PIV Investigation of Flow Behind Surface Mounted Detached Square Cylinder

2006 ◽  
Author(s):  
P. K. Panigrahi
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Velocity Gradient ◽  
Flow Structures ◽  
Wall Region ◽  
Gap Size ◽  
Vorticity Field ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor ◽  
Near Wall

The flow field behind surface mounted detached square ribs over an approaching flat plate turbulent boundary layer has been studied. The Reynolds number based on the rib height has been set equal to 11075. The ratio of gap size from the flat plate surface to the square rib size has been varied between 0.2 and 1.0. The ratio of the approaching boundary layer thickness to the rib height is equal to 0.2. The PIV (2-component and stereo) technique in both stream wise and cross-stream measurement planes have been implemented. The PIV data has been acquired at two different resolutions. The high resolution measurements have been used to show the flow field at immediate downstream of the detached ribs. The oil flow visualization study has been carried out to relate the surface flow patterns to that of the flow structures. The mean and rms velocity field, average stream wise and span wise vorticity field, turbulent energy production and stream traces have been reported. The invariant of the velocity gradient tensor has been calculated to distinguish between the rotational and shear contribution of the vorticity field. The recirculation bubbles with foci like structure behind the detached ribs are displaced upward and its size drops with an increase in the gap size. The flow below the detached rib is film like flow for lower gap size leading to significant near wall modification of the flow structures. For higher gap size, the viscous effect predominates in the near wall region. The stream traces in the cross stream plane show additional node-saddle patterns in the near wall region indicating greater near wall flow structures and hence better mixing. The turbulence intensity, vorticity and velocity gradient tensor invariant results confirm the efficacy of the detached rib with smaller gap to cylinder size as an effective passive flow control tool for near wall mixing enhancement.

PIV Investigation of Flow Behind Surface Mounted Detached Square Cylinder

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
P. K. Panigrahi
Keyword(s):  
Boundary Layer ◽  
Velocity Gradient ◽  
Velocity Fluctuation ◽  
Surface Flow ◽  
Flow Structures ◽  
Energy Budgets ◽  
Gap Size ◽  
Recirculation Bubble ◽  
Oil Film ◽  
Near Wall

The flow field behind surface mounted detached square ribs under the approaching flat plate turbulent boundary layer has been experimentally studied using the particle image velocimetry (PIV) (two-component and stereo) technique in both streamwise and cross stream measurement planes. An oil film visualization study has been carried out for correlating the surface flow patterns to the flow structures. The Reynolds number based on the rib height is equal to 11,075. The ratio of the gap height to the square rib size is set equal to 0.2, 0.37, 0.57, and 1.0. The ratio of approaching boundary layer thickness to rib height is equal to 0.2. The mean and rms velocity fields, streamwise and spanwise vorticity fields, velocity gradient and velocity vector fields, turbulent kinetic energy budgets, and stream trace results are reported. The second invariant of the velocity gradient tensor results are presented to distinguish between the rotational and shear contribution of the vorticity field. The recirculation bubbles with a focilike structure are observed behind the detached ribs. These structures are displaced upward, i.e., away from the wall surface with an increase in gap size of the detached cylinder. The size of the recirculation bubble also drops with an increase in the gap size. The stream traces in the cross stream plane show node-saddle patterns, whose near wall concentration is high for a lower gap size detached cylinder. The oil film visualization images show saddle patterns at the meeting point between the flow through the gap and the reattaching shear layer for the lower gap size detached cylinder. The v-velocity magnitude distribution shows greater wall-normal motion across the wake for the detached cylinder of lower gap size. There is a significant near wall velocity fluctuation for the lower gap size detached cylinder. The higher velocity fluctuation due to the near wall flow structures contributes toward an increase in the near wall mixing of a detached cylinder geometry. Overall, the present study clearly demonstrates the flow structures behind detached ribs, which are responsible for effective near wall mixing. The results from this study provide useful understanding for the design of turbulators in various practical applications.

scholarly journals Evolution of the velocity gradient tensor invariant dynamics in a turbulent boundary layer

2017 ◽  
Vol 815 ◽  
pp. 223-242 ◽  
Author(s):  
P. Bechlars ◽  
R. D. Sandberg
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Gradient ◽  
Evolution Equations ◽  
Turbulence Models ◽  
Turbulent Structures ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor ◽  
The Mean ◽  
The Individual

In order to improve the physical understanding of the development of turbulent structures, the compressible evolution equations for the first three invariants $P$, $Q$ and $R$ of the velocity gradient tensor have been derived. The mean evolution of characteristic turbulent structure types in the $QR$-space were studied and compared at different wall-normal locations of a compressible turbulent boundary layer. The evolution of these structure types is fundamental to the physics that needs to be captured by turbulence models. Significant variations of the mean evolution are found across the boundary layer. The key features of the changes of the mean trajectories in the invariant phase space are highlighted and the consequences of the changes are discussed. Further, the individual elements of the overall evolution are studied separately to identify the causes that lead to the evolution varying with the distance to the wall. Significant impact of the wall-normal location on the coupling between the pressure-Hessian tensor and the velocity gradient tensor was found. The highlighted features are crucial for the development of more universal future turbulence models.

Dual-plane PIV technique to determine the complete velocity gradient tensor in a turbulent boundary layer

2005 ◽  
Vol 39 (2) ◽  
pp. 222-231 ◽  
Author(s):  
Bharathram Ganapathisubramani ◽  
Ellen K. Longmire ◽  
Ivan Marusic ◽  
Stamatios Pothos
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Gradient ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor ◽  
Dual Plane ◽  
Piv Technique

Effect of the eigenvalues of the velocity gradient tensor on particle collisions

2016 ◽  
Vol 792 ◽  
pp. 36-49 ◽  
Author(s):  
Vincent E. Perrin ◽  
Harmen J. J. Jonker
Keyword(s):  
Turbulent Flows ◽  
Velocity Gradient ◽  
Stokes Number ◽  
Negative Eigenvalue ◽  
Collision Kernel ◽  
Flow Structures ◽  
Particle Collisions ◽  
Gradient Tensor ◽  
Local Flow ◽  
Velocity Gradient Tensor

This study uses the eigenvalues of the local velocity gradient tensor to categorize the local flow structures in incompressible turbulent flows into different types of saddle nodes and vortices and investigates their effect on the local collision kernel of heavy particles. Direct numerical simulation (DNS) results show that most of the collisions occur in converging regions with real and negative eigenvalues. Those regions are associated not only with a stronger preferential clustering of particles, but also with a relatively higher collision kernel. To better understand the DNS results, a conceptual framework is developed to compute the collision kernel of individual flow structures. Converging regions, where two out of three eigenvalues are negative, posses a very high collision kernel, as long as a critical amount of rotation is not exceeded. Diverging regions, where two out of three eigenvalues are positive, have a very low collision kernel, which is governed by the third and negative eigenvalue. This model is not suited for particles with Stokes number $St\gg 1$, where the contribution of particle collisions from caustics is dominant.

Lagrangian evolution of the invariants of the velocity gradient tensor in a turbulent boundary layer

2012 ◽  
Vol 24 (10) ◽  
pp. 105104 ◽  
Author(s):  
C. Atkinson ◽  
S. Chumakov ◽  
I. Bermejo-Moreno ◽  
J. Soria
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Gradient ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor

Dynamics of a low Reynolds number turbulent boundary layer

2000 ◽  
Vol 404 ◽  
pp. 87-115 ◽  
Author(s):  
JUAN M. CHACIN ◽  
BRIAN J. CANTWELL
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Boundary Layer ◽  
Turbulent Kinetic Energy ◽  
Reynolds Stress ◽  
Velocity Gradient ◽  
Rapid Change ◽  
Joint Probability ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor

The generation of Reynolds stress, turbulent kinetic energy and dissipation in the turbulent boundary layer simulation of Spalart (1988) is studied using the invariants of the velocity gradient tensor. This technique enables the study of the whole range of scales in the flow using a single unified approach. In addition, it also provides a rational basis for relating the flow structure in physical space to an appropriate statistical measure in the space of invariants. The general characteristics of the turbulent motion are analysed using a combination of computer-based visualization of flow variables together with joint probability distributions of the invariants. The quantities studied are of direct interest in the development of turbulence models. The cubic discriminant of the velocity gradient tensor provides a useful marker for distinguishing regions of active and passive turbulence. It is found that the strongest Reynolds-stress and turbulent-kinetic-energy generating events occur where the discriminant has a rapid change of sign. Finally, the time evolution of the invariants is studied by computing along particle paths in a Lagrangian frame of reference. It is found that the invariants tend to evolve toward two distinct asymptotes in the plane of invariants. Several simplified models for the evolution of the velocity gradient tensor are described. These models compare well with several of the important features observed in the Lagrangian computation. The picture of the turbulent boundary layer which emerges is consistent with the ideas of Townsend (1956) and with the physical picture of turbulent structure set forth by Theodorsen (1955).

Interaction of coherent flow structures in adverse pressure gradient turbulent boundary layers

2019 ◽  
Vol 873 ◽  
pp. 287-321 ◽  
Author(s):  
Matthew Bross ◽  
Thomas Fuchs ◽  
Christian J. Kähler
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Flow Structures ◽  
Wall Region ◽  
Strong Impact ◽  
Wall Flow ◽  
Log Law ◽  
Near Wall

With the aim to characterize the near-wall flow structures and their interaction with large-scale motions in the log-law region, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient turbulent boundary layer at a friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}=5000$. Due to the high spatial and temporal resolution of the measurements, it was possible to resolve and identify uniform-momentum zones in the region $z/\unicode[STIX]{x1D6FF}<0.15$ or $z^{+}<350$ and to relate them with well known coherent flow motions near the wall. The space–time results confirm that the turbulent superstructures have a strong impact even on the very near-wall flow motion and also their alternating appearance in time and intensity could be quantified over long time sequences. Using the time record of the velocity field, rare localized separation events appearing in the viscous sublayer were also analysed. By means of volumetric particle tracking velocimetry their three-dimensional topology and dynamics could be resolved. Based on the results, a conceptual model was deduced that explains their rare occurrence, topology and dynamics by means of a complex interaction process between low-momentum turbulent superstructures, near-wall low-speed streaks and tilted longitudinal and spanwise vortices located in the near-wall region.

Turbulence structures of wall-bounded shear flows found using DNS data

1998 ◽  
Vol 357 ◽  
pp. 225-247 ◽  
Author(s):  
M. S. CHONG ◽  
J. SORIA ◽  
A. E. PERRY ◽  
J. CHACIN ◽  
B. J. CANTWELL ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Gradient Flow ◽  
Flow Structures ◽  
Topological Properties ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor ◽  
Turbulence Structures ◽  
Vortex Loops ◽  
Wall Bounded Flows ◽  
Attached Vortex

This work extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached-eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The direct numerical simulation (DNS) data of wall-bounded flows studied here are from the zero-pressure-gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached eddy hypothesis.

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