ORCID Profile
0000-0001-6058-9935
Current Organisation
University of Oxford
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Publisher: Wiley
Date: 17-03-2021
Abstract: The deformation behavior and evolution of strain distributions of flat metal sheets subjected to the high‐strain forming process of linear flow splitting (LFS) are studied using experimental and numerical techniques. The new tracer gradient method for the mapping of material flow based on diffusional concentration gradients is proposed. The method is validated using theoretical predictions for rolling of a sheet and shown to overcome the limitations of previous techniques. A parametric finite‐element model for LFS of a HC800LA grade steel is developed and validated against the results of the tracer gradient method. A sensitivity study is undertaken to investigate the effects of strain‐hardening behavior and sheet thicknesses on the LFS process. A good agreement between experimental and numerical results is obtained, with the friction between rolls and sheet found to be a critical parameter in the modeling of the process. It is further observed that the formation of the characteristic steady state in the LFS process is linked to the material‐hardening behavior and not the geometry of the sheet.
Publisher: Cambridge University Press (CUP)
Date: 06-10-2016
DOI: 10.1017/JFM.2020.674
Publisher: Cambridge University Press (CUP)
Date: 30-09-2021
DOI: 10.1017/JFM.2021.766
Abstract: The limit of power extraction by a device which makes use of constructive interference, i.e. local blockage, is investigated theoretically. The device is modelled using actuator disc theory in which we allow the device to be split into arrays and these then into sub-arrays an arbitrary number of times so as to construct an $n$ -level multi-scale device in which the original device undergoes $n-1$ sub- isions. The alternative physical interpretation of the problem is a planar system of arrayed turbines in which groups of turbines are homogeneously arrayed at the smallest $n\\mathrm {th}$ scale, and then these groups are homogeneously spaced relative to each other at the next smallest $n-1\\mathrm {th}$ scale, with this pattern repeating at all subsequent larger scales. The scale-separation idea of Nishino & Willden ( J. Fluid. Mech. , vol. 708, 2012 b , pp. 596–606) is employed, which assumes mixing within a sub-array occurs faster than mixing of the by-pass flow around that sub-array, so that in the $n$ -scale device mixing occurs from the inner scale to the outermost scale in that order. We investigate the behaviour of an arbitrary level multi-scale device, and determine the arrangement of actuator discs ( $n\\mathrm {th}$ level devices) which maximises the power coefficient (ratio of power extracted to undisturbed kinetic energy flux through the net disc frontal area). We find that this optimal arrangement is close to fractal, and fractal arrangements give similar results. With the device placed in an infinitely wide channel, i.e. zero global blockage, we find that the optimum power coefficient tends to unity as the number of device scales tends to infinity, a 27/16 increase over the Lanchester–Betz limit of $0.593$ . For devices in finite width channels, i.e. non-zero global blockage, similar observations can be made with further uplift in the maximum power coefficient. We discuss the fluid mechanics of this energy extraction process and examine the scale distribution of thrust and wake velocity coefficients. Numerical demonstration of performance uplift due to multi-scale dynamics is also provided. We demonstrate that bypass flow remixing and ensuing energy losses increase the device power coefficient above the limits for single devices, so that although the power coefficient can be made to increase, this is at the expense of the overall efficiency of energy extraction which decreases as wake-scale remixing losses necessarily rise. For multi-scale devices in finite overall blockage two effects act to increase extractable power an overall streamwise pressure gradient associated with finite blockage, and wake pressure recoveries associated with bypass-scale remixing.
Publisher: Cambridge University Press (CUP)
Date: 17-12-2019
DOI: 10.1017/JFM.2019.816
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Daniel Dehtyriov.