Creating a simple Reynolds number calculator

This time I'm posting a simple Reynolds number calculator, it has been done without using the class mechanism because I was getting some wierd results.

The image that is going to be used is the next one:

The code is the next one:

from Tkinter import *

def calc_Re():
    """
    Re : Reynolds Number
         (Adimensional number used to determine
          if a fluid flow is laminar or turbulent type)
    Re = velocity * diameter / kinematic_viscosity
    Re =  v__val  *  d__val  /      nu_val
    (row = 3, column = 1)
    """
    if nu_val.get() == 0:
        print "Kinematic viscosity must be greater than zero"
        lab4.config(text="Nu = 0")

        lab4.grid(row = 3, column = 1)

    else:
        v_ = v__val.get()
        d_ = d__val.get()
        nu = nu_val.get()
        re_= v_ * d_ / nu
        result = "Re = " + str(re_)
        print "Re = {: f};(U = {: f}, D = {: f}, v = {: f})".format(re_, v_, d_, nu)
        lab4.config(text=result)
        lab4.grid(row = 3, column = 1)


root = Tk()
root.title("Reynolds (Re)")

img_01 = PhotoImage(file = "_Re_01_54x25.gif")
v__val = DoubleVar()
d__val = DoubleVar()
nu_val = DoubleVar()

lab1 = Label(root, text = "velo: U =").grid(row = 0, column = 0)
lab2 = Label(root, text = "diam: D =").grid(row = 1, column = 0)
lab3 = Label(root, text = "nu__: v =").grid(row = 2, column = 0)
lab4 = Label(root, text = " ")

ent1 = Entry(root, textvariable = v__val).grid(row = 0, column = 1)
ent2 = Entry(root, textvariable = d__val).grid(row = 1, column = 1)
ent3 = Entry(root, textvariable = nu_val).grid(row = 2, column = 1)

btn1 = Button(root, image = img_01, command = calc_Re).grid(row = 3, column = 0)

root.mainloop()

As you can see the "grid" system is used for the organization of everything.

The result for when you run the code is:

You can try the consistence of the definitions by seeing that the zero division is being controlled, the message in that situation would be "Nu = 0":

The rest is just a question of using valid values and see the results: