'Bike Aerodynamics' simulation project by Akrem


#1

I created a new simulation project called 'Bike Aerodynamics':

This project shows the effect of the side wind on different wheels designs.


More of my public projects can be found here.


#2

Description


 


 
Regardless of how fast one may pedal, aerodynamics plays a major role in the performance of a cyclist. In fact, biking is considered to be one of the most complex applications of aerodynamics not only because of the bluff body shape of the rider ― which generates the majority of the drag ― but also due to the transient cycling movement. The transient cycling movement engenders a high complexity in the air flow pattern. Moreover, the body shape leads to a dramatic increase in the forces acting on the bike as well as the rider.
 
Computational Fluid Dynamics (CFD) can be used to effectively analyze the airflow pattern around a cyclist, keeping in mind the shape of the bike+body system and the ephemeral manoeuvres in the cycling process. The results of this analysis may be utilized to suggest changes in the posture of the cyclist or the shape of the bike, with an aim of decreasing the air resistance acting on the system.
 

Project Goals


The current project intends to delve into the aerodynamics associated with a cyclist, with an attempt to reduce drag acting on the biomechanical system of the bike+body.
 

Geometry


Symmetrical geometry of cyclist used for Simulation

Meshes


After importing the geometry onto the Simscale platform, a Hex-Dominant Parametric (only CFD) mesh is mapped onto it and several mesh refinements are made on the body+bike system as well as the area surrounding it. The entire mesh geometry is enclosed in a Cartesian Box.
 


 
Mesh refinements within Cartesian Box

Simulations


 
Post meshing operation, multiple simulation runs are carried out with variegated boundary conditions. Some of these include no side wind, side wind at 15o and 20o.
 

Results and Conclusions


 
The following images draw a comparison between the pressure acting on the cyclist with and without side wind
 


 
The image on the left depicts the pressure exerted on the cyclist without side wind. As can be seen, the pressure distribution is almost symmetric. This leads to a high drag acting on the bike+body system.
 
Whereas, the image on the right shows the pressure acting with the presence of side wind. As the component of wind velocity in the direction opposite to the motion of the rider is significantly reduced, the drag is also diminished as compared to the previous scenario. Although, there is a greater pressure (and hence, force) acting on the side (windward) of the cyclist. This stems from the sideways component of the wind velocity.
 
Shown below is the top view of both the cases to help gain a better understanding of the transpiring phenomenon.
 

 
By taking a closer look, we can see that the flow over the bike without a side wind is well aligned and the wake region size is smaller compared to the case with the side wind, where the wake region is larger. A large wake region is beneficial to the cyclist who is riding behind the one currently under study.
 
The following figures manifest the velocity and pressure contours for the cyclist, without side wind. The wake region can be observed in the image on the right.
 

 
Therefore, we may conclude that the wind direction affects the performance of cyclist to a great extent. Also, finding the right position in the racing row gives the rider significant advantage compared to the other riders as every gram of drag has a remarkable effect on the performance of the rider in a long-range race.


#3

Hi Akrem,
Great project, I’ve found it really interesting and your results have helped me to see how the low pressure area created by the body moves towards the ground as it travels away from the rider, I guess because it is influenced by the low pressure area created by the bike.
I’m trying to use your project as a base for my project investigating a design for a rear bicycle fairing.
The project goal is essentially to help me cycle to work! I do a regular 30km commute to and from work by bike, and I want to make a rear fairing to help me do it a bit easier and give me a neat place to carry my stuff.
I’m an electronic/electrical engineer by training so my knowledge of CFD is limited so it is a great help to base my project on an existing one rather than create one from the beginning.
However, I am having some difficulty, In order to simulate the effect of the rear fairing I have to add it to the base geometry. So I download one of your CAD models (mid tyre) and I get a stl file. I use FreeCAD/Blender/text editor (I’ve tried multiple methods) to add a very simple rear fairing to it. However, no matter which method I use, when I import the edited stl file back into the simscale workbench it merges the whole model into a single face, and the number of nodes has increased from ~750k in your model to ~3000k in my one! I then cannot get the simulation to run, I think because I cannot allocate the mesh and simulation refinements to the different faces of the model (rotating elements etc).
If you have any tips on how I could edit and reimport the model that would be great.
Kind Regards
Tom