The drag of three-dimensional rectangular cavities

Cavities and other surface cut-outs are present on aircraft in numerous forms, from cargo bays and landing gear housing to rivet depressions and panel handles. Although these surface imperfections make a significant contribution to the overall drag on an aircraft, relatively little is known about the flow mechanisms associated with cavities, particularly those which have a strongly three-dimensional geometry. The present work is a wind tunnel investigation of the drag forces and flow regimes associated with cavities having a 2:1 rectangular planform geometry. The effects of both the cavity depth and the flow incidence angle have been examined in terms of the overall cavity drag increment and the mean surface pressure distributions. The drag forces have been determined from both integrated pressures and direct force balance measurements. For the model normal to the flow direction the flow within the cavity was remarkably symmetrical in all the configurations examined. In most cases the cavity flow is dominated by a single large eddy. However, for cavities yawed to other incidence angles there is considerable flow asymmetry, with strong vorticity shedding and high drag in some cases, notably with depth/narrowest width ratio of 0.4–0.5 at 45–60° incidence. The present data correspond well with established results and extend the scope of information available for design purposes and for the development of numerical models.Nomenclature A_p planform area of model - C_D pressure drag coefficient (F_D/(A_p · q)) - C_D drag coefficient increase due to cavity (C_D – c_f) - c_f local skin friction coefficient - C_L pressure lift coefficient (F_L/(A_p · q)) - C_p mean surface pressure coefficient (P – P_s)/q) - F_D drag force - F_L lift force - h depth - L longest planform dimension of model - P surface pressure on model - P_s freestream static pressure - P_t freestream total pressure - q freestream dynamic pressure (P_t – P_s) - Re Reynolds number (U_R · W/v) - U_R freestream velocity - W narrowest planform dimension of model - Z vertical cartesian coordinate - incidence angle - kinematic viscosity