Response of the Fully Developed Pipe Flow to Rough Wall Disturbance (Mean Flow Field)

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

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Study of the Standard k-ε Model for Tip Leakage Flow in an Axial Compressor Rotor

International Journal of Turbo and Jet Engines ◽

10.1515/tjj-2015-0039 ◽

2016 ◽

Vol 33 (4) ◽

Cited By ~ 7

Author(s):

Yanfei Gao ◽

Yangwei Liu ◽

Luyang Zhong ◽

Jiexuan Hou ◽

Lipeng Lu

Keyword(s):

Flow Field ◽

Large Scale ◽

Turbulent Viscosity ◽

Axial Compressor ◽

Leakage Flow ◽

Reynolds Stresses ◽

Mean Flow ◽

Tip Leakage Flow ◽

Tip Leakage ◽

Compressor Rotor

AbstractThe standard k-ε model (SKE) and the Reynolds stress model (RSM) are employed to predict the tip leakage flow (TLF) in a low-speed large-scale axial compressor rotor. Then, a new research method is adopted to “freeze” the turbulent kinetic energy and dissipation rate of the flow field derived from the RSM, and obtain the turbulent viscosity using the Boussinesq hypothesis. The Reynolds stresses and mean flow field computed on the basis of the frozen viscosity are compared with the results of the SKE and the RSM. The flow field in the tip region based on the frozen viscosity is more similar to the results of the RSM than those of the SKE, although certain differences can be observed. This finding indicates that the non-equilibrium turbulence transport nature plays an important role in predicting the TLF, as well as the turbulence anisotropy.

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Investigation on hydrodynamic forces leading to coarse particle entrainment using Large-Eddy Simulations

10.5194/egusphere-egu21-13221 ◽

2021 ◽

Author(s):

Gaston Latessa ◽

Angela Busse ◽

Manousos Valyrakis

Keyword(s):

Experimental Data ◽

Flow Field ◽

Large Scale ◽

Large Eddy Simulations ◽

Flow Conditions ◽

Mean Flow ◽

Protection Measures ◽

Practical Applications ◽

Particle Entrainment ◽

Large Eddy

The prediction of particle motion in a fluid flow environment presents several challenges from the quantification of the forces exerted by the fluid onto the solids -normally with fluctuating behaviour due to turbulence- and the definition of the potential particle entrainment from these actions. An accurate description of these phenomena has many practical applications in local scour definition and to the design of protection measures.In the present work, the actions of different flow conditions on sediment particles is investigated with the aim to translate these effects into particle entrainment identification through analytical solid dynamic equations.Large Eddy Simulations (LES) are an increasingly practical tool that provide an accurate representation of both the mean flow field and the large-scale turbulent fluctuations. For the present case, the forces exerted by the flow are integrated over the surface of a stationary particle in the streamwise (drag) and vertical (lift) directions, together with the torques around the particle&#8217;s centre of mass. These forces are validated against experimental data under the same bed and flow conditions.The forces are then compared against threshold values, obtained through theoretical equations of simple motions such as rolling without sliding. Thus, the frequency of entrainment is related to the different flow conditions in good agreement with results from experimental sediment entrainment research.A thorough monitoring of the velocity flow field on several locations is carried out to determine the relationships between velocity time series at several locations around the particle and the forces acting on its surface. These results a relevant to determine ideal locations for flow investigation both in numerical and physical experiments.Through numerical experiments, a large number of flow conditions were simulated obtaining a full set of actions over a fixed particle sitting on a smooth bed. These actions were translated into potential particle entrainment events and validated against experimental data. Future work will present the coupling of these LES models with Discrete Element Method (DEM) models to verify the entrainment phenomena entirely from a numerical perspective.

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CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

Advances in Meteorology ◽

10.1155/2018/2189234 ◽

2018 ◽

Vol 2018 ◽

pp. 1-16 ◽

Cited By ~ 2

Author(s):

Fabio Nardecchia ◽

Annalisa Di Bernardino ◽

Francesca Pagliaro ◽

Paolo Monti ◽

Giovanni Leuzzi ◽

...

Keyword(s):

Flow Field ◽

Water Channel ◽

Flow Regimes ◽

Two Dimensional ◽

Mean Flow ◽

Ansys Fluent ◽

Practical Applications ◽

Aspect Ratios ◽

Starting Point ◽

Step Down

Computational fluid dynamics (CFD) is currently used in the environmental field to simulate flow and dispersion of pollutants around buildings. However, the closure assumptions of the turbulence usually employed in CFD codes are not always physically based and adequate for all the flow regimes relating to practical applications. The starting point of this work is the performance assessment of the V2F (i.e., v2¯ − f) model implemented in Ansys Fluent for simulating the flow field in an idealized array of two-dimensional canyons. The V2F model has been used in the past to predict low-speed and wall-bounded flows, but it has never been used to simulate airflows in urban street canyons. The numerical results are validated against experimental data collected in the water channel and compared with other turbulence models incorporated in Ansys Fluent (i.e., variations of both k-ε and k-ω models and the Reynolds stress model). The results show that the V2F model provides the best prediction of the flow field for two flow regimes commonly found in urban canopies. The V2F model is also employed to quantify the air-exchange rate (ACH) for a series of two-dimensional building arrangements, such as step-up and step-down configurations, having different aspect ratios and relative heights of the buildings. The results show a clear dependence of the ACH on the latter two parameters and highlight the role played by the turbulence in the exchange of air mass, particularly important for the step-down configurations, when the ventilation associated with the mean flow is generally poor.

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An Investigation of Flow Fields Over Multi-Element Aerofoils

Journal of Fluids Engineering ◽

10.1115/1.1431267 ◽

2001 ◽

Vol 124 (1) ◽

pp. 154-165 ◽

Cited By ~ 7

Author(s):

S. R. Maddah ◽

H. H. Bruun

Keyword(s):

Flow Field ◽

Computational Study ◽

Separated Flow ◽

Mean Flow ◽

Model Configuration ◽

Attached Flow ◽

The Mean ◽

Flap Deflection ◽

Comparative Results ◽

Lift And Drag

This paper presents results obtained from a combined experimental and computational study of the flow field over a multi-element aerofoil with and without an advanced slat. Detailed measurements of the mean flow and turbulent quantities over a multi-element aerofoil model in a wind tunnel have been carried out using stationary and flying hot-wire (FHW) probes. The model configuration which spans the test section 600mm×600mm, is made of three parts: 1) an advanced (heel-less) slat, 2) a NACA 4412 main aerofoil and 3) a NACA 4415 flap. The chord lengths of the elements were 38, 250 and 83 mm, respectively. The results were obtained at a chord Reynolds number of 3×105 and a free Mach number of less than 0.1. The variations in the flow field are explained with reference to three distinct flow field regimes: attached flow, intermittent separated flow, and separated flow. Initial comparative results are presented for the single main aerofoil and the main aerofoil with a nondeflected flap at angles of attacks of 5, 10, and 15 deg. This is followed by the results for the three-element aerofoil with emphasis on the slat performance at angles of attack α=10, 15, 20, and 25 deg. Results are discussed both for a nondeflected flap δf=0deg and a deflected flap δf=25deg. The measurements presented are combined with other related aerofoil measurements to explain the main interaction of the slat/main aerofoil and main aerofoil/flap both for nondeflected and deflected flap conditions. These results are linked to numerically calculated variations in lift and drag coefficients with angle of attack and flap deflection angle.

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Investigation of the Flow Field in a HP Turbine Stage for Two Stator-Rotor Axial Gaps: Part II — Unsteady Flow Field

Volume 6: Turbomachinery, Parts A and B ◽

10.1115/gt2006-90556 ◽

2006 ◽

Cited By ~ 1

Author(s):

P. Gaetani ◽

G. Persico ◽

V. Dossena ◽

C. Osnaghi

Keyword(s):

Flow Field ◽

Unsteady Flow ◽

Dominant Role ◽

Outer Part ◽

Secondary Flows ◽

Pressure Probe ◽

Turbine Stage ◽

Mean Flow ◽

Tip Leakage Flow ◽

Flow Measurements

An extensive experimental analysis was carried out at Politecnico di Milano on the subject of unsteady flow in high pressure (HP) turbine stages. In this paper the unsteady flow measured downstream of a modern HP turbine stage is discussed. Traverses in two planes downstream of the rotor are considered and, in one of them, the effects of two very different axial gaps are investigated: the maximum axial gap, equal to one stator axial chord, is chosen to “switch off” the rotor inlet unsteadiness, while the nominal gap, equal to 1/3 of the stator axial chord, is representative of actual engines. The experiments were performed by means of a fast-response pressure probe, allowing for two-dimensional phase-resolved flow measurements in a bandwidth of 80 kHz. The main properties of the probe and the data processing are described. The core of the paper is the analysis of the unsteady rotor aerodynamics; for this purpose, instantaneous snapshots of the rotor flow in the relative frame are used. The rotor mean flow and its interaction with the stator wakes and vortices are also described. In the outer part of the channel only the rotor cascade effects can be observed, with a dominant role played by the tip-leakage flow and by the rotor tip passage vortex. In the hub region, where the secondary flows downstream of the stator are stronger, the persistence of stator vortices is slightly visible in the maximum stator-rotor axial gap configuration, while in the minimum stator-rotor axial gap configuration the interaction with the rotor vortices dominates the flow field. A fair agreement with the wakes and vortices transport models has been achieved. A discussion of the interaction process is reported giving particular emphasis to the effects of the different cascade axial gaps. Some final considerations on the effects of the different axial gap over the stage performances are reported.

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Estimating the mean flow field in combustion chambers

International Journal of Engine Research ◽

10.1177/1468087413485576 ◽

2013 ◽

Vol 15 (3) ◽

pp. 338-345 ◽

Cited By ~ 1

Author(s):

Thad S Morton

Keyword(s):

Flow Field ◽

Mean Flow ◽

Combustion Chambers ◽

The Mean

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Measurement of the Mean Flow Field in a Smooth Rotating Channel With Coriolis and Buoyancy Effects

Volume 5A: Heat Transfer ◽

10.1115/gt2017-63123 ◽

2017 ◽

Author(s):

Ruquan You ◽

Haiwang Li ◽

Zhi Tao ◽

Kuan Wei

Keyword(s):

Flow Field ◽

Coriolis Force ◽

Indium Tin Oxide ◽

Buoyancy Force ◽

Flow Direction ◽

Density Ratio ◽

Mean Flow ◽

The Mean ◽

Static Conditions ◽

Rotating Channel

The mean flow field in a smooth rotating channel was measured by particle image velocimetry under the effect of buoyancy force. In the experiments, the Reynolds number, based on the channel hydraulic diameter (D) and the bulk mean velocity (Um), is 10000, and the rotation numbers are 0, 0.13, 0.26, 0.39, 0.52, respectively. The four channel walls are heated with Indium Tin Oxide (ITO) heater glass, making the density ratio (d.r.) about 0.1 and the maximum value of buoyancy number up to 0.27. The mean flow field was simulated on a 3D reconstruction at the position of 3.5<X/D<6.5, where X is along the mean flow direction. The effect of Coriolis force and buoyancy force on the mean flow was taken into consideration in the current work. The results show that the Coriolis force pushes the mean flow to the trailing side, making the asymmetry of the mean flow with that in the static conditions. On the leading surface, due to the effect of buoyancy force, the mean flow field changes considerably. Comparing with the case without buoyancy force, separated flow was captured by PIV on the leading side in the case with buoyancy force. More details of the flow field will be presented in this work.

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