SMOKE TECHNIQUES

SMOKE TECHNIQUES

Smoke consists of suspensions of small solid or liquid particles in a transparent gas and are usually observed by the scattering and reflection of light by these particles .The principal requirements of a smoke for wind tunnel work are

(a) That it shall not disturb the flow in the tunnel (e.g by the formation of deposits on the surface of a body exposed to the air stream) nor from deposits which lead to blockage of the pipe-work used for ducting the smoke into the tunnel

(b) That it shall be light so that the smoke filament is not influensed appreciably by gravity ,and of low inertia so that transient and unsteady ohenomena may be observed .

(c) That it shall be clearly visible ,non-poisonous, non-corrosive

(d) That it shall be easily produced and controllied and preferably inexpensive.

The most common means of producing smoke for fluid motion visualization are chemical method, the controlled combustion of organic matter, and the vaporization of liquids .Excellent results have been obtained with the compounds of titanium and stannic tetrachloride. These substances, which are liquid at normal temperatures, react with the moisture present in the air stream to form fumes consisting of the oxide of the metal together with hydrogen chloride .The smoke is of dense white appearance and is easily visible .The heavy flocculent nature of the smoke may cause it to sink under gravity, a disadvantage which particularly objectionable at low airspeeds. Moreover, the smoke tends to form deposits on the surface exposed to its action; these deposits must be frequently removed if flow disturbances are to be avoided.The smoke may be introduced into the air stream by a number of methods .For observing the flow down stream of a body, a few drops of liquid may be placed on the surface, or continuously supplied to the groove cut in the surface. An alternative method id to supply the liquid through a small aperture flush with the surface; if this is done, however ,it is usually advisable to mix the liquid with the equal volume of carbon tetrachloride to prevent early blockage of aperture. For observation on other parts of flow field, a filament of smoke may be introduced from a glass rod which has been previously dipped into the liquid.The presence of the hydrochloric acid in the fumes makes them corrosive and generally objectionable, and so smokes of this type are generally unsuitable for use in wind tunnel which is not constructed for this purpose. For example, the tunnel is constructed mainly of wood and special precautions are taken at the exit to neutralize the acidic nature of the smoke. Apart from special applications, therefore, smokes of this type are not often used in current practice

Smoke produced by the combustion of wood ; tobacco and other organic materials have been used for some time .It was found that the best results were obtained by the use of wood rotted to a white silky texture and which could be readily crumbled into small pieces before insertion into the combustion chamber .The smoke supply to the tunnel was controlled by the by-passing a major proportion of smoke to the atmosphere so that the generator could be operated at constant speed independent of the requirements of the experiment. The wood smoke produced by this method is non-toxic and non-corrosive and not unpleasant in use .Nevertheless, the time required for starting up the servicing the apparatus is a considerable disadvantage.Wood smokes contain a large number of small solid particles which tend to form deposits on the surface and so to disturb the flow. This is especially troublesome in boundary-layer observation.

Oil smoke generator is recommended for general application to wind tunnel work except in special cases in which a large and more robust generator may be preferable .The smoke is formed by the evaporation and atomization of mineral oil in an air stream ,and the resultant smoke consists largely of a suspension of small liquid particles. This to a large extent avoids difficulties associated with the formation of deposits on the surface of a model and also in the pipe work .More over the generator can be started up in about five minutes and can then be operated indefinitely .The quantity and rate of supply of the smoke are under close control and the whole generator is quite small and may either be made portable or mounded on a panel attached to the tunnel structure.A heavy mineral oil, highly refined and with a boiling point in excess of 350 degrees centigrade. Was originally used for the production of the smoke, but ordinary commercial kerosene is now employed as it avoids difficulties arising from the carbon deposits caused by the cracking of the oil

APPLICATION OF SMOKE TO OTHER FLOW VISUALISATION PROBLEMS

Apart from its application to boundary layer investigation, smoke may also be used to visualize the flow in many other problems. For instructional purposes in particular many interesting results can be obtained by using smoke to visualize filament lines in the main body of the flow .For this purpose the smoke may be introduced into the tunnel through a comb of tubes placed upstream of the model. The principal requirement of smoke tunnel is that the flow at the working section should be uniform and of low turbulence. An open circuit is u usually essential, or the tunnel would soon become filled with smoke. The air speed should not be so low that difficulties associated with the sinking of the smoke filaments under gravity are encountered. In this connection there is some advantage in a vertical flow arrangement .The smoke should generally be introduced in, or upstream of, the contraction so that disturbance may have an opportunity to decay. Adequate lighting should be arranged and should be suitably reinforced opposite the model to prevent the formation of shadows. A typical arrangement is sketched belowHOT WIRE TECHNIQUES

In these techniques a filament of air of different density from that of the main stream is introduced by inserting a heated wire in to the stream. The changes of refractive index associated with the density changes are then visualized by one of the methods described .The development of the hot wire technique is largely due to Town endThe wire is usually of platinum and about 0.002 in diameter and from 0.5 to 1 m in length. It is heated by current from accumulators at a voltage of about 14 V and controlled by a series rsistance. The wire should be heated to a dull red color: this requires a current of about 1 A at wind speeds of the order of 30ft/sec. A number of wires speed at about 0.5m in intervals across the air stream provide a convenient method for introducing a series of heated filaments. At very low speeds convection effects introduce confusion but at speeds above about 2ft/sec these are usually negligible . In conjunction with a stroboscope the method is particularly useful for the visualization of periodic and transient motions.SPARK TECHNIQUES

In these techniques the paths of particles are visualized by the heating of small volumes of the air stream by the discharge of a series of electric sparks. The thermal capacity of the electrodes between which the spark is discharged also usually produces a continuous filament of heated air in between the spark discharges, so that a stream filament is also visualized. Perhaps the most important advantage of the spark techniques is that the frequency of the spark discharge may be readily controlled so that instantaneous photographs of the resultant flow may be used directly to give quantitative information concerning the variation of the velocity in the field of flow .These techniques are also of value for instructional and demonstration purposesThe sparks may be generated either by an ignition coil or by an alternator. The latter method was found by town end to be preferable, with a 0.5 h.p alternator delivering 150 V at 2000 rpm. The delivery from alternator was then passed through a series resistance of about 100 ohms which controlled the output to a step-up transformer (100:1) used to produce the spark.A spark gap of about 3/8 in was found to be satisfactory .This was formed between 2 spheres of about 1/16 in diameter produced by holding the ends of pieces of steel piano wire of about 0.024 in diameter in the flame of a carbon arc for a few seconds .This procedure has the advantage that the oxidation of the electrodes is at once complete, and that further oxidation of the electrodes is at once complete, and that further oxidation (with the associated variation of nature of the discharge) does not occur when the spark is discharged. TUFTS

Tufts are valuable for the visualization of the flow close to a solid surface. The thickness and length of tuft are determined largely by the scale of the model and the Reynolds number .For usual atmosphere tunnels they are generally about in long and are made of fine silk frayed out at one end and attached smoothly to the surface of the model at the other. If the aerofoil is observed from the directional of the local velocity at the surface may be observed .Moreover .whilst in the laminar regions of the boundary layer the tufts remain steady ,they flutter visibly downstream of the transition position the vibration becoming violet may also be obtained by viewing the tufts from along the spanTufts have been used successfully in flight and in high speed tunnels .They can be generally be observed without special lighting arrangements and are easily photographed. For these reasons they are of great value whenever a raid qualitative examination of the surface flow is required .It is however desirable that whenever they used it should be ascertained that the disturbances introduced into the flow do not have serious aerodynamic effectsdepth tufts (tufts attached to the ends of wires protruding fro the surface) have been used for observing the flow at some distance from the surface there is evidence however that the presence of tufts of this type may lead to appreciable aerodynamic effects and it seems that as a general rule their use is not desirable.

An analogous technique which is valuable for visualizing the flow in ducts and wind tunnel circuits consists of supporting cotton streams from a wire stretched between two opposite walls. By this method the steadiness and direction of the flow may be examined. If the direction of the flow is to be measured at a number of points across the duct it is generally preferable to traverse a single streamer across and to observe it either directly or by projecting its shadow on a screen by passing a parallel beam of light through the duct .With the later arrangement changes the direction of the order of 1 degree may be detected.Tufts are the simplest and most often used. One may use any light and flexible thread (3/4)-oz baby wool or heavy cotton thread, for instance adjusting the length of the tuft according to the size of the model but not exceeding 2 inches in any event in order to avoid flag waving. Usually the tufts are attached to the model surface (cleaned with carbon tetra chloride or benzene) by scotch cellophane tape. When the tape is removed any traces should be washed off either of the above solvents.Possibly the most rapid method of installing tufts is to attach them about every 4 inches to the tape before applications as shown in fig. Whole strips can then be put on at once. For wing tests, streamers at the 15,30,45,60,and 75 percent chord points are adequate and form them the whole stall pattern may be progressively traced. A stall picture is illustrated in fig. This type of set up may not be photographed with the usual high shutter speeds because the movements of the tufts that result from unsteady flow would not be apparent. However at 1/50 sec the moving tufts show up blurred and the stall progress any is noted. Additional visual observations are usually in order to establish the advent of unsteady flow, intermittent stall and full stall.A more detailed stall study that that show in fig. usually drawn for a single angle of attack, would show flow direction by arrows, rough flow by wiggly arrows intermittent shall be circled crosses and full stall by crosses

A more detailed stall study than that shown in fig. usually drawn for a single angle of attack, would show flow direction by arrows, rough flow by wiggly arrows, intermittent stall by circled crosses, and full stall by crosses

A streamer may also be of use mounded at the end of a fish pole so that vortices and roughness may be traced about the model without the proximity of the investor disturbing the flow. if this necessitates the operators entering the tunnel ,goggles are needed although his appearance is a bit silly a particle of dust in his eye at 60 to 100 mhp is no laughing matter.Still another kind of tuft for flow visualization is the tufted wire grid developed by bird at the NACA. Its construction is illustrated in fig. Much of its use has been concerned with checking the theories for the roll-up of the vortices of various wing platforms.

Several chemicals, for example titanium tetra chloride and tin tetra chloride .will produce smoke when brought in contact with damp air, but both compounds are corrosive. Smoke bombs and candles may be obtained from armed services but these are hard to control .Most tunnel operators use the smoke from heated kerosene, rotten wood or smouldering paper. Reference 3 :22 warns of the necessity of providing ample ventilation when smoke which produces carbon monoxide is used .Since a concentration of 4.0 per cent is fatal after less than one hours exposure.The fales method consists of mounding a half model (split through the plane of symmetry) on a glass plate and lightly coating both plate and model with a mixture of lampblack and kerosene .The air flow spreads the mixture along the streamlines so that after the tunnel has been stopped the flow pattern remains. Good pictures can then be made of the flow pattern, and if the lampblack is spread sufficiently thin, only a minimum will be blown into the tunnel. Another good method is to spray the model with fluorescent oil before a run and then examine the flow pattern afterwards under ultra violet light ,perhaps using color film.

A good oil mixture may be made as follows:

a) 3 quarts of SAE 30 wt oilb) 1 quart of SAE 500 to 600 ET of gear oil

c) 4 Tablespoons dow corning mold release

d) 3 Tablespoons kerosene

e) 5 Tablespoons CH 185 per cent fluorescent green dye

THE PHOTO VISCOUS METHOD

In some liquids, viscous shear gives rise to optical effects of stress analysis. In such cases the flow may be observed by passing a beam of polarized light through the field. The method has no applications to routine wind-tunnel work, but may be useful in fundamental researches.METHODS DEPENDING ON CHANGES OF REFRACTIVE INDEX When the refractive index n of an optical medium is close to unity ,it can be related to the density with sufficient accuracy by the expression

(n 1)/ ( = constant Thus if the refractive index of the air can be measured throughout a filed of flow around a body it is possible to deduce the density and hence on certain assumptions to calculate the pressure, speed and mach number .Several methods have been suggested for the measurement of the refractive index (or its first or second derivative) in a wind tunnel by observation of the behavior of a beam of light directed through the working section.

Before describing these methods and seeing to what extent they may be used in practice, it is worth considering their inherent advantages when compared with other method of exploring flow. The alternatives usually involve the introduction of some form of mechanical exploring gear into the field and are therefore subject to the following disadvantages:-

(a) With the exception of the measurement of surface pressure, the exploring apparatus introduces disturbances which may lead to error of observation at high Mach number.(b) Apart from certain types of electrical instruments, the inertia of the measuring apparatus, such as that due to the lag in the pressure leads to the manometer, is usually large: in general therefore, conventional methods are suitable only for measurements in steady flow.

(c) The labour involved in the expansion of extensive fields by conventional methods is considerable .This is particularly serious at high mach number ,where for power economy the tunnel run be as short as possible and where the disturbance introduced by a rake or comb of exploring tubes is often intolerable.

(d) At high mach number ,observations with pitot and static tubes are subjected to uncertainties

Optical methods are essentially applicable to compressible flow only, i.e at high mach number, where the inherent advantages over conventional methods are greatest .It should be noted however that all real flows are compressible to a greater or less extent, and that quantitative results have been obtained with the interferometer, and quantitative results with the schlieren apparatus at air speeds below 200ft/sec.

The principal fundamental objection to these optical methods is that all density changes in the light paths are integrated. In particular, serious errors may arise from phenomena occurring at the ends of aerofoil .The chief practical difficulties are to minimize the effects of vibration and of temperature fluctuations. Moreover they are restricted to a two- dimensional (and also sometimes axi-symmetrical) flow and are often not sufficiently accurate in quantitative use.Suppose a ray of light passing through the undisturbed stream in the working section (empty tunnel) strikes the screen at the point P1. When the model is present, the resultant changes in density, and hence in refractive index, causes a change in the optical path .The ray is therefore deflected through an angle d( and strikes the screen at a point P2distant dQ from P1 , taking an additional time to traverse the new path. Observation may be made of dQ, d( , or dt or a combination of these quantities. In practice dQ is observed in the direct shadow arrangement, d( in the schlieren and dt with the interferometer. It may be shown as a reasonable approximation that the indication given by the interferometer is a function of the density that given by the schieren method a function of the density gradient and that by the direct shadows a function of the rate of change of density gradient. To this extent the methods are complementary rather than alternative. Moreover since the indication given by each method depends on a different derivative of the refractive index, the required quantity of the optical components differ for each .For example wind-tunnel windows which are satisfactory for schieren work may not be suitable for interferometry or vice versa.Let Oz represent the undisturbed path of a ray of light and let A represent the undisturbed path of a ray of light and let A and B represent the points on a element pf the wave-front at O.suppose now that a disturbance is introduced into the light path with a constant gradient of refractive index dn/dx in the direction normal to Oz .If n is the refractive index at O and nA and nB are the refractive indices at A and B respectively, then and the corresponding optical velocities are related by the equations The ray will therefore be deflected through an angle and with a local radius of curvature R given by

d( = 1 dn ds n dx

the total deflection ( in a plane normal to Oy is given by

( x = ( 1 dn ds

n dx

and similarly in a plane normal to Ox by (y = ( 1 dn ds

n dy

For the purpose of calculating the density it is usually assumed that the deflection from the undisturbed path is infinitesimally small so that ds may be replaced by dz in the equations above

The schlieren method

The form of the schlieren method which is most commonly employed is described in ref. in this method some quantity is recorded from which it is possible to deduce the deflection ( of the equations above. An extended line source of light S is usually employed the light being converted into a parallel beam by means of the lens L1 (or a mirror) before passing through the working section of the tunnel. Beyond the working section of second lens L2 produces an image of the source in the plane K beyond which a camera produces an image of the model in the working section on a screen or photographic plate. By a suitable choice of the camera lens the size of the resultant image may be adjusted over a wide range and so it is usually possible to obtain adequate illumination.If the source is of uniform intensity and the angle is sufficiently small (