A variable geometry assembly for modulating a fluid flow in a turbomachine such as a centripetal turboexpander or turbine or a centrifugal compressor, the variable geometry assembly comprising: a first ring (33; 133) comprising a plurality of first wedge-shaped elements (37; 137) and having an axis (A-A); a second ring (35; 135) comprising a plurality of second wedge-shaped elements (39; 139) and having an axis (A-A), the second ring (35; 135) being substantially coaxial to the first ring (33; 133); the second wedge-shaped elements (39; 139) co-acting with the first wedge-shaped elements (37; 137); wherein: flow passages are defined between pairs of sequentially arranged first wedge-shaped elements (37; 137) and second wedge-shaped elements (39; 139), wherein each flow passage is formed between a first airfoil surface formed on the respective first wedge-shaped element and a second airfoil surface formed on the respective second wedge-shaped element; the first ring (33; 133) and the second ring (35; 135) are angularly displaceable one with respect to the other; the first ring (33; 133) and the second ring (35; 135) are configured to move axially with respect to one another when the first ring and the second ring are angularly displaced one with respect to the other; characterized in that the first wedge-shaped elements are comprised of respective trailing edges and the second wedge-shaped elements are comprised of respective leading edges wherein the trailing edges are facing radially inwardly towards the axis (A-A) of the first ring (33; 133) and second ring (35; 135) and the leading edges are facing radially outwardly away from said axis (A-A); wherein each first wedge-shaped element (37; 137) comprises a first sliding surface in sliding contact with a respective second sliding surface of a corresponding one of said second wedge-shaped elements (39; 139) and wherein the first sliding surfaces and the second sliding surfaces are smooth such that the first ring (33; 133) and the second ring (35; 135) slide continuously one over the other when the angular displacement therebetween occurs. a first ring (33; 133) comprising a plurality of first wedge-shaped elements (37; 137) and having an axis (A-A); a second ring (35; 135) comprising a plurality of second wedge-shaped elements (39; 139) and having an axis (A-A), the second ring (35; 135) being substantially coaxial to the first ring (33; 133); the second wedge-shaped elements (39; 139) co-acting with the first wedge-shaped elements (37; 137); wherein: flow passages are defined between pairs of sequentially arranged first wedge-shaped elements (37; 137) and second wedge-shaped elements (39; 139), wherein each flow passage is formed between a first airfoil surface formed on the respective first wedge-shaped element and a second airfoil surface formed on the respective second wedge-shaped element; the first ring (33; 133) and the second ring (35; 135) are angularly displaceable one with respect to the other; the first ring (33; 133) and the second ring (35; 135) are configured to move axially with respect to one another when the first ring and the second ring are angularly displaced one with respect to the other; characterized in that the first wedge-shaped elements are comprised of respective trailing edges and the second wedge-shaped elements are comprised of respective leading edges wherein the trailing edges are facing radially inwardly towards the axis (A-A) of the first ring (33; 133) and second ring (35; 135) and the leading edges are facing radially outwardly away from said axis (A-A); wherein each first wedge-shaped element (37; 137) comprises a first sliding surface in sliding contact with a respective second sliding surface of a corresponding one of said second wedge-shaped elements (39; 139) and wherein the first sliding surfaces and the second sliding surfaces are smooth such that the first ring (33; 133) and the second ring (35; 135) slide continuously one over the other when the angular displacement therebetween occurs.