Flowmeter based on pressure drop across parallel geometry using boundary layer flow including Reynolds numbers above the laminar range

A flowmeter for measuring the flowrate of a fluid inside a flow channel having a cross-section and a principal flow direction, wherein the flowmeter is operable between a lower-limit flowrate and an upper-limit flowrate, comprising

• a flow resistance element located inside the flow channel across substantially the entire cross-section of the flow channel, and
• a differential pressure sensor which measures the difference between the pressure at a first location and the pressure at a second location downstream of the first location,
• and wherein the flow resistance element has an upstream end and a downstream end and
• wherein the pressure at the first location is measured by a static pressure tap inside the flow resistance element preferably close to the upstream end of the flow resistance element and the pressure at the second location is measured by a static pressure tap downstream of the flow resistance element, and wherein the flow resistance element comprises a plurality of objects each having a side surface which is substantially parallel to the principal flow direction and having a frontal surface which is not substantially parallel to the principal flow direction, the side surface having a side surface area and the frontal surface having a frontal surface area, wherein the side surface area is much larger than the frontal surface area,
• and wherein at the upper-limit flowrate at the downstream end of the flow resistance element, a first side surface of said plurality of objects has a first boundary layer having a first boundary layer thickness and a second side surface of said plurality of objects which faces the first side surface has a second boundary layer having a second-boundary-layer thickness, and the first side surface is separated from the second side surface by a distance, and the sum of the first boundary layer thickness and the second-boundary-layer thickness is less than the distance between the first side surface and the second side surface, and wherein a characteristic dimension is the distance between the first side surface and the second side surface, and the flow resistance element has a length from the upstream end to the downstream end, and wherein the ratio of length to characteristic dimension is between about 2 and about 50, and wherein the fluid has a velocity at the upper-limit flowrate and a density and a viscosity and wherein the thickness of each boundary layer is given by the length times 5 divided by the square root of (the density times the velocity times the length divided by the viscosity).