6 Replies Latest reply on Jan 9, 2010 3:49 PM by Bill McEachern

    How to define roughness?

      Hi all,

      how to define different roughness to different surfaces (of different parts) inside an assembly. I have seen when we set uo flow simulation project there you see only one roughness parameter for the entire model. thanks

        • Re: How to define roughness?
          ok here it goes this can be done in boundary conditions and under real wall we specify different wall roughness
            • Re: How to define roughness?
              David Paulson

              Shankar,

               

              I know of no means to actually measure wall roughness.  You might find some published data that is based upon pipe with rough versus smooth walls.  The ability to input a value for wall roughness allows you to estimate a real life value based upon build-up of substances on the pipe wall over time.  Or it allows a means that you can adjust your calculated solution for existing systems so that the measured result is close to your prediction in Flow.  Kind of like calibrating your solution??

               

              I often use Flow to correleate field measurements to theoriticqal calculations.  Sometimes actual performance of systems is not as expected.  Flow can be useful in providing a solution, if you can "calibrate" the Flow output to the field measured condition.  Adjusting the wall roughness may be a means of calibrating the Flow solution to the actual condition.

               

              However, for initial designs, I usually set the value to "0", understanding that performance will deteriorate over time.

                • Re: How to define roughness?

                  hi david

                  yes thats true, but here the case is for me was to vary wall roughness to see weather hadany effet on heat trnsfr coeffiint

                    • Re: How to define roughness?
                      David Paulson

                      Shankar,

                       

                      I do not believe that wall roughness will have an effect on thermodynamic results in Flow.  Wall roughness can be due to either an inherent roughness of the pipe's inner wall or it can also be caused by deposits that build up over time on the pipe's inner wall.  If the roughness is only due to the pipe, then the heat transfer should not be impacted, but it could even be enhanced.  If the rougness is a build-up of deposits, like in the tubes of a heat exchanger, then the heat exchange will be impacted.  For this type of build-up most programs that deal with this allow the input of a "fouling factor" that compensates for the deposits.  So I don't think that wall roughness and fouling factor are the same thing.

                • Re: How to define roughness?
                  Bill McEachern
                  If we assume all the roughness in question are very much rougher than what would be considered aerodynamically smooth then increasing the roughness should increase the heat exchange as the boundary layer will have increased turbulence and hence better momentum and energy transfer to the free stream. The affect might be quite slight depending on the exact conditions - R'number. If it is strongly laminar probably would not see much affect, if it is strongly turbulent to begin with affect might be slight as well, if the increase in roughness causes the B'layer to trip from laminar to turbulent then the difference in rougtness may be a more significant affect.
                  • Re: How to define roughness?
                    Bill McEachern

                    You can also consult the technical reference in the validation examples 2, 3, & 4 - they all deal with wall roughness and one is about pipe wall roughness.