Acoustic radiation pressure can be used to concentrate or remove small particles from an airborne aerosol. In this application, an ultrasonic transducer, mounted flush to one wall of a channel, is used to excite an integer half-wavelength standing wave of high amplitude that propagates perpendicular to the aerosol flow direction. An expression for the Fourier transform of the acoustic pressure in a semi-infinite channel, including the effect of mean fluid flow and finite transducer aperture, has been obtained. A parabolic (laminar) mean flow was assumed. The acoustic pressure was found to be governed by the Mach number of the flow, defined by the projection of the propagation direction relative to the mean flow velocity vector; and the aperture function of the transducer. Near a frequency of 50 kHz, numerical inversions of the acoustic pressure transform showed that the presence of mean flow in the velocity range 0-2 m/s caused changes in acoustic pressure on the order of 1%-4%. Corresponding experimental measurements showed changes in acoustic pressure up to 10%. The highest changes in measured acoustic pressure were found to occur up- and down stream relative to the transducer, and these patterns were in agreement with predictions of the analytical model.