3 results
Flow instabilities in the wake of a circular cylinder with parallel dual splitter plates attached
- Rui Wang, Yan Bao, Dai Zhou, Hongbo Zhu, Huan Ping, Zhaolong Han, Douglas Serson, Hui Xu
-
- Journal:
- Journal of Fluid Mechanics / Volume 874 / 10 September 2019
- Published online by Cambridge University Press:
- 04 July 2019, pp. 299-338
-
- Article
- Export citation
-
In this paper, instabilities in the flow over a circular cylinder of diameter $D$ with dual splitter plates attached to its rear surface are numerically investigated using the spectral element method. The key parameters are the splitter plate length $L$, the attachment angle $\unicode[STIX]{x1D6FC}$ and the Reynolds number $Re$. The presence of the plates was found to significantly modify the flow topology, leading to substantial changes in both the primary and secondary instabilities. The results showed that the three instability modes present in the bare circular cylinder wake still exist in the wake of the present configurations and that, in general, the occurrences of modes A and B are delayed, while the onset of mode QP is earlier in the presence of the splitter plates. Furthermore, two new synchronous modes, referred to as mode A$^{\prime }$ and mode B$^{\prime }$, are found to develop in the wake. Mode A$^{\prime }$ is similar to mode A but with a quite long critical wavelength. Mode B$^{\prime }$ shares the same spatio-temporal symmetries as mode B but has a distinct spatial structure. With the exception of the case of $L/D=0.25$, mode A$^{\prime }$ persists for all configurations investigated here and always precedes the transition through mode A. The onset of mode B$^{\prime }$ occurs for $\unicode[STIX]{x1D6FC}>20^{\circ }$ with $L/D=1.0$ and for $L/D>0.5$ with $\unicode[STIX]{x1D6FC}=60^{\circ }$. The characteristics of all the transition modes are analysed, and their similarities and differences are discussed in detail in comparison with the existing modes. In addition, the physical mechanism responsible for the instability mode B$^{\prime }$ is proposed. The weakly nonlinear feature of mode B$^{\prime }$, as well as that of mode A$^{\prime }$, is assessed by employing the Landau model. Finally, selected three-dimensional simulations are performed to confirm the existence of these two new modes and to investigate the nonlinear evolution of the three-dimensional modes.
Direct numerical simulations of the flow around wings with spanwise waviness
- Douglas Serson, Julio R. Meneghini, Spencer J. Sherwin
-
- Journal:
- Journal of Fluid Mechanics / Volume 826 / 10 September 2017
- Published online by Cambridge University Press:
- 10 August 2017, pp. 714-731
-
- Article
- Export citation
-
The use of spanwise waviness in wings has been proposed in the literature as a possible mechanism for obtaining improved aerodynamic characteristics, motivated by the tubercles that cover the leading edge of the pectoral flippers of the humpback whale. We investigate the effect of this type of waviness on the incompressible flow around infinite wings with a NACA0012 profile, using direct numerical simulations employing the spectral/hp method. Simulations were performed for Reynolds numbers of $Re=10\,000$ and $Re=50\,000$, considering different angles of attack in both the pre-stall and post-stall regimes. The results show that the waviness can either increase or decrease the lift coefficient, depending on the particular $Re$ and flow regime. We observe that the flow around the wavy wing exhibits a tendency to remain attached behind the waviness peak, with separation restricted to the troughs, which is consistent with results from the literature. Then, we identify three important physical mechanisms in this flow. The first mechanism is the weakening of the suction peak on the sections corresponding to the waviness peaks. This characteristic had been observed in a previous investigation for a very low Reynolds number of $Re=1000$, and we show that this is still important even at $Re=50\,000$. As a second mechanism, the waviness has a significant effect on the stability of the separated shear layers, with transition occurring earlier for the wavy wing. In the pre-stall regime, for $Re=10\,000$, the flow around the baseline wing is completely laminar, and the earlier transition leads to a large increase in the lift coefficient, while for $Re=50\,000$, the earlier transition leads to a shortening of the separation bubble which does not lead to an increased lift coefficient. The last mechanism corresponds to a sub-harmonic behaviour, with the flow being notably different between subsequent wavelengths. This allows the wing to maintain higher lift coefficients in some portions of the span.
Wake transition in the flow around a circular cylinder with a splitter plate
- Douglas Serson, Julio R. Meneghini, Bruno S. Carmo, Ernani V. Volpe, Rafael S. Gioria
-
- Journal:
- Journal of Fluid Mechanics / Volume 755 / 25 September 2014
- Published online by Cambridge University Press:
- 22 August 2014, pp. 582-602
-
- Article
- Export citation
-
A simple way to decrease the drag and oscillating lift forces in the flow around a circular cylinder consists of positioning a splitter plate in the wake, parallel to the flow. In this paper, the effect of the splitter plate on the wake dynamics, more specifically on the wake transition, is described in detail. First, two-dimensional and three-dimensional direct numerical simulations (DNS) using the spectral element method were used to observe the behaviour of the wake in the presence of the splitter plate. Then, a linear stability analysis based on the Floquet theory was performed in order to obtain information on how the splitter plate changes the instabilities that lead to wake transition. Simulations were carried out for several gaps between the splitter plate and the cylinder, with the Reynolds number varying in the range between 100 and 350, which corresponds to the wake transition in the flow around a circular cylinder. The results of the simulations showed a discontinuity in the Strouhal number curve that is consistent with the results available in the literature. The stability analysis showed how the splitter plate modifies the transition of the flow to a three-dimensional configuration. The splitter plate has a stabilizing effect on the flow for small gaps, delaying the appearance of three-dimensional structures to higher Reynolds numbers. Mode A and a quasi-periodic (QP) mode are observed for such small gaps. As the gap is increased the discontinuity in the Strouhal number curve also caused a clear change in the characteristics of the neutral stability curve, and the existence of an unstable period-doubling mode was observed. The onset characteristics of the unstable modes are analysed and discussed in depth.