scholarly journals Effects of Rear Angle on the Turbulent Wake Flow between Two in-Line Ahmed Bodies

Atmosphere ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 328
Author(s):  
Ebenezer Essel ◽  
Subhadip Das ◽  
Ram Balachandar
Keyword(s):  
Wake Flow ◽  
Recirculation Region ◽  
Wake Region ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Slant Angle ◽  
The Road ◽  
Rear Angle ◽  
Turbulent Statistics ◽  
The Mean

Understanding the wake characteristics between two in-line vehicles is essential for improving and developing new strategies for reducing in-cabin air pollution. In this study, Ahmed bodies are used to investigate the effects of the rear slant angle of a leading vehicle on the mean flow and turbulent statistics between two vehicles. The experiments were conducted with a particle image velocimetry at a fixed Reynolds number, R e H = 1.7 × 10 4 , and inter-vehicle spacing distance of 0.75 L , where H and L are the height and length of the model. The rear slant angles investigated were a reference square back, high-drag angle ( α = 25 ° ) and low-drag angle ( α = 35 ° ). The mean velocities, Reynolds stresses, production of turbulent kinetic energy and instantaneous swirling strength are used to provide physical insight into the wake dynamics between the two bodies. The results indicate that the recirculation region behind the square back Ahmed body increases while those behind the slant rear-end bodies decreases in the presence of a follower. For the square back models, the dominant motion in the wake region is a strong upwash of jet-like flow away from the road but increasing the rear slant angle induces a stronger downwash flow that suppresses the upwash and dominates the wake region.

Passive pitching of splitters in the trailing edge of elliptic cylinders

2017 ◽  
Vol 826 ◽  
pp. 363-375 ◽  
Author(s):  
Y. Jin ◽  
L. P. Chamorro
Keyword(s):  
Vortex Shedding ◽  
Semimajor Axis ◽  
Wake Flow ◽  
Recirculation Region ◽  
Mean Flow ◽  
Trailing Edge ◽  
Flow Measurements ◽  
High Background ◽  
Aspect Ratios ◽  
The Mean

The distinctive pitching of hinged splitters in the trailing edge of elliptic cylinders was experimentally studied at various angles of attack ($AoA$) of the cylinder, Reynolds numbers, splitter lengths, aspect ratios ($AR$) of the cylinder and freestream turbulence levels. High-resolution telemetry and hotwire anemometry were used to characterize and gain insight on the dynamics of splitters and wake flow. Results show that the motions of the splitters contain various dominating modes, e.g. $f_{p}$ and $f_{v}$, which are induced by the mean flow and wake dynamics. High background turbulence dampens the coherence of the regular vortex shedding leading to negligible $f_{v}$. For a sufficiently long splitter, namely twice the semimajor axis of the cylinder, dual vortex shedding mode exists close to the leading and trailing edges of the splitter. In general, the splitters oscillate around an equilibrium position nearly parallel to the mean direction of the flow; however, a skewed equilibrium is also possible with a strong recirculation region. This is the case with cylinders of low $AR$ and high $AoA$, where higher lift and drag occurs. Flow measurements at various transverse locations within the wake of the cylinder–splitter system indicate that the signature of the low-frequency splitter pitching is shifted in the wake in the cases with non-zero $AoA$ of the cylinder. Although the splitter pitching exhibits two dominant vortex shedding modes in various configurations, only the higher frequency is transmitted to the wake.

Separated and Reattached Flow Over Square, Rectangular and Semi-Circular Blocks

2007 ◽  
Author(s):  
M. Agelinchaab ◽  
M. F. Tachie
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Recirculation Region ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Block Height ◽  
Turbulent Statistics ◽  
Image Velocimetry ◽  
The Mean ◽  
Separated And Reattached Flow

A particle image velocimetry is used to study the characteristics of separated and reattached turbulent flow over two-dimensional transverse blocks of square, rectangular and semi-circular cross-sections fixed to the bottom wall of an open channel. The ratio of upstream boundary layer thickness to block height is considerably higher than in prior studies. The results show that the mean and turbulent statistics in the recirculation region and downstream of reattachment are significantly different from the upstream boundary layer. The variation of the Reynolds stresses along the separating streamlines is discussed within the context of vortex stretching, longitudinal strain rate and wall damping. It appears wall damping is a more dominant mechanism in the vicinity of reattachment. The levels of turbulence diffusion and production by the normal stresses are significantly higher than in classical turbulent boundary layers. The bulk of turbulence production occurs in mid-layer and transported into the inner and outer layers. The results also reveal that the curvature of separating streamline, separating bubble beneath it as well as the mean velocity and turbulent quantities depend strongly on block geometry.

scholarly journals Self-consistent triple decomposition of the turbulent flow over a backward-facing step under finite amplitude harmonic forcing

2019 ◽  
Vol 475 (2225) ◽  
pp. 20190018
Author(s):  
E. Yim ◽  
P. Meliga ◽  
F. Gallaire
Keyword(s):  
Turbulent Flow ◽  
Finite Amplitude ◽  
Navier Stokes ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Backward Facing Step ◽  
Eddy Viscosity Model ◽  
Coherent Interaction ◽  
Self Consistent ◽  
The Mean

We investigate the saturation of harmonically forced disturbances in the turbulent flow over a backward-facing step subjected to a finite amplitude forcing. The analysis relies on a triple decomposition of the unsteady flow into mean, coherent and incoherent components. The coherent–incoherent interaction is lumped into a Reynolds averaged Navier–Stokes (RANS) eddy viscosity model, and the mean–coherent interaction is analysed via a semi-linear resolvent analysis building on the laminar approach by Mantič-Lugo & Gallaire (2016 J. Fluid Mech. 793 , 777–797. ( doi:10.1017/jfm.2016.109 )). This provides a self-consistent modelling of the interaction between all three components, in the sense that the coherent perturbation structures selected by the resolvent analysis are those whose Reynolds stresses force the mean flow in such a way that the mean flow generates exactly the aforementioned perturbations, while also accounting for the effect of the incoherent scale. The model does not require any input from numerical or experimental data, and accurately predicts the saturation of the forced coherent disturbances, as established from comparison to time-averages of unsteady RANS simulation data.

M. J. Hartmann Memorial Session Paper: Turbulence Characteristics in a Supersonic Cascade Wake Flow

1994 ◽  
Vol 116 (4) ◽  
pp. 586-596 ◽  
Author(s):  
P. L. Andrew ◽  
Wing-fai Ng
Keyword(s):  
Boundary Layer ◽  
Incidence Angle ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Shear Stresses ◽  
State University ◽  
Mean Flow ◽  
Flow Measurements ◽  
The Mean ◽  
Supersonic Wake

The turbulent character of the supersonic wake of a linear cascade of fan airfoils has been studied using a two-component laser-doppler anemometer. The cascade was tested in the Virginia Polytechnic Institute and State University intermittent wind tunnel facility, where the Mach and Reynolds numbers were 2.36 and 4.8 × 106, respectively. In addition to mean flow measurements, Reynolds normal and shear stresses were measured as functions of cascade incidence angle and streamwise locations spanning the near-wake and the far-wake. The extremities of profiles of both the mean and turbulent wake properties´ were found to be strongly influenced by upstream shock-boundary -layer interactions, the strength of which varied with cascade incidence. In contrast, the peak levels of turbulence properties within the shear layer were found to be largely independent of incidence, and could be characterized in terms of the streamwise position only. The velocity defect turbulence level was found to be 23 percent, and the generally accepted value of the turbulence structural coefficient of 0.30 was found to be valid for this flow. The degree of similarity of the mean flow wake profiles was established, and those profiles demonstrating the most similarity were found to approach a state of equilibrium between the mean and turbulent properties. In general, this wake flow may be described as a classical free shear flow, upon which the influence of upstream shock-boundary-layer interactions has been superimposed.

scholarly journals A Framework for Parameterizing Eddy Potential Vorticity Fluxes

2012 ◽  
Vol 42 (4) ◽  
pp. 539-557 ◽  
Author(s):  
David P. Marshall ◽  
James R. Maddison ◽  
Pavel S. Berloff
Keyword(s):  
Stress Tensor ◽  
Potential Vorticity ◽  
Large Scale ◽  
Eddy Diffusivity ◽  
Linear Instability ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Conservation Of Energy ◽  
The Mean ◽  
Energy And Momentum

Abstract A framework for parameterizing eddy potential vorticity fluxes is developed that is consistent with conservation of energy and momentum while retaining the symmetries of the original eddy flux. The framework involves rewriting the residual-mean eddy force, or equivalently the eddy potential vorticity flux, as the divergence of an eddy stress tensor. A norm of this tensor is bounded by the eddy energy, allowing the components of the stress tensor to be rewritten in terms of the eddy energy and nondimensional parameters describing the mean shape and orientation of the eddies. If a prognostic equation is solved for the eddy energy, the remaining unknowns are nondimensional and bounded in magnitude by unity. Moreover, these nondimensional geometric parameters have strong connections with classical stability theory. When applied to the Eady problem, it is shown that the new framework preserves the functional form of the Eady growth rate for linear instability. Moreover, in the limit in which Reynolds stresses are neglected, the framework reduces to a Gent and McWilliams type of eddy closure where the eddy diffusivity can be interpreted as the form proposed by Visbeck et al. Simulations of three-layer wind-driven gyres are used to diagnose the eddy shape and orientations in fully developed geostrophic turbulence. These fields are found to have large-scale structure that appears related to the structure of the mean flow. The eddy energy sets the magnitude of the eddy stress tensor and hence the eddy potential vorticity fluxes. Possible extensions of the framework to ensure potential vorticity is mixed on average are discussed.

PIV Study of Shallow Open Channel Flow Over d- and k-Type Transverse Ribs

2007 ◽  
Vol 129 (8) ◽  
pp. 1058-1072 ◽  
Author(s):  
M. F. Tachie ◽  
K. K. Adane
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Skin Friction ◽  
Cross Section ◽  
Open Channel ◽  
Skin Friction Coefficient ◽  
Mixing Length ◽  
Mean Flow ◽  
Turbulent Statistics ◽  
The Mean

A particle image velocimetry was used to study shallow open channel turbulent flow over d-type and k-type transverse ribs of square, circular, and semi-circular cross sections. The ratio of boundary layer thickness to depth of flow varied from 50% to 90%. The mean velocities and turbulent quantities were evaluated at the top plane of the ribs to characterize interaction between the cavities and overlying boundary layer. It was found that the overlying boundary layer interacts more strongly with k-type cavities than observed for d-type cavities. The profiles of the mean velocities and turbulent statistics were then spatially averaged over a pitch, and these profiles were used to study the effects of rib type and cross section on the flow field. The mean velocity gradients were found to be non-negligible across the boundary layer, and the implications of this observation for momentum transport, eddy viscosity, and mixing length distributions are discussed. The results show that the skin friction coefficient, Reynolds stresses and mixing length distributions are independent of rib cross section for d-type. For the k-type ribs, significant variations in skin friction coefficient values, mean flow, and turbulence fields are observed between square ribs and circular/semi-circular ribs.

scholarly journals A Geometric Interpretation of Eddy Reynolds Stresses in Barotropic Ocean Jets

2016 ◽  
Vol 46 (8) ◽  
pp. 2285-2307 ◽  
Author(s):  
Talia Tamarin ◽  
James R. Maddison ◽  
Eyal Heifetz ◽  
David P. Marshall
Keyword(s):  
Stress Tensor ◽  
Ray Tracing ◽  
Analytic Solutions ◽  
Geometric Interpretation ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Nonlinear Simulation ◽  
Fully Nonlinear ◽  
Geometric Decomposition ◽  
The Mean

AbstractBarotropic eddy fluxes are analyzed through a geometric decomposition of the eddy stress tensor. Specifically, the geometry of the eddy variance ellipse, a two-dimensional visualization of the stress tensor describing the mean eddy shape and tilt, is used to elucidate eddy propagation and eddy feedback on the mean flow. Linear shear and jet profiles are analyzed and theoretical results are compared against fully nonlinear simulations. For flows with zero planetary vorticity gradient, analytic solutions for the eddy ellipse tilt and anisotropy are obtained that provide a direct relationship between the eddy tilt and the phase difference of a normal-mode solution. This allows a straightforward interpretation of the eddy–mean flow interaction in terms of classical stability theory: the initially unstable jet gives rise to eddies that are tilted “against the shear” and extract energy from the mean flow; once the jet stabilizes, eddies become tilted “with the shear” and return their energy to the mean flow. For a nonzero planetary vorticity gradient, ray-tracing theory is used to predict ellipse geometry and its impact on eddy propagation within a jet. An analytic solution for the eddy tilt is found for a Rossby wave on a constant background shear. The ray-tracing results broadly agree with the eddy tilt diagnosed from a fully nonlinear simulation.

PIV Study of Adverse and Favorable Pressure Gradient Turbulent Flows Over Transverse Ribs

2008 ◽  
Vol 130 (11) ◽  
Author(s):  
M. Agelinchaab ◽  
M. F. Tachie
Keyword(s):  
Pressure Gradient ◽  
Turbulent Flows ◽  
Reynolds Shear Stress ◽  
Adverse Pressure Gradient ◽  
Bottom Wall ◽  
Combined Effects ◽  
Mean Flow ◽  
Turbulent Statistics ◽  
Favorable Pressure Gradient ◽  
The Mean

This paper reports an experimental study of the combined effects of rib roughness and pressure gradient on turbulent flows produced in asymmetric converging and diverging channels. Transverse square ribs with pitch-to-height ratio of 4 were attached to the bottom wall of the channel to produce the rib roughness. A particle image velocimetry technique was used to conduct measurements at several streamwise-transverse planes located upstream, within, and downstream of the converging and diverging sections of the channel. From these measurements, the mean velocities and turbulent statistics at the top plane of the ribs and across the channel were obtained. The data revealed non-negligible wall-normal motion and interaction between the cavities and overlying boundary layers. The different drag characteristics of the rough bottom wall and the smooth top wall produced asymmetric distributions of mean velocity and turbulent statistics across the channel. The asymmetry of these profiles is most extreme in the presence of adverse pressure gradient. Because of the manner in which pressure gradient modifies the mean flow and turbulence production, it was found that the streamwise turbulence intensity and Reynolds shear stress in the vicinity of the ribs are lower in the adverse pressure gradient than in the favorable pressure gradient channel. The results show also that the combined effects of rib roughness and adverse pressure gradient on the turbulent intensity statistics are significantly higher than when roughness and adverse pressure gradient are applied in isolation.

3D Reynolds Stress Modelling of Turbulent Flow in Linear Turbine Cascade

1997 ◽  
Author(s):  
M. Kanniche ◽  
R. Boudjemadi ◽  
F. Déjean ◽  
F. Archambeau
Keyword(s):  
Reynolds Stress ◽  
Eddy Viscosity ◽  
Strain Tensor ◽  
Secondary Flows ◽  
Turbulence Closure ◽  
Turbine Cascade ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Principal Axes ◽  
The Mean

The flow in a linear turbine cascade (Gregory-Smith et al. (1990)) is numerically investigated using a Reynolds Stress Turbulence closure. A particular attention is given to secondary flows where the normal Reynolds stresses are expected to play an important role. The most classical turbulence closure, the k-epsilon model uses the Boussinesq Eddy Viscosity concept which assumes an isotropic turbulent viscosity. The Reynolds stresses are then related to local velocity gradients by this isotropic eddy viscosity. Corollary, the principal axes of the Reynolds stress tensor are colinear with those of the mean strain tensor. The advantage of Reynolds Stress Turbulence closure is the calculation of Reynolds stresses by their own individual transport equations. This leads to a more realistic description of the turbulence and of its dependance on the mean flow. The most classical Second Order turbulence model (Launder et al. (1975)) is applied to a linear turbine cascade, and the results are compared to secondary velocity and turbulence measurements at cross-passage planes.

Wake Formation for an Oscillating Small-Incidence-Angle Cylinder Pair in Cross-Flow

2011 ◽  
Author(s):  
Mir M. Hayder
Keyword(s):  
Incidence Angle ◽  
Flow Direction ◽  
Incident Angle ◽  
Cross Flow ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Small Incidence ◽  
The Mean ◽  
Wake Formation

The wake region of a pair of equal-diameter staggered circular cylinders in cross-flow is investigated experimentally for Reynolds numbers, based on the mean flow velocity, U, and the cylinder diameter, D, within the range 540 ≤ Re ≤ 755. The centre-to-centre pitch ratio and stagger angle of the cylinders at their mean position are P/D = 2.0 and α = 16°, respectively. In an earlier study, wake formation of a small-incident-angle cylinder pair was investigated for forced oscillation (transverse to the flow direction) of the upstream cylinder only. The present study is aimed to reveal the modification of the wake when the oscillation is shifted from the upstream to downstream cylinder or vice versa. Results with cylinder excitation frequencies in the range 0.07 ≤ feD/U ≤ 1.10 are reported. It is observed that for both upstream and downstream cylinder oscillations with frequency feD/U ≤ 0.10 the wake flow patterns remain essentially the same as those of the corresponding static cases. However, for frequency feD/U > 0.10 the wake undergoes considerable modification vis-a`-vis when the cylinders are stationary, and the flow pattern within the wake is strongly dependent on feD/U value. As also observed in the previous study, there are distinct regions of synchronization between the dominant wake periodicities and the cylinder oscillation over the whole range of feD/U. These synchronizations involve sub- and super-harmonics as well as fundamental synchronizations and are the result of the formation of two rows of vortices, one on either side of the combined wake of the cylinder pair. The manner in which the wake responds to the cylinder oscillation depends strongly on whether it is the upstream or downstream cylinder which is oscillating. Flow-visualization images suggests that the synchronizations on the mean-flow side of the downstream cylinder occur from the outer vortices shed by the downstream cylinder, and those on the mean-flow side of the upstream cylinder occur from the vortices formed by the interaction of the two gap shear layers and the outer shear layer separated from the upstream cylinder.

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