CONTENTS
Circular blade cascade
Background
The research project aims to investigate the boundary layer on the vibrating surface thorough experimental and numerical methods. As investigated in the work from Bergan (2019) and Tengs (2019), the fluid-hydrofoil coupled system can be assumed as a one degree of freedom system (SDOF system). This means that the system is governed by Newton’s second law of motion and is therefore f(mass, damping, stiffness, force). Among these parameters the damping factor has not been much addressed in the literature despite being critical when runner vibration is around resonance frequency (Monette et al., 2014). The theory behind this phenomena has been studied in the paper from Monette et al. (2014). Hydrodynamic damping investigation in singular hydrofoil configuration with different shapes has been carried out by both Bergan (2019) and Coutu et al. (2012). Bergan (2019) have also investigated a linear blade cascade of 3 double-fixed hydrofoils in a cavitation free test rig. The single hydrofoil research has found a linear relationship between damping ratio and water velocity, and a different gradient of this relationship depending if water velocity is below or above the lock in region. When water velocity is below lock-in the linear relationship gradient is slightly positive and almost constant, while above lock in the gradient is largely positive. Moreover the almost linear relationship in the area of velocity below the lock-in is maintained also for the structural natural frequency while is somehow disrupted above lock-in region where no further trends could be founded. Regarding the linear blade cascade work Bergan (2019) has stated that three blade system behaves as a one bladed system while doubts have been raised on the behaviour of a circular cascade configuration. The interaction between fluid and the structure takes place at the interface/boundary layer, moreover a strong interaction between blades and the surrounding water, which led to change of damping characteristics (Trivedi & Cervantes, 2017) has been demonstrated. Further investigations were performed using different trailing edge profiles and their interaction with the vortex shedding (Sagmo, 2021). Flow characteristics were studied in detail including the turbulent properties for different Reynolds numbers. The research clearly indicated a radial arrangement of the hydrofoil is essential to mimic the the turbine blade effect (Pirocca, 2020). The radial cascade, aim of this PhD, will help to understand how blades react to forced excitation and the interaction between neighbouring blades, with a focus on the hydrodynamic damping effect for a circular configuration.
The phase 1 experiments will be carried out on a circular blade cascade which improved version of previous work in the Waterpower laboratory. The original work (2016 - 2019), as stated above, focused on a single hydrofoil test case and the hydrodynamic damping with respect to the flow Reynolds number was studied. Later, the work extended to three hydrofoils arranged parallelly to study the impact of nearby structure on the hydrodynamic damping. Moderate impact on the added mass was seen during this study. However, in the linear arrangement, the acoustic waves are normal to the blade surface. When it comes to hydraulic turbine, the blades are arranged in circular pattern, which is different from the linear arrangement of the hydrofoil. This work further extended to circular arrangement of the hydrofoils to study impact of circular pattern during resonance condition.
Timeline and progress
Rotating disc
Background
Need for robust mechanism to predict the blade resonance.
Available knowledge is limited, specifically vibration induced fatigue.
C. Trivedi, Engineering Failure Analysis, 77 (2017) pp. 1–22
Important objective is to understand the physics focusing on how surrounded bulk flow reacts to the resonating plate, and how wall proximity causes the change of natural frequency (added mass).
This will help us to develop mathematical relation in the context of natural frequency and the nearby structure.
The mathematical relation will be developed further for more complex situation, then turbine blade.
Objectives
- Investigate the change of natural frequency of a plate with respect to the proximity of the rigid wall.
- Investigate the flow physics around the resonating plate, focus on possible source and sink pattern.
- Interpret the flow pattern, wall proximity and the change of natural frequency (added mass).
- Develop correlation of point 3 and check Kwak’s theory holds true (or method presented by Askari et al.) and can be extended to the turbine blades.
- Prepare full scale research proposal using hypothesis of point 4 and outcome of this project.
Timeline and progress
Boundary layer
Background
C. Trivedi, Engineering Failure Analysis, 77 (2017) pp. 1–22
A classic textbook example of failure of large engineering structure is the Tacoma Narrows bridge disaster of 1940. An ultimate failure was related to self-excitation and resonance. The accident led researchers to rethink about the design approach. Although the present project has wide scope in mechanical engineering, the proposed research focuses on hydraulic turbines. Need for energy flexibility and interconnection with wind/solar energy have pushed hydro turbomachines to the limit. Turbines are subject to heavy resonance and forced excitation, which often results in ultimate (premature) failure. Then, the question is how to minimize the damage. Insofar, damping is determined a generic approach, engineering linear relation, based on damped natural frequency. However, boundary layer has essential role to create damping effect. For instance, when a structure reverberates, it dissipates kinetic energy to the fluid through boundary layer, i.e., fluid structure interface, and vice versa. This project aims to determine the damping effect that accounts boundary layer complexities. The project will carry out experimental and numerical investigations of boundary layer at a level of multi physics. Pressure, strain and velocity (PIV) measurements will be conducted on a turbine blade. The project aims to quantify the flow instability, mainly kinetic energy fluctuations, inside the boundary layer, and the role of fluid added damping. Three different test cases will be investigated: (1) circular blade cascade, (2) rotating disc and (3) planar flow on reverberating longitudinal plate.
Timeline and progress
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