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CFD Assists Nozzle Propeller Design
Posted Wed April 02, 2003 @09:35AM
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Application by Moustafa Abdel-Maksoud and Hans-Jürgen Heinke, SVA Potsdam

Nozzle propellers are used in many types of ships and for dynamic positioning of platforms. Their operation at high thrust loadings and low inflow velocities requires high-tech designs. SVA Potsdam specializes in these propulsion systems and is investing in continuous research to ensure that its designs meet expectations.

The flow in the gap between the propeller tip and the inside wall of the nozzle strongly influences the performance of the propeller, and the greater the thrust loadings, the greater the influence. CFD allows visualization of these flows, which are difficult to study experimentally because the gap can be less than one millimeter. Unfortunately, the required computational effort increases rapidly as the thrust loading increases (due to slower convergence as the rotational speed of the propeller increases), and we have realized that combining experimental and numerical investigations can overcome the shortcomings of each method. The experimental data provide a good overview of the forces and moments for the whole range of operational conditions, while CFD reveals the detail of the flow for selected loadings.

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In our CFD model, we use the powerful rotor/stator capabilities of CFX-TASCflow. The stationary part includes the inflow and outflow regions and the nozzle; the propeller blade and part of the propeller shaft are included in the rotating part. The flow in and around the nozzle varies strongly with Reynolds number. As this increases, the pressure reduction near the leading edge is increased and the separation point near the trailing edge moves backwards. This leads to a change in the operating point of the propeller. To understand these phenomena fully, we undertook an extensive CFD analysis of the flow behavior of the nozzle at different thrust loading conditions and Reynolds number, investigating also different nozzle and propeller geometries.

The results compared well with both published data and LDV measurements behind the nozzle. To analyze the effect of scale on the performance characteristic of our design, we compared the numerical results of three full-scale propellers with the corresponding coefficients of a model propeller. The results showed that with increasing Reynolds number, the thrust coefficient of the nozzle increases (this effect is stronger for low thrust loadings), while the torque and thrust coefficients of the propeller decrease.

The flow velocity through the nozzle at full scale is relatively higher than at model scale. This increases the efficiency of the nozzle at full-scale and reduces the propeller loading (thrust and torque coefficients). Additional reduction of the propeller torque takes place due to the lower friction coefficient on the propeller blades at higher Reynolds numbers. This is one possible explanation for the strong Reynolds-number dependency of the torque coefficient of the ducted propeller in comparison to a free-running one.

The quality of propeller design is directly affected by the ability to accurately predict thrust and torque loading at full scale. The results of the numerical study give the allowed range of extrapolation coefficients that can be used to estimate the full-scale propeller performance based on model-scale data. However, because of the strong dependence of the propeller characteristics on the geometry (thickness, camber, pitch and skew distribution), the results of one numerical study cannot generally be applied to a ducted propeller of another design. Therefore, it is expected that CFD methods will be further involved in the propeller design process to assess the extrapolation of model results to the full scale.

nozzle propeller
A nozzle propeller in a Z-drive configuration. Propeller courtesy of Schottel Ruder Propeller system (SRP).

streamlines around hull
Streamlines around a ship hull with a ducted propeller.

pressure distribution
Pressure distribution on a nozzle propeller.

experimental measurement
Measuring the velocity distribution behind a nozzle propeller in the Potsdam cavitation tunnel.

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