LES-based Study of the Roughness Effects on the Wake of a Circular Cylinder from Subcritical to Transcritical Reynolds Numbers

This paper investigates the effects of surface roughness on the flow past a circular cylinder at subcritical to transcritical Reynolds numbers. Large eddy simulations of the flow for sand grain roughness of size k/D = 0.02 are performed (D is the cylinder diameter). Results show that surface roughness triggers the transition to turbulence in the boundary layer at all Reynolds numbers, thus leading to an early separation caused by the increased momentum deficit, especially at transcritical Reynolds numbers. Even at subcritical Reynolds numbers, boundary layer instabilities are triggered in the roughness sublayer and eventually lead to the transition to turbulence. The early separation at transcritical Reynolds numbers leads to a wake topology similar to that of the subcritical regime, resulting in an increased drag coefficient and lower Strouhal number. Turbulent statistics in the wake are also affected by roughness; the Reynolds stresses are larger due to the increased turbulent kinetic energy production in the boundary layer and separated shear layers close to the cylinder shoulders.We acknowledge “Red Española de Surpercomputación” (RES) for awarding us access to the MareNostrum III machine based in Barcelona, Spain (Ref. FI-2015-2-0026 and FI-2015-3-0011). We also acknowledge PRACE for awarding us access to Fermi and Marconi Supercomputers at Cineca, Italy (Ref. 2015133120). Oriol Lehmkuhl acknowledges a PDJ 2014 Grant by AGAUR (Generalitat de Catalunya). Ugo Piomelli acknowledges the support of the Natural Sciences and Engineering Research Council (NSERC) of Canada under the Discovery Grant Programme (Grant No. RGPIN-2016-04391). Ricard Borrell acknowledges a Juan de la Cierva postdoctoral grant (IJCI-2014-21034). Ivette Rodriguez, Oriol Lehmkuhl, Ricard Borrell and Assensi Oliva acknowledge Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (ref. ENE2014-60577-R).Peer ReviewedPostprint (author's final draft

Superposition of AC-DBD plasma actuator outputs for three-dimensional disturbance production in shear flows

This investigation explores the utility of Alternating Current, Dielectric Barrier Discharge (AC-DBD) plasma actuators for producing three-dimensional disturbances of a desired spanwise wavelength via superposition. The technique utilizes two pairs of exposed and covered electrodes on a single dielectric layer arranged in streamwise succession. Two-dimensional forcing is achieved through operation of the upstream, spanwise uniform electrode pair, while three-dimensional forcing at a prescribed spanwise wavelength is attained by operating both electrode pairs simultaneously, with the downstream actuator spanwise modulating the upstream, two-dimensional output. The ability to produce disturbances of different spanwise wavelengths but with equal streamwise wavelength, frequency and total momentum is established through a combined characterization effort that considers quiescent and in-flow conditions. A demonstration of the technique in an exemplary wall-bounded shear flow, a laminar separation bubble, is provided, revealing spanwise wavelength dependent disturbance growth in the flow that could be exploited for performance gains in future flow control endeavours. Graphical abstract: [Figure not available: see fulltext.]</p

Effect of Local DBD Plasma Actuation on Transition in a Laminar Separation Bubble

This work examines the effect of local active flow control on stability and transition in a laminar separation bubble. Experiments are performed in a wind tunnel facility on a NACA 0012 airfoil at a chord Reynolds number of 130 000 and an angle of attack of 2 degrees. Controlled disturbances are introduced upstream of a laminar separation bubble forming on the suction side of the airfoil using a surface-mounted Dielectric Barrier Discharge plasma actuator. Time-resolved two-component Particle Image Velocimetry is used to characterise the flow field. The effect of frequency and amplitude of plasma excitation on flow development is examined. The introduction of artificial harmonic disturbances leads to significant changes in separation bubble topology and the characteristics of coherent structures formed in the aft portion of the bubble. The development of the bubble demonstrates strong dependence on the actuation frequency and amplitude, revealing the dominant role of incoming disturbances in the transition scenario. Statistical, topological and linear stability theory analysis demonstrate that significant mean flow deformation produced by controlled disturbances leads to notable changes in stability characteristics compared to those in the unforced baseline case. The findings provide a new outlook on the role of controlled disturbances in separated shear layer transition and instruct the development of effective flow control strategies.Aerodynamic

Extracting dominant three-dimensional coherent structures from timeresolved planar PIV measurements in the wakes of cylindrical bodies

The present study investigates two techniques for phase averaging time resolved two-dimensional PIV measurements for the purpose of extracting dominant three-dimensional wake characteristics. The first one is the classical phase averaging which uses a reference wake velocity signal measured via Laser Doppler Velocimetry (LDV). The second technique involves obtaining the phase information in each measurement plane using Proper Orthogonal Decomposition (POD) of PIV data and then establishing the relative phase between the planes utilizing the reference LDV signal. These techniques are applied to the results of numerical simulations and experiments on the flow past a circular cylinder immersed in a uniform shear flow. Numerical simulations are completed for ReD = 100 and 300, and experiments are completed for ReD = 2100. The phase-averaged results show that both techniques are able to reconstruct the oblique shedding of dominant spanwise vortices. For the basic case of laminar shedding at ReD = 100, the techniques perform equally well. However, the new POD-based approach is superior when there are temporal variations in the threedimensional wake topology present in the uniform cylinder wake for ReD = 300 and 2100

Spanwise flow structures within a laminar separation bubble on an airfoil

The present study considers the development of a Laminar Separation Bubble on the suction side of a NACA0018 airfoil under natural and forced conditions. Deterministic forcing is applied by means of a two-dimensional plasma actuator installed on the airfoil surface. The spatiotemporal characteristics of the bubble are measured using time-resolved, two-component Particle Image Velocimetry in streamwise and spanwise planes. Analysis of the results shows that while the time-average bubble is strongly two-dimensional, the dominant coherent structures assume three dimensional organisation in the vicinity of laminarturbulent breakdown in both natural and forced conditions.</p

Spanwise flow structures within a laminar separation bubble on an airfoil

The present study considers the development of a Laminar Separation Bubble on the suction side of a NACA0018 airfoil under natural and forced conditions. Deterministic forcing is applied by means of a two-dimensional plasma actuator installed on the airfoil surface. The spatiotemporal characteristics of the bubble are measured using time-resolved, two-component Particle Image Velocimetry in streamwise and spanwise planes. Analysis of the results shows that while the time-average bubble is strongly two-dimensional, the dominant coherent structures assume three dimensional organisation in the vicinity of laminarturbulent breakdown in both natural and forced conditions.Aerodynamic

Vortex merging in a laminar separation bubble under natural and forced conditions

Vortex merging in a laminar separation bubble (LSB) is studied experimentally. The bubble is formed on the suction side of a NACA 0018 airfoil at an angle of attack of 4° and a Reynolds number of 125 000. The merging process in the bubble is manipulated through acoustic forcing applied as a tone at either the fundamental vortex shedding frequency or at the first subharmonic of this frequency. The flow field is assessed using time-resolved, two-component particle image velocimetry. A method for detecting merged structures using wavelet analysis is introduced, allowing for reliable quantification of merging events. The results show that vortex merging occurs naturally in the separation bubble, while forcing at the subharmonic and fundamental frequencies promotes and inhibits merging, respectively. While these trends are similar to those observed for free shear layers, the subharmonic forcing of an LSB is found to directly promote disturbance development at the subharmonic frequency. For all cases, the majority of merging events take place in the aft portion of the bubble, i.e., downstream from the maximum bubble height location and upstream of mean reattachment, with subharmonic forcing causing merging to shift upstream. The merged structures are found to be the most energetic flow features; however, promoting vortex merging through subharmonic forcing does not lead to significant changes in the mean bubble topology. The spanwise behavior of the vortex merging process is studied, revealing that structures merge in a spanwise nonuniform manner, with localized merging occurring away from where forward or rearward streamwise bugles develop in the vortex filaments. Statistical characterization reveals that merging tends to occur more often over some specific spanwise segments, with the number of primary structures that merge varying by as much as 50% between spanwise locations. These findings offer insight into vortex merging in laminar separation bubbles and the attendant influence of forcing, while also highlighting the need to consider spanwise aspects of flow development.Aerodynamic

The effects of three-dimensional forcing on flow development within a laminar separation bubble

This work examines flow development in a laminar separation bubble (LSB) undergoing natural transition and transition controlled with two-dimensional and spanwise modulated disturbances. The investigation is carried out in a series of wind tunnel tests, with the separation bubble formed over a flat plate subjected to an adverse pressure gradient. Velocity field measurements are performed using time-resolved, two-component Particle Image Velocimetry (PIV). Disturbances are produced using surface-mounted plasma actuators in a novel configuration that allows for the introduction of controlled disturbances that are two-dimensional or of a prescribed spanwise wavelength. The natural transition process is dominated by shear layer vortex shedding which is characterized by significant spanwise deformations in the aft portion of the bubble. When the flow is subjected to either two or three-dimensional forcing, vortex formation within the separation bubble is rendered two-dimensional. However, while the two-dimensionally forced perturbations remain largely two-dimensional until breakdown, a clear spanwise wavelength that matches the input wavelength of the forcing develops when the flow is subjected to the spanwise modulated forcing. The reported findings point to the presence of a secondary instability in the separation bubble, which leads to the amplification of the initially weak spanwise component of input disturbances, causing the shear layer vortices to develop significant spanwise undulations.Aerodynamic

Response of a laminar separation bubble to impulsive forcing

The spatial and temporal response characteristics of a laminar separation bubble to impulsive forcing are investigated by means of time-resolved particle image velocimetry and linear stability theory. A two-dimensional impulsive disturbance is introduced with an alternating current dielectric barrier discharge plasma actuator, exciting pertinent instability modes and ensuring flow development under environmental disturbances. Phase-averaged velocity measurements are employed to analyse the effect of imposed disturbances at different amplitudes on the laminar separation bubble. The impulsive disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding in the aft part ceases. Duration of recovery of the bubble to its unforced state is independent of the forcing amplitude. Quasi-steady linear stability analysis is performed at each individual phase, demonstrating reduction of growth rate and frequency of the most unstable modes with increasing forcing amplitude. Throughout the recovery, amplification rates are directly proportional to the shape factor. This indicates that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. The emerging wave packet is characterised in terms of frequency, convective speed and growth rate, with remarkable agreement between linear stability theory predictions and measurements. The wave packet assumes a frequency close to the natural shedding frequency, while its convective speed remains invariant for all forcing amplitudes. The stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop. The results of this study shed light on the response of laminar separation bubbles to impulsive forcing, providing insight into the attendant changes of flow dynamics and the underlying stability mechanisms.Wind EnergyAerodynamic