? Well, of course, you want to do it all (note to future self: bad idea); anyway, moving on. After inputting research into diverse ‘hot topics’, I finally found a project that could make an intriguing assemblage of all my interests, i.e. aerodynamics, 3D-printing, manufacturing, and computer-aided engineering. Et voilà, the output was NACA ducts.
“… But Harris, what are NACA Ducts?!”
My first exposure to NACA ducts was while working with a GT4 car at Spa-Francorchamps. The term NACA duct originates from the predecessor organisation of NASA. In their declassified 1951 report, it revealed the secrets of NACA for public exploitation. However, it was motorsports that initially exploited them five years later. Frank Costin, De Havilland aerodynamicist, replaced protruding air intakes on Vanwall cars with aerodynamically efficient NACA ducts. It then slowly started being utilised for brakes, engines, amongst other cooling purposes. Due to Costin’s association with Lotus, it became the first road car to embrace NACA ducts.
Currently, NACA ducts are often used for aerospace and automotive applications. With proper installation, NACA ducts can enhance the vented airflow into systems with minimal disturbances (turbulence) to the free stream and surrounding boundary layers. Through effective flow area manipulations, NACA ducts create pressure differences to suck air into a system. The combination of diverging curved walls and a shallow ramp design results in causing negligible turbulence. For optimal performance, the three most important factors for NACA ducts is design, placement, and application.
Project Aim and Methodology
Literature review indicated a scarcity of available research on this topic. Also, it highlighted that current NACA models in the market often ignore important design features due to cost, manufacturing, amongst other restraints. This arguably results in inefficient cooling and reduced ergonomics of the system. Therefore, it invited the need for an investigation; hence, the justification for this project.
Being mindful of the research gaps and potential, the main aim of the project was to reverse engineer a design that improves the vented airflow into the cockpit of a Ginetta G55 GT4 car. Due to poor design features, the hypothesis was that the airflow going through the duct into the cockpit was not optimal, and it potentially causes turbulence and overhead pressure on the vehicle. This ultimately leads to inefficient cooling and reduced ergonomics.
The original NACA duct of the Ginetta car was preserved due to functionality purposes. As a result, I had to reconstruct the existing working part through different reverse engineering methods. I opted for the ‘systems thinking’ approach to segment the project into six distinct stages that contained elements of aerodynamics, 3D-printing, manufacturing, and computer-aided engineering. These stages are highlighted in the following outline.
Empirical Description
It is remarked that any good project consists of maths and physics. Therefore, I had to examine basic fluid mechanics and flow modelling in other ducts to develop the new prototype. Two simple flow equations aided the development process for the novel design. These are:
- Continuity equation: ρA1V1=ρA2V2 (concerns with mass conservation)
- Bernoulli’s equation: 1/2 ρv²+P=constant (concerns with pressure changes)
These fundamental equations combined with geometrical iterations, dimensionality manipulations, and a NACA duct calculator helped to construct the final design. The two models were then assessed on ANSYS CFX with a recommended k-omega model. Lastly, wind tunnel testing was conducted to provide further backing to the empirical findings.
CFD Simulation and Wind Tunnel Findings
The simulations were designed to reflect the actual wind tunnel testing. For the simulations, the focus was on four different areas: velocity streamline, pressure differences, turbulence wake profile, and the velocity contour for the near-wall boundary layer. However, the wind tunnel testing only focused on velocities, and that was measured using pitot-tubes and a digital testometer. The side by side illustrations highlights the key observations — the model exhibited on the left is the new prototype model.
Discussion
The new model exhibited 1.8% increase in turbulence, and this was expected due to the new geometry, increased velocity and volume of the air particles. This slight trade-off is arguably acceptable as a minor increase in turbulence does not necessarily correlate to turbulent flow in the boundary layers. This claim can also be backed from the CFD illustrations. The maximum velocity was taken as the average airflow exit velocities of both ducts. The new model showed an enhanced airflow in both CFD (12.75%) and wind tunnel testing (14.62%). One of the key observations was that the old model produced an undesirable negative pressure difference. It potentially creates overhead pressure, non-optimal airflow, and cooling. Therefore, this is in alignment with the initial hypothesis. Additionally, the new model adds a further 2 KPa of positive pressure difference. The increased pressure difference will aid the air suction process, leading to more efficient cooling and ergonomics.
Conclusion
In conclusion, acknowledging the many criticisms and limitations associated with this study, the investigation was arguably successful in achieving its main aim to reverse engineer a new design that improves the vented airflow for the cockpit cooling purposes of the Ginetta G55 GT4 car. The findings from this study can perhaps trigger new research areas or help individuals and businesses to make the best of use of NACA duct technology.
And… That is a wrap for today! Hopefully, this was an informative and interesting read. I will be back soon to explore a new scientific topic. Let me know your thoughts and suggestions in the comments. Sincerely thank you for reading.
