scholarly journals Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

2021 ◽  
pp. 1-20
Author(s):  
M. M. Neamtu-Halic ◽  
J.-P. Mollicone ◽  
M. van Reeuwijk ◽  
M. Holzner
Keyword(s):  
Stable Stratification ◽  
Scalar Transport ◽  
Vortical Structures

On the wake dynamics of a propeller operating in drift

2014 ◽  
Vol 754 ◽  
pp. 263-307 ◽  
Author(s):  
A. Di Mascio ◽  
R. Muscari ◽  
G. Dubbioso
Keyword(s):  
Axisymmetric Flow ◽  
Operating Conditions ◽  
Tip Vortex ◽  
Streamwise Direction ◽  
Vortical Structures ◽  
Actual Operating ◽  
Oblique Flow ◽  
The Subject ◽  
Flow Variables

AbstractThe onset and the nature of dynamic instabilities experienced by the wake of a marine propeller set in oblique flow are investigated by means of detached eddy simulations. In particular, the destabilization process is inspected by a systematic comparison of the wake morphology of a propeller operating in pure axisymmetric flow and in drift with angle of 20°, under different loading conditions. The wake behaviour in oblique flow shows a markedly different character with respect to the axisymmetric condition: in the latter, the destabilization is triggered by an increasing interaction of the main vorticity confined in the tip vortex; whereas, in the former, the role of the secondary vorticity (oriented in the streamwise direction) as well as the hub vortex seems to be crucial. The features of the wake have been investigated by the $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\lambda _{2}$ criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69–94) and typical flow variables (pressure, velocity and vorticity), for both the averaged and instantaneous flow fields. Moreover, in order to further inspect the evolution of the vortical structures, as well as their interaction and destabilization, the spectra of the kinetic energy have been considered. This investigation aims to broaden the knowledge from previous works on the subject of rotor wake instabilities, focusing on the differences between an ideal (axisymmetric) and actual operating conditions occurring in typical engineering applications.

The Role of Wind Speed and Wind Shear for Banner Cloud Formation

2019 ◽  
Vol 77 (4) ◽  
pp. 1199-1212
Author(s):  
Volkmar Wirth ◽  
Pascal Bubel ◽  
Joachim Eichhorn ◽  
Elmar Schömer ◽  
Tobias Kremer ◽  
...  
Keyword(s):  
Wind Speed ◽  
Wind Shear ◽  
Cloud Formation ◽  
Line Of Sight ◽  
Vortical Structures ◽  
Ambient Wind ◽  
Large Eddy ◽  
Flow Geometry ◽  
Wind Lidar

Abstract Banner clouds are clouds that appear to be attached to the leeward face of a steep mountain. This paper investigates the role of wind speed and wind shear for the formation of banner clouds. Large-eddy simulations are performed to simulate the flow of dry air past an idealized pyramid-shaped mountain. The potential for cloud formation is diagnosed through the Lagrangian vertical parcel displacement, which in the case of a banner cloud shows a plume of large values in the lee of the mountain. In addition, vortical structures are visualized through streamlines and their curvature. A series of sensitivity experiments indicates that both the flow and the banner cloud occurrence are largely independent of the ambient wind speed U. On the other hand, the shear of the ambient wind has a profound impact on the location of the stagnation point on the windward face as well as on the flow geometry in the lee of the mountain. The relevant measure for shear is H/Hs, where H denotes the height of the mountain and Hs = U/Uz is the scale height of the shear (with Uz denoting the scale of the shear). The simulations are also used to compute the line-of-sight velocity component seen by a hypothetical Doppler wind lidar positioned in the lee of the mountain; the analysis suggests that such sensitivities can potentially be detected using modern wind lidar technology.

Dynamics of complete turbulence suppression in turbidity currents driven by monodisperse suspensions of sediment

2012 ◽  
Vol 712 ◽  
pp. 384-417 ◽  
Author(s):  
Mrugesh Shringarpure ◽  
Mariano I. Cantero ◽  
S. Balachandar
Keyword(s):  
Reynolds Stress ◽  
Suspended Sediments ◽  
Stable Stratification ◽  
Turbidity Currents ◽  
Vortical Structures ◽  
Threshold Size ◽  
Turbulence Suppression ◽  
Sediment Size ◽  
Ability To Regenerate ◽  
Monodisperse Suspensions

AbstractTurbidity currents derive their motion from the excess density imposed by suspended sediments. The settling tendency of sediments is countered by flow turbulence, which expends energy to keep them in suspension. This interaction leads to downward increasing concentration of suspended sediments (stable stratification) in the flow. Thus in a turbidity current sediments play the dual role of sustaining turbulence by driving the flow and damping turbulence due to stable stratification. By means of direct numerical simulations, it has been shown previously that stratification above a threshold can substantially reduce turbulence and possibly extinguish it. This study expands the simplified model by Cantero et al. (J. Geophys. Res., vol. 114, 2009a, C03008), and puts forth a proposition that explains the mechanism of complete turbulence suppression due to suspended sediments. In our simulations it is observed that suspensions of larger sediments lead to stronger stratification and, above a threshold size, induce an abrupt transition in the flow to complete turbulence suppression. It has been widely accepted that hairpin and quasi-streamwise vortices are key to sustaining turbulence in wall-bounded flows, and that only vortices of sufficiently strong intensity can spawn the next generation of vortices. This auto-generation mechanism keeps the flow populated with hairpin and quasi-streamwise vortical structures and thus sustains turbulence. From statistical analysis of Reynolds stress events and visualization of flow structures, it is observed that settling sediments damp the Reynolds stress events (Q2 events), which means a reduction in both the strength and spatial distribution of vortical structures. Beyond the threshold sediment size, the existing vortical structures in the flow are damped to an extent where they lose their ability to regenerate the subsequent generation of turbulent vortical structures, which ultimately leads to complete turbulence suppression.

Convective Heat Transfer in Turbulent Flow Near a Gap

2006 ◽  
Vol 128 (7) ◽  
pp. 701-708 ◽  
Author(s):  
D. Chang ◽  
S. Tavoularis
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Large Scale ◽  
Stokes Equations ◽  
Reynolds Stress Model ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Vortical Structures

Convective heat transfer in a rectangular duct containing a heated rod forming a narrow gap with a plane wall has been simulated by solving the unsteady Reynolds-averaged Navier-Stokes equations with a Reynolds stress model. Of particular interest is the role of quasi-periodic coherent structures in transporting fluid and heat across the gap region. It is shown that the local instantaneous velocity and temperature vary widely because of large-scale transport by coherent vortical structures forming in pairs on either side of the rod.

Analysis and Active Control of the Flow Around Two Following Ahmed Bodies

2013 ◽  
Author(s):  
Charles-Henri Bruneau ◽  
Khodor Khadra ◽  
Iraj Mortazavi
Keyword(s):  
Drag Coefficient ◽  
Active Control ◽  
Bluff Body ◽  
The Body ◽  
Flow Conditions ◽  
Pressure Force ◽  
Vortical Structures ◽  
The One ◽  
Two Bodies

The aim of this work is on the one hand to show that the drag coefficient of a bluff body is mainly linked to the vortical structures in the close wake of the body. Thus an active control can be used to push away these structures to decrease the pressure forces at the back and consequently to reduce the drag coefficient. On the other hand a study of the platooning is performed. When two bodies are following each other with a short distance between them, the presence of the first body changes significantly the pressure force in front of the second one as a weak flow is present in between. The first body plays the role of a buckler and so the second body has to face completely different flow conditions. Therefore the drag of the second body is drastically reduced as shown on the results. Finally a control of the flow around the first body gives promising and surprising results.

scholarly journals Lift enhancement by bats' dynamically changing wingspan

2015 ◽  
Vol 12 (113) ◽  
pp. 20150821 ◽  
Author(s):  
Shizhao Wang ◽  
Xing Zhang ◽  
Guowei He ◽  
Tianshu Liu
Keyword(s):  
Numerical Simulations ◽  
Nonlinear Interaction ◽  
Mechanical Energy ◽  
Leading Edge ◽  
Direct Numerical Simulations ◽  
Flow Structures ◽  
Vortical Structures ◽  
Geometrical Effect ◽  
Lift Enhancement

This paper elucidates the aerodynamic role of the dynamically changing wingspan in bat flight. Based on direct numerical simulations of the flow over a slow-flying bat, it is found that the dynamically changing wingspan can significantly enhance the lift. Further, an analysis of flow structures and lift decomposition reveal that the elevated vortex lift associated with the leading-edge vortices intensified by the dynamically changing wingspan considerably contributed to enhancement of the time-averaged lift. The nonlinear interaction between the dynamically changing wing and the vortical structures plays an important role in the lift enhancement of a flying bat in addition to the geometrical effect of changing the lifting-surface area in a flapping cycle. In addition, the dynamically changing wingspan leads to the higher efficiency in terms of generating lift for a given amount of the mechanical energy consumed in flight.

The role of coherent vortices near the turbulent/non-turbulent interface in a planar jet

2011 ◽  
Vol 369 (1937) ◽  
pp. 738-753 ◽  
Author(s):  
Carlos Bettencourt da Silva ◽  
Ricardo José Nunes dos Reis
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Vortical Structures ◽  
Micro Scale ◽  
Planar Jet ◽  
Coherent Vortices ◽  
The Mean ◽  
Turbulent Plane ◽  
Plane Jet

The role of coherent vortices near the turbulent/non-turbulent (T/NT) interface in a turbulent plane jet is analysed by a direct numerical simulation (DNS). The coherent vortices near the jet edge consist of large-scale vortical structures (LSVSs) maintained by the mean shear and intense vorticity structures (IVSs) created by the background fluctuating turbulence field. The radius of the LSVS is equal to the Taylor micro-scale R lsvs ≈ λ , while the radius of the IVS is of the order of the Kolmogorov micro-scale R ivs ∼ η . The LSVSs are responsible for the observed vorticity jump at the T/NT interface, being of the order of the Taylor micro-scale. The coherent vortices in the proximity of the T/NT interface are preferentially aligned with the tangent to the T/NT interface and are responsible for the viscous dissipation of kinetic energy near the T/NT interface and to the characteristic shape of the enstrophy viscous diffusion observed at that location.

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