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Separations and secondary structures due to unsteady flow in a curved pipe

Published online by Cambridge University Press:  14 February 2017

C. Vamsi Krishna Open the ORCID record for C. Vamsi Krishna [Opens in a new window] ,
Namrata Gundiah  and
Jaywant H. Arakeri
C. Vamsi Krishna
Affiliation:
Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India
Namrata Gundiah
Affiliation:
Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India
Jaywant H. Arakeri*
Affiliation:
Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India
*
†Email address for correspondence: jaywant@mecheng.iisc.ernet.in
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Abstract

Unsteady flows in highly curved geometries are of interest in many engineering applications and also in physiological flows. In this study, we use flow visualization and computational fluid dynamics to study unsteady flows in a highly curved tube ($\unicode[STIX]{x1D6FD}=0.3$) with square cross-section; here, $\unicode[STIX]{x1D6FD}$ is the ratio of the half edge length to the radius of curvature of the tube. To explore the combined effects of curvature and pulsatility, we use a single flow pulse of duration $T$ and peak area averaged axial velocity $U_{p(max)}$, which are independently varied to investigate a range of Dean and Womersley numbers. This range includes cases corresponding to flows in the ascending aorta. We observe radially inward moving secondary flows which have the structure of wall jets on the straight walls; their subsequent collision on the inner wall leads to a re-entrant radially outward moving jet. The wall jet arises due to an imbalance between the centrifugal force and the radial pressure gradient. During the deceleration phase, the low-axial-momentum fluid accumulated in the jet reverses direction and leads to flow separation near the inner wall. We use boundary layer equations to derive scales, which have not been reported earlier, for the secondary flow velocities, the wall shear stress components and the distance ($\hat{P}$) traversed by the secondary flow structures in the transverse plane. We show that $\hat{P}$ predicts the movement of vortical structures until collision. In the limit $\unicode[STIX]{x1D6FD}\rightarrow 0$, the Reynolds number based on this secondary flow velocity scale asymptotes to the secondary streaming Reynolds number proposed by Lyne (J. Fluid Mech., vol. 45 (01), 1971, pp. 13–31) in loosely curved pipes. The magnitude of the secondary flow velocity is high and ${\sim}40\,\%$ of $U_{p(max)}$ for physiological flow conditions. We show that the flow separation on the inner wall has origins in the secondary flow, which was reported in a few earlier studies, and is not due to the axial pressure gradient in the tube as proposed earlier. The wall shear stress components, hypothesized to be important in arterial mechanobiology, may be estimated using our scaling relations for geometries with different curvatures and varying pulsatilities.

JFM classification

Boundary Layers: Boundary layer separation Vortex Flows: Vortex dynamics
Type
Papers
Information
Journal of Fluid Mechanics , Volume 815 , 25 March 2017 , pp. 26 - 59
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© 2017 Cambridge University Press 

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Krishna et al. supplementary movie

Experimental PLIF flow visualizations and numerical simulations for case 2: Re ~ 3842, De ~ 2104, d/a ~ 0.29, and a ~ 32. We see evolution of the flow structures in the medial and transverse planes for the temporal variation of piston velocity shown in the left panel. The secondary structures impinge on the inner wall where they collide; subsequently the flow separates in the medial plane. Numerical simulations show axial velocity contours and velocity vectors. The colour bar shows the axial velocity normalized by Up(max). This video is slowed by 2x for ease of viewing.

Download Krishna et al. supplementary movie(Video)
Video 6.3 MB

Krishna et al. supplementary movie

Experimental flow visualizations and numerical simulations for case 3: Re ~ 3622, De ~ 1983, d/a ~ 0.20, and a ~ 45. In contrast to Movie 1, the input velocity pulse is shorter, and we see that the secondary structures do not reach the medial plane; hence no separation is observed. However, we see instability on the inner wall at the end of time pulse. This video is slowed by 3.5x for ease of viewing.

Download Krishna et al. supplementary movie(Video)
Video 5.1 MB

Krishna et al. supplementary movie

This case (Re ~ 3622, De ~ 1984, d/a ~ 0.41, and a ~ 23) corresponds to the longest time pulse. In contrast to Movies 1 and 2, the secondary structures impinge on the inner wall prior to stop of the piston; separation is observed at the beginning of the deceleration phase itself. This video is slowed by 1.5x for ease of viewing.

Download Krishna et al. supplementary movie(Video)
Video 10.2 MB