Turbulence

Turbulence or turbulent flow is a flow regime characterized by disordered and stochastic property changes in fluid dynamics and this includes low momentum diffusion, high momentum convection and rapid variation of pressure and velocity in space and time. Due to the action of fluid molecular of viscosity which the kinetic energy dies out the flow in is called laminar flow. Where the flow in which Reynolds number is high it is turbulent flow, and which the flow containing less Reynolds number it is laminar flow. For the pipe flow, a Reynolds number about 4000 will mostly correspond to turbulent flow, and the Reynolds number below 2100 correspond laminar flow. This region between 2100<4000 is="" called="" transition="" region.="" the="" unsteady="" vortices="" appear="" on="" many="" scales="" and="" interact="" with="" each="" other="" in="" turbulence. Drag will increase due to boundary layer skin friction increases. The location and structure of the boundary layer separation changes, and sometimes will result in reduction of overall drag. Reynolds number will not govern the laminar turbulent transition, the same transition occurs if the object size gradually increased, or the fluid viscosity decreased, or density of the fluid is increased. Turbulence is highly characterized by following features; irregularity, diffusivity, rotationality, dissipation, energy cascade, integral length scales, kolmogorov length scales, and Taylor micro scales.<4000>

Examples of Turbulence

• The smoke from the cigarette is turbulent flow, while the smoke rising from the cigarette, for few centimeters it flows laminar, then it becomes turbulent as the Reynolds number increases.
• Placing the object in the moving air, if the object is in the desired smooth shape, there will fewer vertexes. If the object is so shaped, the air flowing over the irregular shape will produce vortex and creates turbulent flow. This result in higher skin friction, but moves the point of boundary layer separation further along, and thus resulting in lower form drag and lower overall drag.
• When the warm air and cold air get mixed up in the atmosphere by wing, this causes the clear-air turbulence experienced during flight.
• Most of the atmospheric circulation.
• The flow conditions in many industrial equipment (such as pipes, ducts) and machines (for instance jet engine).
• The external flows of vehicles such as cars, airplanes, ships, etc.
• The jet exhaust from a nozzle into a quiescent fluid. As the flow emerges into this external fluid, shear layers are created at the lips of nozzle. This creates turbulence.

Questions:

• What is turbulence?
• Explain any one example of turbulence?