John G. Kawall

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 × 10 3 ≤ Re c ≤ 210 × 10 3 , 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.

Coherent structures in an airfoil boundary layer and wake at low Reynolds numbers

Boundary layer and turbulent wake development for a NACA 0025 airfoil at low Reynolds numbers was studied experimentally. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. Laminar boundary layer separation occurs on the upper surface of the airfoil for all Reynolds numbers and angles of attack examined. Two flow regimes are investigated (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results suggest that coherent structures form in the separated flow region and the wake of the airfoil for both flow regimes. The formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin-Helmholtz instability. Linear stability theory can be employed to adequately describe the salient characteristics of such vortices and the initial stage of the separated shear layer transition. The development of the roll-up vortices leads to boundary layer t...

Separated-Shear-Layer Development on an Airfoil at Low Reynolds Numbers

Flow transition in the separated shear layer on the upper surface of a NACA 0025 airfoil at low Reynolds numbers was investigated. The study involved wind-tunnel experiments and linear stability analysis. Detailed measurements were conducted for Reynolds numbers of 100,000 and 150,000 at 0-, 5- and 10-degree angles of attack. For all cases examined, laminar boundary-layer separation takes place on the upper surface of the airfoil. The separated shear layer fails to reattach to the airfoil surface for the lower Reynolds number, but reattachment occurs for the higher Reynolds number. Despite this difference in flow development, experimental results show that a similar transition mechanism is attendant for both Reynolds number flow regimes. Flow transition occurs due to the amplification of natural disturbances in the separated shear layer within a band of frequencies centered at some fundamental frequency. The initial growth of disturbances centered at the fundamental frequency is followed by the growth of a subharmonic component, eventually leading to flow transition. The growing disturbances also cause shear-layer roll-up and the formation of roll-up vortices. The results show that inviscid stability theory can be employed to adequately estimate such salient characteristics as the frequency of the most amplified disturbances and their propagation speed. This implies that the roll-up vortices can be attributed to inviscid instability. However, the results suggest that viscous and nonparallel effects need to be accounted for to effectively model the convective growth of the disturbances in the separated shear layer.

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

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

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Unsteady Separated Flow Characterization on Airfoils Using Time-Resolved Surface Pressure Measurements

Laminar boundary-layer separation and separated shear layer development on a NACA 0025 airfoil at low Reynolds numbers were studied experimentally. Flow visualization, hot-wire velocity measurements, and time-resolved surface pressure measurements were employed in this investigation. The results for two distinct flow regimes, namely, flow separation without subsequent shear layer reattachment and separation bubble formation, are discussed in detail. For both flow regimes, the transition occurs due to the amplification of natural flow disturbances in the separated shear layer. Initially, disturbances within a band of frequencies centered at some fundamental frequency are amplified. Further downstream, nonlinear interactions set in, leading to a breakdown to turbulence. Based on the spectral and correlation analysis of velocity and surface pressure fluctuations, it is demonstrated that the amplification of disturbances and the attendant fluctuations in the flow velocity give rise to distinct surface pressure fluctuations at the fundamental frequency. Thus, time-resolved surface pressure measurements can be employed to estimate important unsteady characteristics of the separated flow region on an airfoil operating in low Reynolds flows.

Effect of acoustic excitation on airfoil performance at low Reynolds numbers

The wake structure, vortex shedding characteristics and boundary-layer separation of a NACA 0025 airfoil and the effect of external acoustic excitations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack. Velocities were measured with hot-wires. Spectral analysis of these data was used in conjunction with complementary surface flow visualization to diagnose the performance of the airfoil at low Reynolds numbers. Evidence of wake vortex shedding and flow separation was obtained for most cases examined, and dependence of these phenomena on Reynolds number and angle of attack was found. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude eliminates 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 the vortex shedding characteristics, decreasing the vortex length scale and the coherency of the vortex structure.

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

The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding of this work.

Separated Shear Layer Transition at Low Reynolds Numbers: Experiments and Stability Analysis

Flow transition in the separated shear layer on the upper surface of a NACA 0025 airfoil at low Reynolds numbers was investigated. The study involved wind tunnel experiments and linear stability analysis. Detailed measurements were conducted for Reynolds numbers of 100,000 and 150,000 at 0, 5, and 10-degrees angles of attack. With laminar boundary layer separation occurring on the upper surface of the airfoil for all cases examined, the separated shear layer fails to reattach to the airfoil surface for the lower Reynolds number but reattachment occurs for the higher Reynolds number. Despite this difference in flow development, experimental results show that a similar transition mechanism is attendant for both Reynolds number flow regimes. Flow transition occurs due to the amplification of natural disturbances in the separated shear layer within a band of frequencies centred at some fundamental frequency. The initial growth of disturbances centred at the fundamental frequency is followed by the growth of a sub-harmonic component, eventually leading to flow transition. The growing disturbances also cause shear layer roll-up and the formation of roll-up vortices. Inviscid stability calculations and experimental results show good agreement, implying that separated shear layer transition on an airfoil at low Reynolds numbers is essentially inviscid in nature. However, the results suggest that the proximity of the airfoil surface to the separated shear layer may cause viscous effects to influence the transition process to some extent at low angles of attack.

Smoke-Wire Flow Visualization on an Airfoil at Low Reynolds Numbers

The application of a smoke wire technique for visualizing flow over an airfoil at low Reynolds numbers is discussed. Detailed recommendations for implementing the smoke wire technique at relatively high free stream velocities of up to 7.8 m/s are provided. An original smoke wire arrangement is proposed, which allows visualizing flow development in a separated shear layer and an airfoil wake. Flow visualization images obtained with the proposed approach shed light on the formation and evolution of coherent structures in the separated flow regions.