extensive excavation are required.

The use of this land

will be the same as it is for the existing facility.

Air Quality The only substance vented into the air from
the NTF will be gaseous nitrogen. There are no toxic or
particulate emissions from this facility. Under some
operating conditions, cold nitrogen is vented in large
quantities and measures have to be taken in the design
and operation of the facility to avoid local areas of oxygen
deficiency and excessive cold temperature. Production of
fog as a result of chilling the ambient air also requires
analysis and control. The area of problems associated
with the cold nitrogen discharge is treated in more detail
later in this document.

Noise The NTF has the potential for greater noise emissions than the wind tunnel facility which it will replace. This is due to the larger, more highly powered tunnel fan and the addition of the nitrogen vent stack system. Two problem areas exist with respect to noise levels; the first is in assuring that personnel in the immediate vicinity of the tunnel are not exposed to harmful noise levels, and the second is in minimizing noise nuisance in nearby areas. The most critical area from the point of noise nuisance is a residential trailer park located across the highway from the LaRC Main Entrance, as shown in Figure 1. The NTF will not be the only source of noise to an otherwise quiet area. This area is adjacent to an automobile race track and is within an area that receives appreciable noise from another LaRC wind tunnel and LAFB flight operations, all of which predated the residential use of this property. The recent assignment of F-15 fighters to LAFB has resulted in this area being classified as not compatible with residential use under Air Force definitions. The control of NTF noise is discussed in more detail later in this document.

Water Quality The construction and operation of the NTF will have no impact on water quality. The only liquid effluents are sewage, storm drainage and cooling tower drain water. These are unchanged from the present use of the facility. Sewage is piped into the Hampton Roads Sanitation District sewage system for treatment and disposal. Existing storm drains, separate from the sewage system, carry rain water to an outfall into Back River after passing through a skimming basin to remove any oils that may have entered the drain system. Cooling tower drain water is treated to remove the chromate corrosion inhibitors before it is discharged.

Solid Wastes No novel solid wastes will result from the

operation of the NTF. The normal refuse will be substantially the same type and quantity as produced in the present facility and it will be collected and disposed of in the same manner as for the rest of the Center.

GN 2 Exhaust to Atmosphere A schematic of the nitrogen exhaust stack is depicted in Figure 2. GN will be exhausted through the exhaust stack at a rate equal to the injection rate into the tunnel, the maximum of which is 1200 lbs/sec flow rate. Four fans positioned at the bottom of the stack will induce 1000 lbs/sec of ambient air flow into the stack to mix with

the cold GN2 such that, upon exit into the atmosphere, the mixture contains air oxygen concentration sufficient to support life. The stack is presently designed with a height of 80 feet. The stack exit area will, during operational flow conditions, produce a minimum exit velocity of 100 ft/sec. The stack height and exit velocity will provide sufficient time for the mixture to mix with the atmosphere to preclude excessive fogging or unacceptable temperatures at ground level.

The results of an analysis of the exhaust plume using Gaussian despersion modeling techniques are shown in Figure 3. The minimum ground level oxygen levels downwind of the exhaust stack never go below an acceptable level of 16.5%. This analysis does not consider either the mixing of the faninduced air with GN2 in the stack, or the minimum 100 ft/sec exit velocity. The analysis was performed using spreading parameters for a Class F or moderately stable atmosphere in which turbulent mixing is a minimum. These assumptions give conservative results, i.e., mixing in the stack and the minimum exhaust velocity of 100 ft/sec_should yield even higher oxygen levels than depicted in Figure 3. It should be noted that other wind velocities, U, were considered, but the case of 1 mph is essentially the worst condition. The case of a wind velocity of 10 mph is also shown and indicates very little change in standard atmospheric ground level oxygen levels. Wind velocity data indicate that wind speeds greater than 1 mph occur in the area for more than 90% of the time. The curve in Figure 3 is for the greatest case mass flow condition (1000 lbs/sec) of GN, into the atmosphere. A more sophisticated analytical model is presently being developed which will consider in detail the negative buoyancy and temperature of the plume and the case of zero wind velocity. This model will use an energy and momentum approach which will assess more comprehensively the plume behavior within a few hundred feet of the stack and will be coupled with the Gaussian analysis to predict plume temperature and nitrogen concentrations as a function of meteorology and operational conditions.

A literature

Data has been sought to validate analysis. search produced some data on LNG spills at ground level. Data were also found from limited tests of a very small scale stack (6 inches in height and up to 1/4 inch in diameter) exhausting Freon 12 in a wind tunnel simulating atmospheric winds. These data are being compared with modeling results. No experimental data have been found which involve field releases of a cold gas into the atmosphere through a stack. Thus, LaRC will carry out stack release experiments to provide necessary data. These experiments will involve releases of cold GN,-air mixtures into the atmosphere at mixture ratios typical of NTF operation. Stack diameters of one to two feet and air and GN2 flow rates capable of producing mixture exit velocities of 100 ft/sec and greater will be used. The resulting plume rise heights and paths as a function of time will be determined. Releases will be made under varying wind velocities. These experiments will provide model data which will verify a detailed prediction capability of plume behavior for a broad range of flow and meterorological conditions.

In summary, the exhaust of GN2 from the NTF for the most severe combination of operational and meteorological conditions has been analyzed using the Gaussian despersion modeling approach. The results indicate no undesirable or hazardous conditions in terms of oxygen depletion at ground level. A more comprehensive analytical model which will better assess the exhaust plume temperature behavior including the effect of negative bouyancy is being developed. Field experiments of cold GN, releases will be carried out at LaRC which will provide data for verification of analysis. These data and the modeling results will specify final design of the exhaust stack height, diameter and exhaust characteristics that will insure non-hazardous exhaust of the GN2.

Noise The NTF will be located adjacent to Langley Air Force Base (LAFB) at Langley Research Center (LaRC). LAFB is the home of several squadrons of F-15 and F-106 fighter aircraft. Noise levels in the area are generally high due to aircraft testing, takeoff and landing. Turboprop transports, for example, on the runway preparing for takeoff produce about 66dB(A) several hundred feet away. Engine noise during takeoff will generally exceed this level. Daytime background noise levels in some nearby communities near LAFB and LaRC range from 49 59 dB(A); nighttime levels from 40 - 57 dB(A).

As previously stated, the primary noise source of the NTF is the fan. Sound pressure levels of the noise inside the tunnel as a function of frequency are shown in Figure 4. These levels are maximum levels and occur during operation of the tunnel at the maximum horsepower of 125,000 HP and pressure of 130 psia. It is expected that the tunnel will operate at these conditions a very small percentage of the total operational time. The maximum sound pressure level occurs at the blade passage frequency of about 145 Hz.

Figure 5 shows the resulting A-weighted sound pressure levels transmitted through the tunnel shell as a function of distance from the tunnel. The transmission loss in the tunnel shell Iwas considered. The location of the nearest residential area within about 3/4 mile of the NTF is shown on the distance axis. This residential area is the trailer park previously mentioned and consists of over 100 trailers. The trailer park is located about 450 - 500 feet south of the NTF site. The noise levels shown in Figure 5 outside the NASA property line (greater than 200 feet from the NTF) do not violate any local, state, or federal regulations but may be considered nuisance levels in the trailer park especially if occurring during the night.

However, design measures will be taken to reduce these levels such that nighttime operation of the NTF will not result in levels excessively above background. These design measures coupled with measures for acoustically insulating workroom walls will result in compliance with OSHA workroom levels which are shown in Table I. A potential design measure for tunnel noise attenuation, for example, is the incorporation of thin lead sheet in the internal insulation system adjacent to the inner surface of the tunnel shell. The effects of this concept are shown in Figure 6, in which the acoustic attenuation properties of the lead sheet are predicted to reduce the noise levels at the trailer park significantly below the measured nighttime background levels of 44 dB(A)

as denoted by the X. Such a design measure appears feasible and practical. Evaluation studies of design concepts such as just cited are currently being carried out. Controlling the external noise levels produced by the NTF to within acceptable levels appears to be within the limits of proven engineering practices and several alternate design concepts are available for solution.

Preliminary estimates of the exhaust stack noise indicate that the primary noise source is the exhaust control valve, which will often be operated at choked conditions. Detailed analyses of the noise produced by the valve are in progress. Preliminary design of the exhaust stack specifies acoustic lining in the stack and acoustic insulation and sound traps in the attached building which houses the GN, exhaust piping and control valve. This design will maintain external stack noise levels below those of the tunnel.

In summary, the control of noise generated by the NTF appears a manageable task. Design measures that reduce noise levels within background at the nearby residential location have been identified. The final design of the NTF will assure that noise levels will be controlled to meet all regulations and to fall within acceptable community levels.