5 Types of Drag
Flow separation - As fluids wraps a tight corner, it will separate from the body leaving a turbulent wake that results in high drag. Mitigated by an aerodynamic body
Skin Friction - Friction between the body and the fluid causes drag. Dominant in streamlines bodies. Mitigated by reducing surface area and surface roughness
Boundary Layer Pressure Loss - Also known as pressure drag. As fluid flows over a body, the fluid right on the surface sticks to the surface (called no-slip condition) and viscous effects in the fluid cause the fluid further from the body to be slowed. The boundary between the affected fluid and the free-stream fluid defines the “boundary layer”. The boundary layer grows as it proceeds down the body. This causes a pressure drop along the body which means that there is a pressure that needs to be overcome.
Induced Drag - All streamlined bodies can generate lift if at a given angle in the airstream. But lift always induces drag on the body (whether up or down). Minimum drag occurs when there is zero lift, so the aerobody should be oriented to minimize lift and therefore drag
Interference drag is caused by imperfections on the body such as joints and seams, or the mating of the canopy or fairings to the body. Reduced by disciplined manufacturing and quality control
Drag Equation
Cd is coefficient of drag and is an encompassing coefficient for all types of drag.
A is the characteristic area and is dependent on the dominant mode of drag. For streamlined bodies, skin friction is the dominant mode and so the area used should be the surface area of the body moving past the fluid. For a bluff body like a regular car, pressure drag is the dominant mode and so a projection of the front facing area is the most useful.
Geometry
Aerodynamic shapes are ones that minimize flow separation. In three dimensions the ideal shape is a cylindrically symmetrical teardrop, however factors require compromise on this design.
A solar car compromises to get power from a solar array, so the teardrop needs to be flattered or have a flat portion added. Bullet designs tend to add a streamlined large surface for the solar array to a general teardrop shape, where catamaran’s are example of where a teardrop is flattened to accommodate the solar array.
Near the ground there is a “ground effect” caused by an airfoil. When there is an aerodynamic shape moving near near the ground, there is a low pressure area created. This is because the flow under the must pass through a minimum ground clearance, as the area the flow can pass through opens up toward the rear, air cannot replace it, which causes low pressure and induced drag. This is fixed with camber, and I believe it can also be applied to the space between two fairings on a catamaran. Camber is a bending of the centerline for an airfoil.