1.Problems in Operating

1.Problems in operating

Transonic wind tunnel

Testing at transonic speeds presents additional problems, mainly due to the reflection of the shock waves from the walls of the test section . Therefore, perforated or slotted walls are required to reduce shock reflection from the walls

Supersonic wind tunnel

1)The power required to run a supersonic windtunnel is enormous, of the order of 50 MW per square meter of test section.:2)adequate supply of dry air 3)wall interference effects 4)high-quality instruments capable of rapid measurements due to short run times on intermittent tunnels

Hypersonic Tunnels

1)supply of high temperatures and pressures for times long enough to perform a measurement

2)reproduction of equilibrium conditions 3)structural damage produced by over-heating

4)fast instrumentation 5) power requirements to run the tunnel

.2.

Schlierenis German for ‘striations’. The term was coined by Albert Töpler, who developed the technique in 1906 from a related technique used to identify figuring errors in telescope mirrors. Schlieren photography is a way of visualizing density variations in a gas, and is useful in wind tunnel studies and investigations into heat flow. It employs ashadowgraphprinciple. A collimated (i.e. parallel) beam of light passes through the test space and is brought to a focus at a knife edge; it then diverges on to a screen or a camera system. Any gas density gradient with a component perpendicular to the knife edge will deviate the light from the region, so that it either clears the edge, giving a bright area on the screen, or is intercepted by it, giving a dark area. The resolution can be improved by a further knife edge at the first focus of the system. Where large spaces are to be imaged, off-axis parabolic mirrors are used rather than lenses to collimate and focus the beam . An alternative to the knife edge is a band of three colour filters, red above and blue below, with a narrow strip of green in between.
Schlieren photography is sensitive enough to record the pattern of warm air rising from a human hand, but a more sensitive test usesinterferometry, in a kind of hybrid of Schlieren photography andholography. A laser beam replaces the white light beam, and a beamsplitter and beam combiner form a Mach-Zehnder interferometer set-up . This shows density differences directly, rather than density gradients

3.Ashock tubeis a device used primarily to study gas phasecombustionreactions. Shock tubes (and relatedimpulse facilities:shock tunnels,expansion tubes, andexpansion tunnels) can also be used to studyaerodynamic flowunder a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities.

A simple shock tube is a tube, rectangular or circular in cross-section, usually constructed of metal, in which a gas at low pressure and a gas at high pressure are separated using some form ofdiaphragm, see for instance texts by Soloukhin, Gaydon and Hurle, and Bradley.[1][2][3]This diaphragm suddenly bursts open under predetermined conditions to produce a wave propagating through the low pressure section. The shock that eventually forms increases the temperature and pressure of the test gas and induces a flow in the direction of theshock wave. Observations can be made in the flow behind the incident front or take advantage of the longer testing times and vastly enhanced pressures and temperatures behind the reflected wave.

The low-pressure gas, referred to as thedriven gas, is subjected to the shock wave. The high pressure gas is known as thedriver gas. The corresponding sections of the tube are likewise called the driver and driven sections. The driver is usually chosen to have a lowmolecular weight,hydrogenorhelium, for safety reasons, with highspeed of sound, but may be slightly diluted to 'tailor' interface conditions across the shock. To obtain the strongest shocks the pressure of the driven gas is well below atmospheric.

The test being conducted begins with the bursting of the diaphragm. Three methods are in common use to burst the diaphragm.

• A mechanically-drivenplungerto pierce it or its destruction by anexplosive chargehave both been practiced.
• Another method is to use diaphragms, either of plastic or metals, generally annealed to ensure closely defined bursting pressures, with plastics for the lowest burst pressures,aluminumandcopperat somewhat higher levels andmild steelandstainless steelfor the highest ones. They are frequently scored in a cross-shaped pattern to a calibrated depth, to rupture evenly, contouring the petals so that the full section of the tube remains open during the test time.
• The third utilizes a mixture of combustible mixture of gases, with an initiator designed to produce adetonationwithin it, producing a sudden and sharp increase in what already may have been a pressurized driver.

The bursting diaphragm produces a series ofpressure waves, each increasing thespeed of soundbehind them, so that they compress into a shock propagating through the driven gas. Thisshock waveincreases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave but at lower velocity than the lead wave. Simultaneously, ararefactionwave, often referred to as thePrandtl-Meyer one, travels back in to the driver gas. The interface, across which a limited degree of mixing occurs, separates driven and driver gases is referred to as the contact surface and follows, at a lower velocity, the lead wave.

Applications

In addition to measurements of rates ofchemical kineticsshock tubes have been used to measuredissociation energiesandmolecular relaxation rates[4][5][6]they have been used inaerodynamic tests. The fluid flow in the driven gas can be used much as awind tunnel, allowing higher temperatures and pressures therein[7]replicating conditions in theturbinesections ofjet engines. However, test times are limited to a few milliseconds, either by the arrival of the contact surface or the reflected shock wave.

They have been further developed intoshock tunnels, with an addednozzleand dump tank. The resultant high temperaturehypersonic flowcan be used to simulateatmospheric re-entryofspace craftorhypersonic craft, again with limited testing times.

4.

On the figure, we show a schematic drawing of ablowdownwind tunnel. Blowdown tunnels are normally used fromhigh subsonictohigh supersonicflow conditions. There are several possible configurations for a blowdown tunnel. On the figure, we show completely closed supersonic configuration. The test section sits at the end of asupersonic nozzle. The Mach number in the test section is determined by pressure and temperature in theplenumand thearea ratiobetween the test section on the nozzlethroat. As the flow expands in the nozzle, thepressure decreasesand any moisture in the tunnel may condense and liquify in the test section. To prevent condensation, air is brought into the tunnel through adryerbed. The air is pumped into a closedhigh pressure chamberupstream of the plenum. At the same time, air is pumped out of a closedlow pressure chamberdownstream of the test section.

Test times are limited in blowdown wind tunnels. At the beginning of the test run,valvesare opened upstream and downstream of the test section. Thepressure ratioestablishes a supersonic flow in the test section and the air flows from the high pressure chamber to the low pressure chamber. As air leaves the high pressure chamber, the pressure in the chamber decreases. Likewise, as air enters the low pressure chamber, the pressure in that chamber increases. Eventually, the pressure in the two chambers equalize, the flow stops, and the test is finished. To provide constant conditions in the test section, apressure regulatorvalve is normally installed in the plenum. Asecond throatis often employed downstream of the test section toshock downthe supersonic flow to subsonic before entering the low pressure chamber.

A closed configuration with both high pressure and low pressure chambers is shown in the figure, but there are other configurations of blowdwon tunnels. Some blowdown tunnels, calledindrafttunnels, do not use a high pressure chamber, but open the plenum chamber to the atmosphere. The indraft tunnel uses the low pressure (vacuum) chamber downstream of the test section to produce flow. The advantage of this configuration is that the conditions in the plenum remain constant and there is no need for a pressure regulator. The disadvantage is that the pressure ratio across the test section is usually lower than a closed confifguration and therefore the maximum Mach number is lower. Another configuration retains the high pressure chamber, but exits to atmosphere instead of into a low pressure chamber. The advantage of this configuration is that it is cheaper than a closed configuration in both construction and operation. But the tunnel is very loud and normally requires some type of muffler downstream of the test section.

The blowdown tunnel has some advantages and some disadvantages relative to aclosedcontinuous flow tunnel.

Advantages of the Blowdown Tunnel

• High Mach capability. Easy tunnel "starting".
• Lower construction and operating costs.
• Superior design for propulsion and smoke visualization. There is no accumulation of exhaust products in an open tunnel.
• Smaller loads on model during startup because of faster starts.

Disadvantages of the Blowdown Tunnel

• Shorter test times require faster (often more expensive) instrumentation.
• Need for pressure regulator valves.
• Noisy operation.

5.Supersonic Wind Tunnels

Supersonic wind tunnels operate differently than subsonic and transonic wind tunnels. First, because fans are inefficient at supersonic speeds, they must run subsonic and the air must make a transition from subsonic to supersonic speeds. Second, supersonic wind tunnels require an enormous amount of power. Supersonic wind tunnels can require so much power that if run during periods of peak electricity demands they can cause a regional brown-out. Very few facilities have continuous supersonic wind tunnels for this reason. The key to making a supersonic wind tunnel is to employ a supersonic venturi. Figure 8.18 shows a schematic of a closed-circuit supersonic wind tunnel. The fan moves the air in a subsonic channel. During startup the subsonic section has been pressurized while the test section remains at a static pressure of 1 atmosphere. The air accelerates in the first venturi until the speed at the throat becomes Mach 1. As the channel opens up, since the air is flowinginto a region of lower pressure it accelerates, producing the supersonic flow in the test section. After the test section the airflow goes through a second venturi. Here the speed decreases until it becomes Mach 1 at the throat. Since the air is going into a region of higher pressure, as the channel opens up the flow slows down, becoming subsonic again. The supersonic wind tunnel has an additional source of power loss. In addition to the friction on the walls and the drag on the models, now there are losses associated with theinevitable shock waves. All of these losses mean a lot of heat is being generated. In order to run continuously, a supersonic wind tunnel must have a large cooler, which is placed in the airflow in the subsonic section.The great amount of power required for supersonic wind tunnels means there are very few continuous wind tunnels and they are not very large. A 3 _ 3 foot (1 _ 1 m) test section is considered very large and requires half a million horsepower (375 megawatts) to operate at Mach 3. But there are other methods to test supersonicaircraft. One method is the “blowdown” supersonic wind tunnel depicted inFigure 8.19. A huge tank is filled with high-pressure air and thenexhausted through a venturi. This kind of wind tunnel works quitewell but will

only allow a few minutes of testing. However, a carefullyplanned test can gather a tremendous amount of data in a very shorttime. With this technique the energy required is generated and stored over time. This type of wind tunnel requires very little power butrequires quite a long time between tests. The NASA HypersonicTunnel Facility at Plum Brook can generate speed up to Mach 7. Thisblowdown facility can accommodate a 5-minute test every 24 hours.The Twenty-Inch Supersonic Wind Tunnel at the Langley ResearchCenter can generate flows with Mach numbers from 1.4 to 5 for 1.5 to5 minutes.Another option, which is more common, is the vacuum supersonicwind tunnel shown schematically in Figure 8.20. Rather than pump achamber to a high pressure, which is dangerous, the chamber isevacuated and the airflow is in the other direction through the testsection. Thus, the upstream reservoir of air is just the atmosphere andthe air is being drawn through the throat and test section into avacuum.In all supersonic venturis, the air expands on the high-speed side

and thus cools. For continuous supersonic wind tunnels this is not a concern because all the energy losses cause the air to be hotto start with. For the blowdown wind tunnels the air is oftenheated before it reaches the venturi so that the test sectionremains at a reasonable temperature. Vacuum wind tunnelshave a problem that the room air is used and thus it is notpractical to preheat the air. Therefore, the test section is verycold. For example, a Mach 3 test section would be _274°F(_170°C) if the air supply were at room temperature

6.Hypersonic Testing

With the incredible power required for supersonic wind tunnels, how can anyone expect to create hypersonic flow conditions, typically above a Mach 5, in a test environment? The only effective method to do this with a stationary model is with the blowdown method, lots of preheating of the air, and a very small test section. The key word in that last sentence was stationary. Some hypersonic facilities actually use a combustion gun, where gases combust in the breach to propel the model. The problem with this technique is that the desired measurements must be made on a nonstationary model, one that is moving very fast. But there is another trick up an engineer’s sleeve. Hypersonic flight implies that the Mach number is typically greater than Mach 5. Up to this point we implicitly assumed that to achieve hypersonic speeds we have to increase the speed in the test section or of the model. What if we were to decrease the speed of sound instead? Sound speed differs for different gases. The speed of sound decreases as the weight of the gas molecules increases. So, instead of using air for our working gas, we could look for a heavier gas, like carbon dioxide, although this will only decrease the sound speed by 14 percent. The advantage of using an alternate gas is that the true speeds can be kept reasonable, while the Mach number is fairly high.

Hypersonic Wind Tunnels

Since air is stored in the high pressure air flasks at ambient temperature, it must be heated in order to avoid condensation during operation of the hypersonic tunnels. This is accomplished by passing the air through a bed of aluminum oxide pebbles which is enclosed in a silicon carbide cylinder surrounded by 12 electrical heating elements (Globars); the bed is maintained at the desired temperature by radiation from these elements. The heater was designed for a maximum operating pressure and temperature of 600 psia and 2500F, respectively. For effective utilization of the tunnels and instrumentation equipment available in this facility, several hypersonic tunnels are permanently connected to the heater.

At present there are three tunnels connected to the collector; these are the Mach 4.4, the Mach 8 and the Mach 12 tunnels. However, additional access ports are available on the collector which have been used in the past for low speed, high temperature tests involving thermal ablation, studies of thermal stresses, and heat transfer in tubes. At the present time the hypersonic tunnels and the heater system are in a stand-down mode. Due to the resurgence of interest in hypersonics there is an ongoing effort to reactivate this part of the facility.

The Mach 8 tunnel has a test section diameter of 2 ft. and consists of an axisymmetric inner contoured nozzle surrounded by a pressure shell. Test models are supported from the horizontal access ports, the vertical windows being used for flow visualization. Free stream static pressures between 0.2 and 3 mm Hg can be obtained with a test duration of up to 90 seconds. Test programs have been carried out in this tunnel relating to near wake studies, nose cone configurations, mass transfer cooling, hypersonic boundary layers, and low density shock layers.

The Mach 12 tunnel is also axisymmetric in the throat region, but at the test section the tunnel has a decagon cross section. The test section diameter is 4 ft. and therefore permits testing of relatively large models. The decagon shape was chosen for economy and ease of construction since an axisymmetric contoured nozzle of this size would be extremely expensive. As a result, the tunnel was manufactured with a monocoque structural design; ribs and stringers were formed with the proper contour and thin plates welded to these ribs form the nozzle. The resulting contour has the same cross sectional area at any position as the corresponding axisymmetric nozzle. A transition between the axisymmetric throat region and the decagon cross section is achieved in the vicinity of the throat at fairly low Mach numbers. Due to the large boundary layer thickness of the flow in this nozzle the test section flow is essentially axisymmetric and quite uniform. The pressure in the test section varies between 0.1 and 0.3 mm Hg and run times on the order of 3 to 4 seconds can be achieved in this tunnel. Experimental studies which have been conducted in this tunnel include the investigation of the near and far wake of blunt and slender bodies and viscous/inviscid interactions occurring in a low density, high Mach number free stream environment. Both the Mach 8 and Mach 12 tunnels exhaust into the vacuum sphere through two large butterfly valves which can be used to isolate either tunnel from the sphere.