Bluff Body AerodynamicsBluff Body

A body moving through a fluid experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag. Frictional 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. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag. Frictional drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body. Cylinders and spheres are considered bluff bodies because at large Reynolds numbers the drag is dominated by the pressure losses in the wake. These flows have large wakes which are considerable to the dimensions of the body. We will study about the flow over a cylinder in-order to understand the effects of the large wakes on the performance of large number of bluff body structures that we encounter in our day today life.

Figure 1 Examples of bluff body

When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or an airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. For example, the drag of a cylinder of diameter $D$ can be ten times larger than a streamlined shape with the same thickness

Figure 2 Drag coefficients of different bodies

Flow over a cylinderCylinders and spheres are considered bluff bodies because at large

Reynolds numbers the drag is dominated by the pressure losses in the wake. The variation of the drag coefficient with Reynolds number is shown in figure 3, and the corresponding flow patterns are shown in figure 4. We see that as the Reynolds number increases the variation in the drag coefficient (based on cross-sectional area) decreases, and over a large range in Reynolds number it is nearly constant. The flow separates as it passes over the cylinder and we can

also observe that as the Reynolds number is increased the vortex/wake behind the cylinder becomes more unstable and results in vortex shedding and finally leads to turbulent flow.

Figure 3 Cd vs Re

Figure 4 Flow over the cylinder with various Re

Vortex sheddingvortex shedding is an oscillating flow that takes place when a fluid such

as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body. In this flow, vortices are created at the back of the body and detach periodically from either side of the body. If the bluff structure is not mounted rigidly and the frequency of vortex shedding matches the resonance frequency of the structure, the structure can begin to resonate, vibrating with harmonic oscillations driven by the energy of the flow. This vibration is the cause of the "singing" of overhead power line wires in a wind, and the fluttering of automobile whip radio antennas at some speeds. Tall chimneys constructed of thin-walled steel tube can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can damage or destroy the chimney.

Strouhal numberThe frequency at which vortex shedding takes place for a cylinder is

related to the Strouhal number by the following equation:

St= fDv

Where St is the strouhal number, f is the vortex shedding frequency, D is the diameter of the cylinder and V is the flow velocity.

Figure 5 Strouhal Number vs Reynolds number

Vehicle AerodynamicsObjectives of improvement of flow past vehicle bodies:

• reduction of fuel consumption • more favourable comfort characteristics (mud deposition on body,

noise, ventilating and cooling of passenger compartment) • improvement of driving characteristics (stability, handling, traffic

safety)

Vehicle aerodynamics includes three interacting flow fields:

• flow past vehicle body • flow past vehicle components (wheels, heat exchanger, brakes,

windshield), • flow in passenger compartment

Development of Aerodynamics design of Vehicles

Drag coefficient of vehicles