Periodic Reporting for period 2 - ImpulsiveFlows (Impulsive Flows - beyond velocity and acceleration)
Reporting period: 2022-06-01 to 2023-11-30
The aim of the project is to study flows around accelerating and decelerating objects, i.e. impulsive flows. The classical description of forces exerted in an accelerating object, such as lift and drag, comprises of a quasi-stationary contribution and a so-called ‘added mass’ contribution; see e.g. [1]. Recent findings [2] indicate that this description is incomplete and that drag forces at high and prolonged levels of acceleration appear to be larger than given by conventional approaches. This has consequences for predicting loads on technical structures, such as wind turbine blades, monopiles, ships and offshore structures, especially when extreme conditions occurs, such as wind gusts.
To gain further insight into these impulsive flows, a novel type of flow facility is constructed that allows to move objects in a fluid at various rates of acceleration and jerk [3]. Robotic arms and gantries are used to move around objects and allow cameras to measure the fluid motion using modern optical flow measurement techniques, such as particle image velocimetry, or PIV [4]. Most research in fluid mechanics is carried out in flow facilities with a defined velocity, such as wind tunnels and towing tanks. The flow velocity, together with the dimension of the object and the kinematic viscosity of the fluid, define the flow Reynolds number. It is this Reynolds number that is used to scale the flow. But what should one do if there is not a well-defined velocity? This typically occurs during an accelerated motion where the velocity changes over a very large range. The flow facility is built to investigate the flow scaling based on acceleration, rather than velocity.
A secondary challenge is to perform optical flow measurements on the flow around a moving object. As the velocity may change rapidly, so does the fluid. Conventional PIV uses a fixed image magnification and exposure timing. This is suitable for flows with a given reference velocity, but unsuited in the case of accelerating flows.
In the project various types of impulse flows are investigated, where not only the linear acceleration is varied, but also the rotational acceleration. The outcome would lead to novel scaling laws for the drag and lift encountered in these highly unsteady motions.
1. Morison et al., J. Petrol. Technol. 2 (1950) 149
2. Grift et al., J. Fluid Mech. 866 (2019) 369
3. The variation of acceleration is called jerk.
4. Adrian & Westerweel, Particle Image Velocimetry, Cambridge University Press (2011)
To gain further insight into these impulsive flows, a novel type of flow facility is constructed that allows to move objects in a fluid at various rates of acceleration and jerk [3]. Robotic arms and gantries are used to move around objects and allow cameras to measure the fluid motion using modern optical flow measurement techniques, such as particle image velocimetry, or PIV [4]. Most research in fluid mechanics is carried out in flow facilities with a defined velocity, such as wind tunnels and towing tanks. The flow velocity, together with the dimension of the object and the kinematic viscosity of the fluid, define the flow Reynolds number. It is this Reynolds number that is used to scale the flow. But what should one do if there is not a well-defined velocity? This typically occurs during an accelerated motion where the velocity changes over a very large range. The flow facility is built to investigate the flow scaling based on acceleration, rather than velocity.
A secondary challenge is to perform optical flow measurements on the flow around a moving object. As the velocity may change rapidly, so does the fluid. Conventional PIV uses a fixed image magnification and exposure timing. This is suitable for flows with a given reference velocity, but unsuited in the case of accelerating flows.
In the project various types of impulse flows are investigated, where not only the linear acceleration is varied, but also the rotational acceleration. The outcome would lead to novel scaling laws for the drag and lift encountered in these highly unsteady motions.
1. Morison et al., J. Petrol. Technol. 2 (1950) 149
2. Grift et al., J. Fluid Mech. 866 (2019) 369
3. The variation of acceleration is called jerk.
4. Adrian & Westerweel, Particle Image Velocimetry, Cambridge University Press (2011)
For this project a novel flow facility is being constructed that can be used to study various impulsive flows with single/multiple bodies. The system nearly completes its construction. At this moment all 3 Ph.D. students are active in the project: Jesse Reijtenbagh, Nicola Savelli, and Lyke van Dalen.
Jesse Reijtenbagh is investigating the scaling of an accelerating object. While the new facility is under construction, a prototype facility was used to investigate the scaling of the drag force for an accelerating plate. A careful experiment was set up, using acceleration rates that span about one decade (0.1-1.6 m/s2). The experiments showed a novel scaling for the drag force that is proportional to the square root of the acceleration; this was not demonstrated before. This result is opposing what is written in all textbooks, namely that for an accelerating object the drag force, given by the added mass, is linearly dependent on the acceleration. The results were presented at the DisCoVor-2022 symposium and published in Physical Review Letters [5]. The new model not only predicts the drag force during acceleration, but also during the relaxation after the acceleration has ceased and the plate is moving at a constant velocity.
Nicola Savelli develops novel methods to extract the forces on a moving object in a non-inertial frame of reference from experimental data of the flow field measured with particle image velocimetry [4]. The main reason for this is that this enables the measurement of very small forces that fall outside the range of suitable force transducers. His first results were presented at the ISPIV-2023 symposium. The novel aspect is that the new method is applicable to accelerating objects and that the measured force is separated into contributions of viscosity, pressure, and instationary motion. The method is currently operational for quasi two-dimensional flows.
Lyke van Dalen will focus on scaling for accelerating and decelerating rotational motion, i.e. spin-up and spin-down. A simple experimental set-up is used, with co-rotating camera and illumination, to study the flow inside a spinning container with a small barrier. This geometry is a model for the inner ear [6]. First results were presented at the DisCoVor-2023 symposium.
In addition there are a number of side-projects that were performed by Bachelor and Master students.
5. Reijtenbagh et al., Phys. Rev. Lett. 130 (2023) 174001
6. Goyens et al., Biomech. Model. Mechan. 18 (2019) 1577
Jesse Reijtenbagh is investigating the scaling of an accelerating object. While the new facility is under construction, a prototype facility was used to investigate the scaling of the drag force for an accelerating plate. A careful experiment was set up, using acceleration rates that span about one decade (0.1-1.6 m/s2). The experiments showed a novel scaling for the drag force that is proportional to the square root of the acceleration; this was not demonstrated before. This result is opposing what is written in all textbooks, namely that for an accelerating object the drag force, given by the added mass, is linearly dependent on the acceleration. The results were presented at the DisCoVor-2022 symposium and published in Physical Review Letters [5]. The new model not only predicts the drag force during acceleration, but also during the relaxation after the acceleration has ceased and the plate is moving at a constant velocity.
Nicola Savelli develops novel methods to extract the forces on a moving object in a non-inertial frame of reference from experimental data of the flow field measured with particle image velocimetry [4]. The main reason for this is that this enables the measurement of very small forces that fall outside the range of suitable force transducers. His first results were presented at the ISPIV-2023 symposium. The novel aspect is that the new method is applicable to accelerating objects and that the measured force is separated into contributions of viscosity, pressure, and instationary motion. The method is currently operational for quasi two-dimensional flows.
Lyke van Dalen will focus on scaling for accelerating and decelerating rotational motion, i.e. spin-up and spin-down. A simple experimental set-up is used, with co-rotating camera and illumination, to study the flow inside a spinning container with a small barrier. This geometry is a model for the inner ear [6]. First results were presented at the DisCoVor-2023 symposium.
In addition there are a number of side-projects that were performed by Bachelor and Master students.
5. Reijtenbagh et al., Phys. Rev. Lett. 130 (2023) 174001
6. Goyens et al., Biomech. Model. Mechan. 18 (2019) 1577
The impulsive flow facility represents a unique and novel type of flow facility that is specifically designed for the investigation of flows around single and multiple moving objects with variable rates of linear and rotational acceleration. This comes with tailored optical methods to measure the instantaneous in a co-moving frame of reference. It is expected that the project results in the description of forces acting on objects that undergo a accelerating or decelerating motion, both at a constant and variable rate.