Hassan M. Nagib | Searcharxiv

arXiv2025

Preliminary Study of the Effects of Leading-Edge Serration on a Two-Section Planar Wing in ground-effect at Low Reynolds Number

A preliminary study has been conducted on the effects of serration on the leading-edge of a two-element trapezoidal wing placed both out-of- and in-ground effect. Aerodynamic performance and flow behaviour were evaluated numerically and validated experimentally. Results indicate an increase in maximum lift coefficient and stall angle obtained implementing a serrated leading-edge geometry due to the flow being re-energized by the formation of a series of counter-rotating pairs of vortices. Results from the analysis of the wing in ground effect appear less well defined. Both leading-edge geometries -- straight and serrated -- show an increase in efficiency due to the proximity to the ground. The wing with the straight leading-edge geometry shows constant improvement up to stall, whilst numerical results show a significant decrease in lift performance at high angles of attack. This may be caused by the lower-fidelity numerical model implemented at higher angles of attack, thus yielding less accurate results.

physics.flu-dyn↗

arXiv2023

The hunt for the Kármán "constant'' revisited

The logarithmic law of the wall, joining the inner, near-wall mean velocity profile (abbreviated MVP) in wall-bounded turbulent flows to the outer region, has been a permanent fixture of turbulence research for over hundred years, but there is still no general agreement on the value of the pre-factor, the inverse of the Kármán ``constant'' $κ$, or on its universality. The choice diagnostic tool to locate logarithmic parts of the MVP is to look for regions where the indicator function $Ξ$ (equal to the wall-normal coordinate $y^+$ times the mean velocity derivative $\dd U^+/\dd y^+$) is constant. In pressure driven flows however, such as channel and pipe flows, $Ξ$ is significantly affected by a term proportional to the wall-normal coordinate, of order $\mathcal{O}(\Reytau^{-1})$ in the inner expansion, but moving up across the overlap to the leading $\mathcal{O}(1)$ in the outer expansion. Here we show that, due to this linear overlap term, $\Reytau$'s well beyond $10^5$ are required to produce one decade of near constant $Ξ$ in channels and pipes. The problem is resolved by considering the common part of the inner asymptotic expansion carried to $\mathcal{O}(\Reytau^{-1})$, and the leading order of the outer expansion. This common part contains a \textit{superposition} of the log law and a linear term $S_0 \,y^+\Reytau^{-1}$, and corresponds to the linear part of $Ξ$, which, in channel and pipe, is concealed up to $y^+ \approx 500-1000$ by terms of the inner expansion. A new and robust method is devised to simultaneously determine $κ$ and $S_0$ in pressure driven flows at currently accessible $\Reytau$'s, yielding $κ$'s which are consistent with the $κ$'s deduced from the Reynolds number dependence of centerline velocities. A comparison with the zero-pressure-gradient turbulent boundary layer further clarifies the issues.

physics.flu-dyn↗