Exercise 4 – Reynold’s Number: a basic class

Published: July 13, 2010

Reynolds number is calculated using formula (D*v*rho)/mu Where D = Diameter, V= velocity, rho = density, mu = viscosity. Write a program that will accept all values in appropriate units (Don’t worry about unit conversion). If number is < 2100, display Laminar flow, If it’s between 2100 and 4000 display ‘Transient flow’ and if more than ‘4000’, display ‘Turbulent Flow’ (If, else, then…)

In working this solution I decided to declare a class to help in the calculation. The solution also shows some details of reading input from the console.

For this post I’ve broken the code into two boxes. The first, as shown by the line numbers, is actually the second part of the program, but I want to explain it first.

class Reynolds
{
    public double density;
    public double diameter;
    public double velocity;
    public double viscosity;

    public double Number
    {
        get
        {
            return density * diameter * velocity / viscosity;
        }
    }

    public string FlowType
    {
        get
        {
            if (Number < 2100)
            {
                return "Laminar";
            }
            else if (Number < 4000)
            {
                return "Transient";
            }
            else
            {
                return "Turbulent";
            }
        }
    }
}

This is the declaration of the Reynolds class. The class has 4 fields which hold values for the four variables in the equation, and two properties, Number and FlowType, which return values based on the calculation of the formula. As you’ll see below, properties of a class are accessed that same way as fields, but as seen above they can contain code to calculate, or otherwise determine the value to be returned. The two properties here are read-only properties because they only have a get clause. It is also possible to create write-only properties (only a set clause) and read-write properties (both get and set). Note that all the fields and properties are declared public so that they’re accessible from outside the class. Common code conventions generally dictate that fields should be declared private, and that applicable public, protected, or internal properties should be created to allow the necessary access.

The second code block shows the main part of the program (no pun intended).

using System;

class MainClass
{
    static double ReadDouble (string inputPrompt = "Enter a number")
    {
        double inputValue;

        do
        {
            Console.Write(inputPrompt + ":  ");
        }
        while (!double.TryParse(Console.ReadLine(), out inputValue));
        return inputValue;
    }

    static void Main()
    {
        Reynolds reynolds = new Reynolds();
        Console.WriteLine("Calculate Reynold's Number and flow characteristic");
        reynolds.density = ReadDouble("Enter the Density (\x03c1)");
        reynolds.diameter = ReadDouble("Enter the Diameter (D)");
        reynolds.velocity = ReadDouble("Enter the Velocity (v)");
        reynolds.viscosity = ReadDouble("Enter the Viscosity (\x03bc)");
        Console.WriteLine("Reynold's Number = " + reynolds.Number.ToString() + " (" + reynolds.FlowType + " flow)");
    }
}

To keep the code simple I created a method (ReadDouble) to prompt the user for a value from the console, and continue to prompt until a valid double value is entered. We’ve seen optional parameters before. Here I’ve declared inputPrompt with a default prompt that will be used if none is specified in the method call. This method also shows use of C#’s do {} while () loop construct, which should be immediately familiar to any C/C++ programmer. C# also has a while () {} construct, with the loop condition at the front-end.

There’s nothing terribly special about the Main method. First thing it does is to create an instance of the Reynolds class, and assign it to a variable called reynolds. Note that the class and variable names differ only by case. As C# is case-sensitive language this isn’t a problem, but it is frowned upon because it can lead to problems when interoperability with a case-insensitive language, like VB.NET, is required.

The ReadDouble method is called a number of times to prompt for each of the required values in turn, and the returned values are assigned to the appropriate fields in the Reynolds class. One thing to note here is the use of Unicode escape sequences in a couple of the prompt strings. These are the codes for the greek letters mu and rho. All strings in C# are Unicode strings, so there’s no need to play with different types and functions as in C/C++.

In reading through the code above you probably thought, “hang about, what happens if I enter zero for the Viscosity (the divisor)?” That’s a good question because most languages would throw an exception in that case, and I haven’t done anything to trap that. You’re going to have to wait for the next post for the answer, because that is actually the subject of Exercise 5.

Andrew Cooper