14:00
Instability, Transition and Control of Turbulent Flows 7
14:00
15 mins
#317
|
Blowup and turbulence in the 3D incompressible Euler equations on a logarithmic lattice
Ciro Campolina, Alexei Mailybaev
Abstract: See pdf.
|
14:15
15 mins
#482
|
Extension of the One-Dimensional Turbulence model towards electrohydrodynamic variable density flows
Juan Alí Medina Méndez, Heiko Schmidt, Ulrich Riebel
Abstract: The unavailability of suitable closure models for electrohydrodynamic (EHD) flows makes numerical simulation research largely dependant on Direct Numerical Simulations (DNSs). Engineering studies also rely on Reynolds-Averaged Navier-Stokes (RANS) simulations in order to predict the flow dynamics, often with little success since the small scale physical processes, which have to be parameterized, are not fully understood. A possible alternative is to resort to stochastic turbulence approaches, e.g. the One-Dimensional Turbulence (ODT) model. In contrast to traditional turbulence modelling, where the modelling focuses on the characterization of certain smaller physical length scale effects, ODT is able to resolve all relevant physical scales on a reduced dimension (Lignell et al., 2012, Lignell et al., 2018). Physical insight can then be gained via detailed parameter studies, which are feasible due to the more economical computational power required.
In this study, a novel formulation for the ODT model is presented. The new approach is based on a previous variable density formulation for closed systems from (Medina et al., 2018), adapted to cylindrical coordinates (Medinaet al., 2019), and recently extended to incorporate EHD effects as part of the eddy selection process in ODT (Medina et al., 2019-2). The variable density formulation for closed systems has allowed reasonable results for characteristic streamwise heat transfer profiles in pipe flows. Preliminary results for the streamwise bulk Nusselt number profile, obtained with a spatial ODT formulation in a heated pipe flow configuration analogous to Bae et al., 2006, are shown in Figure 1(a). The incorporation of EHD effects into the model has also allowed the analysis of incompressible velocity profiles under the influence of electrostatic body forces as in Soldati and Banerjee, 1998, see Figure 1(b). The new model formulation presented here aims to evaluate the heat transfer enhancement produced by EHD body forces in turbulent pipe flows. In this context, relations between the bulk Nusselt and Reynolds number, as well as the EHD number or the electric Reynolds number will be discussed and analysed, as in Ohadi et al., 1991. Results will also be presented and compared with recent experimental measurements (Bacher and Riebel, 2018). This work is yet another contribution towards the milestone of achieving a fully consistent formulation for EHD flows in ODT.
|
14:30
15 mins
#559
|
Experimental investigation of perturbation growth in the submerged jet
Julia Zayko, Alexander Reshmin, Vladimir Trifonov, Linar Gareev, Vasily Vedeneev
Abstract: In this study, we investigate the perturbation growth in the long laminar jet theoretically and experimentally. Linear stability analysis was made. In experiment perturbations were introduced into the jet through an oscillating wire ring. Results of the laser visualization are in agreement with theoretical predictions of the linear stability theory.
|
14:45
15 mins
#232
|
Simultaneous PIV and ultrasound measurements revealing slow mode switching in a von Kármán flow cell
Hanna Berning, He Wang, Thomas Rösgen
Abstract: The von Kármán flow cell is well established in turbulence research for its compact design and the effortless provision of high Reynolds numbers with near isotropic and homogeneous turbulence in the center of the cell [2, 3].
Here, we investigate the feature of homogeneous, isotropic and quasi-stationary turbulence with the commonly utilized constant counter-rotation of the disks. The expected base flow modes (poloidal inward pumping and shearing) can be
observed which correspond to an inward flow in the central plane. Contrary to intuition, we discover also long living flow modes where an outward flow occurs in the central plane [1]. Thus, the generally applied subtraction of the mean will not yield the fluctuating quantities as required. Instead, averaging over the different modes will provide an artificial mean profile that only resembles the inward pumping flow mode. These persistent and recurrent modes, potentially originating from Kelvin-Helmholtz like instabilities, are identified by ultrasound measurements and will be compared both instantaneously and with clustering methods against simultaneous planar PIV measurements.
The measurements are conducted in a cell with an aspect ratio of H/D = 1 at a Reynolds number of 47 000. Ultrasound measurements with a Ultrasound Doppler Velocity Profiler UDOP4000 by Signal Processing SA are compared along the emission line in the center of the axial plane with PIV measurements in this plane. Both the ultrasonic and the optical recordings in PIV mode extend over the width of the cell.
A random forcing method based on a PRBS (pseudo random binary sequence) driving sequence of the discs is applied to counteract the stabilization of the undesirable bifurcations. The time scales of disc reversal are much smaller than the lifetime of the instabilities such that the interfering low wavenumber modes in the velocity spectrum are suppressed. This effective yet simple method is to be characterized again by simultaneous PIV and ultrasound measurements in order to
demonstrate utility both in turbulence research with optical access as well as in magnetohydrodynamics.
[1] T. Grünberg and T. Rösgen. Turbulent flow of a fluid with anisotropic viscosity. Journal of Fluid Mechanics, 792:252–273, 2016.
[2] B. Saint-Michel, B. Dubrulle, L. Marié, F. Ravelet, and F. Daviaud. Influence of reynolds number and forcing type in a turbulent von Kármán flow. New Journal of Physics, 16:063037, 2014.
[3] G. Voth, A. La Porta, A. Crawford, J. Alexander, and E. Bodenschatz. Measurement of particle accelerations in fully developed turbulence. Journal of Fluid Mechanics, 469:121–160, 2002.
|
|