Wind Tunnel Apparatus: Why Strain Gages Preferred Over Pressure Sensors

In summary: 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.Only dynamic pressure. That is why they are designed to be very sensitive.
  • #1
sid_galt
502
1
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?
 
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  • #2
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
 
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  • #3
Astronuc said:
Also, the signal-to-noise ratio of the pressure transducer is much higher than the strain gage.

Thanks for the reply.

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?
 
  • #4
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.
 
  • #5
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.
 
  • #6
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.
 
  • #7
Why would you think they couldn't do both? There's nothing easier than measuring static pressure.
 
  • #8
Well, I read on a site that since the change in capacitance produces a current only for a short amount of time, the capacitor can only measure dynamic pressure. Or is there a way around this?
 

1. Why are strain gages preferred over pressure sensors in wind tunnel apparatus?

Strain gages are preferred over pressure sensors in wind tunnel apparatus because they provide more accurate and precise measurements of the strain and deformation of the object being tested. Pressure sensors measure the force exerted by the air on the object, which can be affected by factors such as air turbulence and the placement of the sensor. Strain gages, on the other hand, directly measure the deformation of the object, providing more reliable data.

2. Can strain gages and pressure sensors be used together in a wind tunnel apparatus?

Yes, strain gages and pressure sensors can be used together in a wind tunnel apparatus. In fact, using both types of sensors can provide a more comprehensive understanding of the aerodynamic forces acting on the object being tested. Strain gages can measure the structural integrity and deformation of the object, while pressure sensors can measure the aerodynamic forces and pressure distribution on the surface of the object.

3. How do strain gages work in a wind tunnel apparatus?

Strain gages work by converting the mechanical strain on an object into an electrical signal. In a wind tunnel apparatus, strain gages are attached to the surface of the object being tested and are connected to a data acquisition system. As the object is subjected to aerodynamic forces in the wind tunnel, the strain gages detect any changes in the object's shape and size, and the data is recorded for analysis.

4. Are there any disadvantages to using strain gages over pressure sensors in a wind tunnel apparatus?

One potential disadvantage of using strain gages over pressure sensors in a wind tunnel apparatus is that strain gages can only measure the deformation of the object, whereas pressure sensors can also measure the distribution of forces on the object's surface. This means that using strain gages alone may not provide a complete picture of the aerodynamic forces acting on the object. Additionally, strain gages require careful calibration and placement, which can be time-consuming and may affect the accuracy of the measurements.

5. Are there any alternatives to using strain gages and pressure sensors in a wind tunnel apparatus?

Yes, there are alternative methods for measuring aerodynamic forces in a wind tunnel apparatus, such as using laser-based techniques or particle image velocimetry. These methods can provide more detailed and accurate measurements, but they may also be more complex and expensive to set up and require specialized equipment and expertise.

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