An experimental study on the wake behind a rectangular forebody with variable inlet conditions

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: The wake behind a rectangular forebody with variable inlet conditions is investigated. The perforated surface of the two-dimensional rectangular forebody, with a smooth leading edge and a blunt trailing edge, allows for boundary layer modification by means of wall suction. The test section, of which the rectangular forebody is the main part, is experimentally evaluated with a series of hot-wire and Prandtl tube measurements in the boundary layer and the wake.For a suction coefficient of ?>9, corresponding to 0.9% suction of the free stream velocity, the asymptotic suction boundary layer (ASBL) is obtained at the trailing edge of the forebody for laminar boundary layers (Rex=1.6×105?3.8×105). The key feature of the ASBL, a spatially invariant boundary thickness which can be modified independent of the Reynolds number, is used to perform a unique parametrical study.Turbulent boundary layers (Rex=4.5×105?3.0×106) subject to wall suction are also investigated. For a critical suction coefficient ?crit, which depends on Rex, the boundary layer relaminarizes. Strong evidence is found to support the hypothesis that turbulent boundary layers will ultimately attain the ASBL as well, provided that the wall suction is strong enough.The effect of the modulated laminar and turbulent boundary layers on the wake characteristics is studied. The shape of the mean wake velocity profile, scaled with the velocity deficit U0and the wake half width ?y1/2, is found tobe independent of x/h, for x/h> 6 and Reh >6.7×103. The wake width is shown to scale with the effective thickness of the body h+2?1, where the ratio is expected to vary with the downstream location.A decrease of the displacement thickness leads to a decrease of the base pressure, with Cp,b = ?0.36 in the ASBL limit. The Strouhal number based on the effective thickness becomes Sth+2?1 ? 0.29 in the ASBL limit and independent of the plate thickness (h) Reynolds number, in the range Reh = 2.9×103 ? 6.7×103. For the turbulent boundary Sth+2?1 is found to be 25% lower, which shows that the wake characteristics depend on the state of the boundary layer at the trailing edge.The total drag is found to be reduced by as much as 30% for Reh = 2.7×104 when a wall normal velocity of only 3.5% of the free stream velocity is applied. Wall suction successively reduces the total drag with increasing wall suction, at least in the Reynolds number rangeReh = 8.0×103?5.5×104.