Wind Tunnel Explained With Computational Fluid Dynamics

Have you ever wondered what are the main features of a wind tunnel and what purpose do they fulfill? Well wonder no more. With the help of Computational Fluid Dynamics (CFD) let's explore a closed return wind tunnel based on the Honda Wind Tunnel at Imperial College, London.

Wind Tunnel CFD SimulationWind Tunnel CFD SimulationVelocity magnitude iso-surfaces

The Honda Wind Tunnel has a historic place in motorsports, having been used by many leading Formula 1 and Le Mans teams. Also it has contributed to the research activities of many trainee aerodynamicists, who later went on to join racecar teams.

The Honda Wind Tunnel is an unusual design in that the closed loop is configured vertically rather than horizontally. Also instead of a single fan it has two. For our CFD model we only need to model half the wind tunnel given that it is symmetric along its center line.

Symmetric Half Model of a Wind TunnelWind Tunnel Symmetric Half ModelBlue faces indicate symmetry joints

Test Section

The main objective of a wind tunnel is to deliver a steady, consistent flow of air to the test section with minimal turbulence. All the aerodynamic apparatus in the tunnel is focused on this one objective.

Wind Tunnel Test SectionWind Tunnel Test SectionVelocity vectors colored by velocity magnitude

You can see from the CFD simulation results the test section flow has very little variation in speed, which is exactly what we are looking for. Note the simulation breaks up the flow domain into multiple-connected volumes to make it easy to replace a volume segment, e.g., switch in a new fan design, or add a model to the test section.

Contraction

The contraction immediately upstream of the test section accelerates the air and delivers it to the test section as a steady, consistent air stream with relatively low turbulence.

Wind Tunnel ContractionWind Tunnel ContractionVelocity vectors colored by velocity magnitude

The CFD results for the contraction show how the air picks up speed as it is squeezed through the contraction.

Straightener

After turning through two 90 degrees bends in a short distance the air develops a large recirculation zone immediately downstream of the final turn. To allow the air to settle there is a length of duct that ends in a flow straightener, immediately upstream of the contraction. The straightener serves to align the air parallel to the wind tunnel walls and reduce turbulence. Typically a straightener consists of a honeycomb section parallel with the tunnel walls and a screen on its exit.

Wind Tunnel StraightenerWind Tunnel StraightenerVelocity vectors colored by velocity magnitude

Rather than model each individual honeycomb passage in the CFD model we chose to use a porous volume with a lower resistance parallel to the wind tunnel walls. The CFD results show how the air upstream of the porous volume is not well aligned with the wind tunnel walls, but once the air exits the porous volume note the better alignment.

Diffuser

The diffuser downstream of the test section slows the air in anticipation of the final two 90 degree turns to minimize pressure loses.

Wind Tunnel DiffuserWind Tunnel DiffuserVelocity vectors colored by velocity magnitude

The CFD results through the diffuser shows the air slowing down.

Turning Vanes

Given the abrupt turns in the wind tunnel, turning vanes are essential to minimize the pressure loses. The required air flow rate and the resulting pressure loss for the wind tunnel determines the fan specification. Therefore, it is usually advantageous to minimize pressure losses to reduce the capital cost of the fan and the long term running cost of the wind tunnel.

Wind Tunnel Turning VanesWind Tunnel Turning VanesStreamlines colored by velocity magnitude

The CFD results show how the air through the bend with turning vanes aligns well with the corner without large recirculation zones that would increase pressure losses. The turning vanes are modeled as double-sided faces to avoid having to resolve their extremely thin cross sections.

Fan

The wind tunnel axial fan pumps air around the closed loop. Immediately downstream of the fan are flow alignment stators.

Wind Tunnel FanWind Tunnel FanStreamlines colored by velocity magnitude

The fan in the CFD simulation is modeled within a moving reference frame (MRF), which is an efficient steady-state simulation technique for rotating machinery. MRF is equivalent to running a rotational simulation and then observing the results at the instant equivalent to the position of the rotor within the MRF.

Notes

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Racecar test in the wind tunnel

What use is a wind tunnel without a model to test? Find out more at "CFD Simulation of a Racecar in a Wind Tunnel".

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