An important factor in quantifying the flow quality of a wind tunnel is the turbulence level. A relatively simple way to determine the
flow quality is to use the technique of turbulence spheres. In this technique, the turbulence level of a freestream may be found through the examination
of the critical Reynolds number for a sphere. This type of study was performed in the Spring of 2000 by the EAE 266 class using three different sphere sizes. The results
show that the turbulence level in the UC Davis AWT is on the order of 0.1%. A brief summary follows and the full report is also available.
- It is known that a sphere in a steady flow will exhibit differing drag coefficient values depending on whether the boundary layer
on the sphere surface is laminar or turbulent. The freestream velocity at which the transition from laminar to turbulent flow occurs depends
on the diameter of the sphere and the turbulence level of the freestream flow. It is also known that in an atmosphereic freestream the Reynolds number
for transition is approximately 385,000. At this critical Reynolds number, the drag coefficient of a sphere drops from a value > 0.4 to a value < 0.2.
By measuring the Reynolds number at which this drop occurs in a wind tunnel for different size spheres, one may determine the turbulence level
at different velocities.
- Test Rig
- The test set-up for this experiment consisted of a vertical 1 inch diameter piece of aluminum bar stock that interfaced with the force balance and a horizontal 0.5 inch aluminum rod
that connected to the vertical piece. The spheres were mounted on the end of the horizontal piece. The photograph below illustrates the test rig. Note the
plexiglass fairing around the vertical piece to reduce excess drag loads.
- Three different size spheres were evaluated. The largest was a 9 inch diameter sphere (pictured above) manufactured expressly as a turbulence sphere by NASA Langley.
It was produced using stereolithography. The other two spheres were actually Christmas tree ornaments and were 5.5 inches and 3.5 inches in diameter.
- The results obtained from this experiment show that the turbulence level in the UC Davis AWT is < 0.1% through a range of velocities.
The following table summarizes the results:
|Sphere Diamter (in)
||Critical Reynolds Number
||Critical Velocity (mph)
The turbulence factor is found by dividing the theoretical critical Reynolds number of 385,000 by the experimentally observed critical
Reynolds number. This factor is then related to turbulence level through an emperically obtained relation.