Fluid Mechanics and Machinery MCQs | Page - 1

 the reynolds number directly depends upon the hydraulic diameter of the pipe. suppose the diameter of the pipe is d, the hydraulic diameter of square pipe is 1.57d. hence, 57% increase.

 a flow through a circular pipe is said to be laminar when the reynolds

 if the reynolds number of the flow is below its lower critical reynolds number, the flow is clearly laminar. the maximum diameter can be found for re = 2100. the diameter comes out to be 2.1 mm.

 the approximate lower critical reynolds number for flow in a pipe, flow between parallel plates, flow in an open channel and flow around the spherical body are 2000, 1000, 500 and 1 respectively.

 adverse pressure gradient takes place when the static pressure increases. it increases the direction of the flow. adverse pressure gradient plays an important role in flow separation. thus, option c is correct.

 vortices are a region in a fluid. it takes place when the flow revolves around an axis line. vortices can be straight or curved. they form shapes like smoke rings and whirlpools.

 eddy viscosity is a turbulent transfer of momentum by eddies. it gives rise to an internal fluid friction. it is in analogous to the action of molecular viscosity in laminar fluid flow. eddy viscosity takes place on a large scale.

 turbulator is a device that converts a laminar flow into a turbulent flow. the turbulent flow can be desired parts of an aircraft or also in industrial applications.

 boundary layer separation undergoes detachment from the surface into a broader wake. it occurs mainly when the portion of the boundary layer is closest to the wall. it leads to reverse in the flow direction.

 with the boundary layer separation, displacement thickness increases sharply. this helps to modify the outside potential flow and its pressure field. thus, option ‘a’ is the correct choice.