Why are you running the simulations in transient? What did you use as an initial condition? It is always good practice to start from a converged steady-state solution to "help" the solver find the correct solution.
My first guess is that the 0.5 [s] is too large a time step to capture any unsteadiness in the higher velocity case. Compare the velocity profiles from two different times. Where are the areas where there are significant differences? If you were to decrease the time step size, you may be able to better capture the unsteadiness. Yes laminar flows can be unsteady.
Think back to y = sin(x) . If you have too big a step in x then you won't recoginze the curve that appears. A similar thing happens when solving an unsteady problem. We and the solver need to be able to resolve the unsteadiness by decreasing Dt.Â
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