As renewable energy sources such as wind and solar power become increasingly integrated into modern power systems, hydropower units are expected to operate with greater flexibility to ensure grid stability. This shift poses new challenges for the stable operation of hydraulic turbines, particularly under wide load conditions where dynamic instabilities—such as pressure pulsations, vortex rope formation, and S-shaped characteristics (in pump-turbine)—can severely affect performance, efficiency, and structural safety. In recent years, advances in control strategies, flow diagnosis, and numerical simulations have provided new insights into the mechanisms and mitigation of these instabilities. This Research Topic aims to gather state-of-the-art studies on key technologies and methodologies that support the safe, efficient, and flexible operation of hydropower units under variable load conditions.

The increasing penetration of intermittent renewable energy sources has imposed stringent demands on the operational flexibility of hydropower units. To meet peak regulation and frequency support needs, hydropower plants are frequently required to operate over a broad load range, often in off-design and low-part-load conditions. However, such conditions can trigger various hydraulic instabilities—such as flow separation, rotating stall, and vortex rope phenomena—leading to increased hydraulic losses, vibrations, cavitation, and even mechanical failures. These challenges have drawn significant attention from both academia and industry. Recent advances in flow control techniques, real-time monitoring, CFD simulations, and model-based regulation strategies have opened new opportunities to improve stability and performance. However, further research is still needed to understand the complex unsteady flow behavior and to develop more robust, adaptive solutions for stable operation. This Research Topic aims to address these issues by collecting original research and reviews on hydraulic analysis, advanced control schemes, structural response evaluation, and innovative diagnostic techniques that contribute to the safe and efficient operation of hydropower units across wide load conditions.

This Research Topic focuses on the key technologies enabling the stable and flexible operation of hydropower units under wide load conditions. We welcome original research articles, reviews, and case studies that explore innovative solutions for hydraulic stability and performance optimization. Specific areas of interest include, but are not limited to:

1. Fluid Dynamic Performance Analysis of Renewable Energy Equipment

2 . Fluid–Structure Interaction Methods for Hydraulic Turbomachinery

3. Unsteady flow mechanisms and vortex dynamics in part-load operation

4 . CFD-based analysis and flow control strategies

5 . Advanced control and regulation technologies

6. Structural responses and fatigue under fluctuating loads

7 . Experimental methods and real-time monitoring techniques

8. Coupling of electrical and hydraulic systems in flexible operation scenarios

We encourage contributions that bridge theory and practical applications, offering insights into current challenges and future trends. Submissions that provide interdisciplinary approaches combining fluid mechanics, control engineering, and system optimization are particularly welcome.

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Advanced Technologies for Ensuring Hydropower Unit Stability under Variable Loads