User:Su Yaris/sandbox/Cobra Probe

Cobra probe is a pressure probe type that specifically measures the pressure and velocity element of a moving fluid. In general, pressure probes are small conductor devices that are used to measure, distinguish, examine or to collect information about the inner flow of a mechanical system. First proposed by Shepherd IC, the Cobra Probe was used in applications for mean flow measurements and then it was improved to resolve turbulence systems. Containing four holes in its design, the cobra probe is a multi-hole pressure probe that enables calculating local static pressures as well as resolving the three components of the fluid’s velocity. Cobra Probe is resilient with its robust design, material selection (stainless steel), calibration, and maintenance. A variety of parameters including 3D velocities, flow angles, turbulence statistics, static pressure and Reynolds stresses can be measured with this device, within the field of aerodynamics.

Table of Contents

1. History
2. Structure & Operation
   1. Structure
   2. Operation
   3. Calibration
      1. Calibration Types
3. Performance
   1. Industry Applications
4. References

1. History

The Cobra Probe was first proposed by Shepherd for mean flow measurements. Later on, the Cobra probe was used by Hooper JD and Musgrove AR for the determination of the total velocity vector in turbulent single phase flow, in 1991.

Initially developed in the 1980s, the Cobra Probe differs from other preceding pressure probes with its structure and its capability of resolving unsteady velocity components. With the help of multi-holes in its structure, Cobra Probe can measure three unsteady velocity components in subsonic flow levels. The device has been widely used for flow angle studies due to its low blockage and relative ease of manufacture.

2.1. Structure

The general structure of a Cobra Probe contains three tubes soldered together side-by-side, facing the gas stream. The shape of the probe head has four pressure holes, giving it the name “cobra”. On top of the head, pressure taps are located 0.5mm diameter from its center. The pressure taps connect together through tubes of four piezo-resistive bridge type pressure transducers. Different from other pressure probes, cobra probe’s the pressure transducers are located close to the pressure ports.

• Pressure transducers

Similar to pressure sensors, pressure transducers also generate an output voltage that changes in response to pressure. In this case, a transducer is a sensing element with signal conditioning circuitry, probably an amplifier to enable signal transmission farther from the source, and possibly some kind of circuitry to adjust for temperature variations.

• Pressure transmitters

Similar to transducers, pressure transmitters produce a current signal over a low-impedance load as opposed to a voltage signal.

The probe’s head is a truncated triangular pyramid and the three tubes of the side face the ground about 45°, which means it has a flow acceptance cone with a 45° half angle. Because of this, some flow situations (such as stagnation pressure, the static pressure, and the flow angle within a fluid stream) would require the probe to be rotated. Cobra probe is intentionally designed to rotate 360° around its shaft axis without altering the position during measurement. This feature enables the device to be used in turbulence measurements as well.

"However, the Cobra probe isn’t suitable for measuring dynamic pressure or estimating static pressure primarily because of the pressure variation between the side holes and the center hole representing only a small fraction of the dynamic force."

2.2. Operation

The Cobra Probe features a multifaceted head with four 0.5 mm pressure taps that are connected to pressure transducers in the Probe's body by tubing. To provide dynamic capabilities, the Probe's frequency response is linearized from the mean velocity component (0 Hz) to over 2000 Hz. The instantaneous velocity vector and static pressure at the Probe head are then linked to the tap pressure ratios using calibration tables.

In short, the probe works by measuring four pressures on different faces of the probe. Using a predetermined transfer function, the signals are adjusted for transmission effects in the fine pressure tubes.

The basic working principle of the high-frequency Cobra Probe is to establish a relationship between the instantaneous static pressure, flow yaw and pitch angles, and the magnitude of the pressure field measured by four pressure tap located on the probe's head.

The calibration surface lookup is applied to all samples, producing time-varying values of pitch and yaw angles, static pressure, and velocity (usually 5000 samples per second). Then, along with mean and time-dependent data, variables like Reynolds stresses and turbulence intensity are shown and saved to disk.

2.3. Calibration

2.3.1. Calibration Types:

• Manufacturer’s Calibration:
  • The probe is shipped fully calibrated to the customers.
  • Within the 45° cone of acceptance, the manufacturer calibrates each Probe for a range of pitch and yaw angles in a steady flow with minimal turbulence. In order to achieve this, the Probe was mounted on a pitch and yaw traverse, with the Probe head situated inside the wind tunnel's core flow.
• In-situ Calibration
  • To perform an in-situ direction calibration, the Cobra Probe was fixed to a yaw table at the bottom of the wind tunnel's floor.
  • An in-situ speed calibration was conducted during two separate wind tunnel runs, each conducted at 0 to 15 m/s of wind speeds.
  • Pressure calibration equipment
    • Dead weight testers
    • Pressure calibrators
  • Method of calibration
    • In the present effort, a single surface method is implemented for the cobra probe in order to simplify the calibration and interpolation process.
    • The four pressures are measured after the probe is positioned at known angles to the flow in order to perform the calibration.Based on combinations of the four measured pressures' differences, two dimensionless pressure coefficients are formed.
      • These pressure coefficients are only dependent on the flow direction and are independent of Reynolds number.
      • Therefore, the inverse relations exist and the calibration functions for two flow directions based on the two pressure coefficients.

3. Performance

It has been demonstrated that the probe has an amplitude response of at least 0.4 in this frequency range and a frequency response of at least 1.5 kHz. Consequently, for flows where the turbulence energy is located in the frequency range below 1 kHz, it ought to be able to resolve the majority of the turbulence spectrum.

3.1. Industry Applications

• General uses in all applications
  • Flow mapping
  • Rapid point (hand-held) measurements
  • Simultaneous multi-probe measurements
  • Turbulent wake measurement and mapping
• CFD boundary & initial conditions determination
  • Vehicle HVAC (air-conditioning) systems
• Industrial/environmental aerodynamics
  • Boundary-layer profiles for industrial and building aerodynamics
  • Pedestrian level flow-field studies
  • On-site measurements in commercial or industrial facilities
• Vehicle aerodynamics
  • Measurement of atmospheric winds during on-road, on-vehicle testing
  • Measurement of vehicle under-body flow
  • Measurements on-road, at test tracks or in wind tunnel facilities
• Aircraft aerodynamics
  • Flow mapping around scale models
  • Atmospheric turbulence characterization on light aircraft
• Wind-tunnel measurements
  • Wind tunnel flow characterization
  • Flow mapping around model-scale vehicles, ships and aircraft.

External links

• https://www.sciencedirect.com/science/article/pii/S0894177700000042