Experimental Investigation On Supersonic Flow Over An Optical Dome With Backward Facing Step And Its Aero-optical Effects

During the high speed flight in the air,the optical dome of the imaging guidance vehicle will interacts with the freestream.The complex flow field will result in severe aero-optical effects on the imaging system such as bore-sight error,jitter,blur and intensity attenuation and reduce the guidance precision greatly.There are many kinds of optical dome with different optical windows such as flat window,concave window,film cooling window,et al.In consideration of the harsh aero-thermal environment over the optical window under the interaction with the high speed flow,in general,supersonic film cooling is an effective method to cool the window surface.Some supersonic flow with low temperature is downstreamly injected tangentially to the window surface from a backward facing step(BFS)on the forehead of the optical dome.Introdcing the BFS will cause the flow field more complex,hence,in order to improve the imaging system performance the investigation on the flow mechanism and aero-optical effects of the supersonic optical dome with BFS is necessary.Primarily based on the experimental techniques of the nano-tracer planar laser scattering(NPLS)and background oriented schlieren based wavefront sensing(BOS-WS),and assistantly using surface pressure measurement and schlieren,a series of experiments have been carried out on the flow mechaism and the aero-optical effects of supersonic flow over a three-dimensional optical dome model with a BFS with or without injection,a two-dimensional BFS model with or without injection,a two-dimensional BFS model without injection but with different step heights and control ways.At first,the wind tunnel and the flow parameters used in these experiments was introduced.NPLS setup was summeried and the flow following ability of the nano-tracer particles was evaluated.We collected some development information of the BOS technique in brief.The principle of the BOS-WS technique and the wavefront reconstruction were discussed clearly.For the three-dimensional optical dome scale model with supersonic injection,respectively,in the Mach 3.8,3.4,3.0 freestream,the fine flow structures measurement was carride out via NPLS.Comparing the impact of different Mach numbers to the instantaneous and time-averaged structures concluded that in lower Mach number flow,the mixed layer between injection and mainstream transits sooner,in the case of without injection the reattachment position is closer to the upstream and the reattached shock angle is greater.Based on BOS-WS technique,under the condition of Mach 3.4 and 3.0,the optical path difference(OPD)distribution of the 532 nm wavelength light passing through the the flow field over the optical window was obtained.Comparing the effects of Mach number and the injection structure on the OPD and optical jitter due to the flow structures varying within microseconds,it is found that: in the same state the OPDrms decreases as the Mach number increases;the OPDrms is smaller when the injection is working and as Mach number increases,the optical jitter effects caused by the flow field structure evolution increase.Using NPLS technique,the experiments have been carried out on the flow over a 2-D BFS model with supersonic injection with or without upstream roughness of 60 level abrasive paper in the freestream of Mach 3.4.The fine flow structures and the evolution characteristics of the turbulent structures were revealed out under different flow conditions.The characteristic time scales and the formation periods of the movement of the turbulent structures were found.Based on the NPLS images,the distribution ranges of the initial formation positions(IFPs)of the structures with different scales in the flow over BFS without injection were caculated out statistically and the distribution characteristics of the large scale structures in the flow field of supersonic BFS were revealed.With roughness effect,the flow structures downstream of the step grow quickly and the structures with different heights concentrate in a short spatial range.The fractal dimensions of the flow in streamwise were caculated via box-counter dimension method and the fractal characteristics were analyzed.The whole field fractal dimension of the 2-D BFS without injection is larger than the case with injection and the fractal dimension with roughness is larger than the case without roughness.By the x-direction the fractal dimension is increasing and without injection and without roughness the fractal dimension of the redeveloped boundary layer downstream of the reattachment is smaller than in the case with roughness.However,it is the same value they are tending to be in the far downstream of BFS where the turbulence is developed sufficiently.In the case with roughness and with injection,the flow transits to be turbulence in a short distance,hence the fractal dimension increases faster and the turning point of the fractal dimension distribution curve appears ealier than in the case without roughness.The turbulence intermittency factors were analyzed statistically.The turbulence interface heights and the turbulence broken levels at different positions were caculated via the distributions of the intermittency factors.Due to the non-uniform of the laser sheet,the flow images were calibrated to deduce the density-gray relations.Then,based on the technique of NPLS-WS,the OPD distributions in the flow region of x=15h-35 h at different moments.Without injection,the OPD in this region decreases as x increasing while with injection it is increasing downstream.With roughness,the OPD in general is lower than that without roughness and within the interval of 5μs the OPD appears moving downstream without obvious deformation in a sense.Changing the step height as 3mm、5mm、10mm and in the Mach 3.4 freestream,the flow structures and surface pressures of the 2-D BFS without control,with roughness of 36 level abrasive paper or with micro-vortex generator(MVG)disturbed were experimentally measured.Based on the amounts of NPLS images of the instantaneous flow structures with high spatio-temporal resolution,the instantaneous and the evolution characteristics of the fine structures were depicted and the time-averaged results were gotten.The fractal dimensions,the turbulence intermittency factors and the spatial-corelations were analyzed statistically.Based on BOS-WS technique,the optical offsets of the light transmit by z direction through the flow field of the supersonic BFS and the OPD contours were reconstructed.In the end,the similarity criterias on the aero-optics experiments study were summaried and investigated priliminarily based on the references collected.The related non-dimensional parameters were deduced via dimensional method and some aero-optical measurements were compared.