“…(2016), and occurrence of an inflection point in the profile in Narsipur et al. 2020). The critical value is not determined from the behaviour of the parameter itself, e.g.…”
Section: Resultsmentioning
confidence: 97%
“…The definition of was revised in Narsipur et al. (2020) but the revision is not relevant for our study (see Appendix D).…”
We propose a more conservative, physically-intuitive criterion, namely, the boundary enstrophy flux (
$BEF$
), to characterise leading-edge-type dynamic stall onset in incompressible flows. Our results are based on wall-resolved large-eddy simulations of pitching aerofoils, with fine spatial and temporal resolution around stall onset. We observe that
$|BEF|$
reaches a maximum within the stall onset regime identified. By decomposing the contribution to
$BEF$
from the flow field, we find that the dominant contribution arises from the laminar leading edge region, due to the combined effect of large clockwise vorticity and favourable pressure gradient. A relatively small contribution originates from the transitional/turbulent laminar separation bubble (LSB) region, due to LSB-induced counter-clockwise vorticity and adverse pressure gradient. This results in
$BEF$
being nearly independent of the integration length as long as the region very close to the leading edge is included. This characteristic of
$BEF$
yields a major advantage in that the effect of partial or complete inclusion of the noisy LSB region can be filtered out, without changing the
$BEF$
peak location in time significantly. Next, we analytically relate
$BEF$
to the net wall shear and show that its critical value (
$=\max (|BEF|)$
) corresponds to the instant of maximum net shear prevailing at the wall. Finally, we have also compared
$BEF$
with the leading edge suction parameter (
$LESP$
) (Ramesh et al., J. Fluid Mech., vol. 751, 2014, pp. 500–538) and find that the former reaches its maximum value between
$0.3^{\circ }$
and
$0.8^{\circ }$
of rotation earlier.
“…(2016), and occurrence of an inflection point in the profile in Narsipur et al. 2020). The critical value is not determined from the behaviour of the parameter itself, e.g.…”
Section: Resultsmentioning
confidence: 97%
“…The definition of was revised in Narsipur et al. (2020) but the revision is not relevant for our study (see Appendix D).…”
We propose a more conservative, physically-intuitive criterion, namely, the boundary enstrophy flux (
$BEF$
), to characterise leading-edge-type dynamic stall onset in incompressible flows. Our results are based on wall-resolved large-eddy simulations of pitching aerofoils, with fine spatial and temporal resolution around stall onset. We observe that
$|BEF|$
reaches a maximum within the stall onset regime identified. By decomposing the contribution to
$BEF$
from the flow field, we find that the dominant contribution arises from the laminar leading edge region, due to the combined effect of large clockwise vorticity and favourable pressure gradient. A relatively small contribution originates from the transitional/turbulent laminar separation bubble (LSB) region, due to LSB-induced counter-clockwise vorticity and adverse pressure gradient. This results in
$BEF$
being nearly independent of the integration length as long as the region very close to the leading edge is included. This characteristic of
$BEF$
yields a major advantage in that the effect of partial or complete inclusion of the noisy LSB region can be filtered out, without changing the
$BEF$
peak location in time significantly. Next, we analytically relate
$BEF$
to the net wall shear and show that its critical value (
$=\max (|BEF|)$
) corresponds to the instant of maximum net shear prevailing at the wall. Finally, we have also compared
$BEF$
with the leading edge suction parameter (
$LESP$
) (Ramesh et al., J. Fluid Mech., vol. 751, 2014, pp. 500–538) and find that the former reaches its maximum value between
$0.3^{\circ }$
and
$0.8^{\circ }$
of rotation earlier.
“…We note that the recent work by Narsipur et al. (2020) has proposed an updated definition for the LESP to take into account the effect of pitch rate on the reference velocity, . Their work considered the critical LESP at the initiation of LEV formation for pitching airfoils with a range of pivot points from leading edge to trailing edge, a range of non-dimensional pitch rates from to 0.6, and two Reynolds numbers of 30 000 and 3 million.…”
Section: Low-order Aerodynamic Modelmentioning
confidence: 99%
“…Narsipur et al. 2020) for the accurate prediction of flow interactions. Figure 18.Effect of the value of on and variations for both phases: ( a ) , ; ( b ) , ; ( c ) , ; ( d ) , .…”
“…Narsipur et al. (2020) modified the formula of the LESP by using a novel reference velocity, further reducing the dependence of the critical value of the LESP on motion kinematics.…”
During a dynamic stall process, various flow structures leave their pressure footprints on a wing surface, through which it is possible to understand the development of different flow stages and predict the critical flow events. Based on a classical airfoil ramp-up pitching motion, proper orthogonal decomposition analysis of pressure on the suction surface is carried out in this study. Accordingly, the surface pressure evolution during dynamic stall is summarized into three basic physical schemas, based on which several real-time critical indicators for predicting the flow events related to the dynamic stall vortex (DSV) are constructed. These critical indicators include the spatial distribution coefficient of pressure (SDCP), the high-order central moment of pressure (HCMP), the location of peak pressure (LPP) and the modulated location of peak pressure (MLPP). These indicators can predict the formation of laminar separation bubbles, DSV initiation, DSV centre position and the detachment of DSV. Therefore, the real-time whole-life monitoring of DSV has been realized. Moreover, the effectiveness of these indicators has also been confirmed under different parameters, and testing using wind tunnel experimental data proves their noise robustness. Studies show that the SDCP and HCMP may also be effective even if only two transducers are used. Finally, a modification method of SDCP based on Z-score standardized pressure is proposed. It is found that the modified SDCP can effectively reduce sensitivity to kinematic parameters and the Reynolds number. The critical indicators in this study can be used as a reference for the standardized mathematical and physical description of DSV evolution, thus laying a foundation for constructing a universal theoretical framework of dynamic stall.
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