scholarly journals Numerical investigation of the wake bi-stability behind a notchback Ahmed body

2021 ◽  
Vol 926 ◽  
Author(s):  
Kan He ◽  
Guglielmo Minelli ◽  
Jiabin Wang ◽  
Tianyun Dong ◽  
Guangjun Gao ◽  
...  
Keyword(s):  
Orthogonal Decomposition ◽  
Large Eddy Simulations ◽  
Flow Structures ◽  
External Perturbations ◽  
Random Fashion ◽  
Large Eddy ◽  
Asymmetric Case ◽  
Wall Flow ◽  
The Asymmetry ◽  
Proper Orthogonal

Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, ${\beta _1} = 17.8\mathrm{^\circ }$ and ${\beta _2} = 21.0\mathrm{^\circ }$ , are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.

Flow Dynamics and Heat Transfer of a Stranded Overhead Conductor Cable

2020 ◽  
Author(s):  
Mohamed Abdelhady ◽  
David H. Wood
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Circular Cylinder ◽  
Coherent Structures ◽  
Carrying Capacity ◽  
Orthogonal Decomposition ◽  
Flow Dynamics ◽  
Large Eddy Simulations ◽  
Large Eddy ◽  
Proper Orthogonal

Abstract Stranded overhead conductor cables are used to transfer electric power, often over large distances. Conductor geometry, as well as environmental conditions, affect the power carrying capacity. This paper studies the flow dynamics and heat transfer for one stranded conductor geometry in air at Reynolds number of 1,000, determined using dynamic Smagorinsky Large Eddy Simulations. Proper Orthogonal Decomposition was used to identify coherent structures. In comparison to a smooth circular cylinder, the conductor strands noticeably affect the flow dynamics and heat transfer, locally and globally.

On the Hybrid Combustion Instability

2014 ◽  
Vol 555 ◽  
pp. 72-77
Author(s):  
Sterian Danaila ◽  
Constantin Leventiu
Keyword(s):  
Proper Orthogonal Decomposition ◽  
Orthogonal Decomposition ◽  
Large Eddy Simulations ◽  
Reduced Order Model ◽  
Chemical Mechanism ◽  
Control Technique ◽  
Order Model ◽  
Reduced Order ◽  
Large Eddy ◽  
Proper Orthogonal

In the present paper, a combined method of large eddy simulations for non-premixed combustion in a turbulent flow coupled with proper orthogonal decomposition of instantaneous velocity, pressure and temperature fields is developed in order to identify the effect of coherent structure and to obtain a reduced order model for control model. First we investigate the reacting flow using Large Eddy Simulations technique. This physical model is pertinent to internal flows inside the hybrid rocket motors. The turbulence-combustion interaction is based on a combination of finite rate/eddy dissipation model applied to a reduced chemical mechanism with four reactions. Next, the paper refers to the derivation of a Reduced Order Model (ROM) for the same problem, based on the Proper Orthogonal Decomposition (POD) technique. ROMs are used to obtain fast and accurate results, needed in the areas of flow control. The flow and thermal fields obtained with ROMs are compared with the ones obtained from the full simulation and an analysis on the number of modes required to achieve the desired accuracy is presented. Finally, a static control technique is proposed.

Study of Unforced and Modulated Inclined Film-Cooling Jets Using Proper Orthogonal Decomposition: Part II—Forced Jets

2011 ◽  
Author(s):  
Guillaume Bidan ◽  
Clementine Vezier ◽  
Dimitris E. Nikitopoulos
Keyword(s):  
Flow Rate ◽  
Film Cooling ◽  
Cross Flow ◽  
Orthogonal Decomposition ◽  
Large Eddy Simulations ◽  
Time Resolved ◽  
Cooling Performance ◽  
Blowing Ratio ◽  
Large Eddy ◽  
Proper Orthogonal

The effects of jet flow-rate modulation were investigated in the case of a 35° inclined jet in cross-flow over a flat plate using Mie scattering visualizations, time-resolved flow rate records and large eddy simulations (LES). In forced experiments, average blowing ratios of 0.3 and 0.4 were investigated with a duty cycle of 50% and pulsing frequencies of St = 0.016 and 0.159. Time-resolved flow rate measurements during the experiments provided precise knowledge of the instantaneous jet blowing ratio and adequate inlet boundary conditions for large eddy simulations. The dynamics of the vortical structures generated during the transient parts of the forcing cycle as well as their impact on film cooling performance were investigated with respect of the forcing parameters. At the considered blowing ratios, a starting ring vortex was consistently generated at the transition from low to high blowing ratio. Ingestion of cross-flow fluid at the transition from high to low blowing ratio was also observed and had a negative impact on film cooling performance. All studied cases exhibited an overall decrease in coverage regardless of pulsing parameters over their corresponding steady jet cases at fixed mass flow rate. Comparisons between pulsed and steady jets at constant pressure supply (same high blowing ratio) did exhibit some film-cooling improvement with pulsing. 3D Proper orthogonal decomposition was performed on LES results at distinct forcing frequencies to provide an analysis of dominant modes in the velocity and temperature fields. Significantly different results were obtained depending on the forcing frequency.

Study of Unforced and Modulated Film-Cooling Jets Using Proper Orthogonal Decomposition—Part II: Forced Jets

2012 ◽  
Vol 135 (2) ◽  
Author(s):  
Guillaume Bidan ◽  
Clementine Vézier ◽  
Dimitris E. Nikitopoulos
Keyword(s):  
Flow Rate ◽  
Film Cooling ◽  
Cross Flow ◽  
Orthogonal Decomposition ◽  
Large Eddy Simulations ◽  
Time Resolved ◽  
Cooling Performance ◽  
Blowing Ratio ◽  
Large Eddy ◽  
Proper Orthogonal

The effects of jet flow-rate modulation were investigated in the case of a 35 deg inclined jet in cross-flow over a flat plate using Mie scattering visualizations, time-resolved flow rate records, and large eddy simulations (LES). In forced experiments, average blowing ratios of 0.3 and 0.4 were investigated with a duty cycle of 50% and pulsing frequencies of St = 0.016 and 0.159. Time-resolved flow rate measurements during the experiments provided precise knowledge of the instantaneous jet blowing ratio and adequate inlet boundary conditions for large eddy simulations. The dynamics of the vortical structures generated during the transient parts of the forcing cycle as well as their impact on film cooling performance were investigated with respect of the forcing parameters. At the considered blowing ratios, a starting ring vortex was consistently generated at the transition from low to high blowing ratio. Ingestion of cross-flow fluid at the transition from high to low blowing ratio was also observed and had a negative impact on film cooling performance. All studied cases exhibited an overall decrease in coverage regardless of pulsing parameters over their corresponding steady jet cases at fixed mass flow rate. Comparisons between pulsed and steady jets at constant pressure supply (same high blowing ratio) did exhibit some film-cooling improvement with pulsing. 3D Proper orthogonal decomposition was performed on LES results at distinct forcing frequencies to provide an analysis of dominant modes in the velocity and temperature fields. Significantly different results were obtained depending on the forcing frequency.

scholarly journals A Reduced Order Model based on Large Eddy Simulation of Turbulent Combustion in the Hybrid Rocket Engine

2019 ◽  
Vol 304 ◽  
pp. 07015
Author(s):  
Sterian Dănăilă ◽  
Dragos Isvoranu ◽  
Constantin Levențiu ◽  
Alina Bogoi
Keyword(s):  
Proper Orthogonal Decomposition ◽  
Orthogonal Decomposition ◽  
Large Eddy Simulations ◽  
Reduced Order Model ◽  
Chemical Mechanism ◽  
Temperature Fields ◽  
Order Model ◽  
Reduced Order ◽  
Large Eddy ◽  
Proper Orthogonal

A combined method of large eddy simulations for non-premixed combustion in a turbulent boundary layer coupled with proper orthogonal decomposition of instantaneous velocity, pressure and temperature fields is developed in order to obtain a reduced order model. First, we investigate a channel turbulent reacting flow using Large Eddy Simulations (LES) technique. Polypropylene/O2 has been considered as fuel/oxidant pair. The turbulence-combustion interaction is based on a combination of finite rate/eddy dissipation model applied to a reduced chemical mechanism with four reactions. The LES numerical results are analyzed with respect to RANS simulations and with other reference data. The second part of the paper refers to the derivation of a Reduced Order Model (ROM) based on proper orthogonal decomposition (POD) technique for the unsteady flow field. In order to achieve that, the eigenmodes of the flow are computed from several snapshots of the instantaneous fields uniformly spaced and the most energetic ones are used to set up the Reduced Order Model. Constant regression rate of the fuel grain is considered. The flow and thermal fields obtained with ROMs are compared with the ones obtained from the full simulation and an analysis on the number of modes required to achieve the desired accuracy is presented.

Analysis of Unsteady Flow Structures in a Radial Turbomachine by Using Proper Orthogonal Decomposition

2018 ◽  
Author(s):  
Matthias Witte ◽  
Benjamin Torner ◽  
Frank-Hendrik Wurm
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Proper Orthogonal Decomposition ◽  
Orthogonal Decomposition ◽  
Mode Shapes ◽  
Flow Structures ◽  
Periodic Flow ◽  
Noise Emission ◽  
Turbulent Flow Field ◽  
Proper Orthogonal

Tonalities in hydro and airborne noise emission are a known problem of turbomachines, wherein the tonalities in the noise spectrum are associated with the different orders of the blade passing frequency (BPF). The proper orthogonal decomposition (POD) method was utilized to find the relationship between the fluctuations in the pressure field at the BPF orders which are the origin of the noise emission and the correlated fluctuations in the turbulent velocity field in terms of coherent, periodic flow structures. In order the provide the input data for the POD analysis, a URANS k-ω-SST scale adaptive simulation (SAS) of the turbulent flow field in a single stage radial pump under part load conditions was performed. Compared to traditional two equation turbulence models this approach is less dissipative and allows the development of small scale turbulence structures and is therefore an appropriate method for this study. In order to compute the POD correlation matrix Sirovich’s “Methods of Snapshots” was applied to the unsteady pressure and velocity fields from the CFD simulation. The discrimination of coherent, periodic flow structures and the incoherent, chaotic turbulence was carried out by analyzing the POD eigenvalue distributions, the POD mode shapes and the spectral properties of the POD time coefficients. Five coupled POD mode pairs were identified in total, which were strictly correlated with the 1st, 2nd, 3rd, 4th and 5th order of the BPF and therefore responsible for the noise emission at these discrete frequencies. The coherent structures were explored on the basis of the spatial POD velocity und pressure mode shapes and in terms of vortical structures after an additional phase averaging. The scope of this study is to introduce an enhanced collection of post processing techniques which are capable of analyzing highly unsteady flow fields from numerical simulations in a better way than is possible by just using traditional techniques like the evaluation of integral or time averaged quantities. The identified coherent flow structures and their associated pressure fluctuations are key elements for a proper comprehension of the internal dynamics of the turbulent flow field in a turbomachine and therefore essential for the understanding of the noise generation processes and the optimization of such machines.

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