If you do not want to lose a large diameter fan operating in the Venturi nozzle, you should use " internal Venturi nozzles ".
These are just two cones inside a circular tube
Their convergence must be in accordance with the Bernoulli's principle, just like a normal venturi nozzle. So must be kept laminar flow. But we gain a large diameter fan. Theoretically, the energy gain obtained is 10 times larger in relation to the windmill in a narrow place of the normal venturi nozzle....
Field cross-sections must be changed in the same function as in classical venturi