Serhiy Yarusevych

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 transition over an airfoil at a low Reynolds number

Shear layer development over a NACA 0018 airfoil at a chord Reynolds number of 100 000 was investigated using a combination of flow visualization, velocity field mapping, surface pressure fluctuation measurements, and stability analysis. The results provide a detailed description of shear layer transition on an airfoil at low Reynolds numbers. An extensive comparison of measured surface pressure and velocity fluctuations demonstrated that time-resolved surface pressure sensor arrays can be used to identify the presence of flow separation, estimate the extent of the separated flow region, and measure disturbance growth rate spectra in significantly less time than is required by conventional techniques. Surface pressure sensor measurements of disturbance growth rate, wave number, and convection speed are found to compare well with predictions of linear stability theory, supporting the claim that convection speeds measured in separation bubbles over low Reynolds number airfoils are associated with wave packe...

Aerodynamic Characterization of a NACA 0018 Airfoil at Low Reynolds Numbers

3to 200x10 3 and angles of attack from 0° to 18°. These data were used to characterize the separation bubble and determine lift coefficients. From these results, two distinct regions in the lift curves can be identified: a region of rapid and linear growth of the lift coefficients at low angles of attack and a region of more gradual and linear growth at higher pre-stall angles. Furthermore, the slope of the lift curve in each region is found to be linked to the rates of change in separation, transition, and reattachment locations with the angle of attack. These findings are substantiated by an analysis of the available experimental data for a NACA 0012 airfoil.

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

Adaptive Compensation for Detuning in Pendulum Tuned Mass Dampers

Detuning, resulting from deterioration, inadvertent changes to structure properties, and design forecasting, can lead to a significant loss of performance in tuned mass dampers (TMDs). To overcome this issue, an adaptive compensation mechanism for suspended pendulum TMDs is proposed. The adaptive pendulum mass damper is a three-dimensional pendulum, augmented with a tuning frame to adjust its natural frequency, and two adjustable air dampers adjust damping. The adjustments for the natural frequency and damping compensation are achieved using a system of stepper motors and a microcontroller. There are two major components in the proposed methodology: identification and control, one followed by the other, in that order. The identification is carried out using spectral information obtained from the structural acceleration responses. The performance of the adaptive pendulum system is studied via both experiments and simulations. The main contribution of this paper is to develop an effective means of compensation for detuning in TMDs, while retaining the simplicity of passive pendulum TMDs. The proposed methodology allows pendulum TMDs to be tuned in place using relatively simple hardware and algorithms, based on ambient vibration measurements only.

Effects of End Plates and Blockage on Low-Reynolds-Number Flows Over Airfoils

Measurements on airfoils at low Reynolds numbers can have a strong dependence on the experimental setup as a result of the sensitivity of the transitioning separated shear layer that develops over the model. In this investigation, the effects of two aspects of the experimental setup, namely end plates and test-section blockage, on lowReynolds-numberairfoilexperiments areexploredthroughmeasurementsonaNACA0018airfoilmodelatachord Reynolds number of 100,000. The improvement in mean spanwise uniformity with end plates installed is quantified and demonstrates the importance of using end plates in two-dimensional airfoil experiments at low Reynolds numbers. Consistent with previous studies on the use of end plates on circular cylinder models, it is found that mean quantities measured on the center-span plane are least sensitive to end-plate spacing for spacings greater than roughly 7 times the projected model thickness. It is shown that the end-plate configuration affects vortex-shedding characteristics and disturbance amplification in the separated shear layer. Blockage effects are investigated by comparing measurements before andafter adaptive-wall test-section streamlining forsolid-blockage ratios between 4 and 8%. These blockage ratios are shown to cause errors in lift as high as 9% of the maximum lift and 3.5% in the wake vortex-shedding frequency. The results of this investigation can be used to estimate the effects of blockage on flow development in low-Reynolds-number airfoil experiments. It is demonstrated that a common blockagecorrectionmethodcanaccuratelycorrectliftmeasurementsformoderateblockagesinlow-Reynolds-numberairfoil experiments for conditions under which a separation bubble forms over the model.

Momentum Coefficient as a Parameter for Aerodynamic Flow Control with Synthetic Jets

The influence of periodic excitation from synthetic jet actuators on boundary-layer separation and reattachment over a NACA 0025 airfoil at a low Reynolds number is studied. Flow-visualization results showed a vertical jet pulse accompanied by two counter-rotating vortices being produced at the exit of the simulated slot, with the vortices shed at the excitation frequency. Hot-wire measurements determined the maximum jet velocity for a range of excitation frequencies and voltages, and were used to characterize the excitation amplitude in terms of the momentum coefficient Cμ. With the synthetic jet actuator installed in the airfoil, flow-visualization results showed that excitation produces boundary-layer reattachment, with the associated significant reduction in wake width. Wake-velocity measurements were performed to characterize the effect of flow-control excitation amplitude and frequency on airfoil drag and wake topology. The results demonstrate that Cμ is the primary governing flow-control parameter....

Vortex shedding in the wake of a step cylinder

Flow past a circular cylinder with a single stepwise discontinuity in diameter was investigated numerically for the diameter ratio D/d=2 and two Reynolds numbers, ReD=150 and 300. The primary focus was on vortex shedding and vortex interactions occurring in the cylinder wake. In agreement with previous experimental findings, three distinct spanwise vortex cells were identified in the step-cylinder wake: a single vortex shedding cell in the wake of the small cylinder (the S-cell) and two vortex shedding cells in the wake of the large cylinder, one in the region downstream of the step (the N-cell) and the other away from the step (the L-cell). Due to the differences in vortex shedding frequencies, complex vortex connections occurred in two vortex interaction regions located between the adjacent cells. However, distinct differences in vortex splitting and vortex dislocations were identified in the two regions. The region at the boundary between the S-cell and the N-cell was relatively narrow and its spanwise...