its lowest spot the first obstacle body 7 a provided as an impact plate 9 has the largest cross section whereas the following obstacle bodies 7 b and 7 c have a cross section that is decreasing in comparison preferably in analogy to the decreasing open diameter inside the flow through chamber 5 so the radial width of the annular flow gap 6 between the obstacle body and the surrounding housing 2 stays constant or even decreases in the flow direction 10 some of the obstacle bodies 7 a d shown as examples have several axially spaced annular separation edges 8 protruding towards the housing 2 the farthest the third obstacle body 7 c when seen in a longitudinal cut view is provided with an annular groove on its outer circumference which abuts to a separation edge 8 in the axial front and back in an axial direction and it has two planar parallel surfaces facing the flow exactly perpendicular to the direction of the axis the flanks of the annular grooves in the enveloping surfaces of these obstacle bodies transition into the enveloping surface in an acute angle forming separation edges 8 whose angle at the front most separation edge this means at the impact surface 9 a is approximately 45 u00b0 wherein both angles of the separation edges can also be under 45 u00b0 the fourth obstacle body 7 c includes several in this case three axially spaced and firmly connected radial discs with constant thickness whose narrow outer enveloping surfaces are parallel to the flow direction 10 and hereby perpendicular to the plane of the discs the obstacle body 7 b is shown.