Experiment: Characteristic of Laminar & Turbulent flow.
Objective:
(i) To study the characteristic of laminar and turbulent flow in a pipe by using simple flow visualization.
(ii)To find out the Critical Reynolds Number for pipe flow.
Apparatus Used:
1. Reynolds Apparatus
2. Measuring Flask
3. Stop Watch
Theory:
The flow of real fluids can basically occur either laminar or turbulent flow. The laminar flow is characterized by fluid particles moving in the form of lamina sliding over each other, such that at any instant the velocity at all the points in particular lamina is the same. The lamina near the flow boundary move at a slower rate as compared to those near the center of the flow passage. This type of flow occurs in viscous fluids, fluids moving at slow velocity and fluids flowing through narrow passages.
The turbulent flow is characterized by constant agitation and intermixing of fluid particles such that their velocity changes from point to point and even at the same point from time to time. This type of flow occurs in low density Fluids flow through wide passage and in high velocity flows. Reynolds conducted an experiment for observation and determination of these regimes of flow. By introducing a fine filament of dye in to the flow of water through the glass tube, at its entrance he studied the different types of flow. At low velocities the dye filament appeared as straight line through the length of the tube and parallel to its axis, characterizing laminar flow. As the velocity is increased the dye filament becomes wavy throughout indicating transition flow. On further increasing the velocity the filament breaks up and diffuses completely in the water in the glass tube indicating the turbulent flow. After conducting his experiment with pipes of different diameters and with water at different temperatures Reynolds concluded that the various parameters on which the regimes of flow depend can be grouped together in a single non dimensional parameter called Reynolds Number.
Reynolds Number is defined as, the ratio of Inertia Force to the Viscous Force, where viscous force is shear stress multiplied surface area and inertia force is mass multiplied acceleration of the fluid particles.
Re= ρUD/μ= UD/ν
where
Re-Reynolds Number,
U - Velocity of flow,
D - Characteristic length = diameter in case of pipe flow,
ρ - Mass density of fluid,
μ - Dynamic viscosity of fluid,
v = μ/ρ - Kinematic viscosity of fluid.
Reynolds observed that in case of flow through pipe for values of
Re<2000 the flow is laminar,
Re>4000 it is turbulent and for,
2000<Re<4000 it is transition flow.
Experimental Procedure:
1. Fill up sufficient amount of water in the upper head tank.
2. Start water flow through pipe and adjust gradually.Start dye injection.
3. Wait for some time. A steady line of dye will be observed. Adjust dye flow, if required.
4. Slowly increases the water flow keeping water level in supply tank constant. At particular flow rate, dye line will be disturbed.Note down this flow rate.
5. Further increase the flow. The disturbances of dye line will go on increasing and at certain flow; the dye line diffuses over the entire cross section. Note down this flow.
6. Slightly increase the flow and then slowly reduce the flow. Note the flow at which diffused dye tends to become steady. (beginning of transition zone while reducing velocity.)
7. Further reduce the flow and wait for the flow at which dye line becomes straight and steady.
Observations:
1. Internal diameter of the glass tube= Ø.025 m
2. Cross sectional area of the glass tube= 4.9087 × 10⁻⁴ m²
3. Temperature of the water= 20°C
4. Density of the water= 998.2 kg/m³
5. Kinematic Viscosity of the water [from standard table]= 1.0035 ×10⁻⁶ m²/Sec
FAQ :
1. Define Critical Reynolds Number and state its physical significance. State the value of Critical Reynolds Number for the present experiment.
Critical Reynolds number is determined as a limit where the laminar flow changes to turbulent flow.
Significance- As this is proportional to the ratio of inertia force to viscous force. it can be used to study the fluid flow whether it is laminar or turbulent.
From our experiment we get 3 different values of Reynolds number for different flow rate as following,
- 1245(Laminar)
- 3462(Transition)
- 5132(Turbulent)
2. List the sources of the error of the experiment.
a. Take this charge reading accurately.
b. Set the discharge value constant during each flow.
c. During whole experiment fluid storage tank should maintain a constant head. But to prevent turbulence that can not be maintained during experiment.
3. Draw the schematic diagram of the experimental set-up.
Written by:- SOUMIK