FY2015 Annual Report

Fluid Mechanics Unit
Associate Professor Pinaki Chakraborty

Abstract

The Fluid Mechanics unit pursued research on turbulent flows, atmospheric flows, planetary flows, and granular flows, and continued to develop a joint fluid mechanics--continuum physics laboratory.

 

1. Staff

  • Dr. Chien-chia liu, Researcher
  • Dr. Rory Cerbus, Researcher
  • Mr. Christian Butcher, Technical Staff
  • Mr. Lin Li, Graduate Student
  • Ms. Yuna Hattori, Graduate Student
  • Ms. Kaori Egashira, Research Unit Administrator

 

2. Collaborations

2.1 Theory of spectral link in turbulent flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Professor Gustavo Gioia, OIST

2.2 Energetics of two-dimensional turbulence in soap films

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Professor Gregory Falkovich, Weizmann Institute of Science, Israel

2.3 Ballistic segregation on asteroid Itokawa

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Professor Troy Shinbrot, Rutgers University
    • Dr. Tapan Sabuwala, OIST

2.4 Granular cratering

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Professor Gustavo Gioia, OIST
    • Professor Susan Kieffer, University of Illinois, USA

2.5 Experiments on turbulent pipe flows

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

2.6 Experiments on Taylor-Couette flows

  • Type of collaboration: Scientific collaboration
  • Researchers:
    • Dr. Yasuo Higashi, OIST
    • Professor Gustavo Gioia, OIST

 

 

3. Activities and Findings

3.1 Janus spectra in two-dimensional turbulent flows

Using experiments of soap-film flows, we show that the mean shear sunders a 2D decaying turbulent flow into two mutually independent fields. These fields, which are the streamwise and transverse velocity fluctuations, exhibit a novel turbulent energy spectra, which we term the Janus spectra. Here, modulated by the interaction between the mean shear and the turbulent vorticity, the two fields concurrently partake in the two disparate cascades of 2D turbulence: the enstrophy cascade and the inverse energy cascade.

3.2 Ballistic segregation in the asteroid Itokawa

Photographs of the asteroid Itokawa reveal unexpectedly strong segregation between regions populated almost entirely by small pebbles and other regions consisting only of larger boulders. We propose that this size segregation may be caused by a very simple and previously unexplored effect, namely that debris collecting on the asteroid will rebound from boulders, but will sink into pebbly regions. We argue that the asteroid must have suffered many orders of magnitude more collisions with pebbles than with boulders, and we predict that seas of pebbles must grow on such asteroids. We have carried out experiments and simulations to evaluate this and related predictions, and we demonstrate that this mechanism of segregation based on simple counting of grains produces segregation that reliably obeys analytic formula. Our findings provide a simple and general framework to interpret surface features of asteroids like Itokawa.

3.3 Granular cratering

The surface of many planetary bodies, such as the Moon, are peppered with craters that stand testament to the violence inflicted by the bombarding meteorites. A seemingly remote problem -- dropping a rigid ball on a bed of grains -- reproduces many features of the planetary analog, thereby providing a handy tool to study the mechanics of cratering. In this continuing project, we are studying, using experiments and simulations, the role of the surface topography in shaping the morphology of the ejecta in granular cratering

3.4 Fully-developed turbulence in slugs of pipe flows

Despite over a century of research, transition to turbulence in pipe flows remains a mystery. In theory, the flow remains laminar for arbitrarily large Reynolds number, Re. In practice, however, the flow transitions to turbulence at a finite Re whose value depends on the disturbance, natural or artificial, in the experimental setup. The flow remains in the transition state for a range of Re ~ 0(1000); for larger Re the flow becomes fully developed. The transition state for Re > 3000 consists of axially segregated regions of laminar and turbulent patches. These turbulent patches, known as slugs, grow as they move downstream. Their lengths span anywhere between a few pipe diameters to the whole length of the pipe. In the OIST pipe-flow experiment, we have perfomed Stereo Particle Image Velocimetry (S-PIV) measurements in the cross-section of the slugs (see the figure below). Notwithstanding the continuous growth of the slugs, we find that the mean velocity and stress profiles in the slugs are indistinguishable from that of statistically-stationary fully-developed turbulent flows. Our results suggest that the phenomenological theory of fully-developed turbulence may provide insight into transitional flows that have long defied theory.

Figure: Velocity vectors in the cross-section of slugs. The arrows represent the in-plane velocity and the colors the streamwise velocity (in cm/s).

3.5 Experiments on turbulent Taylor-Couette flows

We have continued development of the OIST Taylor-Couette experiment. With water as the working fluid, we have conducted preliminary experiments where we measured the torque on the inner cylinder (using a load cell) and the azimuthal velocity profile in the gap between the cylinders (using the technique of Laser Doppler Velocimentry). We have also started work on adapting the experiment for rough inner cylinders.

 

4. Publications

4.1 Journals

  1. Cerbus, R., Goldburg, W.I. Predicting two-dimensional turbulence. Physical Review E, vol. 91, PP.043003

4.2 Books and other one-time publications

Nothing to report

4.3 Oral and Poster Presentations

  1. Chakraborty, P. The spectral link in turbulent frictional drag, Small Meets Large: Connecting Microfluidics with Marine Ecology, OIST, Japan, May 11 (2015).
  2. Cerbus, R., Liu, C., Gioia, G., Chakraborty, P. Cascade transition and anisotropy in decaying 2D turbulence, RIMS Kyoto: Fluid phenomena mediated by turbulence, Kyoto, Japan, July 24 (2015).
  3. Liu, C., Cerbus, R., Chakraborty, P. Interference fringes velocimetry, RIMS Kyoto: Fluid phenomena mediated by turbulence, Kyoto, Japan, July 24 (2015).
  4. Sabuwala, T., Gioia, G., Chakraborty, P. Effect of rainpower on hurricane intensity, 8th European Conference on Severe Storms-ECSS 2015, Vienna, Austria, September 15 (2015).
  5. Cerbus, R., Liu, C., Chakraborty, P., Gioia, G. Evolution of cascades in 2D turbulence, JPS 2015 Autumn Meeting, Osaka, Japan, September 18 (2015).
  6. Liu, C., Cerbus, R., Gioia, G., Chakraborty, P. Mean flow and anisotropic cascades in decaying 2D turbulence, 68th Annual Meeting of the APS Division of Fluid Dynamics, Boston, USA, November 22 (2015).
  7. Cerbus, R., Liu, C., Sakakibara, J., Gioia, G., Chakraborty, P. Fully developed turbulence in slugs of pipe flows, 68th Annual Meeting of the APS Division of Fluid Dynamics, Boston, USA, November 23 (2015).
  8. Shinbrot, T., Chakraborty, P., Sabuwala, T., Itokawa is not Brazil: granular segregation on asteroids, 68th Annual Meeting of the APS Division of Fluid Dynamics, Boston, USA, November 23 (2015).
  9. Goldburg W., Cerbus, R. Determining the direction of a turbulent cascade, 68th Annual Meeting of the APS Division of Fluid Dynamics, Boston, USA, November 24 (2015).
  10. Zhang, D., Gioia G., Chakraborty, P. Non-unique frictional drag in turbulent plane Coutte flows, 68th Annual Meeting of the APS Division of Fluid Dynamics, Boston, USA, November 24 (2015).
  11. Shinbrot, T., Sabuwala, T., Siu, T., Lazo, M., Chakraborty, P. Itokawa: a case for ballistic segregation, APS March meeting, Baltimore, USA, March 15 (2016).
  12. Cerbus, R., Liu, C., Chakraborty, P., Sakakibara, J., Gioia, G. Fully developed turbulence in slugs from pipe flow, JPS Annual Spring Meeting, Miyagi, Japan, March 22 (2016).

 

5. Intellectual Property Rights and Other Specific Achievements

Nothing to report

 

6. Meetings and Events

6.1 Icicle Morphology

  • Date: April 15, 2015
  • Venue: OIST Campus, Center building
  • Speaker: Prof. Stephen W. Morris (University of Toronto)

6.2 Cracking Lava into Columns

  • Date: April 16, 2015
  • Venue: OIST Campus, Center building
  • Speaker: Prof. Stephen W. Morris (University of Toronto)

6.3 Hardwiring: optical Illusions and simulated neuronal architecture

  • Date: September 9, 2015
  • Venue: OIST Campus, Lab 3
  • Speaker: Prof. Troy Shinbrot (Rutgers University)

6.4 Sedimentation and Resuspension from River Plumes

  • Date: February 3, 2016
  • Venue: OIST Campus, Lab 1
  • Speaker: Prof. Bruce R. Sutherland (University of Alberta)

6.5 Results of Hayabusa asteroid mission and future Japanese small-body missions

  • Date: February 26, 2016
  • Venue: OIST Campus, Center building
  • Speaker: Prof. Hirdy Miyamoto (University of Tokyo)

 

7. Other

Nothing to report.