An apparatus for generating pressure variances in a flowing fluid includes a first member having a body and a second member. The second member is displaceable about a rotational axis to at least partially block fluid flow through one or more channels in the first member and at least partially reduce the blockage of this fluid flow. The second member has a hub and at least one section extending axially and radially from the hub relative to the rotational axis of the second member. A related method includes guiding fluid across the first member using the channel(s); selectively blocking the fluid flow through the channel(s) using at least one section of a second member; and moving the second member using an actuator to reduce the blockage of the flow of fluid through the channel(s).