POF Nameplate
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

Flickr Twitter iResearch App Facebook

Search Issue | RSS Feeds RSS
Next Issue

Jan 2006

Volume 18, Issue 1, Articles (01xxxx)

Return to All Sections
back to top
RSS Feeds
back to top Instability and Transition

Development of most amplified wavelength Görtler vortices

H. Mitsudharmadi, S. H. Winoto, and D. A. Shah

Phys. Fluids 18, 014101 (2006); http://dx.doi.org/10.1063/1.2160523 (12 pages) | Cited 3 times

Online Publication Date: 6 January 2006

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The development of most amplified wavelength Görtler vortices is studied by means of varying the spanwise spacing of thin vertical wires located upstream of the leading edge of a concave surface. The free-stream velocity is set so as to provide the value of the dimensionless parameter of that for the most amplified vortex wavelength. The resulting uniform vortex wavelengths were determined by the wire spacings and they were preserved downstream prior to turbulence. The spectrum study of the fluctuating velocity component was able to detect the fundamental frequency of the secondary instability mode with the streamwise wavelengths comparable to the wire spacing, which confirm that the wavelength of the vortices observed is the most amplified one. The intermittency study of the boundary layer flow in the presence of the most amplified wavelength Görtler vortices of 15.0 mm using Turbulent Energy Recognition Algorithm method shows the transition onset in the upwash regions, which coincides with the onset of the secondary instability obtained from the spectrum method. The intermittency factor distributions obtained agree with the single universal distribution with an error of about 5%. The transition Görtler number was also found within the range of that of the boundary layers in the presence of naturally developed Görtler vortices reported earlier.
Show PACS
47.32.cd Vortex stability and breakdown
47.20.Lz Secondary instabilities
47.27.nb Boundary layer turbulence

On unidirectional flight of a free flapping wing

Nicolas Vandenberghe, Stephen Childress, and Jun Zhang

Phys. Fluids 18, 014102 (2006); http://dx.doi.org/10.1063/1.2148989 (8 pages) | Cited 9 times

Online Publication Date: 6 January 2006

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We study the dynamics of a rigid, symmetric wing that is flapped vertically in a fluid. The motion of the wing in the horizontal direction is not constrained. Above a critical flapping frequency, forward flight arises as the wing accelerates to a terminal state of constant speed. We describe a number of measurements which supplement our previous work. These include (a) a study of the initial transition to forward flight near the onset of the instability, (b) the separate effects of flapping amplitude and frequency, (c) the effect of wing thickness, (d) the effect of asymmetry of the wing planform, and (e) the response of the wing to an added resistance. Our results emphasize the robustness of the mechanisms determining the forward-flight speed as observed in our previous study.
Show PACS
47.85.Gj Aerodynamics
47.40.-x Compressible flows; shock waves
47.20.-k Flow instabilities
87.19.rs Movement
87.19.ru Locomotion

Biglobal linear stability analysis of the flow induced by wall injection

F. Chedevergne, G. Casalis, and Th. Féraille

Phys. Fluids 18, 014103 (2006); http://dx.doi.org/10.1063/1.2160524 (14 pages) | Cited 17 times

Online Publication Date: 12 January 2006

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The hydrodynamic stability of the flow in a solid rocket motor is revisited using a general linear stability approach. A harmonic perturbation is introduced into the linearized Navier-Stokes equations leading to an eigenvalue problem posed as a system of partial differential equations with respect to the spatial coordinates. The system is discretized by a spectral collocation method applied to each spatial coordinate and the eigenvalues are determined using Arnoldi’s procedure. A special emphasis is placed on the boundary conditions. The main result is the discrete nature of the eigenvalue set. According to the present theory and the obtained results, only some discrete frequencies may exist in the motor (as eigenmodes). These frequencies only depend on the Reynolds number based on the injection velocity and the radius of the pipe flow. They are compared to measurements that have been performed at ONERA in one case with a cold-gas setup and in another case with a reduced scale live motor. Due to the agreement obtained with both experiments, the biglobal stability approach seems to offer new insight into the unresolved thrust oscillations problem.
Show PACS
47.85.Kn Hydraulic and pneumatic machinery
47.20.Ib Instability of boundary layers; separation
47.10.ad Navier-Stokes equations
47.60.-i Flow phenomena in quasi-one-dimensional systems
02.30.Jr Partial differential equations

Numerical simulation of immiscible two-phase flow in porous media

Amir Riaz and Hamdi A. Tchelepi

Phys. Fluids 18, 014104 (2006); http://dx.doi.org/10.1063/1.2166388 (12 pages) | Cited 12 times

Online Publication Date: 20 January 2006

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Nonlinear evolution of viscous and gravitational instability in two-phase immiscible displacements is analyzed with a high-accuracy numerical method. We compare our results with linear stability theory and find good agreement at small times. The fundamental physical mechanisms of finger evolution and interaction are described in terms of the competing viscous, capillary, and gravitational forces. For the parameter range considered, immiscible viscous fingers are found to undergo considerably weak interaction as compared to miscible fingers. The wave number of nonlinear fingers decreases rapidly due to the shielding mechanism and scales uniformly as t−1 at long times. We have observed that even a small amount of density contrast can eliminate viscous fingers. The dominant feature for these flows is the gravity tongue, which develops a “ridge instability” when capillary and gravity effects are of similar magnitude.
Show PACS
47.55.nb Capillary and thermocapillary flows
47.56.+r Flows through porous media
47.20.Gv Viscous and viscoelastic instabilities
47.55.Ca Gas/liquid flows
47.11.Bc Finite difference methods
47.11.Kb Spectral methods

Holographic particle image velocimetry measurements of hairpin vortices in a subcritical air channel flow

Alexander Svizher and Jacob Cohen

Phys. Fluids 18, 014105 (2006); http://dx.doi.org/10.1063/1.2158429 (14 pages) | Cited 8 times

Online Publication Date: 20 January 2006

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A holographic particle image velocimetry (HPIV) system is employed to study the evolution of coherent structures artificially generated in a plane Poiseuille air flow. As a first step the hot-wire technique and two-dimensional flow visualization are used to determine the generation conditions and dimensions of the coherent structures, their shedding frequency, trajectory, and convection velocity. Then, the HPIV method is utilized to obtain the instantaneous topology of the hairpin vortex and its associated three-dimensional distribution of the two (streamwise and spanwise) velocity components as well as the corresponding wall-normal vorticity. Finally, the experimental data are compared with results of related experimental and numerical studies. The present experimental results support the view that the generation of hairpins under various base flow conditions is governed by a basic mechanism, the important common elements of which are the shear of the base flow and an initial disturbance having a sufficiently large amplitude.
Show PACS
47.32.cb Vortex interactions
47.27.De Coherent structures
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.27.te Turbulent convective heat transfer
47.27.nb Boundary layer turbulence
47.27.nd Channel flow

Lift-up process in a heated-cylinder wake flow

Maosheng Ren, Camilo Rindt, and Anton van Steenhoven

Phys. Fluids 18, 014106 (2006); http://dx.doi.org/10.1063/1.2159031 (12 pages) | Cited 1 time

Online Publication Date: 25 January 2006

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
A three-dimensional flow transition behind a heated cylinder is analyzed at low Reynolds numbers: Re = O(100). Both visualizations and numerical simulations show that the transition manifests itself in the form of mushroom-type structures in the far wake and Λ-shaped structures in the near wake. The legs of the Λ-shaped structures coincide with streamwise vorticity regions. An intermediate stage is observed between the Λ-shaped structures and the escaping mushroom-type structures. This intermediate step is characterized by a lift-up process, which takes place in the center region between the legs and head of the Λ-shaped structures. As a result, hot fluid is being pulled out of the upper vortex core. Due to this lift-up process, mushroom-type structures are generated in the form of escaping vortex rings in the far wake.
Show PACS
47.15.Tr Laminar wakes
47.15.Fe Stability of laminar flows
47.27.Cn Transition to turbulence
47.11.Kb Spectral methods
47.54.De Experimental aspects
47.32.cf Vortex reconnection and rings
Return to All Sections
Close

Google Calendar

ADVERTISEMENT

Go to AIP Home   © AIP Publishing LLC
close