I have met a problem in measuring boundary layer by using hot-wire and sincerely ask for advice: There always exist low frequency disturbacnes ( around 10hz~20hz) when the probe intrude into the boundary layer (the frequency varies a little with change of traverse system ). When the probe is fixed on the plate, there is no this low frequency disturbance. (with Freestream velocity of 8m/s, its turbulent intensity is below 0.15%. In the experiment set-up, the probe used is TSI 1218-T1.5, and traverse rod with elliptic section (a=4mm, b=8mm) is inserted through wind tunnel side wall ) .
What is the most probably factor that causing this low frequency disturbance and how to avoid it? It is suspected that traverse system may be the cause of this problem, then what is the best set-up for traverse system? Thank you very much!
Hi, Jiang:
The technique of hot-wire anemometer depends on the convective heat loss to the surrounding fluid from a probe. If only the fluid velocity varies, then the heat loss can be interpreted as a measure of that variable. As such, when a hot-wire probe is positioned close to a solid surface, an increase in heat transfer occurs. I think that it is the reason for your case.
An example for a 5 um diameter wire, the effect of wall proximity upon the heat transfer extends to 1-2 mm from the surface. You may apply the still-air correction technique to correct your data, which measures the heat transfer from the wire to the surface in still air at the various locations in the experiment, then use the general form of King's law equation to obtain the increased heat transfer.
To simplify the problem, using a Pitot static probe with very fine measurement steps may do the job.
Thank you very much! The issue of heat transfer effect you mentioned is very like to those discussed in paper of Tay 2012 (Meas. Sci. Technol. 23 (2012) 085305 (8pp), dx.doi.org/10.1088/0957-0233/23/8/085305), but the frequency of low disturbance appears at the beginning of probe approaching to the edge of boundary layer (above 3mm from wall), and when it was fix in the boundary layer (such as y = 1.5mm), there is no low frequency spike in frequency spectrum diagram. So it is not likely beeing caused by wall effect of heat transfer. Thanks you all the same!
Hu, Thank you! The separation bubble does influence the results, even initiates rapid transition after bubble. The pressure coefficient (Cp) calculated from the velocity, obtained from hot-wiremeasurements, using Bernoulli’s equation is shown in attachment (the leading edge design and measurement method are the same as Fransson 2004 see http://link.springer.com/article/10.1007/s00348-004-0858-3 ). It found the non-dimensionalized pressure distribution to be within +-0.01 downstream of x = 15 mm from leading edge. I will make sure whether there is separation in that condition though it seems not. Thank you!
Most wind-tunnel experimentalists use a high-pass filter, i.e. they just filter out frequencies lower than, say, 20Hz. In almost any wind-tunnel the Tu level is specified for frequencies above 20Hz only, because the disturbance level strongly goes up with frequency going to zero. This is ok for 2-d boundary layers, but not so simple for 3-d boundary layers with self-induced crossflow: Then zero/low-frequency modes are amplified by crossflow instability whereas those are strongly damped in 2-d boundary layers. If the probe vibrates there are means to avoid this, see the literature, especially for cases with crossflow instability on swept wings.
In my point of view, it could be related to large eddies loop separation if they aren't gross noise, you can investigate it by repeating the tests and do ensemble averaging !
Is hot-wire located at the leading edge area of the flat plate?Possible candidate could be low frequency streaks forced by the freestream turbulence. If so, move the hot wire downstream in the boundary layer away from the leading edge, disturbance level at low frequency should be damped.
Could it be that your probe itself is vibrating with 10 Hz? As soon as you enter the boundary layer the probe vibrates in the velocity gradiënt of the boundary layer which will show up as a fluctuation. When your probe is fixed on the plate, the vibration of the probe will be much smaller and no low frequency fluctuation will be measured.
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