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Compressible flow of gases
Published in Bernard S. Massey, John Ward-Smith, Mechanics of Fluids, 2018
Bernard S. Massey, John Ward-Smith
When a Pitot-static tube is used to determine the velocity of a constant-density fluid the stagnation pressure and static pressure need not be separately measured: it is sufficient to measure their difference. A high-velocity gas stream, however, may undergo an appreciable change of density in being brought to rest at the front of the Pitot-static tube, and in these circumstances stagnation and static pressures must be separately measured. Moreover, if the flow is initially supersonic, a shock wave is formed ahead of the tube, and so results for supersonic flow differ essentially from those for subsonic flow. We first consider the Pitot-static tube in uniform subsonic flow.
Equations of motion
Published in Mohammad H. Sadraey, Aircraft Performance, 2017
There is a convenient way of measuring the dynamic pressure. If a tube is directly pointed into the flow of air, then connect the other end of it to a chamber (pressure measurement device). Thus, the oncoming air is brought to rest relative to the tube as it meets the blocked end of the tube. Since the tube exit is blocked, no air can flow down the tube. Then, the device will read the stagnation or total pressure (Ptot). This type of tube is called a pitot tube and provides a means of measuring the stagnation pressure.
Advances in plant-based quality control practice
Published in Anjan Kumar Chatterjee, Cement Production Technology, 2018
The device was invented by the French engineer Henri Pitot in 1732 and consists of a small cylinder positioned in the pathway of the fluid. One side of the cylinder is open in order to allow the fluid to enter. Once the fluid enters the tube, it cannot flow further because the cylinder does not have an outlet. Inside the tube, a diaphragm enables separate measurement of the static pressure and stagnation pressure. The static pressure is the pressure when the fluid enters the tube, while the stagnation pressure, also known as the total pressure, is the pressure when the fluid comes to rest. The Pitot tube is a double-walled, generally nickel-plated, metal tube with the bottom end bent at a right angle (Figure 8.14). The measurement system also requires a differential pressure transducer and a computer set-up that includes the necessary hardware and software to convert the raw transducer signals into the proper engineering units. The incorporation of sensors to measure the air temperatures, barometric pressure, and relative humidity can further increase the accuracy of the velocity and flow measurements. The Pitot tube measures air or gas velocity directly by means of a pressure transducer, which generates an electrical signal that is proportional to the difference between the total pressure and still air (static pressure). The volumetric flow is then calculated by measuring the average velocity of an air stream passing through a passage of a known diameter. When measuring volumetric flow, the “passage of a known diameter” must be designed to reduce air turbulence as the air mass flows over the Pitot tube. Also, the placement of the Pitot tube in the passage will influence how accurately the measured flow tracks the actual flow through the passage. Calibrating the measurement system in a wind tunnel can further improve the accuracy of the velocity and the flow measurements.
Experimental Study on Heat Transfer Enhancement by Using Textile Flap Oscillation
Published in Heat Transfer Engineering, 2022
Beihan Zhao, Wenshuo Yang, Chaolun Zheng, Yong Pei, Anthony Malatesta, Xinan Liu, Bao Yang
where is the stagnation pressure, is the static pressure, and is the air density. The difference between the stagnation pressure and static pressure was displayed by the height difference of the two water columns in the U-tube manometer. That is, where kN/m3 is the specific weight of the water and H is the height difference of the two water columns. Thus, U can be expressed as The air density was calculated by using the ideal gas law as where patm is local atmospheric pressure measured in the lab, J·kg−1·K−1 is the specific gas constant, and Tinlet is the inlet air absolute temperature in degrees Kelvin. In this study, the height differences of the water columns ranged from 6.0 to 21.0 mm, corresponding to a range of 9.9 to 18.5 m/s for the air velocity.
Investigation of methane-air explosions and its destruction at longwall coalface in underground coalmines
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Yunfei Zhu, Wendong Zhou, Deming Wang, Zhenlu Shao, Chaohang Xu, Min Li, Zhang Yutao
As shown in Figure 15 (a), the peak velocity shows a downtrend in the rail roadway from the lower edge of methane-air could to 3# crosscut. However, the peak velocity at every crosscut entrance has a sharp rise in contrast to the significant drop in peak overpressure. The stagnation pressure is the pressure that a moving fluid would have if it was brought to rest by an isentropic flow against a pressure gradient (Crowl, 2003). This represents the total energy of a fluid as the sum of the dynamic pressure and static pressure. At crosscuts, the free space expands the airflow, causing a decrease in static pressure, as shown in Figure 13. Correspondingly, the dynamic pressure increases. For the same reason, the airflow is blocked and compressed at the upper right corner in the simulation domain; therefore, the static pressure increases and the dynamic pressure drops. Beyond the corner, the compressed airflow expands again, resulting in the acceleration of the airflow, as shown in Figure 15 (b). In comparing two stable pressures before and after every crosscut and the upper right corner as red dots plotted in Figure 15, the velocity drops 48.5 m/s (18.8%) via the 1# crosscut, 36.9 m/s (18.4%) via the 2# crosscut, 36.9 m/s (23.5%) via the 3# crosscut, 20.7 m/s (18.5%) via the 4# crosscut, and 17.4 m/s (just 11.8%) via the corner. Therefore, it is concluded that bifurcations have a significant effect on reducing the dynamic pressure, while corners do not. Along with the conveyance roadway, the peak airflow velocity decays by just 23 m/s (17.7%) in the 440-m-long straight roadway derived from the linear segment in Figure 15 (b). Considering the blast wave velocity is more than 100 m/s in the simulation domain, the dynamic pressure decreases slowly not only in long straight tunnels but the whole simulation domain to act on objects far away from the explosion source.