In Fig. 4, Tesla used a variable inlet nozzle #12, and controlled the amount of gas entering by a movable "block" - #13.
Note that also, in Fig. 4, the inlet and exhaust port size was increased, to allow for more power, on demand.
Tesla, in Fig. 5 used a diverging inlet nozzle - #15, controlled by a "butterfly" valve, #16 .
This design was to be incorporated into Tesla's ''flying machine'', with two 10'' turbines, rated at 400 HP
Inlet nozzles and the Anharmonic Resonator:
Looking at the diagram of the anharmonic resonator (which would include the types of nozzles Tesla was using); Sonic gas speed will occur at the smallest area cross section between a high pressure reservoir and the turbine discs. Supersonic gas flow speeds will only occur immediately downstream of a region of sonic speed (smallest cross section) and only if the cross section gradually increases. This has been proven in laboratories hundreds of times. If all gas flow properties are being regulated at the inlet nozzle immediately upstream of the turbine discs, that will be the region where sonic gas speed will occur. All cross sections upstream of the nozzle and up to the exit of the high-pressure reservoir would have a greater cross-section area. The best location to regulate gas pressure and the mass flow rate of the gas would be at the inlet nozzle, not the reservoir outlet.
The anharmonic resonator (click image thumbnail, left to enlarge) will be used downstream of a regulator valve that would also be the exit of the high pressure gas reservoir. Sonic gas speed will occur in that regulator valve (it will regulate the mass flow rate of the gas and also its pressure) and the gas flow downstream of the regulator valve would go supersonic if cross section area gradually increases. Under these conditions, a device like the aharmonic resonator could break up the supersonic shock waves ahead of a Tesla turbine intake. To slow supersonic gas, the cross section area needs to be reduced. The anharmonic resonator is essentially a modification of the Oswatitsch intake that is used at the entrance to the engines of supersonic aircraft. The Macrosonix-type intake would break up the sound waves so as to reduce the pressure loss as the air slows from supersonic speed to subsonic speed.
This device will only have use if the gas flowing toward the intake nozzle of a Tesla turbine is already traveling at supersonic speed. If the gas speed is subsonic and with no hope of it ever going supersonic, then the device would either do nothing or it will cause problems if it is not designed properly. If you want to run a Tesla turbine when the air at the intake to the nozzle is supersonic, an Oswatitsch intake may involve lower energy losses that this resonator. Both devices would work best in an aircraft traveling at supersonic speed and a Tesla turbine was being used on board to drive electrical generation equipment or hydraulic equipment.
Mass flow rate of gas into a Tesla turbine can be regulated by using a rectangular cross-section of inlet nozzle. It is very easy to achieve adjustable and variable cross-section area from a rectangular nozzle. An alternative would be to use multiple nozzles of varying cross-section areas. Operate some nozzles and keep others shut off so as to achieve the desired mass flow rate of gas into the Tesla Discs. 4-nozzles in a 1:2:4:8 cross section ratio will give a 1:2:4:8 mass flow rate ratio into the Tesla discs, which in turn would yield 15-mass flow settings and also 15-power settings in equal steps.