Most of our military helicopters have centrifugal separators on the air intakes of the engines. Will these work against the ash particles, and if so, is there any risk to any other areas such as rotor blades?
Do we need to ground rotary also?
Those centrifugal separators work adequately for hovering over sand for short periods of time.
However even they only separate out a percent of the ground sand, so too long hovering will still be a problem.
Volcanic ash is made up of a lot more stuff than sand. The problem is that the lighter stuff which is dust of a flour consistency does not have enough enertia to be separated from the air molecules and thus will still turn corners in the separator and enter the engine. OK at least the big stuff causing errosion of the innards will be separated but you still have to contend with the fine stuff that might melt or more likely cause blocking of cooling holes in the turbine.
Whether the stuff melts and plates out on the critical stationary parts of the turbine is a function of the operating temperature in the engine. If you are low enough < 800C (statistically) you will probably have little plating out and immediate power loss but still have to contend with the clogging of cooling holes in the vanes and blades.
The clogging doesn't typically cause a problem in a single flight but over several flights expect to get burning of the blades and vanes and a serious loss of efficency-power.
The flour like particles (small enough to melt) are the stuff that floats at high altitudes and slowly comes down to the ground in those visible blackouts. So you might be able to operate at lower altitudes in helicopters with due care of stirring stuff up off the ground.