Spanwise flow development within a laminar separation bubble under natural and forced transition

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.expthermflusci.2018.02.032 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The variation of streamwise and spanwise characteristic wavelengths of a NACA 0018 laminar separation bubble under natural and periodic excitation conditions is investigated experimentally. Periodic forcing is applied with an AC-DBD plasma actuator, and the response of the bubble is characterised in two orthogonal planes by means of time-resolved particle image velocimetry. Periodic excitation results in substantial time-averaged size reduction of the bubble. Linear stability analysis is used to establish that the most notable flow deformation is achieved when excitation is applied at the most unstable frequency, which does not significantly vary (<4%) for the range of excitation parameters investigated. At excitation frequencies well below the unstable frequency band, the shear layer does not lock to the excitation and is, instead, modulated. Lock-in is achieved at higher forcing frequencies, which are within the unstable band. For the case of modulated shedding, spanwise deformations become more significant than in the natural case; whereas when shedding becomes locked to the excitation frequency, the coherence of the rollers along the span increases. Characteristic streamwise and spanwise wavelengths are statistically quantified by means of spatial wavelet analysis, demonstrating that spanwise deformations attain wider range of wavelengths than the respective streamwise rollers. Analysis of these results suggests that spanwise deformation is associated to both the incoming boundary layer and shear layer stability characteristics

On vortex shedding from an airfoil in low-Reynolds-number flows

Development of coherent structures in the separated shear layer and wake of an airfoil in low-Reynolds-number flows was studied experimentally for a range of airfoil chord Reynolds numbers, 55×103 6 Rec 6 210×103 , and three angles of attack, α = 0◦ , 5◦ and 10◦ . To illustrate the effect of separated shear layer development on the characteristics of coherent structures, experiments were conducted for two flow regimes common to airfoil operation at low Reynolds numbers: (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results demonstrate that roll- up vortices form in the separated shear layer due to the amplification of natural disturbances, and these structures play a key role in flow transition to turbulence. The final stage of transition in the separated shear layer, associated with the growth of a sub-harmonic component of fundamental disturbances, is linked to the merging of the roll-up vortices. Turbulent wake vortex shedding is shown to occur for both flow regimes investigated. Each of the two flow regimes produces distinctly different characteristics of the roll-up and wake vortices. The study focuses on frequency scaling of the investigated coherent structures and the effect of flow regime on the frequency scaling. Analysis of the results and available data from previous experiments shows that the fundamental frequency of the shear layer vortices exhibits a power law dependency on the Reynolds number for both flow regimes. In contrast, the wake vortex shedding frequency is shown to vary linearly with the Reynolds number. An alternative frequency scaling is proposed, which results in a good collapse of experimental data across the investigated range of Reynolds numbers.NSER

Effect of Acoustic Excitation Amplitude on Airfoil Boundary Layer and Wake Development

The effect of acoustic excitation amplitude on boundary layer and wake development for a NACA 0025 airfoil was studied experimentally at low Reynolds numbers. Flow characteristics were investigated with hot-wire anemometry, surface pressure measurements, and flow visualization. A laminar boundary layer separation occurs on the upper surface of the airfoil, forming a separated shear layer, for all situations examined. When the flow is excited at the frequency matching the frequency of the most amplified disturbance in the separated shear layer, natural shear layer disturbances lock onto the excitation frequency and transition is promoted. In the case when the separated shear layer fails to reattach, an increase of the excitation amplitude above a minimum threshold eventually results in shear layer reattachment. The results suggest that an increase of the excitation amplitude not only advances the location of reattachment but also delays boundary layer separation, thereby reducing the extent of the separation region. As a consequence, excitation results in narrowing of the wake and diminishment of the length scales and coherence of the organized wake structures. However, this effect is eventually limited, and a maximum effective excitation amplitude can be identified. It is also shown that an increase of the excitation amplitude has a broadly similar effect on flow patterns to an increase of the chord Reynolds number.The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding of this work

Airfoil Performance at Low Reynolds Numbers in the Presence of Periodic Disturbances

The boundary-layer separation and wake structure of a NACA 0025 airfoil and the effect of external excitations in presence of structural vibrations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack, involving hot-wire measurements and complementary surface flow visualization. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude suppresses or reduces the separation region and decreases the airfoil wake, i.e., produces an increase of the lift and/or decrease of the drag. The acoustic excitation also alters characteristics of the vortical structures in the wake, decreasing the vortex length scale and coherency. Optimum excitation frequencies were found to correlate with the fundamental frequencies of the naturally amplified disturbances in the separated shear layer. The results suggest that acoustic waves play a dominant role in exciting the separated shear layer of the airfoil. Moreover, low-frequency structural vibrations are found to have a significant effect on airfoil performance, as they enhance the sound pressure levels within the test section.The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada NSERC for funding this work

Smoke-Wire Flow Visualization in Separated Flows at Relatively High Velocities

A smoke-wire technique is often used for flow visualization in wind tunnels [1-2], whereby the flow of air isvisualized via evaporating a smoke-generating liquid from a fine wire by means of electric heating. One of the main advantages of this technique is that, when properly implemented, it does not introduce any appreciable disturbances into the investigated flows. Over the past several decades, the technique was applied to study a wide range of both bounded and unbounded shear flows (e.g., [1-9]). Nevertheless, for a variety of fluid phenomena, flow visualization via the smoke-wire technique remains to be a blend of science and art. The present paper discusses the application of the smoke wire technique for visualizing flow over an airfoil at low Reynolds numbers (Rec< 200,000). In such flow, a laminar boundary layer separates on the upper surface of an airfoil, and flow evolution is governed by laminar-to-turbulent transition occurring in a separated shear layer. Since it does not appreciable disturb the flow, smoke-wire flow visualization can be instrumental for gaining new insight into key aspects of flow development. However, the application of the smoke wire technique is commonly limited to free-stream velocities of about 5 m/s [1]. Also, in its common arrangement, a smoke wire placed upstream of the airfoil does not allow visualizing flow inside the separated shear layer. In this paper, the application of a smoke-wire technique for visualizing separated flows at free-stream velocities of up to 7.8 m/s is described.The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding of this work