Take the ratio of specific heats for air as γ = 1.4. Assuming isentropic compressible flow in the tunnel, estimate the speed in the working section where the Mach number is 0.75. What is the free-stream velocity at which compressibility effects begin to become important (i.e., the error in pressure coefficient exceeds 2%) on the airfoil surface when the incidence is 5 degrees? 4.Ī closed-return wind tunnel with a large contraction ratio has air at standard conditions of temperature and pressure in the settling chamber upstream of the contraction to the working section. This maximum increases at the rate of 7% of the free-stream velocity for each degree of incidence. On the basis of Bernoulli's equation, discuss the assumption that the compressibility of air may be neglected for low subsonic speeds.Ī symmetric airfoil at zero lift has a maximum velocity that is 10% greater than the free-stream velocity.
Calculate the speed, Mach number, 0.7 p M 2, and kinematic viscosity of the flow past the model. At the model's position, the pressure coefficient is −0.5. In the wing-flow method of transonic research, an airplane dives at a Mach number of 0.87 at a height where the pressure and temperature are 46,500 N m −2 and −24.6 ☌, respectively. Pressure and temperature in the maximum section are 100,300 N m −2 and 100 ☌, respectively, and the pressure difference across the Pitôt-static tube ≡ 127-mm mercury. Neglecting the effect of the Pitôt-static tube on the flow, estimate the throat diameter under the following conditions: (a)Īir at the maximum section is of standard pressure and density, and the pressure difference across the Pitôt-static tube ≡ 127-mm water. They easily become blocked when used with fluids carrying particles.Ī convergent-divergent duct has a maximum diameter of 150 mm, and a Pitôt-static tube is placed in its throat. They can measure the flow rate only at a particular position in the cross-section of the pipe (iii) They produce only a small pressure loss in the tube (iv)ĭue to the small pressure difference, they are only suitable for high velocity fluids (ii) A development of the Pitot tube, a pitometer, tests the flow of water in water mains and detects leakages. In these cases, the tube is connected to a Bourdon pressure gauge which can be calibrated to read velocity directly.
However, the main use of Pitot tubes is to measure the velocity of solid bodies moving through fluids, such as the velocity of ships and of low speed aircraft. Another use is the measurement of ventilating-duct air flows in mines. The device is generally unsuitable for routine measurements and in industry is often used for making preliminary tests of flow rate in order to specify permanent flow measuring equipment for a pipeline. The tubes canbe made very small compared with the size of the pipeline and the monitoring of flow velocity at particular points in the cross-section of a duct can be achieved. Most tubes incorporate a short external rod to ensure the probe is aligned to the axis of the duct.Ī Pitot-static tube may be used for both turbulent and non-turbulent flow. Alignment of the probe relative to the flow direction affects accuracy, though this amounts to about only 1% error in the recorded velocity pressure for up to 10° for both yaw and pitch. Normally it is not suitable for use below duct velocities of about 3.5 m/s.
For site work, coefficient C may be taken as unity and the other corrections ignored.īecause of its simplicity and ease of use, the Pitot-static tube, connected to a suitable manometer, is used extensively in testing ventilation systems. These corrections, amounting to a total of approximately 2% in terms of the mean velocity resulting from a traverse, are usually reserved for laboratory work. In determining the flow rate from a Pitot-static tube traverse, additional corrections for effects such as tube blockage and turbulence are applied to the mean velocity.
#HOW TO FIND VELOCITY USING A PITOT TUBE ISO#
Values of C are published in ISO 3966:2008. Where C is the calibration coefficient for the tube.
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