Chapter 3 : Kinematics of Fluid
Lecture 7 :


 Problem1: 

A velocity field is given by

 

      a) Find the equation of the streamline at t =t0 passing through the point (x0,y0).

      b) Obtain the path line of a fluid element which comes to (x0, y0) at t=t0.

      c) Show that, if A=0 and B=0 (i.e. steady flow), the streamline and path line coincide.

Solution:

a)      Streamline: Here Ux=(1+At +Bt­­2) and Uy=x.

 Since the slope of the streamline (dy/dx) is the same as the slope (Uy/Ux) of the velocity vector.

Therefore     

 Integrating this with the condition x=x0, y=y0 gives the Streamline

 

 b)      Path line: Consider a fluid element passing through (x0, y0) at t=t0. Its co-ordinates (x,y) at other values of t (which define the pathline) can

                be expressed as

 

          Since,     

 

And,              

 

Integrating  the first equation gives,

Now,    

                  These equations of x, y are parametric equation of path line.

                  The time t can be eliminated between them to give an equation for y in terms of x.

        c)    When A=B=0, then the equation of streamline becomes

 

and the parametric equations of the path line becomes;

 

Therefore,      

 

which is equivalent to streamline.

 

Problem2:

A two-dimensional flow field is defined as

 

Define the equation of Streamline passing through the point (1,0)

Solution:

The equation of Streamline is

 

or,  

 

Hence,   

 

or,

 

Integration of equation above gives

 

where k is constant

 

For stream line passing through (1,0) ,

Hence, the required equation is:

 

 

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