This study presents the results of an experimental work with a smooth pipe at an angle to the flow and in the vicinity of a plane wall in the subcritical range of the Reynolds number. The tested range of the gapto-diameter ratio is o ≤ e/D ≤ 1.8, and that of the angle of attack is 45° ≤ θ ≤ 90°. The hydrodynamic forces on the pipe are investigated in both steady current and uniform oscillatory flow. This has been accomplished by measurements of pressure distribution around the model pipe. The results revealed that the so-called independence principle can be applied to the pipe in the vicinity of the wall for the angles of attack in the tested range.
In a steady current, when a cylinder is placed at an angle to the flow, forces on the cylinder may change. Experiments have shown, however, that in most of the cases the so-called independence or cross-flow principle is applicable (Schlichting 1979, Hoerner 1965). Namely, the component of the force normal to the cylinder may be calculated from Fn = ½CdρDUn2, in which p is the water density, D is the diameter of the cylinder, Un is the velocity component of flow normal to the cylinder axis, and Co is the drag coefficient (obtained for a cylinder perpendicular to the flow). Although theoretically the independence principle is justified only in the subcritical range of Re (Schlichting 1979), it has been proved to hold true also in the postcritical flows (Norton et al., 1981). However there is evidence (Bursnall and Loftin 1951) that for the transcritical values of Re, where the vortex shedding around the pipe undergoes transformation and the drag changes considerably, the independence principle may not be applied.