Measuring and testing – Volume or rate of flow – Using differential pressure
Patent
1988-01-12
1990-05-01
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
Using differential pressure
732025, 73189, G01F 134
Patent
active
049208080
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method for measuring the flow velocity of a free flow in three dimensions and a device with which this method can be carried out.
The method and the device of the invention are used, on the one hand, as a fixed probe for measuring the velocity of the medium which flows around the probe and, on the other hand, as a moving probe, for example on flying bodies, ships, land vehicles or the like while they move through a medium, for example air, to measure the relative velocity between the body carrying the probe and the medium. In the latter case, the probe is therefore used, in other words, to measure the velocity of an aircraft, a ship, a land vehicle or the like. In addition, the measurement of flows in pipework, for example in the chemical field, offers many possibilities of application.
As long as vehicles exist, it is necessary to measure the velocity of the vehicles. Particularly in the case of aircraft, knowledge of the velocity is important. There are also, however, many other technical problems in which flowing media have to be measured. The measurement units which have long been known were: Pressure probes which are used to measure the static pressure at any given point within a flow and consist, for example, of a tube rounded at the front with small holes or an annular slot at the sides. The openings, i.e. the holes or the annular slot, must be located outside the accelerated flow influenced by the tip of the probe, and the probe only measures correctly in the case of exactly axial incident flow. This small blowing angle is often a disadvantage in practical use.
Pitot tubes measure the total pressure in the special case where this is equal to the dynamic pressure (sic) if the flow is completely brought to rest in a stagnation tube.
Prandtl stagnation tubes are combinations of a pitot tube with a pressure probe necessary for measuring the static pressure and permit the determination of the dynamic pressure as the difference between the total pressure and the measured static pressure, from which the incident flow velocity can in turn be determined. It is found computationally as the square root of the pressure difference (total pressure minus static pressure) divided by the density of the medium.
Even in the case of the Prandtl stagnation tubes, accurate measurements can only be achieved with practically axial incident flow. One important reason for this, among others, is that the opening or openings for measuring the static pressure are relatively distant spatially from the dynamic pressure measurement opening. It is therefore only in the case of quite even flow around the Prandtl tube in the axial direction that exact measurement results can be achieved.
Finally, Venturi tubes are also used for measuring the velocities of the flow through them. A Venturi tube is a tube provided with a throat and having axial flow through it. A pressure gauge is used to measure the static pressure difference between a position at which the flow is still substantially laminar and a position at the throat. In the case of a frictionless and inviscid fluid, the flow velocity in the part of the tube with laminar flow is obtained by applying the continuity equation and the Bernoulli equation, as the square root of the difference between the static pressures, divided by the density of the flow medium, and of a geometry factor of the Venturi tube.
On the basis of a known Venturi tube for measuring the flow velocity in a tube, a method and a device are produced for measuring the flow velocity of a free flow in three dimensions, which method and device can be used over a large angular range relative to the flow direction or the blowing direction and which, in special embodiment forms, make it possible to determine the flow velocity without knowledge of the density of the flowing medium.
This objective is achieved by means of a method and device, including advantageous embodiments of this device according to the invention, as described in this specification and claims.
In the method of the invention, st
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