Measuring and testing – Wind tunnel: aerodynamic wing and propeller study
Reexamination Certificate
1999-05-11
2001-07-17
Oen, William (Department: 2855)
Measuring and testing
Wind tunnel: aerodynamic wing and propeller study
Reexamination Certificate
active
06260413
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and to a method for determining aerodynamic properties or characteristics of a wall in contact with a fluid having turbulent flow.
2. Description of the Prior Art
The prior art describes various ways of evaluating the pressure drop of a pressurized gas on turbulent flow in a pipe.
One of these methods consists in stabilizing a flow over a given length, then in measuring the pressure drop downstream, for example between two points of the pipe, the measuring points being sufficiently far from one another. This requires great pipe lengths, mainly with great Reynolds numbers and, for large-diameter pipes, particularly complex procedures (case of hydraulic tests on furrows for example), considerable testing apparatus (high pressure and flow rate), relatively long testing times and therefore high costs.
Viscometers of the prior art are designed with a different aim. They are in fact used to determine the characteristics of a fluid on laminar flow and not the aerodynamic properties of a wall in contact with a fluid on turbulent flow.
SUMMARY OF THE INVENTION
The present invention relates to a device which to determines aerodynamic characteristics of a wall.
The term wall used in the description hereafter refers to the part used for confinement of a fluid and directly in contact with the flow. It can be, for example, the inner part of an unlined channel, a coating or another fluid covering the inner part of a channel.
The term aerodynamic properties refers to, for example, in the case of a channel or a pipe, the pressure drop or pressure loss caused by a fluid flowing along the wall. In the more general case of a plate or of a wall, the drag caused by the flow along its wall is considered.
This pressure drop or drag depends not only on the flow conditions, but also on the characteristics of the wall. The wall is characterized, according to the case, by its surface state, rough patches for example, its geometric shape such as furrows, or by the presence of a film that covers it or of the material that constitutes it.
The aerodynamic properties or characteristics are established in relation to those of a smooth wall made from a standard material such as a steel for which the friction coefficient on turbulent flow in a rectilinear circular pipe is given by the following formula: f=0.316/Re
0.25
, Re being the Reynolds number of this flow. Thus, in the case of a wall having a higher friction factor than that corresponding to a smooth wall (case of rough patches with a quasi-random spatial distribution at the wall surface for example), the wall is characterized by an equivalent aerodynamic roughness. In the case of a wall having a lower friction factor than that corresponding to a smooth wall (case of furrows with an organized geometry showing on the wall surface or made from a particular material for example), the wall will be characterized by an aerodynamic efficiency value defined in the description hereafter.
The present invention advantageously applies to the field of transport of pressurized natural gas over long distances in gas pipelines. Pressure drop can reach multiples of ten bars and it is then necessary to recompress the gas at regular time intervals, onshore for example, by means of high-power recompression stations, according to pressure drop, or offshore for example, by means of pipes of large diameter according to pressure drop. Such stations or pipes contribute to increasing production costs. Proper evaluation of the aerodynamic characteristics of the surface state of a transport pipe advantageously allows selection of material forming its inner wall or optimization of the characteristics of its surface geometry to minimize the pressure drop that may be induced.
The device comprises:
a shaft,
a drum mounted on the shaft,
an enclosure in which the drum is positioned so as to define a space and at least one test zone,
at least one line which delivers a test fluid and commuicates with the test zone, and
devices which determine a parameter representative of the friction factor of the fluid in the test zone.
The said zone has a determined width
1
or a gap width, for a given rotating speed of the drum and a test fluid having known characteristics (viscosity, density . . . ), so that the fluid has turbulent flow in the vicinity of the wall(s) to be tested, the turbulent low corresponding to a given Reynolds number Re.
The drum is cylindrical over at least part of its height.
The device can comprise suitable seals that are arranged to define three zones, a first high-pressure test zone and two low-pressure zones, these zones being physically separated. The physical separation can be provided by the seals.
At least one of the test fluid delivery lines can be placed in the vicinity of the seals.
According to another embodiment, the seals can be situated in the vicinity of the shaft.
The device can be equipped with devices (C
p
, C
r
) which determine the pressure and/or the temperature in the test zone.
It can also comprise Pitot tubes which determine the local velocity of the fluid and deduce the velocity profile in the annular zone.
According to an embodiment, the device comprises a heating and/or cooling system for example.
The invention also relates to a method which determines aerodynamic characteristics of a wall. The method includes:
a test fluid is fed into the test zone comprising the wall(s) to be tested, the test zone being located between a stationary enclosure and a mobile element,
the said mobile element is rotated,
the pressure and the rotating speed of the mobile element are selected so as to obtain the desired Reynolds number,
the dissipative losses in the enclosure are determined,
the shear stress and the friction factor are determined,
the value of the shear stress or that of the friction factor are compared with a set of data obtained from three standard walls for the same value of the Reynolds number as that selected for its characterization, and
the value of the hydraulic characteristic, such as the hydraulic roughness &egr; or the aerodynamic efficiency of the wall(s) tested, is determined.
The dissipative losses in the enclosure can be determined in the vicinity of a test zone defined by a seal which defines three zones, a first test zone and two zones.
The device and the method notably find an application for the study of walls of a pipe intended for transport of a pressurized gas.
The device according to the invention has various advantages, some of which are given by way of non limitative example. By measuring the dissipative losses:
in the case of a rough wall in direct contact with the flow, it is possible to determine the value of the friction factor (parameter depending on the Reynolds number) and that of the equivalent aerodynamic roughness (absolute or relative, parameter independent of the Reynolds number),
in the case of a rough wall covered with a film-forming element, it is possible to determine the value of the friction factor (parameter depending on the Reynolds umber) and that of the equivalent aerodynamic roughness,
in the case of a wall comprising furrows, it is possible to determine the value of the friction factor (parameter depending on the Reynolds number) and that of the hydraulic efficiency of the furrow (parameter defined hereunder),
in the case of a wall made of a particular material, it is possible to determine the value of the friction factor (parameter depending on the Reynolds number) and that of the aerodynamic efficiency of the material as defined hereafter.
REFERENCES:
patent: 4485450 (1984-11-01), Characklis et al.
patent: 4643021 (1987-02-01), Mattout
patent: 4821564 (1989-04-01), Pearson et al.
patent: 5301541 (1994-04-01), Joseph et al.
patent: 5452609 (1995-09-01), Bouis
patent: 5961080 (1999-10-01), Sinha
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Oen William
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