Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

In this study, a water reattachment length was calculated by adopting two different models. The first was based on Unsteady Reynolds-Averaged Navier–Stokes (URANS) k-omega with Shear Stress Transport (SST); the second was a Large Eddy Simulation (LES) with Wall-Adapting Local Eddy-Viscosity (WALE). Both models used the same mesh and were checked with Taylor length-scale analysis. After the analysis, the mesh had 11,040,000 hexahedral cells. The geometry was a symmetrical expansion–contraction tube with a 4.28 expansion ratio that created mechanical energy losses, which were taken into account. Moreover, the reattachment length was estimated by analyzing the speed values; the change of speed value from negative to positive was used as the criterion to recognize the reattachment point.

Turbulent Flow Structure around a Horizontal Circular Cylinder Placed on a Rough Bed

<p>Pipelines that traverse a river are often buried beneath the river bed. However, the pipeline may be exposed due to scoured riverbed during floods. The exposed pipeline vibrates in a frequency band depending upon the flow velocity, size, and shape of the pipe. These vibrations are detrimental to the pipeline safety and stability due to their cyclic nature. In fact, these vibrations are induced by the turbulence around the cylinder which is a function of the flow velocity apart from the diameter of the cylinder and the bed roughness. The main objective of this paper is to investigate the structure of turbulent flow in the recirculation, reattachment and recovery regions behind a horizontal circular cylinder placed on the rough bed. In this direction, different experiments were conducted in a wide flume for various flow Reynolds numbers and cylinder Reynolds numbers. The Acoustic Doppler Velocimetry (ADV) was used for measuring the instantaneous point velocities. The raw velocity data were properly processed before the analysis. The approach flow was found to be a canonical near wall turbulent flow. In the immediate downstream of the cylinder, flow is characterized by recirculation, boundary layer reattachment and recovery. The reattachment length was determined using the established forward fraction method and reattachment length is independent of the flow Reynolds number. In addition, enhanced turbulence intensities, Reynolds shear stress, and turbulent kinetic energy were observed in the separated shear layer and they rapidly decreased in the recovery region. The present investigation will boost the understanding of hydraulics of flow around the horizontal bed-mounted cylindrical objects in rough bed natural streams under different flow conditions.</p><p><strong>Keywords: </strong>Wall mounted horizontal cylinder; Boundary layer; Separated and reattached turbulent flows; Wall Wake flows; ADV; Open channel flow.</p>

Flow responses alteration by geometrical effects of tubercles on plates under the maximal angle of attack

Plates with leading-edge tubercles experience beneficially more gradual aerodynamics stalling when entering the post-stall regime. Little is known, however, about the corresponding aquatic flow responses when these tubercles-furnished plates are subjected to the maximal angle of attack, with the flow direction perpendicular to their planar area. Hence, this study presents numerically, by means of the flow behavior solver ANSYS, the flow responses alteration in terms of the geometrical effects of tubercles on plates through changes in amplitudes (5 mm, 10 mm, 15 mm) and wavelengths (50 mm, 100 mm, 150 mm) under the maximal angle of attack in comparison to a control case, i.e., without tubercles. Additional to the commonly examined flow velocity and pressure, characteristics such as wake (area, reattachment length, flow recirculation intensity) and newly defined downstream vortical parameters (area, perimeter, and Feret diameters) for the vortex region have been proposed and assessed. It is found that the drag increases with the tubercle wavelength but corresponds inversely with the tubercle amplitude. By correlating with the best beneficial velocity and pressure profiles, it has been characterized that the optimally performing plate is the one that generates the greatest flow recirculation intensity, wake area, and reattachment length, corresponding to the capability to produce also the highest vortical area, perimeter, and major Feret diameter. Compared to the control case, all plates with tubercles alter beneficially these flow behaviors. In conclusion, plates with tubercles contribute favorably to the flow behaviors under the maximal angle of attack compared to the control case while the newly proposed downstream parameters could serve capably as alternatives in corroborating the flow physics description in future studies.

Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

Large eddy simulation coupled with immersed boundary method for turbulent flows over a backward facing step

In the present work, large eddy simulation coupled with immersed boundary (LES-IB) method is applied to simulate a backward facing step (BFS) flow, which is a canonical fluid dynamics problem involving flow separation, recirculation and reattachment that are common in many practical applications. The computed reattachment length, a primary parameter to evaluate the overall performance of the numerical method, shows promising accuracy in the present work compared to the alternative numerical simulations. Based on the mean velocity profiles at four representative locations, there is fairly well quantitative agreement among the present LES-IB, DNS and the experiment. The results reveal that the reverse flow in the reattachment region leads to little over-prediction of the reattachment length compared to the DNS result. Furthermore, second-order statistics are in good agreement with the reference data in spite of discrepancies in the recirculation and reattachment region owing to complex flow structure, verifying the accuracy of the present method. In addition, the instantaneous flow fields are also analyzed to show the capability of the present LES-IB method in vortex-capture, and one may see the transient process of flow separation based on the analysis of Lagrangian coherent structure (LCS).

STUDI NUMERIK PERBANDINGAN KARAKTERISTIK ALIRAN BACKWARD FACING STEP DENGAN BODI PENGGANGGU (BUMP) TIPE RECTANGULAR DENGAN BILANGAN REYNOLDS TINGGI

This research uses a backward facing step geometry with additional rectangular bump on the inlet. The rectangular bump with variation mixed length (l) model developed in this study is very important for predicting fluid flow among boundary layer especially in reattachment length, the prediction obtained by applying standard wall function and non-equilibrium wall function with turbulence model RKE Realizable. Furthermore, the results are compared with previous experimental research data Kim dkk. 2005. This research is investigated using 2D simulation Computational Fluid Dynamics (CFD) with Parameter to be used are: step height (h): 1 cm; variation of mixing length (l): 5; 4; 3; and 2 cm; rectangular side (d): 0.5 cm. All investigations are running with Reynolds Number 38,000, based on the step height and the mean stream. From this study we will obtain the best combination of turbulent models and the near-wall treatment method. It also give prediction show the best reattachment length prediction in position l/h 5 from the comparison of the results of the distance variation of the body model.