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<div class="section" id="aerodynamics-of-chimera-evoluzione">
<h1>Aerodynamics of Chimera Evoluzione<a class="headerlink" href="#aerodynamics-of-chimera-evoluzione" title="Permalink to this headline">¶</a></h1>
<img alt="images/design.png" src="images/design.png" />
<div class="section" id="design">
<h2>Design<a class="headerlink" href="#design" title="Permalink to this headline">¶</a></h2>
<div class="section" id="the-concept">
<h3>The Concept<a class="headerlink" href="#the-concept" title="Permalink to this headline">¶</a></h3>
<p>This is the first time in our history that we perform a complete aerodynamics study to improve our
car’s performances. Because of the low speed and high accelerations of our car, we decided to focus
our aerodynamics on creating downforce. Having no experience in aerodynamics devices
manufacturing, our goal was to design and produce an efficient wing out of carbon fiber composites.
Doing so we can directly experiment the design and manufacturing techniques related to the
carbon-fiber composites in aero-devices.
From the literature we got some common lift and drag values, and after some simulations we found
out that adding downforce to the rear axle compensates to the natural over-steering behavior of our
car.
We adopted an unsprung solution for the wing to bring downforce directly to the upright, without passing
through the suspension mechanism, this allows us to keep the car at the lowest possible setup, without
worrying about the additional load coming from the wing. We decided to experiment this kind of solution
also to make easier a future development of an undertray, thanks to the more predictable height and
pitch of the vehicle.</p>
<img alt="images/designAero1.png" src="images/designAero1.png" />
</div>
<div class="section" id="the-s1223-airfoil">
<h3>The [s1223] Airfoil<a class="headerlink" href="#the-s1223-airfoil" title="Permalink to this headline">¶</a></h3>
<p>The airfoil has been chosen from the airfoiltools.com online database, it is the [s1223], which is a
High-lift cambered airfoil, and performs well at low speed in terms of Cl/Cd. The airfoil
performances have been validated in the Ansys Fluent 2D environment.</p>
<img alt="images/designAero2.jpg" src="images/designAero2.jpg" />
<p>Selig S1223 high lift low Reynolds number airfoil
Max thickness 12.1% at 19.8% chord.
Max camber 8.1% at 49% chord</p>
<p>Reynolds number has been estimated to be enclosed between 500.000 and 1.000.000, with the
speed ranging from 40km/h to 90km/h, for a 600mm wing cord.</p>
<p>From the airfoiltools.com, the [s1223] online datasheet:</p>
<p>Violet -> Re = 500.000
Yellow -> Re = 1.000.000</p>
<img alt="images/designAero3.jpg" src="images/designAero3.jpg" />
<img alt="images/designAero4.jpg" src="images/designAero4.jpg" />
<img alt="images/designAero5.jpg" src="images/designAero5.jpg" />
<img alt="images/designAero6.jpg" src="images/designAero6.jpg" />
<p>Due to the experimental purposes of this wing, we made it adjustable on three different angles of
attack. It is not an active wing, the setup must be changed manually before the run, by operating on
six screws.</p>
<p>The three angles have been chosen while keeping high Cl/Cd.</p>
<p>α1 = 12°,
α2 = 7°,
α3 = 2°</p>
<p>In our simulations we used the most critical configuration, at α = 12°, at the most common
cornering speed 17m/s (≈60km/h).</p>
<img alt="images/designAero7.jpg" src="images/designAero7.jpg" />
<img alt="images/designAero8.png" src="images/designAero8.png" />
<img alt="images/designAero9.png" src="images/designAero9.png" />
<img alt="images/designAero10.jpg" src="images/designAero10.jpg" />
<img alt="images/designAero11.jpg" src="images/designAero11.jpg" />
<p>The simulated coefficients are slightly worse than the ones that we expected from the datasheet. In
the stream-line plot we can notice a partial flow detachment at the very tip of the wing, which may
be the cause of our Cl drop and Cd rise.
It’s unclear if the flow detachment is supposed to exist, or if it is the result of an approximation.
Anyway, we headed up with the design of the wing.</p>
</div>
<div class="section" id="going-3d">
<h3>Going 3D<a class="headerlink" href="#going-3d" title="Permalink to this headline">¶</a></h3>
<img alt="images/designAero12.jpg" src="images/designAero12.jpg" />
<p>After the 2D validation, the shape of thw wing has been designed to concentrate the highest angle of attack on the sides (where the airflow is undisturbed by the main-hoop’s turbulence) reducing by 3° in the middle. At the
same time the wing chord increases toward the middle of the wing, in order to increase the flexural resistance, and keep high the lift on the full span.</p>
<img alt="images/designAero13.jpg" src="images/designAero13.jpg" />
<p>After the 3D shaping of the wing, 3D CFD simulations have been done in order to validate the final
geometry.
For calculus simplicity, the wing is suspended in air neglecting of the turbulence of the car in front of
it. The two rod-joints on the top have been suppressed too.</p>
<img alt="images/designAero14.png" src="images/designAero14.png" />
<img alt="images/designAero15.png" src="images/designAero15.png" />
<img alt="images/designAero16.png" src="images/designAero16.png" />
<img alt="images/designAero17.png" src="images/designAero17.png" />
<img alt="images/designAero18.jpg" src="images/designAero18.jpg" />
<p>This performance drop might depend on two factors: the first is that the coarseness of the mesh does not allow to properly model the boundary layer, the second is related to the wing geometry.
If we look at the stream-line graphics we clearly see two counter rotating vortices.</p>
<img alt="images/designAero19.jpg" src="images/designAero19.jpg" />
<img alt="images/designAero20.jpg" src="images/designAero20.jpg" />
<p>These two vortices dissipate energy (increasing Cd), and simultaneously decrease the effective wing
span (dropping Cl).</p>
</div>
</div>
<div class="section" id="dynamics">
<h2>Dynamics<a class="headerlink" href="#dynamics" title="Permalink to this headline">¶</a></h2>
<p>Once the wing aerodynamics properties were validated, we used the calculated coefficients in our
optimal control simulations. We performed the test on a generic autocross circuit with and without
the presence of the wing. We obtained a nice improvement on the fast corners, allowing us to go
even faster and having higher lateral accelerations.</p>
<img alt="images/designAero21.jpg" src="images/designAero21.jpg" />
<p>We have an improvement on the lap time of -1,4457s, which corresponds to a 3,08% faster lap in
optimal conditions.</p>
<img alt="images/designAero22.jpg" src="images/designAero22.jpg" />
<img alt="images/designAero23.jpg" src="images/designAero23.jpg" />
<img alt="images/designAero25.jpg" src="images/designAero25.jpg" />
<p>We can also confirm that the wing compensates to the natural oversteering behavior of our car,
which become almost neutral in the range between 65 and 80km/h. Over 90km/h the behavior is
definitely under-steering. Our plan for the future is to add a front wing to compensate this problem.</p>
<img alt="images/designAero26.jpg" src="images/designAero26.jpg" />
</div>
<div class="section" id="structural-efficiency">
<h2>Structural efficiency<a class="headerlink" href="#structural-efficiency" title="Permalink to this headline">¶</a></h2>
<p>Once the dynamics were validated, it was time to check the structural efficiency of the whole aero
device.</p>
<div class="section" id="the-wing-shell">
<h3>The Wing shell<a class="headerlink" href="#the-wing-shell" title="Permalink to this headline">¶</a></h3>
<p>The most critical part of the wing structure is the wing shell, it must guarantee a maximum
deflection lower than 10mm, with 200N applied vertically.
Since we don’t have precise data about this carbon fiber layout, in our simulation we used a single
0,5mm thick carbon-epoxy layer. After the simulation, to ensure the rigidity, we over-dimensioned
the layout as follows (at the price of the weight):</p>
<p>The wing is then glued to two lateral Al 6082 blades, which are used to screw the wing to the
endplates.</p>
<p>The loading configuration has been exported from the 100km/h CFD simulation as a pressure
distribution.</p>
<img alt="images/designAero27.jpg" src="images/designAero27.jpg" />
<p>Just to have an idea of the loads, we calculated them by fixing the wing by the six mounting holes,
and calculating the reaction forces.</p>
<p>The drag contributes (X Axis) is negligible respect to the lift (Y Axis); because of that for the
structural analysis we modeled the wing as simply fixed by the 6 mounting holes, neglecting of the
two streamlined anchors on the top.
The two Al 6082 side-plates are kept in place to evenly distribute the pressure from the carbon to
the mounting holes.</p>
<img alt="images/designAero28.jpg" src="images/designAero28.jpg" />
<p>The maximum equivalent stress is reached in proximity of the mounting holes, on the aluminum
side-plates; 89MPa is ok for the Al 6082. On the carbon shell we see that 20MPa peaks are
reached, which is ok.</p>
<img alt="images/designAero29.jpg" src="images/designAero29.jpg" />
<img alt="images/designAero30.jpg" src="images/designAero30.jpg" />
<p>The maximum displacement is about 7mm, with about 400N of load. Which is already ok for the
FSG2018 rules. By over dimensioning the carbon fiber layout we are sure that the rigidity
requirements are met.</p>
<img alt="images/designAero31.jpg" src="images/designAero31.jpg" />
</div>
<div class="section" id="upright-attachment">
<h3>Upright attachment<a class="headerlink" href="#upright-attachment" title="Permalink to this headline">¶</a></h3>
<p>The upright attachment is made out of 3 Al 6082 sheets, pressed together by 5 screws around the
wing supports.</p>
<img alt="images/designAero32.jpg" src="images/designAero32.jpg" />
<p>Over the aerodynamics downforce, this part must withstand the inertial loads caused by the wing
structure above it, and road asperity accelerations. Assuming a mass of about 2,5kg for the wing
structure supported by the upright attachment, and a vertical acceleration of about 40G coming
from the tire hitting a curb at 100km/h, we get about 1000N from inertia, plus half of the 400N
from the aero downforce at 100km/h.
Our load is then 1000+200 = 1200N, which is evenly distributed on the 3 upper screws.
For model simplicity, we assumed the force coming from the road, and the part is fixed by the 3
screw holes.</p>
<img alt="images/designAero33.jpg" src="images/designAero33.jpg" />
<p>The force is transmitted to the two lateral plates by two steel pins.</p>
<img alt="images/designAero34.jpg" src="images/designAero34.jpg" />
<img alt="images/designAero35.jpg" src="images/designAero35.jpg" />
<p>As expectable, the maximum stress is located in proximity of the lower attachment hole, anyway it is
limited to 52MPa.
This part has been designed to guarantee a high stiffness, and distribute the tension on the carbon
fiber support, in order to avoid stress concentrations.</p>
<img alt="images/designAero36.jpg" src="images/designAero36.jpg" />
</div>
<div class="section" id="wing-supports">
<h3>Wing Supports<a class="headerlink" href="#wing-supports" title="Permalink to this headline">¶</a></h3>
<p>As for the wing shell, we still made conservative assumptions for this part, and over dimensioned
the carbon layout. In our simulation we still used a single carbon-epoxy layer, instead of the 6 used
on the real part, and a 9mm thick foam core.</p>
<p>The wing’s support has been tested with a load of 1000N, assuming 800N of inertial forces while
hitting a curb, plus 200N from aero downforce</p>
<img alt="images/designAero37.jpg" src="images/designAero37.jpg" />
<img alt="images/designAerodynamics/designAero38.jpg" src="images/designAerodynamics/designAero38.jpg" />
<img alt="images/designAero39.png" src="images/designAero39.png" />
<p>Such a refined mesh was needed to properly model the contact surface between the carbon layer and
the core.</p>
<img alt="images/designAero40.png" src="images/designAero40.png" />
<p>The displacement is ok, simply negligible for our purposes.</p>
</div>
</div>
<div class="section" id="manufacturing-of-the-wing-shell">
<h2>Manufacturing of the wing shell<a class="headerlink" href="#manufacturing-of-the-wing-shell" title="Permalink to this headline">¶</a></h2>
<p>The mold for the wing shell has been obtained from a hydrophobic MFD (Medium density fiber)
block. This choice has been governed by the extremely low cost of the material (600€/m 3 ) compared
to the Ureol (8000€/m 3 ). Turned out that we got some troubles due to the moisture absorbed in the
wood, that developed some bubbles while in the autoclave. We also got some problems since the
wood does not conduce heat very well, and we needed to keep the mold in the autoclave for a
longer period of six hours, instead of 4 hours, to ensure a complete cure of the resin.</p>
<img alt="_images/" src="_images/" />
<img alt="_images/" src="_images/" />
</div>
<div class="section" id="future-developments">
<h2>Future Developments<a class="headerlink" href="#future-developments" title="Permalink to this headline">¶</a></h2>
<p>For our next step we plan to invest more energies into the aerodynamics studies, since we think to have a
large margin to improve our car performances using aerodynamics.</p>
<p><strong>We have 3 main goals:</strong></p>
<blockquote>
<div><ul class="simple">
<li>Development of a fully adjustable aerodynamic pack, complete of undertray, front, and rear wing.</li>
<li>Overall car characterization and drag reduction.</li>
<li>Improvement of the carbon fiber manufacturing techniques.</li>
</ul>
</div></blockquote>
<div class="section" id="development-of-a-full-aerodynamic-pack">
<h3>Development of a full aerodynamic pack<a class="headerlink" href="#development-of-a-full-aerodynamic-pack" title="Permalink to this headline">¶</a></h3>
<p>We aim to the development of a fully adjustable aero pack. This to increase our knowledge of the overall
vehicle behavior, and correction of the actual understeering at high speeds.</p>
<p>Our plan is to have a large use of 3D printed aerodynamics appendix instead of carbon fiber ones.
Additive manufacturing allows us to have extremely complex shapes, at relatively low costs if compared to
the carbon fiber laminated ones.</p>
</div>
<div class="section" id="car-drag-reduction">
<h3>Car drag reduction<a class="headerlink" href="#car-drag-reduction" title="Permalink to this headline">¶</a></h3>
<p>We performed a rough analysis of the car behavior in the free stream using Autodesk Flow Design ©. What
emerged is that the main drag sources in our car are the front wheels, and the very high main hoop.</p>
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