This paper reports the application of optical methods: shadow, schlierenand holographic interferometry in wind tunnel flow visualization. Someexamples obtained in the MTI wind tunnels are presented. Thecomparative advantages of holographic method in regard to shadow andschlieren method for quantitative flow field test are analyzed.
Rapid advances made during the past decades on problems associated with high speed flight have brought into focus the need for competent treatment of the fundamental aspects of the aerodynamics and the need for application of basic sciences in solving actual problems. The different physical methods and techniques are employed to measure density, pressure, velocity and temperature in gas dynamics. Flow visualization is an important tool in experimental fluid mechanics, which can provide the overall picture of an entire flow field [1-6]. Air flow around aerodynamical models is a very complex phenomenon. In optical sense, fluid flow field is a transparent object with complex distribution of light refraction index. Light beam, passing through such an environment, suffers changes in its direction and phase, so that the information in it is carried as displacements or phase modulations. Light refraction index n is the function of air density ρ in each point, ρ ( x, y,z). The density, on the other side, is the function of velocity V, pressure P and air temperature T [2,4]. The three principal optical methods for flow visualization are: shadow, schlieren and interferometry. There are systematic differences between these methods, since shadowgraph is sensitive to changes of the second derivative of density, schlieren to changes of density first derivative, and the interferometry is capable to measure absolute density changes . The examples of the flow visualization, obtained in MTI wind tunnels, with three mentioned methods are analyzed and their advantages are pointed out. In order to demonstrate and compare advantages of optical methods in complex flow field visualization, a series of experiments were performed in trisonic wind tunnel T-36, with M ∞ = 0.7 to 3.24 . Wind tunnel test section has windows with diameter of 300 mm (pair windows are made of glass BK7, schlieren quality andanother pair of klirit) which allows the usage of opticalmethods, for 2D and 3D models.Holographic interferometer with parallel beams is atthe same time a schlieren and a shadow device. Theschematic diagram of the experimental setup is shownin Fig. 1 . The ruby laser (Apollo model 22,E = 3 J , t = 3 ns , lc = 1 m ) (2) is used as a recordinglight source, while 6 mW He-Ne laser (3) is used forinterferometer setting and reconstruction of hologrames(9). The Hg lamp(13), small mirror (14), horizontalknife edge (15) and still camera (16) are the differentparts belonging to schlieren device with Z-shape. Thelarge concave mirrors ( D = 300 m , F = 2750 mm ) (6)are for both systems. The shadow effects are recordedwithout second large mirror (6). Insted of it, a still orvideo camera are placed. Laser and Hg lamp are bothused as light sources for shadow technique. Somemirrors, beam spliters and lenses are used for laserbeams directed, enlarged, colimated and focusing(4,5,7). The lasers and all other mechanical and opticalcomponents are fixed on the adjusting platform (1) withheight equal to height of wind tunnel axis (11).
Double exposition technique is used for holographicinterferograms. Holoplate is exposed two times: windoff (when there exists homogeneous flow fielddistribution) and wind on (when there is a complex flowfield for testing [1-14]. Stagnation pressure ( Po ),atmospheric pressure (Pa), and Mach number ( M ∞ ) are measured by the basic, primary measurement system(PMS) in wind tunnel at the moment of hologram orschlieren recording [4-8] .The shadowgraf is not a method suitable forquantitative density measurements, since suchevaluation would require of one to perform a doubleintegration of the light intensity distribution over thefield picture [1,5,6]. This is a convenient method to geta quick survey of the shock waves geometry, boundarylayers, expansion fans and turbulence. The shadowgrafin this article (Figure 2, 3a and 4a) gives onlyqualitative visualization.The quantitative evaluation of the schlieren image isbased on densitometry. The distribution of the defectionangles in the test field can be calculated fromdensitogram in the parts of the shlieren image which is not disturbed by light diffraction. But, the schlierenmethod produces a two dimensional image of a threedimensional flow field, and the computation of thedensity distribution ρ ( x, y, z ) from the recordedintensity pattern is a more or less insoluble problem[1,3,5]. For schlieren test the optical parts (mirrors,lenses, windows) of the system should be as nearlyperfect as possible. The interpretation of schliereneffects is closely related to knowledge of geometricalsystem characteristic. For those reasons, today,schlieren tests are used often for qualitative flowvisualization in wind tunnels, in the same sense as theshadow method, but with a much higher degree ofresolution and sensitivity [1,5,6]. Contemporaryschlieren devices have a lot of modifications andimprovements described in the literature [1,5].The pair of klirit windows for wind tunnel T-36 isspecially made with the side model holders. 2D models(wedge, cylinder and step) are fixed in windows. Table1. contains the flow parameters during the shadow andholographic recordings. Fig 2 is the combination ofshadowgraf (lower part) and holographic interferogram(upper part) of the flow around wedge model withM ∞ =0.627 . The stagnation point, boundery layer andthe expansion waves on the rear end edges are visible.Figure 3a is the shadowgraf and 3b holographicinterferogram of the flow around the step. The complex flow in two dimensional model ofrocket nozzle is tested by three optical methods.Pressure taps