Research On 3-D Optical Measurements For Ice Shape Growth In Icing Wind Tunnel

Aircraft icing is always a severe threat to the safe flight.Ice accretions on different parts of an airplane all bring about negative impacts on the flight,especially those grown on the wing,the tail and the fuselage that can change the optimal aerodynamic shape of the airplane,leading to a severe deterioration of the flight performance.Ice grown on an aircraft is normally categorized into clear ice,rime ice and mixed ice.To measure ice shapes correctly in icing wind tunnels lays the foundation for the icing research.Because of translucency and textureless of the ice surface,current measuring techniques have to interrupt ice growth,impeding the continuous documentation of ice shape evolution.Therefore,developing a non-contact method to measure the 3-D ice shape is crucial for not only icing experiments and theories but also numerical simulations that predict how ice grows.The goal of this thesis is to explore non-contract methods that can continuously digitize ice shapes when ice accretes on a model in an icing wind tunnel.To avoid any interference in the icing process,no chemical dyestuff or painting is applied to the ice surface.First,taking advantage of that frost and snow almost diffusely reflect visible light,a visible 3-D laser scanning method is developed,and it is applied to measure temporal shape evolution of rime ice and the final shape of clear ice.Then,based on the fact that ice can be approximated as a Lambertian emitter within a particular infrared spectrum,two infrared 3-D laser scanning methods are developed,and they are applied to continuously measure shapes of all ice types.At last,inspired by plant growth measurements through optical flow methods,an exploratory research is performed to study whether optical flow methods can trace ice growth.The main work and conclusions of this thesis are described below: 1)A visible laser line scanning method that is capable of measuring 3-D ice shapes in icing wind tunnels is proposed.Corresponding codes are developed with following key functions: camera calibration using planar patterns,laser line calibration with two reference plates,laser line detection based on spatial temporal analysis,and 3-D point cloud reconstruction by triangulation.Proper post-processing software is used to denoise 3-D point clouds,align multiple scans,and reconstruct a water-tight surface.2)A digital frustum model that simulates both large and small ice features is designed,and then printed out as a plastic frustum by a 3-D printer.A clear ice frustum is made by a mold and casting method from the plastic frustum.Because the reflectivity of clear ice is extremely low across the whole electromagnetic spectrum,while the reflectivity of frost and snow is quite high in the visible light spectrum,frost or snow is proposed to replace opaque chemical dyestuff to serve as a special painting on the clear ice surface.The visible laser line scanning method is successively verified on the plastic frustum,the frost-covered clear ice frustum and a frost-covered clear ice with unknown shape.All measurements agree well with the design data or the photograph.3)A hand tracing method is developed to measure the 2-D ice profile at cross sections of the ice accretion.The visible laser line scanning method is applied to measure rime ice growth on a wing surface in an icing wind tunnel.A single scan is performed at four different times.All reconstructed 3-D ice surfaces fit well with the corresponding photographs.The 2-D ice profiles at two cross sections of the final scan fit well with the results measured by hand trancing.Also,the final shape of clear ice and mixed ice is measured by a single scan.The results agree well with hand tracing results and the photograph.4)An infrared laser point scanning method is proposed,which is based on the observation that ice and water absorbs nearly all the radiation of certain infrared spectrum and a majority of absorbed radiation is re-emitted almost independent on view angles.Corresponding codes are developed containing three key functions: infrared laser spot center detection,system calibration and 3-D point cloud reconstruction.Because the infrared radiation also heats up the object,a still and a moving infrared laser spot is respectively detected by fitting a Gaussian surface and calculating the center of gravity of the image intensity.The infrared laser point scanning method is verified on a transparent glass bowl and a transparent ice bowl.The two measurements agree well with the visible laser line scan on the painted glass bowl.5)An infrared laser line scanning method is adapted from the visible laser line scanning method.The experimental setup is kept almost the same as used for visible laser line scan except that an IR camera,an MIR laser and MIR optics are used.Minor modifications are also made on the codes.According to the characteristic of infrared imaging,only the incoming edge of the infrared laser line is analyzed by the spatial temporal method.Infrared laser line is calibrated by a slightly modified two-reference-plates method.The infrared camera calibration method remains unchanged,except that a chessboard pattern is made from a PCB board.The infrared laser line scanning method is successively validated on the transparent glass bowl,the ice bowl and the ice frustum.All results fit well with the references.6)A mold and casting method is developed to replicate the final ice shape,and the casting shape is measured by the visible laser line scanning method.The infrared laser line scanning method is applied to recover the 3-D shape development of the continuous growth of rime ice,clear ice and mixed ice in an icing wind tunnel.A single scan is performed at 6 different times.All reconstructed 3-D ice surfaces are in good agreement with the corresponding photographs.The final scan also fits well to the result acquired by the mold and casting method.7)Two optical flow methods are explored to trace the shape deformation or growth of an object in wind tunnels,aiming to build a connection between shapes at different times.The two optical flow methods are adapted from LK and HS optical flow methods respectively,with great improvements at textureless regions,motion boundaries and large motion areas.Both optical flow methods introduce techniques such as the eigenvalue analysis of structure tensors,pyramidal coarse-to-fine warping and median filtering.In addition,the LK optical flow method uses non-linear structure tensor diffusion,while the HS optical flow method uses a modified calculation model,multi-cue driven bilateral filtering and weighted median filtering.Accuracy,convergence and allowed maximum displacements of the two optical flow methods are studied on synthetic images.Then,the two optical flow methods are applied to estimate the static 3-D deformation of a birdlike airfoil through a stereoscopic camera setup in a wind tunnel.The results are consistent with those computed by the image correlation method and the numerical simulation.The applicability of optical flow methods to the ice growth measurement is analyzed.