Welcome back to Sports & Building Aerodynamics. This is week 6 on cycling aerodynamics. Let's start again with a module question. Consider two dummy cyclists, equipped with pressure sensors, in the wind tunnel. One in the upright position, and one in the time-trial position. At which cyclist position does the highest pressure occur on the body? Is that the upright position shown in the left figure, the time-trial position shown in the right figure, or is it the same for both positions? Please hang on to your answer, and we'll come back to this question later on. At the end of this module, you will understand how wind-tunnel tests for dummy cyclists are prepared and performed. You will understand also Reynolds number effects. This is still part of a research project that was focused on optimization of power output and aerodynamic drag. And the reason why we also focus on dummy cyclists as opposed to real cyclists is that they can remain static, real static in fixed position. And we can implement a large number of pressure sensors and tubes into the body. More information about the detailed wind-tunnel tests but also the CFD simulations of the dummy cyclist can be found in this article. First, to generate the body geometry, we applied high-resolution 3D laser scanning. We did that for two positions, the upright position, shown here, and the time-trial position, shown in this video. Then based on laser scanning, we obtained a lot of, well we obtained a cloud of data points on the surface, with quite a high spatial resolution. Actually, the spatial resolution was too high for the intended purpose. So in view of a later application of this model in Computational Fluid Dynamics, we also smoothed out some of the details, because here you can indeed see that also the wrinkles in the clothes and on the skin are included in the 3D laser scanning results. But finally, we smoothed that out to some extent, and then this is the final model that we obtained. That is also the model that was used for rapid prototyping, and here the result of that you see next to me. So the upright position, the time-trial position, then we did rapid prototyping of the model at a 1:2 scale. The bicycle was not included. And, here you see the process after the rapid prototyping, actually removing the remaining powder from the body parts. Because we needed the model to be hollow to embed the pressure sensors and the pressure tubes. Some body parts. Then we had a model at a 1:2 scale. The bicycle was not included as you see here. We added some stiffening elements between the hands and the feet, to avoid vibrations of these parts in the wind tunnel at high speeds. Then the surface was treated with resin impregnation to achieve a smooth finishing. Then we had 115 pressure tabs in the scale model which were embedded flush with the surface and then connected by pressure tubes to a pressure transducer. And in order to avoid flow interference effects, we had to have these pressure tubes exit at a strategic position behind the cyclist, as you can see in this photograph. And it was quite a job to identify all the tubes with the right pressure tap. Here, you finally see the cyclist model, the upright position model, on the stand. The stand has an aerodynamic airfoil-like shape to minimize aerodynamic drag. Below it, we have half a sphere where the pressure transducer is included and we needed to put the pressure transducer there to limit the length of the pressure tubes. The tests were again performed, like with the full-scale cyclist, in the closed-circuit wind tunnel in Marknesse in the Netherlands. With an approach-flow wind speed of 20 meters per second, the very low approach-flow turbulence intensity of 0.02%, and a wind direction parallel to the bicycle axis. These are the frontal areas that were obtained for this model. The blockage ratio was only 3% for the model with the highest blockage ratio, that was the upright position, that is the stand included. This is the precision of the force balance; 0.1% for the full-scale range, and we did sampling at 10 Hertz for 30 seconds. And then surface pressures were also measured at the 115 points, with an accuracy of seven Pascal which amounts to 0.1% of the full-scale range. And sampling here was performed at 512 Hertz for 24 seconds. Then we investigated Reynolds number effects, and we found indeed Reynolds number effects, but not beyond 20 meters per second. There, the difference actually only was 2% for the drag force. Here are some additional photographs of the model on the stand. You see the crew actually equipping the model, fixing it at the right position. Some additional photographs. And here the cyclist model is ready to be tested. So we also see that the pressure tubes actually were concentrated in the stand to avoid flow interference. And then of course, we also performed wind-tunnel tests for the stand alone, and also for the half sphere alone to be sure that we could deduce those results, subtract those drag areas from the final drag area that we wanted to obtain, which is that of the cyclist's body alone. Here are some results. Here you see the drag area actually as a function of the wind tunnel speed. And indeed Reynolds number effects are present up to about 20 meters per second. And that's the reason indeed why we, after this preliminary test, performed the measurements above 20 meters per second. So let's get back to the module question now, which relates to some of the results obtained from these wind-tunnel tests. If you have these two dummy cyclists, on which cyclist position, or in which cyclist position do we get the highest pressure on the body? And this actually is answer A; the upright position. So it's a position that presents the largest blockage, largest obstruction to the flow. And that's also the place where the highest pressure will be found. More results on these wind-tunnel tests will be reported in the next module together with the CFD simulation results. In this module, we've learned about how tests, wind-tunnel tests for dummy cyclists are prepared and performed. We've also looked at some Reynolds number effects involved. In the next module, we're going to focus on CFD simulations for a single cyclist. Also, in this module, we'll report the test results of the wind-tunnel experiments that were treated in this module, because we're going to use those as validation for the CFD simulations. So thank you again for watching, and we hope to see you again in the next module.
