# Wind tunnel apparatus

sid_galt
Why aren't pressure sensors used for measuring forces in a wind tunnel? Why are strain gauges preferred? Is it because strain gages are sensitive?

The lift and drag plates attached to the stinger should feel strain in two directions. How come the strain gage manages to measure the strain of only one direction?

Staff Emeritus
sid_galt said:
Why aren't pressure sensors used for measuring forces in a wind tunnel? Why are strain gauges preferred? Is it because strain gages are sensitive?

The lift and drag plates attached to the stinger should feel strain in two directions. How come the strain gage manages to measure the strain of only one direction?
In answer to first question, it is the forces (stresses) on the structure which are of interest to the structural engineer, while the aerodynamicist or fluids engineer is interested in the fluid behavior. The strain gage is directly related to the local stress (think constitutive model). Also, the signal-to-noise ratio of the pressure transducer is much higher than the strain gage.

Regarding second question, a modeler usually wishes to resolve the strains in two or three dimensions (directions) in order to develop a constutive model. Think of how one models in 2D or 3D. However, most often, the maximum stress is in one direction (think von Mises or Tresca).

One can use bi-directional strain gages.

Reference - http://www.isa.org/Template.cfm?Section=View_All_Standards3&template=/Ecommerce/ProductDisplay.cfm&ProductID=3396 [Broken]

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sid_galt
Astronuc said:
Also, the signal-to-noise ratio of the pressure transducer is much higher than the strain gage.

Regarding pressure sensors, how is the performance of a capacitance pressure sensor? I have read they are very sensitive and can measure a large range of pressures.

Astronuc said:
Regarding second question, a modeler usually wishes to resolve the strains in two or three dimensions (directions) in order to develop a constutive model. Think of how one models in 2D or 3D. However, most often, the maximum stress is in one direction (think von Mises or Tresca).

The maximum stress is mostly in one direction. But if unidirectional strain gages are put on an object to measure strains in say the x direction, will the a stress in the y direction affect the measurement?

Staff Emeritus
sid_galt said:
Regarding pressure sensors, how is the performance of a capacitance pressure sensor? I have read they are very sensitive and can measure a large range of pressures.
True perhaps. I am not up on capacitive pressure sensors. However, the key is the structural response to the fluid excitation. One could read local pressure fluctuations, but get very little strain at the location, say on an automobile body or aircraft foil.

sid_galt said:
The maximum stress is mostly in one direction. But if unidirectional strain gages are put on an object to measure strains in say the x direction, will the a stress in the y direction affect the measurement?
One would use an x-oriented strain gage and a y-oriented strain gage on a surface. Ideally, the strain gage has very low stiffness compared to the material substrate, otherwise the strain gage could significantly affect the results and would have to be factored into the analysis.

sid_galt said:
Regarding pressure sensors, how is the performance of a capacitance pressure sensor? I have read they are very sensitive and can measure a large range of pressures.
Capacitive sensors, from what I have seen, are usually used in very high accuracy situations, like calibration standards and such. I can't think of a situation where I have used one myself. I pretty much stick to strain gauge based transducers. I have heard that they tend to be larger because the change in capacitance under load is pretty small, so the need to have a large enough deflection in the diaphragm to get a signal that doesn't get lost in the system noise.

sid_galt said:
The maximum stress is mostly in one direction. But if unidirectional strain gages are put on an object to measure strains in say the x direction, will the a stress in the y direction affect the measurement?
The strain gauge is constructed to try to minimize any Poisson effects. There will be some included in that direction, but the amount is usually pretty small. Usually one uses a rosette in the location of interest to get 3 directions of data and the Poisson effects, as well as other loadings are accounted for.

sid_galt
FredGarvin said:
I have heard that they tend to be larger because the change in capacitance under load is pretty small, so the need to have a large enough deflection in the diaphragm to get a signal that doesn't get lost in the system noise.

I see. Do they measure the dynamic pressure only or can they be modified to sense static pressure too? I am guessing they measure only dynamic pressure.