What Are Flow Rates?

Flow velocity (current velocity) The displacement of a liquid per unit time. Particle velocity is a vector that describes the direction and speed of movement of a liquid particle at a certain instant. Its direction is consistent with the tangent direction of the particle locus. Its size is:

u = lim < t 0> s / t = ds / dt

The unit is m / s, s is the distance that the liquid particle flows in the time t. ^[1]

Chinese name: Flow rate
Foreign name: flow velocity; current velocity

Pinyin: líu sù
Provenance: "Practitioner Pan Yongfu"
Fluid: Liquid and gas with fluidity.

Basic flow information

Word : flow rate

Basic explanation

[velocity of flow] The distance traveled by a flowing object in unit time, expressed in meters per second. ^[2]

Flow citation explanation

1. Refers to the distance the fluid flows in a unit time, usually in meters per second.

Groundwater velocity measurement

Zhao Shuli's "Practitioner Pan Yongfu": "According to the flow velocity of the water in this place, it goes without saying that it is supported by the upstream boat, but it cannot be supported by the east and west.

2. Generally refers to the speed of passing.

Xu Chi's "Goldbach Conjecture Towards the 21st Century": "The flow of time is fast and life is developing rapidly."

Basic meaning of flow rate

Velocity refers to the distance traveled by a gas or liquid mass point in a unit time. The velocity of each point of the flow in the channel and the river is different. The velocity near the bottom of the river (canal) and the side of the river is small. The center of the river is near the water surface. The velocity of flow is the largest. For simplicity of calculation, the average velocity of the cross section is usually used to represent the velocity of the water flow on the section.

Normal units of flow rate are m / s, m / h

Particle velocity is a vector that describes the direction and speed of movement of a liquid particle at a certain instant. Its direction is consistent with the tangent direction of the particle locus. Its size is:

Velocity

The unit is m / s, and s is the distance that the liquid particle flows in the time t. In hydraulics, we usually focus on space points to describe liquid motion. The velocity of a liquid particle passing through a certain space point is the point flow velocity u , which is generally a vector function of the space point position r and time t , that is, u = u ( r , t ) . In turbulence, the point velocity changes irregularly with time. Generally, the average instantaneous average velocity over a period of time, and the difference between the instantaneous velocity and the time-averaged velocity, that is, the pulsating velocity, are taken as the research objects.

Flow rate is the velocity of the fluid. When the flow velocity is very small, the fluid flows in layers without mixing with each other, which is called laminar flow or sheet flow. When the flow rate is gradually increased, the fluid's streamline begins to wavy. The frequency and amplitude of the swing increase with the increase of the flow velocity. Increased, this kind of flow condition is called transitional flow; when the flow velocity is increased, the flow line is no longer clearly identifiable, and there are many small eddies in the flow field, called turbulence, also known as turbulence, turbulence or turbulence .

This change can be quantified by Reynolds number. When the Reynolds number is small, the influence of the viscous force on the flow field is greater than the inertial force. The disturbance of the flow velocity in the flow field will be attenuated by the viscous force, and the fluid flow is stable, which is laminar. On the contrary, if the Reynolds number is large, the inertial force is The effect on the flow field is greater than the viscous force, the fluid flow is relatively unstable, and small changes in the flow rate are easy to develop and enhance, forming a turbulent and irregular turbulent flow field.

Velocity field and streamline

The collection of the velocity of each space point constitutes the velocity field. A streamline is a geometric representation of the velocity field. It is a curve composed of different fluid particles at the same instant, and the velocity vectors of all particles on the line are tangent to the curve. A streamline diagram composed of streamlines passing through points in the flow space at the same instant gives a clear picture of the entire flow at that instant. If the point velocity does not change with time, it is a constant flow; otherwise it is a non-constant flow. Streamlines and streamline diagrams for non-constant flows change over time.

Elementary and total flow

Pitot tube formation principle The cross-section of liquid flow that is perpendicular to the streamline everywhere is called the cross-flow section. The flow with infinitely small cross-sectional area is called elementary flow. The entire liquid flow (such as pipe flow, open channel flow) is composed of countless element flows, and the cross-sectional area is limited. It is a total flow analysis method commonly used in hydraulics to study the size and direction of the average flow velocity of the total flow interruption surface along the flow. The average flow velocity of the section is equal to the flow rate Q passing through the section divided by the section area A , that is, v = Q / A , and the direction is perpendicular to the cross-water section. The velocity profile of the cross section can also be used to indicate the uneven flow velocity distribution on the cross section. Taking tube flow as an example, the velocity on the wall surface is zero, and the velocity gradually increases from the wall surface to the tube axis. Due to the lateral mixing effect in turbulent flow, the velocity distribution is far more uniform than in laminar flow.

Flow rate formula measurement

According to different situations and requirements, different methods and instruments are used for measurement. The commonly used methods and instruments are as follows.

Pitot tube: Pioneered by French engineer H. Pito in 1732, it is still a commonly used instrument for measuring the average point flow rate in the laboratory. The front total pressure hole ( M ) and the side static pressure hole ( N ) are connected to the two ends of the differential pressure gauge through a double-layer sleeve with a round head (see the figure), and the total pressure is measured ( p 0 = P + u 2 / 2) The difference from static pressure (p) (incoming flow pressure). According to Bernoulli's equation, the point velocity is:

Point velocity

In the formula:

is the density of the liquid;

c is the correction coefficient, which must be obtained through calibration.

Rotary propeller (cup) type flow meter: There are many forms, which can be used in the field and laboratory. There are a set of rotatable blades. After the impact of the water flow, there is a fixed relationship between the number of blade rotations and the flow velocity. . Use appropriate equipment to convert the number of revolutions into electrical signals for recording and display. Laser velocimeter: The biggest advantage is that it does not interfere with the measured flow field. Hot wire (membrane) flow meter: It can measure the instantaneous flow rate, pulsating flow rate and turbulent flow rate. Photography method: Calculate the flow velocity based on the exposure time and the length of the bright spot trace.