FY2019 Annual Report

Fluid Mechanics Unit
Professor Pinaki Chakraborty



The Fluid Mechanics unit pursued research on transition to turbulence, fully-developed turbulence, tropical cyclones, and granular flows.

1. Staff

  • Pinaki Chakraborty, Professor
  • Tapan Sabuwala, Group Leader
  • Julio Manuel Barros Junior, Staff Scientist
  • Rory Cerbus, Staff Scientist
  • Christian Butcher, Research Unit Technician
  • Yuna Hattori, PhD Student
  • Lin Li, PhD Student
  • Kalale Chola, Visiting Researcher
  • Tomoe Owan, Research Unit Administrator

2. Collaborations

2.1 Theory of spectral link in turbulent flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Prof. Gustavo Gioia, OIST

2.2 Experiments on transitional pipe flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Prof. Gustavo Gioia, OIST
    • Prof. Jun Sakakibara, Meiji University, Japan

2.3 Frictional drag in two-dimensional flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Prof. Gustavo Gioia, OIST
    • Dr. Alexandre Vilquin, University of Bordeaux, France
    • Prof. Hamid Kellay,  University of Bordeaux, France
    • Prof. Charles-Henri Bruneau, University of Bordeaux, France

2.4 Granular flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Prof. Troy Shinbrot, Rutgers University, USA

3. Activities and Findings

3.1 Transitional pipe flow: the turbulent flow in a slug

The transition to turbulence in pipe flow proceeds through several distinct stages, eventually producing aggressively expanding regions of fluctuations, ‘slugs’, surrounded by laminar flow. We seek to elucidate the statistical nature of the fluctuating flow inside a slug (or slug flow). To that end, we studied the diagnostic profiles (namely, mean-velocity profiles, fluctuating-velocity profiles, and Reynolds stress profiles) in slug flow using experiments and direct numerical simulations (DNS). We conducted the experiments using the OIST pipe-flow setup wherein we measure the velocity field of slug flow using Particle Image Velocimetry (PIV). We conducted the simulations using the open-source, hybrid spectral DNS code, OPENPIPEFLOW. Our focus is on the Reynolds number (Re) regime where pipe flow can be either fully turbulent or transitional with slugs. Note that because no scaling laws exist for the diagnostic profiles in this Re regime, we compared the diagnostic profiles of slug flow and fully-turbulent flow with both flows at the same value of Re. We conclude that slug flow indistinguishable from a fully-turbulent flow proviso the two flows share the same value of Re. Our work highlights the rich Re-dependence of transitional pipe flows.

3.2 Scaling of frictional drag in rough-walled, two-dimensional turbulent flows

At high Re, the frictional drag between a rough wall and the overlaying turbulent flow, f, reaches an asymptotic state where f does not depend on the Re, but only on the roughness, r. To date, the experiments on f have focused on three-dimensional (3D) turbulent flows and have yielded the well-known Strickler scaling, f ~ r^{1/3}. Guided by the spectral theory of frictional drag, and in collaboration with the University of Bordeaux, we have experimentally and numerically studied two-dimensional (2D) turbulent flows over rough walls. Our results show that as in 3D flows, in 2D flows, too, f at high Re becomes independent of Re; but, distinct from 3D flows, f obeys a new scaling: f ~ r. This new scaling is beyond the purview of the standard theory of frictional drag but is in excellent accord with the predictions from the spectral theory.

3.3 Landfalling tropical cyclones

Tropical cyclones (known variously as typhoons, hurricanes, cyclones) are one of the most destructive natural phenomenon on Earth. Although the dynamics of tropical cyclones over oceans has been extensively studied, its past-landfall stage has received little attention. We studied landfalling tropical cyclones under idealized conditions using the computational model `Cloud Model 1,' a state-of-the-art computational tool that has been widely used to study tropical cyclones over oceans. Our preliminary findings suggest that landfalling tropical cyclones manifest a surprisingly rich set of dynamical behavior.

3.4 `Flying hot-wire' experiments for Taylor-Couette flow

We have begun a study of the spectral structure of turbulent Taylor-Couette (TC) flows using the technique of hot-wire annemometry. We have designed and equipped the OIST-TC experimental setup with a custom "flying hot-wire probe." This probe enables measurement of temporally-resolved velocity time series with either or both the cylinders of the TC setup rotating; we compute the spectra from the velocity series. We are benchmarking the data against stationary hot-wire anemometry and laser Doppler velocimetry. The preliminary results show excellent agreement.

4. Publications

4.1 Journals

  1. R. T. Cerbus, C-C. Liu, G. Gioia, and P. Chakraborty. 2020. Small-scale universality in the spectral structure of transitional pipe flows. Science Advances, Vol. 6, no. 4, eaaw6256.
  2. R. T. Cerbus, J. Sakakibara, G. Gioia, and P. Chakraborty. 2020. The turbulent flow in a slug: a re-examination. Journal of Fluid Mechanics,  vol. 883, article A13.
  3. K. A. Flack, M. P. Schultz, J. M. Barros. 2019. Skin Friction Measurements of Systematically-Varied Roughness: Probing the Role of Roughness Amplitude and Skewness. Flow Turbulence and Combustion, DOI: 10.1007/s10494-019-00077-1

4.2 Books and other one-time publications

Nothing to report

4.3 Oral and Poster Presentations

1. C. Butcher, T. McClung, S. Roundy, Code Trafficking: Smuggling Your Best Software, NI Week 2019, Austin, Texas, USA, May 21 (2019)

2. T. Sabuwala, Granular models for regolith distribution on planetary surfaces, Discussion meeting on Granular dynamics & Planetary terrain, Hikone, Shiga, Japan, August 6 (2019)

3. R. Cerbus, Friction and fluctuations in transitional pipe flow, University of Melbourne, Melbourne, Australia, September 24 (2019)

4. R. Cerbus, Friction and fluctuations in transitional pipe flow, Australian National University, Canberra, Australia, October 1 (2019)

5. P. Chakraborty, R. Cerbus, C-C. Liu, G. Gioia, Whorls in transitional pipe flows, Max Planck-Croucher Symposium 2019: Matter to Life, OIST, November 20 (2019)

6. Y. Hattori, Boundary-layer flow of air over a falling soap film, The 72nd Annual Meeting of American Physical Society’s Division of Fluid Dynamics, Seattle, USA, November 25 (2019)

7. P. Chakraborty, T. Sabuwala, C. Butcher, G. Gioia, How the moon got its rays, Tokyo University, Tokyo, Japan, January 9 (2020)

8. P. Chakraborty, R. Cerbus, C-C. Liu, G. Gioia, Whorls in transitional pipe flows, Tokyo University, Tokyo, Japan, January 10 (2020)


5. Intellectual Property Rights and Other Specific Achievements

Nothing to report

6. Meetings and Events

6.1 From Beating Hearts to Fluttering Wings:Flow Physics and Computation at the Intersection of Mechanics and Bioengineering

  • Date: June 13, 2019
  • Venue: OIST Campus Lab3
  • Speaker: Prof. Rajat Mittal (Johns Hopkins University)

6.2 Characterisation and modelling of wall turbulence with strong heat transfer

  • Date: August 19, 2019
  • Venue: OIST Campus Lab1
  • Speaker: Prof. Rene Pecnik (Delft University of Technology)

6.3 An update on K41 vs K62

  • Date: October 7, 2019
  • Venue: OIST Campus Lab1
  • Speaker: Prof. Robert Antonia (The University of Newcastle)

6.4 Impact of Turbulence on Cloud Microphysics

  • Date: November 18, 2019
  • Venue: OIST Campus Lab1
  • Speaker: Prof. Eberhard Bodenschatz (University of Göttingen)

6.5 Self-sustained coherent structures at the Kolmogorov microscale in turbulent shear-flow

  • Date: January 24, 2020
  • Venue: OIST Campus Lab1
  • Speaker: Dr. Ashley Willis (University of Sheffield)

6.6 Numerical simulation of dissipative solutions to the Euler equations

  • Date: January 27, 2020
  • Venue: OIST Campus Lab1
  • Speaker: Prof. Takeshi Matsumoto (Kyoto University)

7. Other

Nothing to report.