Relaminarization of pipe flow by means of 3d-printed shaped honeycombs

Based on a novel control scheme, where a steady modification of the streamwise velocity profile leads to complete relaminarization of initially fully turbulent pipe flow, we investigate the applicability and usefulness of custom-shaped honeycombs for such control. The custom-shaped honeycombs are used as stationary flow management devices which generate specific modifications of the streamwise velocity profile. Stereoscopic particle image velocimetry and pressure drop measurements are used to investigate and capture the development of the relaminarizing flow downstream these devices. We compare the performance of straight (constant length across the radius of the pipe) honeycombs with custom-shaped ones (variable length across the radius). An attempt is made to find the optimal shape for maximal relaminarization at minimal pressure loss. The maximum attainable Reynolds number for total relaminarization is found to be of the order of 10.000. Consequently the respective reduction in skin friction downstream of the device is almost by a factor of 5. The break-even point, where the additional pressure drop caused by the device is balanced by the savings due to relaminarization and a net gain is obtained, corresponds to a downstream stretch of distances as low as approx.\ 100 pipe diameters of laminar flow

Relaminarising pipe flow by wall movement

Following the recent observation that turbulent pipe flow can be relaminarised by a relatively simple modification of the mean velocity profile, we here carry out a quantitative experimental investigation of this phenomenon. Our study confirms that a flat velocity profile leads to a collapse of turbulence and in order to achieve the blunted profile shape, we employ a moving pipe segment that is briefly and rapidly shifted in the streamwise direction. The relaminarisation threshold and the minimum shift length and speeds are determined as a function of Reynolds number. Although turbulence is still active after the acceleration phase, the modulated profile possesses a severely decreased lift-up potential as measured by transient growth. As shown, this results in an exponential decay of fluctuations and the flow relaminarises. While this method can be easily applied at low to moderate flow speeds, the minimum streamwise length over which the acceleration needs to act increases linearly with the Reynolds number.Comment: 13 pages, 9 figure

Relaminarization by steady modification of the streamwise velocity profile in a pipe

We show that a rather simple, steady modification of the streamwise velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarizes. Two different devices, a stationary obstacle (inset) and a device which injects fluid through an annular gap close to the wall, are used to control the flow. Both devices modify the streamwise velocity profile such that the flow in the center of the pipe is decelerated and the flow in the near wall region is accelerated. We present measurements with stereoscopic particle image velocimetry to investigate and capture the development of the relaminarizing flow downstream these devices and the specific circumstances responsible for relaminarization. We find total relaminarization up to Reynolds numbers of 6000, where the skin friction in the far downstream distance is reduced by a factor of 3.4 due to relaminarization. In a smooth straight pipe the flow remains completely laminar downstream of the control. Furthermore, we show that transient (temporary) relaminarization in a spatially confined region right downstream the devices occurs also at much higher Reynolds numbers, accompanied by a significant local skin friction drag reduction. The underlying physical mechanism of relaminarization is attributed to a weakening of the near-wall turbulence production cycle

Destabilizing turbulence in pipe flow

Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities, so far no methods exist to altogether eliminate turbulence if velocities are sufficiently large. We demonstrate for pipe flow that appropriate distortions to the velocity profile lead to a complete collapse of turbulence and subsequently friction losses are reduced by as much as 90%. Counterintuitively, the return to laminar motion is accomplished by initially increasing turbulence intensities or by transiently amplifying wall shear. Since neither the Reynolds number (Re) nor the shear stresses decrease (the latter often increase), these measures are not indicative of turbulence collapse. Instead an amplification mechanism, measuring the interaction between eddies and the mean shear is found to set a threshold below which turbulence is suppressed beyond recovery

Experimental investigation of transition to turbulence in a torus

Zsfassung in dt. SpracheThema der Arbeit ist der Übergang einer laminaren (geordneten) Strömung zu einer turbulenten (chaotischen) Strömung in einem Torus. Die toroidale Geometrie steht hierbei als ein verallgemeinertes Modell für gekrümmte Rohre. Der Übergang zur Turbulenz in gekrümmten Rohren ist grundlegend anders als der Übergang in geraden Rohren. Das Einsetzen turbulenter Eigenschaften in Anwesenheit kleiner Störungen ist in gekrümmten Rohren zu deutlich höheren Reynolds-Zahlen verschoben. Des Weiteren ändert sich auch das gesamte Übergangsszenario unter dem Einfluss der Rohrkrümmung fundamental. Eine vergleichsweise "sanfte" Abfolge von Übergängen statt eines plötzlichen, direkten Sprunges zu turbulenten Charakteristika ist zu beobachten. Ausgehend von einer stationären Grundströmung entwickelt sich bei steigender Reynoldszahl zunächst eine periodisch oszillierende Strömung, welche von einer quasi-periodischen und schließlich einer turbulenten Strömung mit chaotischem Verhalten abgelöst wird. Ein vergleichbares Übergangsszenario ist z.B. von der kanonischen Taylor-Couette-Strömung, der Strömung zwischen zwei konzentrisch rotierenden Zylindern, bekannt.Es entspricht dem Ruelle-Takens-Newhouse-Szenario, welches als eines von mehreren möglichen generischen Übergangsszenarien gilt.Ziel der Arbeit war es, den Übergang zur Turbulenz in gekrümmten Rohren mittels modernster Messtechnik experimentell zu untersuchen. Besonderes Augenmerk wurde dabei auf die Untersuchung sogenannter "wandernder Wellen" bzw. kohärenter Strömungsstrukturen gelegt. Diese entwickeln sich oberhalb des ersten kritischen Transitionspunktes, der Grenze zwischen stationärer Grundströmung und periodisch oszillierender Strömung (primäre Instabilität). Das Hauptziel bestand darin, die physikalischen Eigenschaften und die wichtigsten Charakteristika der primären Instabilität sowie zumindest ansatzweise die nachfolgende Entwicklung bei höheren Reynolds-Zahlen (sekundäre und tertiäre Instabilität) in einem gekrümmten Rohr zu beschreiben.Zu diesem Zweck wurde eine neuartige Versuchsanlage entworfen, aufgebaut, in Betrieb genommen und erfolgreich getestet. Mittels der Versuchsanlage kann eine exakt einstellbare Strömung in einem toroidalen Rohr erzeugt werden. Sie ist dahingehend optimiert, eine gekrümmte Rohrströmung mit nichtintrusiver Messtechnik bei beliebigen Durchflussraten bzw. Geschwindigkeiten zu untersuchen. Visuelle Beobachtungen, Laser-Doppler-Anemometrie, stereoskopische und hochfrequente Particle Image Velocimetry (S-PIV) sowie Druckdifferenzmessungen kamen bei den Experimenten zur Anwendung. Durch die Verwendung von S-PIV war es möglich, alle drei Komponenten der Geschwindigkeitsvektoren in der Messebene, die den gesamten Querschnitt des Rohres umfasste, zu rekonstruieren. Da hydrodynamische Instabilitäten vor allem ein dynamisches Phänomen sind, ergaben die zeitaufgelösten S-PIV-Messungen ein umfassendes und vollständiges Bild des gesamten Strömungsfeldes während des Übergangs zur Turbulenz.Bei der (kritischen) Reynolds-Zahl Re=4080 wurde ein superkritischer Übergang von einer stationären, laminaren Grundströmung zu einer laminaren Strömung, die in Strömungsrichtung periodisch moduliert ist, gefunden. Gleich oberhalb der kritischen Reynoldszahl ist die Amplitude der Modulation in Strömungsrichtung sehr klein. Mit zunehmender Reynolds-Zahl wächst die Modulationsamplitude und die damit verbundenen Strömungsmuster werden deutlicher. Die Intensität der Geschwindigkeitsschwankungen in Strömungsrichtung liegt in der Größenordnung von bis zu 15% der mittleren Geschwindigkeit. Die Modulation des Geschwindigkeitsfeldes in Hauptstromrichtung konnte auch mit Druckdifferenzmessungen nachgewiesen werden. Zeitlich gemittelte Geschwindigkeitsprofile, insbesondere deren Veränderung bei Erhöhung der Reynolds-Zahl, sowie Untersuchungen des zeitabhängigen Strömungsfeldes im Bereich der ersten Instabilität werden in der vorliegenden Arbeit vorgestellt. Die Geschwindigkeitsschwankungen werden beschrieben und die Frequenzspektren mittels Fourieranalyse ausgewertet.Die vorliegenden Ergebnisse erweisen sich als kompatibel mit den - im Vergleich zu geraden Rohren sehr wenigen - publizierten experimentellen Ergebnissen in Bezug auf die allgemeinen Merkmale des laminar-turbulenten Übergangsszenarios in gekrümmten Rohren.The central theme of the present thesis is the process of transition to turbulence in a torus, representing a model for curved pipes. Transition to turbulence in curved pipes is considerably different compared to transition in straight pipes, since the onset of turbulence is significantly delayed to higher Reynolds numbers even for small pipe curvature. Moreover, the whole transition scenario is substantially modified. A sequence of transitions instead of a sudden leap was found, leading from steady basic flow trough periodic and quasi-periodic states to a chaotic behavior, exhibiting analogies with the canonical Taylor-Couette flow and accordance with the Ruelle-Takens-Newhouse-Scenario.The aim of the work was to accurately investigate and measure transition to turbulence in curved pipe flow experimentally. Special attention was paid to the detection and investigation of traveling waves and coherent structures emerging with the first instability. The main objective was to reveal the physics and key points of the primary traveling wave instability and the subsequent development at higher Reynolds numbers in a curved pipe.To that end a novel experiment was designed, set up, put into operation and successfully tested, which realizes a precisely adjustable flow in a toroidal pipe. The facility was designed and optimized to investigate curved pipe flow by state-of-the-art nonintrusive measurement techniques at arbitrary Reynolds numbers, i.e. at different flow rates. Visual observations, laser-Doppler velocimetry, high-speed stereoscopic particle image velocimetry (S-PIV) and pressure drop measurements have been used to investigate and capture the appearance and development of the transitional flow. Using S-PIV it was possible to reconstruct all three components of the velocity vectors in the measurement plane covering the entire cross-section of the tube. As hydrodynamic instability is a dynamic, time variant phenomenon, the time-resolved S-PIV measurements yielded a comprehensive picture of the whole flow field during transition to turbulence.For a Reynolds number of 4080 a supercritical transition from the steady basic flow to a laminar flow which is periodically modulated in the streamwise (toroidal) direction was found. The emerging pattern is almost stationary in a frame of reference moving with the actuator, exhibiting a wave celerity slightly above the mean bulk velocity of the flow depending on the Reynolds number. Right above the critical Reynolds number the amplitude of the streamwise modulation is very small. With increasing Reynolds number, the modulation amplitude of the streamwise velocity grows and the associated flow pattern becomes more distinct.The intensity of the streamwise velocity fluctuations is of the order of up to 15% of the bulk velocity. The modulation of the streamwise velocity field is also distinctly reflected in the streamwise pressure drop. Mean velocity profiles, especially for increasing Reynolds number and investigations of the instantaneous flow field at the first instability are presented in the work. The large scale velocity fluctuations are described and the frequency spectra are analyzed.The present results were found fully compatible with the few published experimental and computational results concerning the general features of transition in curved and helical pipes and related scenarios.11

Relaminarization of pipe flow by means of 3D-printed shaped honeycombs

Based on a novel control scheme, where a steady modification of the streamwise velocity profile leads to complete relaminarization of initially fully turbulent pipe flow, we investigate the applicability and usefulness of custom-shaped honeycombs for such control. The custom-shaped honeycombs are used as stationary flow management devices which generate specific modifications of the streamwise velocity profile. Stereoscopic particle image velocimetry and pressure drop measurements are used to investigate and capture the development of the relaminarizing flow downstream these devices. We compare the performance of straight (constant length across the radius of the pipe) honeycombs with custom-shaped ones (variable length across the radius) and try to determine the optimal shape for maximal relaminarization at minimal pressure loss. The optimally modified streamwise velocity profile is found to be M-shaped, and the maximum attainable Reynolds number for total relaminarization is found to be of the order of 10,000. Consequently, the respective reduction in skin friction downstream of the device is almost by a factor of 5. The break-even point, where the additional pressure drop caused by the device is balanced by the savings due to relaminarization and a net gain is obtained, corresponds to a downstream stretch of distances as low as approximately 100 pipe diameters of laminar flow