AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Fig. 17 Flow visualization by the grid and tuft method, yawed delta wing
Flow Visualization
Flow Field Visualization: Smoke
Methods of Smoke Production: The most common method of flow field visualization after tufts is smoke, which can be produced in a number of ways, although a universally accepted "best" way has yet to be devised. Burning damp straw, rotten wood and tobacco to produce smoke is generally unsatisfactory, even though historic work was accomplished with smoke from such materials. The same is true of pyrotechnic smoke devices (smoke bombs). Chemical methods of producing smoke include both titanium tetrachloride and tin tetrachloride which produce smoke when brought into contact with moist air. These compounds produce the very best dense smoke filaments but the products are corrosive and can be used only in an open circuit facility that can be well vented to an appropriate location. A mixture of anhydrous ammonia and sulfur dioxide produces dense white smoke, odors, and, if the air is damp, sulfuric acid. A significant advantage of the system is the fact that the smoke can be started and stopped in ag y pp fraction of a second, which is not true of the alternatives. This allows the problems associated with it to be minimized if photographic records rather than real-rime human visual inspection can be paramount. Steam and liquid nitrogen produce a good dense
Flow Visualization
Flow Field Visualization: Smoke
A method reported by Shindo and Brask that works at velocities of over 100 mph is a vaporized petroleum product called Type 1962 Fog Juice, which is used in theatrical productions. The smoke generator consists of about 75 in. of 0.060-in. outside-diameter (wall = 0.01 in.) stainless steel tubing. For a probe the tubing is placed inside a 0.375- in diameter steel tube and held by a collet about 8 0 in from the end of the tubing in.-diameter steel tube and held by a collet about 8.0 in. from the end of the tubing, which is bent 90° about 3 in. from the end. Ceramic beads are used to insulate the stainless steel tubing from the outer tube. To vaporize the fluid, 10-15 A is applied to the stainless steel tubing from the collet to a point about 60.0 in. away inside a non-heat- conducting handle. This allows the stainless steel tube to expand. The power unit consists of a variac whose output is connected to the 230-V windings of a 1.5-kVA 115:230 transformer. The variac is used to control the temperature on the stainless steel tubing by applying 0-50 V. The tubing has about 3 resistance, so the current is limited to a maximum of 15 A
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
current is limited to a maximum of 15 A.
Flow Visualization
Flow Field Visualization: Smoke
The fuel reservoir is airtight and has a pressure regulator used to set plant air pressure at about 30 lb/in2to feed the fuel to the probe and a needle valve to control
the fuel flow. Plastic tubing connects the reservoir to the probe. It takes some experience to obtain the desired volume of smoke which is affected by the air pressure fuel flow and voltage The following values arc approximate With the pressure, fuel flow and voltage. The following values arc approximate. With the power switch off, set the variac at 70%, apply 30 lb/in2 to the reservoir, and crack
open the needle valve. When a small stream of fluid comes out the end, turn on the power. When turning off, cut the power, and when a stream of fluid leaves the tube, shut the needle valve off. This is done to reduce carbon formation in the stainless steel tubing. If hot fluid is emitted, the temperature is too low; either increase the voltage or reduce the fuel flow. If the smoke pulsates, it is generally a sign that the air pressure is too low.
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
The most widely used method for producing smoke at the larger wind tunnels today is a wand system similar to that described above with polyethylene glycol as the "fuel." A useful alternative to air pressure to drive the fuel through the wand is a peristaltic pump. These are sold by suppliers to chemistry and life science laboratories and have very tine volume flow control. An example of such a system is shown in Fig. 18.
Flow Visualization
Flow Field Visualization: Smoke
The fuel reservoir is airtight and has a pressure regulator used to set plant air pressure at about 30 lb/in2to feed the fuel to the probe
and a needle valve to control the fuel flow. Plastic tubing connects the reservoir to the probe. It takes some experience to obtain the desired volume of smoke which is affected by the air pressure, fuel desired volume of smoke which is affected by the air pressure, fuel flow and voltage. The following values are approximate. With the power switch off, set the variac at 70%, apply 30 lb/in2 to the
reservoir, and crack open the needle valve. When a small stream of fluid comes out the end, turn on the power. When turning off, cut the power, and when a stream of fluid leaves the tube, shut the needle valve off. This is done to reduce carbon formation in the stainless steel tubing. If hot fluid is emitted, the temperature is too low; either increase the voltage or reduce the fuel flow. If the smoke pulsates, it is generally a sign that the air pressure is too low.
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
pulsates, it is generally a sign that the air pressure is too low. The most widely used method for producing smoke at the larger wind tunnels today is a wand system similar to that described above with polyethylene glycol as the "fuel." A useful alternative to air pressure to drive the fuel through the wand is a peristaltic pump. These are sold by suppliers to chemistry and life science laboratories and have very tine volume flow control. An example of such a system is shown in Fig. 18.
Fig. 18 Smoke generator and delivery wand used at the Glenn L. Martin wind tunnel, University of Marylan
Flow Visualization
Flow Field Visualization: Smoke
F N M Brown developed a system of smoke generation in which kerosene is vaporized and the vapor then cooled to the temperature of the airstream before being emitted into the stream. The systems are designed to be used with special purpose flow visualization wind tunnels rather than in larger general-purpose wind tunnels. They have been used in many studies
They have been used in many studies.
Using Smoke: Smoke filaments can be used to find key locations such as stagnation points. Smoke can easily show the extent of separated regions and the size of a separation bubble. Smoke can also be used to show and track strong features such as tip or leading-edge vortices from lifting surfaces that may impinge on downstream parts of a vehicle with deleterious effects. Strong stable light sources are required. If a periodic phenomenon is being investigated, then a stroboscope will be needed that can be synchronized to the period of the phenomenon being studied. Common subjects requiring such an arrangement are propellers and rotors Regions of separated flow can requiring such an arrangement are propellers and rotors. Regions of separated flow can be detected in two opposite ways. Smoke filaments can be introduced upstream of the model and located in a series of positions so that the boundary of essentially undisturbed flow can be detected. Any volume into which smoke does not go readily is