Steady Vs. Unsteady Computational Fluid Dynamics (CFD)

Rotating machinery like pumps, compressors, and turbines are the workhorses of numerous industries, driving everything from power generation to fluid transport. Optimizing their performance is paramount, and Computational Fluid Dynamics (CFD) has become an indispensable tool in this endeavor. However, the decision between steady and unsteady CFD simulations can significantly impact the accuracy and depth of the analysis, not to mention the cost and schedule. Let's explore the differences and applications of these approaches in the context of these critical machines.

MSI regularly collaborates with machinery manufacturers and system integrators by providing unsteady CFD analysis services or training.

Figure 1. Unsteady Simulation of Cavitation in a Boiler Feed Pump (Link to case study: https://www.mechsol.com/case-study/boiler-feed-pump-optimization-to-reduce-cavitation

The Dynamic Dance of Rotating Machinery:

Pumps, compressors, and turbines all share a common characteristic: rotating components that induce complex, often time-dependent flow patterns. This inherent dynamism necessitates a careful consideration of whether a steady-state or unsteady CFD approach is most appropriate. 

Steady-State CFD: A Snapshot of Average Performance

What it offers:

• Provides a time-averaged picture of flow behavior.
• Determines key performance parameters like efficiency, head, pressure ratio, and flow rate at specific operating points.
• Relatively computationally efficient, allowing for rapid design iterations.
• Useful for initial design assessments and approximate performance curve generation.

 Applications:

• Generating performance maps (e.g., head vs. flow rate for pumps, pressure ratio vs. flow rate for compressors).
• Evaluating overall efficiency and performance at design operating conditions.
• Analyzing pressure and velocity distributions.
• Identifying potential areas of high-pressure loss or flow separation.

Limitations:

• Ignores time-dependent phenomena like rotor-stator interactions, blade passing effects, and transient flow variations.
• Will have difficulty accurately predicting performance under off-design conditions or during start-up/shut-down.
• Cannot capture dynamic instabilities like cavitation, vorticity, surge, stall, or flutter.

Unsteady CFD: Capturing the Rhythms of Flow

What it offers:

• Captures the dynamic evolution of flow behavior over time.
• Reveals transient phenomena like rotor-stator interaction, vortex shedding, and blade vibration.
• Provides detailed insights into fluctuating pressures and velocities.
• Essential for analyzing performance under transient operating conditions.

Applications:

• Analyzing rotor-stator interaction and its impact on noise and vibration.
• Predicting cavitation inception and dynamics in pumps and turbines.
• Simulating surge and stall in compressors.
• Analyzing blade flutter and vibration in turbines.
• Investigating transient performance during start-up and shut-down.
• Analyzing the effect of variable speed operation.

Challenges:

• Significantly higher computational cost compared to steady-state simulations.
• Requires careful selection of time step and turbulence models to ensure accuracy and stability.
• Demands substantial computational resources and time.
• Post processing the large amount of data generated.

Figure 2: Cavitation-Induced Unsteady Pressure Auto-Oscillation in a Rocket Turbopump 

Choosing the Right Approach:

Steady-State:

• For initial design evaluations and performance mapping.
• When computational resources are limited.
• When the primary interest is in average performance at design conditions.

 Unsteady:

• For analyzing dynamic phenomena and transient behavior.
• When predicting performance under off-design or transient conditions, especially when a customer is looking for performance guarantees at off-design conditions.
• When evaluating the impact of instabilities and vibrations.
• When needing to analyze noise generation.

 Specific Considerations:

• Pumps: Cavitation, rotor-stator interaction, and transient start-up/shut-down are often critical, making unsteady CFD valuable.
• Compressors: Surge, rotating stall, and blade flutter are significant concerns, necessitating unsteady analyses.
• Turbines: Blade flutter, cavitation (especially in hydraulic turbines), and transient load changes are key areas where unsteady CFD is crucial.

Conclusion:

The choice between steady and unsteady CFD in pumps, compressors, and turbines hinges on the specific objectives of the analysis. While steady-state simulations provide valuable insights into average performance, unsteady simulations are essential for capturing the complex dynamic behavior that governs the operation of these machines. By carefully considering the flow physics and computational resources, engineers can leverage the power of CFD to optimize designs and ensure reliable, efficient operation.

Please contact MSI if you are interested in contracting a specific unsteady CFD analysis project or if your company wants to add, or update, your own unsteady CFD analysis capability.

Selected MSI case histories:

Optimization of Hydroturbine with CFD Modeling of Discharge Chamber

Flood Event Simulation On a Stop Log

Automated Design Optimization Of A Hydroturbine

CFD Analysis To Resolve Suction Induced Vibration Problems

Boiler Feed Pump Optimization to Reduce Cavitation

Design Optimization Of A Low Pressure Steam Turbine Stage

Heat Transfer Analysis Of Liquid Cooled Rocket Engine Assembly

Development Of A Vacuum Blower

Fluid-Structure Interaction and Hydraulic Analysis of A Pelton Turbine

Analysis Of Multiple Hydro Turbines In A Parallel Array