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High Reynolds Number Flow Over A Backward-Facing Step

Electronic Theses of Indian Institute of Science

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Title High Reynolds Number Flow Over A Backward-Facing Step
 
Creator Nadge, Pankaj M
 
Subject Fluid Mechanics
High Reynolds Number Flow
Three-Dimensional Vortical Structures
Bubble Structures
Backward-facing Step Flow
Flow Field Measurements
Unsteady Three-dimensional Flow Downstream
Three-dimensional Unsteady Structures
Particle Image Velocimetry (PIV)
Hotwire Anemometry
Flow Separation
Reynolds Number
Bubble Schedule
Applied Mechanics
 
Description Flow separation and reattachment happens in many fluid mechanical situations occurring in engineering applications as well as in nature. The flow over a backward-facing step represents a geometrically simple flow situation exhibiting both flow separation and reattachment. Broadly speaking there are only two important parameters in the problem, the Reynolds number(Re) based on the step height(h),and a geometrical parameter, referred to as the Expansion ratio(ER), defined as the downstream channel height to the upstream channel height. In spite of the relative simplicity of this geometry, the flow downstream is quite complex. The main focus of the present work is to elucidate the unsteady three-dimensional coherent structures present in this flow at large Re, Re>36,000,based on the step height(h). For this, we use velocity field measurements from Particle Image Velocimetry (PIV)in conjunction with hotwire anemometry measurements.
The time-averaged structure of this flow is first studied in detail, including the effect of Reynolds number(Re) and Expansion Ratio(ER), on it. These studies show that at sufficiently large Re (Re>20,000), the reattachment length becomes independent of Re. The detailed internal structure of the separation bubble is also found to be independent of Re, but for Revalues that are relatively larger(Re>36,000). At large Re, the main effect of ER ,is found to be on the reattachment length, which increases with ER and saturates for ER values greater than about 1.8. The detailed internal structure of the separation bubble has been mapped at high Re and is found to be nearly the same for all ER, when the streamwise length is normalized by the reattachment length.
In order to elucidate the unsteady coherent vortical structures, PIV measurements are done in two orthogonal planes downstream of the backward-facing step. These measurements are done for ER= 1.50 at large Re(Re=36,000) and in a large aspect ratio facility(AR= span length/step height= 24); the latter being important to avoid any effects due to span-wise confinement. In the spanwise plane parallel to the lower wall(x-z plane),instantaneous velocity fields show counter rotating vortex pairs, which is a signature of the three-dimensional vortical structures in this plane. Using conditional averaging, this counter-rotating vortex pair signature is captured right from upstream of the step, to well after reattachment. Spatial correlations are used to get the length scale of these coherent vortical structures, which varies substantially from the attached boundary layer before separation to the region after reattachment. The variation of these structures in the cross-stream (vertical) direction at reattachment and beyond gives an idea about their three dimensional shape. The circulation of these counter-rotating pairs is measured from the conditionally aver-aged fields, and is found to increase with streamwise distance reaching normalized circulation values (Γ/Uoh) of about 0.5 around reattachment.
Velocity spectra downstream of the step show peaks corresponding to both the shear layer frequency(Stsl)and a relatively lower frequency that corresponds to large-scale shedding from the separation bubble (Stb); the latter in particular being quasi-periodic. Small amplitude sinusoidal forcing at the shedding frequency(Stb) is applied close to the step, by blowing and suction, to make the quasi-periodic shedding more regular. Measurements show that this has a very small effect on both the mean separation bubble and on the counter-rotating structures in the x-z plane. This mild forcing however enables phase locked PIV measurements to be made which shows the bubble shedding phenomenon in the cross-stream plane(side view or x-y plane).
The phase-averaged velocity fields show significant variations from phase to phase. Although there is some hint of structures being shed, from these phase-averaged fields, it is not very clear. One of the primary reasons is the fact that the flow is effectively spanwise averaged, as the three-dimensional structures are not locked in the spanwise direction. To get a three dimensional view of the sheddin gphenomenon, it is necessary to lock the spanwise location with respect to the three-dimensional vortical structures before averaging across the different phases. We use the condition, u’
 
Contributor Govardhan, R N
 
Date 2016-06-21T07:23:14Z
2016-06-21T07:23:14Z
2016-06-21
2012-12
 
Type Thesis
 
Identifier http://etd.iisc.ernet.in/handle/2005/2542
http://etd.ncsi.iisc.ernet.in/abstracts/3297/G25638-Abs.pdf
 
Language en_US
 
Relation G25638