Fluiddynamik und Turbulenz (B1) - Transition in Rohrströmung

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Experimentelle Untersuchungen der Kinematik und Dynamik von transitionalen durch optimierte Störungen erzeugte Strömungsstrukturen in einem Rohr

Projektleitung:Dipl.-Ing. Hermann Lienhart, Prof. Dr.-Ing. habil. Antonio Delgado
Beteiligte:Dipl.-Ing. Hermann Lienhart, Dipl.-Technomath. Jens Krauss, Prof. Dr.-Ing. habil. Antonio Delgado, ,
Förderer:
DFG
Stichwörter:Transition, kohärente Strukturen, TW (traveling waves)
Laufzeit:1.1.2010 - 31.12.2015
Inhalt und Ziele:Trotz der Tatsache, dass die voll entwickelte laminare Rohrströmung (RS) linear stabil ist, wird in der Realität eine Transition der laminaren Strömung zu einem turbulenten Strömungszustand beobachtet. Wird in der Strömung bei niedriger Re-Zahl eine lokalisierte Störung induziert, treten bestimmte Strömungsstrukturen vor und während der Transition auf. Als solche Strömungsstrukturen sind seit geraumer Zeit Puff- und Slug-Strukturen bekannt. Vor kurzem wurden numerisch vor der Transition großskalige Strömungsstrukturen, sogenannten Traveling Waves (TW), die exakte instationäre numerische Lösungen der Navier-Stokes Gleichungen sind, nachgewiesen. Es ist jetzt von großem Interesse, diese Strömungsstrukturen auch experimentell zu untersuchen, da noch keinerlei Erkenntnisse über die Art und die Stärke der Störungen vorliegen, welche zu diesen Strukturen führen. Ferner ist die Entwicklung der Transitionsstrukturen bei zunehmender Re-Zahl, noch nicht vollständig klar. So ist zum Beispiel unbekannt, ob sich nur bestimmte TW-Typen zu turbulenten Puffs oder Slugs weiterentwickeln. Unabhängig von dieser dynamischen Beschreibung der Turbulenzvorgänge haben kinematische Untersuchungen der Anisotropie von Geschwindigkeitsfluktuationen gezeigt, dass Anisotropie- Invarianten von Geschwindigkeitsfluktuationen während der Transition bestimmten Pfaden folgen. Was nun aussteht, ist eine Zusammenführung der Ergebnisse der Betrachtungen zur Dynamik der transitionalen Strömungsstrukturen mit denen der kinematischen Untersuchungen. Das vorgeschlagene Projekt zielt deshalb darauf ab, den Mechanismus der Entstehung von TW in Rohren, den Zusammenhang zwischen den TW und den turbulenten transitionalen Puff- bzw. Slug-Strukturen sowie die Grenzen zwischen ihnen experimentell zu untersuchen. Darüber hinaus sollen die Verbindung zwischen den dynamischen und kinematischen Befunden über transitionale Strukturen hergestellt und daraus Strategien zur Kontrolle der Transition konzipiert werden. Dazu werden Geschwindigkeitsmessungen mit der Hitzdrahtanemometrie und mit Stereo-PIV durchgeführt und für die direkte Bestimmung der Lebensdauer von Puff-Strukturen werden Drucktransienten gemessen. Mit Hilfe von Strömungsvisualisierungen in Verbindung mit Mustererkennung und Optimierungsalgorithmen werden die optimalen Störungsparameter zur Erzeugung der verschiedenen TW-Typen identifiziert. Die Entwicklung einer Technik zur gezielten Erzeugung von verschiedenen Formen von TW auf eine reproduzierbare Art und Weise, ermöglicht es, in der zweiten Projektphase das Geschwindigkeitsfeld in den TW quantitativ zu untersuchen und die Entwicklung von TW im Rohr bis ins turbulente Stadium zu verfolgen. Schließlich werden die gemessenen Profile der turbulenten Spannungen und deren Anisotropie, die Wahrscheinlichkeit des Auftretens und die Lebensdauerstatistik sowie die räumliche Topologie der kohärenten Strömungsstrukturen analysiert im Vergleich mit denjenigen, die in Taylor-Couette-Strömungen (TC) und der Rayleigh-B\'enard-Konvektion (RB) vorgefunden werden. Übergreifendes Ziel ist es, die gemeinsamen Transportmechanismen in diesen Strömungen aufzuzeigen.
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Experimental investigations on the dynamics and kinematics of transitional flow structures in a pipe generated by optimized disturbances

I.Summary

Despite the fact that fully-developed laminar pipe flow (PF) is linearly stable, in reality laminar-to-turbulent transition of flow start to occur in pipes at various Reynolds numbers (Re), depending on the existing disturbances in the flow. When a localized disturbance is induced in the flow, at low Re, certain flow structures starts to occur prior to transition and during transition. Among those flow structures, turbulent puff and slug structures have been known for a long time. Recently, it was shown numerically that large scale flow structures, the so called traveling waves (TW), occur prior to transition. Despite the numerical discovery of TW, it is still a challenge to investigate these flow structures experimentally, because of the lack of knowledge on the types and the amplitude of disturbances generating such structures. Moreover, the evolution of the transitional structures with increasing Re has not been fully understood. For example, whether TW evolve to a turbulent puff is not known, if it evolves, next it should be found whether only certain type of TW develops to a turbulent puff or slug. In the last decade at LSTM-Erlangen, the kinematical consideration on the anisotropy of velocity fluctuations have shown that anisotropy invariants of velocity fluctuations follow certain paths during transition. However, no effort has been made to converge the results obtained from the dynamics of the transitional flow structures and the kinematical considerations. Hence, the proposed project aims at investigating the generation mechanism of TW in pipes, their connection to turbulent transitional structures puff and slug and the border between them, the connection between the dynamical and kinematical findings on transitional structures, and at establishing strategies to control transition. The investigations will be of experimental nature. Velocity measurements with hot-wire anemometry and stereo PIV will be conducted, pressure transients will be measured for the direct determination of lifetime of puffs. Flow visualization coupled with pattern recognition and optimization algorithm will be employed to find optimum disturbance parameters for each type of TW, so that the connection between the disturbance and the generated TW will be efficiently established. This data would help to construct a map showing the border between the laminar and turbulent states. Being able to generate TW of different shapes in a reproducible manner allows us, at the second phase of the project, to investigate quantitatively the velocity field within the TW and in the development of TW along the pipe till it becomes turbulent. Hence, the data made available by the experiments would help to bridge the dynamical and kinematical considerations and construct strategies to control transition. Ultimately, the measured profiles of turbulent stresses and their anisotropy, probability and lifetime statistics, and the peculiarities of transitional flow structures will be analyzed together with those found in Taylor-Coutte flows (TC) and Rayleigh-Bennard convection (RB), to show the common transport mechanisms in those flows.

Mapping  the anisotropy invariants of turbulent stresses may reveal the connection between the three types of flows

Mapping  the anisotropy invariants of turbulent stresses may reveal the connection between the three types of flows

II.Objectives

Accordingly, there are five major objectives of the proposed experimental investigations:

  1. To conduct measurements of the turbulent velocity fluctuations and pressure transients and to construct profiles of Reynolds stresses and their anisotropies within puff, splitting puff and slug structures, and the probability, lifetime, propagation speeds of those transitional structures.
  2. To investigate the generation of TW of different types in the low Re range by the selection of proper disturbance parameters with the help of optimization algorithms. This study reveals the border between stable laminar states of the flow and the chaotic turbulent states of the flow, and the connection between the disturbance and the shapes of TW.  As a result, each type can be generated reproducibly and studied separately. Ultimately, the path toward transition can be better described.
  3. To investigate the velocity field and its statistics within the transitional structures, especially in TW, in order to establish the connection between findings made for laminar TW structures and for the turbulent puff and slug structures.
  4. To find out the similarities between the observed flow structure in PF, and RB and TC flows on the basis of turbulent stress profiles, their path on the AI-map, probability and lifetime statistics.
  5. To find out proper strategies for the control of transitional structures.

III.Work plan and methods

This project comprises three parts, which will be accomplished within 6 years.

Part 1

The dynamics and kinematics of puff, puff splitting and slug structures by hot-wire and pressure measurements with the available  PF transition facility will be studied.  A pipe with larger diameter will be employed with the same test-rig to have better spatial resolution of the hot-wire probes so that profiles of turbulent stresses and their anisotropies within transitional structures can be precisely measured.

 

Transition pipe flow facility at LSTM-Erlangen of FAU Erlangen-Nürnberg

Transition pipe flow facility at LSTM-Erlangen of FAU Erlangen-Nürnberg

Part 2

Investigations on the generation of TW's of different shapes with disturbances optimized by genetic algorithms will be conducted. For the measurements, visualizations made my two high-speed cameras will be used together with birefringent flow visualization method.


Strategy to investigate the travelling waves The schematics of the  pipe flow facility for the investigations of travelling waves
The strategy to investigate the travelling waves The schematics of the  pipe flow facility for the investigations of travelling waves
The swirling pipe flow facility of LSTM-Erlangen will be modified for the investigations of travelling waves
The swirling pipe flow facility of LSTM-Erlangen will be modified for the investigations of travelling waves

 

Part 3:

Investigations of the dynamics and kinematics within each kind of TW and other transitional structures with the help of stereo PIV measurements will be performed.

 

IV.Current activities achievements and publications

Hierarchical occurrence of turbulent transitional structures

Hierarchical occurrence of turbulent transitional structures at the end of 566 D pipe(presented at EFMC-8 & STAB-2010)
Hierarchical occurrence of turbulent transitional structures at the end of 566 D pipe(presented at EFMC-8 & STAB-2010)

The first part of the project was planned to be conducted by the existing transition pipe-flow facility of LSTM-Erlangen. However, preliminary studies have shown that determination of Reynolds number and reliable operation of disturbance unit  during a measurement campaign, which lasts more than a week,  becomes a bottle neck. Hence, the facility is equipped with extra pressure transducer for accurate determination of the Reynolds number and the disturbance unit is further modified to enhance its reliability. Hence, through integration of all equipments via a comprehensive DAQ program, it was possible to conduct long time measurements. With the introduction of the automatic structure recognition algorithm, the occurrence probability of different types of turbulent transitional flow structures could be detected at the end of pipes with different pipe lengths

Tracking the turbulent transitional structures

The probability statistics obtained with the present facility were not matching to those available in the literature. It was suspected that the kind of disturbance introduced in our facility may not be generating each time turbulent structure. Hence,  it was decided to follow each disturbance along the pipe and increase the consistency  in our evaluation. For this purpose a hot-wire sensor is developed. This sensor is simply a wire stretched along the complete diameter and had its heated portion only at the center of the pipe. By selecting a very thin wire, the maximum Re number about the wire is kept below 100 hence,  flow is not allowed to generate a unsteady wake.  At the moment only 8 sensors and bridges are available. Measurements conducted with this system show how a disturbance evolves along the pipe. It is observed that disturbances can decay (Re < 2200) start to grow (Re > 2200) through multiple splitting and merging.  Furthermore, the dependency on the disturbance type cannot be neglected.

Our preliminary experiments have shown that the length of our pipe is not sufficient to investigate the dissipation regime (Re < 2200). Moreover, splitting is a question of travel time of the structures. Therefore, we are now building a 48 m pipe (3200 D) along which 35 sensors will be installed with which we can track each disturbance induced on the flow.


Schematics of the flow facility for tracking the transitional flow structures as they travel downstream of the disturbance unit Normalized hot-wire signals showing the evolution of one-disturbance to a slug structure via multiple splitting in the pipe at Re=2740. Blue lines are the signals recorded by different sensors, red dashed line is showing the disturbance signal (will be presented at ETC-13, Warsaw and FOR 1182 Workshop Durkheim)
Schematics of transition pipe flow facility used for tracking transitional flow structures as they travel downstream of the disturbance unit
Picture of the experimental facility for tracking the transitional flow structures in a pipe
Picture of the transition pipe flow facility for tracking transitional flow structures Normalized hot-wire signals showing the evolution of one-disturbance to a slug structure via multiple splitting in the pipe at Re=2740. Blue lines are the signals recorded by different sensors, red dashed line is showing the disturbance signal (will be presented at ETC-13, Warsaw and FOR 1182 Workshop Durkheim)

 

Generation of travelling waves

Two more PhD students  were engaged to conduct the investigations in Part 2 and Part 3. They are financed by Erlangen Graduate School of Optical Technologies and DAAD.  They are going to modify the swirling pipe flow facility for the study of travelling waves. Initial focus will be on the optical visualization  technique, pattern recognition and  development of the disturbance unit.

V.Team

Name Role
Dr.-Ing. Özgür Ertunç Project leader / researcher
Dipl.-Ing. Hermann Lienhart Project leader / researcher
Prof. Dr.-Ing. Antionio Delgado Project leader
Dipl.-Technomath.  Jens Krauß PhD candidate
Dipl.-Phys. Alena Taranka, MSc. PhD candidate
Nesrin Talat, MSc. PhD candidate
Horst Weber Electronic technician
Christina Ostwald Student
Hannes Schweiger Student
Pablo Ruiz Miranda Student

VI.Publications

[1] Ertunç, Ö., Krauß, J., Lienhart, H., Miranda, P.R., Delgado, A.: “Tracking the transitional flow structures in low Reynolds number pipe flow”, 13th European Turbulence Conference (ETC 13), Warsaw,  Polland, September, 2010.

[2] Krauß, J., Ertunç, Ö., Lienhart, H., Ostwald, C., Delgado, A.: “ Evolution of transitional structures from puff to slug through multiple splitting in a pipe flow at low Reynolds number“,13th European Turbulence Conference (ETC 13), Warsaw,  Polland, September, 2010.

[3] Krauß, J., Ertunç, Ö., Lienhart, H., Ostwald, C., Delgado, A.: “Charakterisierung der Strukturen in transitionalen Rohrströmungen“, 17. DGLR-Fach-Symposium der STAB, Berlin, November, 2010.

[4] Ertunç, Ö., Krauß, J., Nishi, M., Ostwald, C.,  Lienhart, H., Delgado, A. : “Statistics of single and splitting puffs in a pipe flow at low Reynolds numbers”, EFMC 8, Bad Reichenhall, 2010.

[5] Nishi, M., Ertunç, Ö.,  Delgado, A. : “On the direct lifetime measurement of transitional flow structures and puff splitting in low Reynolds number pipe flows “, under consideration, J. Fluid Mech, 2010.

[6] Kauß, J.,  Ertunç, Ö., Ostwald, Ch., Lienhart, H., Delgado, A. : “ Probability of Occurrence and Propagation Speed of the Turbulent Transitional Flow Structures in Low Reynolds number Pipe Flows“,under consideration,  New Results in Numerical & Experimental Fluid Mechanics, 2010.

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