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Research Papers: Fundamental Issues and Canonical Flows

# Investigation of Flow Dynamics Over Transitional-Type Microcavity

[+] Author and Article Information
Paulius Vilkinis

Laboratory of Heat-Equipment Research
and Testing,
Lithuanian Energy Institute,
Breslaujos Street 3,
Kaunas LT 44403, Lithuania
e-mail: paulius.vilkinis@lei.lt

Nerijus Pedišius

Laboratory of Heat-Equipment Research
and Testing,
Lithuanian Energy Institute,
Breslaujos Street 3,
Kaunas LT 44403, Lithuania
e-mail: nerijus.pedisius@lei.lt

Mantas Valantinavičius

Laboratory of Heat-Equipment Research
and Testing,
Lithuanian Energy Institute,
Breslaujos Street 3,
Kaunas LT 44403, Lithuania
e-mail: mantas.valantinavicius@lei.lt

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 27, 2017; final manuscript received January 4, 2018; published online March 13, 2018. Assoc. Editor: Pierre E. Sullivan.

J. Fluids Eng 140(7), 071203 (Mar 13, 2018) (7 pages) Paper No: FE-17-1617; doi: 10.1115/1.4039159 History: Received September 27, 2017; Revised January 04, 2018

## Abstract

Flow over a transitional-type cavity in microchannels is studied using a microparticle image velocimetry system (μPIV) and commercially available computational fluid dynamics (CFD) software in laminar, transitional, and turbulent flow regimes. According to experimental results, in the transitional-type cavity (L/h1 = 10) and under laminar flow in the channel, the recirculation zone behind the backward-facing step stretches linearly with $ReDh$ until the reattachment point reaches the middle of the cavity at xr/L = (0.5 to 0.6). With further increase in $ReDh$, the forward-facing step lifts the reattaching flow from the bottom of the cavity and stagnant recirculation flow fills the entire space of the cavity. Flow reattachment to the bottom of the cavity is again observed only after transition to the turbulent flow regime in the channel. Reynolds-averaged Navier–Stokes (RANS) equations and large eddy simulation (LES) results revealed changes in vortex topology, with the flow regime changing from laminar to turbulent. During the turbulent flow regime in the recirculation zone, periodically recurring vortex systems are formed. Experimental and computational results have a good qualitative agreement regarding the changes in the flow topology. However, the results of numerical simulations based on RANS equations and the Reynolds-stress-baseline turbulence model (RSM-BSL), show that computed reattachment length values overestimate the experimentally obtained values. The RSM-BSL model underestimates the turbulent kinetic energy intensity, generated by flow separation phenomena, on the stage of transitional flow regime.

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## Figures

Fig. 1

Dependence of the recirculation zone length on Re from various studies in different geometries: axisymmetric expansion (curves 1–3), backward-facing step (curves 4–7 and 10), obstacles (curves 8–9). 1—[16], 2—[17], 3—[18], 4—[20], 5—[21], 6—[22], 7—[23], 8—[24], 9—[25], 10—[19]. Note: Re is determined according to characteristic geometric parameters used by the authors.

Fig. 2

Side view of the investigated microchannel with transitional-type cavity on one wall

Fig. 3

CFD geometry and grid structure at the bottom of cavity and around cavity edge. Note: For the sake of visibility presented grid contains smaller amount of cells than used for computation.

Fig. 4

Dependence of the reattachment length on ReDh for the investigated cavity

Fig. 5

Experimental time-averaged velocity vectors fields at ReDh: (a) 95, (b) 280, (c) 800, (d) 950, and (e) 1600

Fig. 6

Comparison between instantaneous experimental (a) and computational and (b) velocity fields at ReDh = 800: 1—elongated stagnant recirculation loop, 2—vortex at forward-facing step, 3—saddle zone, 4—mixing layer on the interface between the channel and cavity flows

Fig. 7

Instantaneous simulated velocity flow fields in a turbulent flow regime at ReDh = 1600: 1, 2, 3—dominant vortices in recirculation zone, 4—vortex on the upper wall of cavity, 5—pathways of vortices (1) and (4) downstream of the reattachment, 6—averaged position of the reattachment according to experimental data (Fig. 4)

Fig. 8

Comparison between experimental and computational values of the reattachment length as function of ReDh

Fig. 9

Comparison between experimental and simulated velocity profiles at different locations behind the backward-facing step at ReDh: (a) 95, (b) 280, (c) 630, (d) 950, and (e) 1600. Note: Circles indicate the reattachment point according to the experimental results.

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