There are a variety of designs offering free air gravity driven ventilation.
Until recently the American suppliers offered ventilators with flat top rain hats with sides that force the air to go down before it can go up. LorenCook is most proud of their curled throat top edge that they say allows the air to move over and down with less turbulence.
Edmonds in Australia has one of the highest performance rooftop gravity ventilators in the world, their Hurricane, which uses a vertical bladed turbine to enable air to flow nearly perfectly up and out.
Invisco Industrial in California has an improved version of the Hurricane that is called the Tornado. It incorporates turbine blades that are 20% taller than the Hurricane thus offering 20% more air flow.