10 Replies Latest reply on Nov 17, 2014 1:55 PM by Txon Aguirre

    Marine propeller simulation

    Choong Waiheng

      Hi forum,

       

      I having a problem with the marine propeller simulation.   As the propeller simulated without local mesh refining (pic 1), the result shown a ideal flow pattern.   As the local mesh refinement been applied, the flow pattern (pic 2) has resulted unlogical.   Can anyone help?  

       

      Without local mesh refinement

      Untitled.png

       

      With local mesh refinement

      Untitled2.png

       

      Can anyone help?

        • Re: Marine propeller simulation
          Chris Michalski

          My 1st thought is that the rest of your mesh is too coarse or your local mesh is too fine.  When it refines the local mesh it is seeing a large amount of recirculation.  This is likely due to the pressure difference from upstream to downstream across much smaller cells.

            • Re: Marine propeller simulation
              Choong Waiheng

              Thank you for your response.   I will try to readjust the meshing configuration.   Shall update the result.    Thank you.

                • Re: Marine propeller simulation
                  Jared Conway

                  Also, can you describe the setup? Local or global rotation, internal or external, size of comp domain....etc.

                   

                  It might also be good to look at cut plots. Flow trajectories can throw you off sometime. Velocity, pressure and mesh are good ones to look at.

                   

                  You may also want to check the swx kb. There are a couple of good articles on rotation. It may be a rotating reference frame check box that is the issue. Looking at cut plots would show you that the results are right and that it was just an option with the flow trajectories.

              • Re: Marine propeller simulation
                Txon Aguirre

                It seems there is no flow incoming velocity. Looks you've set up a stationary propeller condition. Under that condition, most flow is recirculated around the propeller, as shown in your pic number 2. Your propeller blades are over loaded, cavitation can be very big as well as structural stress.

                 

                Without entering complex fluid dynamics, take a brief look, from a simple geometrical point of view.

                 

                The pitch angle at the root of that propeller is around 45º, and can be assumed to be zero at the tip, with a linear distribution. Hence at 70%R, it can be taken as 17º.

                 

                From the simple mechanical screw approach: tan(17º)= L/(2*PI*0.7R); L= Pitch lenght; R=Propeller radius

                 

                tan(17º)= L/(2*PI*0.7R): L= 2*PI*0.7R*tan(17º)

                 

                Rotational and axial speeds, keep the same relation, hence ....... (TAKE CARE. THIS IS ONLY A VERY,VERY ROUGH APPROACH)

                 

                Va= 2*PI*0.7*tan(17º)*R*N: Va= advance speed, N= propeller Revs/second.

                 

                Va= 1.334 R*N

                 

                This speed, calculated in a very simple way, gives an idea about the required advance speed of your model for reasonable working conditions.

                 

                 

                Regards

                • Re: Marine propeller simulation
                  Txon Aguirre

                  Yes, of course.

                  What is happening?:

                  If no axial component is provided, the blades work in the "stall" condition(angle of attack too big, as if a stopped plane was horizontally falling down).

                  Ahead of the prop. a very low relative pressure area develops, that cannot be "refilled" on time, leading to high cavitation.

                  Astern of the prop. expulsion flow is inmersed into a static fluid field, that exerts a very high resistance for moving, and makes pressure rise very much.

                   

                  As there is no axial flow, the only way both pressure fields (high astern and low ahead) can get balanced, is by means of a parasitic flow, runing around the propeller tips from the compression to the suction area.(The toroidal flow pic attached by Choong).

                   

                  If instead of water, the fluid was air, the effect would be less noticiable. Air is compressible, while water isn`t.

                   

                  Why?:

                  The thrust is provided by the blades, and the blades develop the thrust by means of LIFT, as in a wing. In order to produce this lift, the fluid must enter the blade withing a range of angles that is limited by the design (As the angle of attack in a plane's wing).

                   

                  This entering speed is a vector, resulting from the blade motion and has two main components. Va= Advance velocity (The axial component), and Vr=tangential speed (coming from the blade rotation and its radius). Both in blue at the pic.

                  From the point of view of the blade (relative motion), Both components have inverse direction. Va in yellow and Vr in red, giving a resultant speed vector, in green. The blade inflow speed and angle (hydrodynamic pitch).

                   

                  Depending on the value of each one (Va and Vt), the value and angle of the green vector will change.

                   

                  By combination of the Inflow vector (green) and the geometric pitch, in red, the angle of attack results, in blue.

                   

                  The LIFT, and hence the THRUST results from the circulation of flow on each blade profile, and this, only happens if the angle of attack is inside the limits given by each profile specifications.

                  If Va=0, No axial flow, the flow hydrodynamic pitch will be 0, and hence the angle of attack= geometric pitch.

                   

                  Propellers are designed for best performance at a given angle of attack, that is achieved when the ship or the plane is moving (There is a "Velocity of advance"). It makes no sence designing a propeller for static conditions. This would be a fan ¡

                  PropSpeeds.jpg

                   

                  My recomendation.

                  Yes, test propellers for the designed speed. Static condition tests are conducted for "Bollard pull" (BHP) measurements, acceleration capacities, manouverability ....... in all those cases, a general use, FPP (Fixed pitch propeller) will work very badly.

                   

                  The first stage must be getting the best propeller under the designed cruising conditions.

                   

                  The propeller that Choong has presented is a light boat high speed one. Thats normal that under static conditions, the wake looks like a mess.

                   

                   

                  Kind regards

                    • Re: Marine propeller simulation
                      Jared Conway

                      interesting, so you're saying when physically tested there is a flow rate applied to the "water tunnel"? i've only had one experience where we tested prop for a customer on a load cell type device where the prop spun and generated the flow and they measured the thrust.

                        • Re: Marine propeller simulation
                          Txon Aguirre

                          Hi again Jared:

                          I say that there are several tests to be performed for studying a propeller, or for final design stages.

                           

                          Static condition (Turning ahead and astern):

                          STATIC: No advance velocity.

                          For ship acceleration,  towing and manouverability performances. Maximum blades structural stress condition.

                           

                          SERVICE: Range of advance coeficient "J = Va/(ND)": Va= advance velocity, N= RPM/second, D= Propeller diameter.

                          Propeller performance for a variety of RPM vs Speed.

                           

                          CRUISE: Projected Va, RPM

                          For a good design, best efficiency is achieved under this conditions.

                           

                          Standard propeller testing channels, are designed more or less as a squared tube, with two vertical and two horizontal "pipes", forming a closed loop.

                          At the lower horizontal leg, there is located a water pump, for providing the desired advance velocity.

                          At the upper branch, is where the propeller to be tested is placed.

                           

                          Most times, those tests are performed for the "open water" condition, where no ship's hull influence is present. Hence a kind of fluid fairing arrangement is needed ahead of the prop.

                          From another point of view, for naval propellers, cavitation is a major issue, hence, the whole device internal environment can be controlled (pressure/temperature), in order to simulate different, working conditions.

                           

                          What I mean in relation to the simulation presented by Choong, is that under static conditions, the results from Pic-2 are logical and not those from Pic-1.

                          In the second case(Pic-1), a too coarse mesh has not been able to show the recirculation, that certainly is happening.

                           

                          Regards