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)
