TRL Aerodynamic Multi functional Platform

TRLAerodynamic multifunctional experimental platform (wind tunnel)

When a turbulent jet is injected into a stationary environment at a uniform velocity from the nozzle outlet, it forms a velocity discontinuous discontinuity with the surrounding stationary fluid. The discontinuity becomes unstable due to interference and generates vortices, which suck up the surrounding fluid and enter the jet. At the same time, it continuously moves, deforms, and splits to produce turbulence, and its influence gradually develops towards the inner and outer sides; Forming two freely turbulent mixing layers inside and outside, there is no external force along the flow direction because the static pressure of the entire jet field is equal.

Due to the lateral transfer of momentum, the fluid sucked in gains momentum and flows forward along with the originally ejected fluid. The original fluid loses momentum and its velocity decreases, while the momentum of the free jet remains unchanged. A certain velocity gradient is formed in the mixed layer, also known as the shear layer. The result of entrainment and mixing causes the jet cross-section to continuously expand, while the flow velocity decreases and the flow rate increases along the way.

Due to the intermittent phenomenon of turbulence, the jet boundary is actually an irregular surface composed of turbulent eddies and surrounding potential flows. However, in practical analysis, the jet boundary is regarded as a linearly extended interface from a statistical average perspective. Extend the boundary line of the main section and intersect it withOPoint,OThe point is the jet pole.

After forming a stable flow pattern, the turbulent jet is divided into several sections. The turbulent mixing part that extends from the nozzle boundary to the inside and outside is called the boundary layer. The central part maintains the original outlet velocity without being affected by mixing, which is called the jet core. The flow from the outlet to the section where the core disappears is called the starting section. The part where turbulence is fully developed is called the main section.

The longitudinal velocity distribution of each section of the jet body is similar and can be expressed using a dimensionless radius verification formula.

（21）

Where:UMeasurement point flow velocity,m/s;

U_mMeasure the flow velocity at the center point of the cross-section,m/s;

yThe distance from the measuring point to the axis,m;

y₀Jet half width can be estimated using the following formula

Where:RNozzle radius,m;

XThe distance from the nozzle outlet to the experimental section,m

θ jet diffusion angle.

The estimated value is for reference only, and the measurement point should be used during the experimentU=0To determine the conditions.

1experimental setup

The damping network voltage stabilizing box is equipped with this device2Different jet nozzles can be used, and one to two of them can be chosen during the experiment.In diameter30mmBoth cylindrical and convergent nozzle test sections can be used for circular jet experiments; In addition, it is equipped with a flat nozzle (wide)15mm, Long150mm）Can conduct flat jet experiments. The nozzle is placed at the bottom of the pressure regulator box on the test bench, and a pitot tube and measuring device are installed on the right side of the nozzleX ruler y rulerConnect the pitot tube and micro pressure gauge to measure the velocity distribution of each section.

Cylindrical nozzle radius γ₀ =15mm

Flat (slit shaped) nozzle

Half width of nozzle b₀ =15mm

Spray nozzle length i=150mm

IIBasic principle of the experiment:

㈠Determination of turbulence coefficient

⑴According to the motion characteristics of the jet，The non dimensional (dimensionless) velocity on each section of the jet starting section and basic section (range) has similarity with the distribution of non dimensional coordinates. Represented by the formula of jet motion characteristics, it is:

（1）

Where:yDistance between the measuring point and the axis（m）；y₀The half width of the jet can be calculated using the following formula:

Boundary layer thickness on the same cross-section (m);

υMeasurement point flow velocity（m/s）；

υ_m cross-sectionaxisFlow rate（m/s）。

This can verify the similarity of dimensionless velocity distributions.

⑵Formula for axial velocity of jet basic section

①Circular jet

（2）

②Flat jet

（3）

Where:γ₀Circular jet outlet radius (m)

b₀Half width of flat jet outlet (m)

Range of S-jet (m)

υ₀Flow velocity at the nozzle (m/s)

Select within the basic rangeThree cross-sections, measure the υ of each cross-section obtained₀ ，S and nozzle's υ₀andγ₀(orb₀）Substitute equation（2) Or equation (3), three alpha values are obtained, and the average value is taken to obtain the turbulence coefficient alpha.

㈡Measure flow velocity

（4）

Where:y₀Jet radius

r₀Nozzle radius

XDistance from nozzle outlet to experimental section

θdiffusion angle

5）

Where:uMeasurement point flow velocity

cBi custody correction factor0.98~1

Bi custody pressure difference

ρ air density

（6）

Where: u_mMeasure the flow velocity at the center of the cross-section

yThe distance between the measuring point and the center

1. Use a pitot tube to determine the center coordinate Y on the nozzle outlet section_centerAnd record the value.