The Form Drag of Three-Dimensional Bluff Bodies Immersed in Turbulent Boundary Layers

Measurements of the pressure distributions on the three-dimensional bluff bodies are correlated with the characteristics of the smooth-wall turbulent boundary layers in which the bodies are immersed. The bluff bodies selected for measurement were a cube and a vertical circular cylinder which can be considered as typical examples of three-dimensional bluff bodies. Experimental data were collected to investigate the effects of (1) the variation of the height of bluff bodies h, (2) the characteristics of the smooth-wall boundary layers in which they are immersed, on the form drag acting on the three-dimensional bluff bodies. For flow with zero-pressure gradient, the form drag coefficients of the cube and the vertical circular cylinder defined by C_Dτ=D/(1/2ρu_τ²h²) are found to be expressed as a power-law function of hu_τ/ν in the range of h/δ less than about 1.0, where D is the form drag, u_τ the shear velocity, ν the kinematic viscosity and δ the thickness of the undisturbed boundary layer at the location of the bluff bodies. For h/δ>1.0, the drag coefficients are independent of the parameter u_τ/U₀, being uniquely related to h/δ. Further, the pressure distributions along the front centerline of each bluff body can be expressed by a single curve irrespective of both the height of the bluff body and the boundary layer characteristics and show a good agreement with the dynamic pressure in an undisturbed boundary layer at the location of the bluff bodies in the range of about 0.2<y/h<0.7, where y is the distance from the wall.