arXiv subjects

Daniel Feldmann

Publications and source records attributed to Daniel Feldmann.

7 recordsLinked to original sources

arXiv2023

Towards indirect assessment of surface anomalies on wind turbine rotor blades

We present results from novel field, lab and computer studies, that pave the way towards non-invasive classification of localised surface defects on running wind turbine rotors using infrared thermography (IRT). In particular, we first parametrise the problem from a fluid dynamical point of view using the roughness Reynolds number ($Re_k$) and demonstrate how the parameter regime relevant for modern wind turbines translate to parameter values that are currently feasible in typical wind tunnel and computer experiments. Second, we discuss preparatory wind tunnel and field measurements, that demonstrate a promising degree of sensitivity of the recorded IRT data w.r.t. the key control parameter ($Re_k$), which is a minimum requirement for the proposed classification technique to work. Third, we introduce and validate a local domain ansatz for future computer experiments, that enables well-resolved Navier-Stokes simulations for the target parameter regime at reasonable computational costs.

physics.flu-dyn↗

arXiv2022

Routes to turbulence in Taylor-Couette flow

Fluid flows between rotating concentric cylinders exhibit two distinct routes to turbulence. In flows dominated by inner-cylinder rotation, a sequence of linear instabilities leads to temporally chaotic dynamics as the rotation speed is increased. The resulting flow patterns occupy the whole system and sequentially lose spatial symmetry and coherence in the transition process. In flows dominated by outer-cylinder rotation, the transition is abrupt and leads directly to turbulent flow regions that compete with laminar ones. We here review the main features of these two routes to turbulence. Bifurcation theory rationalises the origin of temporal chaos in both cases. However, the catastrophic transition of flows dominated by outer-cylinder rotation can only be understood by accounting for the spatial proliferation of turbulent regions with a statistical approach. We stress the role of the rotation number (the ratio of Coriolis to inertial forces) and show that it determines the lower border for the existence of intermittent laminar-turbulent patterns.

physics.flu-dyn↗

arXiv2022

Taylor rolls on tour: Slow drift of turbulent large-scale structures in flows with continuous symmetries

In Rayleigh-Benard convection and Taylor-Couette flow cellular patterns emerge at the onset of instability and persist as large-scale coherent structures in the turbulent regime. Their long-term dynamics has been thoroughly characterised and modelled for the case of turbulent convection, whereas turbulent Taylor rolls have received much less attention. Here we present direct numerical simulations of axisymmetric Taylor-Couette flow and show a discontinuous phase-transition to spatio-temporal chaos as the system size increases. Beyond this transition, Taylor rolls suddenly undergo erratic drifts evolving on a very slow time scale. We estimate an effective diffusion coefficient for the drift and compare the dynamics to analogous motions in Rayleigh-Benard convection and Poiseuille flow, suggesting that this spontaneous diffusive displacement of large coherent structures is common among different types of wall-bounded turbulent flows.

physics.flu-dyn↗

arXiv2020

Effect of filter kernel on scale-energetics of near-wall turbulent structures

Inter-scale energy fluxes, $\Pi^\lambda$, are widely used as a diagnostic tool to analyse energy transfer across length scales, $\lambda$, in turbulence data. Here, we investigate how the choice of filter kernel (sharp spectral, Gaussian, box) affects the computed energy fluxes at constant filter width. We apply spatial filtering to a turbulent pipe flow simulation dataset and assess the effect on the local structure of $\Pi$. While the mean energy flux profile at each wall-normal distance is qualitatively robust across kernels, we observe significant differences in the intensity and spatial distribution of localised $\Pi$ events. Correlations between typical flow structures in the buffer layer (streaks, vortices, and Q-events) and regions of forward/backward transfer in the instantaneous $\Pi$ field differ markedly between kernel types. Cross-correlations appear strongly upstream--downstream symmetric when using the sharp spectral kernel, but asymmetric for the Gaussian and box kernels. For the Gaussian and box kernels $\Pi$ events tend to localise along the inclined meander of streaks, while they are centred on top of the streaks for the sharp spectral kernel. Moreover, using the sharp spectral kernel, we observe a coincidence of backward scatter and fluid transport away from the wall ($Q_1$), which does not appear with the Gaussian and box kernels. All kernels, however, predict backward scatter directly downstream of $Q_1$ events. The results suggest that interpretations of inter-scale energy flux based on sharp spectral scale separation should be treated with caution, since such kernels act non-local in physical space, whereas $\Pi$ events are inherently localised. Our python post-processing tool eFlux for scale separation and energy flux analysis in pipe flows is freely available and readily adaptable to other flow configurations and filter widths.

physics.flu-dyn↗

arXiv2020

Turbulent kinetic energy transport in oscillatory pipe flow

Laminar as well as turbulent oscillatory pipe flows occur in many fields of biomedical science and engineering. Pulmonary air flow and vascular blood flow are usually laminar, because shear forces acting on the physiological system ought to be small. However, frictional losses and shear stresses vary considerably with transition to turbulence. This plays an important role in cases of e.g. artificial respiration or stenosis. On the other hand, in piston engines and reciprocating thermal/chemical process devices, turbulent or transitional oscillatory flows affect mixing properties, and also mass and heat transfer. In contrast to the extensively investigated statistically steady wall bounded shear flows, rather little work has been devoted to the onset, amplification and decay of turbulence in pipe flows driven by an unsteady external force. Experiments [1, 3, 6] indicate that transition to turbulence depends on only one parameter, i.e. Re_{\delta} \sim Re/Wo with a critical value of about 550, at least for Womersley numbers Wo > 7. We perform direct numerical simulations (DNS) of oscillatory pipe flows at several combinations of Re and Wo to extend the validity of this critical value to higher Wo. To better understand the physical mechanisms involved during decay and amplification of the turbulent flow, we further analyse the turbulent kinetic energy distribution and its budgets terms.

physics.flu-dyn↗

arXiv2020

On phase asymmetries in oscillatory pipe flow

We present results from direct numerical simulations (DNS) of oscillatory pipe flow at several dimensionless frequencies Wo = {6.5, 13, 26} and one fixed shear Reynolds number Re_{\tau} = 1440. Starting from a fully-developed turbulent velocity field at that Re_{\tau}, the oscillatory flow either relaminarises or reaches a conditionally turbulent or strongly asymmetric state depending on Wo. The numerical method is validated by demonstrating excellent agreement of our DNS results with experimental data and analytical predictions from literature for the limiting cases of non-oscillating but turbulent and oscillating but laminar pipe flow. For an oscillating turbulent pipe flow we further found a very good agreement between qualitative descriptions of the characteristic flow features observed in experiments and our DNS. Here, we focus on the observation of a strongly asymmetric behaviour between the positive and the negative half-cycles of the oscillatory pipe flow at Wo=6.5.

physics.flu-dyn↗

arXiv2019

nsCouette -- A high-performance code for direct numerical simulations of turbulent Taylor-Couette flow

We present nsCouette, a highly scalable software tool to solve the Navier-Stokes equations for incompressible fluid flow between differentially heated and independently rotating, concentric cylinders. It is based on a pseudospectral spatial discretization and dynamic time-stepping. It is implemented in modern Fortran with a hybrid MPI-OpenMP parallelization scheme and thus designed to compute turbulent flows at high Reynolds and Rayleigh numbers. An additional GPU implementation (C-CUDA) for intermediate problem sizes and a basic version for turbulent pipe flow (nsPipe) are also provided.

physics.flu-dyn↗