A combustor structure includes a combustor fuel-air mixing apparatus having a mainstream airflow region. One or more concavities are disposed within the mainstream airflow region of the fuel-air mixing apparatus. At least one fuel injection hole is disposed within an upstream base region of at least one concavity. Passing a stream of air through the main airflow region causes fuel injected into the upstream base region of at least one concavity to passively mix with a naturally oscillating vortex of air within the concavity.