Variable Transport Delay Irregularities

I was wondering if anyone has encountered any irregularities when using Simulink's Variable Transport Delay block. I am modeling flow in a pipe, where the flow rate in the pipe will periodically slow to zero, thus creating a momentary infinite delay. According to the MATLAB's documentation, the only limitation on the Max Delay of the block is that it be non-negative. However, I have recently learned from the Mathworks that this block can actually only handle delays that are greater than zero and smaller than the current simulation time.

"I was finally able to get some input from the developer who wrote the Variable Transport Delay block. He says that the block is intended to work with delays greater than zero and smaller than the current simulation time. Outside that, the behavior is not well-defined. I will submit a suggestion to our documentation team to have this limitation more explicitly described in the page for the Variable Transport Delay block.

For applications (such as fluid flow dropping to zero) where the delay goes outside this range, you would need to implement your own algorithm to work for the specific problem. I am sorry that this is what the answer turned out to be after such a long time."

Has anyone ever attempted to address this problem with custom code? Thanks.

-Jon

Answer 1

[Guy Rouleau]

The limitation mentioned here applies to the the Variable Time Delay, not the Variable Transport Delay. For the Time Delay, a delay that brings you to a negative time is undefined.

With the Variable Transport Delay, to model a flow dropping to zero, you need to set the Maximum Delay in the block dialog to a very large value. This will not be an ideal zero, but a small negligible leakage.