Patent Description:
An object (e.g., an aircraft or a marine vessel) that moves through a fluid (e.g., air or water) experiences a drag force. An increase in the drag force experienced by the object increases the energy required for the object to move through the fluid. For example, an increase in the drag force experienced by an aircraft moving at an established speed may increase the power required by the aircraft to move through the air at the same established speed. Thus, drag force has a significant impact of aircraft fuel consumption and aircraft range.

<CIT> relates to a method and a device for producing riblets into an already painted and cured surface by means of laser interference structuring or DLIP - Direct Laser Interference Patterning.

According to a first aspect of the invention, there is provided a method reducing drag according to appended independent claim <NUM>. According to a second aspect of the invention, there is provided a physical object according to appended independent claim <NUM>. According to a third aspect of the invention, there is provided a method of manufacturing a physical object according to appended independent claim <NUM>. Preferable features of the invention are defined in the appended dependent claims.

According to an embodiment, a method for reducing drag comprises forming smooth surfaces on a plurality of first portions of a physical object. The method also includes forming periodic riblets on a plurality of second portions of the physical object, wherein each riblet of the periodic riblets is the same length. The smooth surface of each of the plurality of first portions of the physical object is the same length as the periodic riblets and adjacent first portions of smooth surfaces and second portions of riblets form an intermittent repeating riblet pattern along a predetermined length of the physical object and along a predetermined direction of flow. Each riblet of the periodic riblets of the plurality of second portions of the physical object is depressed below a plane of the smooth surface of the plurality of first portions of the physical object. The method also includes generating a flow over the periodic riblets of the plurality of second portions of the physical object and over the smooth surface of the plurality of first portions of the physical object, wherein a length of each riblet of the periodic riblets runs parallel to the direction of the flow.

According to another embodiment, a physical object includes a plurality of first portions comprising a smooth surface and a plurality of second portions comprising periodic riblets. Each riblet of the periodic riblets has a same length and the length of the periodic riblets is the same as a length of the smooth surface of each of the plurality of first portions of the physical object. Adjacent first portions of smooth surfaces and second portions of riblets form an intermittent repeating riblet pattern along a predetermined length of the physical object and along a predetermined direction of flow. Each riblet of the periodic riblets of each of the plurality of second portions of the physical object is depressed below a plane of the smooth surface of each of the one or more first portions of the physical object and runs parallel to the predetermined direction of flow.

According to yet to another embodiment, a method of manufacturing a physical object includes forming smooth surfaces on a plurality of first portions of the physical object and forming periodic riblets on a plurality of second portions of the physical object. Each riblet of the periodic riblets is the same length and the smooth surface of each of the plurality of first portions of the physical object is the same length as the periodic riblets. Adjacent first portions of smooth surfaces and second portions of riblets form an intermittent repeating riblet pattern along a predetermined length of the physical object and along a predetermined direction of flow and each riblet of the periodic riblets of the plurality of second portions of the physical object is depressed below a plane of the smooth surface of the plurality of first portions of the physical object and runs parallel to the predetermined direction of flow.

Technical advantages of this disclosure may include one or more of the following. The use of submerged ribbed surfaces on physical objects reduces overall drag (which includes pressure and viscous drag) experienced by the physical object as compared to physical objects having a smooth surface, which may significantly reduce fuel costs since less power is required to move the object through the fluid (e.g., gas or liquid). The drag reduction experienced by a physical object such as an aircraft that uses submerged ribbed surfaces may also increase the range (i.e., the maximum distance the aircraft can fly between takeoff and landing) of the physical object as compared to physical objects that have a smooth surface. In certain embodiments, the drag reduction may allow higher maximum speeds to be obtained for a fixed propulsion input. In some embodiments, submerged periodic riblets may reduce heat transfer on a hot or cold surface adjacent to a turbulent boundary layer, which may reduce the insulation required in particular applications. The use of submerged periodic riblets may delay or prevent the separation of the flow in a turbulent boundary layer from the surface, which may reduce aerodynamic drag, increase lift on a physical object (e.g., an aircraft wing), and/or improve the performance of propulsion systems.

Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages.

To assist in understanding the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:.

Embodiments of this disclosure describe physical objects having submerged periodic riblets that may be used to reduce total drag, which includes pressure drag and friction drag, over the surfaces of the physical objects. Riblets are very small (e.g., less than a hundredth of an inch (less than <NUM>) in depth) grooves or channels on a surface of a physical object (e.g., a vehicle). The riblets run parallel to the direction of flow. Submerged periodic riblets are regions with riblets that are submerged below a smooth surface of the physical object. The regions having submerged periodic riblets are followed by a section of a smooth surface. This intermittent pattern is repeated for the length of the surface of the physical object.

While conventional riblets that protrude above the surface of the physical object reduce drag by suppressing near wall turbulent structures, conventional riblets also increase the wetted area. The concept of the submerged periodic riblets disclosed herein relies on the fact that that the damping of turbulent structures persists beyond the end of the riblet section, which reduces drag over the riblet and smooth regions. Alternating the smooth and riblet regions reduces drag by reducing the wetted area.

Submerging the riblets reduces pressure drag in the transition regions between the smooth surface and the riblet surface as compared to protruding periodic or variable height riblets. As such, embodiments of this disclosure use submerged periodic riblets to reduce the pressure drag penalty of the periodic riblet concept relative to the protruding periodic riblets.

<FIG> show example apparatuses and methods associated with submerged periodic riblets. <FIG> shows an example physical object with submerged periodic riblets and <FIG> shows an example longitudinal section of the physical object of <FIG>. <FIG> shows an example cross section of a protruding riblet pattern and <FIG> shows an example cross section of a submerged riblet pattern. <FIG> shows an example pressure output pattern associated with a physical object having a protruding riblet pattern and <FIG> shows an example pressure output pattern associated with a physical object having a submerged riblet pattern. <FIG> shows an example bar chart that compares drag produced by a physical object having a protruding riblet pattern to a physical object having a submerged riblet pattern. <FIG> shows an example method for reducing drag on a surface using a submerged riblet pattern.

<FIG> illustrates an example physical object <NUM> having submerged periodic riblets <NUM>. Physical object <NUM> with submerged periodic riblets <NUM> may be used to reduce overall drag (e.g., aerodynamic or hydrodynamic drag) over a surface as compared to physical object <NUM> without submerged periodic riblets <NUM> or physical object <NUM> with protruding periodic riblets (e.g., protruding riblets <NUM> of <FIG>. ) One or more portions of physical object <NUM> may be made of steel, aluminum, copper, titanium, nickel, plastic, fiberglass, a combination thereof, or any other suitable material.

Physical object <NUM> is any object that is susceptible to drag (e.g., skin friction drag and pressure drag. ) For example, physical object <NUM> may be a component (e.g., a portion of an outer body) of an aircraft (e.g., an airplane, a helicopter, a blimp, a drone, etc.), a component of a marine vessel (e.g., a cargo ship, a passenger ship, a canoe, a raft, etc.), a component of a motorized vehicle (e.g., a truck, a car, a train, a scooter, etc.), a component of a non-motorized vehicle (e.g., a bicycle, a skateboard, etc.), a component of a spacecraft (e.g., a spaceship, a satellite, etc.), a wind turbine, a projectile (e.g., a missile), or any other physical object that is capable of experiencing drag. In certain embodiments, drag is generated by a force acting opposite to the relative motion of physical object <NUM> (e.g., a wing of an aircraft) moving with respect to a surrounding fluid (e.g., air). In some embodiments, drag is generated by the viscosity of gas. In certain embodiments, drag is generated due to the viscosity of a fluid (e.g., water) near the surface of physical object <NUM> (e.g., a section of a pipe or duct.

Physical object <NUM> of <FIG> includes a first portion <NUM>, a second portion <NUM>, and a third portion <NUM>. First portion <NUM> has a smooth surface <NUM>, second portion <NUM> has a ribbed surface <NUM>, and third portion <NUM> has a smooth surface <NUM>. In certain embodiments, smooth surface <NUM> of first portion <NUM> and/or smooth surface <NUM> of third portion <NUM> is flat. In some embodiments, smooth surface <NUM> of first portion <NUM> and/or smooth surface <NUM> of third portion <NUM> may have a curvature. In the illustrated embodiment of <FIG>, smooth surface <NUM> of first portion <NUM> of physical object <NUM> is along a same plane (e.g., plane <NUM> of <FIG>) as smooth surface <NUM> of third portion <NUM> of physical object <NUM>. Second portion <NUM> of physical object <NUM> includes submerged periodic riblets <NUM>. Submerged periodic riblets <NUM> of second portion <NUM> form ribbed surface <NUM>.

Submerged periodic riblets <NUM> of second portion <NUM> of physical object <NUM> span from first portion <NUM> of physical object <NUM> to third portion <NUM> of physical object <NUM>. Each submerged periodic riblet <NUM> of physical object <NUM> is depressed below a plane of smooth surface <NUM> of first portion <NUM> such that no portion of submerged periodic riblet <NUM> extends beyond the plane of smooth surface <NUM> of first portion <NUM> in a direction away from physical object <NUM>.

Each submerged periodic riblet <NUM> includes a peak <NUM>. Each peak <NUM> of each submerged periodic riblet <NUM> is a location (e.g., a point, a plane, a ridge, etc.) along an exterior surface of submerged periodic riblet <NUM> that is closest to the plane of smooth surface <NUM>. In the illustrated embodiment of <FIG>, peaks <NUM> of submerged periodic riblets <NUM> reach the plane of smooth surface <NUM> of physical object <NUM>. In some embodiments, one or more peaks <NUM> of one or more submerged periodic riblets <NUM> may form a pointed tip. In some embodiments, one or more peaks <NUM> of one or more submerged periodic riblets <NUM> may form a flat or rounded peak surface.

The intersections of adjacent submerged periodic riblets <NUM> create valleys <NUM>. Each valley <NUM> between adjacent submerged periodic riblets <NUM> is a location along an exterior surface of submerged periodic riblet <NUM> that is farthest away from the plane of smooth surface <NUM>. The valleys <NUM> of submerged periodic riblets <NUM> are located below the plane of smooth surface <NUM> of physical object <NUM>. In some embodiments, submerged periodic riblets <NUM> may be spaced apart such that adjacent submerged periodic riblets <NUM> do not intersect. For example, each valley <NUM> between each submerged periodic riblet <NUM> may be a flat or rounded valley surface. In certain embodiments, one or more valleys <NUM> of one or more submerged periodic riblets <NUM> may form a pointed tip.

Although physical object <NUM> of <FIG> illustrates a particular number of submerged periodic riblets <NUM>, peaks <NUM>, valleys <NUM>, first portions <NUM>, smooth surfaces <NUM>, second portions <NUM>, ribbed surfaces <NUM>, third portions <NUM>, and smooth surfaces <NUM>, this disclosure contemplates any suitable number of submerged periodic riblets <NUM>, peaks <NUM>, valleys <NUM>, first portions <NUM>, smooth surfaces <NUM>, second portions <NUM>, ribbed surfaces <NUM>, third portions <NUM>, and smooth surfaces <NUM>. For example, physical object <NUM> of <FIG> may include a fourth portion with a submerged ribbed surface adjacent to third portion <NUM> having smooth surface <NUM>.

<FIG> illustrates an example longitudinal section <NUM> of physical object <NUM> of <FIG>. Longitudinal section <NUM> of <FIG> is cut through surface <NUM> of first portion <NUM>, valley <NUM> of second portion <NUM>, and smooth surface <NUM> of third portion <NUM>. First portion <NUM> of physical object <NUM> has a length L1, second portion <NUM> of physical object <NUM> has a length L2, and third portion <NUM> of physical object <NUM> has a length L3. In the illustrated embodiment of <FIG>, each submerged periodic riblet <NUM> of second portion <NUM> has approximately (within ten percent) a same length L2.

In the illustrated embodiment of <FIG>, length L1 of first portion <NUM> is approximately the same as length L2 of second portion <NUM> and length L3 of third portion <NUM> is approximately the same as length L2 of second portion <NUM>. Length L1, length L2, and length L3 in the illustrated embodiment of <FIG> are approximately the same length.

Length L2 of submerged periodic riblets <NUM> of physical object <NUM> runs parallel to a flow direction <NUM>. For example, physical object <NUM> may be a wing of an aircraft, and length L2 of submerged periodic riblets <NUM> of physical object <NUM> runs parallel to flow direction <NUM> generated by the aircraft when the aircraft is in flight. In the illustrated embodiment of <FIG>, peak <NUM> of submerged periodic riblet <NUM> is level with plane <NUM> of smooth surface <NUM> of physical object <NUM>.

First portion <NUM> of physical object <NUM> includes a transition surface <NUM>. An angle <NUM> between transition surface <NUM> of first portion <NUM> of physical object <NUM> and smooth surface <NUM> of first portion <NUM> of physical object <NUM> is within a range of <NUM> degrees to <NUM> degrees (e.g., <NUM> degrees. ) Each submerged periodic riblet <NUM> terminates, in a first direction, at transition surface <NUM> of first portion <NUM> of physical object <NUM>. Third portion <NUM> of physical object <NUM> includes a transition surface <NUM>. An angle <NUM> between transition surface <NUM> of third portion <NUM> of physical object <NUM> and smooth surface <NUM> of third portion <NUM> of physical object <NUM> is within a range of <NUM> degrees to <NUM> degrees (e.g., <NUM> degrees. ) Each submerged periodic riblet <NUM> terminates, in a second direction opposite the first direction, at transition surface <NUM> of third portion <NUM> of physical object <NUM>.

According to the invention, the intermittent pattern created by smooth surface <NUM> of first portion <NUM>, submerged periodic riblets <NUM> of second portion <NUM>, and smooth surface <NUM> of third portion <NUM> repeats along a predetermined length. For example, this intermittent pattern may repeat along the width of an airplane wing. Length L1, length L2, and length L3 are measured from the center of transition surfaces between each portion of physical object <NUM>. For example, as illustrated in <FIG>, length L2 is measured from the center of transition surface <NUM> to the center of transition surface <NUM> along plane <NUM> of smooth surface <NUM> of physical object <NUM>.

<FIG> illustrates an example cross section <NUM> of a protruding riblet pattern <NUM> not according to the invention. Protruding riblet pattern <NUM> is a pattern of protruding riblets <NUM> that protrude above a plane of an adjacent surface. For example, referring to the illustrated embodiment of <FIG>, protruding riblets <NUM> would be located above the plane of smooth surface <NUM> of first portion <NUM> of physical object <NUM> such that each valley <NUM> between adjacent protruding riblets <NUM> may be located along the plane of smooth surface <NUM> of first portion <NUM> of physical object <NUM>. A physical object (e.g., physical object <NUM> of <FIG>) having a ribbed surface of protruding riblets <NUM> experiences less friction drag when subjected to dynamic (e.g., aerodynamic or hydrodynamic) flow than a comparable physical object having a smooth surface. However, due to the geometry of protruding riblet pattern <NUM>, a physical object having a ribbed surface of protruding riblets <NUM> experiences higher pressure drag when subjected to dynamic flow than a comparable physical object having a smooth surface.

In <FIG>, protruding riblet pattern <NUM> protrudes above baseline <NUM>. Baseline <NUM> is equivalent to a plane of an adjacent surface (e.g., smooth surface <NUM> of <FIG>). Protruding riblet pattern <NUM> of <FIG> is a sawtooth pattern. Each protruding riblet <NUM> of protruding riblet pattern <NUM> has a peak <NUM>. Peak <NUM> of each protruding riblet <NUM> has a height relative to baseline <NUM> of less than <NUM> inches (<NUM>). In certain embodiments, the height of each peak <NUM> of each protruding riblet <NUM> may be within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)). Each peak <NUM> of each protruding riblet <NUM> forms an angle <NUM>. Angle <NUM> may range from <NUM> degrees to <NUM> degrees. In the illustrated embodiment of <FIG>, angle <NUM> is <NUM> degrees. In certain embodiments, each protruding riblet <NUM> may be a two-dimensional (2D), thin plate riblet that is perpendicular to and located above baseline <NUM> of cross section <NUM>. The 2D, thin plate riblets may create a series of channels with thin blades defining the channel walls.

Adjacent protruding riblets <NUM> of protruding riblet pattern <NUM> form valleys <NUM>. Each valley <NUM> of each protruding riblet <NUM> is located at baseline <NUM>. Each valley <NUM> forms an angle <NUM>. Angle <NUM> may range from <NUM> degrees to <NUM> degrees. In the illustrated embodiment of <FIG>, angle <NUM> is <NUM> degrees. One or more valleys <NUM> may be located above baseline <NUM>. For example, each valley <NUM> of each protruding riblet <NUM> may be located <NUM> inches (<NUM>) above baseline <NUM>.

Each protruding riblet <NUM> of cross section <NUM> may be approximately equal in size, shape, and/or orientation relative to baseline <NUM>. In <FIG>, each protruding riblet <NUM> of protruding riblet pattern <NUM> is in the shape of a triangle having two sides <NUM> and a base <NUM>. Each side <NUM> of each protruding riblet <NUM> has a length less than <NUM> inches (<NUM>). The length of each side <NUM> of each protruding riblet <NUM> of protruding riblet pattern <NUM> may be within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)). Each base <NUM> of each protruding riblet <NUM> has a length less than <NUM> inches (<NUM>). The length of each base <NUM> of each protruding riblet <NUM> of protruding riblet pattern <NUM> may be within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)).

Although cross section <NUM> of <FIG> illustrates a particular number of protruding riblets <NUM>, peaks <NUM>, and valleys <NUM>, this disclosure contemplates any suitable number of protruding riblets <NUM>, peaks <NUM>, and valleys <NUM>. For example, protruding riblet pattern <NUM> of <FIG> may include more or less than seven protruding riblets <NUM>. Although cross section <NUM> of <FIG> illustrates a particular arrangement of protruding riblets <NUM>, peaks <NUM>, and valleys <NUM>, this disclosure contemplates any suitable arrangement of protruding riblets <NUM>, peaks <NUM>, and valleys <NUM>. For example, two or more protruding riblets <NUM> of <FIG> may have different sizes, shapes, and/or orientations relative to baseline <NUM>. As another example, two or more peaks <NUM> of two or more protruding riblets <NUM> may have different heights above baseline <NUM>. As still another example, one or more peaks <NUM> and/or valleys <NUM> of one or more protruding riblets <NUM> may be rounded or flat. As yet another example, the length of sides <NUM> and base <NUM> may be the same to form equilateral triangles.

As illustrated in cross section <NUM> of <FIG>, the geometry of a physical object that uses protruding riblet pattern <NUM> increases the wetted area as compared to a smooth surface. As such, while a physical object with a protruding riblet pattern <NUM> experiences less friction drag when subjected to dynamic flow than a comparable physical object having a smooth surface, the pressure drag increases due to the increased projected area in the flow direction.

<FIG> illustrates an example cross section <NUM> of a submerged riblet pattern <NUM> according to the invention. Submerged riblet pattern <NUM> is a pattern of repeating submerged riblets that are located below a plane (e.g., plane <NUM> of <FIG>) of an adjacent surface. For example, submerged periodic riblets <NUM> may be equivalent to submerged periodic riblets <NUM> of <FIG>, which are located below the plane of smooth surface <NUM> of first portion <NUM> of physical object <NUM>. A physical object with a submerged ribbed surface that uses submerged riblet pattern <NUM> experiences less friction drag when subjected to dynamic flow than a comparable physical object having a smooth surface. While a physical object with a submerged ribbed surface that uses submerged riblet pattern <NUM> experiences higher pressure drag when subjected to dynamic flow than a comparable physical object having a smooth surface, the pressure drag created by submerged riblet pattern <NUM> is significantly less than the pressure drag created by protruding riblet pattern <NUM> of <FIG>.

In the illustrated embodiment of <FIG>, submerged riblet pattern <NUM> is recessed below baseline <NUM>. Baseline <NUM> is equivalent to a plane of an adjacent surface (e.g., smooth surface <NUM> of <FIG>). Submerged riblet pattern <NUM> of <FIG> is a sawtooth pattern. Each submerged riblet <NUM> of submerged riblet pattern <NUM> has a peak <NUM>. Each peak <NUM> of each submerged riblet <NUM> is located at baseline <NUM>. Each peak <NUM> forms an angle <NUM>. Angle <NUM> may range from <NUM> degrees to <NUM> degrees. In the illustrated embodiment of <FIG>, angle <NUM> is <NUM> degrees.

Adjacent submerged riblets <NUM> of submerged riblet pattern <NUM> form valleys <NUM>. Each valley <NUM> of each submerged riblet <NUM> has a depth relative to baseline <NUM> of less than <NUM> inches (<NUM>). In certain embodiments, the depth of each valley <NUM> of each submerged riblet <NUM> may be within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)). Each valley <NUM> of each submerged riblet <NUM> forms by an angle <NUM>. Angle <NUM> may range from <NUM> degrees to <NUM> degrees. In the illustrated embodiment of <FIG>, angle <NUM> is <NUM> degrees. In certain embodiments, each submerged riblet <NUM> may be a two-dimensional (2D), thin plate riblet that is perpendicular to and located below baseline <NUM> of cross section <NUM>. The 2D, thin plate riblets may create a series of channels with thin blades defining the channel walls.

Each submerged riblet <NUM> of cross section <NUM> may be equal in size, shape, and/or orientation. In the illustrated embodiment of <FIG>, each submerged riblet <NUM> of submerged riblet pattern <NUM> is in the shape of a triangle having two sides <NUM> and a base <NUM>. Each side <NUM> of each submerged riblet <NUM> has a length less than <NUM> inches (<NUM>). In certain embodiments, the length of each side <NUM> of each submerged riblet <NUM> of submerged riblet pattern <NUM> is within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)). Each base <NUM> of each submerged riblet <NUM> has a length less than <NUM> inches (<NUM>). In certain embodiments, the length of each base <NUM> of each submerged riblet <NUM> of submerged riblet pattern <NUM> is within a range of <NUM> inches (<NUM>) to <NUM> inches (<NUM>) (e.g., <NUM> inches (<NUM>)).

The sizes of submerged riblets <NUM> depends on the application of submerged riblet pattern <NUM>. For example, the sizes of each submerged riblets <NUM> may depend on the speed of fluid, the viscosity and/or density of the fluid, the scale of the object (e.g., physical object <NUM> of <FIG>), etc. In certain applications, submerged riblets <NUM> are less than a hundredth of an inch (<NUM>) in depth. For a highly viscous fluid (e.g., oil), submerged riblets <NUM> may be greater than a hundredth of an inch (<NUM>) in depth. In certain embodiments, submerged riblets <NUM> may be sized using turbulent wall scaling. For example, submerged riblets <NUM> may be sized according to the following formula: non-dimensional scaling h+ = (height)*sqrt((density)*(wall shear stress))/(viscosity), where h+ may be set to a value between <NUM> and <NUM>. As another example, submerged riblets <NUM> may be sized according to the following formula: nondimensional spanwise spacing s+ = (spanwise spacing)*sqrt((density)*(wall shear stress))/(viscosity), where s+ may be set to a value between <NUM> and <NUM>.

Although cross section <NUM> of <FIG> illustrates a particular number of submerged riblets <NUM>, peaks <NUM>, and valleys <NUM>, this disclosure contemplates any suitable number of submerged riblets <NUM>, peaks <NUM>, and valleys <NUM>. For example, submerged riblet pattern <NUM> of <FIG> may include more or less than seven submerged riblets <NUM>. Although cross section <NUM> of <FIG> illustrates a particular arrangement of submerged riblets <NUM>, peaks <NUM>, and valleys <NUM>, this disclosure contemplates any suitable arrangement of submerged riblets <NUM>, peaks <NUM>, and valleys <NUM>. For example, two or more submerged riblets <NUM> of <FIG> may have different sizes, shapes, and/or orientations. As another example, two or more valleys <NUM> between adjacent submerged riblets <NUM> may have different depths below baseline <NUM>. As still another example, two or more peaks <NUM> of two or more submerged riblets <NUM> may be located below baseline <NUM>. As yet another example, the length of sides <NUM> and base <NUM> of one or more submerged riblets <NUM> may be the same to form equilateral triangles.

<FIG> illustrates an example pressure pattern <NUM> associated with a protruding riblet pattern <NUM> (e.g., protruding riblet pattern <NUM> of <FIG>). Pressure pattern <NUM> was created using a simulation of a small scale structure <NUM> representative of a physical object (e.g., physical object <NUM> of <FIG>). The simulation was performed in a low Reynolds number channel with limited spanwise and streamwise extent. The effects of the pressure gradients illustrated in <FIG> were assessed from a highly resolved computational large eddy simulation of the riblet configuration in a channel flow. The simulation mimics the flow of a fluid (e.g., a liquid or gas) on a surface having a protruding riblet pattern. The flow direction <NUM> is parallel to the protruding riblets of protruding riblet pattern <NUM>. The output of the simulation is displayed in <FIG> as pressure pattern <NUM>.

Structure <NUM> of pressure pattern <NUM> includes smooth surfaces <NUM> similar to smooth surfaces <NUM> and <NUM> of <FIG>. Structure <NUM> of pressure pattern <NUM> includes protruding ribbed surfaces <NUM> that protrude above the plane of smooth surfaces <NUM>. Protruding ribbed surfaces <NUM> form protruding riblet pattern <NUM>. In <FIG>, protruding riblet pattern <NUM> is a sawtooth pattern similar to protruding riblet pattern <NUM> of <FIG>.

Pressure pattern <NUM> of <FIG> shows a distribution of time averaged pressure coefficient (Cp) as generated by the simulation. Cp is a non-dimensional parameter defined as the ratio of a difference between a local pressure and a free stream pressure and a free stream dynamic pressure. A Cp value of zero indicates that the pressure at a particular point is the same as the free stream pressure, a Cp value of one indicates a stagnation point, and a CP value less than zero indicates that the local velocity is greater than the free stream velocity. In <FIG>, Cp represents a time average pressure taken over a predetermined amount of time. Cp is represented as a grayscale in <FIG>. The lowest Cp value (i.e., -<NUM>) is the darkest shade in the grayscale and the highest Cp value (i.e., <NUM>) is the lightest shade in the grayscale. As such, the grayscale lightens in shade as the Cp value increases.

As indicated by the different shades of gray in pressure output pattern <NUM> of <FIG>, protruding riblet pattern <NUM> produces a large variant of Cp values ranging from - <NUM> to <NUM>. The highest Cp values are generated in forward facing transition regions <NUM> between smooth surfaces <NUM> and protruding riblet surfaces <NUM> as the flow travels in flow direction <NUM> from smooth surfaces <NUM> to protruding riblet surfaces <NUM>. The lowest Cp values are generated in aft facing transition regions <NUM> between smooth surfaces <NUM> and protruding riblet surfaces <NUM> as the flow travels in flow direction <NUM> from protruding ribbed surfaces <NUM> to smooth surfaces <NUM>. The submerged riblet pattern mitigates these pressure differentials by producing a more constant pressure over the surfaces of the structure, as described below in <FIG>.

<FIG> illustrates an example pressure pattern <NUM> associated with a submerged riblet pattern <NUM> (e.g., submerged riblet pattern <NUM> of <FIG>). Pressure pattern <NUM> was created using the same simulation technique of <FIG>. Flow direction <NUM> is parallel to the submerged riblets of submerged riblet pattern <NUM>. The output of the simulation is displayed in <FIG> as pressure pattern <NUM>.

Structure <NUM> of pressure pattern <NUM> includes smooth surfaces <NUM> similar to smooth surfaces <NUM> and <NUM> of <FIG>. Structure <NUM> of pressure pattern <NUM> includes submerged ribbed surfaces <NUM> that are recessed below the plane of smooth surfaces <NUM>. Submerged ribbed surfaces <NUM> form submerged riblet pattern <NUM>. In the illustrated embodiment of <FIG>, submerged riblet pattern <NUM> is a sawtooth pattern similar to submerged riblet pattern <NUM> of <FIG>.

As indicated by the different shades of gray in pressure output pattern <NUM> of <FIG>, submerged riblet pattern <NUM> produces a small variant of Cp values ranging from - <NUM> to <NUM>. Positive Cp values of approximately <NUM> are generated along submerged ribbed surfaces <NUM> and negative Cp values of approximately -<NUM> are generated along smooth surfaces <NUM>. As such, submerged riblet pattern shown <NUM> in <FIG> mitigates the pressure differentials shown in pressure pattern <NUM> of <FIG> by producing more constant pressures over the surfaces of structure <NUM>.

<FIG> illustrates an example bar chart <NUM> that compares drag produced by a physical object having a protruding riblet pattern to a physical object having a submerged riblet pattern. The pressure drag and viscous drag increments are calculated from a time average of the forces in a computational large eddy simulation of the flow in a channel with constant cross section in the spanwise direction. Periodic boundary conditions are applied in the spanwise direction to approximate a 2D channel flow of infinite span. The simulation includes a smooth surface on one wall of the channel and a riblet wall on the opposing channel wall. The difference in the drag components between the smooth wall and the riblet wall provides the increments shown in <FIG>.

Bar chart <NUM> includes drag differences for a protruding riblet pattern <NUM> and a submerged riblet pattern <NUM>. Protruding riblet pattern <NUM> is equivalent to protruding riblet pattern <NUM> of <FIG>. Submerged riblet pattern <NUM> is equivalent to submerged riblet pattern <NUM> of <FIG>. The drag for protruding riblet pattern <NUM> and submerged riblet pattern <NUM> is measured as a percentage difference from the drag generated by a smooth surface without riblets. Pressure drag differences, friction (e.g., viscous) drag differences, and total drag differences are provided in bar chart <NUM>.

Protruding riblet pattern <NUM>, as illustrated in bar chart <NUM> of <FIG>, generates a percent pressure drag difference <NUM> of positive seven percent, which indicates that protruding riblet pattern <NUM> generates a pressure drag that is seven percent greater than the negligible pressure drag generated by a smooth surface. Protruding riblet pattern <NUM> generates a percent viscous drag difference <NUM> of negative five percent, which indicates that protruding riblet pattern <NUM> generates a viscous drag that is five percent lower than the viscous drag generated by a smooth surface. The total drag difference, which is calculated by adding pressure drag difference <NUM> and viscous drag difference <NUM> of protruding riblet pattern <NUM>, is positive two percent, which indicates that protruding riblet pattern <NUM> generates a total drag that is two percent higher than the total drag generated by a smooth surface. Thus, while protruding riblet pattern <NUM> is effective at reducing viscous drag as compared to a smooth surface without riblets, protruding riblet pattern <NUM> increases the overall drag when taking into consideration pressure drag.

Submerged riblet pattern <NUM>, as illustrated in bar chart <NUM> of <FIG>, generates a percent pressure drag difference <NUM> of positive two percent, which indicates that submerged riblet pattern <NUM> generates a pressure drag that is two percent greater than the pressure drag generated by a smooth surface. Submerged riblet pattern <NUM> generates a percent viscous drag difference <NUM> of negative four percent, which indicates that submerged riblet pattern <NUM> generates a viscous drag that is four percent lower than the viscous drag generated by a smooth surface. The total drag difference, which is calculated by adding pressure drag difference <NUM> and viscous drag difference <NUM> of submerged riblet pattern <NUM>, is negative two percent, which indicates that submerged riblet pattern <NUM> generates a total drag that is two percent lower than the total drag generated by a smooth surface. Thus, submerged riblet pattern <NUM> is effective at reducing viscous drag as compared to a smooth surface without riblets and is also effective at reducing the overall drag when taking into consideration both viscous drag and pressure drag.

<FIG> illustrates an example method <NUM> for reducing drag on a surface having a submerged riblet pattern, in accordance with an example embodiment. Method <NUM> starts at step <NUM>. At step <NUM>, a smooth surface (e.g., smooth surface <NUM> of <FIG>) is formed on a first portion (e.g., first portion <NUM> of <FIG>) of a physical object (e.g., physical object <NUM> of <FIG>). The physical object may be a component (e.g., a portion of an outer body) of an aircraft (e.g., an airplane, a helicopter, a blimp, a drone, etc.), a component of a of a marine vessel (e.g., a cargo ship, a passenger ship, a canoe, a raft, etc.), a component of a motorized vehicle (e.g., a truck, a car, a a train, a scooter, etc.), a component of a non-motorized vehicle (e.g., a bicycle, a skateboard, etc.), a component of a spacecraft (e.g., a spaceship, a satellite, etc.), a wind turbine, a projectile (e.g., a missile), or any other physical object that is capable of experiencing drag. Method <NUM> then moves from step <NUM> to step <NUM>.

At step <NUM> of method <NUM>, periodic riblets (e.g., submerged periodic riblets of <FIG>) are formed on a second portion (e.g., second portion <NUM> of <FIG>) of the physical object. The second portion of the physical object is adjacent to the first portion of the physical object. Each riblet of the periodic riblets has a same length. The smooth surface of the first portion of the physical object may have a same length as the periodic riblets, as measured in the direction of the length of the periodic riblets.

Method <NUM> then moves from step <NUM> to step <NUM>, where each riblet of the periodic riblets of the second portion of the physical object is depressed below a plane of the smooth surface of the first portion of the physical object. The peak of each riblet of the periodic riblets may be at a same level as the plane of the smooth surface of the first portion of the physical object. In certain embodiments, a constant distance is formed between each peak (e.g., peaks <NUM> of <FIG>) of each riblet of the periodic riblets such that that the distance between each peak is the same. In certain embodiments, a constant distance is formed between each valley (e.g., valleys <NUM> of <FIG>) of each riblet of the periodic riblets such that that the distance between each valley is the same. Method <NUM> then moves from step <NUM> to step <NUM>.

At step <NUM>, a flow is generated over the periodic riblets of the second portion of the physical object and over the smooth surface of the first portion of the physical object. For example, the flow may be generated by an airplane moving through the air at a predetermined speed. The flow direction (e.g., flow diction <NUM> of <FIG>) runs parallel to the length of each riblet of the periodic riblets. Method <NUM> then moves from step <NUM> to step <NUM>, where method <NUM> determines whether the flow is a gas or a liquid.

If the flow is a gas (e.g., air), method <NUM> moves from step <NUM> to step <NUM>, where an aerodynamic drag is generated over the submerged riblet pattern that is less than the total aerodynamic drag (i.e., pressure drag and viscous drag) produced by generating a flow over a smooth surface without riblets. As indicated in <FIG> above, the aerodynamic drag generated over the submerged riblet pattern that is less than the total aerodynamic drag produced by generating a flow over a protruding riblet pattern (e.g., protruding riblet pattern <NUM> of <FIG>). As such, the submerged riblet pattern reduces drag over aerodynamic surfaces, which may reduce fuel costs and increase range in vehicles (e.g., aircraft) utilizing the submerged riblet pattern.

If the flow is a liquid (e.g., water), method <NUM> advances from step <NUM> to step <NUM>, where a hydrodynamic drag is generated over the submerged riblet pattern that is less than the total hydrodynamic drag (i.e., pressure drag and viscous drag) produced by generating a flow over a smooth surface without riblets. As indicated in <FIG> above, the hydrodynamic drag generated over the submerged riblet pattern that is less than the total hydrodynamic drag produced by generating a flow over a protruding riblet pattern (e.g., protruding riblet pattern <NUM> of <FIG>). As such, the submerged riblet pattern reduces drag over hydrodynamic surfaces, which may reduce fuel costs and increase range in vehicles (e.g., marine vessels) utilizing the submerged riblet pattern. Method <NUM> then moves from steps <NUM> and <NUM> to step <NUM>, where method <NUM> ends.

Method <NUM> may include forming each peak of each riblet of the periodic riblets at an angle between <NUM> degrees and <NUM> degrees (e.g., <NUM> degrees. ) As another example, method <NUM> may include forming each valley between adjacent riblets of the periodic riblets at an angle between <NUM> degrees and <NUM> degrees (e.g., <NUM> degrees. ) Method <NUM> repeats steps <NUM> through <NUM> to form an intermittent pattern along a predetermined length of a component (e.g., an aircraft wing.

Steps of method <NUM> depicted in <FIG> may be performed in parallel or in any suitable order. For example, step <NUM> directed to forming a smooth surface on a first portion of a physical object and step <NUM> directed to forming periodic riblets on a second portion of the physical object may be reversed. Any suitable component may perform any step of method <NUM>. For example, one or more machines (e.g., robotic machines) may be used to form one or more surfaces of the physical object.

Embodiments of this disclosure may be applied to any fluid flow application where the boundary layer is turbulent and skin friction is significant. For example, embodiments of this disclosure may be used to reduce internal flow drag in propulsion systems, reduce pipe flow drag, reduce drag in automotive systems, and the like.

Claim 1:
A method for reducing drag, comprising:
forming smooth surfaces (<NUM>, <NUM>) on a plurality of first portions (<NUM>, <NUM>) of a physical object (<NUM>);
forming periodic riblets (<NUM>) on a plurality of second portions (<NUM>) of the physical object (<NUM>), wherein:
each riblet of the periodic riblets (<NUM>) is the same length;
the smooth surface (<NUM>, <NUM>) of each of the plurality of first portions (<NUM>, <NUM>) of the physical object (<NUM>) is the same length as the periodic riblets (<NUM>);
adjacent first portions (<NUM>, <NUM>) of smooth surfaces (<NUM>, <NUM>) and second portions (<NUM>) of riblets (<NUM>) form an intermittent repeating riblet pattern along a predetermined length of the physical object (<NUM>) and along a predetermined direction of flow; and further wherein
each riblet of the periodic riblets (<NUM>) of the plurality of second portions (<NUM>) of the physical object (<NUM>) is depressed below a plane of the smooth surface (<NUM>, <NUM>) of the plurality of first portions (<NUM>, <NUM>) of the physical object (<NUM>);
generating a flow over the periodic riblets of the plurality of second portions (<NUM>) of the physical object (<NUM>) and over the smooth surface (<NUM>) of the plurality of first portions (<NUM>, <NUM>) of the physical object (<NUM>), wherein a length of each riblet of the periodic riblets (<NUM>) runs parallel to the direction of the flow.