Why chimneys have spiral fins at its outer side ?

        We may have seen that some chimneys have spiral shaped fin outside its cylindrical wall. This spiral is seen, especially, in tall chimneys.

Chimney with spiral fins

The main reason behind the use of this spiral fin is to avoid the vibration of it, thereby avoiding the failure of the chimney.

Here is the animation of the von karman vortex street, when fluid passes through the cylindrical object.

See the von karman vortex street during flow past the cylinder

 In this animation we can see that there is the vortex formations in the wake regions. And it is changing. This flow is due to symmetry of the object. If symmetry is not there the vortex get vanished. When  Eddy formation at two sides of the cylinder, interacts with each other, then this phenomenon happens. There is fluctuation of force acting on the chimney due to change in position of the wake, this leads to the vibration of the chimney.

To avoid the fluctuating force, we have to avoid interaction of the two eddies.. To avoid interaction of the eddies, we have to remove the cause it - symmetry of the object. So we have to make the cylinder asymmetric.  By attaching the spiral fins we are making it asymmetric.Spiral fins effectively create  asymmetric 3D flow, thereby discouraging the alternate shedding of vortices.

That's why tall chimneys have spiral fins attached to it. 

Why bulb of mercury thermometer is kept cylindrical ?

We all know that mercury thermometer is used for measurement of temperature. And our prime intention is to get accurate temperature measurement.

Mercury thermometer

Principle of Temperature measurement in mercury thermometer is thermal expansion. If temperature increases mercury expands and if it decreases mercury contracts. So basically temperature difference causes the transfer of heat and there is the process leading towards the thermal equilibrium. And the duration in which process of reaching the equilibrium is there is time dependent heat transfer - very well know as transient heat transfer.

For the analysis of transient heat transfer there is one method- lumped mass system. Based on it following equation is obtained.

 Equation 1

Where,

T              = Temperature of body at time 't'

Tinf          = Temperature of surrounding of the body,

Ti             = Initial Temperature of body 

t               = time(seconds)

h              = convective heat transfer coeffienct

A             = surface area of the body 

Rho         = density of the body material

c              = specific heat of the body

V             = volume of the body 

   Here, ( Rho*c*V/(h*A) )= time constant =Tau

So from Equation 1, we can see that to reach the temperature Tinf , we should have theoretically infinite time. But to reach specific temperature, Tau plays a major role. If Tau(=time constant) is less then time required to reach the specific temperature is less, if it is high then so time is also high. 

Graph 1

Now we can understand that to measure the temperature we have to wait for infinite time(see graph 1), which is impossible. So we should measure the temperature as near as actual temperature with possible time.  i.e. we should have as minimum as possible the time constant(refer graph 2). For given material with same convective heat transfer coefficient,  Spherical shape has minimum ratio of A/V . Here we want high A/V, to make (Rho*c/h) *(V/A) minimum.  The cylindrical one has high (A/V).

Graph 2

Thus we should have minimum time constant to measure the temperature by thermometer. And for that we should have cylindrical bulb. That's why bulb of mercury thermometer is kept cylindrical. hence more sensitive.  

See above video for: How to plot transient temperature in scilab and view the temperature variation.

             

Unbelievable !!! Metal needle can float on the water

If you disturb the water very much then your  metal needle will sink into the water.



Higher the surface tension of the liquid more weight it can float.

How to make paper frog- An origami technique

This video shows how to make a paper frog, which can jump, from a single piece of paper without cutting or sticking any other paper. And when this frog is ready then when you press at the back, it will jump. If you use stiffer paper then frog will jump high as well as long.

If you have any suggestions please  comment.

Thank you for watching.