Patent Application
20180268085
The present invention relates to a turbulence-generating grid and a method of manufacturing a turbulence-generating grid.
Turbulence-generating barriers or meshes or grids or the like are often used in wind tunnels and the like to produce turbulent boundary layer flows that correspond to or mimic actual flow over a real structure. For example, in civil engineering experiments to test large scale structures, e.g. skyscrapers and other large buildings and long span bridges, etc., it is important to reproduce the corresponding turbulent boundary layer flows over a model of the proposed structure in a wind tunnel. Many such turbulence-generating barriers or meshes or grids or the like are known. Typically, however, the design and production of such generating barriers or meshes or grids or the like is effectively done on a trial-and-error basis, with a first prototype being built and used in an initial experiment, and the prototype then being modified manually to try to adjust the properties of the flow that is produced in order to meet design requirements. This is a slow and laborious process, which relies heavily on personal experience and expertise of the individuals designing the grid or the like to manually adjust the prototypes, who will often have to work by “feel” in modifying the design of the prototypes.
According to a first aspect of the present invention, there is provided a turbulence-generating grid for producing a turbulent flow, the grid having a top and a bottom and opposed sides and comprising a number N of layers, each layer being defined between respective pairs of horizontal bars and the sides of the grid, each layer being subdivided by a number cn of vertical bars so as to define a plurality of respective through holes between at least adjacent pairs of the vertical bars and the horizontal bars,
In an embodiment, the aspect ratios of each of the vertical bars across a layer are the same.
In an embodiment, each layer has a respective blockage ratio σn, and the blockage ratio σn for at least some of the layers are different from each other.
In an embodiment, the difference of the blockage ratio between two adjacent layers is the same for all pairs of adjacent layers of the grid.
In an embodiment, the dimensions and the spacings of the vertical bars of one of the layers are different from the dimensions and the spacings of the vertical bars of another of the layers.
In an embodiment, at least one of the dimensions and the spacings of the vertical bars of each of the layers differ from layer to layer.
In an embodiment, at least one of the dimensions and the spacings of the vertical bars of the layers changes monotonically from layer to layer.
According to a second aspect of the present invention, there is provided a method of manufacturing a turbulence-generating grid which in use produces a turbulent flow having a mean velocity profile and a turbulence intensity profile, the grid having a top and a bottom and opposed sides and comprising a number N of layers, wherein n is the layer number for the respective layers, each layer being defined between respective pairs of horizontal bars and the sides of the grid, each layer being subdivided by a number cn of vertical bars so as to define a plurality of respective through holes between at least adjacent pairs of the vertical bars and the horizontal bars, each layer having a respective blockage ratio σn, the method comprising:
Embodiments of this aspect of the present invention provide a method of manufacturing a turbulence-generating grid which enables desired flow characteristics to be achieved in a straightforward and systematic manner. In practice, little or no modification of the initially-produced turbulence-generating grid is required to obtain the desired flow characteristics.
In an embodiment, the aspect ratio of the vertical bars within at least one of the layers are the same for all vertical bars across said layer.
In an embodiment, the number cn of vertical bars and the dimensions and spacings of the vertical bars for each of the layers is calculated so as to maintain constant the aspect ratio of each of the vertical bars across a layer.
In an embodiment, the method comprises attaching blocks of varying thicknesses to the grid in order to maintain the constant aspect ratio of each of the vertical bars across a layer.
In an embodiment, for each of the layers, the width wn of the vertical bars of the layer is calculated according to:
wherein:
In an embodiment, the difference of the blockage ratio between two adjacent layers is the same for all pairs of adjacent layers of the grid.
In an embodiment, the dimensions and the spacings of the vertical bars of one of the layers are different from the dimensions and the spacings of the vertical bars of another of the layers.
In an embodiment, at least one of the dimensions and the spacings of the vertical bars of each of the layers differ from layer to layer.
In an embodiment, at least one of the dimensions and the spacings of the vertical bars of the layers changes monotonically from layer to layer.
There may be provided a turbulence-generating grid manufactured in accordance with a method as described above.
According to a third aspect of the invention there is provided a computer-implemented method of designing a turbulence-generating grid which in use produces a turbulent flow having a mean velocity profile and a turbulence intensity profile, the grid having a top and a bottom and opposed sides and comprising a number N of layers, wherein n is the layer number for the respective layers, each layer being defined between respective pairs of horizontal bars and the sides of the grid, each layer being subdivided by a number cn of vertical bars so as to define a plurality of respective through holes between at least adjacent pairs of the vertical bars and the horizontal bars, each layer having a respective blockage ratio σn, the method comprising:
Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.
In examples of embodiments of the present invention, a number of parameters are effectively input to the design process so as to produce a turbulence-generating grid which provides desired flow characteristics in use. The design process enables grids to be manufactured that generate shear flows with different desired gradients and enables flow quantities like turbulence level and length scale to be obtained as desired. In some examples, the blockage ratio of each layer of the grid is varied according to design requirements for the shear flow gradient, which may be linear or non-linear and which is controlled in some examples by the height of each layer. While maintaining the blockage ratio of each layer, the width and gap distance of the vertical bars can be varied as well, producing different turbulence and integral length scales in the flow. In order to have further control over the turbulence intensity, the thickness of the vertical bars can be easily changed to alter the drag coefficient. This third dimension of the grid provides further tuning capabilities of the flow. In general, it has been found that slight changes to the design parameters enable very different turbulent flows to be obtained in a wind tunnel, providing for enormous flexibility in the flow characteristics to be obtained in use.
The turbulence-generating grid may be made of for example acrylic or some other plastics, though other materials such as wood, metal, etc. may be used. It is convenient to manufacture the turbulence-generating grid by for example laser cutting or water jet cutting depending on the material, though other techniques may be used, including moulding for example.
The steps relating to the design of the grid described herein may be computer-implemented or computer-assisted. For example, software written in a computing language such as MATLAB may be used together with user-input parameters to facilitate the design process.
Table 1 below sets out a number of parameters relevant for present purposes.
The first parameters are the external dimensions H and W of the grid 10. In practice, these will often be the same as or close to the cross-sectional dimensions of the wind tunnel's test section (where by convention the flow is in the x-direction along the tunnel, y is along the height of the wind tunnel and z is across the width of the wind tunnel). In these particular examples which have been subject to testing, H=W=3 feet (approximately 90 cm), corresponding to the cross-sectional dimensions of the wind tunnel in which the testing took place.
The next parameter to consider is the number N of horizontal layers 12 of the grid 10. In the examples shown in
The height of each layer 12 (in the y direction) is hn. The height of each layer 12 is set so that the grid 10 has an overall desired height, which will typically be the same as or close to height of the wind tunnel's test section. In general, the heights of the layers 12 may all be the same, or may all be different, or some may be the same and one or more may be different. In the case of the specific examples of actual grids 10 which have been subject to experiment, the height of each layer 12 was the same, and was h=101.67 mm.
An important parameter in order to obtain a desired mean flow profile produced by a turbulence-generating grid 10 is the blockage ratios for each layer 12. (In general, the blockage ratio is the ratio of the frontal area of the blockage to the cross-sectional area of the wind tunnel in which the blockage is present. In this case, the blockage ratio of a layer 1 is the ratio of the total frontal area of the horizontal and vertical bars 14, 16 of the layer 12 to the total cross-sectional area of the layer 12.) There are therefore a number N of blockage ratios. Together they form a blockage ratio profile, that is, a profile of blockage ratios across the grid 10. The blockage ratio profiles for each of the grids 10 shown by way of example in
The next step is to determine the dimensions and spacings of the vertical bars 16 in each horizontal layer 12 in a way which achieves the blockage ratio of that layer 12 and at the same time allows the desired turbulence intensity and integral length scale profiles to be obtained. The turbulence intensity, also often referred to as turbulence level, is defined as I=u′/U where u′ is the root-mean-square of the turbulent velocity fluctuations at a particular location over a specified period of time and U is the mean velocity at the same location over the same time period.
While the blockage profile controls the mean velocity profile of the flow produced by the grid 10, the vertical bars 16 are intended principally to control and vary the wake interaction mechanisms in order to control the turbulence characteristics of the flow. Four parameters are introduced here, namely cn the number of the vertical bars 16 in a layer 12, gn the gap or spacing between two adjacent bars 16 in a layer 12, wn the transverse or lateral width of each individual bar 16, and an the aspect ratio of the bars 16, which is defined as the ratio of depth or chord dn over width wn. In one example, the number of the bars 16 is set for each layer to be cn=2n+5 for n=1 to N in the examples shown in
In an example, to calculate the width wn of the vertical bars 16, the following equations may be used:
The purpose behind the second equation for wn is to remove the effect of the thickness of the horizontal bars 14 on the wall of the wind tunnel at the top and bottom of the grid 10, so as not to trip the boundary layer on the wall of the wind tunnel and cause unwanted interactions with the turbulent flow from the grid 10. In the examples shown in
By way of illustrative example, the dimensions of the vertical bars 16 and the corresponding blockage ratio σnfor each layer 12 of the grids 10 shown in
(For completeness, it is mentioned that the values of blockage ratio σn shown in Table 2 are obtained using the calculated values of wn and gn which yields discrepancies between design values of the blockage ratio σn due to round off errors arising from keeping just two decimal digits of w and g for manufacturing purposes. However, the differences between the obtained values and the desired values for the blockage ratio σn are usually smaller than 0.01%.)
It may be noted that since the thicknesses dn of the vertical bars 16 in the streamwise direction are all equal to each other and to the thickness D of the grid 10 in the streamwise direction in the particular examples of grids 10 shown in
Blocks of different thicknesses may be attached to the lee side of the grid to achieve this uniform aspect ratio for all layers 12, and consequent a uniform drag coefficient. The blocks may be of foam or acrylic, for example. Since the width of the vertical bars 16 in each layer 12 may be different, the resulting thicknesses of the modified vertical bars 16 may vary from layer 12 to layer 12.
The corresponding drag coefficients Cd for the vertical bars 16 for these example grids 10 before and after the thickness modification are shown in
Before showing the measured vertical mean velocity profiles produced by the grids, we refer to previous works which used wire gauze to produce arbitrary flow profiles. Following the paper “The effect of wire gauze on small disturbances in a uniform stream” by Taylor, G. I., Batchelor, G. K., Dryden, H. L. & Schubauer, G. B., published Q. J. Mech. Appl. Math. 2 (1) in 1949, and the paper “Steady flow past non-uniform wire grids” by McCarthy, J. H., published J. Fluid Mech. 19 (04), in 1964, the entire content of each of which is incorporated herein by reference, the velocity field near the grid can be expressed as:
where u+0 denotes the downstream velocity, u−0 denotes the upstream velocity, the subscript n denotes the layer number, i.e. different height, αn is the refraction angle, and Kn is the resistance coefficient of the grid, which can be calculated from
where σn is the local blockage ratio of the grid, and r is an empirical constant chosen to be r=0.7 in this example. Finally, an empirical expression of the refraction angle caused by the presence of the grid is given by both Taylor and McCarthy as
αn=1.1(1+Kn)−1/2.
In
In the paper “Turbulence Without Richardson-Kolmogorov Cascade” by N. Mazellier and J. C. Vassilicos, published Physics of Fluids 22, 075101 (2010), and the paper “Particle Image Velocimetry Study of Fractal-Generated Turbulence” by R. Gomes-Fernandes, B. Ganapathisubramani and J. C. Vassilicos, published Journal of Fluid Mechanics, September 2012, pages 1 to 31, the entire content of each of which is incorporated herein by reference, there are reports and discussions of experimental results concerning wind tunnel turbulence generated by multiscale/fractal grids. Examples of such multiscale/fractal grids are disclosed in our WO2007/113335A2 and WO2009/124939A1, the entire contents of which are incorporated herein by reference.
u′/U
∞˜1/β(Cdwn/x*peak)−1/2
where α=8.3 is a constant for laminar incoming flow condition. Then it was shown that the scaled turbulence intensity (u′/U∞)β(Cdwn/x*peak)−1/2 collapsed over several experiments. Note however, this scaling was proposed for fractal square grids, and the flow was homogeneous and isotropic. In the current case, there is a mean shear rate and the local convection velocity at each layer is different (for grid 2 and grid 3). Therefore, we include the mean streamwise velocity in the new scaling equation as
u*=(u′/U∞)Un,pβ(Cdwn/x*peak)−1/2,
where Un,p is the local mean velocity calculated from the mean velocity equations. With this method of scaling u*, it is possible at least to prescribe the shape of the turbulence intensity profile by calculating x*peak from the geometries of the grid and the desired mean velocity profile Un,p. The results are shown in
The longitudinal integral length scale L as a function of the streamwise direction for all grids is presented in
The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1520479.5 | Nov 2015 | GB | national |
This application is a continuation under 35 U.S.C. § 120 of International Application No. PCT/EP2016/078209, filed Nov. 18, 2016, which claims priority to United Kingdom Application No. GB 1520479.5, filed Nov. 20, 2015 under 35 U.S.C. § 119(a). Each of the above-referenced patent applications is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2016/078209 | Nov 2016 | US |
| Child | 15983393 | US |
