Laminar-Turbulent Transition of an Inlet Boundary Layer in a Circular Pipe Induced by Periodic Injection (Turbulence within Isolated Turbulent Patches and Its Growth Mechanism)

The Proceedings of Conference of Chugoku-Shikoku Branch

2018

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The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. The turbulence induced by the jet within the boundary layer developed into turbulent patches which then grew in the axial, circumferential and radial directions downstream. A single hotwire probe was used in the measurements. The axial velocity component was measured at four downstream stations, one within the inlet region and the other within the development region. Output voltage of the hot-wire was digitized and data was processed by a PC. An ensemble-averaging based on the signal from the jet trigger was performed. Mean and fluctuating velocity in the axial direction and intermittency factor which is the fraction of turbulence were obtained. In this research, three conclusions were obtained. First, the fluctuating velocity near the leading edge at the opposite side of jet hole is caused due to the crush of the jet on the wall. The fluctuation by the crush decreases downstream, though the fluctuation itself is sustained by the entrainment of the non-turbulent fluid. Second, the entrainment of the non-turbulent fluid to the turbulent patch near the trailing edge contributes to the axial growth of the patch. Third, an increase of the velocity in non-turbulent area between two patches contributes to the increase of the fluctuating velocity within and the axial growth of the turbulent patch.

Section: Experimental Apparatus and Methodsmentioning

confidence: 94%

Section: Experimental Apparatus and Methodsmentioning

confidence: 99%

Section: Experimental Apparatus and Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Laminar-Turbulent Transition in an Inlet Region of a Circular Pipe Induced by the Jet Disturbance

Nakatsu

The Proceedings of Conference of Chugoku-Shikoku Branch

2018

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“…For a long time, there has been interest in various phenomena in fluid science, e.g., visualization and visual output of hot-wires at the instantaneous clear boundary between the turbulent boundary layer and outer laminar region, wall-and free-shear turbulent flows exhibiting "coherent structure" (4) in random environments; and its strong relationship with the Reynolds shear stress is recognized in wall turbulence, turbulent puffs or slugs appearing in transitional circular pipes and turbulent spots in the transitional boundary layer and their boundaries are clear but irregular in time and space (5)(7)~ (9) . This paper concerns identification of the above nature of the turbulence.…”

Section: Overview Of Distinction Between Laminar and Turbulent Flowsmentioning

confidence: 99%

Randomness Representation in Turbulent Flows with Kolmogorov Complexity (In Mixing Layer)

Nakamura

2013

JFST

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Investigations of the definition and randomness of turbulence were reviewed. The Kolmogorov complexity measure of randomness was then introduced. Numerical and graphic data in the mixing layer formed downstream of a two-dimensional nozzle exit were compressed with the aid of a compression program. Approximated Kolmogorov complexity, AK, and normalized compression distance, NCD, were obtained. The AK indicated the regularity of the laminar flow and the randomness of the turbulent flow quantitatively. The NCD of the numerical value varied with data length. Between the same data, it approached zero, yet, between different data, it approached unity as the data length increased. The NCD of the numerical value in the natural transition process in the mixing layer increased monotonically downstream. Thus, the NCD appears to be the measure of the transition process. In the natural transition process in the mixing layer, the AK of the numerical value and the NCD of the graphic data did not change monotonously in the downstream direction. They therefore contain some uncertainty for measurement of the transition process.

Randomness representation in turbulent flows with Kolmogorov complexity (In laminar-turbulent transition due to periodic injection in an inlet boundary layer in a circular pipe)

Transactions of the JSME (In Japanese)

Nakamura

Tamura

2015

The complexity or randomness of the laminar-turbulent transition process of an inlet boundary layer in a circular pipe was analyzed by use of Kolmogorov complexity which is a very important measure of the randomness of various events. In the inlet region of the circular pipe, a jet that triggered a turbulent patch in the flow was periodically injected radially, and then propagated downstream periodically. Periodic velocity data which satisfies the phase constancy obtained from a hot-wire anemometer were compressed using a compression program running on a Windows PC. From the compressed data, the approximated Kolmogorov complexity, named AK, and normalized compression distance NCD were obtained. Although the distribution of AK of irregularly fluctuating velocity in the flow showed a slight discrepancy from the profile of fluctuating velocity intensity and intermittency factor, they exhibited qualitative consistency. Therefore, the objective judgment of the laminar-turbulent transition process based on the approximated Kolmogorov complexity mostly supports the conventional judgment. In conclusion, our study established the validity and usefulness of Kolmogorov complexity or algorithmic complexity to describe objectively the laminar-turbulent transition process in the mixing layer, flat plate boundary layer and pipe flow. With the aid of the AK we can distinguish the complexity within the turbulent region; on the other hand, the intermittency factor cannot.