|
" Streamwise Vortices in a Convex Wall Jet "
Pandey, Anshuman
James W., Gregory
| Document Type
|
:
|
Latin Dissertation
|
| Language of Document
|
:
|
English
|
| Record Number
|
:
|
1052869
|
| Doc. No
|
:
|
TL51986
|
| Main Entry
|
:
|
Pandey, Anshuman
|
| Title & Author
|
:
|
Streamwise Vortices in a Convex Wall Jet\ Pandey, AnshumanJames W., Gregory
|
| College
|
:
|
The Ohio State University
|
| Date
|
:
|
2019
|
| Degree
|
:
|
Ph.D.
|
| student score
|
:
|
2019
|
| Note
|
:
|
250 p.
|
| Abstract
|
:
|
The outer part of a wall jet streaming on a convex surface exhibits a centrifugal instability because of an imbalance between the radial pressure gradient and centrifugal forces on the curving fluid particles. This results in the generation of streamwise vortices – a phenomenon observed in several centrifugally unstable flows. In the convex wall jet (CWJ), secondary flow produced by these vortices creates inflection points in the spanwise and radial profiles of streamwise velocity that can then support the development of secondary instabilities. These secondary instabilities are believed to be the major source of unsteadiness of streamwise vortices (primary instabilities) in a turbulent CWJ, however their characteristics are not well known. Using steadily forced streamwise vortices and stereo particle image velocimetry (stereo-PIV) based investigations, this work identifies three dominant secondary instabilities: sinuous, varicose, and Eckhaus. The sinuous and varicose streak instabilities are both due to the presence of inflection points in the modified velocity profiles, and the Eckhaus instability is proposed as an explanation for the spanwise wavelength selection of the vortices in the downstream direction. Secondary flow produced by counter-rotating pairs of vortices results in low-speed and high-speed streak regions, which are dominated by the sinuous and varicose instabilities, respectively. The Eckhaus instability is related to the tendency of the flow to maintain circular shape of the vorticity cells such that their spanwise wavelength is twice the local jet half-width. When the local spanwise wavelength does not conform to this condition, this instability is activated and causes merging/splitting of vortices because it is the only method of changing the spanwise wavelength in an experimental setup of finite span. For steadily forced vortices of fixed upstream wavelength, this explains the experimentally observed splitting behavior close to the nozzle where the jet is thin, and merging behavior farther downstream where the jet is expanding. Analysis by streamwise two-point correlation shows that both the local streamwise wavelength and the local wave speed of the dominant instability scale as about 0.85 times the local spanwise wavelength and local maximum jet velocity, respectively, thereby giving a local Strouhal number (defined using these two scales) of 1. This interesting behavior has previously been observed in centrifugally unstable curved channel flows as well. Due to consecutive mergers, the strength of streamwise vortices increases in the downstream direction. These strong vortices are expected to play a dominant role in the separation of the jet from the convex surface, however this process is not well understood. Since separation is a three-dimensional process, crossplane and streamwise stereo-PIV measurements have been made in this work to understand it. Under the action of steady streamwise vortices, the mean separation line becomes wavy in the spanwise direction because separation occurs preferentially at the upwash (radially positive flow by counter-rotating pair of vortices) locations and is delayed at the downwash locations. Separation happens when the upwash velocity becomes larger than the local azimuthal velocity of the jet. Spanwise meandering of vortices due to secondary instabilities causes spanwise movement of the upwash and downwash locations. Upstream and downstream motion of separation point in a streamwise plane is a result of this movement.
|
| Descriptor
|
:
|
Aerospace engineering
|
| Added Entry
|
:
|
James W., Gregory
|
| Added Entry
|
:
|
The Ohio State University
|
| |