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4 years ago

Passive flow control around a wall-mounted finite cylinder - Pegasus

Passive flow control around a wall-mounted finite cylinder - Pegasus

Passive flow control around a wall-mounted finite cylinder -

Passive flow control around a wall-mounted finite cylinder Diogo C. Barros 1 École Nationale Supérieure de Mécanique et d’Aérotechnique - ENSMA, Poitiers, 86360 The present work relates the effect of a straight perturbation on a finite cylinder mounted over a wall. The aspect ratios studied (length/diameter) are 3 and 6 at Reynolds’ numbers Re of 5.10 4 , 7.10 4 e 1.10 5 . The influence of a trip wire on the vortex shedding frequencies and flows modifications generated by different aspect ratios has been analyzed. Averaged drag and lift coefficients have been measured and the existence of a critical position of the perturbation has been observed, in which the Strouhal number St and the coefficients vary substantially. Non-stationary efforts coupled with the fluctuant pressure field on the wall have been measured. The results show an important correlation between efforts and pressures. The POD (Proper Orthogonal Decomposition) has been used for the reconstruction of the pressure field and the EPOD (Extended Proper Orthogonal Decomposition) have pointed the correlated part of the efforts with the pressure signals. It has been observed that the aspect ratio introduces important differences in the efforts and pressures measures, changing the recirculation zone around the cylinder. The position of the perturbation changes significantly the vortex shedding frequency and the mean values of the forces. The energy of the proper modes and the extended ones shows that a reconstruction of the pressure or effort field can be done by using two modes. Nomenclature D = cylinder’s diameter [mm] L = cylinder’s length [mm] d = trip wire’s diameter [mm] AR = aspect ratio (L/D) U ∞ = free stream velocity [m/s] Re = Reynolds number based on D St = Strouhal number based on D C x ,C D = drag coefficient C y = lift coefficient C p = pressure coefficient C pb = base pressure coefficient C p ’ = fluctuating pressure coefficient R xy = cross-correlation of x and y PSD = power spectral density (nondimensionalization by the power) [1/Hz] θ = angular position of the trip wire [°] θ cr = critical angular position [°] δ = boundary layer thickness [mm] = coherence of x and y γ xy T I. Introduction HE study of surface-mounted obstacles is of great interest in many projects on the aeronautical and terrestrial transport sectors. These bodies are responsible for the generation of fluctuating pressure in addition to transmitting vibrations to the structures on which they are mounted. A classical effect of this is the noise generation originated from wall-vibrations and unsteady pressure created, like the case of landing gear’s structures in the airplanes. These aspects are intimately linked to wake characteristics, such that its comprehension allows the creation of models capable of controlling them, modifying, for instance, the vortex shedding frequency, aerodynamic loads and maybe the suppression of this shedding. The flow control around finite cylinders using a straight perturbation is, thus, a simple system whose properties allow the comprehension of these phenomena and enable a general view of these problems. 1 Graduate Student, student member, AIAA. E-mail adress: diogocmbarros@gmail.com. 1 American Institute of Aeronautics and Astronautics

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