Flow structure and dynamics behind cylinder arrays at Reynolds number ∼100

Abstract

The flow behind nine different arrays of cylinders is experimentally investigated via Particle Image Velocimetry (PIV) at a Reynolds number of Re ∼100 based on the diameter of the cylinders. Each array consists of a column of four cylinders in front and three in the rear. The horizontal distance between the two columns and the vertical distance between the cylinders within each column are varied for H/D=[2,4,8] and V/D=[2,4,6], resulting in nine different arrays denoted as mVnH, where m corresponds to V/D and n stands for H/D. The PIV measurements are conducted for 15 s at 200 Hz frequency, corresponding to 39 to 360 vortex shedding events for the wakes in this study. Then, proper orthogonal decomposition is applied to the velocity fields to analyze the flow dynamics. All arrays show unsteady flow, and based on their flow structures, they are classified in to three main categories of single bluff body (SBB), transitional (TR), and co-shedding (CS) flow. SBB characteristics can be seen for 2V2H and 2V4H arrays, but the latter has more steady vortex shedding as the H/D increases from 2 to 4. Then, 2V8H and 4V2H have an asymmetric flow with several vortex streets and act as an intermediary stage in the shift from SBB to CS flow structure when the distances are increased. The highest total kinetic energy values and widest probability density functions of the velocity components are observed for this group. The five remaining arrays in the CS group have symmetric flow, with three or five vortex streets present behind. However, based on the distances, the frequency and phase synchronization of the vortex streets change considerably, which might have an important effect on, for example, the heat transfer or the structural load of the cylinders.

Spatial prediction of the turbulent unsteady von Kármán vortex street using echo state networks

“…In this way, each method can benefit from the strength of its counterparts to compensate for its weakness. Furthermore, it is desirable to further extend the application of spatial prediction to more complex turbulent flows, such as flow behind arrays of multiple cylinders 74 or even turbulent Rayleigh B enard convection. 75 Another possible field of application is to complement the information in the case of a low number of measurement positions, for example, in weather or ocean flow data.…”

On the prediction of the turbulent flow behind cylinder arrays via echo state networks

“…The network is trained for 700 time steps and tested for another 700 time steps to assess its capability in prediction of the TCPM. For more details of the flow physics and vortex shedding for the different cases the interested reader is referred to Ghazijahani and Cierpka [29]. There are three main hyperparameters that are believed to have the most important role in the performance of an ESN, their values are varied to investigate how they affect the predictions.…”

“…However, there is still a lot to be learned about the application of ESNs to more complex turbulent flows. In this regard, the flow past an array of cylinders is an ideal test case [29]. Varying the distance between the cylinders, the dynamics of the turbulent wake significantly changes and shifts from single bluff body flow to transient flow and finally to co-shedding flow.…”

“…Furthermore, it is interesting to investigate the possible impact of the array variation and, therefore, the dynamics on the performance of ESNs in general and using optimized hyperparameters in particular. Thus, three different arrangements of cylinder arrays (see Ghazijahani and Cierpka [29] for more details) with single bluff body, transient and co-shedding flow regimes are chosen for the current study. They are named in the following mVnH, where the first number stands for the vertical distance and the second for the horizontal distance in diameter of the cylinders, respectively.…”

Impact of tracer particles on the electrolytic growth of hydrogen bubbles

“…Our reasoning focuses on the so far ignored detail that specific tracer particles were intentionally added to the electrolyte in order to enable the simultaneous measurements of temperature and electrolyte velocity in the referenced experiments of Massing et al 5 Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods are standard techniques frequently used in fluid mechanics to measure the velocity field in liquid or gas flows. [15][16][17] In addition, they are also applied to multiphase flows, sometimes even to accurately determine the shape and the position of interfaces. [18][19][20][21] A possible impact of the particles on the interface is typically not considered, supported by the argument that the particle concentration is low.…”