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  1. J

    CFD analysis of a starfish?

    Even if you are only interested in a comparison between two competing designs, I strongly recommend you to go for a full scale test, specially because it is possible to do. This will give much more substance to your research and will probably save you time and criticism.
  2. J

    Fatigue analysis:frequency domain vs time domain

    I might have, what are you looking for?
  3. J

    Fatigue analysis:frequency domain vs time domain

    I will try to clarify based on DNV standard OS-F201 Dynamic Risers (http://exchange.dnv.com/publishing/Codes/download.asp?url=2010-10/os-f201.pdf" [Broken]). In fact, there are no explicit requirements to design based solely on a time domain analysis, but there are three interesting tables on...
  4. J

    Fatigue analysis:frequency domain vs time domain

    I did not state that, what I am saying is: "Often, frequency domain analysis cannot be replaced by a time-domain analysis". You are very right here, except that you are implying that by linearizing you are inevitably\permanently losing the analysis accuracy, which is not always true...
  5. J

    Fatigue analysis:frequency domain vs time domain

    The decision whether to go for a frequency domain analysis or a time domain analysis is not straightforward, and there are some main factors you should account for. I will try to summarise, but beware they will not be enough to assure anyone that the "best" approach will be attained. Often...
  6. J

    Help with plot for mystery

    Given the right conditions, this "jetstream" from a blowout, have a greater probability of sinking the overpassing boat, due to a sudden loss of buoyancy caused by the water vapor and the mixed expanding gases reaching the surface, rather than actually sand blasting the hull.
  7. J

    Project help

    The homepage is now available at: http://hovercrafthomepage.blogspot.com/p/hovercraft-design-calculator.html
  8. J

    Contact Stress - Can we add it up?

    Also, look for Hertzian Stress, Tribology and Wear to get a better view about this interesting subject. What is the required factor of safety for the contact stress you found? Usually, the allowable stress is 4 for linear contact and 5.5 for point contact, but this varies a lot depending...
  9. J

    Plate deflection theory vs FEA: same problem, same parameters, different results

    You're welcome. For this to work, you cannot restrain more than one degree of freedom. A simply supported boundary condition will restrain only the vertical direction.
  10. J

    Plate deflection theory vs FEA: same problem, same parameters, different results

    No, it is different. Clamped-Clamped will restrain the rotations also, therefore reducing the maximum deflection. *Re-run as simply supported.
  11. J

    Plate deflection theory vs FEA: same problem, same parameters, different results

    I tried http://www.efunda.com/formulae/solid_mechanics/plates/calculators/SSSS_PUniform.cfm#Results and got 53.3nm for mxn=32x32 which agrees with your result. I solved the problem with CosmosWorks shell elements and got 53.35nm. Are you working with shell or solid elements ? I tried with...
  12. J

    Designing Moon Buggy frame need help

    According to the rules, flywheels are not allowed. If you implement a propeller, it will act as a flywheel also, even with a very low energy storage capacity and your design might not be allowed in the competition. Also, the power source is the same both for the wheels and the propeller...
  13. J

    Designing Moon Buggy frame need help

    The same for me, I had the wrong impression by the project title and thought about something much more substantial, I should have researched or asked for the rules first. About FSAE: http://students.sae.org/competitions/formulaseries/
  14. J

    Designing Moon Buggy frame need help

    Yes, during launch every gram counts, but he mentioned FSAE, which is for students. This is a chance to get involved in a real project before graduation, and certainly there are rules establishing all the requirements, which probably take into account everything you pointed out including self...
  15. J

    Engines: Torque Lost To Friction

    The speed I'm talking about is the primary chain linear speed, not the final bike speed, just to make it clear in case there is any misunderstanding. The chain maximum linear speed is related to the engine maximum rpm and primary sprocket size. Above 6000rpm is recommended to swap the...
  16. J

    Engines: Torque Lost To Friction

    Another interesting point is the engine rotation direction that will be reversed depending on whether you are using a chain driven primary or a gear mesh.
  17. J

    Engines: Torque Lost To Friction

    Usually transmission chains should not be applied where the chain link will achive speeds beyond 25m/s (56mph), so for motorcycle engines capable of +6000rpm a geared primary is more common. If the sprocket is 80mm in diameter and the engine achieves 6000rpm the chain speed will be close to...
  18. J

    How to counteract the rotation?

    You asked "How to counteract", but actually you are asking about the needed power to counteract. The answer is it is much less than the total available power. The demanding torque to keep the rotor at a constant given rotating velocity is mainly related to the drag force component generated by...
  19. J

    Engines: Torque Lost To Friction

    I never saw a motorcycle engine with a chain driven primary. From which bike is the first picture ? What is the maximum RPM ?
  20. J

    Designing Moon Buggy frame need help

    I think they are supposed to demonstrate a fully working vehicle with all equipment and accessories, also there are good natural and artificial test beds on Earth that almost mimic the conditions on mars or the moon. The extra frame weight should not be a drawback as it is much less than the...
  21. J

    Designing Moon Buggy frame need help

    The frame geometry is usually a consequence of the components you want to carry, suspension type, driver and passenger seats and envelope, drivetrain supports, motor supports, drive controls, batteries, antennas, instruments, equipments, payload area, crash area and so on. You might start by...
  22. J

    Using golf ball dimples to decrease vaccuum drag on motor vehicles and aircraft.

    mugaliens: Insects versus birds: the great divide http://www.citeulike.org/group/918/article/790560
  23. J

    Torque Calculation to Open A Door

    I suggest you to adopt linear angular acceleration\deceleration phases with constant angular velocity in-between and ignore energy losses related to the interaction with the surrounding air or other fluid, hence considering the only parasitic resistance to movement is the hinge friction. Assign...
  24. J

    Question about velocity and motors

    It looks like this is a gear\rack mechanism, a very simplified approach will be: Considering the mass path is perfectly horizontal, negligible rotational inertia, no friction, starting at max power and max rpm... Fmax = Tin x i x n / r F -> Max Traction Force [N] Tin -> Motor Input...
  25. J

    Question about velocity and motors

    Hi FES, if this is your homework, it should go to the homework section. Add a sketch or drawing if possible. Think different, if you had to lift the same mass 0.45m in the vertical plane, frictionless and disregarding inertia, how would you solve the problem ? What would be the acceleration...
  26. J

    A fatigue analysis problem

    Based on Peterson's Stress Concentration Factors 2nd Ed. I'm getting Kt between 2.51 and 2.58 under bending only, for D=25mm, d=17mm and r=0.5mm: https://efatigue.com/ (click on stress concentration) According to the drawing, it seems that the bearings are positioned where d=17mm, is this...
  27. J

    Fatigue Analysis problem

    Don't expect to grab the results so fast, either learning your way trough a suitable hybrid analysis or sticking to the time domain will cost you some time. About 15+ years ago, time domain analysis were almost unbearable even with relatively small load histories and simple models, so several...
  28. J

    Engg Mechanics

    Now you are being specific. I'm sure you will get replies from the physicists in this forum, if you direct your thread to them.
  29. J

    Need information how with simple technology make sticky fly paper

    Thanks, now I get the picture. Unfortunately, I'm not of much use here, so good luck then.
  30. J

    Fatigue Analysis problem

    That will probably be your best option, changing back to the time domain for the rainflow cycle counting. If you stick to the frequency domain, there are other stochastic approachs for keeping the analysis reliability when computing structural damage, like working with a Rayleigh distribution.
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