Sub Systems

Hot Wheels

March 14, 2017 | Sharan Kishore

When people talk about Hyperloop, friction-less locomotion such as magnetic levitation or air bearings is what comes to a person’s mind apart from the name Elon Musk of course. Even though the Hyperloop will run on friction-less systems at high speeds but it still needs some system to function at low speeds. As magnets and air bearings have very little effect at low speeds thus wheels are a viable and currently the only option. Wheels will be used for the initial run i.e. until the speed required to levitate successfully is achieved. And if the levitation is active like ARXPAX, still wheels will be viable so as to conserve energy at lower speeds. Also, wheels may be required to balance certain moments generated.  Now imagine a scenario where the pod is travelling at a very high speed and the levitation fails due to unforeseen circumstances. In this event, the wheels must thermally survive the heat generated due to the friction from the aluminum sub track.

Keeping this scenario in mind, we have performed thermal simulations on the wheels to analyze the maximum temperature developed. To maximize safety in our design, we have assumed that the wheels are used for the entire track length at the speed of 150m/s.

Now the simulation can be performed using multiple methods. One method is to:

  1. Make a CAD file of track along with the wheel

  2. Provide frictional contacts between the track and the wheel

  3. Apply the pod’s weight to get the proper normal force

  4. Input the required rpm/velocity

  5. Run the simulation as transient structural along with temperature degree of freedom(an extra ANSYS command snippet is required) of course for a shorter length of the track

The above method will give accurate results and ANSYS will calculate the heat flux etc. but it is computational expensive.

An alternative and easier method that we used is to:

  1. Make a CAD file of the wheel only

  2. Analytically calculate the heat flux for two rubbing surfaces (rotating and planar in our case) over the entire track length

  3.  Apply the calculated heat flux value over the entire circumference of the wheel. Even though the wheel will have a line contact always but at very high rpm we can assume that every ‘line’ on the circumference of the wheel will be at the same temperature/heat flux which basically becomes the entire circumference(surface) of the wheel

  4. Apply convection on the required regions and run the simulation as Transient thermal

 

Transient simulation makes more sense because the pod will not reach a steady state as it is constantly in contact with the track. Also, with transient thermal simulation, data for the temperature of the wheel for the entire track duration can be obtained.

The material used for the wheel is Aluminum 6061. A separate layer of polyurea is added on to the circumference of the aluminum. The thickness of the extra layer is 2cm. This layer is required to prevent the rubbing of Aluminum wheel and Aluminum sub track surface.

Setup Bonded contacts were defined between the aluminum and polyurea surface. Tetrahedron mesh was generated and further refinement was performed using ‘body sizing’ option. An initial temperature was set to the model. This value was obtained from the CFD simulation. Please refer aerodynamics blog to check out our shape iterations and results. Convection was applied on all the faces and heat flux calculated from the analytical equations was applied on the circumference. The model was solved for 5 s and the heat flux was activated for all the 5 sec mimicking the worst case situation.

 The maximum temperature plot for 5 sec is shown below. We observe that at the end of 5 seconds polyurea reaches a max temperature of 434.56  ̊C. This shows that the wheel can withstand thermally in the event of failure of levitation wheels because ployurea becomes soft only at temperatures above 600   ̊C.  We also observe that the conductivity of aluminium is  8000 times  the  conductivity of polyurea. Due to such vast difference in thermal conductivity, the aluminium doesn’t receive any heat during the 5 second run and the temperature remains  at approx 35   ̊C. From the fig 4 we also observe that 2cm of poly urea is not required and the thickness can be reduced to save material costs. 

Transient thermal simulations was performed on a aluminium wheel with polyurea on the outer surface. The wheel was assumed to be in contact with the track and translating at 150m/s for the entire track length(worst case scenario). Heat flux generated on the wheel due to friction was calculated analytically and applied on the surface of the wheel. Maximum temperature obtained was 434.56  ̊C which is well below the softening temperature of polyurea. The thickness of the polyurea can also be optimized to save the material costs. Now it’s time to check the stress distribution on the wheel due to high centrifugal forces.

Chassis – Fluid Structure Interaction

March 12, 2017 | Rohan Thakker

One of the initial iterations of the chassis design is shown above. It has 4 longerons with ribs placed in the front, middle and the back of the pod. Supports are adequately provided on the front and back as well. The weight of this design was 70.23 kg.

Stresses generated in the chassis due to pressure distribution of fluid on the pod and uniformly distributed load of different systems of the pod are calculated through ANSYS Workbench. The resultant deformation is shown in the figure above.

The analysis of the frame yields a maximum deformation of 1.8 mm. The factor of safety achieved is 3.5, which is very promising. The weight of the chassis is high and could be reduced without compromising structural integrity. Additionally, it was necessary to add more support elements at certain locations and remove support elements from others to obtain a more optimized chassis design.

The new design had a weight of 57 kg which is a reduction of 20% from first design shown.

Design 2 gave us a factor of safety of 2.68 and a maximum deformation of 2.4 mm. Weight could further be reduced by strategically reducing material from certain locations.

The new design had material removed at strategic locations in the base plate. The weight of the new chassis frame was 40 kg which was 43% weight reduction from the 1st design and 30% when compared to design 2.

 

The factor of safety of this design turned out to be 2.26. This was still a good factor of safety and it had the added benefit of being very light in weight. The next step would be to conduct stress analysis due to acceleration and deceleration of the pod on this frame.

Under an initial acceleration of 18.62 m/s2, the pod’s frame experiences a maximum Von Mises Stress of 55.18 MPa, resulting in a factor of safety of 5 relative to the yield strength of Al6061-T6. Whereas under a nominal deceleration of 21.19 m/s2, the pod’s primary structure experiences a maximum Von Mises Stress of 62.79 MPa, resulting in a factor of safety of 4.4 relative to the yield strength of Al6061-T6.

 Some advantage of this design:

  • Strong frame with factor of safety of 2.26 and anoptimized low weight design of 40 kg

  • Provides support for the carbon fiber shell at strategic locations

  • Designed in a way that makes it very easy to inspect, replace or repair components

  • Modular design

  • Easy and economical to manufacture

Chassis Design Consederations

March 12, 2017 | Rohan Thakker

Designing the chassis is one of the critical parts of our pod design as the chassis supports the entire structure of the pod and distributes the loads acting on the pod to the external forces and vibrations. For the designing process of the chassis, it is therefore important to select the optimum material, place the chassis supports in planned and strategic locations, maintain structural integrity and make it as light-weight as possible. We don’t want a very heavy chassis because that will increase the overall weight of our pod, resulting in a reduction of top speed and acceleration (Our eventual goal is to make the pod go as fast as possible). This post discusses the considerations our team has taken while designing the chassis, the evolution of our chassis system, the pros and cons of our system and future scope of work.

We have made the following considerations for our design:

1. Forces: The forces that act on the pod and how they are determined :

  • Pressure Force: The flow of air in the tube exerts a pressure on a moving pod. This pressure depends on the outer profile of the pod and the velocity of the pod. CFD simulations give us the pressure distribution over a pod moving through the tube at specific speeds. The pressure distribution is then imported into the Static Structural Solver of ANSYS Workbench to give us the stresses and deformations of the chassis caused due to these forces.

  • Forces due to acceleration and deceleration: The acceleration profile is taken into consideration to calculate forces due to acceleration and deceleration. The maximum values are extracted for each of them and are multiplied by the weight of the pod to determine the maximum force acting on the chassis.

  • Weight of the systems: The weight of different systems of the pod (Propulsion, Levitation, Braking, etc.) is assumed to be uniformly distributed over the chassis base plate for simplicity while extra weight is added for the calculations to take into consideration any access weight.

  • Frictional Resistance: The forces of friction generated due to the air drag and the wheels has been simulated and included in the chassis design. Forces caused due to friction while braking has also been taken into consideration.

2. Material Selection: It is critical to select a material that has the following properties:

  • Lightweight

  • Strong

  • Economical

  • Easy to manufacture

  • Machinable

  • Readily available

Al 6061-T6 Aluminum offers all these benefits and thus was selected as the material for the chassis.

3. Placement of supports and structural members: The simulations were started with analysis of a shell of the pod to assess the major areas of stress concentration. For the next design, supports and reinforcements were added at these locations to distribute stress and reduce material used. Analysis was again performed and this process was repeated till we got a highly-optimized design for the chassis.

4. Maximum deformation: Not only the maximum deformation should be within the elastic limit of the material but it should also ensure that the chassis does not come in contact with the I- beam or the track as this clearance is very low.

5. Maximum Von Mises Stress: The Maximum Von Mises Stress of the chassis should be much lesser than the yielding stress of the material and the design should a considerable factor of safety.

 

The primary components of the chassis and their functions are given below:

  • Ribs: Ribs provide lateral structural strength, structural connection points for longerons, and a frame to which carbon fiber shell can be mounted. The thickness of the ribs is taken to be 5 mm.

  • Longerons: Longerons provide longitudinal structural strength and distribute external loads. The cross section of the longerons is taken to be 12.7 mm x 12.7 mm square tubes.

  • Base plate: The Base Plate provides longitudinal structural strength and a mounting surface for all internal components of the pod. Thickness of base plate is taken as 5 mm.