40 research outputs found
Virtual wall oscillations forced by a dbd plasma actuator operating under beat frequency – a concept for turbulent drag reduction
No full textA modified concept for generating DBD-based virtual spanwise wall oscillations is introduced in continuation of earlier efforts by Hehner et al. (2019) ["Stokes-layer formation under absence of moving parts – A novel oscillatory plasma actuator design for turbulent drag reduc-tion", Phys. Fluids 31, 051701]. Four groups of a multi-electrode actuator array are operated at interfering high-voltage signals to achieve an oscillating discharge intensity at 50 Hz beat frequency. The resulting velocity fields have been recorded with time-resolved planar high-speed particle image velocimetry so as to analyse the induced flow topology and wall-normal velocity profiles. A direct comparison of the results to Hehner et al. (2019) has indicated favourable effects of the new concept in terms of flow topology (i.e improved spanwise flow homogeneity, velocity magnitude increase and reduced lift-off), which renders the new operation concept of the actuator array particularly promising for Stokes-layer-like flow formations and, consequently, for turbulent drag reduction over a range of Reynolds numbers.Aerodynamic
Riblets in fully developed turbulent channel flow
No full textThis database features high-precision pressure measurements in a wind tunnel facility used to determine the skin friction drag reduction of ribelts. Please refer to the README below for further information.Neuer Forschungsdatensatz zu finden unter https://publikationen.bibliothek.kit.edu/1000152366
## RIBLETS IN FULLY DEVELOPED TURBULENT CHANNEL FLOW
This database features high-precision pressure drop and flow rate measurements in an air channel flow facility with aspect ratio 1:12 which were taken to determine the skin friction drag reduction of riblets. In this facility the channel flow is fully turbulent for bulk Reynolds numbers above 5000. The dataset includes values for the smooth wall reference case and three geometrically different sets of riblets. Multiple flow rate measurement devices are used (see table1), namely an inlet nozzle and an orifice flow meter. The measurements were taken by Andreas Güttler, Lars von Deyn and Luigi Coppini.
table1:
| type | index | diameter | range in Reb |
| -------- -- |-------|-----------|--------------|
| orifice | 1 | 60[mm] |3000<Reb<13000
| orifice | 2 | 105[mm] |7000<Reb<38000
| orifice | 3 | 150[mm] |30000<Reb<89000
| inlet nozzle| 4 | 60[mm] |6000<Reb<24000
___
References:
A. Güttler. High accuracy determination of skin friction differences in an air
channel flow based on pressure drop measurements. PhD thesis, Karlsruhe Institute
of Technology, 2015.
DIN EN ISO 5167:Measurement of fluid flow by means of pressure differential devices
inserted in circular cross-section conduits running full, 2004.
___
Content:
* reference results of smooth turbulent channel flow measured in the range of 5000<Reb<89000.
* three geometrically different sets of ribelts measured in the range of 5000<Reb<89000.
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_____s_____
/\ /\ |
/ \ / \ |h
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table2:
| | s[mu] | h[mu] | tip angle | Reb_opt |
| ------- |-------|---------|-------------|-----------|
|set1 |614 |294 |53.5 | 11400 |
|set2 |170 |160 |50 | 50000 |
|set3 |86 |83 |51 | 97000 |
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Riblets in fully developed turbulent channel flow
No full textThis database features high-precision pressure measurements in a wind tunnel facility used to determine the skin friction drag reduction of ribelts. Please refer to the README below for further information
Dielectric-barrier discharge plasma actuators for turbulent friction-drag manipulation via spanwise oscillations
No full textEin Plasmaaktuator wird über instationäre Betriebsmodi angesteuert, um wandnahe
Fluidoszillationen zu erzeugen. Das Ziel ist es, spannweitig oszillierende
Wände zugunsten einer Verringerung des turbulenten Reibungswiderstands
nachzuahmen. Da der Aktuator keine beweglichen Teile besitzt, könnte er
sich als nicht-mechanischer Ersatz der oszillierenden Wand eignen. Die
Kombination von Betriebsmodus und zugrundeliegender Elektrodenanordnung
ist eine Neuerung, welche die spannweitige Homogenität der Strömung
solcher virtuellen Wandoszillationen verbessert. Die mechanische Charakterisierung
wird mittels eines planaren Feldmessverfahrens durchgeführt, um
sowohl die induzierten Strömungstopologien als auch die Effekte von Volumenkraft
und „virtueller Wandgeschwindigkeit“, d.h. Reaktion des Fluids,
aufzuzeigen. Daraus wird zur Bewertung und Optimierung der Leistungsfähigkeit
des Aktuators ein universelles Diagramm hinsichtlich aktuatorspezifischer
Parameter abgeleitet. Da die berechnete Volumenkraft die Art der
Kraftausübung gut widerspiegelt, kann diese modellhaft zu verbesserten numerischen
Simulationen der Aktuatorik dienen. Ferner wird eine neue Vorgehensweise
für die Bestimmung der elektrischen Leistung von Aktuatoren mit
mehreren Hochspannungselektroden bereitgestellt, welche die potenzielle Abschätzung
des Nettogewinns in aktiven Kontrollszenarien ermöglicht. Zuletzt
wird die unmittelbare Auswirkung der oszillatorischen Kraftausübung auf den
Reibungswiderstand in der Querebene einer voll entwickelten turbulenten
Kanalströmung mittels einer stereoskopischen Feldmesstechnik untersucht.
Im Wesentlichen verbleibt die Strömung im sich entwickelnden Stadium und
erfährt auf dem Aktuator eine Erhöhung des Reibungswiderstands, während
sich dieser stromab des Aktuators verringert
Effects of actuation mode on plasma-induced spanwise flow oscillations
No full textTwo different plasma actuation strategies for producing near-wall flow oscillations, namely the burst-modulation and beat-frequency mode, are characterized with planar particle image velocimetry in quiescent air. Both concepts are anticipated to work as non-mechanical surrogates of oscillating walls aimed at turbulent flow drag reduction, with the added benefit of no moving parts, as the fluid is purely manipulated by plasma-generated body forces. The current work builds upon established flow-control and proof-of-concept demonstrators, as such, delivering an in-depth characterization of cause and impact of the plasma-induced flow oscillations. Various operational parameter combinations (oscillation frequency, duty cycle and input body force) are investigated. A universal performance diagram that is valid for plasma-based oscillations, independent of the actuation concept is derived. Results show that selected combinations of body force application methods suffice to reproduce oscillating wall dynamics from experimental data. Accordingly, the outcomes of this work can be exploited to create enhanced actuation models for numerical simulations of plasma-induced flow oscillations, by considering the body force as a function of the oscillation phase. Furthermore, as an advantage over physically displaced walls, the exerted body force appears not to be hampered by resonances and therefore remains constant independent of the oscillation frequency. Hence, the effects of individual parameter changes on the plasma actuator performance and fluid response as well as strategies to avoid undesired effects can be determined. AerodynamicsFlow Physics and Technolog
Plasma-Based Forcing Strategies for Control of Crossflow Instabilities
No full textThe present work experimentally investigates two forcing strategies toward controlling stationary crossflow instability (CFI) induced transition manifesting on a swept wing at subsonic conditions. The effectiveness of upstream flow deformation (UFD) and the base-flow modification strategies, realized through the application of spanwise-modulated and spanwise-uniform dielectric barrier discharge plasma actuation, respectively, is compared experimentally. Specialized, patterned actuators that generate spanwise-modulated plasma jets have been fabricated using a spray-on technique and positioned near the leading edge. An array of discrete roughness elements (DREs) is installed upstream of the plasma forcing to lock the origin and evolution of the critical stationary CFI vortices in the boundary layer. The impact of the phase relation between the spanwise-modulated plasma jets and the incoming CFI vortices is inspected. Infrared thermography is employed to detect and quantify the transition location. A delay in transition is observed with all tested forcing configurations. However, as the incoming CFI vortices are highly amplified due to the application of DREs, the acquired results suggest that with spanwise-modulated forcing the control mechanism responsible for the observed transition delay is not purely UFD; rather the beneficial effects observed leverage on a combination of direct attenuation of the CFI vortices and localized base-flow modification, depending on the aforementioned phase relation. For all forcing strategies and configurations, a simplified drag reduction efficiency estimation is performed using the experimentally measured transition location and the electrical power use of the actuators. A net gain is found for selected configurations
An experimental investigation into Stokes-layer formation with oscillating dielectric barrier discharges
No full textAn AC-DBD plasma actuator is experimentally characterized, proposing a novel flow-control concept for turbulent drag reduction, yet decisively enhancing its control authority compared to former strategies. Essentially, the exposed and encapsulated silver electrodes of 1 and 3 mm width (6 µm thickness), respectively, are adjacently placed on a 500 µm thick PET dielectric, featuring a spanwise wavelength λz=4 mm. The plasma-generation system comprises three HV transformers that switch the electric field on this multi-electrode array with a duty cycle of 50 %, hence exerting opposed body-force oscillations. High-speed PIV is used to acquire phase-resolved velocity data with two different magnifications (40 and 80 px mm-1). Based on Reτ = 250, an optimal oscillation period T+ = 125 (λz+ = 80) is applied (Gatti & Quadrio, JFM 2016). The momentum transfer to the near-wall fluid above the dielectric results in a Stokes-layer-like flow of a wall-normal range y+ < 10. Variations of the wall-parallel velocity magnitude are analyzed in terms of spanwise homogeneity across the electrodes and found to be significantly reduced. As such, the presented study demonstrates a valuable step towards an ideal Stokes layer, mimicking moving parts with plasma discharges
Swept-wing transition control using DBD plasma actuators
No full textIn the present work, laminar flow control, following the discrete roughness elements (DRE) strategy, also called upstream flow deformation (UFD) was applied on a 45◦ swept-wing at a chord Reynold’s number of Rec = 2.1 · 106 undergoing cross-flow instability (CFI) induced transition. Dielectric barrier discharge (DBD) plasma actuation was employed at a high frequency (fac = 10kHz) for this purpose. Specialized, patterned actuators that generate spanwinse-modulated plasma jets were fabricated using spray-on techniques and positioned near the leading edge. An array of DREs was installed upstream of the plasma forcing to lock the origin and evolution of critical stationary CFI vortices in the boundary layer. Two forcing configurations were investigated-in the first configuration the plasma jets were directly aligned against the incoming CF vortices while in the second the CF vortices passed between adjacent plasma jets. Infrared thermography was used to inspect transition location, while quantitative measurements of the boundary layer were obtained using particle image velocimetry. The obtained results show that the plasma forcing reduces the amplitude of stationary CF modes, thus delaying laminar-to-turbulent transition. In contrast to previous efforts [1], the plasma forcing did not introduce unsteady fluctuations into the boundary layer. The mechanism responsible for the observed transition delay appears to leverage more on localised base-flow modification rather than the DRE/UFD control strategy.Aerodynamic
