When an unsteady wake from an upstream body impinges on a downstream body, the resultant interaction noise can be significant. The use of distributed blowing through the surface of a cylinder to reduce this source of noise was investigated in a series of experiments. The two bluff bodies in tandem were a cylinder and an H-beam. Two configurations were tested, one with the cylinder upstream of the H-beam (OH configuration) and the other with the H-beam upstream of the cylinder (HO configuration). The default separation distance was / = 2. These two configurations modelled the interaction noise due to large perturbations in the wake generated by an upstream component inducing unsteady pressure fluctuations on a downstream component. Blowing was used to break down the large flow structures in the wake and to modify the shear layers. The mean velocities and velocity fluctuations were determined in the flowfield. The application of blowing to the OH configuration reduced the ′ ′ component of the stress term. This resulted in a peak reduction of 9.3 dB at a Strouhal number of 0.2. There was a broadband noise reduction of 3.2 dB averaged over the frequency range 0.05 < < 5. The effect of blowing on the HO configuration was to inhibit the strong crossflow fluctuations ( ′ ′ ) between the Hbeam and the cylinder by delaying the reattachment of the shear layers onto the surface of the cylinder. This resulted in a large noise reduction of 13.2 dB at a Strouhal number of 0.8. There was a broadband noise reduction of 4.3 dB averaged over the frequency range 0.05 < < 6.3. The effect of blowing produced additional high frequency noise. This additional noise was minimised with blowing applied through a sintered plate with a very small pore diameter. NomenclatureBlowing area, m 2 Pressure coefficient Blowing coefficient Pore diameter, ṁ Volume flow rate, m 3 /s Distance to microphone, m Reference area, m 2 Strouhal number based on cylinder diameter Component separation distance, m Lighthill stress tensor, N/m 2 Perforated plate thickness, m , , Non-dimensional cartesian components of velocity vector ′ ′ , ′ ′ , ′ ′ Non-dimensional components of Reynolds stress tensor ∞ Freestream velocity, m/s , , Cartesian coordinates, positive downstream, positive to port, positive up Distance from separation line in direction, m

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AIAA 34th Aerospace Sciences Meeting and Exhibit, Reno, NV Jan 15-18, 1996An experimental study into the rationale of wind tunnel wall correction is under way in an adaptive flexible walled test section. The investigation has initially concentrated on deliberately introducing nominally uniform streamwise gradients of Mach number or wall-induced upwash into the vicinity of a 2D aft cambered aerofoil, in order to reveal the sensitivities of the model's aerodynamic behavior to such flow imperfections. Previously reported tests at a relatively low transonic Mach number have generated a valuable body of data, which can be used to guide the streamlining policy in an adaptive flexible walled test section and can also aid the application of wall corrections in conventional wind tunnels. Recent tests have concentrated on determining the effects of upwash gradients on lift at a higher transonic speed and form the prime subject of this paper. The most notable finding is that the '3/4 chord theorem' appears to apply to the nonlinear situations found with transonic flows as well as to flows that satisfy the restrictions of linear theory. The evidence suggests that the theorem can be used as a basis for the correction of model incidence at a given lift in order to account for the effects of gradient in upwash.