In the literature I read that the frequency of Vortex shedding is measured using Hot wire probe. What is the principle and method of measurement ? For a bluff body which is allowed to vibrate in the cross flow direction the Straouhal's law is not valid in some region, can we get the Vortex shedding frequency using Hot wire probe in this case also ?
There are many ways that can measure the frequency of vortex shedding. The vortex shedding can induce velocity fluctuations in the wake. The hotwire with sufficient sampling frequencies can capture the signal of velocity fluctuations in the wake. The Fourier analysis of the signal can determine the dominant frequencies, corresponding to the frequencies associated with vortex shedding.
This was done by my BSc final year students project in 1985. We had a pipe which was cut in half and the cut part is covered with an aluminum foils with a suitable sealing glue. Inside the pipe they stuck strain gauges on the foil surface. You can use any other sensor for vibration. The strain gauges then gave the oscillation on an oscilloscope and you can measure the frequency this way and relate it to the flow velocity.
Since then many instruments appeared in the market to use this flow phenomenon to measure the flow. One insertion application that is suitable for large pipes is to make a disk with hard surface facing the flow and the aluminum foil on the back of it. The holder of the disc must bend away/curve out from the flow region not disturb the flow. A thermistor can also be used to sense the vibration as there will be some small temperature fluctuations accompanying the flow oscillations.
Hope this is useful.
Using a hot wire system would be helpful it provides u with FFT and Strouhal number
Thinking simple, a hot wire would be too complicated for the job in my opinion. Besides, it is vibration and pressure variations is what you are measuring. The change in temperature is a secondary phenomena. By the way, further simple well known circuitry can easily be added to the strain gauge, thermistor or whatever, to find out the oscillations, filter them, then derive a number for the frequency from the detected signal. I just kept to the basic in my previous reply and the use of the oscilloscope is only for development purposes.
Along the lines of simplicity, do you have visual access? Why not smoke or dye flow visualization coupled with a high-speed camera? With some form of spatial calibration scale/image and knowledge of the frame rate you can achieve the same ends. I suppose it depends upon your desired experimental uncertainty.
Heck, if the flow is slow enough (in water?) the frame rate on the new iPhones could be fast enough.
Dear,
This thesis gives you detail measurements of vortex shedding behind bluff body:
Measuring the Vortex-Shedding Frequency Behind
Staggered Cylinders in Cross-Flow by Christopher Clark
Santa Clara University, [email protected]
Besides HWA measurements, measuring the wake velocity fluctuations can be done using Lidar instrument, you can estimate the velocity deficits in along wind stream component. Subsequently, it is possible to find the vortex shedding frequencies by taking the FFT of measured data.
Note that Lidar is expensive proposition when compared to HWA or other optical particle tracking methods, but provides highly accurate results if experiments are done correctly.
Using the smoke flow visualization method along with high quality camera would be fruitful in this case as it gives you not only flow behaviors but also the frequency
You can use a single hot-wire without calibration and monitoring only the tension. Place the sensor with the wire // to the cylindre axis z at x/D~2 and y/D~1, with x the streetwise direction and (0,0) the axis of the cylinder. With this setting you can estimate the Strouhal frequency with or without cylinder vibration.
- Badges
- Science topic