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Project:         Study of boundary layer on reverberating hydrofoil at natural frequency 
Project acronym: BOUNDARYLAYER
Type:            Internally finance project
Call:            HORIZON-CL5-2023-D3-01-13
Finance:         NTNU (EPT)
TRL:             3 - 5

Start:           01 July 2020
End:             31 December 2030
Project cost:    1.3 Million Euro
Coordinator:     Chirag Trivedi
Email:           chirag.trivedi@ntnu.no


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.


CONTENTS

Project team

Chirag Trivedi leads the Fluid Structure Interaction Research Group, based in the Waterpower Laboratory. He joined the Waterpower Laboratory in April 2012 as an exchange doctoral researcher, then as a Postdoctoral Fellow in August 2014, and later as an Associate Professor in July 2020. With over 15 years of experience in Francis turbines and reversible pump-turbines, Chirag coordinates the group’s activities and supervises students. He is actively involved in both numerical and experimental research, as well as in developing the test rig for the project.

Gabriele Gaiti is is a PhD candidate who joined the research group in August 2021. He conducts fundamental research on fluid-structure interactions, focusing on hydrodynamic damping on hydrofoils. Gabriele has developed a circular blade cascade for damping and vibration measurements, enabling the measurement of pressure, velocity, and strain in hydrofoil test sections. Additionally, he is conducting numerical simulations of one-way fluid-structure interactions on hydrofoils.

Dadi Ram Dahal is a PhD candidate who joined the research group in September 2022. He conducts fundamental research on fluid-structure interactions, with a focus on the boundary layer on resonating surfaces. Dadi has developed test rigs for boundary layer research, enabling the measurement of pressure, velocity, and strain in hydrofoil test sections. Additionally, he is conducting numerical simulations of one-way and two-way fluid-structure interactions on geometrical shapes such as circular discs and rectangular plates.

Jørgen Heggeseth Bakkeng writes master's thesis during spring 2025. The title of the thesis is "Study performance characteristics of benchmark test rig at different Reynolds number". Jørgen will carry out the experiments on the test rig designed for planar flow at different Reynolds numbers, including calibration of the important instruments equipped on the test rig. Main task is to determine the performance of the rig and create benchmark measurement data. This data will help to understand the rig characteristics during the FSI measurements with PIV and boundary layer.

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 experiments will be conducted in a blade cascade, an improved version of previous work in the Waterpower Laboratory. The original work (2016 - 2019) focused on a single hydrofoil test case, studying hydrodynamic damping with respect to the flow Reynolds number. Later, the research extended to three hydrofoils arranged in parallel to examine the impact of nearby structures on hydrodynamic damping. This study observed a moderate impact on the added mass. However, in the linear arrangement, the acoustic waves are normal to the blade surface. In hydraulic turbines, the blades are arranged in a circular pattern, which differs from the linear arrangement of the hydrofoil. This work further extends to a circular arrangement of the hydrofoils to study the impact of the circular pattern during resonance conditions.

Timeline and progress


Rotating disc

Background

Our prior studies in the Waterpower Laboratory clearly indicated that a nearby wall significantly impacts the added mass of neighboring structures. Hydraulic turbines consist of several components, each playing a critical role in altering the added mass and eigenfrequencies. Additionally, the rotating turbine runner adds a new dimension to the complexity of the current fluid-structure interaction. Available knowledge is limited, particularly regarding vibration-induced fatigue. We need a robust mechanism to predict blade resonance. This research gap is addressed in the article by C. Trivedi (Engineering Failure Analysis, 77, 2017, pp. 1–22). To tackle these challenges, we have developed a simplified test rig with a rotating disc that allows us to study various influencing parameters. The test rig enables us to change the distance of the nearby wall, angular speed, and submergence level.

Determine the impact of nearby wall on the natural frequency of the blade type structure.

The primary objective is to understand the physics of how the surrounding bulk flow reacts to the resonating plate and how wall proximity affects the natural frequency (added mass). This understanding will help us develop a mathematical relationship between natural frequency and nearby structures. This mathematical relationship will be further refined for more complex situations, such as turbine blades.

Objectives

  1. Investigate the change of natural frequency of a plate with respect to the proximity of the rigid wall.
  2. Investigate the flow physics around the resonating plate, focus on possible source and sink pattern.
  3. Interpret the flow pattern, wall proximity and the change of natural frequency (added mass).
  4. 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.

Timeline and progress


Boundary layer

Background

Damping is divided into three categories: (1) fluid added (hydrodynamic) damping, (2) structural (friction) damping, and (3) material damping. Hydrodynamic damping depends on changes in mode shape due to fluid pressure, convection through vortex shedding, viscous effects within the boundary layer, flow velocity, surface roughness, proximity of nearby structures, and submergence level. When a structure is subjected to a specific mode shape, it deforms, affecting the flow field, particularly near the antinode boundary of the vibrating structure. This results in rapid changes in pressure and velocity. It is crucial to understand what happens in the boundary layer when a structure vibrates at resonant frequency and undergoes different mode shapes, such as changes in viscosity, inertia, and shear stress within the boundary layer. During resonance, amplitudes near the vibrating wall follow a sinusoidal pattern, with small amplitudes at the node point and high amplitudes at the antinode point. Consequently, the pressure gradient constantly shifts from favorable to adverse, along with changes in Reynolds stresses and viscous effects. In the boundary layer, a three-dimensional fluid element travels upstream and downstream as the flow accelerates and decelerates depending on the mode shape. Reverse flow occurs in regions of low kinetic energy (near the vibrating node point), and large eddies bring outer-region momentum towards the wall, supplying some downstream flow. Generally, three regions are created when backflow occurs.

The goal of this work is to close the knowledge gap on the behavior of hydrodynamic damping, focusing mainly on changes in the boundary layer during resonance and its effect on damping. Boundary layer flow instability, primarily caused by kinetic energy functions resulting from high-frequency vibration of the blade structure, will be studied to understand the relationship between the boundary layer and the damping effect. To achieve this, coupled fluid-structure interaction simulations using ANSYS will be conducted to understand the flow physics around the vibrating body. Then, a small test rig with simple geometry will be built in the laboratory, where PIV measurements will be implemented for flow characteristics. Excitation will be provided by piezoelectric patches, and the response will be registered with the help of strain gauges. Later, the experiments will be scaled to a hydrofoil-type structure to study the impact of the pressure gradient. Stepped sine frequency excitation will be used to avoid transients. For numerical investigation, high-quality simulations will be used. Initially, the simulation will be carried out using a relatively simple model, and complexity will gradually be increased to achieve the desired results.

The boundary layer plays an essential role in creating the damping effect. We have developed a dedicated benchmark test rig in the Waterpower Laboratory to study flow phenomena in an isolated environment. The test rig is highly versatile, allowing us to conduct numerous experiments addressing the fundamentals of fluid dynamics and fluid-structure interactions. We plan to use this test rig with a rectangular cross-section and aim to integrate a reverberating longitudinal plate into the test section to investigate the boundary layer at different Reynolds numbers.

Investigate the effect of wall vibration on the instability of boundary layer with the variation of, velocity, and static pressure of flow, and study the relation of instability in boundary layer and hydro dynamic damping to establish an empirical relationship between them.

Timeline and progress

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