Thurst and Power coefficient calculation in helicopter
 

Hi
I am simulating a flow over a helicopter blades in hover condition using overset mesh technique in STARCCM+. I have created two seperate region- outer domain and inner region with overset mesh BC. The inner region is rotated with specific rpm. Can anyone help me how to calculate power coefficient and thurst coefficient (Ct/Sigma) ( Ct-Thrust Coeff and sigma- solidity) from the simulation:confused:. Thanks in advance

The short answer is to use expression reports. This will allow you to setup monitors and plots to track the convergence and values of your thrust and power coefficients.

From your description of your model, you already know your fixed rotor RPM. For the sake of argument let's assume that your rotor has a 3.5m radius (38.5 m^2 area) and is operating at 850 RPM (312 m/s tip speed). Now you should either know your free-stream density or be able to determine it from a surface average report of your far-field boundaries. For the sake of argument let's assume it is 1 kg/m3.

To setup the thrust coefficient report, first create a force report with the axis aligned with the rotor axis to capture the thrust. Call this 'thrust'. Next, create an expression report and call it 'thrust_coeff'. The definition will look something like this:

$thrust_Report/(1*38.5*312*312)

You can then create a monitor and plot from this report to keep tabs on this value like you would any other solution parameter.

The power coefficient will be similar, the key difference is that you need a moment report about the rotor axis rather than a force report aligned with the rotor axis. Call this one 'torque'. Power is torque x rotational speed which we already know is 850 rpm (~89 rad/sec). Therefore we can define a second expression report for power coefficient as follows:

$torque_Report*89/(1*38.5*312*312*312)

As for solidity, this is not something that will fall out of your solution. It is a geometric parameter you should calculate independently of your flow solution. There are a few different ways to approach this, but I typically use N*c*R/A where N is the number of blades, c is the mac of the blade, R is radius and A is total swept area. If you need this in your solution to help calculate other parameters (like figure of merit) then I would just treat it like a constant. It's not worth the effort of creating an expression report to calculate the same value at each iteration.

Thank you for the response

Thrust coefficient
 

Hey,

How do your thrust coefficient results look like. Are they perioidic ?
Star literature suggests that we obtain periodic results in around 2 - 5 rotor revolutions.

How is the thrust coefficient value fares when compared to the experimental values. How close are you to the experimental value.

Any leads appreciated.