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Systems, devices and methods are disclosed for controlling the flow of a fluid over the window of an optical instrument housing in a freestream flow field. For example, the flow upstream of the housing may be split to create a flow region over the window that is conducive to successful operation of

Systems, devices and methods are disclosed for controlling the flow of a fluid over the window of an optical instrument housing in a freestream flow field. For example, the flow upstream of the housing may be split to create a flow region over the window that is conducive to successful operation of the instrument. The flow region may be maintained for various rotations of the housing about yaw, pitch, and roll axes. The disclosed features in some embodiments induce flow regions with reduced spatial and temporal density gradients of the flow over the window.

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1. An apparatus for controlling fluid flow over a window of a moveable optical instrument housing, the housing comprising a perimeter and at least partially protruding from a vehicle into a freestream flow field, the apparatus comprising: an inner perimeter configured to complement the perimeter of

1. An apparatus for controlling fluid flow over a window of a moveable optical instrument housing, the housing comprising a perimeter and at least partially protruding from a vehicle into a freestream flow field, the apparatus comprising: an inner perimeter configured to complement the perimeter of the housing;an arcuate top surface comprising a curvature and configured to extend into an upstream portion of the freestream flow field; andan arcuate outer perimeter coupled to the arcuate top surface,wherein the apparatus is configured to couple with the housing and to split the freestream flow field into a first flow field that is at least partially above the apparatus and window and a second flow field that is at least partially under the apparatus and around the housing;wherein the first flow field comprises an aero-optical flow region that extends at least partially over the arcuate top surface and continues at least partially over the window, andwherein the aero-optical flow region is maintained within a range of angular orientations of the apparatus relative to the freestream flow. 2. The apparatus of claim 1, further comprising: an inner portion of the arcuate top surface configured to be substantially flush with an outer portion of the window when the apparatus is coupled with the housing. 3. The apparatus of claim 1, further comprising: an inner portion of the arcuate top surface configured to be substantially contiguous with an outer portion of the window when the apparatus is coupled with the housing. 4. The apparatus of claim 3, wherein a slope of the inner portion of the arcuate top surface is configured to substantially match a slope of the outer portion of the window, wherein the outer portion of the window is adjacent to the inner portion of the arcuate top surface. 5. The apparatus of claim 1, further comprising: an inner portion of the arcuate top surface,wherein the apparatus is configured such that a boundary layer in the first flow field over the inner portion comprises a velocity profile substantially similar to a downstream velocity profile of the boundary layer over an outer portion of the window, andwherein the outer portion of the window is adjacent to the inner portion of the arcuate top surface. 6. The apparatus of claim 1, wherein the inner perimeter is substantially impermeable to the freestream flow field, the first flow field and the second flow field. 7. The apparatus of claim 1, wherein the inner perimeter further comprises: a seal,wherein the seal is substantially impermeable to the freestream flow field, the first flow field and the second flow field. 8. The apparatus of claim 1, wherein the arcuate outer perimeter is rounded. 9. The apparatus of claim 1, wherein the arcuate outer perimeter comprises a radius. 10. The apparatus of claim 1, wherein the arcuate top surface and the outer perimeter form a substantially smooth surface. 11. The apparatus of claim 1, wherein the curvature curves toward the second flow field. 12. The apparatus of claim 1, further comprising: a bottom surface, wherein the bottom surface and the arcuate top surface define a thickness therein. 13. The apparatus of claim 12, wherein the thickness is substantially uniform. 14. The apparatus of claim 12, wherein the thickness is substantially nonuniform. 15. The apparatus of claim 1, wherein the inner perimeter, the arcuate top surface, the outer perimeter and the bottom surface form a plurality of cross sections of the apparatus. 16. The apparatus of claim 15, wherein at least one of the cross sections is configured to arcuately extend at least partially around the perimeter of the window, and wherein at least one of the cross sections is configured to extend at least partially toward the freestream flow field. 17. The apparatus of claim 15, wherein at least one of the cross sections is in a plane such that at least part of the freestream flow field in the plane is perpendicular to a line that is tangent to a point on an edge of the cross section. 18. The apparatus of claim 15, wherein at least one of the cross sections is configured to extend at least 180 degrees around the perimeter of the window. 19. The apparatus of claim 1, wherein the optical instrument housing is moveable in at least two dimensions. 20. The apparatus of claim 19, wherein the housing is rotatable about a pitch axis and a yaw axis. 21. The apparatus of claim 20, wherein the aero-optical flow region is maintained for rotations about the pitch and yaw axes of at least +/−135 degrees. 22. The apparatus of claim 1, wherein a Strehl ratio in the aero-optical flow region is at least 0.7 for electromagnetic radiation consisting of a wavelength of about 1 micrometer. 23. The apparatus of claim 1, wherein the optical instrument housing is substantially spherical and the window is substantially planar. 24. The apparatus of claim 23, wherein the optical instrument housing comprises a turret housing on the vehicle, wherein the vehicle is an aircraft. 25. The apparatus of claim 24, wherein the apparatus is coupled to the turret housing. 26. A method for controlling fluid flow over the window of an optical instrument housing on a vehicle in a freestream flow field, the method comprising: splitting an upstream portion of the freestream flow field into a first flow field comprising an aero-optical flow region and a second flow field using an apparatus comprising: an inner perimeter configured to complement a perimeter of the housing,an arcuate top surface comprising a curvature and configured to extend into the upstream portion of the freestream flow field, andan arcuate outer perimeter coupled to the arcuate top surface,wherein the apparatus is configured to couple with the housing;wherein the aero-optical flow region extends at least partially over the top surface and continues at least partially over the window, andwherein the second flow field is at least partially under the apparatus and around the housing,wherein the aero-optical flow region is maintained over a range of angular orientations of the apparatus relative to the freestream flow. 27. The method of claim 26, further comprising: coupling the apparatus to the housing. 28. The method of claim 27, further comprising: forming a substantially contiguous surface with an inner portion of the top surface and the perimeter of the housing. 29. The method of claim 27, further comprising: matching the slope of an outer portion of the housing with the inner portion of the top surface, wherein the outer portion of the housing is adjacent to the inner portion of the top surface. 30. A system for controlling fluid flow over a window of a moveable optical instrument housing that protrudes from a vehicle into a freestream flow field, the system comprising: a housing defining an axis; andan arcuate lip comprising a curved top surface and outwardly extending away from the axis,wherein the lip is configured to split the freestream flow field into a first flow field above the lip and window and a second flow field under the lip and around the housing,wherein the first flow field comprises an aero-optical flow region that extends at least partially over the top surface of the lip and continues at least partially over the window, andwherein the aero-optical flow region is maintained over a range of angular orientations of the axis relative to the freestream flow.