2 Replies Latest reply on Nov 9, 2016 11:03 AM by Bill McEachern

    propeller - weird forces explaining weird results

    Flegmatoid Zoid

      While doing a number of CFD simulations on variety of propellers, I've observed that:

      - Solidworks results differ from theoretical (ideal) calculations by ~ 50%

      - Solidworks results differ from real calculations by ~30%

       

      I.e., 48" diameter (1.2m), 8.5" pitch (21cm), 2400RPM.

      Theoretical maximum (Abbott f-la): 153N (or 17.2 kg) of Thrust.

      By considering the propeller efficiency (70%-80%), expected results are in range from 107N to 122N

      Solidworks gave 87N.

       

      To further debug the problem, I've ran a force analysis on each of the propeller surfaces.

       

      The results were rather surprising. The bottom surfaces seem to generate the opposite thrust of ~1/3 of upper surfaces' thrust.

      My physics knowledge can't explain this.

       

      By excluding these opposing (2x 25N) values, we would get ~138N.

      That would be a fairly good estimate, because it would indicate a propeller efficiency of ~90%.

      This is ok, because FEM cannot account for non-linear losses.

       

      Are my assumptions reasonable?

        • Re: propeller - weird forces explaining weird results
          Amit Katz

          If you want to "debug" the force calculation results, you need to take a look at the pressure profiles, since that's how the software calculates the forces. I guess the only way to judge the results would be to compare the CFD data to some experimental smoke visualizations or PIV analysis.

          • Re: propeller - weird forces explaining weird results
            Bill McEachern

            Well, actually a CFD calc is inherently non-linear as it iterates to get a solution. The guy that did this calc here had some rather reasonable success in comparing to experimental data (https://forum.solidworks.com/message/684433?et=watches.email.thread#comment-684433). And again the rotating frame approximation is not an ideal approximation for axial flow machines though it should be not a rally bad estimate. In my experience, which I have to admit is not extensive in axial flow devices but it isn't zero either, it is a non trivial effort to come up with a good estimate via a hand calc. When I have done it in the past you essentially build a momentum model of the rotor using the 2D lift and drag coefficients. then you put in "factors" to account for the tip loses and pressure distribution roll off at the tip. You used to be able to get a program from DOE/ NREL - I think it was called PROP PC. It works as outlined above but with a higher degree of sophistication. I just don't know how reasonable it is to just do a swag (scientific wild ass guess) and say the code doesn't work. Not saying that your calc was a swag but it kind of sounds like one since no mention is given to what sections comprise the spanwise twist, thickness & chord distributions which all need to be considered in some regard in making a an estimate of the thrust and torque at least in my humble opinion. But maybe it is a more sophisticated approximation than it appears.