Daniel Feldmann

Direct numerical simulation of fully developed turbulent and oscillatory pipe flows at

Shear flow turbulence in oscillatory fluid motions is of theoretical interest and practical relevance, since the onset of turbulence can drastically change the transport properties and mixing efficiency. To supplement former theoretical and experimental investigations on the transition to turbulence in Sexl–Womersley (SW) flows, we perform three-dimensional direct numerical simulations (DNS) of oscillatory pipe flows at three Womersley numbers (Wo∈{26, 13, 5}) and one constant Reynolds number () based on the friction velocity and the pipe diameter. For this, the incompressible Navier–Stokes equations are solved in cylindrical coordinates using a fourth-order-accurate finite-volume method on staggered grids, motivated by Schumann’s volume balance procedure. We generate a well-correlated high-Reynolds-number initial flow field for the oscillatory flows by means of a DNS of a statistically steady pipe flow at . To underline the reliability of the DNS results for the oscillatory pipe flows, we validate the fi...

On the convergence and scaling of high-order statistical moments in turbulent pipe flow using direct numerical simulations

Direct numerical simulations of turbulent pipe flow in a flow domain of length L = 42R, friction Reynolds number in the range of 180 ≤ Reτ ≤ 1500, and two different wall-normal grid refinements were carried out and investigated in terms of high-order turbulence statistics. The phenomenology of large local wall-normal velocity fluctuations (velocity spikes) was discussed by means of time series and instantaneous flow-field realisations. Due to their rare appearance both in space and time, statistical high-order moments take a long time to converge. A convergence study was performed and for fully converged statistics the sensitivity of the grid resolution on the wall-normal kurtosis component value at the wall as well as the scaling behaviour of high-order statistics was investigated. The streamwise Reynolds stress as well as the streamwise skewness and the wall-normal flatness exhibited logarithmic Reynolds number dependencies in the vicinity of the wall and scaling laws were derived accordingly. In the bu...

On Phase Asymmetries in Oscillatory Pipe Flow

We present results from direct numerical simulations (DNS) of oscillatory pipe flow at several dimensionless frequencies \(W\!o\in \{6.5,13,26\}\) 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 \(W\!o\). 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 \(W\!o=6.5\).

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.

Numerical study on the decay and amplification of turbulence in oscillatory pipe flow

Results of three-dimensional direct numerical simulations of oscillatory pipe flows at two high dimensionless frequencies Wo ∈ {13,26} are discussed. Starting from a fully-developed turbulent velocity field, the flow either relaminarise or reach a conditionally or fully turbulent state, depending on Re and Wo. Analysing the temporal evolution of several phase-averaged quantities, reveals different behaviour of the wall shear stress and the RMS velocity profiles, while the integral bulk flow only reflects minor differences. Utilising two different initial conditions do not significantly influence the developing oscillatory bulk flow.

Numerical Simulation of the Oscillatory Ventilation in Simplified Human Lung Models

The high-frequency oscillation ventilation (HFOV) is a mechanical respiration technique which unlikely produces any cyclic alveolar recollapse and recruitment. Hence, HFOV was proposed to reduce ventilator associated lung injury (VALI) and might advance from being a rescue proceeding to being an accepted alternative ventilation strategy for patients suf fering from acute respiratory distress syndrom (ARDS) [1, 4]. But optimising HFOV certainly requires more detailed knowledge of the governing transport mechanisms in the lung, which are rather complex and yet not fully understood [3]. Within the framework of the Protective Artificial Respirati on project (PAR), supported by the German Research Foundation (DFG), we aim to analyse the air flow under the conditions of spontaneous breathing and artificial respiration.

Revisiting the Higher-Order Statistical Moments in Turbulent Pipe Flow Using Direct Numerical Simulations

Direct numerical simulations of turbulent pipe flow in a flow domain of length \(L=42R\) and a friction Reynolds number of \(Re_{\tau } =180\) and two different wall normal grid refinements were carried out and investigated in terms of turbulent high-order statistics. The phenomenology of large local wall-normal velocity fluctuations (velocity spikes), is discussed through time series and instantaneous flow field realizations. Due to their rare appearance both in space and time, statistical high-order moments take long time to converge. A convergence study is performed and for fully converged statistics the sensitivity of the wall-normal kurtosis component value on the wall is investigated. The fourth order statistical moment of the wall-normal velocity needs to be integrated at least \(\varDelta \tau ^+ = 2.4\cdot 10^{11}\) units of an averaging coordinate in time and space to obtain a fully converged flatness value in the very vicinity of the wall. Furthermore, a near-wall grid spacing of at least \(\varDelta r^+=0.11\) is needed to fully satisfy the no-slip boundary condition in terms of wall-normal flatness.