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.
Side Profile of an Airfoil
Camber: A measure of the curvature of the airfoil
The camber of the upper surface is more pronounced than the camber of the lower surface (usually somewhat flat).
Leading Edge: Forward-facing end that is rounded
Trailing-Edge: Rear-facing end that is narrow and tapered towards the rear
Chord Line: Reference line drawn from the centre of the leading edge straight through the wing till the trailing edge.
This reference line is used to find the magnitude of the upper or lower camber at any point along the wing (by measuring the distance between the chord line and the upper and lower surfaces of the wing).
Mean Camber Line: Another reference line drawn from the leading edge to the trailing edge.
However, unlike the chord line, this line is equidistant at all points along the wing from the upper and lower surfaces.
Airfoils are designed in such a way that the shape takes advantage of the air’s response to certain physical laws. Hence, two actions from the air are developed as the wing passes through. A positive or high pressure lifting action from the air mass below the wing, and a negative pressure lifting action from low pressure above the wing.
Teardrop wing profile results in the speed and pressure changes of the air passing over the top and under the bottom to be the same.