7.l.2 Velocity Magnitude from Pitot-Static Tube(皮托静压管) 11/48 Assuming steady one-dimensional flow of an incompressible frictionless fluid: V= Pstag-Pstat (7.1) P Pstag:stagnation(淤塞；停滞；驻点)or total pressure,free stream Pstat:static pressure,free stream V:flow velocity Static tops(several,equally p fluid mass density Free streom spaced around circumference) If p is accurately known,the Pstat,m Measured deviation from the idea Pstag.m values theoretical result of Eq.(7.1) can be traced to inaccurate p.V Pstot Psto Stagnaticn measurement of P_stag True values point and Pstat. Tube support-一 The static pressure is usually the more difficult to measure accurately. Inclined Figure 7.3 Pitot-Static Tube differential manometer 12/48 7.1.2 Velocity Magnitude from Pitot-Static Tube The difference between true (Pstat) and measured(Pstat,m)values of static pressure may be due to the following: 1 Misalignment of the tube axis Flow Pstot,mPstot Influence of and velocity vector. pVy2 hole-tip spacing 2 Nonzero tube diameter. 0 1012 3 Influence of stagnation point on -0.0 the tube-support leading edge. -0.02 Incompressible turbulent flow While errors in the stagnation pressure are likely to be smaller than Support those in the static pressure,several possible sources of error are present: Pstat,m Pstat Flow Influence of ①Misalignment. PVY hole-support spacing 0.05 2 Two-and three-dimensional 0.04 velocity fields.The stagnation 0.03 0.02 Incompress ble turbulent flow pressure measured correspond to 0.01 ■一=一。》 some sort of average velocity. 0 0 6 8 0 12名8 ③Effect of viscosity.. Figure 7.4 Static Pressure Errors
Assuming steady one-dimensional flow of an incompressible frictionless fluid: ܸ ൌ ଶ ೞೌିೞೌ ఘ (7.1) ܲ௦௧ : stagnation(淤塞;停滞;驻点) or total pressure, free stream ܲ௦௧௧ : static pressure, free stream V : flow velocity ρ : fluid mass density Figure 7.3 Pitot-Static Tube If ρ is accurately known, the deviation from the idea theoretical result of Eq.(7.1) can be traced to inaccurate measurement of ܲ_ݐݏ݃ܽ .ݐܽݐݏ_ܲ and The static pressure is usually the more difficult to measure accurately. 7.1.2 Velocity Magnitude from Pitot-Static Tube(皮托静压管) Figure 7.3 Pitot-Static Tube 11/48 The difference between true (ܲ௦௧௧) and measured (ܲ௦௧௧, m) values of static pressure may be due to the following: ① Misalignment of the tube axis and velocity vector. ② Nonzero tube diameter. ③ Influence of stagnation point on the tube-support leading edge. While errors in the stagnation pressure are likely to be smaller than those in the static pressure, several possible sources of error are present: ① Misalignment. ② Two- and three-dimensional velocity fields. The stagnation pressure measured correspond to some sort of average velocity. ③ Effect of viscosity. 7.1.2 Velocity Magnitude from Pitot-Static Tube Figure 7.4 Static Pressure Errors 12/48
13/48 7.1.2 Velocity Magnitude from Pitot-Static Tube An important application of the Pitot-static tube is found in aircraft and missiles( 弹，导弹).The stagnation-and static-pressure readings of a tube fastened to a vehicle are used to determine the airspeed and Mach number while the static reading alone is utilized to measure altitude. When a pitot for subs-static tube is employed in a compressible fluid,Eq.(7.1)no longer applies.There are special equations for subsonic(亚音速的)flow(Mach number<马赫数>NM<1)and supersonic(超音速的)fow(Mach number NM>1) calculation respectively. The measurement of stagnation and static pressures may be combined in a single probe, or two separate probes,one for stagnation and the other for static,may be employed. Two independent static taps Wedge static-pressure probe Used to measure velocity direction as well. 0.05in. Flow To align with flow, rotate to balance two static-tap readings Figure 7.8 Several Examples of Commonly Used Forms of Pitot-Static Probe 14/48 7.1.2 Velocity Magnitude from Pitot-Static Tube Simple total-pressure Venturi shield total tube tube Flow (b) Flow (d) Total Pressure Tube also used for sub-and supersonic flow View 4-4 Boundary-loyer 222ZZ2Z222 total tube 0.001in.L0.003in. Flow d Figure 7.8 Several Examples of Commonly Used Forms of Pitot-Static Probe
An important application of the Pitot-static tube is found in aircraft and missiles (飞 弹,导弹). The stagnation- and static- pressure readings of a tube fastened to a vehicle are used to determine the airspeed and Mach number while the static reading alone is utilized to measure altitude. When a pitot for subs-static tube is employed in a compressible fluid, Eq. (7.1) no longer applies. There are special equations for subsonic(亚音速的) flow (Mach number <马赫数> NM <1) and supersonic(超音速的) flow (Mach number NM >1) calculation respectively. The measurement of stagnation and static pressures may be combined in a single probe, or two separate probes, one for stagnation and the other for static, may be employed. 7.1.2 Velocity Magnitude from Pitot-Static Tube Figure 7.8 Several Examples of Commonly Used Forms of Pitot-Static Probe Used to measure velocity direction as well. 13/48 Figure 7.8 Several Examples of Commonly Used Forms of Pitot-Static Probe 7.1.2 Velocity Magnitude from Pitot-Static Tube Total Pressure Tube also used for sub- and supersonic flow 14/48
7.1.3 Velocity Direction from Yaw Tube(偏航管)，Pivote/4s Vane(回转叶片)，and Servoed Sphere(伺服球) Flow-velocity direction information is of interest in flight vehicles where angle-of-attack measurements are utilized in attitude measurement and control,stability augmentation( 大，增加)，and gust(一阵强风)alleviation(缓解)systems. Yaw tubes are employed to determine the direction of local flow velocity. Angle of Attack:the angle between the object's reference line and the oncoming flow. The simplest form of yaw tube is useful for finding the angular inclination in one plane only. Subsonic Taps 1 3 are connected to a differential- pressure instrument that reads zero when the 7 tube is aligned with the flow; A center tap 2 is often included to read the m stagnation pressure after alignment is attained 之=Angle of attack (valid only if the angle of attack is zero). 中=Angle of yaw Tops 1 and 3 each 40 from 2 Figure 7.10 Yaw Tubes 16/48 7.1.3 Velocity Direction from Yaw Tube,Pivoted Vane,and Servoed Sphere (b):Operates on similar principle with(a).It may be utilized in regions where the flow direction changes greatly,since its sensing holes may be located very close together. (c)&(d):The two-axis probes could be designed to allow rotation about each axis;however,the complexity and size of such a design are generally prohibitive (价格或费用高昂得令人难以承受). ld) Subsonic or Subsonic or supersonic supersonic Subsonic or supersonic 20 (5o 4 Claw probe Double-claw probe 04 with total top no total tap Figure 7.10 Yaw Tubes
Flow-velocity direction information is of interest in flight vehicles where angle-of-attack measurements are utilized in attitude measurement and control, stability augmentation(扩 大,增加), and gust(一阵强风) alleviation (缓解) systems. 7.1.3 Velocity Direction from Yaw Tube(偏航管), Pivoted Vane(回转叶片), and Servoed Sphere(伺服球) Figure 7.10 Yaw Tubes Yaw tubes are employed to determine the direction of local flow velocity. Angle of Attack: the angle between the object's reference line and the oncoming flow. The simplest form of yaw tube is useful for finding the angular inclination in one plane only. ¾ Taps 1 & 3 are connected to a differentialpressure instrument that reads zero when the tube is aligned with the flow; ¾ A center tap 2 is often included to read the stagnation pressure after alignment is attained (valid only if the angle of attack is zero). 15/48 Figure 7.10 Yaw Tubes (b): Operates on similar principle with (a). It may be utilized in regions where the flow direction changes greatly, since its sensing holes may be located very close together. (c) & (d): The two-axis probes could be designed to allow rotation about each axis; however, the complexity and size of such a design are generally prohibitive (价格或费用高昂得令人难以承受). 7.1.3 Velocity Direction from Yaw Tube, Pivoted Vane, and Servoed Sphere 16/48