Flow over submerged bodies
Introduction:External flows past objects have been studied extensively because of their many practical applications. For example, airfoils are made into streamline shapes in order to increase the lifts, and at the same time, reducing the aerodynamic drags exerted on the wings. On the other hand, flow past a blunt body, such as a circular cylinder, usually experiences boundary layer separation and very strong flow oscillations in the wake region behind the body. In certain Reynolds number range, a periodic flow motion will develop in the wake as a result of boundary layer vortices being shed alternatively from either side of the cylinder.
This regular pattern of vortices in the wake is called a Karman vortex street. It creates an oscillating flow at a discrete frequency that is correlated to the Reynolds number of the flow. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure. Two distinct regimes exist in an external flow: (a) Flow outside the boundary layer. Here viscosity is negligible. The velocities and pressures are affected by the physical presence of the body together with its boundary layer. Hence, the theories of ideal flow may be used. (b) Flow immediately
Drag and Lift:
A body moving through a fluid experiences a resistant force due to viscous action. The component of the force parallel to undisturbed initial velocity is called the drag force (FD) and the component perpendicular to the approach direction of the flowing fluid is called the lift force (FL).
Depending on the nature of the fluid flow and shape of the body, drag force may be classified as follows:
i. Shear (skin friction) drag
ii. Form (pressure) drag
iii. Profile drag
iv. Wave drag
v. Induced drag
Drag coefficient = Total drag force/ (0.5ρV2 A) Where A = frontal area of the body
Shear drag: Shear drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of
boundary layers, and it scales with Reynolds number. Shear drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow.
Occurs in flow over a surface which is not parallel to the stream everywhere. It results from differences of pressure over the surface.
The fluid has a tendency to stick to the body surface over which it is flowing (due to its viscosity). The fluid impacting on the surface at point A would stagnate. As a result, the K.E is converted to pressure energy. The shear stress does not contribute to drag force since it acts normal to incoming stream
Streamlined body: Separation occurs at point B further down stream
Bluff body: Separation occurs at B. Larger wake, large pressure drag (large pressure difference A, C) Downstream of separation point, flow gets disturbed by
large scale eddies known as wake. Energy is dissipated by highly turbulent motion in a wake and pressure there is reduced. The magnitude of pressure drag depends on the position of separation. On streamlined bodies, the shape is such that separation occurs well towards the rear. The wake is small, pressure drag is also small. In a bluff body, flow is separated over most of its surface, wake is large, and hence the pressure drag is large.
Profile drag: This is the sum of skin friction drag and pressure drag. It arises due to existence of boundary layer on 2D or 3D objects and formation of wakes caused by its separation. Body aligned normal to flow (Drag completely due to pressure)