Flightstream：Why does the reference velocity have such a large impact on the calculation result Fx?

Dear

When I simulated the propeller, the thrust varied significantly with the reference velocity. When I set the velocity to be the same as the free flow velocity, the thrust was smaller than the simulated value. Another issue was that as J increased, the thrust did not decrease as expected, but instead increased.

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Accepted answers

rgiri

Hello,

Thanks for contacting Siemens/Altair FlightStream support. Is your propeller simulation steady/unsteady? The reference velocity is a critical parameter. It is also used in the computation of coefficient of pressure values that are then used to compute aerodynamic loads and hence you need to be very careful with the value you set.

Generally, for steady-state simulations, the reference velocity is typically matched to be the free-stream velocity. If the user changes the latter, they must also remember to change this reference velocity, or else the coefficients will be incorrect.

For unsteady simulations i.e. rotors in hover/propeller, the reference velocity should be set to the tip speed. This prevents the “minimum Cp” from getting clipped prematurely since we calculate surface Cp as dP/(0.5*rho*v_ref^2). Alternatively, you could set the min. Cp in the settings to a really small number like -1e5. But the former is preferred. Typically, the user can choose the largest velocity of the simulation (free-stream, rotary motion, slipstream etc.) and use that as the reference velocity. This ensures that all coefficient of pressure values based on reference velocity remain small and reasonable numbers.

You'll find all the information about reference velocity in the FlightStream manual under Solver→ RunTime Settings → Reference velocity

There is a meshing propeller in HyperMesh for use in FlightStream tutorial that you can follow

Meshing Propellers for FlightStream using Altair Hypermesh

While there is no specific propeller analysis in FlightStream, you can follow the unsteady analysis of a hovering rotor in FlightStream (certain aspects might be similar to your case) tutorial.

FlightStream Tutorial: Rotor in Axial and Edgewise Flight

Let me know if you have any further questions.

Thanks & Regards

Ritangshu Giri

All comments

whzhao

Could you provide a tutorial on propeller simulation?

rgiri

Hello,

You'll find all the information about reference velocity in the FlightStream manual under Solver→ RunTime Settings → Reference velocity

There is a meshing propeller in HyperMesh for use in FlightStream tutorial that you can follow

Meshing Propellers for FlightStream using Altair Hypermesh

While there is no specific propeller analysis in FlightStream, you can follow the unsteady analysis of a hovering rotor in FlightStream (certain aspects might be similar to your case) tutorial.

FlightStream Tutorial: Rotor in Axial and Edgewise Flight

Let me know if you have any further questions.

Thanks & Regards

Ritangshu Giri

whzhao

Thank you so much for your answer, I learned a lot.

Because FlightStream takes viscosity into account. I'd like to simulate a propeller with ducts and a front stator under water. Do you think this is feasible? My main goal is to calculate the thrust and torque coefficients under different advance rates. I tried the simulation, and the wake was very beautiful, but the thrust coefficient differed significantly from the CFD simulation value.

I'd prefer to use a steady-state method for calculation because it's very fast.

Thanks & Regards

whzhao

rgiri

FlightStream can simulate a propeller with ducts and a front stator under water but it affects the simulation, if you are close to the surface of the water where the free surface has an impact. Then you need to use the free surface modeling in FlightStream. If you assume the domain to be >3 diameters away from the free surface, you don't need that feature and can use normal FlightStream.

Can you share your FlightStream simulation file (*.fsm)? What are the values of thrust co-efficient you get from CFD and FlightStream respectively?

Thanks & Regards

Ritangshu Giri

whzhao

Thank you very much for your answer. Below is my rotor model, which I modeled using OpenVSP. The scenario I want to simulate is one where the rotor is completely submerged underwater, but not near the surface of water. At a rotational speed of 1200 r/min and a advance rates(J) of 0.38, the free-flow velocity is 1.35 m/s, and the thrust coefficient of CFD is 0.45,but the thrust coefficient of FlightStream is 0.06.When the advance rates(J) is 1, the free-flow velocity is 3.54 m/s, and the thrust coefficient obtained from the CFD simulation is 0.226,the thrust coefficient of FlightStream is 0.206.

Thank you so much for your help, I am extremely grateful.

Thanks & Regards

whzhao

rotor.fsm

rgiri

Hello,

Unfortunately, I cannot download your model. Can you reattach it again? Since, it's a propeller with ducts and a front stator, it would be more advisable to run an unsteady simulation and then compare it with CFD results at different advance rates.

You can follow the rotor motion tutorials that I'd shared earlier.

Thanks & Regards

Ritangshu Giri

whzhao

I'm very sorry, I don't know why the upload failed. I've re-uploaded it now, please try downloading again. I'm using FlightStream 2026.I also uploaded the .P3d file.

Thanks & Regards

whzhao

rotor.fsm

rotor.p3d

whzhao

Are you able to successfully download the uploaded file now? If so, could you please take a look at this example when you have time?

Thanks & Regards

whzhao