The Flow Visualization Facility (FVF) is one of the specialized aerodynamic research facilities at NASA Dryden Flight Research Center at Edwards Air Force Base, California. Built in 1983, the FVF is a convenient, inexpensive tool that provides a means by which complex flow fields can be observed. As the designs of advanced technology vehicles become more complex, water tunnels have become increasingly useful as a time- and cost-effective flow diagnostic tool.
Since the start of its operation, the FVF's primary use has been to study highly three-dimensional vortex flow on aircraft configurations. Because of the low Reynolds numbers obtainable in a water tunnel, it is best used to investigate flow regimes where the vortex flow is dominant over viscous flow effects.
Ideally, a water tunnel is best used in the infant stages of aircraft development. Potential problems can be identified and a variety of fixes may be implemented before a substantially more expensive wind tunnel model is built and tested. Flow visualization in a water tunnel, however, is useful at any stage of aircraft development. Observation of the flow is a good first step toward providing physical explanations for anomalies or phenomena discovered during wind tunnel or flight tests. Once the causes are understood, possible configuration modifications can easily be investigated.
The FVF is a continuous flow, closed return water tunnel. It has a vertical test section in which the water flows downward. The cross-sectional dimensions of the test section are 16 x 24-in. and it is 72-in. long. The effective contraction ratio is 5:1.
Three honeycomb flow straighteners are used. The test section is made of 2-in. thick acrylic plastic which allows the user a 360 degree view of the test in progress to observe and record events. Access to the interior of the test section for model installation or model changes is provided by a 16-in. diameter door located on the side of the test section. There is no access to the model during a test run. The model can be positioned in angle of attack and sideslip. For a typical model (18-in. long), the limits on angle of attack are from -30 to +90 degrees and on angle of sideslip are -10 to +15 degrees and the maximum angular rate is 30 deg/sec.
Flow is provided by a 25 h.p. motor driving a centrifugal type pump. The motor is equipped with speed control which can vary the flow rate from 0.5 in/sec to 18 in/sec. The typical speed at which flow visualization tests are conducted is 4.5 in/sec. At this speed, the turbulence at the test section centerline is less than 0.5 percent.
Since most of the tests performed in the FVF are visual in nature, it follows that the primary method of data acquisition in the FVF is the use of photo/video documentation equipment. To provide high-quality documentation at a particular time, digital photographs are taken. To document the motion of the flow field, video cameras are used. A screen splitter is also available so that the planform and side views can be recorded simultaneously.
Propulsion System Simulation
Past experience has shown that it is important to model the flow properly through the inlets to simulate mass flow through an engine. This is typically accomplished by incorporating flow through inlets in the model and ducting the water from the model to the exterior of the test section (depending on the water level in the water tunnel, the test section is pressurized from approximately 3 to 6 psi). The flow rate is controlled by a flow meter and a valve allowing matching of a desired capture area ration that corresponds to a specified flight condition. In a similar fashion, an engine exhaust jet is modeled by ducting water from an external pressurized water source to the model and controlling the flow rate by a flow meter and a valve.
Due to the low flow rates used in the water tunnel, the resulting dynamic pressure is low enough such that relatively delicate models can be mounted in the test section. Many of the models used in the water tunnel started out as commercially available plastic kits which were modified to meet the test requirements.
During model construction, internal dye lines and a rigid model mount or sting are incorporated. The sting is mounted parallel and as close to the center water line as possible. Dye ports are flush with the model surface and normally do not affect the model's flow field. External dye lines are sometimes attached to the model surface to provide dye to portions of the flow field not covered by dye emanating from the model's fixed dye ports. Models often incorporate flow through inlets. Water is drawn through the inlets and the flow rate can be varied from outside the tunnel.
Quick configuration modifications can be made to the model using a variety of adhesive tapes, modeling clay, metals, or plastics. Typical modifications consist of adding or removing spoilers, vortex generators, strakes, fences, and external stores. Also modifications that change the model mold-line itself can be accomplished quickly such as re-shaping of fairings, the canopy, the forebody, or surface contours.
FLOW VISUALIZATION TECHNIQUES
Dye injection is the most common flow visualization technique used in the FVF. Typically, a mixture of vegetable food coloring and water is injected from ports on the model surface and acts as a tracer. Six pressurized dye reservoirs, each controlled by their own valve, provide the model with different colors of dye. The user has a choice of six different dye colors. Several of the model's dye lines can be flowed to a single dye reservoir to use one color. An external dye probe is also available to visualize the flow away from the model surface.
NASA Dryden conducted flight tests on an F-18 to study its flow field at high angles of attack. To support this and other similar efforts, several water tunnel flow visualization tests were performed on F-18 models built from a variety of commercially available kits. Results from flight, water tunnel, and computational fluid dynamics codes were all correlated.
Trapped Vortex Investigation
In another study, the FVF was used to observe the effects of a trailing disk behind an ogive cylinder. Flight and wind tunnel testing had previously documented the reduction of base drag by such an arrangement. Flow visualization in the FVF illustrated the mechanism responsible for that reduction, an annular vortex trapped between the cylinder and the disk. Laser light sheet techniques were used to provide a cross-sectional view of the trapped vortex.
Vertical Jet and Ground Interaction
Studies have also been conducted in the FVF illustrating the behavior of a vertical jet flow near the ground. Water was pumped through nozzles which were mounted close to the water tunnel wall. Again the laser light sheet technique was used to provide a cross-sectional view of the flow. This test provided an understanding of what happens when a vertical take-off and landing aircraft is in ground effect.
F-8 Oblique Wing
At one time, there were plans to flight test an oblique wing design on an F-8 aircraft. Since one wing is swept forward while one is swept back in this design, there is an asymmetry in the vortex flow at moderate to high AOAs which can cause roll asymmetries. Several studies were conducted in the FVF to investigate different wing designs, wing mountings, and wing flow control devices such as vortex generators. Results from these flow visualization studies helped to define the final wing and wing mounting design, although the flight test program was cancelled before the oblique wing and mounts were built.
The MCAIR 279 design was a supersonic V/STOL fighter design. It was fabricated by in-house model makers and was representative of a complex model. This particular model was fabricated from scratch from detailed drawings and photographs of a larger wind tunnel model. It was used to investigate the effects of different components and their locations on the overall flow field at high angles of attack. The model was tested with and without movable canards, with the wing in a fore or aft position and with and without the movable horizontal stabilizer.
The NASA Dryden Flow Visualization Facility is a convenient and inexpensive tool that can provide a qualitative description of complex fluid phenomena. As the designs of advanced technology vehicles become more complex, water tunnels have become increasingly useful as a flow diagnostic tool. The visualization and interpretation of complicated fluid motions about isolated vehicle components and complete configurations in a time- and cost-effective manner can be key elements in the development of flow control concepts.
The experienced FVF staff provides expertise in testing and interpretation of flow visualization results. The FVF is prepared to provide useful insight into the complex flow phenomena.
Further information on the Flow Visualization Facility may be obtained by contacting:
National Aeronautics and Space Administration
Dryden Flight Research Center
Mail Stop B4820
P.O. Box 273
Edwards, California 93523-0273