Acoustic Doppler velocimetry measurements of flow over a backward-facing step

Abstract

The application of the acoustic Doppler velocimetry (ADV) technique to backward-facing step flows is very scarce in the literature. This work aims at investigating the applicability of the ADV technique to flows over a backward-facing step, at various Reynolds numbers to capture different flow regimes, and to provide reliable data which can be used to validate numerical models. The ADV ${\mathrm{V}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{o}}^{+}$ was used in a laboratory flume and a customized numerical code was developed, which implements a methodology to post-process the ADV records. The normalized mean streamwise velocity and turbulence profiles were compared with classical results measured using a laser Doppler velocimeter and a high correlation was found. The ADV measurements showed accurate results regarding the streamwise velocity profiles, the turbulent shear stress and the reattachment length. Thus, ADVs can be used to characterize separated flows with good accuracy providing that an adequate post-processing is made to the acquired data. The errors obtained by the uncertainty analysis were relatively low, in general.

Keywords:

• ADV
• backward-facing step
• experimental data
• multiple ${\mathsf{R}}_{\mathsf{e}}$
• post-processing
• uncertainty analysis

Acknowledgments

The authors would like to thank Professor Joe Desloges for providing access to the laboratory flume, and Mr Mircea Pilaf and Professor Mathew Wells for their assistance. The first author acknowledges the Fundação para a Ciência e a Tecnologia (FCT) for supporting this work through scholarship funding.

ORCID

Maria Amélia V. C. Araújo  http://orcid.org/0000-0003-3983-5577

Supplemental data

Supplemental data for this article can be accessed http://doi.org/10.1080/00221686.2019.1671521.

Notation

$C^{+}$	=	constant (–)
$E_{11}$	=	normalized one-dimensional velocity spectrum of component 1 (m${}^2$ s${}^{-1}$)
h	=	step height (m)
H	=	water flow depth (m)
${\mathsf{R}}_{\mathsf{e}}$	=	step height Reynolds number (–)
U	=	mean streamwise velocity (m s${}^{-1}$ )
$U_e$	=	upstream free-stream reference velocity (m s${}^{-1}$ )
$U^{+}$	=	mean streamwise velocity in wall coordinates (–)
$⟨u^{\prime }{u}^{\prime }⟩$	=	normal Reynolds stresses (m${}^2$ s${}^{-2}$)
$⟨-u^{\prime }{v}^{\prime }⟩$	=	Reynolds shear stress (m${}^2$ s${}^{-2}$)
$u_{\tau }$	=	friction or shear velocity (m s${}^{-1}$)
V	=	mean cross-wise velocity (m s${}^{-1}$ )
$⟨v^{\prime }{v}^{\prime }⟩$	=	normal Reynolds stresses (m${}^2$ s${}^{-2}$)
x	=	horizontal position from the step (m)
$x_r/h$	=	reattachment length (–)
y	=	vertical position (m)
$y^{+}$	=	vertical position in wall coordinates (–)
κ	=	von Karman constant (0.41)
ν	=	kinematic viscosity of the water (m${}^2$ s${}^{-1}$)

Additional information

Funding

This work was partly funded by: Fundação para a Ciência e a Tecnologia (FCT), equipment and operating grants from the Natural Sciences and Engineering Research Council of Canada, Cefas Seedcorn and the EU Interreg Atlantic Area EAPA -285/2016 MyCoast.